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Zhiyong Dai, Chao Zhou, Chenyang Guo, Kang Cao, Ruisheng Zhang, Tieyan Chang, [Yoshitaka Matsushita](https://orcid.org/0000-0002-4968-8905), Adil Murtaza, Fanghua Tian, Wenliang Zuo, Yin Zhang, Sen Yang, Xiaoping Song

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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 Zhiyong Dai, Chao Zhou, Chenyang Guo, Kang Cao, Ruisheng Zhang, Tieyan Chang, Yoshitaka Matsushita, Adil Murtaza, Fanghua Tian, Wenliang Zuo, Yin Zhang, Sen Yang, Xiaoping Song; Giant enhancement of magnetostriction in Pt doped FeGa ribbons. Appl. Phys. Lett. 21 August 2023; 123 (8): 082402. and may be found at https://doi.org/10.1063/5.0151431.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Giant enhancement of magnetostriction in Pt doped FeGa ribbons](https://mdr.nims.go.jp/datasets/574dae17-abe0-4a16-b24f-604179599781)

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Giant enhancement of magnetostriction in Pt doped FeGa ribbonsZhiyong Dai1,#, Chao Zhou1,#,*, Chenyang Guo1, Kang Cao1, Ruisheng Zhang1, Tieyan Chang2, Yoshitaka Matsushita3, Adil Murtaza1, Fanghua Tian1, Wenliang Zuo1, Yin Zhang1, Sen Yang1,*, Xiaoping Song1School of Physics, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an 710049, China2 NSF’s ChemMatCARS, The University of Chicago, Lemont, Illinois 60439, USA3National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan AbstractFeGa alloys are among the most promising magnetostrictive materials. How to improve the magnetostriction of FeGa has been a long-term concern. In this work, trace amount of Pt doped FeGa - (Fe0.83Ga0.17)100-xPtx (x=0, 0.2, 0.4 and 0.6) were fabricated using melt-spinning technique. The magnetostriction, crystal structure and magnetocrystalline anisotropy of the ribbon samples were studied, revealing that the conforming between the crystal growth direction and the easy magnetization axis, and the enlarged lattice parameter both contribute to the enhancement of magnetostriction, which reached the maximum  of -1369 ppm at the optimum composition x=0.4. Finally, transmission electron microscopy was performed to show that, nanoinclusions exist not only in Pt-doped but also in Pt-free samples, demonstrating that the nanoinclusions do not play the dominant role in magnetostriction improvement. This work may accelerate the design of highly magnetostrictive FeGa alloys and other magnetic functional materials which are required of large magnetostriction. Keywords: magnetostriction; FeGa; magnetocrystalline anisotropy; crystal growth direction; nanoinclusion._____________________________Authors to whom correspondence should be addressed: chao.zhou@xjtu.edu.cn; yang.sen@xjtu.edu.cn; #Zhiyong Dai and Chao Zhou contributed equally to this work.Magnetostrictive materials are widely used in actuators, sensors, and transducers. Compared with intermetallic Terfenol-D alloys, FeGa alloy has attracted much interest in the last decades because of good mechanical properties, lower cost, high Curie temperature (Tc > 923 K), good thermal stability and high permeability [1, 2]. However, their magnetostriction (usually 120~240 ppm for rods or sheets of polycrystals) is still not high enough to become an alternative to Terfenol-D alloys (usually 800~1200 ppm) [2]. Then ribbons of FeGa alloy were found to have giant magnetostriction  along the ribbon length when the magnetic field was perpendicular to the plane of ribbons [3, 4]. For long, the introduction of nanoinclusions via trace amount of rare-earth doping, such as Tb [5], Dy [6], La [7], Ce [8], has been regarded as one of the few effective routes to improve the magnetostriction of FeGa ribbons. The investigation of microstructure reveals that tetragonally modified D03 (m-D03) nanoinclusions are largely responsible for the enhanced, heterogeneous magnetostriction [7, 9, 10]. Density Functional Theory (DFT) calculation