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A
REPORT
ONNANOMATERIALS & NANOMAGNETISM
Submitted
By
GAGANJOT SINGH
University roll no. - 601202004
Under the esteemed guidance of
Dr. POONAM UNIYAL
(Assistant Professor, SPMS, Thapar University.)
School of Physics and Materials Science,
Thapar University, Patiala [Pb.]
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Contents
1. Introduction to materials..........3
1.1 General.......3
1.2 Classification of materials .3
1.3 Selection of materials ....4
2. Nano Materials.....5
2.1 Introduction...............5
2.2 Improved properties ofnano over bulk.6
2.3 Characterization of nano materials...7
2.3.1. Structural Characterization7 2.3.2. Chemical Characterization7
2.4 Properties of nano materials .....9
2.5 Applications of nano materials ......10
3. Nano Magnetism........11
3.1 Introduction.....11
3.2 Challenges in nano magnetism.......11
3.3 Dimensionality in nano magnetism ...........13
3.4 Magnetic Anisotropy .....15
3.5 Single Domain....16
3.6 Domain walls..........19
3.7 Applications....19
3.8 Future......21
3.9 Bismuth Ferrite.......213.10 Summary...........23
References.....24
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Chapter 1
Introduction to materials
1.1General
Material is anything made of matter, constituted of one or more substances. Wood,
cement, hydrogen, air, water and any other matter are all examples of materials. Materials
science and engineering plays a vital role in this modern age of science and technology.
Various kinds of materials are used in industry, housing, agriculture, transportation, etc.
to meet the plant and individual requirements. The rapid developments in the field of
quantum theory of solids have opened vast opportunities for better understanding and
utilization of various materials. The spectacular success in the field of space is primarilydue to the rapid advances in high - temperature and high-strength materials. The selection
of a specific material for a particular use is a very complex process. However, one can
simplify the choice if the details about (i) operating parameters, (ii) manufacturing
processes, (iii) functional requirements and (iv) cost considerations are known.
1.2. Classification of materials
The factors which form the basis of various systems of classifications of materials inmaterial science and engineering are: (i) the chemical composition of the material, (ii) the
mode of the occurrence of the material in the nature, (iii) the refining and the
manufacturing process to which the material is subjected prior it acquires the required
properties, (iv) the atomic and crystalline structure of material and (v) the industrial and
technical use of the material.
Common engineering materials that fall within the scope of material science and
engineering may be classified into one of the following six groups:
1) Metals (ferrous and non-ferrous) and alloys
2) Ceramics
3) Organic Polymers
4) Composites
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5) Semi-conductors
6) Biomaterials
7) Advanced Materials
Table 1: Different classes of materials
1.3. Selection of Materials
One of the most challenging tasks of an engineer is the proper selection of the material
for a particular job, e.g., a particular component of a machine or structure. An engineer
must be in a position to choose the optimum combination of properties in a material at the
lowest possible cost without compromising the quality. The properties and behavior of a
material depends upon the several factors, e.g., composition, crystal structure, conditions
during service and the interaction among them. The performance of materials may be
found satisfactory within certain limitations or conditions. However, beyond these
conditions, the performance of materials may not be found satisfactory.
Table 2: Factors affecting the material selection
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Chapter 2
Nano Materials
2.1. Introduction
Nanomaterialsis a term that includes all nano sized materials, including
engineered nanoparticles, incidental nanoparticles and other nano-objects, like those that
exist in nature.
The European Commission adopted the definition of nano materials as given below.
A natural, incidental or manufactured material containing particles, in an unbound state
or as an aggregate or as an agglomerate and where, for 50% or more of the particles in
the number size distribution, one or more external dimensions is in the size range 1 nm
100 nm. In specific cases and where warranted by concerns for the environment, health,
safety or competitiveness the number size distribution threshold of 50% may be replaced
by a threshold between 1 and 50%. The European Commission adopted the above
definition of a nano material.
Nanomaterials are a field that takes materials science-based approach on nanotechnology.
It studies materials with morphological features on the nanoscale, and especially those
that have special properties stemming from their nanoscale dimensions. Nanoscale isusually defined as smaller than a one tenth of a micrometer in at least one dimension [1],
though sometimes includes up to a micrometer. An important aspect of nanotechnology is
the vastly increased ratio of surface area to volume present in many nanoscale materials,
which makes possible new quantum mechanical effects. One example is the
quantum size effect where the electronic properties of solids are altered with great
reductions in particle size. Nanoparticles, for example, take advantage of their
dramatically increased surface area to volume ratio. Their optical properties,
e.g. fluorescence, become a function of the particle diameter. This effect does not come
into play by going from macro to micro dimensions. However, it becomes pronounced
when the nanometer size range is reached.
