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[2025 ogawa APL final.pdf](https://mdr.nims.go.jp/filesets/2339f839-3a47-4754-99bc-d8f783714a4e/download)

## Creator

[Daisuke Ogawa](https://orcid.org/0000-0002-4373-6435), [Yuma Iwasaki](https://orcid.org/0000-0002-7117-277X), [Jun Uzuhashi](https://orcid.org/0000-0003-2023-8158), [Yuta Sasaki](https://orcid.org/0000-0002-9192-4799), Masato Kotsugi, [Yukiko K. Takahashi](https://orcid.org/0000-0001-9197-7236)

## 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 Daisuke Ogawa, Yuma Iwasaki, Jun Uzuhashi, Yuta Sasaki, Masato Kotsugi, Yukiko K. Takahashi; Er-driven magnetic tunability in FePt thin films investigated via high-throughput experiments and microstructure analysis for future HAMR media. Appl. Phys. Lett. 23 June 2025; 126 (25): 252405 and may be found at https://doi.org/10.1063/5.0273511. [In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Er-driven magnetic tunability in FePt thin films investigated via high-throughput experiments and microstructure analysis for future HAMR media](https://mdr.nims.go.jp/datasets/7e7a6053-dcc9-4024-836f-d6089f2bd883)

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Intrinsic magnetic properties and microstructural characteristics of FePtthin films with segregated rare earth elements prepared by acombinatorial high-throughput sputtering systemD. Ogawa,1, a) Y. Iwasaki,2 J. Uzuhashi,1 Y. Sasaki,1 M. Kotsugi,3 and Y. K. Takahashi1, a)1)National Institute for Materials Science, Tsukuba 305-0047, Japan2)Center for Basic Research on Materials (CBRM), National Institute for Materials Science (NIMS), Tsukuba 305-0047,Japan3)Department of Materials Science and Technology, Tokyo University of Science, Tokyo 125-8585,Japan(Dated: 9 July 2025)This study undertakes comprehensive experimental validations based on theoretical predictions of the impact of Er andTm doping on the magnetic properties of FePt thin films. Initial theoretical investigations indicate that doping with rareearth elements may result in promising alterations to the magnetic properties of the FePt thin films, with Er doping inparticular offering a promising avenue for further study. Experimental synthesis via a combinatorial high-throughputsputtering system, which enables precise control over the composition of FePt thin films, achieves the desired magneticproperties. Small quantities of dopants, specifically 0.35 at.% Er, substantially enhance the key magnetic properties ofsaturation magnetization (µ0Ms), anistropy constant (Ku) at room temperature, and the Curie temperature (TC). Precisemicrostructural observations of a sample show that Er segregates at grain boundaries, voids, and the substrate/FePtinterface, where Er preferentially replaces Fe sites. In other regions of the FePt grains, Er is not solid-soluble, andpure FePt and FePtEr form a composite material in the order of tens of nm. The incorporation of Er also influences thedamping constant α . The findings of this study substantiate the intrinsic characteristics of Er-doped films, particularlythe enhanced µ0Ms, Ku and TC attainable with nominal dopant concentrations, and facilitate the realization of ultimatemagnetic recording densities anticipated for future data storage technologies.Keywords: heat-assisted magnetic recording, FePt, combinatorial, high throughput, machine learning, rare-earth dop-ing, intrinsic magnetic properties, microstructure, damping constantI. INTRODUCTIONHeat-assisted magnetic recording (HAMR)1,2 has emergedas a pivotal technology in the evolving landscape of high-density data storage, particularly in the era of big data. Thistechnology offers a promising alternative to traditional record-ing methods by potentially exceeding areal densities of 4Tb/in2, considering the anisotropy constant of 7 MJ/m3 andsaturation magnetization of 1.4 T for FePt grains3. However,the transition of HAMR technology from 1 Tb/in2 to 4 Tb/in2involves several significant challenges. These challenges in-clude reducing the size of magnetic grains, enhancing thermalgradients, improving