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## Creator

[P. D. Bentley](https://orcid.org/0000-0003-4160-2449), [Y. Sasaki](https://orcid.org/0000-0002-9192-4799), [I. Suzuki](https://orcid.org/0000-0002-8932-8226), [S. Isogami](https://orcid.org/0000-0001-7230-6090), [Y. K. Takahashi](https://orcid.org/0000-0001-9197-7236), [H. Suto](https://orcid.org/0000-0003-4387-5862)

## Rights

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in P. D. Bentley, Y. Sasaki, I. Suzuki, S. Isogami, Y. K. Takahashi, H. Suto; Development of L1<sub>0</sub>-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructure. Appl. Phys. Lett. 13 January 2025; 126 (2): 022404 and may be found at https://doi.org/10.1063/5.0246369.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Development of L10-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructure](https://mdr.nims.go.jp/datasets/60955540-f737-46b3-932f-3d18df25ddf1)

## Fulltext

Development of L10-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructureDevelopment of L10-ordered FePt with low damping and largeperpendicular magnetic anisotropy by engineering the nanostructureP. D. Bentley,1, 2, a) Y. Sasaki,1, b) I. Suzuki,1 S. Isogami,1 Y. K. Takahashi,1 and H. Suto1, c)1)National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan2)Kansai Institute for Photon Science, National Institutes for Quantum Science and Technology, 8-1-7 Umemidai Kizugawa,Kyoto 619-0215, Japan(Dated: 11 December 2024)THz spintronics is an emergent area of research aimed at bridging the gap between 5th and 6th Generation wirelesstelecommunications by utilizing new spintronic devices such as magnetic spin torque oscillators as a source of lowpowered THz emission. The realization of such devices using ferromagnetic metal thin films however requires magneticmaterials with both large perpendicular magnetic anisotropy and low Gilbert damping constants. In this letter we reporton the development of L10-ordered FePt with an effective Gilbert damping constant as low as 0.033. Using time-resolved magneto-optical Kerr effect, we characterized the magnetization dynamics of continuous L10-ordered FePtgrown on MgO and SrTiO3 substrates. By changing the substrate on which FePt is grown, the lattice mismatch andsubsequent number of misfit dislocations at the interface and L10-ordering can be controlled. We found that fewermisfits and improved ordering in FePt lead to a reduced Gilbert damping constant due to reduced electron scattering butthat FePt grown on SrTiO3 also shows robust perpendicular magnetic anisotropy. Importantly, these results demonstratethe ability to control the damping in FePt and similar materials by changing the number of misfit dislocations at theinterface and the smaller damping in FePt opens up the possibility of using this material in spintronic materials in theTHz wave range.In the current realm of big data acquisition and utilization,where data density requirements have escalated to new levels,there have been increasing demands for improved high-speedand-capacity communication network technologies. These re-quirements drive the advancement of innovative solutions toensure seamless and efficient data handling in an increasinglyconnected world. Sixth-generation wireless technology (6G)is the next stage in mobile communications, where the electro-magnetic wave frequency range used for 6G partially includesterahertz waves which are those within the frequency range of100 GHz to 10 THz.1,2 Therefore, the realization of oscillatorsand detectors which operate in the THz wave range are an im-portant building block in the future of the Internet of Things.Recently, using magnetoresistance in ferromagnetic (orferrimagnetic) metal, insulator, ferromagnetic thin filmheterostructures, spin-torque/spin-Hall nano-oscillators havebeen developed which act as micro to millimeter wave emit-ters and or detectors.3–10 To achieve spintronic devices withTHz operating frequencies, theoretical studies have mainlyfocused on using antiferromagnetic materials owing to theirantiferromagnetic resonance frequencies in the THz gap.11–14However in terms of experimental devices, spintronic emit-ter and detector devices have been demonstrated primarily us-ing ferromagnetic materials because of their large magnetore-sistance effect, large saturation magnetization, and efficientcontrol of their magnetization owing to their high spin injec-tion efficiency and small damping constant α .15–17 Unfortu-nately ferromagnetic-based systems have struggled to achieveTHz operation so far as these spintronic devices require mag-netic materials with effective anisotropy fields above 3.0 Ta)Electronic mail: bentley.phillip@qst.go.jpb)Electronic mail: sasaki.yuta@nims.go.jpc)Electronic mail: suto.hirofumi@nims.go.jpwhich