A certain number of physical properties also alter with the change from macroscopic to
nanoscopic system. Novel mechanical properties of nanomaterials are a subject of nano
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mechanics research. When brought into a bulk material, nanoparticles can strongly
influence the mechanical properties of the material, like stiffness or elasticity. For
example, traditional polymers can be reinforced by nanoparticles resulting in novel
materials which can be used as lightweight replacements for metals. Such nano
technologically enhanced materials may enable a weight reduction accompanied by an
increase in stability and improved functionality. Catalytic activities also reveal new
behavior in the interaction with biomaterials.
2.2. Improved Properties of Nanomaterials over Bulk Materials
Nanomaterials offer better properties over their bulk counterparts. It has been observed
that changes in particle properties can be observed when particle size is less than a
particular level, called the critical size. Table presents the feature sizes for significant
changes in properties in nanomaterials.Table 3: Sizes for changes in properties in nano composites
Additionally, as dimensions reach the nanometer level, interactions at phase interfaces
become largely improved, and this is important to enhance material properties. In this
context, the surface area / volume ratio of reinforcement materials employed in the
preparation of nanomaterials is crucial to understand their structure-property
relationships. One of the major factors which alter the properties in nanomaterials is the
increase in ratio of surface area to volume. The surface area of particle increases
exponentially, creating more sites for bonding, catalysis or reaction with surroundingmaterial, resulting in improved properties such as increased strength or chemical or heat
resistance. Hence due to the high surface to volume ratio associated with nanometer sized
particles, it is possible to control the fundamental properties of materials through
surface/size effect.
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2.3. Characterization of nanomaterials
Materials in the nanometer scale, such as colloidal dispersions and thin films, have been
studied over many years and many physical properties related to the nanometer size, such
as coloration of gold nanoparticles, have been known for centuries. One of the critical
challenges faced currently by researchers in the nanotechnology and nano science fields
is the inability and the lack of instruments to observe, measure and manipulate the
materials at the nanometer. In the past, the studies have been focused mainly on the
collective behaviors and properties of a large number of nano structured materials. A
better fundamental understanding and various potential applications increasingly demand
the ability and instrumentation to observe measure and manipulate the individual
nanomaterials and nanostructures.
Characterization and manipulation of individual nanostructures require not only extremesensitivity and accuracy, but also atomic-level resolution. It therefore leads to various
microcopies that will play a central role in characterization and measurements of nano
structured materials and nanostructures. The development of novel tools and instruments
is one of the greatest challenges in nanotechnology.
In this chapter, various structural characterization methods that are most widely used in
characterizing nanomaterials and nano structures are mentioned. These include: X-ray
diffraction (XRD) [2] [3], various electron microscopy (EM) including scanning electron
microscopy (SEM) and transmission microscopy (TEM) [4] [5] [6] [7], and scanning probe
microscopy (SPM) [8].
2.3.1. Structural CharacterizationCharacterization of nanomaterials and nanostructures has been largely based on the
surface analysis techniques and conventional characterization methods developed for
bulk materials. For example, XRD has been widely used for the determination of
crystallinity, crystal structures and lattice constants of nanoparticles, nano wires and thin
films; SEM and TEM together with electron diffraction have been commonly used in
characterization of nanoparticles; optical spectroscopy is used to determine the size of
semiconductor quantum dots. SPM is a relatively new characterization technique and has
found wide spread applications in nanotechnology. The two major members of the SPM
family are scanning tunneling microscopy (STM) and atomic force microscopy (AFM).
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Although both STM and AFM are true surface image techniques that can produce
topographic images of a surface with atomic resolution in all three dimensions,
combining with appropriately designed attachments, the STM and AFM have found a
much broadened range of applications, such as nano indentation, nanolithography (as
discussed in the previous chapter), and patterned self-assembly. In the following, we
mentioned characterization techniques in nanotechnology.