magnetic anisotropy, and reducing noise.Addressing these challenges requires complex solutions thatare supported by comprehensive research and innovation inthe fields of materials science and magnetic recording tech-nologies. A significant challenge lies in reducing the mag-netic grain size to increase the areal density while maintainingthe thermal stability and signal-to-noise ratio (SNR). The pro-posed media specifications for advancing towards from 2 to4 Tb/in2 include reducing the center-to-center grain distance(Dp) from 7.0 nm to approximately 5.1 nm and the magneticcore size (Dcore) from 6.0 nm to 4.3 nm. The variability inthe grain diameter (σ /mean grain diameter) should be finelycontrolled within a range of 10-15%, ensuring uniformity ina)Corresponding authorsize across the media4. Furthermore, a film thickness (t) of8.2 nm is necessary to guarantee a sufficient SNR. This im-plies that a highly severe microstructure is required, specifi-cally, an aggregate structure (columnar structure) of FePt coregrains with an aspect ratio (t/D) of 1.604. Recently, h-BNhas been extensively studied as a promising material to satisfythese requirements5–8. In addition to the microstructure con-trol, another approach to maintain the SNR while achieving4Tb/in2 is to increase the µ0Ms of the FePt core grains. Byincreasing the µ0Ms of the FePt grains, the film thickness formaintaining the SNR can be reduced, which is expected to re-lax the above aspect ratio requirement and simplify achievinga higher density. It has been reported that the addition of Ndincreases the spin and orbital moment of Fe, with a high or-bital moment of Nd and an increase in µ0Ms9. The additionof Gd, and Tb10,11 is of interest not only for HAMR applica-tions but also for so-called all-optical switching (AOS), wherenon-collinear antiferromagnetic coupling with Fe is reportedto lower µ0Ms. However, the mechanism by which these ad-ditive elements alter the intrinsic magnetic properties has notyet been explained. The latest developments in first-principlescalculations have demonstrated that the magnetocrystallineanisotropy energy (∆E) of FePt is predominantly attributableto the robust spin-orbit coupling derived from heavy elements,such as Pt12. Despite the inability to explain the tempera-ture dependence using the general K(T )/M(T )3 relationship,first-principles approaches have been developed to elucidatethe behavior at finite temperatures by incorporating trans-spinfluctuations13. Furthermore, hard X-ray photoelectron spec-2ErFe PtNo.1 No.2No.4 No.5No.8 No.9No.11 No.12No.3No.7No.6No.10FeErPt(a) (b)FIG. 1. (a) Target configuration and substrate location in the sput-tering chamber. (b) Schematic of the cathode and substrate from theside.troscopy (HAXPES) has revealed that in L10-ordered FePtthin films, the Pt 5d orbitals exert a significant influence onthe perpendicular magnetic anisotropy (PMA) owing their ro-bust hybridization with the Fe 3d state14. Thus, the origin ofthe intrinsic magnetic properties of L10-FePt is gradually be-ing understood; however, much remains unknown. Therefore,this study will focuses on the improvement and evaluation ofthe intrinsic magnetic properties (µ0Ms, Ku, and TC) when ad-ditive elements are added to the FePt core grains, exploringmaterials that increase µ0Ms while maintaining Ku by addinga third element to the FePt grains. First-principles calcula-tions are highly effective tools for the mechanistic elucida-tion of material properties but they are also useful in mate-rial exploration. Machine learning has recently emerged as avaluable tool for material exploration. By incorporating ma-chine learning into first-principles data for prediction, the pos-sibility of exploring optimal compositions in a wide compo-sition space, such as the addition of two or more elements,that cannot be followed only by first-principles calculations,has been proposed15,16. This study initially concentrates onthe elements Er, and Tm, which have been predicted to en-hance µ0Ms through first-principles calculations? . The damp-ing