give rise to the high magnetization precession frequen-cies needed.Due to its large magnetoresistance, high spin current in-jection efficiency,18 and anomalous spin Hall effect,19 L10-ordered FePt is a promising candidate material for variousspintronic applications. Its huge perpendicular magneticanisotropy (PMA) energy above 4.5 MJ m−3 is already beingused in magnetic recording media applications such as harddisk drives, and its anisotropy field is high enough in princi-ple for THz wave use.20–22Since the magnetization dynamics of L10-FePt are inthe THz wave range, conventional electrical measurementsare challenging therefore necessitating the use of all-opticalmethods such as time-resolved magneto-optical Kerr effect(TRMOKE) measurements with ultrashort laser pulses tomeasure the damping constant of this material.23,24 Conse-quently, there are very few studies which report on the Gilbertdamping constant in L10-FePt, and there is no research specif-ically focused on reducing the damping of this material. Al-though previous measurements of the damping constant ofL10-FePt have reported a relatively large value ranging from0.05−0.26,25–28 overcoming this drawback and reducing thedamping in L10-FePt could lead to the realization of practi-cal ferromagnetic metal thin films for THz spintronic devices,thus making this material a desirable candidate for these ap-plications.In this study, we aimed to reduce the damping constant ofL10-FePt by controlling the fine nanostructure and reducingthe number of misfit dislocations observed using transmissionelectron microscopy (TEM). Theoretically, the damping con-stant of L10-FePt is primarily influenced by electron impu-rity scattering, and both theoretically and experimentally, it isexpected to vary with changes in the electron impurity scat-tering rate, however this is difficult to control or modify.29,30Through fine structural evaluation and terahertz magnetiza-mailto:bentley.phillip@qst.go.jpmailto:sasaki.yuta@nims.go.jpmailto:suto.hirofumi@nims.go.jpDevelopment of L10-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructure 2FIG. 1. (a) XRD profiles of 30 nm continuous FePt thin films grown on MgO and STO substrates showing the presence of L10-ordered FePt,(b) and (c) cross-sectional STEM images of FePt/MgO and FePt/STO, respectively, where the change in contrast represents the differencebetween the substrate and the FePt thin film, the white markers highlight misfit dislocations, and the lower subfigure of (b) and (c) are inverseFourier filtered transformation images of the STEM images shown in the upper subfigure.tion dynamics analysis using light, we successfully reducedthe damping constant from the previously reported value of0.0528 to 0.033 by controlling the number of misfit disloca-tions. These results suggest that reducing the damping con-stant through fine structural control of the nanostructure, en-able the application of materials such as L10-FePt which havenot been extensively explored for spintronic devices beyondmagnetic recording, for applications in the THz wave range.Continuous thin films of FePt were deposited on single-crystalline MgO(001) and SrTiO3(001) (STO) substrates ata deposition temperature of 400°C via magnetron sputter-ing in an ultrahigh vacuum chamber with a base pressure of8× 10−7 Pa and an Ar pressure of 0.5 Pa during sputtering.Both substrates were annealed at 650 °C for 1 hour beforedeposition of the FePt film to remove contamination at thesurface of the substrate. A Fe51Pt49 target was used and com-positional analysis of each of the samples showed a stoichio-metric Fe50Pt50 composition. Structural properties of thesethin films were evaluated using X-ray diffraction (XRD) usingCu Kα X-rays. Cross-sectional scanning transmission elec-tron microscopy (STEM) measurements were carried out us-ing a SpectraUltra S/TEM (Thermo Fisher Scientific) and theSTEM samples were fabricated by focused ion beam (FIB)with a scanning electron microscopy (SEM) dual-beam sys-tem Helios5UX (Thermo Fisher Scientific). Magnetic proper-ties of the FePt thin films were evaluated using two supercon-ducting quantum interference device (SQUID) systems, onemeasured up to 7 T at room temperature and the other up to14 T to evaluate the anisotropy field and PMA of these thinfilms. TRMOKE was performed using a Quantum DesignOpticool system with laser pump and probe beam diameters120 µm and 80 µm, and laser fluences of 1.33 mJ cm−2 and0.99 mJ cm−2, respectively, and fixed magnetic field anglesof 80° and 45° to measure closer to and further away fromthe easy magnetization axis of the sample. The magnetic fieldangle is measured is relative to the normal of the film planeand the optical setup used in these experiments is described ingreater detail elsewhere.28Figure 1(a) shows