1. X-ray diffraction (XRD)
2. Scanning electron microscopy (SEM)
3. Transmission electron microscopy (TEM)
4. Scanning probe microscopy (SPM)
5. Gas adsorption
2.3.2. Chemical CharacterizationChemical characterization is to determine the surface and interior atoms and compounds
as well as their spatial distributions. As mentioned in the introduction section, many
chemical analysis methods have been developed for the surface analysis or thin films, but
are readily applicable to the characterization of nanostructures and nanomaterials.
2.3.2.1.Optical Spectroscopy
1. Absorption and transmission spectroscopy
2.
Photoluminescence
3. Infrared spectroscopy
4. Raman spectroscopy
2.3.2.2.Electron spectroscopy
1. Energy dispersive X-ray spectroscopy
2. Auger electron spectroscopy
3. X-ray photo electron spectroscopy
2.3.2.3.Ionic spectroscopy1. Rutherford back scattering spectroscopy2. Secondary ion mass spectroscopy
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2.4.Properties of nano materials
Between the dimensions on an atomic scale and the normal dimensions, which
characterize bulk material is a size range where condensed matter exhibits some
remarkable specific properties that may be significantly different from the physical
properties of bulk materials. Some such peculiar properties are known, but there may be a
lot more to be discovered. Some known physical properties of nanomaterials are related
to different origins: for example, (i) large fraction of surface atoms, (ii) large surface
energy, (iii) spatial confinement, and (iv) reduced imperfections. The following are just a
few examples:
(1)Nano materials may have a significantly lower melting point or phase transition
temperature and appreciably reduced lattice constants, due to a huge fraction of
surface atoms in the total amount of atoms.(2)Mechanical properties of nanomaterials may reach the theoretical strength, which are
one or two orders of magnitude higher than that of single crystals in the bulk form.
The enhancement in mechanical strength is simply due to the reduced probability of
defects.
(3)Optical properties of nanomaterials can be significantly different from bulk crystals.
For example, the optical absorption peak of a semiconductor nano particle shifts to a
short wavelength, due to an increased band gap. The color of metallic nanoparticles
may change with their sizes due to surface Plasmon resonance.
(4)Electrical conductivity decreases with a reduced dimension due to increased surface
scattering. However, electrical conductivity of nano materials could also be enhanced
appreciably, due to the better ordering in microstructure, e.g. in polymeric fibrils.
(5)Magnetic properties of nano structured materials are distinctly different from that of
bulk materials. Ferromagnetism of bulk materials disappears and transfers to super
paramagnetism in the nanometer scale due to the huge surface energy.
(6)
Self-purification is an intrinsic thermodynamic property of nanostructures and
nanomaterials. Any heat treatment increases the diffusion of impurities, intrinsic
structural defects and dislocations, and one can easily push them to the nearby
surface. Increased perfection would have appreciable impact on the chemical and
physical properties. For example, chemical stability would be enhanced.
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2.5.Applications of Nanomaterials
1. Nano electronics
2. Nano medicines
3. Biological applications of nano particles
4. Catalysis by Gold particles
5. Band gap engineered quantum devices
6. Quantum well and quantum dot devices
7. Carbon nano tubes emitter
8. Photo-electro chemical cells
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Chapter 3
Nano Magnetism
3.1. Introduction
Nanomagnetism describes the science and technology underlying the magnetic behavior
of nano structured (1100 nm) systems. It focuses on the magnetic behavior of individual
building blocks of nano structured systems as well as on combinations of individual
building blocks that display collective magnetic phenomena. A fundamental
understanding of nanomagnetism can be exploited to yield integrated systems with
complex structures and architectures that possess new functionalities. A nanomagnet is a
sub micro metric system that presents spontaneous magnetic order (magnetization) atzero applied magnetic fields (remanence). The small size of nano magnets prevents the
formation of magnetic domains.
The magnetization dynamics of sufficiently small nano magnets at low temperatures,
typically single-molecule magnets, presents quantum phenomena, such as macroscopic
spin tunneling. At larger temperatures, the magnetization undergoes random thermal
fluctuations (super paramagnetism) which present a limit for the use of nano magnets for
permanent information storage.
Canonical examples of nano magnets are grains [9] [10]of ferromagnetic metals
(iron, cobalt, and nickel) and single-molecule magnets. [11]The vast majority of nano
magnets feature transition metal (titanium, vanadium, chromium, manganese, iron, cobalt
or nickel) or rare earth (Gd, Eu, and Er) magnetic atoms.