constant α is also a crucial parameter because high mag-netic damping is preferred to achieve a higher SNR17, andfaster writing times18. In particular, the temperature depen-dence of α and its behavior near TC are important for thepractical applications of HAMR19–22, and attempts have beenmade to understand these properties from experimental23 andtheoretical24 perspectives.In this study, the effects of Er and Tm doping on FePt thinfilms, which has been predicted to increase µ0Ms by first-principles calculations and has not been previously reported.To ensure efficient data collection, samples are prepared byusing a combinatorial high-throughput sputtering system, inwhich multiple film compositions are obtained in a single de-position process. The first aim of this study is to comprehen-sively investigate the compositional dependence of the intrin-sic magnetic properties of FePt thin films, such as µ0Ms, Ku,TC. The second aim is to investigate the microstructure anddamping constant α in detail, focusing on compositions thatshowed improve the magnetic properties.II. EXPERIMENTAL DETAILSThe films were deposited using a combinatorial high-throughput sputtering system, with an FePt-Er layer ofapproximately 30-40 nm deposited on a single-crystalMgO(001) substrate, which is well known to promote c-axisorientation and L10 ordering of FePt25,26. A 5 nm carbon capwas applied to all films to prevent oxidation. The substratetemperature and argon gas pressure employed during the de-position of FePt-Er were 500 ◦C and 10 mTorr, respectively.In the Fe, Pt, and Er target arrangements shown in Fig. 1,sputter discharges were performed at the sputter power anddeposition time indicated in the Supplementary Material. Thetarget diameter was φ 76.2 mm. The distance between the tar-get and the substrate was 250 mm, and the angle of incidencewas 45 ◦. For the sputtering of Er or Tm, the rate was con-trolled by placing a masking plate directly over the target tosuppress the excessively high rates. A combinatorial methodwas employed to fabricate films with multiple compositionsusing a single deposition process. This involved the placementof 10 mm square MgO(001) substrates in the configurationshown in Fig. 1(a) on the substrate holder, with no rotationof the substrates during deposition. The concept of employ-ing a combinatorial approach by using a thin film process hasa substantial history27–29 and has recently demonstrated con-siderable potential as a highly effective tool for the acquisitionof large datasets in conjunction with machine learning30,31.Magnetization curves were analyzed utilizing a vibratingsample magnetometer (VSM) using TM-VSM211483ASE(Tamagawa) and a superconducting quantum interference de-vice (SQUID) using MPMS3 (Quantum Design). The in-plane (IP) hysteresis loops were measured using a Dynacool(Quantum Design) with a 14 T maximum magnetic field,which was equipped with a large bore coil set. The uniax-ial anisotropy constant Ku was estimated by anomalous holeeffect (AHE) torque measurements using a Dynacool (Quan-tum Design) with 9 T maximum magnetic field, which wasequipped with a standard manufacturer’s rotator and resis-tance measurement options. Further details regarding theAHE torque measurements and analyses can be found in therelevant literature32,33. The temperature dependence of mag-netization was measured using a SQUID-VSM (Quantum De-sign). The Curie temperatures (Tc) were determined by fittingM-T curves using the Kuz’min formula34.Regression analysis with using the random forest35 modelwas performed using the software WAVEBASE36 (ToyotaMotor Corporation) to complementarily predict the µ0Ms be-tween the gaps in the experimental data points for compo-sitions (x,y) in the (Fe50−xPtx)100−y-Ery (X=Er,Tm) system,based on the experimental data gathered by the combinatorialmethod.For the analysis of the crystal structures, X-ray diffraction(XRD) profiles were obtained, and the L10 ordering parame-ter, S and lattice parameters were evaluated using a Smartlab(Rigaku). The method employed for estimating S was in ac-cordance with that described in previous literature37–39. Thefilm thicknesses of all samples were estimated by X-ray re-flectivity (XRR) measurements and fitting the obtained data.3(a)(b)(c)(FePt)-X (X=Er,Tm)X content (at.