Hindawi Publishing CorporationJournal of NanomaterialsVolume 2013 Article ID 952540 22 pageshttpdxdoiorg1011552013952540
Review ArticleMagnetic Nanoparticles A Subject for Both FundamentalResearch and Applications
S Bedanta1 A Barman2 W Kleemann3 O Petracic4 and T Seki5
1 School of Physical Sciences National Institute of Science Education and Research (NISER) IOP CampusBhubaneswar 751005 India
2Department of Condensed Matter Physics and Material Sciences S N Bose National Centre for Basic SciencesBlock-JD Sector-III Salt Lake City Kolkata 700098 India
3 Department of Physics University Duisburg-Essen 47057 Duisburg Germany4 Julich Centre for Neutron Science JCNS and Peter Grunberg Institute PGI JARA-FIT Forschungszentrum Julich GmbH52425 Julich Germany
5 Institute for Materials Research Tohoku University 2-1-1 Katahira Aoba-ku Sendai 980-8577 Japan
Correspondence should be addressed to S Bedanta sbedantaniseracin
Received 16 July 2013 Accepted 19 October 2013
Academic Editor Gaurav Mago
Copyright copy 2013 S Bedanta et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
Single domain magnetic nanoparticles (MNPs) have been a vivid subject of intense research for the last fifty years Preparationof magnetic nanoparticles and nanostructures has been achieved by both bottom-up and top-down approaches Single domainMNPs show Neel-Brown-like relaxation The Stoner-Wohlfarth model describes the angular dependence of the switching of themagnetization of a single domain particle in applied magnetic fields By varying the spacing between the particles the inter-particle interactions can be tuned This leads to various supermagnetic states such as superparamagnetism superspin glass andsuperferromagnetism Recently the study of the magnetization dynamics of such single domain MNPs has attracted particularattention and observations of various collective spin wave modes in patterned nanomagnet arrays have opened new avenues foron-chip microwave communications MNPs have the potential for various other applications such as future recording media andin medicine We will discuss the various aspects involved in the research on MNPs
1 Introduction
Modern technologies aided the invention of various newmagnetic materials synthetic structures micro- and nanos-tructures and metamaterials Magnetism has come a longway and found applications in a range of multidisciplinaryfields in present and future nanotechnologies like nonvolatilemagnetic memory [1] magnetic storage media [2] magneticrecording heads [3] magnetic resonance imaging [4] and inbiomedicine and health science [5] Emerging technologiessuch as spin logic [6 7] spin torque nano-oscillators (STNOs)[8] and magnonic crystals [9] have also become very active
The new technologies demand the invention of newmaterial properties which requires structuring of knownmaterials in all three dimensions at various length scalesand exploiting dynamical magnetic properties over various
timescales For various applications exploration of a vari-ety of new phenomena is required and this ranges fromslower processes such as domain wall and magnetic vortexdynamics to faster processes such as spin wave propagationand localization ultrafast demagnetization and relaxationThis introduces magnetic structures at various length scalessuch as nanodots microdisks magnetic nanowires andnanostripes Here we shall review the experimental andnumerical studies of properties of magnetic nanoparticles(MNPs) together with their leading preparation techniques
Research on MNPs has been a vivid research subjectover the last few decades not only for technological reasonsbut also from the fundamental research point of viewThe field of MNPs has been an interdisciplinary subjectwhere researchers from physics chemistry and biology putequal interest in synthesizing understanding and moving
2 Journal of Nanomaterials
forward for various applications In the last decade thoroughinvestigations have been made in the field of nanosizedmagnetic particles because of their potential for applicationssuch as data storage memory magnonic crystals permanentmagnets in biology for example improving the quality ofmagnetic resonance imaging (MRI) hyperthermic treatmentfor malignant cells site-specific drug delivery and manip-ulating cell membranes [10 11] Frenkel and Dorfman [12]first predicted that a particle of a FM material is expectedto consist of a single magnetic domain below a critical sizeKittel [13] made rough estimates of critical particle sizes Anapproximate radius of 10ndash1000 nm is found for a sphericalMNP of a FM material The magnitude of the magneticmoment 119898 of a particle is proportional to its volume Suchmonodomain FM particles can be viewed as a large magneticunit each having a magnetic moment of thousands of 120583
119861
Therefore these single domain magnetic nanoparticles arecalled ldquosuperspinsrdquo or ldquomacrospinsrdquo Usually an ellipsoidalshape of the particles is assumed where the magneticmoments have the tendency to align along the longest axiswhich defines the direction of lowest ldquoshaperdquo anisotropyenergy [14]
The critical radius 119903119888belowwhich a particle acts as a single
domain particle is given by [15]
119903119888asymp 9
(119860119870119906)12
12058301198722
119904
(1)
where 119860 is the exchange 119870119906is the uniaxial anisotropy
constant 1205830is the vacuum permeability and 119872
119904is the
saturation magnetization Typical values for 119903119888are about
15 nm for Fe 35 nm for Co and 30 nm for 120574-Fe2O3 while for
SmCo5it is as large as 750 nm [16] Depending on size and
material the magnetic moments of single domain particlescan be 103ndash105 120583B [17]
There are various models for the magnetization reversalof single domain particles A model for the coherent rotationof the magnetization was developed by Stoner andWohlfarth[18] They assumed noninteracting particles with uniaxialanisotropy inwhich the spins are parallel and rotate at unison
In this paper we highlight the state-of-the-art prepara-tion techniques for MNPs the magnetic states observed inensembles of MNPs and also some future applications ofMNPs In the beginning we go through both bottom-up andtop-down approaches for preparing magnetic nanoparticlesand nanostructures We also briefly discuss nanoparticlesuperlattices or supracrystals and templated self-organizationof nanoparticles Then we review some of the magneticground states such as superparamagnetism superspin glasssurface spin glass and superferromagnetism in ensemblesof MNPs Further we will discuss the recent progress infemto- and picosecondmagnetization dynamics in nanomag-nets particularly the ultrafast demagnetization relaxationprecession of magnetization and damping in single andarrays of nanomagnets Later the applications of magneticnanoparticles in various fields particularly in biology willbe highlighted Finally we discuss several major issues andchallenges in this field of research
2 Synthesis of Magnetic Nanoparticles
There are basically two types of approaches (i) bottom-up and (ii) top-down approaches to synthesize magneticnanoparticles and nanostructures
There are several important issues of nanoparticle syn-thesis [19] such as (i) obtaining a monodisperse particle sizedistribution (ii) control of the particle size in a reproduciblemanner (iii) obtaining materials with satisfactory high crys-tallinity and the desired crystal structure (iv) control over theshape of nanoparticles (v) stability of the nanoparticles overlong time
Synthesis of MNPs by chemical methods has been widelyused in the last few decades because it is one of the cheapestways of producing large quantities of the desired MNPs Inthe chemical method the particle size can be well controlledin the range from a few nanometers to micrometers Inchemical synthesis a short burst of nucleation followed byslow controlled growth is critical to produce monodisperseparticles [19]
Figure 1 shows the representative procedure for thepreparation of MNPs by chemical synthesis In the firstpart the rapid injection of the reagents often organometalliccompounds into hot surfactant solution induces the simul-taneous formation of nuclei [20] In the second part reagentsare mixed at low temperature and the resulting reactionmixtures are slowly heated in a controlledmanner to generatenuclei The particle growth occurs by subsequent additionof reactive species The particle size can also be increasedby aging at high temperature by Ostwald ripening duringwhich smaller nanoparticles dissolve and deposit at the biggerones [19] In chemical synthesis the particle size can becontrolled by systematically adjusting the various reactionparameters such as time temperature and concentration ofreagents and stabilizing surfactants Also during this processorganic reagents can be added whichwill form a shell aroundthe magnetic core an option to avoid any agglomeration ofMNPs
Another important technique to synthesize MNPs is themicroemulsion approach In this technique two immiscibleliquids form a thermodynamic stable isotropic dispersiondenominatedmicroemulsion where themicrodomain of oneor both liquids is stabilized by an interfacial film of surfactantmolecules [21] Various magnetic nanoparticles have beenprepared by the microemulsion technique some bodies ofliterature about which are given by references of Lu et al [21]Details of various approaches of chemical synthesis of MNPsare described in various recent articles [19 21ndash26]
Sun et al [27] synthesized FePt nanoparticles using achemical method FePt is a highly interesting alloy sinceL10-ordering of FePt leads to the large uniaxial magnetic
anisotropy (119870119906
sim 7 times 106 Jm3) which is comparable to
the values for rare earth-based permanent magnets Theysuccessfully prepared monodisperse FePt nanoparticles viaa reduction process The MNPs were regularly aligned onthe substrate However the nanoparticles still included thedisordered phase that is the L1
0-ordering was not complete
because of the limited annealing temperature In addition theimprovement of hard magnetic properties is indispensable
Journal of Nanomaterials 3
Injection ofreagents
at high temperature
Hotsurfactantsolution
Aging
Surfactant
Size-selectionprocess
Figure 1 The procedure for synthesis of monodisperse nanoparti-cles by injecting reagents into the hot surfactant solution followed byaging and size-selection process Reproduced with permission fromHyeon [19] copy 2003 Royal Society of Chemistry
10nm
Figure 2 Scanning tunneling microscope image of Fe particlesdeposited on top of an insulating MgO layer Reproduced withpermission from Ernult et al [34] copy 2005 AIP Publishing LLC
for the applications Following the above report by Sun etal [27] many experiments on the preparation of L1
0-FePt
nanoparticles have been carried out using similar solutionprocesses [28ndash30] Nanoparticles consisting of exchange-coupled nanocomposites were also fabricated by Zeng etal [31] A different chemical method was reported usingnanospheres Self-assembled nanospheres are utilized as atemplate for the preparation of nanoparticle arrays [32 33]Albrecht et al [32] formed a topographic pattern of themagnetic multilayer on spherical nanoparticles where thenanostructures were both monodisperse and magneticallyisolated They found an unexpected switching behaviorinduced by their spherical shape
The use of film growth via vapor deposition techniquesis another method for self-assembly of nanoparticles TheVolmer-Weber (V-W)mode is a film growth giving an island-like morphology This growth mode is achieved by selectingsubstrate material layer thickness and growth temperatureadequately Ernult et al prepared self-aligned Fe nanoparti-cles on MgO employing a molecular beam epitaxy apparatus[34] Figure 2 shows a typical scanning tunnelingmicroscope
image of Fe particles deposited on top of an MgO layer Theyalso observed a transition from a three-dimensional V-Wtype growth mode to a two-dimensional growth when thelayer thickness was increased Such metallic nanoparticleson an insulating layer become an important componentas a structure of electrodeinsulating layernanoparticlesThis allows the injection of an electrical current into thenanoparticles through the insulating layer playing a majorrole in devices such as single electron transistors Black et al[35] reported the transport measurement performed in self-assembledCoparticles In the case ofmagnetic nanoparticlesspin-dependent single electron tunneling is expected [36]and Yakushiji et al [37] found an enhancement of spinlifetime in MNPs
A particulate film also provides us with an interestingchange in the magnetization reversal behavior from theincoherent reversal mode such as domain wall motion to thecoherent rotation mode This change sometimes enhanceshardmagnetic properties such as coercivity (119867
119888) Shima et al
[38] deposited FePt on MgO at high substrate temperatureWhen the FePt layer thickness was reduced from 100 nm to10 nm the film morphology was drastically changed from acontinuous state to the particulate one as shown in Figure 3The formation of the particulate film significantly enhanced119867119888of FePt They also achieved the huge 119867
119888= 105 kOe at
42 K for the FePt particulate film [39] Okamoto et al [40]reported that themorphology change with the FePt thicknessled to a change in themagnetization reversal process Furthergranular films in which the MNPs are embedded into anonmagneticmatrix can isolate theMNPswhich are suitablefor next-generation ultrahigh density magnetic recordingmedia
There are several other methods to prepare MNPs viabottom-up approach such as electrochemical reactions [41]sol-gel processing [42] plasma or flame spraying synthesis[43] chemical vapor deposition [44] atomic or molecularcondensation [20] sputtering and thermal evaporation [45]and bio-assisted synthesis [11] The top-down approach viavarious types of lithography will be discussed in the nextsection of this paper
3 Magnetic Nanostructures by Lithography
In contrast to the self-assembly of the bottom-up techniquesthe top-down techniques employ the microfabrication pro-cesses such as lithographical patterning liftoff ion millingor wet etching The top-down techniques have advantagesthat we can accurately control the size and the shape ofnanostructures and it is easy to align the nanostructures asdesired However high cost and slow manufacturing of thetop-down techniques are drawbacks for themass productionIn addition the size limitation of the nanostructure dependson the kind of lithography If one uses an electron beam(EB) as a writer that is EB lithography the size can bereduced down to a few tens of nanometer although it takeslong time to make patterns Photolithography saves the timefor patterning owing to the large area exposure while the
4 Journal of Nanomaterials
400nm
(a)
(c)
(e)
(b)
(d)
(f)
Figure 3 Transmission electron microscope images for FePt thin films with thicknesses of (a) 10 nm (b) 15 nm (c) 20 nm (d) 45 nm (e)50 nm and (f) 60 nm Reproduced with permission from Shima et al [38] copy 2002 AIP Publishing LLC
Film
Resist
Substrate
(a) Etching (milling)
Resist
Substrate
Film
(b) Liftoff
Figure 4 Schematic illustrations of the microfabrication processes(a) etching (milling) and (b) liftoff
minimum size is mainly determined by the wavelength of thelight source
Figure 4 illustrates the steps for microfabrication pro-cesses using (a) etching or milling and (b) liftoff In thecase of the etching process a film is first deposited on asubstrate Then a resist for lithographical patterning is spin-coated onto the film and a desired pattern of the resist isformed Finally the film is etched through the resist maskand the desired pattern is obtained Contrastingly a resistis first spin-coated onto a substrate in the case of the lift-off process After patterning the resist a film is depositedon the substrate with the patterned resist The thin filmdeposited on the region without resist finally remains afterlifting the resist off from the substrate Both methods haveadvantages and disadvantages For example although thelift-off process is free from etching damage the substratesurface is contaminated by the resist that can be suppressedin the etching process There are many reports on themagnetic properties for nanostructured elements of NiFeFe Co and CoPt [47ndash50] Such nanostructures showed thecharacteristic magnetization reversal behavior and magneticdomain structures For example Shinjo et al [51] reported
Journal of Nanomaterials 5
(a) D = 500nm (b) D = 100nm
(c) D = 30nm (d) D = 30nm
1000nm 300nm
100nm 500nm
Figure 5 Atomic force microscope images for the L10-FePt circular dots with diameters (119863) of (a) 500 nm (b) 100 nm and (c) 30 nm and
(d) a scanning electron microscope image for119863 = 30 nm Reproduced with permission from Seki et al [46] copy 2011 IOP Publishing
the first observation of magnetic vortex core in NiFe disksprepared using the top-down technique The formation ofthe magnetic vortex is attributable to the balance betweenmagnetic exchange energy and magnetostatic energy in aconfined magnetic disk The top-down techniques are alsoused to fabricate magnetic nanopillars and nanowires whichare key elements for future spintronic devices such as mag-netic random access memories [52] and racetrack memories[53] respectively
As well as the bottom-up technique one can control themagnetization reversal mode in a magnetic nanostructurefabricated by the top-down technique Figure 5 displays theatomic force microscope images and the scanning electronmicroscope image for L1
0-FePt circular dots with various
diameters which were fabricated through the use of EBlithography and Ar ion milling [46] These images supportthat FePtwas patterned intowell-defined circular shapes evenin the case of the diameter of 30 nm As the dot diameterwas reduced from 1 120583m to 30 nm the magnetization reversalmode was changed as in the case of L1
0-FePt nanoparticles
prepared by the bottom-up technique Figure 6 shows the
magnetic field angular dependence of normalized119867119888for the
perpendicularly magnetized L10-FePt dots with various dot
diameters where 120579 is the polar angle of the magnetic fieldand 120579 = 0
∘ and 90∘ correspond to the normal and paralleldirections to the plane of the device 119867
119888was normalized
by the value of 119867119888at 120579 = 0
∘ The magnetization reversalfor the dots with a large diameter is governed by domainwall motion where the angular dependence follows therelationship of 1 cos 120579 On the other hand the 120579 dependenceof normalized 119867
119888deviates from the tendency of 1 cos 120579
with decreasing dot diameter and the local minimum of 119867119888
appears around 120579 = 45∘ This implies that the magnetization
reversal for dots with a small diameter occurs through theincoherent magnetization rotation A detailed analysis ofmagnetization reversal was also carried out for a singleFePt dot using the anomalous Hall effect [54] In addi-tion the nucleation phenomenon of the reversed magneticdomains was discussed using microfabricated FePt dots [55]Consequently nanostructures fabricated by the top-downtechnique are useful for doing systematic investigations ofthe magnetization reversal behavior and magnetic domain
6 Journal of Nanomaterials
30
25
20
15
10
05
000 20 40 60 80
DW motion
S-W rotation
30nm50nm100nm
500nm1120583m
120579 (∘)
HcH
(120579=0∘)
c
Figure 6 Magnetic field angular dependence of normalized coer-civity (119867
119888) for L1
0-FePt dots with diameters of 1120583m (solid squares)
500 nm (open triangles) 100 nm (solid triangles) 50 nm (opencircles) and 30 nm (solid circles) The dashed line denotes 1 cos 120579whereas the dashed-dotted line denotes the Stoner-Wohlfarth-typecoherent rotation
structures at the nanometer scale owing to their well-definedgeometries
4 Nanoparticle Superlattices or Supracrystals
A particularly exciting novel approach in NP research is tofabricate and to study self-organized assemblies of MNPs[56ndash58] They constitute an analogy to conventional crys-talline materials because atoms are replaced by MNPs andspins are replaced by superspins Instead of an atomic crystalone rather speaks of so-calledMNP ldquosupracrystalsrdquo or ldquosuper-latticesrdquo [58 59] Such systems thus represent a new classof materials where their properties are determined on theone hand by the properties of the individual MNPs as theirbuilding blocks and on the other hand by themutual interac-tions Therefore interesting behavior and novel applicationscan be expected from the controlled tuning of individual andcollective properties of the superlattices
NP superlattices can be fabricated by various techniquesBasically one has to distinguish four different classes ofmethods namely as follows
(i) Physical vapor deposition methods they include forexample thermal vapor deposition or sputtering Inthis case the MNPs of magnetic metals such asCo80Fe20
are formed spontaneously during deposi-tion by nonwetting Volmer-Weber-type growth ontoa suitable substrate material like SiO
2and Al
2O3 The
advantage is a completely ldquoorganics-freerdquo procedure
500nm
Figure 7 Scanning electron microscopy image of a 3-dimensionalassembly of iron oxide nanoparticles with 20 nm diameter on a Sisubstrate [62]
however the order of self-organization of these dis-continuous metal-insulator multilayers (DMIMs) isusually relatively weak and the shape of the MNPs isnot well controllable [60 61]
(ii) MNPs from the gas phase here MNPs are formedfor example by nucleation and growth from a super-saturated vapor In a subsequent step the MNPs areldquolandedrdquo onto a substrate [63 64] Advantages are theability to form spherical MNPs from many differentmaterials with relatively well-controllable diametersHowever also here the order of self-organizationonce deposited is relatively poor
(iii) Chemically prepared MNPs in this case the MNPsare synthesized by chemical processes and are dis-persed in a solvent Self-assembly onto a substratethen occurs by involving colloidal ordering processesduring evaporation of the solvent [21 56ndash58 6265] In this case superlattices of excellent qualitycan be prepared Figure 7 shows an example of a3-dimensional assembly of 20 nm iron oxide MNPs[62]
This third class of methods one could call it ldquocolloidal self-assembly techniquesrdquo comprises a large number of variousapproaches Tomention only a few examples there are [66 67]the following
(i) ldquoDrop-castingrdquo namely applying a droplet of theMNP dispersion onto a substrate and letting thesolvent evaporate [68] This method often producesvery thick 3-dimensional superlattices of relativelyhigh quality However the thickness of the MNPfilm varies significantly over the substrate surface andbasically no control of the self-assembly process ispossible
(ii) ldquoSpin-coatingrdquo or rotation-coating which is similar tocoating a substrate with photosensitive or electron-beam sensitive resist in nanolithography This pro-duces films of very good homogeneity Even sub-monolayer MNP films are possible by this approach
Journal of Nanomaterials 7
Figure 8 TEM image of a binaryMNP superlattice composed of 56and 105 nm Fe
3O4NPsThe inset shows the structural model of the
[001] projection of this lattice The scale bar corresponds to 20 nmReproducedwith permission fromChen et al [71] copy 2010 AmericanChemical Society