and tests of magnetization versus temperature indicates that exchange couplings between the nearest Tb-Fe or Tb-Tb pairs of the tetragonal nanoinclusions might generate strong localized magnetocrystalline anisotropy [11]. By phase field model, it is concluded that a mixture of cubic matrix and tetragonal coherent nanoinclusions exhibits larger magnetostriction than cubic matrix [7, 12, 13]. However, it is hard to explain why quite few localized nanoinclusions results in the change of volume effect (magnetostriction).Recently, it was reported that the doping of Pt into FeGa, can tune the crystal growth direction along the easy magnetization axis, thus the magnetostriction is greatly improved via 90° domain switching [14]. Nevertheless, those samples only undergo as-cast process or directional solidification, so the impact from the solidification process was inadvertently ignored, although it should have been considered. In this research, we added trace amount of Pt element to rapid solidification of FeGa alloy. The magnetostriction is greatly enhanced, which is close to the effects of trace amount of rare-earth doping. The magnetic properties and microstructure are investigated, and the origin of magnetostriction enhancement is discussed. Alloying of high-purity constituents (Fe 99.95%, Ga 99.95%, Pt 99.99%) with nominal compositions of (Fe0.83Ga0.17)100-xPtx (0 ≤ x ≤ 0.6), was conducted using arc melting in an argon atmosphere. The ingots were melted four times to get chemical homogeneity. Then, the ribbons were prepared by single roller melt-spinning method with a linear velocity of 12 m/s in an argon atmosphere. Each cut-out sample of ribbons is ~ 120 μm thick, 5 mm wide, and 12 mm long.Magnetostriction was measured at 300 K using the standard strain gauge method with a magnetic field perpendicular to the plane of ribbons, as shown in supplementary figure S1. The magnetic field is up to 20 kOe. Since the thickness of the ribbons is very thin, about 120 μm, and the parallel magnetostrictions  is difficult to be tested, we used  as a characterization of magnetostrictive properties. X-ray diffraction (XRD, Bruker D8-ADVANCE) analysis with Cu Kα radiation (λ = 0.15406 nm) at room temperature was used for crystal structure characterization. The magnetic hysteresis loops were conducted at 300 K by using superconducting quantum interference device (SQUID, Quantum Design, MPMS-3). The magnetic field of up to 15 kOe was in the plane of ribbons when measured. The sample morphology and structure were observed at room temperature through transmission electron microscopy (TEM, JEOL JEM-2100). Sample foils for TEM observation, with a diameter of 3 mm, were punched out and two-jet thinning operated at 243 K and a voltage of 25 V in an electrolyte of 5 vol.% perchloric acid and 95 vol.% methanol. The Selected Area Diffraction (SAED) patterns was labeled by CrysTBox [15]. The high-resolution TEM (HRTEM) images were analyzed using DigitalMicrograph software by Gatan.Fig. 1(a) shows the magnetostriction  of (Fe0.83Ga0.17)100-xPtx ribbons (x=0, 0.2, 0.4, 0.6) at 300 K. The magnetostriction  of Fe83Ga17 ribbon is -400 ppm, which is consistent with the previous study [3], and between -800 ppm [16] and -220 ppm [9, 17]. For FeGa system with random texture configuration, magnetostriction measured parallel to the field  () is positive. Because of the Joule magnetostrictive behavior in FeGa alloys [18, 19], the linear magnetostriction is volume-conserving () and the magnetostriction perpendicular to the field, , is negative. It is noticed that the magnetostriction  of Fe83Ga17 ribbon is larger than magnetostriction constants of bulk single crystals 20 and thin films [20, 21]of Fe83Ga17 alloy. This can be understood as follows. Ribbons are composites of disordered BCC (A2) matrix and nano modified-D03 phase. Nano modified-D03 phases are coherent in the matrix and aids to enhance lattice