%)FIG. 2. X (X=Er,Tm) content dependence of the (a) saturation mag-netization µ0Ms, (b) anisotropic field µ0Hk, and (c) Curie tempera-ture TC for FePt-X thin films. Dotted lines are predicted values whenthe Er was substituted at the Fe site in Fe50−xPt50Erx by regressionanalysis.The compositional analysis was conducted using X-ray flu-orescence (XRF) spectroscopy, with a ZSX Primus II system(Rigaku).The plane view and cross-sectional scanning transmis-sion electron microscopy (STEM) observations with energy-dispersive X-ray spectroscopy (EDS) were performed usinga Spectra Ultra S/TEM (Thermo Fisher Scientific). Planeview TEM lamellae were prepared by chemical etching,and cross-sectional TEM lamellae and needle-shaped spec-imens for atom probe tomography (APT) were fabricatedusing a focused ion beam (FIB) with a scanning electronmicroscopy (SEM) dual-beam system Helios5UX (ThermoFisher Scientific). APT measurements were carried out us-ing LEAP5000XS (CAMECA) in the 355 nm UV laser puls-ing mode with 30 pJ energy at a specimen temperature of 30K. Data were visualized and analyzed using the APsuite 6.1program (CAMECA).The damping constant was evaluated by using the all-optical time-resolved magneto-optical Kerr effect (AO-TRMOKE). The laser pulse source was a Yb:KGW laser sys-tem with a wavelength, pulse width, and pulse repetition rate(a)(c)(e)⏊(b)(d)(f)Magnetization, μ0M (T)Torque (MJ/m3)Saturation magnetizaiton,μ0M (T)Temperature (K)Magnetic field, μ0H (T)θM (deg.)(FePt)-Er0.35FePt(FePt)-Er0.35FePt(FePt)-Er0.35FePtKu = K1 + K2      =5.63 MJ/m3Ku = K1 + K2      =6.43 MJ/m3FIG. 3. (a) Magnetization curves in the out-of-plane (⊥) and in-plane (//) directions at 300 K for (a) FePt thin film without Er additionand (b) FePt thin film with 0.35 at.% Er addition. AHE torque andfitting curves at 300 K for (c) FePt thin film without Er addition and(d) FePt thin film with 0.35 at.% Er addition. M-T curve for for (e)FePt thin film without Er addition and (f) FePt thin film with 0.35at.% Er addition. Black dashed lines are Kuz’min fitting functions.of 1028 nm, 290 fs, and 10 kHz, respectively. The wave-length of the probe laser pulse was converted to 514 nm byusing a BaB2O4 (BBO) crystal. The pump beam amplitudewas modulated using a mechanical chopper with a modula-tion frequency of 870 Hz. The pump induced change in theKerr rotation angle of the reflected probe laser pulse that wasdetected using a balanced photodiode detector and lock-in am-plifier by varying the optical delay ∆t between the pump andprobe laser pulse with a delay line. The laser spot size for thepump and probe laser beam were 126 and 63 µm in diameter,respectively. The pump fluence was fixed to be 2.4 mJ/cm2and the probe fluence was below 0.5 mJ/cm2. A magneticfield of 7 T was applied at 80 ◦ with respect to the film normalduring the TRMOKE measurements40.4III. RESUSTS AND DISCUSSION:A. Magnetic PropertiesFig. 2 illustrates the (a) saturation magnetization (µ0Ms) at300 K, (b) anisotropy field (µ0Hk) at 300 K, and (c) Curietemperature of the FePt-X films (X=Er, Tm) as a function ofthe Er and Tm atomic contents. When the configuration ofthe cathodes in Fig. 1 was used, the Er and Tm contents andthe Fe and Pt composition ratios change naturally. L10-FePtvaries in µ0Ms and Ku as the ratio of Fe to Pt changes, withthe composition of x = 50 ~60 in FexPt100−x having the largestvalue41–43. Consequently, a regression analysis utilizing thecomposition (x, y) as a descriptor and µ0Ms as the target vari-able was used to predict µ0Ms when the atomic ratio percent-age of Pt was assumed to be 50%. The blue and green dottedlines in Fig. 2(a) correspond to the predicted values of µ0Msassuming that the atomic percentage of Pt is 50% in the Er-and Tm-doped samples was 50%, respectively. As a result,an increase in µ0Ms was observed with very small additionsof Er ≤ 0.5 at.