3.2.Challenges in nano magnetism
The grand challenges in nanomagnetism can be expressed in a number of ways
depending on the audience.[12] At the most basic level the challenge is to create, explore
and understand new materials that exhibit unexpected collective behavior, also known as
emergent behavior. The creation of emergent matter energizes the synthesis community,
and high-quality synthesis of new materials is critical to the future of all of condensed
matter and materials physics. Nano science offers exciting new routes to create advanced
materials and hierarchically assemble systems in a non-Edisonian manner. Utilizing the
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three basic tools of nano science - geometric confinement, physical proximity and self-
organization - one can create materials by design. Confinement creates new properties
from known materials via manipulation of dimensionality. Proximity effects allow multi-
component composites to behave as new materials that embrace properties that are often
mutually exclusive and thus not found in single component systems. Self-organization
utilizes rules of Nature to segregate and/or assemble systems on ultra small length scales
that transcend the limits of present-day Litho graphic definition. Self-assembly shows
promise of becoming a powerful approach to miniaturize structures in an affordable
manner with very high uniformity. A challenge is to utilize self-organization, as is
accomplished in biological systems, to create not just component materials, but fully
functional, integrated systems. One can imagine pulling an entire computer processor out
of a test tube in the future. Achieving such goals certainly encompasses non-equilibriumsynthetic routes, and the creation of multifunctional materials. Competing interactions
and the preponderance of low-lying energetic states and quantum fluctuations help create
the complexity that gives rise to unanticipated phenomena in magnetic nano systems. We
note that, during the exploration of magnetic surfaces, interfaces and multi layers
undertaken a few decades ago, the discoveries of spin-valve read heads or magnetic
random access memory was completely unanticipated. This realization underscores the
fact that fundamental studies involving the creation of emergent matter is an important
stimulant of advanced technologies. The second grand challenge in nanomagnetism,
beyond the creation of emergent matter, is the exploration of its magnetic properties. This
task requires all levels of characterization tools. Essential in this is the utilization of the
advanced user facilities that are available to the research community. This underscores an
important synergy between materials creation and exploration that requires national
investments both in synthesis and in major user facilities. Additionally, understanding
emergent behavior requires the talents of the theory community and the utilization of
high-performance computing to simulate complex behavior. This challenge has many
facets. It includes: (i) defining the rules that govern self-organization; (ii) understanding
spin dynamics and equilibrium states at the spatial and temporal limits of interest; (iii)
understanding spin transport phenomena, spin-torque effects, spin accumulation, spin
diffusion, etc.; and (iv) relating such effects to the detailed structure of the material. This
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challenge also involves understanding how to create entangled spin systems, and how to
control the process as well as read the states. A longer-term aspect of the challenge is to
foster communication of information via spin currents and excitations without the flow of
charge. This would be a novel approach to complement existing electronic circuitry that
depends on charge flow.
Why create, explore and understand nano magnetic emergent matter? An important
reason is that the more detailed understanding which is the goal of basic science will
enable the community to address the strategic needs of our society at large. It is vital to
stimulate the economy and create new jobs by ushering in new technologies and
innovations. New diagnostic tools and treatments for diseases will preserve health and
wellbeing, as well as heal the ill and help the impaired. New sensors will help ensure
homeland security, and advanced materials and approaches will provide for nationaldefense needs. It is important to create materials and harness phenomena that will
advance transportation requirements, and it is imperative to take leadership in meeting
national goals of energy independence. Additionally, all of these initiatives must be
advanced in a manner that is respectful of the environment. These are goals for the
nanomagnetism community, for the nano science and nanotechnology communities, and
for the larger materials research communities. The nanomagnetism community is in a
particularly advantageous position to rise to these challenges because of the enormous
potential derived from recent breakthroughs. Although magnetism is one of the oldest
sciences known, it is a driving force in the new scientific era of nanotechnology. This
statement is highlighted quite succinctly by examining the evolution of the Giant
Magneto Resistance (GMR) effect, which made the transition from a laboratory curiosity
to a technology driver within the short span of about ten years. From GMR, a host of new
phenomena, materials, and potential technologies are blossoming. [13]
3.3.Dimensionality in nano magnetismDimensionality is a general term which is used to describe the effect of size confinement
on the physical properties of matter. It becomes very important, when the properties of
matter are discussed on the nanometer scale. In general, one divides nanostructures into
three different groups including: thin films (2D structures), nano wires (1D structure),
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and nano dots or nanoparticles (0D structures). The magnetization of these different
groups is significantly influenced by confinement.