distortion in the matrix, thus enhancing magnetostriction [7]. Moreover, the strong shape anisotropy of ribbons forces the magnetic domains magnetized in directions closer to the ribbon plane as much as possible, whereas the deviation between the orientation (statistical) of most domains and the normal direction of the ribbon plane, can be tuned by trace amount doping of elements, as observed in the present study (Fig. 2) which will be discussed later in this work. When the applied field is perpendicular to the field, nearly all magnetic domains rotate to the direction parallel to the field, so the magnetostriction of ribbon samples is usually larger than that of the bulk single crystals with the same composition. Thin films of FeGa alloys are normally composed of nano polycrystalline, which are random in-plane textures of polycrystals [22], so the magnetostriction of film sample is usually less than that of the ribbon sample with the same composition. The magnetostrictions  saturate at lower magnetic fields by doping Pt and exhibits low hysteresis when x is less than or equal 0.4. Saturated magnetostriction firstly increases, then decreases, and has a maximum at x = 0.4, as shown in Fig. 1(b). It is noteworthy that the magnetostriction has been enhanced from -391 ppm to -1369 ppm by doping 0.4% Pt. The enhancement is similar in magnitude to that reported in the literature for doping 0.2 % La (from -176 ppm to -650 ppm) [7]. As shown in Fig. 1(b), saturation field decreases a lot after doping 0.2% Pt and remains at a constant level with doping more Pt.Fig. 1. (Fe0.83Ga0.17)100-xPtx ribbons (x=0, 0.2, 0.4, 0.6). (a) magnetostriction hysteresis curves at 300K, (b) saturation magnetostriction and saturation field vs Pt content x. The magnetostriction was measured along the length direction of ribbons while magnetic field was perpendicular to the plane of ribbons.XRD patterns are performed to analyze the phase structure and crystal orientation of the ribbons. Fig. 2(a) presents XRD patterns of (Fe0.83Ga0.17)100-xPtx (x = 0, 0.2, 0.4, 0.6) ribbons. Only A2 phase, a disordered body-centered cubic structure, is detected and peaks are indexed as (1 1 0), (2 0 0), (2 1 1) planes. Trace amount of Pt shows good solid solubility in FeGa alloys, which is indicated in Supplementary Figure S2, unlike forming precipitates at grain boundaries in FeGa alloys as white areas in BSE images [23]. The calculated results of lattice parameters and relative intensity ratio I200/I110 of peaks (200) and (110) are shown in Fig. 2(b). The lattice parameters monotonically increase with Pt content x, also indicating good solid solubility of trace amount of Pt in FeGa alloys. Atom radius of Pt, 138.5 pm, is larger than Ga 135 pm, and Fe 126 pm [24]. Here the enhancement of the lattice parameter and the improvement of magnetostriction can be correlated, analogous to the solid solution of Tb in the A2 matrix [23, 25]. Enlarged lattice parameters give rise to large lattice distortion in the matrix and induces a larger magnetostrictive effect. The ratio I200/I110 firstly increases from 0.40 to 0.67 at x=0.2 and then decreases linearly, with Pt content further increasing. Because ribbons were quenched by the rotating copper roller, the growth directions of grains were perpendicular to the plane of ribbons. Higher ratio I200/I110 indicates stronger <100> texture perpendicular to the plane of ribbons, and the conforming between the crystal growth direction and the easy magnetization axis. More magnetization directions (i.e., <100> directions of grains) of magnetic domains are oriented in the ribbon plane. Then, when the magnetic field is perpendicular to the plane of ribbons, 90 degrees rotation of domains are more likely to occur. Thus, the magnetostrictions  perpendicular to the plane of the ribbons and  along the length direction of ribbons are both enhanced [9]. Stronger <100> texture induced by trace doping is