%. In the composition region of Er ≥ 1 at.%,µ0Ms decreased monotonically. In contrast, the µ0Ms valuedecreased linearly with an increasing Tm content across allcomposition ranges, exhibiting no µ0Ms peaks, as seen in theEr-doped samples. It was observed that the µ0Hk and TC in-creased in the region of a very low composition, ≤ 1 at.% forboth Er- and Tm-doped samples. Increases in µ0Ms, Ku andTC were observed with the addition of a very small amount ofEr (0.35 at%). Therefore, focusing on this composition, Fig. 3shows: (a) the magnetization curves with an induced externalfield along the out of plane (⊥) and in-plane (//) directions at300 K with and without an Er-doped sample, (b) the magnetictorque measurement and fitting results using AHE at 300 K,and (c) the M-T curves. Fig. 3(a) shows that µ0Ms was 1.39T at 300 K without the addition of Er, whereas it increased to1.54 T with the addition of 0.35 at% Er. µ0Hk also increasedfrom 10.0 T to 11.8 T with the addition of Er. Fig. 3(b) showsthat the anisotropy constant Ku (K1+K2) also increased from5.63 MJ/m3 without Er addition to 6.43 MJ/m3 with a verysmall Er addition of 0.35 at%. From Fig. 3(c), the Curie tem-perature increased from 679 K to 716 K with the addition ofEr. However, no difference in µ0Ms was observed betweenthe two samples at temperatures close to 0 K. Therefore, theincrease in µ0Ms and Ku at 300 K owing to the addition of0.35 at.% Er can be attributed mainly to the increase in theCurie temperature, which reduced the rate of decrease withtemperature. The L10 ordering parameter S, estimated fromXRD, was 0.956 and 0.926 for samples with and without 0.35at.% Er, respectively. The observation that there is minimalalteration in the L10 ordering parameter S with the incorpora-tion of Er indicates that the observed changes in the magneticproperties are a consequence of an intrinsic modification ofthe magnetic properties of the FePt grains. The detailed infor-mation concerning the composition, thickness, lattice constantc, L10 order parameter, µ0Ms, TC, and µ0Hk of two samplesshown in Fig. 3 is listed in Table I, and all the samples de-scribed in this paper is listed in Table S1 of the supplementalmaterial.TABLE I. Lattice constants a, c, L10 ordering parameter, and mag-netic properties of FePt thin films without Er addition and with Er0.35 at.% addition.without Er Er 0.35 at.%Lattice constants a (nm) 0.387 0.387c (nm) 0.372 0.371L10 order parameter 0.926 0.956Ms (T) 50 K 1.64 1.66300 K 1.39 1.54Ku (MJ/m3)50 K 8.00 7.34300 K 5.63 6.43Hk (T) 300 K 10.0 11.8TC (K) 679 716B. Microstructure AnalysisEvaluation of the magnetic properties revealed that the sam-ple with an addition of 0.35 at.% Er exhibited the most sig-nificant improvement. Therefore, microstructure observationswere conducted for this sample. Fig. 4 shows a cross-sectionalhigh-angle annular dark-field (HAADF)-STEM image of asample with 0.35 at.% Er incorporated. As shown in Fig. 4(a),the image reveals the formation of a continuous film of a FePt-Er film on the MgO substrate. However, an examination ofthe upper portion of the film revealed the formation of val-leys in certain areas, despite its continuous nature. Fig. 4(b)-(d) show the magnified HAADF-STEM image and EDS ele-mental maps of FePt and Er, respectively. The white arrowsin Fig. 4(c) and (d) indicate the locations of the valleys. InFig. 4(d), pink arrows indicate Er segregation. Small voidswere formed just below the valleys Er was preferentially seg-regated near these voids. EDS maps of FePt and Er in differentregions are shown in Fig. 4(e). The EDS line profile of whitearea in Fig. 4(e) is shown in Fig. 4(f). The results showed thatFe, Pt and Er were uniformly distributed; however, 1.0 at.% ofEr tended to be segregated near the interface between the MgOsubstrate and the FePt layer. Fig. 4(g)-(i) show the HAADF-STEM image focused on near the valley, and EDS maps ofFePt and Er. The dark-contrast region in Fig. 4(g) correspondsto a void. A comparison with the Er distribution in Fig. 4(i)revealed that Er was preferentially segregated around the void.Fig. 5 shows the 3D atom maps for 0.35at% Er-doped FePt-Erfilm. The detection limit of EDS is typically approximately1 at.