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Thermal stability of L10-FePt nanodots patterned by self-assembledblock copolymer lithographyTo cite this article: Eduardo Fernandez et al 2018 Nanotechnology 29 465301
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Thermal stability of L10-FePt nanodotspatterned by self-assembled blockcopolymer lithography
Eduardo Fernandez1,2, Kun-Hua Tu1, Pin Ho1,3 and Caroline A Ross1
1Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77Massachusetts Ave, Cambridge, MA 02139, United States of America2 BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park,48940 Leioa, Spain
E-mail: [email protected]
Received 6 July 2018, revised 17 August 2018Accepted for publication 31 August 2018Published 21 September 2018
AbstractArrays of 14 nm thick L10-FePt nanodots with diameter of 27 nm and center-to-center spacing of39 nm were produced by block copolymer patterning of an FePt film and their magnetic reversaland thermal stability were characterized. A self-assembled polystyrene-b-polydimethylsiloxanediblock copolymer film was used as a lithographic mask and a pattern transfer process based onion beam etching and rapid thermal annealing of the sputtered FePt film was developed. The dotarrays exhibited perpendicular magnetic anisotropy with K=4.8×107 erg cm−3 and asaturation magnetization of 960 emu cm−3. First order reversal curves indicate a softer magneticcomponent attributed to the ion-milled material at the edges of the dots. The switching volumeand the thermal stability were obtained from relaxation measurements and DC demagnetizationcurves. Micromagnetic simulations reproduce the magnetic domain structure obtained for thecontinuous and patterned FePt thin film.
Keywords: magnetic switching volume, L10 structure, block copolymer lithography, PS-b-PDMS, pattern transfer
(Some figures may appear in colour only in the online journal)
1. Introduction
The thermal stability of magnetic nanostructures is anessential characteristic of magnetic memory, biologicallabeling, hyperthermia treatments and other applications[1–4]. Thermal stability is related to the probability thatthermal fluctuations can induce spontaneous reversal of themagnetization, which occurs when the thermal energy kBT isa large enough fraction of the energy barrier for reversal [5].Raising the temperature facilitates reversal, both by increasingthermal energy and by reducing the energy barrier. Forexample in heat assisted magnetic recording [6, 7], local
heating of a region of a high anisotropy magnetic recordingmedium lowers the energy barrier to the point that the field ofthe write head is capable of writing data on the medium. Thisenables the use of hard magnetic materials such as CoCrPt,FePt, CoPt, or FePd [5–11] for recording media which canhave coercive fields over μ0Hc=3 T after patterning, abovethe maximum field of ∼2.4 T that a write head can apply.
Thermal stability may be expressed by a factorΔ=KV*/kT, where K is the magnetic anisotropy, V* is theswitching volume [5, 12], and kT is thermal energy. V*
represents the volume of magnetic material involved in thereversal process. For a uniaxial particle undergoing coherentrotation, V*=V, where V is the physical volume of theparticle, but incoherent reversal leads to V*<V. In systemswhere particles are magnetically coupled, V* may exceed V asmultiple particles switch together. A criterion for stability is
Nanotechnology
Nanotechnology 29 (2018) 465301 (9pp) https://doi.org/10.1088/1361-6528/aade2f
3 Present address: Institute of Materials Research and Engineering, Agencyfor Science, Technology and Research, 2 Fusionopolis Way, #09-01Kinesis, Singapore 138635.
0957-4484/18/465301+09$33.00 © 2018 IOP Publishing Ltd Printed in the UK1
Δ>40, ensuring that the magnetic nanostructure will maintainits magnetization direction for ∼10 years. There have beennumerous reports on the thermal stability of nanoparticle sys-tems [13], but lithographically patterned 2D arrays of thin filmmagnetic nanostructures provide a particularly well controlledmodel system to examine switching mechanism and thermalstability. 2D arrays are also of interest for bit patterned media,where the magnetization direction of each particle representsone bit of data [14, 15]. This article examines the thermal sta-bility and magnetic reversal of 2D arrays of high anisotropy L10structure FePt thin film dots.