3.3.1. Thin magnetic filmsThe electronic structure at the surface of a given material differs considerably from the
bulk. Early studies of nickel mono layers on copper substrates show significant changes
in the density of states as a function of surface energy. The different coordination number
of atoms and the number of incomplete bonds at the surface modifies the local density of
states. The d orbital is more localized and interacts more strongly with spin magnetic
moments giving a higher net magnetization at the surface. This is an example of size
effect in one dimension. The magnetization reversal in thin ferromagnetic films is
affected by such size confinement. The nucleation and propagation of magnetic domains
which involve Bloch walls in the bulk is more likely to involve Nel walls when thethickness decreases. Thickness confirms the tendency of the formation of Nel walls in
the thinner film. In thin films with uniaxial in-plane anisotropy, magnetic domains form
stripes and are separated by Nel walls for thicknesses below 50 nm. In thin films with
perpendicular anisotropy, circular magnetic domains form which are highly mobile.
These magnetic bubbles hold potential for magnetic storage technology.[14]
3.3.2. Nano wires or one dimensional magnetsFundamental interest in arrays of ferromagnetic nano wires lies in the emergence of novel
magnetic and transport properties as the dimension approaches the length scale of a few
nano meters to a few tens of Nano meters. Current interest in research on ferromagnetic
nano wires is stimulated by the potential application to future ultra-high-density magnetic
recording media.[15]The controlled production of magnetic nano wire arrays with
outstanding characteristics is important to control the magnetization process. Free
standing nanowires can be fabricated by different methods such as lithography and
template electrodeposition [16] [17] [18] [19] [20]. The smaller diameter wires have high out-of-
plane remanence while larger diameter wires have low remanence. This is consistent with
the predictions of a micro magnetic model, which indicates a change from a flower (or
single domain) to a vortex remanent state with increasing diameter. The flowervortex
transition occurs at a diameter of 3.5 times of exchange length for cylinders without any
magneto crystalline anisotropy [21]. However, it is important to note that the remanence
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only decreases slowly as the vortex develops, and high aspect ratio vortex state particles
can still have significant remanence.
3.3.3. Nanodots and superparamagnetismCompetition between exchange and anisotropy constants [22] induces a critical size of
nanodots. The state of lowest free energy of a ferromagnetic particle is one of uniform
magnetization below a certain critical size. For larger particles, the magnetization is non-
uniform. These two modes are known to correspond to single domain and vortex states
which have been studied extensively over last decade [23] [24]. Arrays of magnetic
nanodots have been fabricated, the magnetization curves obtained for the arrays of
circular dots exhibiting the two reversal modes. The effect of diameter and thickness is
quite important. As the size approaches the critical radius, the single domain mode is
predominant [25]. Nanodots smaller than the critical radius, tend to give up ferromagneticalignment, becoming superparamagnetic.
Figure: 1 M vs. H curve for superparamagnetism
This behavior takes place when the fluctuations of magnetic moments caused by the
thermal energy distort the intrinsic alignment of the ferromagnet. Considering a single
domain particle with two distinct magnetic states, when the thermal energy becomes
comparable with the energy barrier between the two states, superparamagnetism is
established.
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3.4.Magnetic anisotropy
The term magnetic anisotropy is used to describe the dependence of the internal energy
on the direction of the spontaneous magnetization, creating easy and hard directions of
magnetization. The total magnetization of a system will prefer to lie along the easy axis.
The energetic difference between the easy and hard axis results from two microscopic
interactions: the spin-orbit interaction and the long-range dipolar coupling of magnetic
moments. The anisotropy energy arises from the spin-orbit interaction and the partial
quenching of the angular momentum. The spin-orbit coupling is responsible for the
intrinsic (magneto crystalline) anisotropy, surface anisotropy, and magneto striction,
while the shape anisotropy is a dipolar contribution and is calculated e.g. by assuming a
uniform distribution of magnetic poles on plane surfaces. Anisotropy energies are usually
in the range 102-107 Jm-3. This corresponds to energy per atom in the range 10-8-10-3eV. The anisotropy energy is larger in lattices (of magnetic ions) of low symmetry and
smaller in lattices of high symmetry. In bulk materials, magneto crystalline and magneto
static energies are the main source of anisotropy whereas in fine particles, thin films and
nano structures, other kinds of anisotropies such as shape and surface anisotropy are
relevant in addition to these usual anisotropies. In the following we will discuss four
different contributions to magnetic anisotropy: magneto crystalline anisotropy, shape
anisotropy, strain anisotropy and surface anisotropy.