%; however, APT has a significantly higher chemicalsensitivity44. Although dependent on the target materials andanalysis conditions, 0.002at% of Mg dopant in GaN was suc-cessfully measured using APT45. Fig. 5(a) shows Fe, Pt, Mg,and O atoms displayed as red, green, purple, and blue dots,respectively. In contrast, Fig. 5(b) shows the Er atoms as pinkdots. The preferentially segregated Er at the interface betweenthe MgO substrate and the FePt layer can also be seen here.In addition, Er was preferentially present in the upper regionsof the FePt layer. Fig. 5(c) shows the 3D atom probe analy-sis region selected in the blue frame of Fig. 5(b) from the top.As indicated by the cross-sectional image in Fig. 4, Er wassegregated along grain boundaries and voids in a network-likepattern. From the mass spectrum in Fig. 5(d), the Pt+++ peak,5(a)(c) (d) (e)(g) (h) (i)ErFePtErFePtDistance (nm)Atomic content(a.u.)(f)FePtEr(b)(FePt)-Er0.35FIG. 4. (a) Low-magnification cross-sectional HAADF-STEM image of FePt-Er thin film with 0.35 at.% Er addition, (b) higher magnificationHAADF-STEM image, (c) EDS elemental map of FePt corresponding to the field of view in (b), (d) EDS elemental map of Er correspondingto the field of view in (b), (e) EDS elemental map of FePtEr for another area and EDS line profile of FePtEr in the white frame, (f) EDS lineprofile in the white region shown in (e), (g) HAADF-STEM image focusing on the valley, (h) EDS elemental map of FePt corresponding tothe region in (g), and (i) EDS elemental map of Er corresponding to the region in (g).Er+++ and Er++ peaks were identified, suggesting that Er wasindeed present, at least as a divalent or trivalent ion. How-ever, no peaks attributable to Er were observed from the massspectrum of the region excluding Er segregation at the grainboundaries, near voids, and at the FePt/MgO interface. Thisfinding demonstrates that Er was not uniformly solid-solublein the FePt grains. Fig. 5(c) shows that pure FePt and FePt-Er are intermingled to a depth of approximately tens of nm.Mass spectrum analysis demonstrated that the Er content inthe region where Er was present was approximately 1 at.%.Fig. 6 illustrates the results of the top-view HAADF-STEMobservations conducted on a sample containing 0.35 at.% Er.Fig. 6(a)-(c) show the HAADF-STEM image, and FePt andEr EDS elemental maps, respectively. Fig. 6(d) shows theEDS line profiles of Fe, Pt, and Er in the areas in Fig. 6(c).The upper and lower graphs illustrate the intensity and atomiccontent of each element, respectively. The dark-imaged areain Fig. 6(a) was confirmed to be a void based on the reducedintensity observed in Fig. 6(d). Near this void, there was anincrease in the atomic content of Er and a corresponding de-crease in the atomic concentration of Fe. The Er atoms weresegregated preferentially in the region near the void, and theFe sites were preferentially substituted by Er in that region.Fig. 6(e) shows a HAADF-STEM image of a separate areaof the sample with 0.35 at.% Er added. Fig. 6(h) shows theline profiles of Fe, Pt, and Er in the white region of Fig. 6(g)in the same format in Fig. 6(d). The black dotted lines inFig. 6(e) indicate the grain boundaries. Fig. 6(g) shows thatEr was also segregated near the vicinity of the grain bound-ary. The magnified sections of the atomic resolution on theleft and right serve to highlight the areas in Fig. 6(e) where Erwas present and absent, respectively. A comparison of thesehighlighted areas shows that at least the same periodic f ctstructure was maintained in the regions with and without Er.It was not possible to ascertain from this image whether thedegree of L10 ordering was preserved in these areas. How-ever, the overall L10 order of the sample, as estimated fromthe XRD results, showed minimal variation with or withoutEr. This indicates that the L10 structure was maintained evenin regions where Er was segregated. It was