XRD profiles of the FePt/MgO and theFePt/STO sample where L10 order is observed in both FePtthin films.31,32 The (001) and (002) peak positions for theFePt/STO sample are shifted to slightly higher angles rela-tive to the FePt/MgO sample indicating a smaller out-of-plane(OOP) lattice constant (c), which is a result of the smaller in-plane (IP) lattice constant (a) of STO (3.91 Å) relative to MgO(4.20 Å).33 Similar to previous reports,34 the c for FePt/MgOand FePt/STO were found to be 3.73 Å and 3.71 Å, respec-tively. Using STEM, the in-plane lattice constant for bothsamples was measured to be 3.85 Å and therefore the c/aratio for the FePt/MgO and FePt/STO sample are ≈ 0.969and ≈ 0.964, whilst mismatch between FePt and the sub-strate are 8.33% and 1.41%, respectively. Using the peakintensities of the (001) superlattice peak and the (002) fun-damental peak and the formula described in,35–37 the orderparameter, defined as SL10 =√Iexp100/Iexp200/√Icalc100 /Icalc200 whichis ≈ 0.85√Iexp100/Iexp200 where I is the experimental (exp) andcalculated (calc) intensities of the (100) and (200) reflec-tions, for these two samples were calculated to be ≈ 0.70 forthe FePt/MgO sample and ≈ 0.82 for the FePt/STO sample.High-resolution cross-sectional STEM seen in Figure 1(b) and1(c) show sharp interfaces for both FePt samples but a smallernumber of misfit dislocations for the FePt/STO sample.Development of L10-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructure 3FIG. 2. In-plane (IP) and out-of-plane (OP) magnetization curvesmeasured at room temperature for 30 nm continuous FePt thin filmsgrown on (a) MgO and (b) STO.Figure 2(a) and 2(b) show the magnetization curves forthe FePt/MgO and FePt/STO sample, respectively. The OPmagnetization curve for both samples exhibit a similar shapeand are consistent with continuous FePt thin films prepared atthese temperatures.33,38 The anisotropy field of the FePt/MgOand the FePt/STO sample are found to be µ0Hk = 6.80 Tand µ0Hk = 5.57 T and by calculating the area underneaththe first quadrant of the magnetization curve of both sam-ples, the PMA was determined to be ≈ 2.26 MJ m−3 and≈ 1.79 MJ m−3, respectively. According to Sakuma et al.,the weaker PMA in the FePt/STO sample can be explained tobe a result of a smaller lattice mismatch between the FePt andthe substrate.39Typical TRMOKE signals obtained for the FePt/MgO sam-ple measured at magnetic field angles θH = 80° and 45° canbe seen in Figure 3(a) and 3(b), respectively. Damped oscil-lation signals relating to magnetization precessional motionwere clearly observed for both magnetic field angles. To fitthe TRMOKE spectra at different magnetic field strengths andθH of both samples as seen in Figure 3(a) and 3(b) for theFePt/MgO sample, and Figure 4(a) and 4(b) for the FePt/STOsample, the following function was used which is a leastsquares method accounting for the exponential recovery of thebackground signal:∆φk = A0 +A1e−ν∆t +B0e−∆t/τ sin(2π f ∆t +φ) (1)where the first and second terms represent the magnetiza-tion recovery process and the third term represents the mag-netization precession in the damped oscillation signal.28,40 Inthis equation, B0 is the precession amplitude, τ lifetime, fFIG. 3. TRMOKE spectra at different magnetic field strengths for thecontinuous 30 nm FePt/MgO film at magnetic field angles of (a) 80°and (b) 45°, respectively. The solid red curves represent the fitting ofthe blue dot raw data. Calculated (c) precession frequency f and (d)effective damping constant αeff as a function of the applied magneticfield at these different magnetic field angles. Solid curves here rep-resent the fitting using the second-order Kittel functions. The dashedline highlights the minimum αeff which for the FePt/MgO sample isaround 0.045.frequency, and φ the initial phase. The effective dampingαeff was then calculated using the equation αeff = 1/2π f τ .TRMOKE spectra between 4 T and 7 T were fitted and theprecession frequency and effective damping for these differ-ent magnetic field strengths plotted as seen in Figure 3(c) and3(d), respectively.To fit the parameters extracted from the raw spectra,second-order equations derived from the Landau-Lifshitz-Gilbert (LLG) equation f = (γµ0/2π)√H1H2, where γ =gµB/h̄ is the gyromagnetic ratio. g, µB, and h̄ are the spectro-scopic splitting g-factor, the Bohr magneton, and the reducedPlanck’s constant, respectively, were used. H1 = H cos(θH−θM) + Heffk cos2(θM)− Hk2 cos4(θM) and H2 = H cos(θH −θM) + Heffk cos(2θM)− 12 Hk2(cos(2θM) + cos(4θM)), whereH, Heffk , Hk2 are the external magnetic field, effective perpen-dicular magnetic anisotropy (PMA) field, and second orderPMA field, respectively.41,42 Simultaneously, the magnetiza-tion angle θM was calculated using the relation 2H sin(θH−θM)−Heffk