3.4.1. Magneto crystalline anisotropyMagnetic anisotropy is meant as the dependence of the internal energy on the direction of
spontaneous magnetization. An energy term of this kind is called as magnetic anisotropy
energy. Generally the magnetic anisotropy energy term possesses the crystal symmetry of
the material, and known as crystal magnetic anisotropy or magneto crystalline anisotropy[26]. The simplest forms of crystal anisotropies are the uniaxial anisotropy in the case of a
hexagonal and the cubic anisotropy in the case of a cubic crystal. For example, hexagonal
cobalt exhibits uniaxial anisotropy, which makes the stable direction of internal
magnetization (or easy direction) parallel to the c axis of the crystal at room temperature.
3.5. Single domain
In magnetism, it refers to the state of a ferromagnet [27] in which the magnetization does
not vary across the magnet. A magnetic particle that stays in a single domain state for all
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magnetic fields is called a single domain particle [28]. They are very important in a lot of
applications because they have a high coercivity. They are the main source of hardness in
hard magnets, the carriers of magnetic memory in tape drives, and the best recorders of
the ancient Earth's magnetic field.
3.5.1. History & Definitions of a single-domain particleEarly theories of magnetization in ferromagnets assumed that ferromagnets are divided
into magnetic domains and that the magnetization changed by the movement of domain
walls. However, as early as 1930, Frenkel and Dorfman predicted that sufficiently small
particles could only hold one domain, although they greatly overestimated the upper size
limit for such particles [29]. The possibility of single domain particles received little
attention until two developments in the late 1940s:
(1)
Improved calculations of the upper size limit by Kittel and Nel, and(2)A calculation of the magnetization curves for systems of single-domain particles by
Stoner and Wohlfarth [30] [31].
Early investigators pointed out that a single-domain particle could be defined in more
than one way [32]. Perhaps most commonly, it is implicitly defined as a particle that is in a
single-domain state throughout the hysteresis cycle, including during the transition
between two such states. However, it might be in a single-domain state except during
reversal. Often particles are considered single-domain if their saturation remanence is
consistent with the single-domain state. More recently, it was realized that a particle's
state could be single-domain for some range of magnetic fields and then change
continuously into a non-uniform state [33]. Another common definition of single-domain
particle is one in which the single-domain state has the lowest energy of all possible
states.
3.5.2. Limits on the single-domain sizeExperimentally, it is observed that though the magnitude of the magnetization is uniform
throughout a homogeneous specimen at uniform temperature, the direction of the
magnetization is in general not uniform, but varies from one region to another, on a scale
corresponding to visual observations with a microscope. Uniformity of direction is
attained only by applying a field, or by choosing as a specimen, a body which is itself of
microscopic dimensions (a fine particle). The size range for which a ferromagnet become
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single-domain is generally quite narrow and first quantitative results in this direction are
due to William Fuller Brown, Jr. who, in his fundamental paper [34]rigorously proved (in
the framework of Micro magnetic. This paper states the existence of a critical radius such
that the state of lowest free energy is one of uniform magnetization if the existence of a
critical size under which spherical ferromagnetic particles stay uniformly magnetized in
zero applied field.
Although pure single-domain particles (mathematically) exist for some special geometry
only, for most ferromagnets a state of quasi-uniformity of magnetization is achieved
when the diameter of the particle is in between about atomic meters and nanometers. The
size range is bounded below by the transition to super paramagnetism and above by the
formation of multiple magnetic domains.
1) Lower limit: super paramagnetismThermal fluctuations cause the magnetization to dance around in a random manner. In the
single domain state, the moment rarely strays far from the local stable state. Energy
barriers prevent the magnetization from jumping from one state to another. However, if
the energy barrier gets small enough, the moment can jump from state to state frequently
enough to make the particle super paramagnetic. The frequency of jumps has a strong
exponential dependence on the energy barrier, and the energy barrier is proportional to
the volume, so there is a critical volume at which the transition occurs. This volume can
be thought of as the volume at which the blocking temperature is at room temperature.