determined that Erwas not uniformly solid-soluble in the FePt grains. Addition-ally, it was observed that L10-FePtEr was preferentially seg-regated around the void/grain boundaries and the MgO/FePtinterface, with a size of approximately tens of nm. Generally,examples of nanoscale void formation and segregants aroundvoids by doping additives are common and have been reportedin other material systems. For example, it has been reportedthat doping Sb into p-type ZnO nanowires may improve p-type properties owing to the formation of nanoscale voids asthe Sb segregates around voids during the growth process, andthe surrounding ZnO grows more slowly than normal46. In aCoSi2 sample, the formation of voids at the interface by BF2doping contributes to increased sheet resistance and thermalstability, and is expected to be used in ultra large scale integra-tion (ULSI) devices47. In these cases, the formation of voidsand segregants around voids has a positive effect on improv-6Er++Er+++Pt+++(a)(b)(c)(d)Count (arb.unit)Mass to Charge State Ratio (Da)Including Er segregationExcluding Er segregation50 60 70 80 90(FePt)-Er0.35(FePt)-Er0.35FIG. 5. 3D atom probe tomography reconstruction for 0.35 at.% Er-doped FePt-Er film. (a) Fe, Pt, Mg, and O atoms displayed as red,green, purple, and blue dots, respectively; (b) Er atoms displayed as pink dots; (c) Er atoms displayed as pink dots from the top view; (d)Mass spectrum from the entire FePt-Er film including Er segregation displayed as a blue line and the region excluding Er segregation at grainboundaries and the FePt/MgO interface displayed as a black line.ing properties, and it may not be necessary to suppress them.However, void formation can generally be controlled by opti-mizing the amount of additives and the heat treatment process.As the heat-treatment temperature was fixed at 500 ◦C in thisstudy, further improvements may be required. This is an issuefor future research.Regarding the magnetic properties of the FePt-Er films, itcan be surmised that at 300 K, the µ0Ms and Ku values of pureL10-FePt grains and L10-FePtEr near voids/grain boundariesand the MgO/FePt interface should differ. Despite the pres-ence of disparate Ms and Ku values, the single-phase behaviorobserved in the magnetization curves of the Er-doped samplesdepicted in Fig. 3(b) may be attributed to the exchange cou-pling between the two phases. In composites with two µ0Msand Ku values, the magnetization curve behaves in a single-phase manner, and the µ0Ms and Ku are averaged out; if theexchange coupling between the two phases is sufficiently ac-tive, is experimentally real48,49 and theoretically explained50.Therefore, it is possible that the averaged µ0Ms and Ku ofL10-FePt and L10-FePtEr may be present in Fig. 3, whereasµ0Ms and Ku in the L10-FePtEr single-phase are expected tobe larger than those in Fig. 3. To evaluate the intrinsic mag-netic properties, a continuous film sample was analyzed. Mi-crostructural analysis of this film showed that Er was pref-erentially segregated around the void/grain boundaries andMgO/FePt interface.C. DynamicsFinally, in order to ascertain the damping constant of a sam-ple with 0.35 at.% Er, TRMOKE measurements were con-ducted. Figs.7(a) and 7(b) show the TRMOKE data in thetime- and frequency-domains,respectively, measured at roomtemperature. Time-domain data were analyzed using the leastsquares method through the following equation, as shown bythe red curves in Fig. 7(a):∆ϕK =A0 +A1 exp(−ν ·∆t)+B0 exp(−∆tτ)sin(2π f ·∆t +δ0) ,(1)The first and second terms are background signals, with anoffset A0, intensity of magnetization recovery process A1, andits recovery rate ν . The third term with a precession amplitudeB0, decay time τ , frequency f , and initial phase δ 0 describesthe magnetization precessional motion. The effective damp-ing constant αeff can be evaluated using αeff =12 π f τ . The pre-7(a)(b)5nmvoid5nmGBGBGBFePt-Er FePtvoidvoid(b) (c)(d)(e) (f) (g) (h)(FePt)-Er0.35(FePt)-Er0.35FIG. 6. (a) HAADF-STEM image of FePt-Er film with 0.35 at.