In order to pattern 2D arrays of magnetic nanostructures,several methods have been used including embedded maskpatterning, electron beam lithography, nanoimprint litho-graphy, and block copolymer (BCP) self-assembly [16–18].The latter process, which relies on the microphase separationof a BCP thin film, is a promising strategy for cost-effectivefabrication of nanoscale structures with periodicities below100 nm. The BCP consists of two covalently bonded butimmiscible blocks, and annealing leads to the formation ofe.g. spheres, cylinders or lamellae of one block within thesecond block, driven by the segregation strength, χN, whereχ represents the segment–segment Flory–Huggins interactionparameter and N represents the degree of polymerization [19].To control the long-range order and orientation of the self-assembled nanostructures in BCP thin films several annealingand ordering techniques have been applied including thermalannealing [20, 21], electric fields [22], the use of chemicaland topographical substrate patterns [23–26], shear alignment[27] and solvent vapor annealing [28, 29]. The microphase-separated structure can be used for additive or subtractivelithography processes by typically removing one block andusing the remaining structure as a mask.
Prior work has demonstrated the use of BCP lithographyto pattern arrays of magnetic materials including NiFe,CoCrPt or Co/Pd [30–33], but there has been little work onpatterned arrays of L10 materials. FePt L10 dots were madepreviously using BCP lithography [34] but the structural andmagnetic characteristics were not explored in detail. Werecently described patterned line arrays of FePt made usingBCP lithography [35], but the process was limited by theroughness of the FePt after annealing. Here we use BCPlithography to make arrays of L10 FePt dots and measure thethermal stability from the time-dependent coercivity and theinterparticle interactions via the first order reversal curve(FORC) method. Application of BCP patterning technologyto high anisotropy thin film materials such as FePt willfacilitate the development of high density recording media aswell as an improved understanding of the thermal stability ofhard magnetic nanostructures.
2. Fabrication and characterization methods
The dot arrays are prepared from L10 FePt films in a processshown schematically in figure 1. The 14 nm thick FePt filmswere deposited on MgO (100) substrates using ion beamsputtering with a base pressure of 5×10−8 Torr. The
samples were grown at room temperature leading to a che-mically disordered face centered cubic (fcc) A1 phase. Inorder to produce the ordered face centered tetragonal (fct) L10phase with alternating Fe and Pt atomic planes along the[001] c-axis, the samples were subjected to a rapid thermalanneal (RTA) at 700 °C for 300 s in N2 ambient at differentstages of the fabrication process. The transformation from theA1 to L10 structure was characterized by x-ray ω-2θ dif-fraction measurements (PANalytical X’Pert PRO x-ray dif-fractometer with Cu Kα radiation).
16 nm of carbon was deposited by electron beam eva-poration on the FePt films. This carbon layer is used as amask for the subsequent patterning process via ion beametching. A 30 nm thick film of polystyrene-block-poly-dimethylsiloxane (PS-b-PDMS, purchased from PolymerSource) was spin coated on top of the carbon layer. The BCPhad a molecular weight of 56.1 kg mol−1 with a poly-dispersity of 1.1, and a PDMS volume fraction (fPDMS) of0.161. The BCP wets the carbon, and there was no need tocoat a brush layer of hydroxyl-terminated PS or PDMS toavoid dewetting as has been performed in previous work[36–38]. The BCP film was annealed at room temperature in avapor of pure toluene for 10 min in a closed glass chamberwith a volume of 88 cm3 containing 3 cm3 of liquid toluene.Based on in situ spectral reflectometry measurements (Fil-metrics, Inc. F20-UV 250–1500 nm), the BCP swelled from30 to 52 nm thickness on exposure to the vapor, giving aswelling ratio of 1.7 (figure 2(a)). After the vapor annealingthe sample was heated for 2 min at 60 °C to remove the sol-vent. The BCP films were then reactive-ion etched for 5 s inCF4 to remove the PDMS layer at the air interface, followedby 80 s of O2 plasma to remove the PS block leaving an arrayof oxidized PDMS close packed spherical microdomains. TheO2 plasma process also etches the carbon layer beneath the PScreating the hard mask, consisting of bilayer dots of oxidizedPDMS/carbon on top of the magnetic film [39].