[69] However one achieves hereby only short-rangeordered superlattices probably due to the extremelyquick process where the MNPs have little time toassemble The control parameter ismdashapart from thechoice of the solvent as in any other techniquemdashbasically only the spin-speed
(iii) ldquoLangmuir-Blodgettrdquo technique in this method asubstrate is initially immersed into the MNP dis-persion Using a high-precision motorized stage thesubstrate is slowly pulled out of the dispersion at aconstant angle and at a constant speed Both parame-ters are control parameters of the process [66] Thismethod is in principle capable of producing high-quality monolayer MNP films
(iv) ldquoSedimentationrdquo the substrate is placed at the bottomof the container where initially theMNPdispersion isfilled inThe solvent evaporates slowly and the MNPsldquosedimentrdquo onto the substrate Control parameters arethe evaporation rate the type of the vessel (eg itsmaterial) the temperature and the use of sonicationor not
Further methods are for example ldquoLangmuir-Schaferrdquo andldquocapillaryrdquo techniques [67] and various methods exploitingthe self-assembly at a liquid-liquid or liquid-air interface [70]By such methods it is possible to fabricate MNP films ofexcellent order extending over several micrometers and eveninvolving particles of two or three different sizes (see egFigure 8)
The magnetic properties of such MNP superlattices havebeen in the focus of many current studies [27 56 68 72ndash76] In most cases the collective magnetic behavior of thesuperlatticemdashbeing a consequence of dipolar interactionsmdashis intensely investigated Although the physics of dipolarlyinteracting 2D or 3D lattices seems to be a ldquoclassicrdquo andsolved topic in magnetism the community is far from aconsensus This is due to the huge complexity of the systemwhere dipolar interactions are long-range and anisotropicand thus lead to both frustrated and highly correlatedbehavior like superspin glass or superferromagnetism (seeSection 7) Therefore the behavior of such arrays is hard to
predict and many parameters like shape of the entire systemanisotropies order versus disorder andmanymore influencethe properties significantly
5 Superparamagnetism
Small enough FM particles will be single domain sincethe energy cost of domain wall formation outweighs anysaving of demagnetizing energy In these single domainFM particles the magnetization is often considered to lieparallel or antiparallel to a particular direction called the easyaxis This can be due to different anisotropy contributionsfor example magnetocrystalline shape strain and surfaceanisotropies [77] Let us consider an assembly of uniaxialsingle domain particles each with an anisotropy energydensity 119864 = 119870119881sin2120579 For a particle the energy barrierΔ119864119861
= 119870119881 separates the two energy minima at 120579 = 0
and 120579 = 120587 corresponding to the magnetization parallelor antiparallel to the easy axis as shown in Figure 9 Neelpointed out [78] that for small enough single domain particlesKV may become so small that energy fluctuations canovercome the anisotropy energy and spontaneously reversethe magnetization of a particle from one easy direction tothe other even in the absence of an applied field In thelimit 119896
119861119879 ≫ 119870119881 the particle can be considered freely
fluctuating (119896119861= Boltzmannrsquos constant) A FM nanoparticle
is defined as superparamagnetic (SPM) when the energybarrier E
119861 for a magnetization reversal is comparable to
the thermal energy k119861T This behavior has been discussed
in the literature also under several other names includingldquoapparent paramagnetismrdquo [79] ldquocollective paramagnetismrdquo[80] ldquoquasiparamagnetismrdquo [81] and ldquosubdomain behaviorrdquo[82] This is the isotropic SPM limit The direction of thesuperspin or macrospin fluctuates with a frequency 119891 or acharacteristic relaxation time 120591 = (2120587119891)
minus1 The Neel-Brownexpression [78 83] for the relaxation time is given by
120591 = 1205910exp(119870119881
119896119861119879
) (2)
where 1205910sim 10minus10 s is the inverse angular attempt frequency
The fluctuations thus slow down (120591 increases) as the sampleis cooled to lower temperatures The system appears staticwhen the SPM relaxation time 120591 becomes much longerthan the experimental measuring time 120591
119898 Only if 120591 is
shorter compared to 120591119898 one may observe an average value
of the magnetization When this SPM relaxation time 120591
becomes comparable to 120591119898 the particle is said to be blocked
The magnetic behavior of the particle is characterized bythe so-called ldquoblockingrdquo temperature 119879
119887 below which the
particle moments appear frozen on the time scale of themeasurement 120591
119898asymp 120591 Inverting (2) one obtains
119879119887asymp
119870119881
119896119861ln (1205911198981205910)
(3)
Equation (3) is valid for individual particles or a system ofnoninteracting particles with the same size and anisotropyIf the particles are not monodisperse the distribution of
8 Journal of Nanomaterials
Free
ener
gy
0 1205872 120587
120579
ΔEB
Easy axis
M
120579
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
forward for various applications In the last decade thoroughinvestigations have been made in the field of nanosizedmagnetic particles because of their potential for applicationssuch as data storage memory magnonic crystals permanentmagnets in biology for example improving the quality ofmagnetic resonance imaging (MRI) hyperthermic treatmentfor malignant cells site-specific drug delivery and manip-ulating cell membranes [10 11] Frenkel and Dorfman [12]first predicted that a particle of a FM material is expectedto consist of a single magnetic domain below a critical sizeKittel [13] made rough estimates of critical particle sizes Anapproximate radius of 10ndash1000 nm is found for a sphericalMNP of a FM material The magnitude of the magneticmoment 119898 of a particle is proportional to its volume Suchmonodomain FM particles can be viewed as a large magneticunit each having a magnetic moment of thousands of 120583
119861
Therefore these single domain magnetic nanoparticles arecalled ldquosuperspinsrdquo or ldquomacrospinsrdquo Usually an ellipsoidalshape of the particles is assumed where the magneticmoments have the tendency to align along the longest axiswhich defines the direction of lowest ldquoshaperdquo anisotropyenergy [14]
The critical radius 119903119888belowwhich a particle acts as a single
domain particle is given by [15]
119903119888asymp 9
(119860119870119906)12
12058301198722
119904
(1)
where 119860 is the exchange 119870119906is the uniaxial anisotropy
constant 1205830is the vacuum permeability and 119872
119904is the
saturation magnetization Typical values for 119903119888are about
15 nm for Fe 35 nm for Co and 30 nm for 120574-Fe2O3 while for
SmCo5it is as large as 750 nm [16] Depending on size and
material the magnetic moments of single domain particlescan be 103ndash105 120583B [17]
There are various models for the magnetization reversalof single domain particles A model for the coherent rotationof the magnetization was developed by Stoner andWohlfarth[18] They assumed noninteracting particles with uniaxialanisotropy inwhich the spins are parallel and rotate at unison
In this paper we highlight the state-of-the-art prepara-tion techniques for MNPs the magnetic states observed inensembles of MNPs and also some future applications ofMNPs In the beginning we go through both bottom-up andtop-down approaches for preparing magnetic nanoparticlesand nanostructures We also briefly discuss nanoparticlesuperlattices or supracrystals and templated self-organizationof nanoparticles Then we review some of the magneticground states such as superparamagnetism superspin glasssurface spin glass and superferromagnetism in ensemblesof MNPs Further we will discuss the recent progress infemto- and picosecondmagnetization dynamics in nanomag-nets particularly the ultrafast demagnetization relaxationprecession of magnetization and damping in single andarrays of nanomagnets Later the applications of magneticnanoparticles in various fields particularly in biology willbe highlighted Finally we discuss several major issues andchallenges in this field of research
2 Synthesis of Magnetic Nanoparticles
There are basically two types of approaches (i) bottom-up and (ii) top-down approaches to synthesize magneticnanoparticles and nanostructures
There are several important issues of nanoparticle syn-thesis [19] such as (i) obtaining a monodisperse particle sizedistribution (ii) control of the particle size in a reproduciblemanner (iii) obtaining materials with satisfactory high crys-tallinity and the desired crystal structure (iv) control over theshape of nanoparticles (v) stability of the nanoparticles overlong time
Synthesis of MNPs by chemical methods has been widelyused in the last few decades because it is one of the cheapestways of producing large quantities of the desired MNPs Inthe chemical method the particle size can be well controlledin the range from a few nanometers to micrometers Inchemical synthesis a short burst of nucleation followed byslow controlled growth is critical to produce monodisperseparticles [19]
Figure 1 shows the representative procedure for thepreparation of MNPs by chemical synthesis In the firstpart the rapid injection of the reagents often organometalliccompounds into hot surfactant solution induces the simul-taneous formation of nuclei [20] In the second part reagentsare mixed at low temperature and the resulting reactionmixtures are slowly heated in a controlledmanner to generatenuclei The particle growth occurs by subsequent additionof reactive species The particle size can also be increasedby aging at high temperature by Ostwald ripening duringwhich smaller nanoparticles dissolve and deposit at the biggerones [19] In chemical synthesis the particle size can becontrolled by systematically adjusting the various reactionparameters such as time temperature and concentration ofreagents and stabilizing surfactants Also during this processorganic reagents can be added whichwill form a shell aroundthe magnetic core an option to avoid any agglomeration ofMNPs
Another important technique to synthesize MNPs is themicroemulsion approach In this technique two immiscibleliquids form a thermodynamic stable isotropic dispersiondenominatedmicroemulsion where themicrodomain of oneor both liquids is stabilized by an interfacial film of surfactantmolecules [21] Various magnetic nanoparticles have beenprepared by the microemulsion technique some bodies ofliterature about which are given by references of Lu et al [21]Details of various approaches of chemical synthesis of MNPsare described in various recent articles [19 21ndash26]
Sun et al [27] synthesized FePt nanoparticles using achemical method FePt is a highly interesting alloy sinceL10-ordering of FePt leads to the large uniaxial magnetic
anisotropy (119870119906
sim 7 times 106 Jm3) which is comparable to
the values for rare earth-based permanent magnets Theysuccessfully prepared monodisperse FePt nanoparticles viaa reduction process The MNPs were regularly aligned onthe substrate However the nanoparticles still included thedisordered phase that is the L1
0-ordering was not complete
because of the limited annealing temperature In addition theimprovement of hard magnetic properties is indispensable
Journal of Nanomaterials 3
Injection ofreagents
at high temperature
Hotsurfactantsolution
Aging
Surfactant
Size-selectionprocess
Figure 1 The procedure for synthesis of monodisperse nanoparti-cles by injecting reagents into the hot surfactant solution followed byaging and size-selection process Reproduced with permission fromHyeon [19] copy 2003 Royal Society of Chemistry
10nm
Figure 2 Scanning tunneling microscope image of Fe particlesdeposited on top of an insulating MgO layer Reproduced withpermission from Ernult et al [34] copy 2005 AIP Publishing LLC
for the applications Following the above report by Sun etal [27] many experiments on the preparation of L1
0-FePt
nanoparticles have been carried out using similar solutionprocesses [28ndash30] Nanoparticles consisting of exchange-coupled nanocomposites were also fabricated by Zeng etal [31] A different chemical method was reported usingnanospheres Self-assembled nanospheres are utilized as atemplate for the preparation of nanoparticle arrays [32 33]Albrecht et al [32] formed a topographic pattern of themagnetic multilayer on spherical nanoparticles where thenanostructures were both monodisperse and magneticallyisolated They found an unexpected switching behaviorinduced by their spherical shape
The use of film growth via vapor deposition techniquesis another method for self-assembly of nanoparticles TheVolmer-Weber (V-W)mode is a film growth giving an island-like morphology This growth mode is achieved by selectingsubstrate material layer thickness and growth temperatureadequately Ernult et al prepared self-aligned Fe nanoparti-cles on MgO employing a molecular beam epitaxy apparatus[34] Figure 2 shows a typical scanning tunnelingmicroscope
image of Fe particles deposited on top of an MgO layer Theyalso observed a transition from a three-dimensional V-Wtype growth mode to a two-dimensional growth when thelayer thickness was increased Such metallic nanoparticleson an insulating layer become an important componentas a structure of electrodeinsulating layernanoparticlesThis allows the injection of an electrical current into thenanoparticles through the insulating layer playing a majorrole in devices such as single electron transistors Black et al[35] reported the transport measurement performed in self-assembledCoparticles In the case ofmagnetic nanoparticlesspin-dependent single electron tunneling is expected [36]and Yakushiji et al [37] found an enhancement of spinlifetime in MNPs
A particulate film also provides us with an interestingchange in the magnetization reversal behavior from theincoherent reversal mode such as domain wall motion to thecoherent rotation mode This change sometimes enhanceshardmagnetic properties such as coercivity (119867
119888) Shima et al
[38] deposited FePt on MgO at high substrate temperatureWhen the FePt layer thickness was reduced from 100 nm to10 nm the film morphology was drastically changed from acontinuous state to the particulate one as shown in Figure 3The formation of the particulate film significantly enhanced119867119888of FePt They also achieved the huge 119867
119888= 105 kOe at
42 K for the FePt particulate film [39] Okamoto et al [40]reported that themorphology change with the FePt thicknessled to a change in themagnetization reversal process Furthergranular films in which the MNPs are embedded into anonmagneticmatrix can isolate theMNPswhich are suitablefor next-generation ultrahigh density magnetic recordingmedia
There are several other methods to prepare MNPs viabottom-up approach such as electrochemical reactions [41]sol-gel processing [42] plasma or flame spraying synthesis[43] chemical vapor deposition [44] atomic or molecularcondensation [20] sputtering and thermal evaporation [45]and bio-assisted synthesis [11] The top-down approach viavarious types of lithography will be discussed in the nextsection of this paper
3 Magnetic Nanostructures by Lithography
In contrast to the self-assembly of the bottom-up techniquesthe top-down techniques employ the microfabrication pro-cesses such as lithographical patterning liftoff ion millingor wet etching The top-down techniques have advantagesthat we can accurately control the size and the shape ofnanostructures and it is easy to align the nanostructures asdesired However high cost and slow manufacturing of thetop-down techniques are drawbacks for themass productionIn addition the size limitation of the nanostructure dependson the kind of lithography If one uses an electron beam(EB) as a writer that is EB lithography the size can bereduced down to a few tens of nanometer although it takeslong time to make patterns Photolithography saves the timefor patterning owing to the large area exposure while the
4 Journal of Nanomaterials
400nm
(a)
(c)
(e)
(b)
(d)
(f)
Figure 3 Transmission electron microscope images for FePt thin films with thicknesses of (a) 10 nm (b) 15 nm (c) 20 nm (d) 45 nm (e)50 nm and (f) 60 nm Reproduced with permission from Shima et al [38] copy 2002 AIP Publishing LLC
Film
Resist
Substrate
(a) Etching (milling)
Resist
Substrate
Film
(b) Liftoff
Figure 4 Schematic illustrations of the microfabrication processes(a) etching (milling) and (b) liftoff
minimum size is mainly determined by the wavelength of thelight source
Figure 4 illustrates the steps for microfabrication pro-cesses using (a) etching or milling and (b) liftoff In thecase of the etching process a film is first deposited on asubstrate Then a resist for lithographical patterning is spin-coated onto the film and a desired pattern of the resist isformed Finally the film is etched through the resist maskand the desired pattern is obtained Contrastingly a resistis first spin-coated onto a substrate in the case of the lift-off process After patterning the resist a film is depositedon the substrate with the patterned resist The thin filmdeposited on the region without resist finally remains afterlifting the resist off from the substrate Both methods haveadvantages and disadvantages For example although thelift-off process is free from etching damage the substratesurface is contaminated by the resist that can be suppressedin the etching process There are many reports on themagnetic properties for nanostructured elements of NiFeFe Co and CoPt [47ndash50] Such nanostructures showed thecharacteristic magnetization reversal behavior and magneticdomain structures For example Shinjo et al [51] reported
Journal of Nanomaterials 5
(a) D = 500nm (b) D = 100nm
(c) D = 30nm (d) D = 30nm
1000nm 300nm
100nm 500nm
Figure 5 Atomic force microscope images for the L10-FePt circular dots with diameters (119863) of (a) 500 nm (b) 100 nm and (c) 30 nm and
(d) a scanning electron microscope image for119863 = 30 nm Reproduced with permission from Seki et al [46] copy 2011 IOP Publishing
the first observation of magnetic vortex core in NiFe disksprepared using the top-down technique The formation ofthe magnetic vortex is attributable to the balance betweenmagnetic exchange energy and magnetostatic energy in aconfined magnetic disk The top-down techniques are alsoused to fabricate magnetic nanopillars and nanowires whichare key elements for future spintronic devices such as mag-netic random access memories [52] and racetrack memories[53] respectively
As well as the bottom-up technique one can control themagnetization reversal mode in a magnetic nanostructurefabricated by the top-down technique Figure 5 displays theatomic force microscope images and the scanning electronmicroscope image for L1
0-FePt circular dots with various
diameters which were fabricated through the use of EBlithography and Ar ion milling [46] These images supportthat FePtwas patterned intowell-defined circular shapes evenin the case of the diameter of 30 nm As the dot diameterwas reduced from 1 120583m to 30 nm the magnetization reversalmode was changed as in the case of L1
0-FePt nanoparticles
prepared by the bottom-up technique Figure 6 shows the
magnetic field angular dependence of normalized119867119888for the
perpendicularly magnetized L10-FePt dots with various dot
diameters where 120579 is the polar angle of the magnetic fieldand 120579 = 0
∘ and 90∘ correspond to the normal and paralleldirections to the plane of the device 119867
119888was normalized
by the value of 119867119888at 120579 = 0
∘ The magnetization reversalfor the dots with a large diameter is governed by domainwall motion where the angular dependence follows therelationship of 1 cos 120579 On the other hand the 120579 dependenceof normalized 119867
119888deviates from the tendency of 1 cos 120579
with decreasing dot diameter and the local minimum of 119867119888
appears around 120579 = 45∘ This implies that the magnetization
reversal for dots with a small diameter occurs through theincoherent magnetization rotation A detailed analysis ofmagnetization reversal was also carried out for a singleFePt dot using the anomalous Hall effect [54] In addi-tion the nucleation phenomenon of the reversed magneticdomains was discussed using microfabricated FePt dots [55]Consequently nanostructures fabricated by the top-downtechnique are useful for doing systematic investigations ofthe magnetization reversal behavior and magnetic domain
6 Journal of Nanomaterials
30
25
20
15
10
05
000 20 40 60 80
DW motion
S-W rotation
30nm50nm100nm
500nm1120583m
120579 (∘)
HcH
(120579=0∘)
c
Figure 6 Magnetic field angular dependence of normalized coer-civity (119867
119888) for L1
0-FePt dots with diameters of 1120583m (solid squares)
500 nm (open triangles) 100 nm (solid triangles) 50 nm (opencircles) and 30 nm (solid circles) The dashed line denotes 1 cos 120579whereas the dashed-dotted line denotes the Stoner-Wohlfarth-typecoherent rotation
structures at the nanometer scale owing to their well-definedgeometries
4 Nanoparticle Superlattices or Supracrystals
A particularly exciting novel approach in NP research is tofabricate and to study self-organized assemblies of MNPs[56ndash58] They constitute an analogy to conventional crys-talline materials because atoms are replaced by MNPs andspins are replaced by superspins Instead of an atomic crystalone rather speaks of so-calledMNP ldquosupracrystalsrdquo or ldquosuper-latticesrdquo [58 59] Such systems thus represent a new classof materials where their properties are determined on theone hand by the properties of the individual MNPs as theirbuilding blocks and on the other hand by themutual interac-tions Therefore interesting behavior and novel applicationscan be expected from the controlled tuning of individual andcollective properties of the superlattices
NP superlattices can be fabricated by various techniquesBasically one has to distinguish four different classes ofmethods namely as follows
(i) Physical vapor deposition methods they include forexample thermal vapor deposition or sputtering Inthis case the MNPs of magnetic metals such asCo80Fe20
are formed spontaneously during deposi-tion by nonwetting Volmer-Weber-type growth ontoa suitable substrate material like SiO
2and Al
2O3 The
advantage is a completely ldquoorganics-freerdquo procedure
500nm
Figure 7 Scanning electron microscopy image of a 3-dimensionalassembly of iron oxide nanoparticles with 20 nm diameter on a Sisubstrate [62]
however the order of self-organization of these dis-continuous metal-insulator multilayers (DMIMs) isusually relatively weak and the shape of the MNPs isnot well controllable [60 61]
(ii) MNPs from the gas phase here MNPs are formedfor example by nucleation and growth from a super-saturated vapor In a subsequent step the MNPs areldquolandedrdquo onto a substrate [63 64] Advantages are theability to form spherical MNPs from many differentmaterials with relatively well-controllable diametersHowever also here the order of self-organizationonce deposited is relatively poor