% Er added, observed from the top view; (b) EDS elemental map of FePt inthe field of view of (a); (c) EDS elemental map of Er; (d) EDS line profiles of Fe, Pt, and Er corresponding to the white area in (c), wherethe upper and lower graphs correspond to the intensity and atomic concentration, respectively; (e) HAADF-STEM image observed from thetop view in another area, (f) EDS elemental map of FePt in the field of view of (e); (g) EDS elemental map of Er; (h) EDS line profiles ofFe, Pt, and Er corresponding to the white area in (g), where the upper and lower graphs correspond to the intensity and atomic concentration,respectively.0 10 20 30Δt (ps)0 200 400 600f (GHz)0102030400123Δφk (a.u.)I FFT (a.u.)(a)(b)(FePt)-Er0.35FePt(FePt)-Er0.35FePtFIG. 7. (a) Time-domain TRMOKE data, (b) frequency-domaindata for FePt thin films without Er and with a 0.35 at.% Er addition.cession frequency increased from f = 203 GHz to 220 GHz byintroducing 0.35 at.% of Er for the FePt film, which is consis-tent with the increase of perpendicular magnetic anisotropy.The αeff for the FePt film without Er was estimated to be0.17 which is nearly equal to a FePt continuous film withlarge extrinsic contributions owing to the two-magnon scat-tering and PMA distribution in the film measured at 300 K23.However, the αeff for the FePt film with 0.35 at.% Er was de-creased to 0.10. In General, Gilbert damping constants arisefrom electronic transitions due to spin-orbit interactions51,52.Er belongs to the heavy rare earth group, which contains asubstantial number of electrons in the f-orbital. Consequently,the spin-orbit interaction should be considerable. There havebeen experimental instances where the damping constant hasbeen observed to increase with the addition of rare earthelements53–55.However, due to the large extrinsic contributions to the αeff,it is difficult to discuss the intrinsic damping modification ow-ing to the electrons in the f-orbital. Therefore, it was consid-ered that the decrease in αeff by Er-doping may be attributedto extrinsic contributions modified by microstructural alter-ations, and magnetic di-polar and exchange coupling changes.Further detailed investigations are required to separate the ex-ternal factors from the intrinsic Gilbert damping constant αby measuring the angular dependence of the applied magnetic8field.IV. CONCLUSIONFePt-X (X = Er, Tm) films were deposited using a combi-natorial high-throughput method. This method wass demon-strated to facilitate a comprehensive investigation of the de-pendence of µ0Ms, µ0Hk and TC on the concentration of X.These findings indicate that the doping with a small amountof Er (0.35 at.%) enhanced µ0Ms at 300 K from 1.39 T to1.54 T, the Ku at 300 K from 5.63 MJ/m3 to 6.43 MJ/m3, andthe TC from 679 K to 716 K. TEM observations and 3D atomprobe analysis demonstrated that Er was segregated at approx-imately 1 at.% around voids, grain boundaries, and near theMgO substrate/FePt interface. It was assumed that Er was notsolidl-soluble in the FePt grains and that the L10-FePt grainsand L10-FePtEr were mixed in a complex manner on the or-der of tens of nm. The EDS line profiles from the top-viewTEM images showed that Er preferentially replaced the Fesites. Furthermore, Er-doping of FePt films had a significanteffect on the damping constant.In this study, the intrinsic magnetic properties of a contin-uous film sample were ealuated. Based on microstructuralanalysis, it was found that Er was preferentially segregatedaround the void/grain boundaries and the MgO/FePt interface.For HAMR applications with an FePt granular structure, moreinterfaces should exist than in continuous films; therefore, itis expected that Er will exist preferentially and that granularfilms of FePt core material with improved µ0Ms can be pro-duced, contributing to further density increases.V. ACKNOWLEDGMENTSThe authors thank T. Hiroto, S. Nimori (NIMS) for their as-sistance with the use of the experiment machines. The authorsalso thank K. Suzuki (NIMS) for her contribution in conduct-ing experiments. 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