The samples were imaged using a Zeiss Merlin high-resolution scanning electron microscope (SEM) at 2 kV, andImageJ software [40] was used to obtain the size and dis-tribution of the dots. The oxidized PDMS/carbon dots had adiameter of 27 nm and center-to-center distance of 39 nm(figure 2(b)). The dots had a narrow distribution of diameterswith a standard deviation of 1.4 nm, shown in the inset offigure 2(b). The dots occupy 56.5% of the total surface andhave a density of 0.63 Tdot in–2.
The sample was then etched using an Ar+ ion beam inincrements of 30 s. Since the MgO is non-conductive, mea-surements of the electrical resistance after each ion beametching step indicated the completion of the etching process.An etching time of 2 min led to a resistance increase from0.3 kΩ to 0.8 MΩ. The resulting metal dot array is shown infigure 2(c). Magnetic measurements were made using avibrating sample magnetometer (VSM, ADE model 1660),a SQUID magnetometer (MPMS3-Quantum Design) and atorque magnetometer (ADE model 1660), and surface topo-graphy was measured using a Nanoscope IV atomic forcemicroscope (AFM).
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3. Results and discussion
3.1. Structure and magnetic hysteresis of L10 FePt continuousthin film
The cubic MgO has a lattice parameter of 4.212 Å (figure 3,red line). The as-deposited FePt showed no film peaks in thediffraction scans. However, diffraction peaks from theannealed FePt yielded an out-of-plane (OP) lattice parameterof 3.70 Å (figure 3, black line). This lattice parameter is closeto the bulk lattice parameter c for L10 FePt, which is a P4/mmm fct structure with c=3.713 Å and a=b=3.853 Å.There are no (h00) peaks in the thin film scan correspondingto the a spacing, which is consistent with the film having a c-axis out of plane texture. The high perpendicular magneticanisotropy, discussed below, supports the interpretation of theannealed structure as textured fct rather than a cubic dis-ordered phase.
VSM measurements show that for the unannealed A1phase sample the magnetization is mainly in plane due toshape anisotropy (figure 4(a), red line). The small hysteresisin the hard-axis OP loop may indicate that a fraction of theL10 phase has crystallized. After annealing, the sample shows
a sheared OP hysteresis loop (figure 4(b), black line) withcoercivity 2 kOe. The saturation magnetization (Ms) for theannealed film was 960 emu cm−3, compared to the Ms valueof 1150 emu cm−3 reported for epitaxial L10 FePt films [41].The in-plane (IP) loop also shows hysteresis with a coercive
Figure 1. A schematic of the FePt dot array fabrication process. In the optimum process the FePt rapid thermal anneal is performed afterstep 6.
Figure 2. (a) BCP thickness during the annealing process as function of time; (b) SEM image of the BCP after etching the PS leavingoxidized PDMS spheres (step 5 of figure 1). The inset shows an analysis of the distribution of dot diameters; (c) SEM image of FePt dots afteretching.
Figure 3. X-ray diffraction data from the FePt/MgO after the rapidthermal anneal is shown in black. The film peaks, indicated witharrows, can be indexed as (00l). Diffraction data from the uncoatedMgO substrate is shown in red.
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field of 900 Oe. This may originate from the presence ofmisoriented grains such as (111) textured grains [42], but thiscannot be verified by XRD since the FePt (111) peak overlapswith the MgO (200) peak. However, FORC data (describedbelow in section 3.4) did not show evidence of additionalmagnetic distributions.
To estimate the anisotropy constant K, a magnetic torquemeasurement was performed on the ordered continuous thinfilm, applying a maximum magnetic field of 16 kOe. Thisyielded an anisotropy constant of K=4.8×107 erg cm−3,close to values previously reported [43]. The torque mea-surement provides a lower limit for the anisotropy constant ifthe sample is not fully saturated.
3.2. BCP lithography process
A key issue in patterning the FePt film arises from the filmroughening during the RTA required to form the orderedphase. As shown in the insets of figure 3, an unpatterned filmbefore RTA has a smooth surface with a root mean squareroughness of 0.2 nm (figure 4(a)), but the roughness increasedto 3 nm after it was annealed (figure 4(b)). The surfaceroughness interferes with the self-assembly of the BCP,leading to poorly ordered dot arrays. To attempt to limitroughening, a thin Ta layer was deposited on top of the FePtprior to annealing, but this diffused during RTA and loweredMS of the FePt significantly.