(iii) Chemically prepared MNPs in this case the MNPsare synthesized by chemical processes and are dis-persed in a solvent Self-assembly onto a substratethen occurs by involving colloidal ordering processesduring evaporation of the solvent [21 56ndash58 6265] In this case superlattices of excellent qualitycan be prepared Figure 7 shows an example of a3-dimensional assembly of 20 nm iron oxide MNPs[62]
This third class of methods one could call it ldquocolloidal self-assembly techniquesrdquo comprises a large number of variousapproaches Tomention only a few examples there are [66 67]the following
(i) ldquoDrop-castingrdquo namely applying a droplet of theMNP dispersion onto a substrate and letting thesolvent evaporate [68] This method often producesvery thick 3-dimensional superlattices of relativelyhigh quality However the thickness of the MNPfilm varies significantly over the substrate surface andbasically no control of the self-assembly process ispossible
(ii) ldquoSpin-coatingrdquo or rotation-coating which is similar tocoating a substrate with photosensitive or electron-beam sensitive resist in nanolithography This pro-duces films of very good homogeneity Even sub-monolayer MNP films are possible by this approach
Journal of Nanomaterials 7
Figure 8 TEM image of a binaryMNP superlattice composed of 56and 105 nm Fe
3O4NPsThe inset shows the structural model of the
[001] projection of this lattice The scale bar corresponds to 20 nmReproducedwith permission fromChen et al [71] copy 2010 AmericanChemical Society
[69] However one achieves hereby only short-rangeordered superlattices probably due to the extremelyquick process where the MNPs have little time toassemble The control parameter ismdashapart from thechoice of the solvent as in any other techniquemdashbasically only the spin-speed
(iii) ldquoLangmuir-Blodgettrdquo technique in this method asubstrate is initially immersed into the MNP dis-persion Using a high-precision motorized stage thesubstrate is slowly pulled out of the dispersion at aconstant angle and at a constant speed Both parame-ters are control parameters of the process [66] Thismethod is in principle capable of producing high-quality monolayer MNP films
(iv) ldquoSedimentationrdquo the substrate is placed at the bottomof the container where initially theMNPdispersion isfilled inThe solvent evaporates slowly and the MNPsldquosedimentrdquo onto the substrate Control parameters arethe evaporation rate the type of the vessel (eg itsmaterial) the temperature and the use of sonicationor not
Further methods are for example ldquoLangmuir-Schaferrdquo andldquocapillaryrdquo techniques [67] and various methods exploitingthe self-assembly at a liquid-liquid or liquid-air interface [70]By such methods it is possible to fabricate MNP films ofexcellent order extending over several micrometers and eveninvolving particles of two or three different sizes (see egFigure 8)
The magnetic properties of such MNP superlattices havebeen in the focus of many current studies [27 56 68 72ndash76] In most cases the collective magnetic behavior of thesuperlatticemdashbeing a consequence of dipolar interactionsmdashis intensely investigated Although the physics of dipolarlyinteracting 2D or 3D lattices seems to be a ldquoclassicrdquo andsolved topic in magnetism the community is far from aconsensus This is due to the huge complexity of the systemwhere dipolar interactions are long-range and anisotropicand thus lead to both frustrated and highly correlatedbehavior like superspin glass or superferromagnetism (seeSection 7) Therefore the behavior of such arrays is hard to
predict and many parameters like shape of the entire systemanisotropies order versus disorder andmanymore influencethe properties significantly
5 Superparamagnetism
Small enough FM particles will be single domain sincethe energy cost of domain wall formation outweighs anysaving of demagnetizing energy In these single domainFM particles the magnetization is often considered to lieparallel or antiparallel to a particular direction called the easyaxis This can be due to different anisotropy contributionsfor example magnetocrystalline shape strain and surfaceanisotropies [77] Let us consider an assembly of uniaxialsingle domain particles each with an anisotropy energydensity 119864 = 119870119881sin2120579 For a particle the energy barrierΔ119864119861
= 119870119881 separates the two energy minima at 120579 = 0
and 120579 = 120587 corresponding to the magnetization parallelor antiparallel to the easy axis as shown in Figure 9 Neelpointed out [78] that for small enough single domain particlesKV may become so small that energy fluctuations canovercome the anisotropy energy and spontaneously reversethe magnetization of a particle from one easy direction tothe other even in the absence of an applied field In thelimit 119896
119861119879 ≫ 119870119881 the particle can be considered freely
fluctuating (119896119861= Boltzmannrsquos constant) A FM nanoparticle
is defined as superparamagnetic (SPM) when the energybarrier E
119861 for a magnetization reversal is comparable to
the thermal energy k119861T This behavior has been discussed
in the literature also under several other names includingldquoapparent paramagnetismrdquo [79] ldquocollective paramagnetismrdquo[80] ldquoquasiparamagnetismrdquo [81] and ldquosubdomain behaviorrdquo[82] This is the isotropic SPM limit The direction of thesuperspin or macrospin fluctuates with a frequency 119891 or acharacteristic relaxation time 120591 = (2120587119891)
minus1 The Neel-Brownexpression [78 83] for the relaxation time is given by
120591 = 1205910exp(119870119881
119896119861119879
) (2)
where 1205910sim 10minus10 s is the inverse angular attempt frequency
The fluctuations thus slow down (120591 increases) as the sampleis cooled to lower temperatures The system appears staticwhen the SPM relaxation time 120591 becomes much longerthan the experimental measuring time 120591
119898 Only if 120591 is
shorter compared to 120591119898 one may observe an average value
of the magnetization When this SPM relaxation time 120591
becomes comparable to 120591119898 the particle is said to be blocked
The magnetic behavior of the particle is characterized bythe so-called ldquoblockingrdquo temperature 119879
119887 below which the
particle moments appear frozen on the time scale of themeasurement 120591
119898asymp 120591 Inverting (2) one obtains
119879119887asymp
119870119881
119896119861ln (1205911198981205910)
(3)
Equation (3) is valid for individual particles or a system ofnoninteracting particles with the same size and anisotropyIf the particles are not monodisperse the distribution of
8 Journal of Nanomaterials
Free
ener
gy
0 1205872 120587
120579
ΔEB
Easy axis
M
120579
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 1 The procedure for synthesis of monodisperse nanoparti-cles by injecting reagents into the hot surfactant solution followed byaging and size-selection process Reproduced with permission fromHyeon [19] copy 2003 Royal Society of Chemistry
10nm
Figure 2 Scanning tunneling microscope image of Fe particlesdeposited on top of an insulating MgO layer Reproduced withpermission from Ernult et al [34] copy 2005 AIP Publishing LLC
for the applications Following the above report by Sun etal [27] many experiments on the preparation of L1
0-FePt
nanoparticles have been carried out using similar solutionprocesses [28ndash30] Nanoparticles consisting of exchange-coupled nanocomposites were also fabricated by Zeng etal [31] A different chemical method was reported usingnanospheres Self-assembled nanospheres are utilized as atemplate for the preparation of nanoparticle arrays [32 33]Albrecht et al [32] formed a topographic pattern of themagnetic multilayer on spherical nanoparticles where thenanostructures were both monodisperse and magneticallyisolated They found an unexpected switching behaviorinduced by their spherical shape
The use of film growth via vapor deposition techniquesis another method for self-assembly of nanoparticles TheVolmer-Weber (V-W)mode is a film growth giving an island-like morphology This growth mode is achieved by selectingsubstrate material layer thickness and growth temperatureadequately Ernult et al prepared self-aligned Fe nanoparti-cles on MgO employing a molecular beam epitaxy apparatus[34] Figure 2 shows a typical scanning tunnelingmicroscope
image of Fe particles deposited on top of an MgO layer Theyalso observed a transition from a three-dimensional V-Wtype growth mode to a two-dimensional growth when thelayer thickness was increased Such metallic nanoparticleson an insulating layer become an important componentas a structure of electrodeinsulating layernanoparticlesThis allows the injection of an electrical current into thenanoparticles through the insulating layer playing a majorrole in devices such as single electron transistors Black et al[35] reported the transport measurement performed in self-assembledCoparticles In the case ofmagnetic nanoparticlesspin-dependent single electron tunneling is expected [36]and Yakushiji et al [37] found an enhancement of spinlifetime in MNPs
A particulate film also provides us with an interestingchange in the magnetization reversal behavior from theincoherent reversal mode such as domain wall motion to thecoherent rotation mode This change sometimes enhanceshardmagnetic properties such as coercivity (119867
119888) Shima et al
[38] deposited FePt on MgO at high substrate temperatureWhen the FePt layer thickness was reduced from 100 nm to10 nm the film morphology was drastically changed from acontinuous state to the particulate one as shown in Figure 3The formation of the particulate film significantly enhanced119867119888of FePt They also achieved the huge 119867
119888= 105 kOe at
42 K for the FePt particulate film [39] Okamoto et al [40]reported that themorphology change with the FePt thicknessled to a change in themagnetization reversal process Furthergranular films in which the MNPs are embedded into anonmagneticmatrix can isolate theMNPswhich are suitablefor next-generation ultrahigh density magnetic recordingmedia
There are several other methods to prepare MNPs viabottom-up approach such as electrochemical reactions [41]sol-gel processing [42] plasma or flame spraying synthesis[43] chemical vapor deposition [44] atomic or molecularcondensation [20] sputtering and thermal evaporation [45]and bio-assisted synthesis [11] The top-down approach viavarious types of lithography will be discussed in the nextsection of this paper
3 Magnetic Nanostructures by Lithography
In contrast to the self-assembly of the bottom-up techniquesthe top-down techniques employ the microfabrication pro-cesses such as lithographical patterning liftoff ion millingor wet etching The top-down techniques have advantagesthat we can accurately control the size and the shape ofnanostructures and it is easy to align the nanostructures asdesired However high cost and slow manufacturing of thetop-down techniques are drawbacks for themass productionIn addition the size limitation of the nanostructure dependson the kind of lithography If one uses an electron beam(EB) as a writer that is EB lithography the size can bereduced down to a few tens of nanometer although it takeslong time to make patterns Photolithography saves the timefor patterning owing to the large area exposure while the
4 Journal of Nanomaterials
400nm
(a)
(c)
(e)
(b)
(d)
(f)
Figure 3 Transmission electron microscope images for FePt thin films with thicknesses of (a) 10 nm (b) 15 nm (c) 20 nm (d) 45 nm (e)50 nm and (f) 60 nm Reproduced with permission from Shima et al [38] copy 2002 AIP Publishing LLC
Film
Resist
Substrate
(a) Etching (milling)
Resist
Substrate
Film
(b) Liftoff
Figure 4 Schematic illustrations of the microfabrication processes(a) etching (milling) and (b) liftoff
minimum size is mainly determined by the wavelength of thelight source
Figure 4 illustrates the steps for microfabrication pro-cesses using (a) etching or milling and (b) liftoff In thecase of the etching process a film is first deposited on asubstrate Then a resist for lithographical patterning is spin-coated onto the film and a desired pattern of the resist isformed Finally the film is etched through the resist maskand the desired pattern is obtained Contrastingly a resistis first spin-coated onto a substrate in the case of the lift-off process After patterning the resist a film is depositedon the substrate with the patterned resist The thin filmdeposited on the region without resist finally remains afterlifting the resist off from the substrate Both methods haveadvantages and disadvantages For example although thelift-off process is free from etching damage the substratesurface is contaminated by the resist that can be suppressedin the etching process There are many reports on themagnetic properties for nanostructured elements of NiFeFe Co and CoPt [47ndash50] Such nanostructures showed thecharacteristic magnetization reversal behavior and magneticdomain structures For example Shinjo et al [51] reported
Journal of Nanomaterials 5
(a) D = 500nm (b) D = 100nm
(c) D = 30nm (d) D = 30nm
1000nm 300nm
100nm 500nm
Figure 5 Atomic force microscope images for the L10-FePt circular dots with diameters (119863) of (a) 500 nm (b) 100 nm and (c) 30 nm and
(d) a scanning electron microscope image for119863 = 30 nm Reproduced with permission from Seki et al [46] copy 2011 IOP Publishing
the first observation of magnetic vortex core in NiFe disksprepared using the top-down technique The formation ofthe magnetic vortex is attributable to the balance betweenmagnetic exchange energy and magnetostatic energy in aconfined magnetic disk The top-down techniques are alsoused to fabricate magnetic nanopillars and nanowires whichare key elements for future spintronic devices such as mag-netic random access memories [52] and racetrack memories[53] respectively
As well as the bottom-up technique one can control themagnetization reversal mode in a magnetic nanostructurefabricated by the top-down technique Figure 5 displays theatomic force microscope images and the scanning electronmicroscope image for L1
0-FePt circular dots with various
diameters which were fabricated through the use of EBlithography and Ar ion milling [46] These images supportthat FePtwas patterned intowell-defined circular shapes evenin the case of the diameter of 30 nm As the dot diameterwas reduced from 1 120583m to 30 nm the magnetization reversalmode was changed as in the case of L1
0-FePt nanoparticles
prepared by the bottom-up technique Figure 6 shows the
magnetic field angular dependence of normalized119867119888for the
perpendicularly magnetized L10-FePt dots with various dot
diameters where 120579 is the polar angle of the magnetic fieldand 120579 = 0
∘ and 90∘ correspond to the normal and paralleldirections to the plane of the device 119867
119888was normalized
by the value of 119867119888at 120579 = 0
∘ The magnetization reversalfor the dots with a large diameter is governed by domainwall motion where the angular dependence follows therelationship of 1 cos 120579 On the other hand the 120579 dependenceof normalized 119867
119888deviates from the tendency of 1 cos 120579
with decreasing dot diameter and the local minimum of 119867119888
appears around 120579 = 45∘ This implies that the magnetization
reversal for dots with a small diameter occurs through theincoherent magnetization rotation A detailed analysis ofmagnetization reversal was also carried out for a singleFePt dot using the anomalous Hall effect [54] In addi-tion the nucleation phenomenon of the reversed magneticdomains was discussed using microfabricated FePt dots [55]Consequently nanostructures fabricated by the top-downtechnique are useful for doing systematic investigations ofthe magnetization reversal behavior and magnetic domain
6 Journal of Nanomaterials
30
25
20
15
10
05
000 20 40 60 80
DW motion
S-W rotation
30nm50nm100nm
500nm1120583m
120579 (∘)
HcH
(120579=0∘)
c
Figure 6 Magnetic field angular dependence of normalized coer-civity (119867
119888) for L1
0-FePt dots with diameters of 1120583m (solid squares)
500 nm (open triangles) 100 nm (solid triangles) 50 nm (opencircles) and 30 nm (solid circles) The dashed line denotes 1 cos 120579whereas the dashed-dotted line denotes the Stoner-Wohlfarth-typecoherent rotation
structures at the nanometer scale owing to their well-definedgeometries
4 Nanoparticle Superlattices or Supracrystals
A particularly exciting novel approach in NP research is tofabricate and to study self-organized assemblies of MNPs[56ndash58] They constitute an analogy to conventional crys-talline materials because atoms are replaced by MNPs andspins are replaced by superspins Instead of an atomic crystalone rather speaks of so-calledMNP ldquosupracrystalsrdquo or ldquosuper-latticesrdquo [58 59] Such systems thus represent a new classof materials where their properties are determined on theone hand by the properties of the individual MNPs as theirbuilding blocks and on the other hand by themutual interac-tions Therefore interesting behavior and novel applicationscan be expected from the controlled tuning of individual andcollective properties of the superlattices
NP superlattices can be fabricated by various techniquesBasically one has to distinguish four different classes ofmethods namely as follows
(i) Physical vapor deposition methods they include forexample thermal vapor deposition or sputtering Inthis case the MNPs of magnetic metals such asCo80Fe20
are formed spontaneously during deposi-tion by nonwetting Volmer-Weber-type growth ontoa suitable substrate material like SiO
2and Al
2O3 The
advantage is a completely ldquoorganics-freerdquo procedure
500nm
Figure 7 Scanning electron microscopy image of a 3-dimensionalassembly of iron oxide nanoparticles with 20 nm diameter on a Sisubstrate [62]
however the order of self-organization of these dis-continuous metal-insulator multilayers (DMIMs) isusually relatively weak and the shape of the MNPs isnot well controllable [60 61]
(ii) MNPs from the gas phase here MNPs are formedfor example by nucleation and growth from a super-saturated vapor In a subsequent step the MNPs areldquolandedrdquo onto a substrate [63 64] Advantages are theability to form spherical MNPs from many differentmaterials with relatively well-controllable diametersHowever also here the order of self-organizationonce deposited is relatively poor
(iii) Chemically prepared MNPs in this case the MNPsare synthesized by chemical processes and are dis-persed in a solvent Self-assembly onto a substratethen occurs by involving colloidal ordering processesduring evaporation of the solvent [21 56ndash58 6265] In this case superlattices of excellent qualitycan be prepared Figure 7 shows an example of a3-dimensional assembly of 20 nm iron oxide MNPs[62]
This third class of methods one could call it ldquocolloidal self-assembly techniquesrdquo comprises a large number of variousapproaches Tomention only a few examples there are [66 67]the following
(i) ldquoDrop-castingrdquo namely applying a droplet of theMNP dispersion onto a substrate and letting thesolvent evaporate [68] This method often producesvery thick 3-dimensional superlattices of relativelyhigh quality However the thickness of the MNPfilm varies significantly over the substrate surface andbasically no control of the self-assembly process ispossible
(ii) ldquoSpin-coatingrdquo or rotation-coating which is similar tocoating a substrate with photosensitive or electron-beam sensitive resist in nanolithography This pro-duces films of very good homogeneity Even sub-monolayer MNP films are possible by this approach
Journal of Nanomaterials 7
Figure 8 TEM image of a binaryMNP superlattice composed of 56and 105 nm Fe
3O4NPsThe inset shows the structural model of the
[001] projection of this lattice The scale bar corresponds to 20 nmReproducedwith permission fromChen et al [71] copy 2010 AmericanChemical Society
[69] However one achieves hereby only short-rangeordered superlattices probably due to the extremelyquick process where the MNPs have little time toassemble The control parameter ismdashapart from thechoice of the solvent as in any other techniquemdashbasically only the spin-speed
(iii) ldquoLangmuir-Blodgettrdquo technique in this method asubstrate is initially immersed into the MNP dis-persion Using a high-precision motorized stage thesubstrate is slowly pulled out of the dispersion at aconstant angle and at a constant speed Both parame-ters are control parameters of the process [66] Thismethod is in principle capable of producing high-quality monolayer MNP films
(iv) ldquoSedimentationrdquo the substrate is placed at the bottomof the container where initially theMNPdispersion isfilled inThe solvent evaporates slowly and the MNPsldquosedimentrdquo onto the substrate Control parameters arethe evaporation rate the type of the vessel (eg itsmaterial) the temperature and the use of sonicationor not
Further methods are for example ldquoLangmuir-Schaferrdquo andldquocapillaryrdquo techniques [67] and various methods exploitingthe self-assembly at a liquid-liquid or liquid-air interface [70]By such methods it is possible to fabricate MNP films ofexcellent order extending over several micrometers and eveninvolving particles of two or three different sizes (see egFigure 8)
The magnetic properties of such MNP superlattices havebeen in the focus of many current studies [27 56 68 72ndash76] In most cases the collective magnetic behavior of thesuperlatticemdashbeing a consequence of dipolar interactionsmdashis intensely investigated Although the physics of dipolarlyinteracting 2D or 3D lattices seems to be a ldquoclassicrdquo andsolved topic in magnetism the community is far from aconsensus This is due to the huge complexity of the systemwhere dipolar interactions are long-range and anisotropicand thus lead to both frustrated and highly correlatedbehavior like superspin glass or superferromagnetism (seeSection 7) Therefore the behavior of such arrays is hard to
predict and many parameters like shape of the entire systemanisotropies order versus disorder andmanymore influencethe properties significantly
5 Superparamagnetism
Small enough FM particles will be single domain sincethe energy cost of domain wall formation outweighs anysaving of demagnetizing energy In these single domainFM particles the magnetization is often considered to lieparallel or antiparallel to a particular direction called the easyaxis This can be due to different anisotropy contributionsfor example magnetocrystalline shape strain and surfaceanisotropies [77] Let us consider an assembly of uniaxialsingle domain particles each with an anisotropy energydensity 119864 = 119870119881sin2120579 For a particle the energy barrierΔ119864119861
= 119870119881 separates the two energy minima at 120579 = 0
and 120579 = 120587 corresponding to the magnetization parallelor antiparallel to the easy axis as shown in Figure 9 Neelpointed out [78] that for small enough single domain particlesKV may become so small that energy fluctuations canovercome the anisotropy energy and spontaneously reversethe magnetization of a particle from one easy direction tothe other even in the absence of an applied field In thelimit 119896
119861119879 ≫ 119870119881 the particle can be considered freely
fluctuating (119896119861= Boltzmannrsquos constant) A FM nanoparticle
is defined as superparamagnetic (SPM) when the energybarrier E
119861 for a magnetization reversal is comparable to
the thermal energy k119861T This behavior has been discussed