also reported in other studies in Fe-17%Ga, such as 0.2% Tb [26], 0.42% Dy [6], 0.8% Ce [8], 0.32% Y [27], and the contribution of trace doping to enhancement of magnetostriction were also confirmed. When the tendencies of lattice parameter and ratio I200/I110 are both considered, the maximum point of magnetostriction should be 0.4, which is consistent with the experimental results. So, the increased <100> texture and the enlarged lattice parameter both contribute to the enhancement of magnetostriction. That is to say that the conforming between the crystal growth direction and the easy magnetization axis, and the enlarged lattice parameter both contribute to the enhancement of magnetostriction.Fig. 2. Melt-spun (Fe0.83Ga0.17)100-xPtx ribbons (x=0, 0.2, 0.4, 0.6). (a) X-ray diffraction patterns at 300K; (b) Lattice parameter and relative intensity ratio I200/I110 vs Pt content x.The in-plane magnetic hysteresis loops of (Fe0.83Ga0.17)100-xPtx ribbons (x=0, 0.2, 0.4, 0.6) are shown in Fig. 3(a). Magnetizations quickly saturate at ~ 2 kOe applied magnetic field and magnetic hysteresis are small, exhibiting a typical soft magnetic behavior. Magnetization saturation Ms and magnetocrystalline anisotropy energy K1 are derived from these curves according to the law of approach to ferromagnetic saturation [28] and shown in Fig. 3(b). In the disordered Fe100-xPtx alloys with x < 20, total magnetic moment obeys a law of mixing of atomic moments of Fe and Pt being 2.75 μB and 0.3 μB, respectively [29]. What's more, Pt atoms are heavier than Fe atoms. So, saturation magnetization Ms generally decreases with Pt content increasing and Fe content decreasing. However, Pt metal is an exchange-enhanced paramagnet [29] and α-Fe lattice expansion was accompanied by an increase in magnetic moment [30]. Consequently, magnetization saturation doesn’t decrease linearly with Pt doping. Here K1 of Fe-17%Ga is 36.5 kJ/m3, which is consistent with the previous study [31]. An earlier study reports a calculation of 100 kJ/m3 by minimizing energy of perpendicular magnetization [9]. Therefore, the law of approach to ferromagnetic saturation provides a better calculation here. At x = 0.4, K1 has a minimum value of 14.0 kJ/m3, which is analogous to Terfenol-D [32]. For Terfenol-D, Tb1-xDyxFe2 with x = 0.7 ~ 0.73, a minimum magnetocrystalline anisotropy occurs near room temperature [32-34]. The tendency of K1 of FeGa ribbons is different from the earlier study of (Fe0.83Ga0.17)100-xPtx bulk samples [14]. This difference may originate from preparation methods. Rapid solidification aids in the formation of nanostructure, which generally reduces magnetocrystalline anisotropy [35, 36]. The large spin-orbit coupling interaction of the Pt 5d electrons and Fe 3d electrons strongly influences the magnetic anisotropy [37, 38], and texture and crystal defects also influence the calculation of K1. So, K1 do not decrease monotonously. It is noteworthy that, the tendency of K1 is also different from earlier reports in Fe-Ga ribbons [7, 9]. Fig. 3 (Fe0.83Ga0.17)100-xPtx ribbons (x=0, 0.2, 0.4, 0.6). (a) in-plane magnetic hysteresis loops at 300K and the inset shows a zoom of the region of first quadrant less than 10 kOe. (b) saturation magnetization Ms and magnetocrystalline anisotropy energy K1 vs Pt content.It should also be noted that, the difference of saturation field of magnetostriction between Pt-free and Pt-doped FeGa alloys is greater than those between Pt-doped ones. This phenomenon can be understood as follows. By doping 0.2at% Pt, stronger <100> textures are obtained, which exhibit higher permeability than random oriented polycrystal [39], so the saturation field decreases sharply after trace amount of Pt is introduced into FeGa. Such regularity is consistent with FeGa alloys doped with Tb [40], Er [41]. With doping more Pt, the effect of stray field Hd should be considered: if a ribbon is approximately seen