The strategy adopted was therefore to form the BCPpattern consisting of carbon/oxidized PDMS dots on the FePtprior to annealing. In our first attempt the RTA was per-formed between etching the BCP and the ion beam etching(i.e. RTA between steps 5 and 6 of figure 1), but the resultingdot arrays did not exhibit a high perpendicular magnetization.Both the IP and OP hysteresis loops exhibit high coercivitybut the magnetization reached only about 400 emu cm−3,which is lower than expected from the areal coverage of thedots. The low saturation magnetization is attributed toincomplete saturation of the sample at the maximum fieldused in the VSM measurement, i.e. the loops represent minorloops. To obtain dots with a higher PMA, the process wasmodified by performing the RTA after ion beam etching (i.e.RTA after step 6). The optimum process is therefore: FePtdeposition, carbon deposition, BCP deposition and annealing,
reactive ion etching of BCP and carbon, ion beam etching ofFePt to form the dot array then RTA to crystallize FePt. Themagnetic results from this improved process are described inthe next subsection.
3.3. Magnetic hysteresis of L10 FePt patterned film
SQUID magnetometry measurements are shown in figure 5for a dot array sample made by ion beam etching followed byRTA. The measurements were made on a 5 mm diametersection cut from the central area of the original 10 mm squaresample. Based on the dimensions of the dots (diameter of27 nm and center to center distance of 39 nm), the OP curve(figure 5, black line) indicates Ms=930 emu cm−3, similarto that of the annealed continuous thin film and consistentwith formation of the L10 phase. The OP loop exhibits a highremanence and a coercivity of 14.8 kOe, in comparison with 2kOe for the continuous thin film (figure 4(b), black line). Thelarger coercivity of the patterned sample compared with theunpatterned film originates from a different reversal processfor the dots (e.g. single domain behavior in the dots versusdomain wall propagation in the unpatterned film), or a higheranisotropy in the L10 phase arising from the lower in planestress of the dots during the annealing process. The IP loop(figure 5, red line) was not saturated even at 80 kOe.
Figure 4. Magnetic hysteresis loops of an unpatterned sample of FePt/MgO for an in-plane field (red line) and an out-of-plane field (blackline). Left: before rapid thermal annealing, and right: after rapid thermal annealing. The insets show the corresponding AFM images.
Figure 5. Magnetic hysteresis loops of FePt dots annealed after ionbeam etching for an in plane field (red line) and an out of plane field(black line).
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There is a low field step in both OP and IP loops whoseorigin may be one of the following: regions where patterningwas incomplete and the film coercivity is lower; a region withreduced anisotropy at the edges of the dots where the che-mical ordering was degraded; or incomplete conversion ofgrains of the A1 phase to the L10 phase. (A similar step wasobserved for the sample that was annealed before ion beametching.) FORC measurements were used to help distinguishbetween these possibilities. The FORC plots are constructedfrom analysis of the ascending branches of the hysteresisloops and are plotted in terms of the coercivity Hc1 and bias(or interaction) field Hb of the population of hysterons thatconstitute the hysteresis loops.
The FORC measurement of the annealed unpatternedfilm (figure 6(a)) exhibited only one peak, and together with
the x-ray data, argues against the presence of substantialamounts of the A1 phase after annealing. The FORC of thepatterned sample (figures 6(b) and (c)) exhibits two con-tributions, the softer one with Hc1 around 5 kOe, and theharder one with Hc1 around 20 kOe. The dominant feature inthe FORC is a narrow ridge along the Hc1 axis with zero biasHb. This pattern is characteristic of an ensemble of non-interacting single domain particles [44–46]. The lowerintensity peak at 5 kOe shows a non-zero Hb indicating amagnetic interaction with the high coercivity material. Thissuggests it does not originate from regions of incompletepatterning, which would only interact with the dots via weakdipolar interactions, but instead it is attributed to a lowanisotropy region at the boundaries of the dots, which formsas a result of Ar ion damage and implantation [47]. The softermaterial at the boundary is expected to be exchange-coupledto the harder material at the center of the dot, leading to thebias field. In this model, only 1.5 nm thickness of lowanisotropy material at the dot edges would be needed toaccount for the height of the low field step in the hysteresisloops. Preparing dots of different sizes would enable thishypothesis to be validated, since the fraction of low-aniso-tropy surface material is lower for larger dots.