sin(2θM)+Hk2 cos2(θM)sin(2θM) = 0.28 The lifetime was calculated using the equation 1/τ = αγ(H1 +H2)/2π , where α is the intrinsic magnetic damping constant.As a small laser fluence was used in all measurements, weobserved no temperature variance in any of our measure-ments and therefore the Landau-Lifshitz-Bloch equation wasnot used in our analysis.28Development of L10-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructure 4FIG. 4. TRMOKE spectra at different magnetic fields for the contin-uous 30 nm FePt/STO film at magnetic field angles of (a) 80° and (b)45°, respectively. Calculated (c) f and (d) αeff as a function of theapplied magnetic field at these different magnetic field angles. Theminimum αeff for the FePt/STO sample is around 0.033.The solid lines seen in Figure 3(c) represent the fitting ofthe precession frequency using the above equations for theMgO sample at both angles, whilst the solid curves seen inFigure 3(d) were calculated by approximating the magnitudeof the PMA distribution ∆Heffk as:1/τHk = π∣∣∣∣ d fdHeffk∣∣∣∣∆Heffk (2)where the final ∆Heffk for the FePt/MgO and FePt/STO sam-ple were 250 mT and 136 mT, respectively. The precessionfrequency monotonically increases with magnetic field forboth magnetic field angles 80° and 45°28,43 whilst the effec-tive damping constant was found to increase with decreasingmagnetic field strength showing a minimum value at a mag-netic field strength of 7 T (upper limit of the equipment usedhere) of ≈ 0.045 as highlighted by the dashed line in Fig-ure 3(d); this was similarly performed for the FePt/STO sam-ple as seen in Figure 4(d). The relationship between αeff andµ0H is a result of the dephasing effect and fewer magnetic do-mains at higher applied magnetic fields since at 7 T, the FePtthin film approaches a single domain state and there is conse-quently less dephasing of the spins (spins precessing at differ-ent phases) between the few remaining magnetic domains.44From the fitting of the magnetization dynamics parameters,the µ0Heffk , µ0Hk2, and g-factor of the FePt/MgO sample weredetermined to be 6.34±0.57 T, 0.33±0.04 T, and 2.29±0.02,respectively.As seen in Figure 4(c) an 4(d), TRMOKE at magnetic fieldangles 80° to 45° was also performed for the FePt/STO sam-ple. Similar to the FePt/MgO sample, the precession fre-quency increases monotonically with magnetic field whilstthe damping constant decreases. The µ0Heffk , µ0Hk2, and g-factor for FePt/STO were determined to be 4.72± 0.01 T,1.14± 0.001 T, and 2.09± 0.01, respectively. The smallerµ0Heffk in the FePt/STO continuous thin film is consistent witha smaller PMA observed in this sample with respect to thatseen in FePt/MgO. Comparing the calculated g-factor of bothsamples, the smaller constant observed in the FePt/STO sam-ple is a result of differences in L10 order where increased dis-order in FePt has been found to lead to a monotonic increasein the g-factor.44 Since the L10 order parameter in FePt/STOis greater than that in the MgO sample there is therefore lessdisorder in this system and consequently it has a smaller g-factor.As seen in Figure 3(d) and Figure 4(d), the effective damp-ing constant αeff at different magnetic field strength evalu-ated from the TRMOKE data at θH = 80°, is well reproducedby considering the magnitude of the PMA distribution usingEquation 2. From this fitting, the intrinsic damping constantfor the FePt/MgO sample and the FePt/STO sample were cal-culated to be α = 0.045 and 0.033, respectively. Since αeffcorresponds to the upper-bound value of α , the minimum ef-fective damping constant is identical to or larger than the in-trinsic damping constant. Therefore, as highlighted by thedashed line in both Figure 3(d) and Figure 4(d), we have alsoplotted the minimum value of the effective damping constantas a function of both θH for both samples which is found to bethe same as the intrinsic damping constant in both cases.From a recent theoretical study, it has been reported that thedamping constant in L10-ordered FePt is largely influenced byelectron impurity scattering29 instead of thermal lattice expan-sion or c/a modification.45–47 From our TEM measurements,we observed a far higher number of misfit dislocations nearthe interface for the FePt/MgO sample than for the FePt/STOsample. These misfit dislocations can be considered as impu-rity scattering centers which increase electron impurity scat-tering in L10-ordered FePt and therefore modify α resulting inthe observation of a lower magnetic damping constant for theFePt/STO sample since it has fewer misfit dislocations thanthe FePt/MgO sample. It should be noted that previously ithas been suggested that a self-assembled network of misfitdislocation leads to an increase in extrinsic effects such astwo-magnon scattering which