in the literature also under several other names includingldquoapparent paramagnetismrdquo [79] ldquocollective paramagnetismrdquo[80] ldquoquasiparamagnetismrdquo [81] and ldquosubdomain behaviorrdquo[82] This is the isotropic SPM limit The direction of thesuperspin or macrospin fluctuates with a frequency 119891 or acharacteristic relaxation time 120591 = (2120587119891)
minus1 The Neel-Brownexpression [78 83] for the relaxation time is given by
120591 = 1205910exp(119870119881
119896119861119879
) (2)
where 1205910sim 10minus10 s is the inverse angular attempt frequency
The fluctuations thus slow down (120591 increases) as the sampleis cooled to lower temperatures The system appears staticwhen the SPM relaxation time 120591 becomes much longerthan the experimental measuring time 120591
119898 Only if 120591 is
shorter compared to 120591119898 one may observe an average value
of the magnetization When this SPM relaxation time 120591
becomes comparable to 120591119898 the particle is said to be blocked
The magnetic behavior of the particle is characterized bythe so-called ldquoblockingrdquo temperature 119879
119887 below which the
particle moments appear frozen on the time scale of themeasurement 120591
119898asymp 120591 Inverting (2) one obtains
119879119887asymp
119870119881
119896119861ln (1205911198981205910)
(3)
Equation (3) is valid for individual particles or a system ofnoninteracting particles with the same size and anisotropyIf the particles are not monodisperse the distribution of
8 Journal of Nanomaterials
Free
ener
gy
0 1205872 120587
120579
ΔEB
Easy axis
M
120579
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 3 Transmission electron microscope images for FePt thin films with thicknesses of (a) 10 nm (b) 15 nm (c) 20 nm (d) 45 nm (e)50 nm and (f) 60 nm Reproduced with permission from Shima et al [38] copy 2002 AIP Publishing LLC
Film
Resist
Substrate
(a) Etching (milling)
Resist
Substrate
Film
(b) Liftoff
Figure 4 Schematic illustrations of the microfabrication processes(a) etching (milling) and (b) liftoff
minimum size is mainly determined by the wavelength of thelight source
Figure 4 illustrates the steps for microfabrication pro-cesses using (a) etching or milling and (b) liftoff In thecase of the etching process a film is first deposited on asubstrate Then a resist for lithographical patterning is spin-coated onto the film and a desired pattern of the resist isformed Finally the film is etched through the resist maskand the desired pattern is obtained Contrastingly a resistis first spin-coated onto a substrate in the case of the lift-off process After patterning the resist a film is depositedon the substrate with the patterned resist The thin filmdeposited on the region without resist finally remains afterlifting the resist off from the substrate Both methods haveadvantages and disadvantages For example although thelift-off process is free from etching damage the substratesurface is contaminated by the resist that can be suppressedin the etching process There are many reports on themagnetic properties for nanostructured elements of NiFeFe Co and CoPt [47ndash50] Such nanostructures showed thecharacteristic magnetization reversal behavior and magneticdomain structures For example Shinjo et al [51] reported
Journal of Nanomaterials 5
(a) D = 500nm (b) D = 100nm
(c) D = 30nm (d) D = 30nm
1000nm 300nm
100nm 500nm
Figure 5 Atomic force microscope images for the L10-FePt circular dots with diameters (119863) of (a) 500 nm (b) 100 nm and (c) 30 nm and
(d) a scanning electron microscope image for119863 = 30 nm Reproduced with permission from Seki et al [46] copy 2011 IOP Publishing
the first observation of magnetic vortex core in NiFe disksprepared using the top-down technique The formation ofthe magnetic vortex is attributable to the balance betweenmagnetic exchange energy and magnetostatic energy in aconfined magnetic disk The top-down techniques are alsoused to fabricate magnetic nanopillars and nanowires whichare key elements for future spintronic devices such as mag-netic random access memories [52] and racetrack memories[53] respectively
As well as the bottom-up technique one can control themagnetization reversal mode in a magnetic nanostructurefabricated by the top-down technique Figure 5 displays theatomic force microscope images and the scanning electronmicroscope image for L1
0-FePt circular dots with various
diameters which were fabricated through the use of EBlithography and Ar ion milling [46] These images supportthat FePtwas patterned intowell-defined circular shapes evenin the case of the diameter of 30 nm As the dot diameterwas reduced from 1 120583m to 30 nm the magnetization reversalmode was changed as in the case of L1
0-FePt nanoparticles
prepared by the bottom-up technique Figure 6 shows the
magnetic field angular dependence of normalized119867119888for the
perpendicularly magnetized L10-FePt dots with various dot
diameters where 120579 is the polar angle of the magnetic fieldand 120579 = 0
∘ and 90∘ correspond to the normal and paralleldirections to the plane of the device 119867
119888was normalized
by the value of 119867119888at 120579 = 0
∘ The magnetization reversalfor the dots with a large diameter is governed by domainwall motion where the angular dependence follows therelationship of 1 cos 120579 On the other hand the 120579 dependenceof normalized 119867
119888deviates from the tendency of 1 cos 120579
with decreasing dot diameter and the local minimum of 119867119888
appears around 120579 = 45∘ This implies that the magnetization
reversal for dots with a small diameter occurs through theincoherent magnetization rotation A detailed analysis ofmagnetization reversal was also carried out for a singleFePt dot using the anomalous Hall effect [54] In addi-tion the nucleation phenomenon of the reversed magneticdomains was discussed using microfabricated FePt dots [55]Consequently nanostructures fabricated by the top-downtechnique are useful for doing systematic investigations ofthe magnetization reversal behavior and magnetic domain
6 Journal of Nanomaterials
30
25
20
15
10
05
000 20 40 60 80
DW motion
S-W rotation
30nm50nm100nm
500nm1120583m
120579 (∘)
HcH
(120579=0∘)
c
Figure 6 Magnetic field angular dependence of normalized coer-civity (119867
119888) for L1
0-FePt dots with diameters of 1120583m (solid squares)
500 nm (open triangles) 100 nm (solid triangles) 50 nm (opencircles) and 30 nm (solid circles) The dashed line denotes 1 cos 120579whereas the dashed-dotted line denotes the Stoner-Wohlfarth-typecoherent rotation
structures at the nanometer scale owing to their well-definedgeometries
4 Nanoparticle Superlattices or Supracrystals
A particularly exciting novel approach in NP research is tofabricate and to study self-organized assemblies of MNPs[56ndash58] They constitute an analogy to conventional crys-talline materials because atoms are replaced by MNPs andspins are replaced by superspins Instead of an atomic crystalone rather speaks of so-calledMNP ldquosupracrystalsrdquo or ldquosuper-latticesrdquo [58 59] Such systems thus represent a new classof materials where their properties are determined on theone hand by the properties of the individual MNPs as theirbuilding blocks and on the other hand by themutual interac-tions Therefore interesting behavior and novel applicationscan be expected from the controlled tuning of individual andcollective properties of the superlattices
NP superlattices can be fabricated by various techniquesBasically one has to distinguish four different classes ofmethods namely as follows
(i) Physical vapor deposition methods they include forexample thermal vapor deposition or sputtering Inthis case the MNPs of magnetic metals such asCo80Fe20
are formed spontaneously during deposi-tion by nonwetting Volmer-Weber-type growth ontoa suitable substrate material like SiO
2and Al
2O3 The
advantage is a completely ldquoorganics-freerdquo procedure
500nm
Figure 7 Scanning electron microscopy image of a 3-dimensionalassembly of iron oxide nanoparticles with 20 nm diameter on a Sisubstrate [62]
however the order of self-organization of these dis-continuous metal-insulator multilayers (DMIMs) isusually relatively weak and the shape of the MNPs isnot well controllable [60 61]
(ii) MNPs from the gas phase here MNPs are formedfor example by nucleation and growth from a super-saturated vapor In a subsequent step the MNPs areldquolandedrdquo onto a substrate [63 64] Advantages are theability to form spherical MNPs from many differentmaterials with relatively well-controllable diametersHowever also here the order of self-organizationonce deposited is relatively poor
(iii) Chemically prepared MNPs in this case the MNPsare synthesized by chemical processes and are dis-persed in a solvent Self-assembly onto a substratethen occurs by involving colloidal ordering processesduring evaporation of the solvent [21 56ndash58 6265] In this case superlattices of excellent qualitycan be prepared Figure 7 shows an example of a3-dimensional assembly of 20 nm iron oxide MNPs[62]
This third class of methods one could call it ldquocolloidal self-assembly techniquesrdquo comprises a large number of variousapproaches Tomention only a few examples there are [66 67]the following
(i) ldquoDrop-castingrdquo namely applying a droplet of theMNP dispersion onto a substrate and letting thesolvent evaporate [68] This method often producesvery thick 3-dimensional superlattices of relativelyhigh quality However the thickness of the MNPfilm varies significantly over the substrate surface andbasically no control of the self-assembly process ispossible
(ii) ldquoSpin-coatingrdquo or rotation-coating which is similar tocoating a substrate with photosensitive or electron-beam sensitive resist in nanolithography This pro-duces films of very good homogeneity Even sub-monolayer MNP films are possible by this approach
Journal of Nanomaterials 7
Figure 8 TEM image of a binaryMNP superlattice composed of 56and 105 nm Fe
3O4NPsThe inset shows the structural model of the
[001] projection of this lattice The scale bar corresponds to 20 nmReproducedwith permission fromChen et al [71] copy 2010 AmericanChemical Society
[69] However one achieves hereby only short-rangeordered superlattices probably due to the extremelyquick process where the MNPs have little time toassemble The control parameter ismdashapart from thechoice of the solvent as in any other techniquemdashbasically only the spin-speed
(iii) ldquoLangmuir-Blodgettrdquo technique in this method asubstrate is initially immersed into the MNP dis-persion Using a high-precision motorized stage thesubstrate is slowly pulled out of the dispersion at aconstant angle and at a constant speed Both parame-ters are control parameters of the process [66] Thismethod is in principle capable of producing high-quality monolayer MNP films
(iv) ldquoSedimentationrdquo the substrate is placed at the bottomof the container where initially theMNPdispersion isfilled inThe solvent evaporates slowly and the MNPsldquosedimentrdquo onto the substrate Control parameters arethe evaporation rate the type of the vessel (eg itsmaterial) the temperature and the use of sonicationor not
Further methods are for example ldquoLangmuir-Schaferrdquo andldquocapillaryrdquo techniques [67] and various methods exploitingthe self-assembly at a liquid-liquid or liquid-air interface [70]By such methods it is possible to fabricate MNP films ofexcellent order extending over several micrometers and eveninvolving particles of two or three different sizes (see egFigure 8)
The magnetic properties of such MNP superlattices havebeen in the focus of many current studies [27 56 68 72ndash76] In most cases the collective magnetic behavior of thesuperlatticemdashbeing a consequence of dipolar interactionsmdashis intensely investigated Although the physics of dipolarlyinteracting 2D or 3D lattices seems to be a ldquoclassicrdquo andsolved topic in magnetism the community is far from aconsensus This is due to the huge complexity of the systemwhere dipolar interactions are long-range and anisotropicand thus lead to both frustrated and highly correlatedbehavior like superspin glass or superferromagnetism (seeSection 7) Therefore the behavior of such arrays is hard to
predict and many parameters like shape of the entire systemanisotropies order versus disorder andmanymore influencethe properties significantly
5 Superparamagnetism
Small enough FM particles will be single domain sincethe energy cost of domain wall formation outweighs anysaving of demagnetizing energy In these single domainFM particles the magnetization is often considered to lieparallel or antiparallel to a particular direction called the easyaxis This can be due to different anisotropy contributionsfor example magnetocrystalline shape strain and surfaceanisotropies [77] Let us consider an assembly of uniaxialsingle domain particles each with an anisotropy energydensity 119864 = 119870119881sin2120579 For a particle the energy barrierΔ119864119861
= 119870119881 separates the two energy minima at 120579 = 0
and 120579 = 120587 corresponding to the magnetization parallelor antiparallel to the easy axis as shown in Figure 9 Neelpointed out [78] that for small enough single domain particlesKV may become so small that energy fluctuations canovercome the anisotropy energy and spontaneously reversethe magnetization of a particle from one easy direction tothe other even in the absence of an applied field In thelimit 119896
119861119879 ≫ 119870119881 the particle can be considered freely
fluctuating (119896119861= Boltzmannrsquos constant) A FM nanoparticle
is defined as superparamagnetic (SPM) when the energybarrier E
119861 for a magnetization reversal is comparable to
the thermal energy k119861T This behavior has been discussed
in the literature also under several other names includingldquoapparent paramagnetismrdquo [79] ldquocollective paramagnetismrdquo[80] ldquoquasiparamagnetismrdquo [81] and ldquosubdomain behaviorrdquo[82] This is the isotropic SPM limit The direction of thesuperspin or macrospin fluctuates with a frequency 119891 or acharacteristic relaxation time 120591 = (2120587119891)
minus1 The Neel-Brownexpression [78 83] for the relaxation time is given by
120591 = 1205910exp(119870119881
119896119861119879
) (2)
where 1205910sim 10minus10 s is the inverse angular attempt frequency
The fluctuations thus slow down (120591 increases) as the sampleis cooled to lower temperatures The system appears staticwhen the SPM relaxation time 120591 becomes much longerthan the experimental measuring time 120591
119898 Only if 120591 is
shorter compared to 120591119898 one may observe an average value
of the magnetization When this SPM relaxation time 120591
becomes comparable to 120591119898 the particle is said to be blocked
The magnetic behavior of the particle is characterized bythe so-called ldquoblockingrdquo temperature 119879
119887 below which the
particle moments appear frozen on the time scale of themeasurement 120591
119898asymp 120591 Inverting (2) one obtains
119879119887asymp
119870119881
119896119861ln (1205911198981205910)
(3)
Equation (3) is valid for individual particles or a system ofnoninteracting particles with the same size and anisotropyIf the particles are not monodisperse the distribution of
8 Journal of Nanomaterials
Free
ener
gy
0 1205872 120587
120579
ΔEB
Easy axis
M
120579
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 5 Atomic force microscope images for the L10-FePt circular dots with diameters (119863) of (a) 500 nm (b) 100 nm and (c) 30 nm and
(d) a scanning electron microscope image for119863 = 30 nm Reproduced with permission from Seki et al [46] copy 2011 IOP Publishing
the first observation of magnetic vortex core in NiFe disksprepared using the top-down technique The formation ofthe magnetic vortex is attributable to the balance betweenmagnetic exchange energy and magnetostatic energy in aconfined magnetic disk The top-down techniques are alsoused to fabricate magnetic nanopillars and nanowires whichare key elements for future spintronic devices such as mag-netic random access memories [52] and racetrack memories[53] respectively
As well as the bottom-up technique one can control themagnetization reversal mode in a magnetic nanostructurefabricated by the top-down technique Figure 5 displays theatomic force microscope images and the scanning electronmicroscope image for L1
0-FePt circular dots with various
diameters which were fabricated through the use of EBlithography and Ar ion milling [46] These images supportthat FePtwas patterned intowell-defined circular shapes evenin the case of the diameter of 30 nm As the dot diameterwas reduced from 1 120583m to 30 nm the magnetization reversalmode was changed as in the case of L1
0-FePt nanoparticles
prepared by the bottom-up technique Figure 6 shows the
magnetic field angular dependence of normalized119867119888for the
perpendicularly magnetized L10-FePt dots with various dot
diameters where 120579 is the polar angle of the magnetic fieldand 120579 = 0
∘ and 90∘ correspond to the normal and paralleldirections to the plane of the device 119867
119888was normalized
by the value of 119867119888at 120579 = 0
∘ The magnetization reversalfor the dots with a large diameter is governed by domainwall motion where the angular dependence follows therelationship of 1 cos 120579 On the other hand the 120579 dependenceof normalized 119867
119888deviates from the tendency of 1 cos 120579
with decreasing dot diameter and the local minimum of 119867119888
appears around 120579 = 45∘ This implies that the magnetization
reversal for dots with a small diameter occurs through theincoherent magnetization rotation A detailed analysis ofmagnetization reversal was also carried out for a singleFePt dot using the anomalous Hall effect [54] In addi-tion the nucleation phenomenon of the reversed magneticdomains was discussed using microfabricated FePt dots [55]Consequently nanostructures fabricated by the top-downtechnique are useful for doing systematic investigations ofthe magnetization reversal behavior and magnetic domain
6 Journal of Nanomaterials
30
25
20
15
10
05
000 20 40 60 80
DW motion
S-W rotation
30nm50nm100nm
500nm1120583m
120579 (∘)
HcH
(120579=0∘)
c
Figure 6 Magnetic field angular dependence of normalized coer-civity (119867
119888) for L1
0-FePt dots with diameters of 1120583m (solid squares)
500 nm (open triangles) 100 nm (solid triangles) 50 nm (opencircles) and 30 nm (solid circles) The dashed line denotes 1 cos 120579whereas the dashed-dotted line denotes the Stoner-Wohlfarth-typecoherent rotation
structures at the nanometer scale owing to their well-definedgeometries
4 Nanoparticle Superlattices or Supracrystals
A particularly exciting novel approach in NP research is tofabricate and to study self-organized assemblies of MNPs[56ndash58] They constitute an analogy to conventional crys-talline materials because atoms are replaced by MNPs andspins are replaced by superspins Instead of an atomic crystalone rather speaks of so-calledMNP ldquosupracrystalsrdquo or ldquosuper-latticesrdquo [58 59] Such systems thus represent a new classof materials where their properties are determined on theone hand by the properties of the individual MNPs as theirbuilding blocks and on the other hand by themutual interac-tions Therefore interesting behavior and novel applicationscan be expected from the controlled tuning of individual andcollective properties of the superlattices
NP superlattices can be fabricated by various techniquesBasically one has to distinguish four different classes ofmethods namely as follows
(i) Physical vapor deposition methods they include forexample thermal vapor deposition or sputtering Inthis case the MNPs of magnetic metals such asCo80Fe20
are formed spontaneously during deposi-tion by nonwetting Volmer-Weber-type growth ontoa suitable substrate material like SiO
2and Al
2O3 The
advantage is a completely ldquoorganics-freerdquo procedure
500nm
Figure 7 Scanning electron microscopy image of a 3-dimensionalassembly of iron oxide nanoparticles with 20 nm diameter on a Sisubstrate [62]
however the order of self-organization of these dis-continuous metal-insulator multilayers (DMIMs) isusually relatively weak and the shape of the MNPs isnot well controllable [60 61]
(ii) MNPs from the gas phase here MNPs are formedfor example by nucleation and growth from a super-saturated vapor In a subsequent step the MNPs areldquolandedrdquo onto a substrate [63 64] Advantages are theability to form spherical MNPs from many differentmaterials with relatively well-controllable diametersHowever also here the order of self-organizationonce deposited is relatively poor
(iii) Chemically prepared MNPs in this case the MNPsare synthesized by chemical processes and are dis-persed in a solvent Self-assembly onto a substratethen occurs by involving colloidal ordering processesduring evaporation of the solvent [21 56ndash58 6265] In this case superlattices of excellent qualitycan be prepared Figure 7 shows an example of a3-dimensional assembly of 20 nm iron oxide MNPs[62]
This third class of methods one could call it ldquocolloidal self-assembly techniquesrdquo comprises a large number of variousapproaches Tomention only a few examples there are [66 67]the following
(i) ldquoDrop-castingrdquo namely applying a droplet of theMNP dispersion onto a substrate and letting thesolvent evaporate [68] This method often producesvery thick 3-dimensional superlattices of relativelyhigh quality However the thickness of the MNPfilm varies significantly over the substrate surface andbasically no control of the self-assembly process ispossible
(ii) ldquoSpin-coatingrdquo or rotation-coating which is similar tocoating a substrate with photosensitive or electron-beam sensitive resist in nanolithography This pro-duces films of very good homogeneity Even sub-monolayer MNP films are possible by this approach
Journal of Nanomaterials 7
Figure 8 TEM image of a binaryMNP superlattice composed of 56and 105 nm Fe
3O4NPsThe inset shows the structural model of the
[001] projection of this lattice The scale bar corresponds to 20 nmReproducedwith permission fromChen et al [71] copy 2010 AmericanChemical Society
[69] However one achieves hereby only short-rangeordered superlattices probably due to the extremelyquick process where the MNPs have little time toassemble The control parameter ismdashapart from thechoice of the solvent as in any other techniquemdashbasically only the spin-speed
(iii) ldquoLangmuir-Blodgettrdquo technique in this method asubstrate is initially immersed into the MNP dis-persion Using a high-precision motorized stage thesubstrate is slowly pulled out of the dispersion at aconstant angle and at a constant speed Both parame-ters are control parameters of the process [66] Thismethod is in principle capable of producing high-quality monolayer MNP films
(iv) ldquoSedimentationrdquo the substrate is placed at the bottomof the container where initially theMNPdispersion isfilled inThe solvent evaporates slowly and the MNPsldquosedimentrdquo onto the substrate Control parameters arethe evaporation rate the type of the vessel (eg itsmaterial) the temperature and the use of sonicationor not