as an infinite plate, the saturation field of magnetostriction is mainly determined by stray field Hd which is equal to saturation magnetization Ms [42]. Because Ms changes not much with doping more Pt (as evidenced in Fig.3, ΔM/Ma<2.3%, where Ma denotes the average value for Ms of x=0.2, 0.4 and 0.6), the saturation field of magnetostriction remains at a constant level for 0.2≤x≤0.6.TEM results of (Fe0.83Ga0.17)100-xPtx ribbons (x=0, 0.4) are present in Fig. 4. From the SAED patterns in Fig. 4(a), (b), {1 0 0} superlattice reflections (calculated in the A2 crystal lattice) can hardly be observed, suggesting that ordering degree of the m-D03 structure is low or the density/domain size of m-D03 phase are quite small. Like the previous studies [43, 44], line intensity profiles between (2 0 0) and (-2 0 0) spots marked in Fig. 4(a), (b) are shown in Fig. 4(c), (d). Minor peaks of {1 0 0} superlattice reflections can be observed and are indicated by black arrows. However, their intensities are very weak, less than 1/100 of {2 0 0} spots of A2 phase. Regions where nanoinclusions exist are indicated by yellow dotted circles in Fig. 4 (e), (f). Nanoinclusions are so few in the matrix, and the low volume fraction observed in this work differ from the earlier report, 5% [9, 11]. The results explain why it cannot be detected by XRD in Fig. 2(a). The sizes of nanoinclusions in Fig. 4 (e), (f) are less than 5 nm, and thus better for enhancing magnetostriction than bigger sizes [13]. However, the volume fraction between nanoinclusions and the matrix remains extremely low after doping 0.4% Pt. Therefore, based on our results, we can not claim that the nanoinclusions play the key role in the enhancement of magnetostriction after the doping of Pt.Fig. 4 (Fe0.83Ga0.17)100-xPtx (x=0, 0.4) ribbons. (a), (b) SAED patterns along [001] zone axis at 300K. (c), (d) Line intensity profiles between (2 0 0) and (-2 0 0) spots marked in (a) and (b), respectively. The {1 0 0} superlattice diffraction spots are indicated by black arrows. (e), (f) HR-TEM images along [0 0 1] zone axis in (a), (b). Regions where nanoinclusions exist are indicated by yellow dotted circles. The method to distinguish regions is explained in Supplementary Figure S3.In conclusion, trace amount of Pt doping acts significantly on improving magnetostriction of Fagan alloy, as manifested in (Fe0.83Ga0.17)100-xPtx (x = 0, 0.2, 0.4, 0.6) ribbons, with the optimum composition x=0.4 showing the magnetostriction of -1369 ppm, 2.5 times that of the Pt-free sample. Experimental results of the crystal structure, lattice parameters, texture alignment, nanoinclusions and magnetocrystalline anisotropy reveal that the increased lattice parameter and the gradual conformation of the crystal growth direction align the easy magnetization axis with Pt doping, contribute mainly to the enhancement of magnetostriction of FeGa. SUPPLEMENTARY MATERIALSee supplementary material for the method of magnetostriction measurements, BSE images and the method to distinguish m-D03 regions.The data that support the findings of this study are available from the corresponding author upon reasonable request.This work was financially supported by the National Key R&D Program of China (2022YFE0109500, 2021YFB3501401), the Natural Science Foundation of Shaanxi Province (2023-JC-ZD-24), the Space Science and Application Project of China Manned Space Engineering (Grant No. KJZ-YY-NCL10), Key Scientific and Technological Innovation Team of Shaanxi Province (2020TD-001), Innovation Capability Support Program of Shaanxi (Nos. 2018PT-28, 2017KTPT-04), the Fundamental Research Funds for the Central Universities (China) and the World-Class Universities (Disciplines), the Characteristic Development Guidance Funds for the Central Universities (China).References[1] ZHANG H B. High-throughput design of magnetic materials [J]. 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