2) Upper limit: transition to multiple domainsAs size of a ferromagnet increases, the single-domain state incurs an increasing energy
cost because of the demagnetizing field. This field tends to rotate the magnetization in a
way that reduces the total moment of the magnet, and in larger magnets the
magnetization is organized in magnetic domains. The demagnetizing energy is balanced
by the energy of the exchange interaction, which tends to keep spins aligned. There is a
critical size at which the balance tips in favor of the demagnetizing field and the multi
domain state is favored. Most calculations of the upper size limit for the single-domain
state identify it with this critical size [35] [36] [37].
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3.5.3. Domain formationNext is to focus on how domain formation occurs in an initially saturated specimen. In
general this process constitutes a very considerable resistance to the process of
demagnetization in many specimens. Saturation is expected in a magnetic specimen when
the demagnetizing fields can be overcome in certain magnetic fields. However, the real
demagnetizing fields are non-uniform over the volume of the specimen. Usually the end
regions are much more difficult to saturate than the bulk of the crystal, and residual
domains persist near the ends until external magnetic fields can completely make
saturation [38]. The demagnetizing effect of the end surfaces can be eliminated assuming a
ring-shaped specimen; however residual domains are still expected to be stabilized by
pores, inclusions and grain boundaries. Considerable demagnetizing fields can arise from
grain boundaries and this requires higher fields which are considerably higher than bulk
saturation. A critical field designated as Hn (nucleation fields) may be needed some time
to start nucleation of domains. However it is quite possible that critical fields may
represent the initiation of wall motion rather than the nucleation of the walls, and in this
case they may be designated as starting fields (Hs).
3.6.Domain walls
Domain walls are interfaces between regions in which the spontaneous magnetization has
different directions. At or within the wall the magnetization must change direction. A
simplistic picture of a domain wall which makes an abrupt change between two domains.
For this ferromagnetic specimen the easy axis is y and a row of atoms is shown parallel
to x-axis, with the 180 domain wall lying in the y-z plane. In this case the domain wall
will have a large exchange energy associated with it because the spins adjacent to the
wall are anti-parallel and the exchange energy in a ferromagnet is a minimum only when
adjacent spins are parallel.
3.7.Applications3.7.1. Applications in R&DMagnetic nano-particle applications are being investigated in multiple disciplines from
biomedical sensors, drug delivery, magnetic resonance imaging, data storage, nano-
electronics, etc. Two applications, which are being heavily investigated by several
research groups, are: detoxification of contaminated personnel for the military and drug
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delivery and treatment. Current techniques in both of these fields are very limited and
minimally robust and nano-technology has the potential to enhance the capabilities in
both fields immensely.
3.7.2. Detoxification of Contaminated Personnel MilitaryBiochemical warfare is steadily becoming a greater threat worldwide. In the case of a
conflict where a soldier is exposed to toxins there are very few options for survival.
Current medical detoxification treatment methods available are dialysis and blood
purification, both of which have low rates of success and take extensive periods of time
to undergo. Magnetic nano-particles are being investigated for their capability to bind
with toxins within the body. The various synthesis techniques including the thermal
decomposition of metal-carbonyl complexes have shown that magnetic nano-particles are
stabilized by an organic surfactant which acts as a capping layer. If a chemically activecapping layer, which will functionally bind to the toxins within the body, can be utilized,
then magnetic nano-particles can be sent into the bloodstream of a contaminated person.
Then, by utilizing a magnetic field gradient, toxins can be extracted from the body. In
research this methodology has been labeled tag and drag and is depicted in Figure.
Figure 2: Tag and Drag Detoxification of Contaminated Personnel via Magnetic Nano-particles.
Current research has shown success in this tag and drag approach using gold coated iron,
nickel, and cobalt ferromagnetic nano-particles. The next phase of research is identifying
pre-dominant toxins, functional organic capping layers, and bio-compatibility.
3.7.3. Drug Delivery and EfficacyDrug delivery and effective application of drugs within the body is another heavily
investigated application for magnetic nano-particles. Researchers are particularly
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investigating ferromagnetic nano-particles with respect to treatment of various cancers.
The goal is to have medicines functionally bind to magnetic nano-particles and utilize a
magnetic gradient to guide the nano-particle to the affected region. Once at the affected
region, the hysteretic behavior of ferromagnetic particles is utilized to provide thermal
activation. Hysteresis occurs in ferromagnets because when an external field is applied
and removed, a portion of the field is absorbed by the ferromagnet, called the remnant
magnetization. When a reverse field is applied, this remnant magnetization has to be
overcome. This effectively yields an energy loss in the form of heat which is
proportional to the area within the hysteresis loop. Researchers are utilizing this thermal
energy for localized heating within the body. Once the medicine has been guided via a
magnetic gradient to a desired location, the magnetic field is varied to create hysteresis.