Further methods are for example ldquoLangmuir-Schaferrdquo andldquocapillaryrdquo techniques [67] and various methods exploitingthe self-assembly at a liquid-liquid or liquid-air interface [70]By such methods it is possible to fabricate MNP films ofexcellent order extending over several micrometers and eveninvolving particles of two or three different sizes (see egFigure 8)
The magnetic properties of such MNP superlattices havebeen in the focus of many current studies [27 56 68 72ndash76] In most cases the collective magnetic behavior of thesuperlatticemdashbeing a consequence of dipolar interactionsmdashis intensely investigated Although the physics of dipolarlyinteracting 2D or 3D lattices seems to be a ldquoclassicrdquo andsolved topic in magnetism the community is far from aconsensus This is due to the huge complexity of the systemwhere dipolar interactions are long-range and anisotropicand thus lead to both frustrated and highly correlatedbehavior like superspin glass or superferromagnetism (seeSection 7) Therefore the behavior of such arrays is hard to
predict and many parameters like shape of the entire systemanisotropies order versus disorder andmanymore influencethe properties significantly
5 Superparamagnetism
Small enough FM particles will be single domain sincethe energy cost of domain wall formation outweighs anysaving of demagnetizing energy In these single domainFM particles the magnetization is often considered to lieparallel or antiparallel to a particular direction called the easyaxis This can be due to different anisotropy contributionsfor example magnetocrystalline shape strain and surfaceanisotropies [77] Let us consider an assembly of uniaxialsingle domain particles each with an anisotropy energydensity 119864 = 119870119881sin2120579 For a particle the energy barrierΔ119864119861
= 119870119881 separates the two energy minima at 120579 = 0
and 120579 = 120587 corresponding to the magnetization parallelor antiparallel to the easy axis as shown in Figure 9 Neelpointed out [78] that for small enough single domain particlesKV may become so small that energy fluctuations canovercome the anisotropy energy and spontaneously reversethe magnetization of a particle from one easy direction tothe other even in the absence of an applied field In thelimit 119896
119861119879 ≫ 119870119881 the particle can be considered freely
fluctuating (119896119861= Boltzmannrsquos constant) A FM nanoparticle
is defined as superparamagnetic (SPM) when the energybarrier E
119861 for a magnetization reversal is comparable to
the thermal energy k119861T This behavior has been discussed
in the literature also under several other names includingldquoapparent paramagnetismrdquo [79] ldquocollective paramagnetismrdquo[80] ldquoquasiparamagnetismrdquo [81] and ldquosubdomain behaviorrdquo[82] This is the isotropic SPM limit The direction of thesuperspin or macrospin fluctuates with a frequency 119891 or acharacteristic relaxation time 120591 = (2120587119891)
minus1 The Neel-Brownexpression [78 83] for the relaxation time is given by
120591 = 1205910exp(119870119881
119896119861119879
) (2)
where 1205910sim 10minus10 s is the inverse angular attempt frequency
The fluctuations thus slow down (120591 increases) as the sampleis cooled to lower temperatures The system appears staticwhen the SPM relaxation time 120591 becomes much longerthan the experimental measuring time 120591
119898 Only if 120591 is
shorter compared to 120591119898 one may observe an average value
of the magnetization When this SPM relaxation time 120591
becomes comparable to 120591119898 the particle is said to be blocked
The magnetic behavior of the particle is characterized bythe so-called ldquoblockingrdquo temperature 119879
119887 below which the
particle moments appear frozen on the time scale of themeasurement 120591
119898asymp 120591 Inverting (2) one obtains
119879119887asymp
119870119881
119896119861ln (1205911198981205910)
(3)
Equation (3) is valid for individual particles or a system ofnoninteracting particles with the same size and anisotropyIf the particles are not monodisperse the distribution of
8 Journal of Nanomaterials
Free
ener
gy
0 1205872 120587
120579
ΔEB
Easy axis
M
120579
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 6 Magnetic field angular dependence of normalized coer-civity (119867
119888) for L1
0-FePt dots with diameters of 1120583m (solid squares)
500 nm (open triangles) 100 nm (solid triangles) 50 nm (opencircles) and 30 nm (solid circles) The dashed line denotes 1 cos 120579whereas the dashed-dotted line denotes the Stoner-Wohlfarth-typecoherent rotation
structures at the nanometer scale owing to their well-definedgeometries
4 Nanoparticle Superlattices or Supracrystals
A particularly exciting novel approach in NP research is tofabricate and to study self-organized assemblies of MNPs[56ndash58] They constitute an analogy to conventional crys-talline materials because atoms are replaced by MNPs andspins are replaced by superspins Instead of an atomic crystalone rather speaks of so-calledMNP ldquosupracrystalsrdquo or ldquosuper-latticesrdquo [58 59] Such systems thus represent a new classof materials where their properties are determined on theone hand by the properties of the individual MNPs as theirbuilding blocks and on the other hand by themutual interac-tions Therefore interesting behavior and novel applicationscan be expected from the controlled tuning of individual andcollective properties of the superlattices
NP superlattices can be fabricated by various techniquesBasically one has to distinguish four different classes ofmethods namely as follows
(i) Physical vapor deposition methods they include forexample thermal vapor deposition or sputtering Inthis case the MNPs of magnetic metals such asCo80Fe20
are formed spontaneously during deposi-tion by nonwetting Volmer-Weber-type growth ontoa suitable substrate material like SiO
2and Al
2O3 The
advantage is a completely ldquoorganics-freerdquo procedure
500nm
Figure 7 Scanning electron microscopy image of a 3-dimensionalassembly of iron oxide nanoparticles with 20 nm diameter on a Sisubstrate [62]
however the order of self-organization of these dis-continuous metal-insulator multilayers (DMIMs) isusually relatively weak and the shape of the MNPs isnot well controllable [60 61]
(ii) MNPs from the gas phase here MNPs are formedfor example by nucleation and growth from a super-saturated vapor In a subsequent step the MNPs areldquolandedrdquo onto a substrate [63 64] Advantages are theability to form spherical MNPs from many differentmaterials with relatively well-controllable diametersHowever also here the order of self-organizationonce deposited is relatively poor
(iii) Chemically prepared MNPs in this case the MNPsare synthesized by chemical processes and are dis-persed in a solvent Self-assembly onto a substratethen occurs by involving colloidal ordering processesduring evaporation of the solvent [21 56ndash58 6265] In this case superlattices of excellent qualitycan be prepared Figure 7 shows an example of a3-dimensional assembly of 20 nm iron oxide MNPs[62]
This third class of methods one could call it ldquocolloidal self-assembly techniquesrdquo comprises a large number of variousapproaches Tomention only a few examples there are [66 67]the following
(i) ldquoDrop-castingrdquo namely applying a droplet of theMNP dispersion onto a substrate and letting thesolvent evaporate [68] This method often producesvery thick 3-dimensional superlattices of relativelyhigh quality However the thickness of the MNPfilm varies significantly over the substrate surface andbasically no control of the self-assembly process ispossible
(ii) ldquoSpin-coatingrdquo or rotation-coating which is similar tocoating a substrate with photosensitive or electron-beam sensitive resist in nanolithography This pro-duces films of very good homogeneity Even sub-monolayer MNP films are possible by this approach
Journal of Nanomaterials 7
Figure 8 TEM image of a binaryMNP superlattice composed of 56and 105 nm Fe
3O4NPsThe inset shows the structural model of the
[001] projection of this lattice The scale bar corresponds to 20 nmReproducedwith permission fromChen et al [71] copy 2010 AmericanChemical Society
[69] However one achieves hereby only short-rangeordered superlattices probably due to the extremelyquick process where the MNPs have little time toassemble The control parameter ismdashapart from thechoice of the solvent as in any other techniquemdashbasically only the spin-speed
(iii) ldquoLangmuir-Blodgettrdquo technique in this method asubstrate is initially immersed into the MNP dis-persion Using a high-precision motorized stage thesubstrate is slowly pulled out of the dispersion at aconstant angle and at a constant speed Both parame-ters are control parameters of the process [66] Thismethod is in principle capable of producing high-quality monolayer MNP films
(iv) ldquoSedimentationrdquo the substrate is placed at the bottomof the container where initially theMNPdispersion isfilled inThe solvent evaporates slowly and the MNPsldquosedimentrdquo onto the substrate Control parameters arethe evaporation rate the type of the vessel (eg itsmaterial) the temperature and the use of sonicationor not
Further methods are for example ldquoLangmuir-Schaferrdquo andldquocapillaryrdquo techniques [67] and various methods exploitingthe self-assembly at a liquid-liquid or liquid-air interface [70]By such methods it is possible to fabricate MNP films ofexcellent order extending over several micrometers and eveninvolving particles of two or three different sizes (see egFigure 8)
The magnetic properties of such MNP superlattices havebeen in the focus of many current studies [27 56 68 72ndash76] In most cases the collective magnetic behavior of thesuperlatticemdashbeing a consequence of dipolar interactionsmdashis intensely investigated Although the physics of dipolarlyinteracting 2D or 3D lattices seems to be a ldquoclassicrdquo andsolved topic in magnetism the community is far from aconsensus This is due to the huge complexity of the systemwhere dipolar interactions are long-range and anisotropicand thus lead to both frustrated and highly correlatedbehavior like superspin glass or superferromagnetism (seeSection 7) Therefore the behavior of such arrays is hard to
predict and many parameters like shape of the entire systemanisotropies order versus disorder andmanymore influencethe properties significantly
5 Superparamagnetism
Small enough FM particles will be single domain sincethe energy cost of domain wall formation outweighs anysaving of demagnetizing energy In these single domainFM particles the magnetization is often considered to lieparallel or antiparallel to a particular direction called the easyaxis This can be due to different anisotropy contributionsfor example magnetocrystalline shape strain and surfaceanisotropies [77] Let us consider an assembly of uniaxialsingle domain particles each with an anisotropy energydensity 119864 = 119870119881sin2120579 For a particle the energy barrierΔ119864119861
= 119870119881 separates the two energy minima at 120579 = 0
and 120579 = 120587 corresponding to the magnetization parallelor antiparallel to the easy axis as shown in Figure 9 Neelpointed out [78] that for small enough single domain particlesKV may become so small that energy fluctuations canovercome the anisotropy energy and spontaneously reversethe magnetization of a particle from one easy direction tothe other even in the absence of an applied field In thelimit 119896
119861119879 ≫ 119870119881 the particle can be considered freely
fluctuating (119896119861= Boltzmannrsquos constant) A FM nanoparticle
is defined as superparamagnetic (SPM) when the energybarrier E
119861 for a magnetization reversal is comparable to
the thermal energy k119861T This behavior has been discussed
in the literature also under several other names includingldquoapparent paramagnetismrdquo [79] ldquocollective paramagnetismrdquo[80] ldquoquasiparamagnetismrdquo [81] and ldquosubdomain behaviorrdquo[82] This is the isotropic SPM limit The direction of thesuperspin or macrospin fluctuates with a frequency 119891 or acharacteristic relaxation time 120591 = (2120587119891)
minus1 The Neel-Brownexpression [78 83] for the relaxation time is given by
120591 = 1205910exp(119870119881
119896119861119879
) (2)
where 1205910sim 10minus10 s is the inverse angular attempt frequency
The fluctuations thus slow down (120591 increases) as the sampleis cooled to lower temperatures The system appears staticwhen the SPM relaxation time 120591 becomes much longerthan the experimental measuring time 120591
119898 Only if 120591 is
shorter compared to 120591119898 one may observe an average value
of the magnetization When this SPM relaxation time 120591
becomes comparable to 120591119898 the particle is said to be blocked
The magnetic behavior of the particle is characterized bythe so-called ldquoblockingrdquo temperature 119879
119887 below which the
particle moments appear frozen on the time scale of themeasurement 120591
119898asymp 120591 Inverting (2) one obtains
119879119887asymp
119870119881
119896119861ln (1205911198981205910)
(3)
Equation (3) is valid for individual particles or a system ofnoninteracting particles with the same size and anisotropyIf the particles are not monodisperse the distribution of
8 Journal of Nanomaterials
Free
ener
gy
0 1205872 120587
120579
ΔEB
Easy axis
M
120579
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 8 TEM image of a binaryMNP superlattice composed of 56and 105 nm Fe
3O4NPsThe inset shows the structural model of the
[001] projection of this lattice The scale bar corresponds to 20 nmReproducedwith permission fromChen et al [71] copy 2010 AmericanChemical Society
[69] However one achieves hereby only short-rangeordered superlattices probably due to the extremelyquick process where the MNPs have little time toassemble The control parameter ismdashapart from thechoice of the solvent as in any other techniquemdashbasically only the spin-speed
(iii) ldquoLangmuir-Blodgettrdquo technique in this method asubstrate is initially immersed into the MNP dis-persion Using a high-precision motorized stage thesubstrate is slowly pulled out of the dispersion at aconstant angle and at a constant speed Both parame-ters are control parameters of the process [66] Thismethod is in principle capable of producing high-quality monolayer MNP films
(iv) ldquoSedimentationrdquo the substrate is placed at the bottomof the container where initially theMNPdispersion isfilled inThe solvent evaporates slowly and the MNPsldquosedimentrdquo onto the substrate Control parameters arethe evaporation rate the type of the vessel (eg itsmaterial) the temperature and the use of sonicationor not
Further methods are for example ldquoLangmuir-Schaferrdquo andldquocapillaryrdquo techniques [67] and various methods exploitingthe self-assembly at a liquid-liquid or liquid-air interface [70]By such methods it is possible to fabricate MNP films ofexcellent order extending over several micrometers and eveninvolving particles of two or three different sizes (see egFigure 8)
The magnetic properties of such MNP superlattices havebeen in the focus of many current studies [27 56 68 72ndash76] In most cases the collective magnetic behavior of thesuperlatticemdashbeing a consequence of dipolar interactionsmdashis intensely investigated Although the physics of dipolarlyinteracting 2D or 3D lattices seems to be a ldquoclassicrdquo andsolved topic in magnetism the community is far from aconsensus This is due to the huge complexity of the systemwhere dipolar interactions are long-range and anisotropicand thus lead to both frustrated and highly correlatedbehavior like superspin glass or superferromagnetism (seeSection 7) Therefore the behavior of such arrays is hard to
predict and many parameters like shape of the entire systemanisotropies order versus disorder andmanymore influencethe properties significantly
5 Superparamagnetism
Small enough FM particles will be single domain sincethe energy cost of domain wall formation outweighs anysaving of demagnetizing energy In these single domainFM particles the magnetization is often considered to lieparallel or antiparallel to a particular direction called the easyaxis This can be due to different anisotropy contributionsfor example magnetocrystalline shape strain and surfaceanisotropies [77] Let us consider an assembly of uniaxialsingle domain particles each with an anisotropy energydensity 119864 = 119870119881sin2120579 For a particle the energy barrierΔ119864119861
= 119870119881 separates the two energy minima at 120579 = 0
and 120579 = 120587 corresponding to the magnetization parallelor antiparallel to the easy axis as shown in Figure 9 Neelpointed out [78] that for small enough single domain particlesKV may become so small that energy fluctuations canovercome the anisotropy energy and spontaneously reversethe magnetization of a particle from one easy direction tothe other even in the absence of an applied field In thelimit 119896
119861119879 ≫ 119870119881 the particle can be considered freely
fluctuating (119896119861= Boltzmannrsquos constant) A FM nanoparticle
is defined as superparamagnetic (SPM) when the energybarrier E
119861 for a magnetization reversal is comparable to
the thermal energy k119861T This behavior has been discussed
in the literature also under several other names includingldquoapparent paramagnetismrdquo [79] ldquocollective paramagnetismrdquo[80] ldquoquasiparamagnetismrdquo [81] and ldquosubdomain behaviorrdquo[82] This is the isotropic SPM limit The direction of thesuperspin or macrospin fluctuates with a frequency 119891 or acharacteristic relaxation time 120591 = (2120587119891)
minus1 The Neel-Brownexpression [78 83] for the relaxation time is given by
120591 = 1205910exp(119870119881
119896119861119879
) (2)
where 1205910sim 10minus10 s is the inverse angular attempt frequency
The fluctuations thus slow down (120591 increases) as the sampleis cooled to lower temperatures The system appears staticwhen the SPM relaxation time 120591 becomes much longerthan the experimental measuring time 120591
119898 Only if 120591 is
shorter compared to 120591119898 one may observe an average value
of the magnetization When this SPM relaxation time 120591
becomes comparable to 120591119898 the particle is said to be blocked
The magnetic behavior of the particle is characterized bythe so-called ldquoblockingrdquo temperature 119879
119887 below which the
particle moments appear frozen on the time scale of themeasurement 120591
119898asymp 120591 Inverting (2) one obtains
119879119887asymp
119870119881
119896119861ln (1205911198981205910)
(3)
Equation (3) is valid for individual particles or a system ofnoninteracting particles with the same size and anisotropyIf the particles are not monodisperse the distribution of
8 Journal of Nanomaterials
Free
ener
gy
0 1205872 120587
120579
ΔEB
Easy axis
M
120579
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 9 Schematic picture of the free energy of a single domainparticle with uniaxial anisotropy as a function of magnetizationdirection 119864
119861is the energy barrier hindering the free rotation of the
magnetization and 120579 is the angle between the magnetizationM andthe easy axis
particle sizes results in a distribution of blocking tempera-tures The experimental measuring time 120591
119898is different from
one measurement technique to another For example 120591119898
is in the range 10minus12ndash10minus10 s for inelastic neutron scatteringand time-resolvedmagneto-optical Kerr effect measurement10minus10ndash10minus7 s for Mossbauer spectroscopy (comparable to thedecay time of the nuclear Mossbauer transition) and 10minus10ndash10minus5 s for 120583SR (a measurable fraction of muons live for upto sim10 120591
120583 where 120591
120583= 22 120583s is the average muon lifetime)
while 119886119888 susceptibility typically probes 10minus1ndash10minus5 sThereforeit should be noted that for a specific sample the blockingtemperature is not uniquely defined but for each appliedexperimental technique a related blocking temperature mustbe defined Brown [84] has shown that 120591
0depends on the
material parameters (size and anisotropies) field and evenon temperature From (3) it is clear that 120591 depends on 119881
and 119879 so that by varying the volume of the particles or themeasurement temperature 120591 can be in the order from 10minus9 sto several years (Figure 9)
It should be noted that the SPM blocking phenomenacan be observed in ensembles of MNPs with negligible orvery weak interparticle interactions However interactionscan affect the inter-particle magnetic states such as superspinglass and superferromagnetism which will be discussed inthe next two sections
6 Superspin Glass and Surface Spin Glass
Spin glasses (SG) belong to the most prominent disorderedsystems in solid state magnetism and have thoroughly beeninvestigated for decades [85] With a few quite generalingredients such as site disorder and frustrated interactionthey generate an amazing wealth of properties They cul-minate in the definition of the SG order parameter whichsharply contrasts to that of ferro- or antiferromagnets butmeaningfully describes the random distribution of frozen
spin orientations [86] An often studied example is thedisordered alloy Cu
1minus119909Mn119909with 119909 ≪ 1 [87] whose random
Mn spin distribution and oscillating indirect RKKY exchangeinteraction provides the key ingredients of glassiness
Analogously to the SG state of dilute spins in bulkmaterials a collective glassy magnetic state can also occurin ensembles of single domain NPs in which the inter-particle interaction is nonnegligible Under the condition of anonvanishing NPmagnetization (ldquosuperspinrdquo) ⟨119878⟩ = 0 beinginterpreted either by that of a finite-size ferromagnet or by theground state of a magnetic macromolecule superspin glass(SSG) states have been considered for example in frozenferrofluids [88] or discontinuous metal-insulator multilayers(DMIMs) [75] From the beginning the crucial ingredientsmdashspatial randomness and frustrationmdashwere evident whenacceptingmagnetic dipolar interaction to prevail between thesuperspins Similar arguments as in classic dipolar glasses[89] were accepted by most researchers with very few excep-tions [90]
Figure 10 shows some of the key signatures of a typicalDMIMSSG namely [Co
80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
[91] The typical low-119879 shift of the peak temperature 119879119898
of the broad glassy susceptibility response under decreasingfrequencies 10minus2 le 119891 le 10
2Hz is shown in Figure 10(a) Thedominating relaxation time 120591 = (2120587119891)
minus1 turns out to obey acritical power law
120591 = 120591lowast(
119879119898
119879119892
minus 1)
minus119911]
(4)
with reasonable parameters referring to the glass temperature119879119892
= 61K the dynamic critical exponent 119911] = 102and the relaxation time of an individual particle moment120591lowast= 10minus8 s Figure 10(c) An Arrhenius ansatz Figure 10(b)
120591 = 1205910exp(119870119881119896
119861119879119898) also seems to fit with a reasonable
anisotropy parameter 119870119881 = 24 times 10minus20 J but yields an
unreasonably small value of the inverse angular attemptfrequency 120591
0= 10minus23 s and can thus be excluded
Nearly simultaneously with the first report on a SSG [88]another nanoparticular peculiarity was reported by Kodamaet al [92] namely the surface spin disorder in NiFe