3.4. Magnetic switching volume of L10 FePt patterned film
Assuming coherent reversal of the dots with anisotropyK=4.8×107 erg cm−3, the thermal stability ofΔ=KV*/kTmay be estimated as >9000, assuming V* equals the physicalvolume V of the dots for a coherent reversal process(V=8.0× 10−18 cm3 from the diameter of 27 nm and thick-ness of 14 nm). However, incoherent reversal as well as thepresence of material with lower anisotropy at the edges of thedots will reduce the thermal stability. To obtain a measurementof the switching volume, time-dependent measurements of themagnetization were carried out. Magnetization relaxation dueto irreversible jumps of single domain macrospins is assumedto follow an Arrhenius-type relaxation. The magnetizationdecreases logarithmically with time asM(t)=M(t0) – S ln(t/t0)(figure 7(a)), with the slope defined as the magnetic viscosity,S. S depends on the distribution of the activation energies of themacrospins under a given external magnetic field. In addition,the application of a reverse field (oriented opposite to the localmagnetic moment) induces reversals whose number dependson the field itself and also on the distribution of activationenergies.
By recording the remanent magnetization as a function ofreverse fields of increasing amplitude (the demagnetizationcurve (DCD) figure 7(b), green line), the number of irreversiblejumps per unit field (the irreversible susceptibility χDCD) can beobtained. This parameter contains an explicit dependence on V*,which can be extracted from the ratio χDCD/S, where the valuesof χDCD and S correspond to the reversal field for which bothquantities are maximum [48, 49]. To measure DCD as well asviscosity, the sample is driven to positive saturation prior to theapplication of a reverse magnetic field. In the inset of figure 7(a)we show the magnetic viscosity and on figure 7(b)-blue line theirreversible susceptibility at room temperature, both having
Figure 6. (a) FORC distribution of the unpatterned annealed FePtfilm. (b) FORC distribution of the annealed FePt dots. The axesare the coercivity Hc1 and the bias field or interaction field Hb.(c) a family of first-order reversal curves for the FePt dots.
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maximum values at 20 kOe and as a function of thereverse field.
We obtained a switching volume of 3.7×10−25 m3 (thevolume of a cube with a 7 nm edge) and a thermal stabilityΔ=428. For a data retention time of about 10 years,Δ=40 [50] is required, which would correspond to a(3.7 nm)3 cube, assuming the same anisotropy and neglectinglow-anisotropy material at the edges. BCP lithography couldbe extended to making arrays with period below 10 nm [51]which would be close to the limit of thermally stable FePtdots presented in this work.
3.5. Micromagnetic simulations
Micromagnetic simulations of the domain structure and strayfield of the FePt film and dots were carried out. The micro-magnetic simulations were performed using the finite
difference OOMMF code [52]. The film plane (the xy plane)was modeled using 2D periodic boundaries with a box size of1 μm×1 μm×30 nm, consisting of 10 repeats of the xyplane, to avoid effects from the edges of the film. The meshused had a lateral cell size of 2.5 nm and cell thickness of2 nm. The model used values of MS=960 emu cm−3 andK=4.9×107 erg cm−3 based on our experimental mea-surements, an exchange stiffness of A=1×10−5 erg cm−1
[53], and a damping parameter of 0.1. The FePt layer wasmodeled as a set of grains, each with 20 nm diameter (area of400 nm2), and each of which had a common anisotropy axiswith a random misalignment of 5° with respect to the OPdirection. The FePt was 14 nm thick, and the model included16 nm of non-magnetic material above the film in which thestray field was calculated.
Figure 8(a) shows the OOMMF simulation of the strayfield 16 nm above the surface of the film after initially
Figure 7. (a) Magnetization evolution with time for different reverse fields. The inset shows magnetic viscosity S for different reverse fields.(b) DC demagnetization curve (green line) and the irreversible susceptibility (blue line).
Figure 8. Continuous FePt film. (a) Stray field simulation from OOMMF, (b) MFM image, and (c) a cross section of part of the simulation(black dotted line) showing the magnetization direction in the domains and the structure of the domain walls. The length of the cross-sectionin (c) is 180 nm.