modify the damping in mag-netic metal thin films.48,49 However, since the measurementspresented here were carried out at high magnetic fields, theseeffects can be ignored.42 Another possible explanation as whythe damping constant is smaller in the FePt/STO sample is dueto its smaller PMA since both PMA and damping are relatedto the spin-orbit interaction. However, we have managed togrow FePt on STO which has a similar frequency and PMA tothat grown on MgO. Whilst the damping constant in this sam-ple is not as small as 0.033, it still shows a smaller dampingconstant than the MgO substrate sample (see the Supplemen-tal Material 50) Importantly, our results demonstrate that bylimiting the amount of misfit dislocations and other nanostruc-tural factors that may occur during the manufacturing processDevelopment of L10-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructure 5of magnetic materials such as L10-ordered FePt, we can con-trol their damping opening up the possibility of using this ma-terial and similar materials for spintronic applications in theTHz wave range. In particular, α as low as 0.01 has been re-ported theoretically in L10-FePt,51 however these calculationsas well as other calculations which have focused on the damp-ing in magnetic materials, did not incorporate misfit disloca-tions. Therefore, given the results presented here even smallerdamping in L10-FePt may be achievable experimentally.In summary, we have demonstrated that the damping con-stant in L10-ordered FePt can be modified by controlling thenumber of misfit dislocations at the interface of the material.By changing the substrate which FePt is grown on from MgOto STO, the lattice mismatch and subsequent number of misfitdislocations are reduced. We find that the FePt/STO sampleshows improved L10 order compared to the FePt/MgO sampleand only a small difference in the PMA between both samples.TRMOKE of the FePt/STO sample show an αeff as small as0.033, smaller than that of the FePt/MgO sample, 0.045, asa result of smaller number of misfit dislocations at the inter-face which subsequently modify the number of inter-band andintra-band electron transitions and the electron scattering ratein FePt. Importantly, we show that the damping in FePt can bereduced which thus opens up the possibility of using this ma-terial and similar materials for spintronic applications in theTHz wave range.We acknowledge Yukie Mori for the preparation of the sam-ples for cross-sectional TEM measurements. We also ac-knowledge support from the Japan Science and TechnologyAgency (JST) Core Research for Evolutional Science andTechnology (CREST) (Grant No. JPMJC22C3), the Min-istry of Education, Culture, Sports, Science and Technology(MEXT) Leading Initiative for Excellent Young Researchers(Grant No. JPMXS0320230032), and the Japan Society forthe Promotion of Science (JSPS) KAKENHI (Grant Nos.JP21K14218 and JP18H03787). A part of this work was sup-ported by the Electron Microscopy Unit, National Institute forMaterials Science (NIMS).AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts to disclose.Author ContributionsPhillip David Bentley: Conceptulization (supporting);Data curation (equal); Formal analysis (equal); Investigation(equal); Methodology (equal); Project administration (sup-porting); Validation (equal); Visualization (lead); Writing –original draft (lead); Writing – review & editing (equal). YutaSasaki: Conceptulization (lead); Data curation (equal); For-mal analysis (equal); Funding acquisition (equal); Investi-gation (equal); Methodology (equal); Project administration(lead); Supervision (lead); Validation (equal); Visualization(supporting); Writing – original draft (supporting); Writing –review & editing (equal). Ippei Suzuki: Resources (lead).Shinji Isogami: Validation (supporting). Yukiko Takahashi:Data curation (equal); Funding acquisition (equal); Investiga-tion (equal); Project Administration (supporting); Supervision(lead); Validation (supporting); Writing – original draft (sup-porting); Writing – review & editing (equal). Hirofumi Suto:Project Administration (supporting); Supervision (lead); Val-idation (supporting); Writing – original draft (supporting);Writing – review & editing (equal).DATA AVAILABILITYThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.1C.-X. Wang, X. You, X. Gao, X. Zhu, Z. Li, C. Zhang, H. Wang,Y. Huang, Y. Chen, H. Haas, J. S. Thompson, E. G. Larsson, M. D.Renzo, W. Tong, P. Zhu, X. Shen, H. V. Poor, and L. Hanzo, IEEECommun. Surv. Tutor. 25, 905 (2023).2Z. Zhang, Y. Xiao, Z. Ma, M. Xiao, Z. Ding, X. Lei, G. K. Kara-giannidis, and P. Fan, IEEE Veh. Technol. Mag. 14, 28 (2019).3S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, N. C. Emley, R. J.Schoelkopf, R. A. 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