2O4NPs
A model of the intrananoparticular magnetization involvingferrimagnetically aligned core spins and a spin-glass-likesurface layer with a canted spin structure (Figure 11(a)) wasproposed on the basis of anomalous magnetic low temper-ature properties and numerical calculations Ever since thispioneering and highly cited publication the subject has beenin the centre of attention [93] Only recently important detailsof the surface spin dynamics in dilutely dispersed NiFe
2O4
NPs were disclosed by Nadeem et al [94] They drew theattention to the core-shell structure where the ferrimagneticcore is completely blocked at 119879
119892asymp 15K and the shell
encounters a classic spin glass transition with aging memoryand dynamic scaling effects
Disorder and frustration the classic ingredients of SGformation are readily available at surfaces of ferrimagnetic(such as NiFe
2O4) or antiferromagnetic particles (such as
NiO [95]) Figure 11(b) shows its separate peak-like anomaliesof the complex ac susceptibility 1205941015840 minus 119894120594
10158401015840 at the blocking
Journal of Nanomaterials 9
2
0
log10
(120591s
)
minus2
101
10minus1
10minus3
120591(s
)
0012 00154
2
0
120594998400(102SI
)1
2
0
120594998400998400(102SI
)
40 60 80 100 120T (K)
(a)
(b)
01 03 05TmTg minus 1
(c)
f
1Tm (Kminus1)
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 10 (a) Real and imaginary components1205941015840(119879) and12059410158401015840(119879) of the 119886119888 susceptibilitymeasured on [Co80Fe20(119905119899= 09 nm)Al
2O3(3 nm)]
10
at frequencies 10minus2 le 119891 le 102Hz (b) Arrhenius law fit (straight line) to log
10[120591119904] versus 1119879
119898 where 119879
119898corresponds to the peak position of
1205941015840(119879) (c) Double logarithmic plot of 120591 versus 119879
119898119879119892minus 1 and best fit to a power law (straight line) Reproduced with permission from Sahoo
et al [91] copy 2003 AIP Publishing LLC
temperature 119879119861
asymp 85K (inset) and at 119879119892
= 159K asextrapolated from (4) with 119911] = 8 and 120591
0= 10minus12 s Even
the probably oldest magnetic material in history magnetiteFe3O4 has recently disclosed a surface SG transition whose
frequency-dependent peak of the out-of-phase susceptibility12059410158401015840(119879) has been located at asymp35K in NPs sized 40 nm [96]
7 Superferromagnetism
In the superparamagnetic (SPM) state ofMNPs no collectiveinterparticle order exists while the intraparticle spin struc-ture gives rise to individual net magnetic ldquosupermomentsrdquo(ldquosuperspinsrdquo or ldquomacrospinsrdquo) However for increasing par-ticle concentration the magnetic inter-particle interactionsbecome nonnegligible and one may find a crossover fromsingle-particle blocking to collective freezing As describedabove for an intermediate strength of magnetic interactionsrandomness of particle positions and sufficiently narrowsize distribution one can observe a SSG state With furtherincrease of concentration but prior to physical percolationthe inter-particle interactions become stronger and finallycan lead to a kind of FM domain state FM-like correlationswill arise between the ldquosupermomentsrdquo of the nanoparticlesin addition to those between the atomic moments withinthe particles The FM state of nanoparticle ensembles mighttherefore be called ldquosuperferromagneticrdquo (SFM) Conse-quently a SFM domain is defined like a FM domain the onlydifference being that the atomic moments is replaced by thesupermoments of the individual nanoparticles
The term ldquosuperferromagnetismrdquo was first introduced byBostanjoglo and Roehkel [97] LaterMoslashrup observed it whenstudying microcrystalline goethite FeO(OH) by Mossbauerspectroscopy [98] Afterwards the same terminology hasbeen used in different magnetic systems [99] However a
SFM domain state has scarcely been evidenced up to now innanoparticle systems For example Sankar et al [100] havestudied nonpercolated Co-SiO
2granular films and evidenced
FM-like correlations between the nanoparticles by small-angle neutron scattering In their case the observedmagneticcorrelations were extracted from the ZFC state in zero mag-netic field That is why they attributed the FM correlations tomagnetic interactions among the nanoparticles On differentconcentrations of nanoparticles they found that the FMcorrelations disappear for lower metallic volume fractionsthat is among others for weaker dipolar interactions
There has been indication or evidence for the exis-tence of SFM domains observed by various experiments orexperimental protocols such as dynamic hysteresis [101 102]polarized neutron reflectometry [103] Cole-Cole diagrams[104 105] and aging and memory effects [104] Howeverreal time imaging of SFM domains was only possible by Kerrmicroscopy and X-ray photoemission electron microscopy(X-PEEM) [105] Figures 12(a)ndash12(f) show LMOKE micro-graphs following the temporal evolution of the switchingprocess at room temperature after saturating the negativemagnetization (dark) and subsequently exposing the sampleto a positive supercoercive field of 120583
0119867 = 06mT The
first stripe-like domains with reversed magnetization (light)appear at time 119905 asymp 2 s as seen in Figure 12(a) In the nextfew seconds they are observed to expand simultaneouslysideways and along the easy (=field) direction while furtherdomains nucleate at other sample regions These sidewayssliding [102 103] and nucleation processes continue underthe same constant field until all of the downmagnetizationis reversed after 9 seconds Systematic investigations haveshown that the domain nucleation rate and the velocity ofsubsequent viscous slide motion of the walls can accuratelybe controlled by the magnitude of the external field [105]
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 11 (a) Calculated spin configuration at119867 = 0 for a cross-section of aNiFe2O4particle with diameter 25 nm Circles indicate extremely
canted orientations Reproduced with permission from Kodama et al [92] copy 1996 APS (b) Temperature dependence of 1205941015840 and 12059410158401015840 (solid and
open symbols resp) of NiO particles with mean diameter 65 nm under an ac field of 1198670= 10Oe measured at frequencies 10Hz le 119891 le
10 kHz with 1205941015840 peaking at 119879
119892= 159K as f rarr 0The inset shows the high temperature region with 120594
1015840 peaking at 119879119861asymp 85K Reproduced with
permission fromWinkler et al [95] copy 2008 IOP Publishing
8 Magnetic Core-Shell Nanoparticles
Core-shell magnetic nanoparticles have gained particularattention because of their physical and chemical propertiesthat are strongly dependent on the structure of the coreshell and interface It is now known that the values of mag-netic anisotropies in small particle exceed the correspondingbulk magnetocrystalline ones by orders of magnitude [106]Another interest in magnetic core-shell nanoparticles arisesbecause of the possibility to tune the surface strain anisotropyon themagnetic core through coreshell interfacemicrostruc-ture manipulation
The subject of magnetic core-shell nanoparticles can bedivided into three parts (i) a magnetic core with nonmag-netic shell (ii) a ferromagnetic core with ferromagnetic shelland (iii) a ferromagnetic (FM) core with antiferromagnetic(AFM) shell or vice versa So far it has been successfullyshown in recent years that a nonmagnetic coating can be usedfor magnetic core stabilization and surface functionalizationfor applications for example in biology [107 108]
An AFM shell coated over a FM core leads to theso-called exchange bias effect This effect is basically aninterfacial undercompensation of spins which may induceunidirectional anisotropy and is noticed as a shift of thehysteresis loop along the field axis depending on the history ofthe direction of the cooling field119867
119865119862 In the AFMFM core-
shell structure the Curie temperature 119879119888 of the FM has to
be higher than the Neel temperature (119879119873) and the system
has to be cooled from a starting temperature in betweenin the presence of an applied field 119867
119865119862 It has been shown
that exchange bias in core-shell magnetic particles provides apath for the improvement in the thermal stability of the core[109] EB has been observed in nanoparticles for a variety of
materials and morphologies which can be divided in threecategories [110]
(i) single phase ferromagnetic or antiferromagneticoxides
(ii) NPs deposited in AFMmatrices(iii) NPs with core-shell structure
It has also been shown that EB-like hysteresis shifts can beobserved in FM nanoparticles in which the surface behaveslike a spin glass which is formed due to finite-size and surfaceeffects [111 112] (see also Section 6) Exchange bias in core-shell NPs has been found in many types of systems forexample in CoCoO [109 113] NiCoNiCoO [114] CoMnO[115] Co
80Ni20oxide [116] CoPtCoO [117 118] and so forth
Exchange bias has been reported in unconventionalAFM core with ferromagnetic shell for example innanoparticles with MnO (core)Mn
3O4
(shell) [119]Similar unconventional systems are ferrimagnetic CoFe
[120] and so forth For details of such exchange bias incore-shell magnetic nanoparticles readers are referred toreferences [109 121]
9 Applications of Magnetic Nanoparticles
The unique chance to control coercivity in magnetic nano-materials has led to a number of significant technologicalapplications particularly in the field of information storageSmall magnetic particles are promising candidates for afurther increase of the density of magnetic storage devicestowards 100Gbitinch2 up to a few Tbitinch2 [122] Apartfrom data storage there are potential other applications of
Journal of Nanomaterials 11
(a) (b)
(c) (d)
(e) (f)
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 12 Longitudinal MOKE microscopy images of superferromagnetic [Co80Fe20(13 nm)Al
2O3(3 nm)]
10taken at room temperature
under supercoercive fields 1205830119867 = 06mT at 119905 = 2 s (a) 3 s (b) 4 s (c) 5 s (d) 6 s (e) and 9 s (f) Reproduced with permission from Bedanta
et al [20] copy 2010 IOP Publishing
magnetic nanoparticles for example in ferrofluids high-frequency electronics high performance permanent mag-nets and magnetic refrigerants Magnetic particles are alsopotential candidates to be used in biology and medicalapplications such as drug-targeting cancer therapy lymphnode imaging or hyperthermia [10 11 123]
In recent years researchers have tried to fabricate MNPbased multifunctional nanostructures There are basicallytwo types of approaches (i) molecular functionalizationwhich involves attaching the magnetic nanoparticles to anti-bodies proteins and dyes and so forth and (ii) integration ofMNPs with other functional nanoparticles such as quantum
dots or metallic nanoparticles [23] For example semicon-ducting chalcogenides have been grown by using magneticnanoparticles as seeds In this case the final product iscore-shell or heterodimer nanostructures with bothmagneticand fluorescent properties This leads to the demonstra-tion of intracellular manipulation of nanoparticles and apromising candidate for dual-functional molecular imaging(ie combined MRI and fluorescence imaging) MNPs canbe used as MRI contrast enhancement agents since thesignal of magnetic moments of protons around magneticnanoparticles can be captured by resonant absorption [24]These multifunctional MNPs could be used in biological
12 Journal of Nanomaterials
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
applications such as protein purification bacteria detectionand toxin decorporation [23] Figure 13 illustrates these twoapproaches for making multifunctional MNPs and theirvarious biological applications
In the last three decades magnetic data storage has seena linear rise in terms of storage capacity The physics ofmagnetic nanostructures is at the heart of magnetic harddisk drive technology In the future it is very probable thatareal densities will increase well beyond 1 Terabitinch2 byemploying new technologies like bit patterned media (BPM)or heat assisted magnetic recording [122 124]
Patterned magnetic nanostructures such as two-dimensional dot-arrays have attracted the interest ofresearchers due to their potential applications such asmagnetic information storage [125] or nonvolatile magneticrandom access memory (MRAM) [126] The demand forultrahigh density magnetic storage devices drives the bit sizeinto the nanometer scale As the volume 119881 = 120587119863
21199054 (where
119863 and 119905 are the diameter and thickness resp) of the grainsis reduced in the scaling process the magnetization of thegrains may become unstable due to thermal fluctuationsand data loss may occur [122] As the physical size ofthe nanostructures in the patterned array decreases lossof data due to the thermal instability (also known asldquosuperparamagnetic (SPM) effectrdquo) would become a verycrucial issue [127] Therefore future data storage technologyhas to overcome the SPM effect In this regard the L1
0-FePt
alloy is one of the most promising materials for futureultrahigh density magnetic storage devices because itpossesses a huge uniaxial magnetocrystalline anisotropy(119870119906= 7 times 10
7 ergcc) [128] which leads to a high thermalstability of magnetization
Also the present longitudinal data storage media may beconsidered as a collection of independent particles because oftheir weak intergranular exchange coupling However as wehave discussed in the superferromagnetic section (Section 7)strong intergranular interactions can drive the system to formlong-range ordered SFM domains which are clearly unsuit-able for applications in data storage Also the SFM alignmentcounteracts large tunnelingmagnetoresistance (TMR) valuesso magnetic random access memory applications are notpromising for SFM systems However SFM materials aresoft magnetics which make them nearly ideal materials forhigh permeability low-loss materials for microelectronicspower management and sensing devices designed for highfrequencies
10 Simulations and Modeling ofMagnetic Nanostructures
It is well known that numerical simulations constitute thethird pillar of condensed matter physics besides experimen-tal exploration and analytical theoretical description Withsimulations one models either the static spin structure inequilibrium or the spin dynamics as function of time orfrequency In the context of MNPs usually two types ofnumerical simulations are employed namely micromagnetic[129] andMonte-Carlo simulations [130]
In micromagnetic simulations the system under study isconsidered in the ldquocontinuum approximationrdquo [17] Insteadof assuming localized moments the magnetization is ratherdescribed by a continuous vector fieldM(r) For the practicalsimulation the system has to be subdivided into cells Thecell size should be smaller than the exchange length ofthe material Typical cell sizes are in the order of a fewnanometers Before the start a certain user-defined initialmagnetization configuration is chosen Then during the runof the simulation the magnetization of each cell is updatedbased on a physical model For micromagnetic simulationsthis is (in most cases) the Landau-Lifshitz-Gilbert equationwhich is an equation of motion of the magnetization vectorin an effective field [130 131]
119889M119894
119889119905
= minus1205741015840M119894timesHeff minus
1205721205741015840
M119904
M119894times (M119894timesHeff) (5)
where Heff = 119867 + 119867demag + 119867119870
+ 119867ex + ℎ(119905) is theeffective magnetic field with contributions from Zeemandemagnetizing anisotropy exchange and time-dependentmagnetic fields and M
119894is the magnetization of cell i 1205741015840 =
120574(1 + 1205722) with 120574 being the gyromagnetic ratio M
119904=
saturation of magnetization of the material under study and120572 the damping constant of the specific system All energycontributions like Zeeman energy anisotropy energy andinteractions act in the form of an effective field Heff onto themagnetization vector [129] Sincemicromagnetic simulationsare explicitly referring to 119879 = 0 they are usually employedto larger nanomagnets where thermal fluctuations are lessrelevant to calculations of the ground state at low enoughtemperatures
Contrastingly if the temperature behavior of MNPs hasto be calculated Monte-Carlo simulations are used [130 132133] In this type of simulations the system is consideredin the model of localized moments Hence each moment isupdated in the course of the simulationTheupdating is basedon Monte-Carlo techniques The most popular approach ishereby the Metropolis algorithm [130] which calculates thestatistical probability of a spin-flip or rotation based on thecomparison of a random number with the Boltzmann factorFor the calculation of the Boltzmann factor the Hamiltonianof the system needs to be known
11 Magnetization Dynamics and LLGModeling of Magnetic Nanoparticles
The quasistatic and ultrafast magnetic properties of magneticnanostructures are different from their bulk counterpartsMagnetization dynamics of these systems strongly dependsupon theirmagnetic ground states which depend not only ontheir intrinsic material parameters such as exchange stiffnessconstant saturation magnetization and magnetocrystallineanisotropy but also on their physical structures as well asthe external parameters such as the strength and orien-tation of the bias magnetic field To study the quasistaticand ultrafast dynamic properties of nanomagnets differentkinds of sensitive characterization techniques have beendeveloped in last few decades Electron and force based
Journal of Nanomaterials 13
Potentialapplications
Specificbinding
drug delivery
Bacteriadetection
protein
Multimodalimaging
Antibodies
or DNAs
Ligands orreceptors
Dyes
Magnetic nanoparticles
QDs
Metal
Multimodalimaging
Drug deliveryMRI
Multimodalimaging
multivalency
ldquoNanocomponentsrdquo
nm5
nm5
nm5
PotentialapplicationsldquoBiomoleculesrdquo
ldquoNanodrugsrdquo
separation
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 13 Various potential applications of multifunctional magnetic nanoparticles in biology Reproduced with permission from Gao et al[24] copy 2009 American Chemical Society
magnetic microscopy such as magnetic force microscopy(MFM) Lorentz force microscopy (LFM) photoemissionelectronmicroscopy (PEEM) spin polarized low energy elec-tron microscopy (SPLEEM) scanning electron microscopywith polarization analysis (SEMPA) spin polarized scanningtunneling microscopy (SP-STM) electron holography andballistic electron magnetic microscopy all provide excellentspatial resolution but offer very poor or moderate temporalresolution
Consequently different kinds of techniques have emergedto investigate the fast magnetization dynamics of magneticthin films and confined magnetic structures Out of thosetechniques ferromagnetic resonance (FMR) and Brillouinlight scattering (BLS) provide information on the dynamics inthe frequency and wave-vector domain respectively Recentdevelopments of spatially resolved FMR [134] and BLS [135]techniques have emerged as powerful tools to study theprofiles of the dynamic modes in confined magnetic struc-tures The magnetoresistive method [136] X-ray microscopy[137] and pulse inductive magnetometry [138] also havethe potential to emerge as powerful techniques On theother hand the time-resolved magneto-optical Kerr effecthas emerged as one of the most powerful techniques tostudy the femto- and picosecond magnetization dynamicsof magnetic nanostructures due to simultaneous spatio-temporal resolution Here we will review the time-resolvedstudy of femto- and picosecond magnetization dynamics ofmagnetic nanostructures
111 Background Theory Magnetization dynamics can occurover a wide range of timescales Laser induced ultrafastdemagnetization occurs within a few hundreds of femtosec-onds The fast remagnetization time following the ultrafastdemagnetization covers the time scale of 1ndash10 picosecondsThe precession of magnetization occurs within few picosec-onds to few hundreds of picoseconds The damping ofmagnetization precession occurs on sub-nanosecond to tens
of nanoseconds time scalesThe slowest process is the domainwall dynamics which occurs between a few nanoseconds andmicroseconds
The time evolution of magnetization under the applica-tion of a time-dependent magnetic field ℎ(119905)may bemodeledby the Landau-Lifshitz-Gilbert equation of motion [130 131]given by (5)
Under the macrospin model and assuming a uni-form ellipsoidal particle with demagnetizing tensor axes119873119909 119873119910 and 119873
119911(119873119909
+ 119873119910
+ 119873119911
= 4120587) Kittel[13] derived the frequency for the uniform precessionalmode also known as ferromagnetic resonance mode as1205960
= 120574([119867119911+ (119873119910minus 119873119911)119872119911] times [119867
119911+ (119873119909minus 119873119911)119872119911])12 If
the system has two- and fourfoldmagnetic anisotropies givenby the energies 119865
2= minus119870
2sin 120579cos2120601 and 119865
4= (119870
44)
sin4120579(3 + cos 4120601) the resonant frequencies will take forms as
1205960= 120574([119867
119911+ (119873119910minus 119873119911+
21198702
119872119904
cos 2120601)119872119911]
times [119867119911+ (119873119909minus 119873119911+
21198702
119872119904
cos2120601)119872119911])
12
1205960= 120574([119867
119911+ (119873119910minus 119873119911minus
41198704
119872119904
cos 4120601)119872119911]
times [119867119911+ (119873119909minus 119873119911minus
1198704
119872119904
(3 + cos 4120601))119872119911])
12
(6)
These equations are valid for the case of vanishing wave-vector 119896 = 0 For finite wave-vector spin waves a dispersionof the spin wave frequencies with the wave-vector can befound In the exchange dominated regime (very large 119896)120596(119896)is proportional to (1 minus cos 119896119886) and in the long wavelengthlimit (119896119886 ≪ 1) this becomes 120596(119896) = 120574(119867 + 119863119896
2) where
14 Journal of Nanomaterials
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
119863 = 2119860119872119904and 119860 is the exchange stiffness constant The
perpendicular standing spin wave (PSSW)mode observed ina thin film is an example of exchange spinwaveThe spinwavepropagates along the thickness of the film and reflects back toform a standing spin wave The wave-vector is quantized forpinned or unpinned boundary condition The wave-vectorcan have values 119899(120587119889) where 119889 is the film thickness and 119899
is a positive integerIf an in-plane magnetic field is applied to an infi-
nite ferromagnetic thin film long wavelength spin waves(120582 sim 1 120583m) dominated by dipolar interaction are observedThe dispersion relations of dipolar modes can be calculatednumerically by solving the Landau-Lifshitz equation afterconsidering Maxwellrsquos equations in the magnetostatic limitThe dispersion relation of dipole-exchange spin wave in aninfinite ferromagneticmaterial can bewritten by theHerring-Kittel formula [139]
120596 = 120574[(119867 +
2119860
119872119904
1198962)(119867 +
2119860
119872119904
1198962+ 4120587119872
119904sin2120579119896)]
12
(7)
where 120579119896is the angle between 119896 and119872
119904 Damon and Eshbach