This in effect causes localized heating that helps in the activation and completion of achemical reaction between the applied medicine and the adversely affected area. Current
research is attempting to develop magnetic nano-particles with large magnetic moments
and resistance to physical breakdown within the body.
3.7.4. Information storageResearch is going into the use of using MNPs for magnetic recording media. The most
promising candidates for high-density storage are the face-centered tetragonal phase Fe
Pt alloy. Grain sizes can be as small as 3 nanometers. If it is possible to modify the MNPs
at this small scale, the information density that can be achieved with this media could
easily surpass 1 Terabyte per square inch.
3.8.Future Research and Development
For magnetic nano-particles, current and future research is geared towards size
distribution control. For most applications it is critical to have a specific size as well as
very tight distribution. Current research is pushing to synthesize super-lattices of
magnetic nano-particles which show size distribution of less than 5%. For theapplications discussed in this paper, future research is being geared towards discovering
biocompatible capping layers. This is essential in order to utilize magnetic nano-
particles in vivo.
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3.9. Bismuth ferrite
3.9.1. Introduction toBiFeO3BiFeO3 (BFO) is an inorganic chemical compound with aperovskite structure. It is one
of the most promising lead-free piezoelectric materials by exhibiting multi ferroic
properties at room temperature. Multiferroic materials exhibit ferroelectric or anti
ferroelectric properties in combination with ferromagnetic (or antiferromagnetic)
properties in the same phase. BiFeO3 is the only prototype among all other multi ferroic
oxides which shows both ferromagnetism and ferro electricity in a single crystal above
room temperature. It has ferroelectric Curie temperature Tc = 1143K and
antiferromagnetic Nel temperature TN= 643K. The ions responsible for the production of
ferro electricity and magnetism are Bi3+ and Fe+3 ions. Ferro electricity is produced due to
Bi3+ and anti ferromagnetism is due to Fe+3 ions [39].
3.9.2. Crystal structure of BiFeO3 It is having rhombohedrally distorted perovskite structure with R3c space group at
room temperature.
Figure 3: BiFeO3 in Perovskite structure
Bi3+ ion occupy the corner position, Fe3+ in the body centered position, and O2- in all
face centered position. The lattice parameters are a = 5.587 , b = 5.587 and c =
13.867 with = = 900and =1200. The hexagonal unit cell contains 6 formulas.
Generally ferroelectricity is produced by vacant d0 orbital and ferromagnetism is
observed due to partially filled dn orbital. In BiFeO3, ferroelectricity is produced due to
stereo chemical activity of Bi3+ ion and ferromagnetism is due to Fe+3ions[39].
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Production of ferroelectricity in bismuth ferrite
Bi3+ and Fe3+cations displaced along the [1 1 1] threefold polar axis and off centered with
respect to the body centre of the oxygen octahedron, which in turn gives rise to
ferroelectricity.
Production of ferromagnetism in bismuth ferrite
It is produced due to rotations of adjacent oxygen FeO6 octahedral around the [1 1 1]
pseudo cubic direction (figure 3).
3.9.3. Applications of BiFeO31. Due to multiferroic nature of BiFeO3 it has broader applications in the field of
transducers, magnetic field sensors and information storage industry.
2. Due to its magneto electric coupling it has the advantage that data can be written
electrically and read magnetically. It exploits the best aspects of ferroelectricrandom access memory (Fe - RAM) and magnetic data storage. Multiferroism
leads to fast, low-power consumption, multifunctional memory devices exploiting
the best attributes of conventional ferroelectric and magnetic random-access
memories.
3. Recently, ferroelectric random access memories (Fe RAMs) have achieved fast
access speeds (5 ns), high densities (64 Mb)[39].
3.10. SummaryOngoing research has shown that synthesis of magnetic nano-particles is achievable. The
applications for magnetic nano-particles are very diverse. In order to make use of the
potential of magnetic nano-particles, work in synthesis refinement is necessary, to control
geometric and magnetic properties. From an application aspect, research is being geared
to find functional surfactants which serve to meet application requirements.
Biocompatibility in the case of drug deliver and detoxification applications and
conductivity in the case of electronic applications are examples of such. As researchevolves and solutions to current limitations are resolved, the integration of magnetic
nano-particles into everyday use becomes a clearer reality.
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