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randomizing the directions of the magnetic moments in thecells then allowing the model to relax. This may be comparedwith figure 8(b) which shows an experimental magnetic forcemicroscope image of an unpatterned annealed film sampleafter ac-demagnetization, taken by a Quadrexed D3100Nanoscope IV AFM with a low moment CoCr magneticprobe at a working distance of 10 nm and a scanning area of1 μm2. The experimental ac-demagnetization was carried outin an OP field of up to ±14 kOe with the field decreasing in10 Oe steps. Both the experiment and simulation show alabyrinth pattern of domains with widths on the order of50 nm. Figure 8(c) shows the domain wall structure, in whichabout 90% of walls are 180° Bloch domain walls with widthof ∼5 nm, which is characteristic of samples with highperpendicular anisotropy. Figure 8(c) also shows a sectionthrough a small bubble domain in which part of the wall hasNéel character. We also simulated a full hysteresis loopincluding the virgin curve using the previous simulation asthe starting condition. The field was increased up to 12 kOethen cycled to −12 kOe to obtain a switching field of about
3.8 kOe for the continuous FePt film, compared to theexperimental value of 2 kOe from figure 3(b).
The FePt dots were modeled using the same parametersfor K, Ms, exchange and damping as the unpatterned film,but taking a 1 μm×1 μm SEM image of the dot array(figure 9(a)) as a mask to define the FePt dots in the model.Each dot had a 5° deviation of its anisotropy axis assignedrandomly along an IP direction. The different directions areshown in color in figure 9(b). A hysteresis loop was obtainedfor an out of plane field of up to 120 kOe, figure 9(c). Thisyielded a switching field of 60 kOe which is four times largerthan the measured value of 14.8 kOe. The high value foundfor the model may be a result of not including the lowanisotropy material at the edges of the dots, but the cell size inOOMMF was too large to allow the thin layer at the edges tobe included. On cycling through the hysteresis loop, mostdots switched at approximately the same field but there was astep in the loop representing a state in which a few of the dotswere stabilized in the unreversed state. This configuration isshown in figure 9(d) where the reversed dots are blue andunreversed dots are red. The model suggests that a coherent
Figure 9. (a) SEM image of FePt dots, (b) and bitmap image used for the OOMMF simulation that assigns a different 5° misorientation ofthe easy axis of each dot with respect to the out-of-plane direction. The easy axis directions are shown in different colors as depicted inthe inset: the black dots have an OP easy axis and the colored dots have easy axes tilted 5° along x (blue), y (green), –x (yellow) or –y (red).(c) Modeled hysteresis loop including the virgin curve calculated from (b); (d) dot magnetization along the z direction at the point in thehysteresis loop indicated in (c). Blue dots have reversed; red have not.
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reversal process occurs, and domain walls were not presentwithin the dots during reversal.
4. Conclusions
The magnetic behavior of 14 nm thick L10 FePt films and dotarrays was investigated. Annealing the as-grown FePt filmproduced the ordered fct L10 phase. Close packed dot arrayswere made using BCP lithography with a carbon hard mask.In the optimum BCP lithography process, the RTA was car-ried out after the FePt had been patterned by ion beametching, producing 27 nm diameter dots with perpendicularmagnetic anisotropy. Hysteresis measurements and FORCplots indicate a soft magnetic phase in the dots which wasattributed to a poorly-ordered region at the edges of the dots.An analysis of the switching volume gave a thermal stabilityof KV*/kT=428 at room temperature. This suggests thatpatterned L10 FePt dots could be scaled to few-nm dimen-sions and still retain sufficient thermal stability for a 10 yearsdata retention time.
Acknowledgments
The authors gratefully acknowledge the support of C-SPIN, aSTARnet Center of the Semiconductor Research Corporationsponsored by DARPA and MARCO. EF acknowledges theBasque Government Fellowship grant and PH acknowledgesthe Agency of Science, Technology and Research (A*STAR)International Fellowship grant. Shared experimental facilitiesof CMSE, an NSF MRSEC under award DMR1419807, wereused. Part of the magnetic measurements were performed atSGIKER (UPV/EHU) with the help of Dr Iñaki Orue.
ORCID iDs
Caroline A Ross https://orcid.org/0000-0003-2262-1249
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