[140] first calculated the dispersion of dipolar modes for athin magnetic film They found two kinds of solutions thesurface or Damon-Eshbach (DE) and the volume mode Ingeneral when the surface mode propagates perpendicularlyto the magnetization it is called magnetostatic surface wave(MSSW) mode Considering negligible anisotropy the dis-persion relation of MSSWmode is given by
There is a manifold of volume or bulk modes When thedirections ofM and k are identical and both lie in the plane ofthe film the spin wave is called the backward volumemagne-tostatic mode (BWVMS) Considering negligible anisotropythe dispersion relation of lowest order BWVMS spin waves isgiven by
(
120596119861
120574
)
2
= 119867[119867 + 4120587119872119904(
1 minus 119890minus2119896119889
119896119889
)] (9)
The negative slope of the dispersion implies that the phasevelocity and group velocity are in opposite directions Whenthe magnetization is along the normal to the film and propa-gation direction is in the plane of the film a forward volumemagnetostatic (FWVMS) mode is observed The dispersionrelation can be expressed after neglecting anisotropy as
(
120596119865
120574
)
2
= (119867 minus 4120587119872119904) [119867 minus 4120587119872
119904(
1 minus 119890minus119896119889
119896119889
)] (10)
In a confined magnetic structure spin waves can also bequantized in the plane of the film If 119908 is the width ofnanostructure then the values of quantized wave-vector spinwaves may be written as 119896
119899= 2120587120582
119899= 119899120587119908 The
nonuniform demagnetization field and the edge effect mustbe considered to calculate the confined spin wave modesA convenient alternative to calculate the quantized spinwaves is solving the Landau-Lifshitz equation (5) within theframework of micromagnetism
112 Time-Resolved Magneto-Optical Kerr Effect Measure-ments Time-resolved magneto-optical Kerr effect measure-ments are based upon an optical pump-probe experimentIn its most general form a laser pulse (typical pulse widthof 100 fs) is divided into a strong pump beam and a weakprobe beam by a beam splitter The pump is used to excitethe magnetization dynamics while the probe is time-delayedwith respect to the pump beam by an optical delay generatorand is used to probe the dynamics in a noninvasive mannerby measuring the magneto-optical Kerr (or Faraday) effectUpon reflection (transmission) from the sample the planepolarized probe beam becomes elliptically polarized wherethe rotation angle or ellipticity gives a measure of themagnetization state of the sample In some cases secondharmonic MOKE [141] is also used which is highly sensitiveat the surface of thin film samples The Kerr (Faraday)rotationellipticity is measured by either using a photoelasticmodulator and an analyzer or a balanced photodiode detector[142] both of which provide very sensitive detection of rota-tion and ellipticity respectively down to microdegrees Thetime-delay between the pump and probe beams is scannedand theKerr (Faraday) rotationellipticity ismeasured at eachscan point to build the time evolution of magnetization afterexcitation by the pump pulseThe excitation can be electronicor purely optical In 1991 Freeman et al [143] reported thefirst time-resolved magneto-optical Kerr effect measurementof magnetization evolution and relaxation dynamics in pureand Tb doped EuS thin filmThey used an optically triggeredphotoconductive switch [144] to generate a current pulsewhich flows through a transmission line structure to producea pulsed magnetic field The pulsed magnetic field excites thedynamics and the corresponding relaxation is studied Laterin 1992 [145] they showed clear precessional dynamics inan yttrium iron garnet (YIG) film using the same techniqueUsing this technique Elezzabi et al [146] reported the directmeasurement of conduction electron spin-lattice relaxationtime T1 in gold film as 45 plusmn 5 ps suggesting that Matthiessenrsquosrule is not obeyed for conduction electron spin scatteringIn the same year Beaurepaire et al [147] demonstrated sub-picosecond demagnetization of metallic Ni film by directexcitation with a femtosecond laser pulse The measurementalso allowed to deduce electron and spin temperatures andgives a value to the electron-spin coupling constant In 1997the first time-resolved stroboscopic imaging of nonuniformprecessional dynamics in a microscale permalloy dot wasreported [148]Theseworks triggered a flurry of experimentaland theoretical works in the investigation of optical and fieldpump induced spin andmagnetization dynamics inmagneticthin films and patterned structures Even though doubts wereraised on whether the optically induced ultrafast magneto-optics in Ni is pure magnetism or optics [149] van Kampenet al demonstrated an all-optical method to excite and detectcoherent spin waves in magnetic materials in 2002 [150] Theapproach is based upon the temperature dependence of theanisotropy which allows one to use the heat froman absorbedlaser pulse to generate a fast anisotropy field pulse triggeringa precession of magnetization
Journal of Nanomaterials 15
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
113 Magnetization Dynamics of Nanomagnets The initialworks on the magnetization dynamics of nanomagnets havestarted to appear in early 2000 by FMR [151] and BLStechniques [152] The first time-resolved measurement ofprecessional dynamics in nanomagnet arrays induced bymagnetic field pulse was reported by Kruglyak et al in2005 [153] where they showed a size dependent variationof precession frequency and a cross-over to nonuniformprecession as the size of the permalloy nanodots is reduced tobelow 220 nm These experiments were performed on arraysof nanomagnets where the magnetostatic-interaction effectsfrom the neighboring elements cannot be ignored In 2006Barman et al [154] used a novel technique known as thetime-resolved cavity enhanced MOKE to study the intrin-sic femto- and picosecond dynamics of single cylindricalshaped Ni nanomagnets with diameter varying from 5 120583mdown to 125 nm The idea was based upon enhancing themagneto-optical Kerr rotation by coating the nanomagnetswith a dielectric enhancement layer By properly choosingthe thickness and material index of the dielectric layer theKerr signal reflected off the magnetic surface was enhancedby five times through constructive multiple reflections Avery interesting size dependence of precession frequency wasobserved due to the variation of magnetic ground states fromin-plane multidomain to vortex and finally to out-of-planemagnetized quasisingle domain state (Figure 14) The damp-ing also showed a strong size dependence with a transitionto a small damping value at the nanoscale as opposed toa large value at micron and submicron scales [155] Latera dynamic configurational anisotropy was shown in arraysof 220 nm permalloy nanomagnets [156] originating fromthe variation of both the static and dynamic magnetizationconfiguration and the associated dynamic effective magneticfield In 2007 Laraoui et al [157] presented ultrafast thermalswitching relaxation and precession of individual CoPt
3
nanodisks and permalloy microdisks with diameter down to200 nm A coherent thermal switching using 8mJcm2 pumppulses and a bi-exponential relaxation inCoPt
3was observed
A fast remagnetization (120591spin-lat = 52 ps) is associated withthe equilibrium between the spin and lattice while the slowremagnetization (120591diff = 530 ps) corresponds to the thermaldiffusion to the surrounding of the disk deposited on asapphire substrate In 2008 Liu et al [158] showed that theshape of the nanodisks significantly affects the vortex-to-quasisingle domain state transitions and the time-resolvedmagnetization dynamics showed different modal frequenciesdue to the transition between different domain states Theyobserved that this transition can also be spontaneouslytriggered when the bias field is kept fixed at a critical fieldregion and the disk is driven into dynamics In 2008 Keatleyet al [159] showed that at large amplitude precession the edgemode can be suppressed and dynamics is dominated by thecenter mode which is a useful result from the viewpointof nanoscale spin transfer torque oscillators and bistableswitching devices where large amplitude dynamics happens
114 Collective Magnonic Modes in Arrays of NanomagnetsAn array of dipolar coupled nanomagnets may show long
wavelength collective dynamics where the dynamics of theconstituent nanomagnets maintain constant amplitude andphase relationships similar to the acoustic and optical modesof phononsThese longwavelength collective dynamics in theform of Bloch waves defined in the Brillouin zone (BZ) of anartificial lattice can be manipulated by tailoring the lattice toform magnonic crystals the magnetic analogue of photonicand phononic crystals
In 2010 Kruglyak et al [160] reported the measurementand imaging of collective magnonic modes in arrays of 80 times
40 nm2 Co50Fe50(07 nm)Ni
92Fe8(4 5 nm) elements with
20 nm interdot separationThe pulsed field excited dynamicsshowed a broad single peak at higher bias field and splittinginto three narrower peaks at smaller bias field values dueto the appearance of the collective nonuniform precessionalmodes such as quasiuniform and backward volume-like andDamon-Eshbach-likemodes In 2011 Rana et al [161] showedan all-optical excitation and detection of collective modes inarrays of 200 nm square dots with varying interdot separation(119878) from 400 nm down to 50 nm The dynamics showed asystematic transition from a strongly collective regime (119878 le
75 nm) to a weakly collective regime (100 nm le 119878 le 300 nm)
to a completely isolated regime (119878 ge 400 nm) An anisotropyof the strongly collective mode (119878 = 50 nm) was observedwhen the orientation of the bias field is rotatedwith respect tothe lattice symmetry [162] In addition to a fourfold symmetryof the mode frequency a transition from a strongly collectiveto an isolated regime was also observed as the bias field isrotated from 0 to 45∘ with respect to the symmetry axisIn 2011 Rana et al [163] showed the detection of 50 nmpermalloy dots in the single nanodot regime where the dotsare magnetostatically isolated (Figure 15) The dynamics ofthe isolated 50 nm permalloy dot is dominated by the edgemode while the center mode becomes almost nonexistentThe damping of this mode is close to the thin film valueWhen these dots are arranged in arrays the frequencies ofthe modes increase with the decrease in interdot separationprimarily due to the quadrupolar interaction followed bya mode splitting where a collective-like backward volume-like mode appears in addition to the coherent mode of thearray The damping also shows an increase with the decreasein interdot separation due to the dynamic dephasing of thedots predicted earlier in 2009 [164] In 2013 Saha et al[165] showed how the collective magnonic modes of a two-dimensional nanomagnetic lattice can be tuned by the latticesymmetryThe interdot magnetostatic interaction is tuned byvarying the lattice symmetry which results in new collectivemagnonic modes as the symmetry reduces from square tooctagonal through rectangular hexagonal and honeycombsymmetry Further works on nanodots with different shapes[166 167] have shown various anisotropic spin waves whichcan form building blocks for two-dimensional magnoniccrystals
The collective magnetization dynamics of arrays of mag-netic nanodots is an important problemwith future prospectsof applications in on-chip microwave communication andspin-logic devices Magnonic band formation tunabilityof bandgaps control on propagation velocity anisotropy
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 14 Theoretical modeling of the diameter and bias-field dependence of experimentally obtained precession frequencies (a)Experimentally obtained frequencies of the uniform precession modes from uncoated and 70 nm SiN-coated magnets as a function ofmagnet diameter (aspect ratio) at 119867bias 168 kOe Solid line calculated precession frequencies of single magnets from (1) The gray shadedregion shows the possible range of precession frequencies in arrays of magnets separated by 56 nm (b) Experimental (points) and calculated(curves) precession frequencies as a function of bias magnetic field (119867bias) (c)119872119911 and contributions of different contributions to the effectivemagnetic field as a function of magnet diameter Reproduced with permission from Barman et al [154] copy 2006 American Chemical Society
in the spin wave propagation and variation in Gilbertdamping and extrinsic contributions to the damping aresome of the important issues to be dealt with Hencemore detailed understanding of the spin wave dynamicsin magnetic nanodot arrays and possible control of aboveproperties would open up exciting new prospects in thesefields On the other hand further development of powerfultechniques such as time-resolved near field MOKE and X-ray microscopy for studying magnetization dynamics withvery high spatiotemporal resolution will enable the study ofthe intrinsic dynamics of smaller single nanomagnets as wellas their dynamics under the magnetostatic interaction of theneighboring elements when placed in arrays of various latticeconstants and lattice symmetries
12 Summary and Outlook
We have discussed various aspects concerning the researchon MNPs and nanostructures The research on synthesisof magnetic nanoparticles has made a significant progressin particular with chemical routes By coating the MNPswith organic ligands the agglomeration and oxidation issuecan be taken care of easily Lithography techniques haveseen significant developments for making patterning ofnanostructures and in particular of magnetic materials forfuture data storage We have discussed that nanoparticleassemblies may show different magnetic states such as SPMSSG or SFM depending on the strength and symmetryof interparticle interactions While the structural charac-terization of nanoparticle assemblies is usually performed
Journal of Nanomaterials 17
0 250 500 750 1000Time (ps)
Kerr
rota
tion
(au
)S = 100nm
S = 75nm
S = 50nm
S = 150nm
S = 200nm
(a)
0 5 10 15 20
1
12
1
Frequency (GHz)Po
wer
(au
)
1
1
(b)
5 10 15Frequency (GHz)
Pow
er (a
u)
1
2
1
1
1
1
(c)
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
Figure 15 (a) Experimental time-resolved Kerr rotations and (b) the corresponding FFT spectra are shown for arrays of permalloy dots withwidth = 50 nm thickness = 20 nm andwith varying interdot separation 119878 at119867 = 25 kOe (c)The FFT spectra of the simulated time-resolvedmagnetization are shownThe peak numbers are assigned to the FFT spectraThe dotted line in (c) shows the simulated precession frequencyof a single permalloy dot with width = 50 nm and thickness = 20 nm Reproduced with permission from Rana et al [163] copy 2011 AmericanChemical Society
with transmission electron scanning electron or atomicforce microscopy X-ray diffraction and so forth the mag-netic states are characterized by different techniques suchas SQUID magnetometry ac susceptometry Mossbauer ormuon spectroscopy neutron scattering and magnetic forcemicroscopy
Still the field of magnetic nanoparticles has several openchallenges which should be addressed in the next couple ofyears
(i) Study of the Curie temperature of nanomagnets
(ii) Synthesis of monodisperse MNPs with well-controlled shape size and crystallinity
(iii) Study of supracrystals in order to understand theorigin of 3D ordering
(iv) Study of SSG correlations
(v) Study of SFM systems with different magnetic mate-rials particle sizes and so forth
(vi) Understanding the origin of superferromagnetismcan SFM be observed in a purely dipolarly coupledsystem
(vii) Getting insight into the SFM domain wall picture arethey real and do they have finite width
(viii) Understanding the dynamics of nanomagnets byspectroscopy and micromagnetic modeling going forapplications in data storage and biology
Conflict of Interests
The authors declare no financial conflict of interests
References
[1] S Tehrani E Chen M Durlam et al ldquoHigh density submicronmagnetoresistive random access memory (invited)rdquo Journal ofApplied Physics vol 85 no 8 pp 5822ndash5827 1999
[2] T Thomson G Hu and B D Terris ldquoIntrinsic distributionof magnetic anisotropy in thin films probed by patternednanostructuresrdquo Physical Review Letters vol 96 no 25 ArticleID 257204 4 pages 2006
[3] J R Childress and R E Fontana Jr ldquoMagnetic recording readhead sensor technologyrdquo Comptes Rendus Physique vol 6 no9 pp 997ndash1012 2005
[4] S H Chung AHoffmann S D Bader et al ldquoBiological sensorsbased on Brownian relaxation of magnetic nanoparticlesrdquoApplied Physics Letters vol 85 no 14 pp 2971ndash2973 2004
[5] M Arruebo R Fernandez-Pacheco M R Ibarra and JSantamarıa ldquoMagnetic nanoparticles for drug deliveryrdquo NanoToday vol 2 pp 22ndash32 2007
[6] D A Allwood G Xiong C C Faulkner D Atkinson D Petitand R P Cowburn ldquoMagnetic domain-wall logicrdquo Science vol309 no 5741 pp 1688ndash1692 2005
[7] A Imre G Csaba L Ji A Orlov G H Bernstein and WPorod ldquoMajority logic gate for magnetic quantum-dot cellularautomatardquo Science vol 311 no 5758 pp 205ndash208 2006
[8] S Kaka M R PufallW H Rippard T J Silva S E Russek andJ A Katine ldquoMutual phase-locking of microwave spin torquenano-oscillatorsrdquo Nature vol 437 no 7057 pp 389ndash392 2005
18 Journal of Nanomaterials
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
[9] B Lenk H Ulrichs F Garbs and M Munzenberg ldquoThebuilding blocks of magnonicsrdquo Physics Reports vol 507 no 4-5pp 107ndash136 2011
[10] C C Berry and A S G Curtis ldquoFunctionalisation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R198ndashR206 2003
[11] P Tartaj M del Puerto Morales S Veintemillas-Verdaguer TGonzalez-Carreno and C J Serna ldquoThe preparation of mag-netic nanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 36 no 13 pp R182ndashR197 2003
[12] J Frenkel and J Dorfman ldquoSpontaneous and induced magneti-sation in ferromagnetic bodiesrdquo Nature vol 126 no 3173 pp274ndash275 1930
[13] C Kittel ldquoTheory of the structure of ferromagnetic domains infilms and small particlesrdquo Physical Review vol 70 no 11-12 pp965ndash971 1946
[14] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo IEEE Transactions on Mag-netics vol 27 no 4 pp 3475ndash3518 1991
[15] R C OrsquoHandley Modern Magnetic Materials Principles andApplications Wiley-VCH Weinheim Germany 2000
[16] D Givord Q Lu andM F Rossignol Science and Technology ofNanostructured Materials G C Hadjipanayis and G A PrinzEds Plenum Press New York NY USA 1991
[17] S BlundellMagnetism in CondensedMatter Oxford UniversityPress New York NY USA 2001
[18] E C Stoner and E P Wohlfarth ldquoA mechanism of magnetichysteresis in heterogeneous alloysrdquo Philosophical Transactionsof the Royal Society A vol 240 pp 599ndash642 1948
[19] T Hyeon ldquoChemical synthesis of magnetic nanoparticlesrdquoChemical Communications vol 9 no 8 pp 927ndash934 2003
[20] C B Murray D J Norris and M G Bawendi ldquoSynthesisand characterization of nearly monodisperse CdE (E = S SeTe) semiconductor nanocrystallitesrdquo Journal of the AmericanChemical Society vol 115 no 19 pp 8706ndash8715 1993
[21] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie (International Edition) vol 46 no 8 pp 1222ndash1244 2007
[22] S Singamaneni V N Bliznyuk C Binek and E Y TsymballdquoMagnetic nanoparticles recent advances in synthesis self-assembly and applicationsrdquo Journal of Materials Chemistry vol21 no 42 pp 16819ndash16845 2011
[23] L E Euliss S G Grancharov S OrsquoBrien et al ldquoCooperativeassembly of magnetic nanoparticles and block copolypeptidesin aqueous mediardquo Nano Letters vol 3 no 11 pp 1489ndash14932003
[24] J Gao H Gu and B Xu ldquoMultifunctional magnetic nanopar-ticles design synthesis and biomedical applicationsrdquo Accountsof Chemical Research vol 42 no 8 pp 1097ndash1107 2009
[25] U Jeong X Teng Y Wang H Yang and Y Xia ldquoSuperpara-magnetic colloids controlled synthesis and niche applicationsrdquoAdvanced Materials vol 19 no 1 pp 33ndash60 2007
[26] S Laurent D Forge M Port et al ldquoMagnetic iron oxidenanoparticles synthesis stabilization vectorization physico-chemical characterizations and biological applicationsrdquo Chem-ical Reviews vol 108 pp 2064ndash2110 2008
[27] S Sun C B Murray D Weller L Folks and A MoserldquoMonodisperse FePt nanoparticles and ferromagnetic FePtnanocrystal superlatticesrdquo Science vol 287 no 5460 pp 1989ndash1992 2000
[28] B Jeyadevan A Hobo K Urakawa C N Chinnasamy KShinoda and K Tohji ldquoTowards direct synthesis of fct-FePtnanoparticles by chemical routerdquo Journal of Applied Physics vol93 no 10 pp 7574ndash7576 2003
[29] S Momose H Kodama T Uzumaki and A Tanaka ldquoMagneticproperties of magnetically isolated L1
0-FePt nanoparticlesrdquo
Applied Physics Letters vol 85 no 10 pp 1748ndash1750 2004[30] S Yamamoto Y Morimoto T Ono and M Takano ldquoMag-
netically superior and easy to handle L10-FePt nanocrystalsrdquo
Applied Physics Letters vol 87 no 3 Article ID 032503 2005[31] H Zeng J Li J P Liu Z L Wang and S Sun ldquoExchange-
coupled nanocomposite magnets by nanoparticle self-assemblyrdquo Nature vol 420 no 6914 pp 395ndash398 2002
[32] M Albrecht G Hu I L Guhr et al ldquoMagnetic multilayers onnanospheresrdquoNature Materials vol 4 no 3 pp 203ndash206 2005
[33] F Q Zhu G W Chern O Tchernyshyov X C Zhu J G Zhuand C L Chien ldquoMagnetic bistability and controllable reversalof asymmetric ferromagnetic nanoringsrdquo Physical Review Let-ters vol 96 no 2 Article ID 027205 4 pages 2006
[34] F Ernult S Mitani K Takanashi et al ldquoSelf-alignment of Fenanoparticles on a tunnel barrierrdquo Applied Physics Letters vol87 no 3 Article ID 033115 2005
[35] C T Black C B Murray R L Sandstrom and S Sun ldquoSpin-dependent tunneling in self-assembled cobalt-nanocrystalsuperlatticesrdquo Science vol 290 no 5494 pp 1131ndash1134 2000
[36] S Mitani S Takahashi K Takanashi K Yakushiji S Maekawaand H Fujimori ldquoEnhanced magnetoresistance in insulatinggranular systems evidence for higher-order tunnelingrdquoPhysicalReview Letters vol 81 no 13 pp 2799ndash2802 1998
[37] K Yakushiji F Ernult H Imamura et al ldquoEnhanced spinaccumulation and novel magnetotransport in nanoparticlesrdquoNature Materials vol 4 no 1 pp 57ndash61 2005
[38] T Shima K Takanashi Y K Takahashi and K Hono ldquoPrepa-ration and magnetic properties of highly coercive FePt filmsrdquoApplied Physics Letters vol 81 no 6 pp 1050ndash1052 2002
[39] T Shima K Takanashi Y K Takahashi andKHono ldquoCoerciv-ity exceeding 100 kOe in epltaxially grown FePt sputtered filmsrdquoApplied Physics Letters vol 85 no 13 pp 2571ndash2573 2004
[40] S Okamoto O Kitakami N Kikuchi T Miyazaki Y Shimadaand Y K Takahashi ldquoSize dependences of magnetic propertiesand switching behavior in FePt L1
0nanoparticlesrdquo Physical
Review B vol 67 no 9 Article ID 094422 2003[41] C Pascal J L Pascal F Favier M L Elidrissi Moubtassim and
C Payen ldquoElectrochemical synthesis for the control of 120574-Fe2O3
nanoparticle size Morphology microstructure and magneticbehaviorrdquo Chemistry of Materials vol 11 pp 141ndash147 1999
[42] Z Dai F Meiser and H Mohwald ldquoNanoengineering ofiron oxide and iron oxidesilica hollow spheres by sequentiallayering combinedwith a sol-gel processrdquo Journal of Colloid andInterface Science vol 288 no 1 pp 298ndash300 2005
[43] R N Grass and W J Stark ldquoGas phase synthesis of fcc-cobaltnanoparticlesrdquo Journal ofMaterials Chemistry vol 16 no 19 pp1825ndash1830 2006
[44] T Jaumann E M M Ibrahim S Hampel D Maier A Leon-hardt and B Buchner ldquoThe synthesis of superparamagneticcobalt nanoparticles encapsulated in carbon through high-pressure CVDrdquoChemical Vapor Deposition vol 19 pp 228ndash2342013
[45] G N Kakazei Y G Pogorelov A M L Lopes et al ldquoTunnelmagnetoresistance and magnetic ordering in ion-beam sput-tered Co
80Fe20Al2O3discontinuous multilayersrdquo Journal of
Applied Physics vol 90 no 8 pp 4044ndash4048 2001
Journal of Nanomaterials 19
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
[46] T SekiH Iwama T Shima andK Takanashi ldquoSize dependenceof the magnetization reversal process in microfabricated L1
0-
FePt nano dotsrdquo Journal of Physics D vol 44 no 33 Article ID335001 2011
[47] M Schneider H Hoffmann and J Zweck ldquoLorentzmicroscopyof circular ferromagnetic permalloy nanodisksrdquoApplied PhysicsLetters vol 77 no 18 pp 2909ndash2911 2000
[48] M Hanson O Kazakova P Blomqvist R Wappling andB Nilsson ldquoMagnetic domain structures in submicron-sizeparticles of epitaxial Fe (001) films shape anisotropy andthickness dependencerdquoPhysical Review B vol 66 no 14 ArticleID 144419 2002
[49] M Hehn K Ounadjela J-P Bucher et al ldquoNanoscale magneticdomains inmesoscopicmagnetsrdquo Science vol 272 no 5269 pp1782ndash1785 1996
[50] J Moritz L Buda B Dieny et al ldquoWriting and reading bits onpre-patterned mediardquo Applied Physics Letters vol 84 no 9 pp1519ndash1521 2004
[51] T Shinjo T Okuno R Hassdorf K Shigeto and T Ono ldquoMag-netic vortex core observation in circular dots of permalloyrdquoScience vol 289 no 5481 pp 930ndash932 2000
[52] D C Ralph and M D Stiles ldquoSpin transfer torquesrdquo Journal ofMagnetism andMagneticMaterials vol 320 no 7 pp 1190ndash12162008
[53] S S P Parkin M Hayashi and L Thomas ldquoMagnetic domain-wall racetrack memoryrdquo Science vol 320 no 5873 pp 190ndash1942008
[54] N Kikuchi S Okamoto O Kitakami Y Shimada and KFukamichi ldquoSensitive detection of irreversible switching in asingle FePt nanosized dotrdquo Applied Physics Letters vol 82 no24 pp 4313ndash4315 2003
[55] D Wang T Seki K Takanashi and T Shima ldquoMagnetizationreversal process in microfabricated L1
0-FePt dotsrdquo Journal of
Physics D vol 41 no 19 Article ID 195008 2008[56] O Hellwig L J Heyderman O Petracic and H Zabel ldquoCom-
peting interactions in patterned and self-assembled magneticnanostructuresrdquo in Springer Tracts in Modern Physics vol 246pp 189ndash234 Springer Berlin Germany 2013
[57] S A Claridge A W Castleman Jr S N Khanna C B MurrayA Sen and P S Weiss ldquoCluster-assembled materialsrdquo ACSNano vol 3 no 2 pp 244ndash255 2009
[58] M P Pileni ldquoSupracrystals of inorganic nanocrystals an openchallenge for new physical propertiesrdquo Accounts of ChemicalResearch vol 41 no 12 pp 1799ndash1809 2008
[59] F X Redl K-S Cho C B Murray and S OrsquoBrien ldquoThree-dimensional binary superlattices of magnetic nanocrystals andsemiconductor quantum dotsrdquo Nature vol 423 no 6943 pp968ndash971 2003
[60] D Babonneau F Petroff J-L Maurice F Fettar A Vaures andA Naudon ldquoEvidence for a self-organized growth in granularCoAl
2O3multilayersrdquo Applied Physics Letters vol 76 no 20
pp 2892ndash2894 2000[61] X Chen S Bedanta O Petracic et al ldquoSuperparamagnetism
versus superspin glass behavior in dilute magnetic nanoparticlesystemsrdquo Physical Review B vol 72 no 21 Article ID 2144362005
[62] O Petracic D Mishra D Greving et al ldquoStructural andmagnetic correlations in iron oxide nanoparticle superlatticesrdquoUnpublished
[63] C Binns K N Trohidou J Bansmann et al ldquoThe behaviourof nanostructured magnetic materials produced by depositing
gas-phase nanoparticlesrdquo Journal of Physics D vol 38 no 22pp R357ndashR379 2005
[64] B Balasubramanian R Skomski X Li et al ldquoCluster synthesisand direct ordering of rare-earth transition-metal nanomag-netsrdquo Nano Letters vol 11 no 4 pp 1747ndash1752 2011
[65] F Li D P Josephson andA Stein ldquoColloidal assembly the roadfrom particles to colloidal molecules and crystalsrdquo AngewandteChemie (International Edition) vol 50 no 2 pp 360ndash388 2011
[66] K J M Bishop C E Wilmer S Soh and B A GrzybowskildquoNanoscale forces and their uses in self-assemblyrdquo Small vol 5no 14 pp 1600ndash1630 2009
[67] F Marlow M Muldarisnur P Sharifi R Brinkmann and CMendive ldquoOpals status and prospectsrdquo Angewandte Chemie(International Edition) vol 48 no 34 pp 6212ndash6233 2009
[68] S Disch E Wetterskog R P Hermann et al ldquoShapeinduced symmetry in self-assembledmesocrystals of iron oxidenanocubesrdquo Nano Letters vol 11 no 4 pp 1651ndash1656 2011
[69] DMishra DGreving G A Badini Confalonieri et al ldquoGrowthmodes of nanoparticle superlattice thin filmsrdquo Unpublished
[70] A Dong J Chen P M Vora J M Kikkawa and C B MurrayldquoBinary nanocrystal superlattice membranes self-assembled atthe liquid-air interfacerdquoNature vol 466 no 7305 pp 474ndash4772010
[71] J Chen X Ye and C B Murray ldquoSystematic electron crystallo-graphic studies of self-assembled binary nanocrystal superlat-ticesrdquo ACS Nano vol 4 no 4 pp 2374ndash2381 2010
[72] M J Benitez D Mishra P Szary et al ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physics Condensed Matter vol 23 no 12Article ID 126003 2011
[73] J W Cheon J I Park J S Choi et al ldquoMagnetic superlatticesand their nanoscale phase transition effectsrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 9 pp 3023ndash3027 2006
[74] D Parker I Lisiecki and M P Pileni ldquoStructural and mag-netic characterization of self-assembled iron oxide nanoparticlearraysrdquo Journal of Physical Chemistry Letters vol 23 no 12Article ID 126003 2011
[75] W Kleemann O Petracic C Binek et al ldquoInteracting fer-romagnetic nanoparticles in discontinuous Co
80Fe20Al2O3
multilayers from superspin glass to reentrant superferromag-netismrdquo Physical Review B vol 63 no 13 Article ID 1344232001
[76] O Petracic X Chen S Bedanta et al ldquoCollective states ofinteracting ferromagnetic nanoparticlesrdquo Journal of Magnetismand Magnetic Materials vol 300 no 1 pp 192ndash197 2006
[77] S Bedanta and W Kleemann ldquoSupermagnetismrdquo Journal ofPhysics D vol 42 no 1 Article ID 013001 2009
[78] L Neel ldquoTheorie du trainagemagnetique des ferromagnetiquesen grains fins avec applications aux terres cuitesrdquo AnnalesGeophysique vol 5 pp 99ndash136 1949
[79] L Weil L Gruner and A Deschamps ldquoOrientation desprecipitations du cobalt dans un alliageCuCordquoComptes Rendusvol 244 p 2143 1957
[80] A Knappwost ldquoKollektivparamagnetismus und volumen mag-netisierter aerosolerdquo Zeitschrift fur Elektrochemie vol 61 pp1328ndash1334 1957
[81] R Hahn and E Kneller Zeitschrift fur Metallkunde vol 49 pp426ndash441 1958
[82] A E Berkowitz and P J Flanders ldquoPrecipitation in a beta-brass-fe alloyrdquo Journal of Applied Physics vol 30 Article ID S111 1959
20 Journal of Nanomaterials
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
[83] W F Brown ldquoThermal fluctuations of a single-domain particlerdquoPhysical Review vol 130 no 5 pp 1677ndash1686 1963
[84] W F Brown Jr ldquoRelaxational behavior of fine magnetic parti-clesrdquo Journal of Applied Physics vol 30 Article ID S130 1959
[85] K Binder and A P Young ldquoSpin glasses experimental factstheoretical concepts and open questionsrdquo Reviews of ModernPhysics vol 58 no 4 pp 801ndash976 1986
[86] G Parisi ldquoOrder parameter for spin-glassesrdquo Physical ReviewLetters vol 50 no 24 pp 1946ndash1948 1983
[87] J Mydosh Spin Glasses An Experimental Introduction Taylor ampFrancis London UK 1993
[88] C Djurberg P Svedlindh P NordbladM F Hansen F Boslashdkerand S Moslashrup ldquoDynamics of an interacting particle systemevidence of critical slowing downrdquo Physical Review Letters vol79 no 25 pp 5154ndash5157 1997
[89] K Binder and J D Reger ldquoTheory of orientational glassesmodels concepts simulationsrdquo Advances in Physics vol 41 no6 pp 547ndash627 1992
[90] R Skomski ldquoAre there superspin glassesrdquo Journal of AppliedPhysics vol 109 no 7 Article ID 07E149 2011
[91] S Sahoo O Petracic W Kleemann et al ldquoCooperative versussuperparamagnetic behavior of dense magnetic nanoparticlesin Co
no 23 pp 4116ndash4118 2003[92] R H Kodama A E Berkowitz E J McNiff Jr and S Foner
ldquoSurface spin disorder in NiFe2O4nanoparticlesrdquo Physical
Review Letters vol 77 no 2 pp 394ndash397 1996[93] D Fiorani Ed Surface Effects in Magnetic Nanoparticles
Springer New York NY USA 2005[94] K NadeemH Krenn T Traussing and I Letofsky-Papst ldquoDis-
tinguishing magnetic blocking and surface spin-glass freezingin nickel ferrite nanoparticlrdquo Journal of Applied Physics vol 109no 1 Article ID 013912 2011
[95] E Winkler R D Zysler M Vasquez Mansilla et al ldquoSurfacespin-glass freezing in interacting core-shell NiO nanoparticlesrdquoNanotechnology vol 19 no 18 Article ID 185702 2008
[96] K L Lopez Maldonado P de la Presa E Flores Taviz et alldquoMagnetic susceptibility studies of the spin-glass and Verweytransitions in magnetite nanoparticlesrdquo Journal of AppliedPhysics vol 113 Article ID 17E132 2013
[97] O Bostanjoglo and K Roehkel ldquoSuperferromagnetism ingadolinium filmsrdquo Physica Status Solidi A vol 11 no 1 pp 161ndash166 1972
[98] S Moslashrup M Bo Madsen J Franck J Villadsen and CJ W Koch ldquoA new interpretation of Mossbauer spectra ofmicrocrystalline goethite ldquosuper-ferromagnetismrdquo or ldquosuper-spin-glassrdquo behaviourrdquo Journal of Magnetism and MagneticMaterials vol 40 no 1-2 pp 163ndash174 1983
[99] D G Rancourt and J M Daniels ldquoInfluence of unequalmagnetization direction probabilities on theMossbauer spectraof superparamagnetic particlesrdquo Physical Review B vol 29 no5 pp 2410ndash2414 1984
[100] S Sankar A E Berkowitz D Dender et al ldquoMagnetic corre-lations in non-percolated Co-SiO
2granular filmsrdquo Journal of
Magnetism and Magnetic Materials vol 221 no 1-2 pp 1ndash92000
[101] S Bedanta J Rhensius W Kleemann P Parashar S Car-doso and P P Freitas ldquoDynamic magnetization properties ofa superferromagnetic metal-insulator multilayer observed bymagneto-optic Kerrmicroscopyrdquo Journal of Applied Physics vol105 no 7 Article ID 07C306 2009
[102] X Chen O Sichelschmidt W Kleemann et al ldquoDomain wallrelaxation creep sliding and switching in superferromagneticdiscontinuous Co
80Fe20Al2O3multilayersrdquo Physical Review
Letters vol 89 no 13 Article ID 137023 2002[103] S Bedanta O Petracic E Kentzinger et al ldquoSuperferro-
magnetic domain state of a discontinuous metal insulatormultilayerrdquo Physical Review B vol 72 no 2 Article ID 0244192005
[104] X Chen W Kleemann O Petracic O Sichelschmidt SCardoso and P P Freitas ldquoRelaxation and aging of a superferro-magnetic domain staterdquo Physical Review B vol 68 no 5 ArticleID 054433 2003
[105] S Bedanta T Eimuller W Kleemann et al ldquoOvercomingthe dipolar disorder in dense CoFe nanoparticle ensemblessuperferromagnetismrdquo Physical Review Letters vol 98 no 17Article ID 176601 2007
[106] J Dorman and D Fiorani Eds Magnetic Properties of FineParticles North-Holland Amsterdam The Netherlands 1991
[107] L A Harris J D Goff A Y Carmichael et al ldquoMagnetitenanoparticle dispersions stabilized with triblock copolymersrdquoChemistry of Materials vol 15 no 6 pp 1367ndash1377 2003
[108] A F Thunemann D Schutt L Kaufner U Pison andH Mohwald ldquoMaghemite nanoparticles protectively coatedwith poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)rdquo Langmuir vol 22 no 5 pp 2351ndash23572006
[109] V Skumryev S Stoyanov Y Zhang G Hadjipanayis D Givordand J Nogues ldquoBeating the superparamagnetic limit withexchange biasrdquo Nature vol 423 no 6942 pp 850ndash853 2003
[110] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
[111] A Punnoose H Magnone M S Seehra and J BonevichldquoBulk to nanoscale magnetism and exchange bias in CuOnanoparticlesrdquo Physical Review B vol 64 no 17 Article ID174420 2001
[112] R D Zysler E Winkler M Vasquez Mansilla and D FioranildquoSurface effect in the magnetic order of antiferromagneticnanoparticlesrdquo Physica B vol 384 no 1-2 pp 277ndash281 2006
[113] J Nogues V Skumryev J Sort S Stoyanov and D GivordldquoShell-driven magnetic stability in core-shell nanoparticlesrdquoPhysical Review Letters vol 97 no 15 Article ID 157203 2006
[114] B Jeyadevan C N Chinnasamy O Perales-Perez et alldquoSynthesis and magnetic properties of core-shell structured(NiCo)O(AFM)-NiCo(FM) magnetic nanoparticlesrdquo IEEETransactions on Magnetics vol 38 no 5 pp 2595ndash2597 2002
[115] J van Lierop M A Schofield L H Lewis and R J GambinoldquoExchange bias in a thin film dispersion of MnO nanocrystal-lites in Cordquo Journal of Magnetism and Magnetic Materials vol264 no 2-3 pp 146ndash152 2003
[116] C Luna M del Puerto Morales C J Serna and M VazquezldquoExchange anisotropy in Co
80Ni20oxide nanoparticlesrdquo Nan-
otechnology vol 15 no 4 pp S293ndashS297 2004[117] A Tomou D Gournis I Panagiotopoulos Y Huang G
C Hadjipanayis and B J Kooi ldquoWeak ferromagnetism andexchange biasing in cobalt oxide nanoparticle systemsrdquo Journalof Applied Physics vol 99 no 12 Article ID 123915 2006
[118] O Iglesias A Labarta and X Batlle ldquoExchange bias phe-nomenology andmodels of coreshell nanoparticlesrdquo Journal ofNanoscience and Nanotechnology vol 8 no 6 pp 2761ndash27802008
Journal of Nanomaterials 21
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
[119] G Salazar-Alvarez J Sort S Surinach M D Baro andJ Nogues ldquoSynthesis and size-dependent exchange bias ininverted core-shell MnOmdashMn3O4 nanoparticlesrdquo Journal ofthe American Chemical Society vol 129 no 29 pp 9102ndash91082007
[120] E L Salabas A Rumplecker F Kleitz F Radu and F SchuthldquoExchange anisotropy in nanostructured porous Co
3O4rdquo Nano
Letters vol 6 pp 2977ndash2981 2006[121] J Nogues J Sort V Langlais et al ldquoExchange bias in nanostruc-
turesrdquo Physics Reports vol 422 no 3 pp 65ndash117 2005[122] AMoser K TakanoD TMargulies et al ldquoMagnetic recording
advancing into the futurerdquo Journal of Physics D vol 35 no 19pp R157ndashR167 2002
[123] W Andra U Hafeli R Hergt and R Misri ldquoApplication ofmagnetic particles in medicine and biologyrdquo in Handbook ofMagnetism and Advanced Magnetic Materials Vol 4 NovelMaterials H Kronmuller and S P S Parkin Eds pp 2536ndash22568 John Wiley amp Sons Chichester UK 2007
[124] Y Shiroishi K Fukuda I Tagawa et al ldquoFuture options forHDD storagerdquo IEEE Transactions on Magnetics vol 45 no 10pp 3816ndash3822 2009
[125] B D Terris and T Thomson ldquoNanofabricated and self-assembled magnetic structures as data storage mediardquo Journalof Physics D vol 38 no 12 pp R199ndashR222 2005
[126] Y Guo PWangM-M Chen et al ldquoMRAMarray with coupledsoft-adjacent magnetic layerrdquo Journal of Applied Physics vol 97no 10 Article ID 10P506 2005
[127] D Weller and A Moser ldquoThermal effect limits in ultrahigh-density magnetic recordingrdquo IEEE Transactions on Magneticsvol 35 no 6 pp 4423ndash4439 1999
[128] O A Ivanov L V Solina V A Demshina and L M MagatldquoDetermination of the anisotropy constant and saturationmagnetization and magnetic properties of an iron-platinumalloyrdquo Physics of Metals and Metallography vol 35 no 1 pp 81ndash85 1973
[129] J Fidler andT Schrefl ldquoMicromagneticmodellingmdashthe currentstate of the artrdquo Journal of Physics D vol 33 no 15 pp R135ndashR156 2000
[130] D P Landau and K BinderAGuide toMonte Carlo Simulationsin Statistical Physics Cambridge University Press CambridgeUK 2005
[131] T L Gilbert ldquoA lagrangian formulation of the gyromagneticequation of themagnetic fieldrdquo Physical Review vol 100 ArticleID 1243 1955
[132] J-O Andersson C Djurberg T Jonsson P Svedlindh andP Nordblad ldquoMonte Carlo studies of the dynamics of aninteracting monodispersive magnetic-particle systemrdquo PhysicalReview B vol 56 no 21 pp 13983ndash13988 1997
[133] O Iglesias and A Labarta ldquoFinite-size and surface effects inmaghemite nanoparticles Monte Carlo simulationsrdquo PhysicalReview B vol 63 no 18 Article ID 184416 2001
[134] S Tamaru J A Bain R J M van de Veerdonk T M CrawfordM Covington and M H Kryder ldquoImaging of quantizedmagnetostatic modes using spatially resolved ferromagneticresonancerdquo Journal of Applied Physics vol 91 no 10 p 80342002
[135] G Gubbiotti G Carlotti M Madami S Tacchi P Vavassoriand G Socino ldquoSetup of a new Brillouin light scattering appa-ratus with submicrometric lateral resolution and its applicationto the study of spin modes in nanomagnetsrdquo Journal of AppliedPhysics vol 105 no 7 Article ID 07D521 2009
[136] I N Krivorotov N C Emley J C Sankey S I Kiselev D CRalph and R A Buhrman ldquoTime-domain measurements ofnanomagnet dynamics driven by spin-transfer torquesrdquo Sciencevol 307 no 5707 pp 228ndash231 2005
[137] Y Acremann J P Strachan V Chembrolu et al ldquoTime-resolvedimaging of spin transfer switching beyond the macrospinconceptrdquo Physical Review Letters vol 96 no 21 Article ID217202 2006
[138] T J Silva C S Lee TM Crawford andC T Rogers ldquoInductivemeasurement of ultrafast magnetization dynamics in thin-filmPermalloyrdquo Journal of Applied Physics vol 85 no 11 pp 7849ndash7862 1999
[139] C Herring and C Kittel ldquoOn the theory of spin waves inferromagnetic mediardquo Physical Review vol 81 no 5 pp 869ndash880 1951
[140] R W Damon and J R Eshbach ldquoMagnetostatic modes of aferromagnet slabrdquo Journal of Physics andChemistry of Solids vol19 pp 308ndash320 1961
[141] R-P Pan H DWei and Y R Shen ldquoOptical second-harmonicgeneration from magnetized surfacesrdquo Physical Review B vol39 no 2 pp 1229ndash1234 1989
[142] P Kasiraj R M Shelby J S Best and D E Horne ldquoMagneticdomain imaging with a scanning Kerr effect microscoperdquo IEEETransactions on Magnetics vol 22 no 5 pp 837ndash839 1986
[143] M R Freeman R R Ruf and R J Gambino ldquoPicosecondpulsed magnetic fields for studies of ultrafast magnetic phe-nomenardquo IEEE Transactions on Magnetics vol 27 no 6 pp4840ndash4842 1991
[144] D H Auston ldquoPicosecond optoelectronic switching and gatingin siliconrdquo Applied Physics Letters vol 26 no 3 pp 101ndash1031975
[145] M R Freeman M J Brady and J Smyth ldquoExtremely highfrequency pulse magnetic resonance by picosecond magneto-optic samplingrdquo Applied Physics Letters vol 60 no 20 pp2555ndash2557 1992
[146] A Y Elezzabi M R Freeman and M Johnson ldquoDirectmeasurement of the conduction electron spin-lattice relaxationtimeT1 in goldrdquoPhysical Review Letters vol 77 no 15 pp 3220ndash3223 1996
[147] E Beaurepaire J-C Merle A Daunois and J-Y Bigot ldquoUltra-fast spin dynamics in ferromagnetic nickelrdquo Physical ReviewLetters vol 76 no 22 pp 4250ndash4253 1996
[148] W K Hiebert A Stankiewicz and M R Freeman ldquoDirectobservation of magnetic relaxation in a small permalloy diskby time-resolved scanning kerr microscopyrdquo Physical ReviewLetters vol 79 no 6 pp 1134ndash1137 1997
[149] B Koopmans M van Kampen J T Kohlhepp and W J Mde Jonge ldquoUltrafast magneto-optics in nickel magnetism oropticsrdquo Physical Review Letters vol 85 no 4 pp 844ndash8472000
[150] M vanKampen C Jozsa J T Kohlhepp et al ldquoAll-optical probeof coherent spin wavesrdquo Physical Review Letters vol 88 no 22Article ID 227201 2002
[151] S Jung B Watkins L DeLong J B Ketterson and VChandrasekhar ldquoFerromagnetic resonance in periodic particlearraysrdquo Physical Review B vol 66 no 13 Article ID 1324012002
[152] G Gubbiotti G Carlotti T Okuno T Shinjo F Nizzoli and RZivieri ldquoBrillouin light scattering investigation of dynamic spinmodes confined in cylindrical Permalloy dotsrdquo Physical ReviewB vol 68 no 18 Article ID 184409 2003
22 Journal of Nanomaterials
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
[153] V V Kruglyak A Barman R J Hicken J R Childress and J AKatine ldquoPicosecond magnetization dynamics in nanomagnetscrossover to nonuniform precessionrdquo Physical Review B vol 71no 22 Article ID 220409 2005
[154] A Barman S Wang J D Maas et al ldquoMagneto-opticalobservation of picosecond dynamics of single nanomagnetsrdquoNano Letters vol 6 no 12 pp 2939ndash2944 2006
[155] A Barman S Wang J Maas et al ldquoSize dependent damping inpicosecond dynamics of single nanomagnetsrdquo Applied PhysicsLetters vol 90 no 20 Article ID 202504 2007
[156] VVKruglyak P S Keatley R J Hicken J R Childress and J AKatine ldquoDynamic configurational anisotropy in nanomagnetsrdquoPhysical Review B vol 75 no 2 Article ID 024407 2007
[157] A Laraoui J Venuat V Halte M Albrecht E Beaurepaireand J-Y Bigot ldquoStudy of individual ferromagnetic disks withfemtosecond optical pulsesrdquo Journal of Applied Physics vol 101no 9 Article ID 09C105 2007
[158] Z Liu R D Sydora and M R Freeman ldquoShape effects onmagnetization state transitions in individual 160-nm diameterPermalloy disksrdquo Physical Review B vol 77 no 17 Article ID174410 2008
[159] P S Keatley P Gangmei M Dvornik R J Hicken J RChildress and J A Katine ldquoLarge amplitude magnetizationdynamics and the suppression of edge modes in a singlenanomagnetrdquo Applied Physics Letters vol 98 no 8 Article ID082506 2011
[160] V V Kruglyak P S Keatley A Neudert R J Hicken JR Childress and J A Katine ldquoImaging collective magnonicmodes in 2D arrays ofmagnetic nanoelementsrdquo Physical ReviewLetters vol 104 no 2 Article ID 027201 2010
[161] B Rana S Pal S Barman Y Fukuma Y Otani and A BarmanldquoAll-optical excitation and detection of picosecond dynamicsof ordered arrays of nanomagnets with varying areal densityrdquoApplied Physics Express vol 4 no 11 Article ID 113003 2011
[162] B Rana D Kumar S Barman et al ldquoAnisotropy in collectiveprecessional dynamics in arrays of Ni
80Fe20
nanoelementsrdquoJournal of Applied Physics vol 111 Article ID 07D503 2012
[163] B Rana D Kumar S Barman et al ldquoDetection of picosecondmagnetization dynamics of 50 nm magnetic dots down to thesingle dot regimerdquoACSNano vol 5 no 12 pp 9559ndash9565 2011
[164] A Barman and S Barman ldquoDynamic dephasing of magnetiza-tion precession in arrays of thin magnetic elementsrdquo PhysicalReview B vol 79 no 14 Article ID 144415 2009
[165] S Saha RMandal S Barman et al ldquoTunablemagnonic spectrain two-dimensional magnonic crystals with variable latticesymmetryrdquo Advanced Functional Materials vol 23 pp 2378ndash2386 2013
[166] B K Mahato B Rana D Kumar et al ldquoConfigurationalanisotropic spin waves in cross-shapedNi
80Fe20nanoelementsrdquo
Applied Physics Letters vol 102 Article ID 192402 2013[167] S Saha S Barman J Ding A O Adeyeye and A Bar-
man ldquoTime-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structuresrdquoApplied Physics Letters vol 102 Article ID 242409 2013
![Engineered Multifunctional Nanocarriers for Cancer Diagnosis ...homepages.uc.edu/~shid/publications/PDFfiles/Engineered...[55–59 ] Magnetic nanoparticles (MNPs) or magnetic nanospheres](https://static.documents.pub/doc/80x56/603a819c2af00a55936733e1/engineered-multifunctional-nanocarriers-for-cancer-diagnosis-shidpublicationspdffilesengineered.jpg)
![Biogenic Synthesis of Silver Nanoparticles with ...nanomaterial, such as metal nanoparticles [7]. Typically, MNPs can be synthesized using various approaches, such as biological, physical,](https://static.documents.pub/doc/80x56/5f2c4a5b59327467e05d1364/biogenic-synthesis-of-silver-nanoparticles-with-nanomaterial-such-as-metal.jpg)
