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Budai, N.*; 一色 弘成; 大谷 義近
Physical Review Applied (Internet), 25(3), p.L031002_1 - L031002_7, 2026/03
被引用回数：1 パーセンタイル：0.00(Physics, Applied)We present a magnetic imaging technique to investigate materials with perpendicular magnetic anisotropy, using a conventional atomic force microscope (AFM). In this method, the Si tip acts as a thermal anchor, generating localized in-plane temperature gradients in a heated nanowire. The resulting anomalous Nernst effect (ANE) is measured at both ends of the wire. For uniform out-of-plane magnetization, the anomalous Nernst signal reverses the sign as the tip scans from edge to edge, reflecting the direction of the local temperature gradient. By simulating this gradient, the coefficient of the ANE can be quantitatively determined. Furthermore, this technique enables the visualization of magnetic multi-domain structures. This method offers a simple, quantitative, and practical approach to magnetic imaging using widely available AFM setups.
一色 弘成; 鈴木 和也; 今井 正樹; Dong, Z.*; 中堂 博之; 大谷 義近; 高梨 弘毅; 家田 淳一
High Entropy Alloys & Materials, 8 Pages, 2026/00
High-entropy alloys, defined as single-phase solid solutions composed of five or more elements, have attracted considerable attention for their exceptional mechanical properties. However, their electronic and magnetic transport properties remain largely unexplored. In this study, we focus on the anomalous Nernst effect, a magnetic thermoelectric phenomenon, in FeCoNi-based ferromagnetic high-entropy alloy thin films at room temperature. We systematically examined the influence of heavy-element concentration in FeCoNiCuPd. Increasing the Pd content to 10, 20, and 30 at% led to anomalous Nernst conductivities of 0.53, 0.61, and 0.74 A m
K, respectively, attributable to the strong spin-orbit coupling of Pd. These results indicate that adding heavy elements effectively enhances transverse transport in high-entropy alloys. To further enhance the anomalous Nernst effect, we introduced an even heavier element, Pt, into the alloy. The resulting FeCoNiPdPt thin film exhibited a high anomalous Nernst conductivity of approx. 0.97 A m K together with a large anomalous Nernst thermopower of approx. 0.9 uV K, demonstrating the thermoelectric potential of these alloys.
Hwang, Y.*; Puebla, J.*; 近藤 浩太*; Gonzalez-Ballestero, C.*; 一色 弘成*; S
nchez Mu
oz, C.*; Liao, L.*; Chen, F.*; Luo, W.*; 前川 禎通*; et al.
Physical Review Letters, 132(5), p.056704_1 - 056704_7, 2024/01
被引用回数：54 パーセンタイル：98.30(Physics, Multidisciplinary)Here, we report the observation of strong coupling between magnons and surface acoustic wave (SAW) phonons in a thin CoFeB film constructed in an on-chip SAW resonator by analyzing SAW phonon dispersion anticrossings. We employ a nanostructured SAW resonator design that, in contrast to conventional SAW resonators, allows us to enhance shear-horizontal strain. Crucially, this type of strain couples strongly to magnons. Our device design provides the tunability of the film thickness with a fixed phonon wavelength, which is a departure from the conventional approach in strong magnon-phonon coupling research. We detect a monotonic increase in the coupling strength by expanding the film thickness, which agrees with our theoretical model. Our work offers a significant way to advance fundamental research and the development of devices based on magnon-phonon hybrid quasiparticles.
Lyons, T. P.*; Puebla, J.*; 山本 慧; Deacon, R. S.*; Hwang, Y.*; 石橋 幸治*; 前川 禎通*; 大谷 義近*
Physical Review Letters, 131(19), p.196701_1 - 196701_6, 2023/11
被引用回数：35 パーセンタイル：94.13(Physics, Multidisciplinary)Harnessing the causal relationships between mechanical and magnetic properties of van der Waals materials presents a wealth of untapped opportunity for scientific and technological advancement, from precision sensing to novel memories. This can, however, only be exploited if the means exist to efficiently interface with the magnetoelastic interaction. Here, we demonstrate acoustically-driven spin-wave resonance in a crystalline antiferromagnet, chromium trichloride, via surface acoustic wave irradiation. The resulting magnon-phonon coupling is found to depend strongly on sample temperature and external magnetic field orientation, and displays a high sensitivity to extremely weak magnetic anisotropy fields in the few mT range. Our work demonstrates a natural pairing between power-efficient strain-wave technology and the excellent mechanical properties of van der Waals materials, representing a foothold towards widespread future adoption of dynamic magneto-acoustics.
Liao, L.*; Puebla, J.*; 山本 慧; Kim, J.*; 前川 禎通*; Hwang, Y.*; Ba, Y.*; 大谷 義近*
Physical Review Letters, 131(17), p.176701_1 - 176701_6, 2023/10
被引用回数：16 パーセンタイル：82.06(Physics, Multidisciplinary)Phonons and magnons are engineered by periodic potential landscapes in phononic and magnonic crystals, and their combined studies may enable valley phonon transport tunable by the magnetic field. Through nonreciprocal surface acoustic wave transmission, we demonstrate valley-selective phonon-magnon scattering in magnetoelastic superlattices. The lattice symmetry and the out-of-plane magnetization component control the sign of nonreciprocity. The phonons in the valleys play a crucial role in generating nonreciprocal transmission by inducing helical strains that couple with the magnons. The transmission spectra show a nonreciprocity peak near a transmission gap, matching the phononic band structure. Our results open the way for manipulating valley phonon transport through periodically varying magnon-phonon coupling.
Puebla, J.*; Hwang, Y.*; 前川 禎通*; 大谷 義近*
Applied Physics Letters, 120(22), p.220502_1 - 220502_9, 2022/05
被引用回数：64 パーセンタイル：94.83(Physics, Applied)Surface acoustic waves (SAWs) are elastic waves propagating on the surface of solids with the amplitude decaying into the solid. The well- established fabrication of compact SAW devices, together with well-defined resonance frequencies, places SAWs as an attractive route to manipulate the magnetization states in spintronics, all of which is made possible by the magnetostriction and magnetoelastic effects. Here, we review the basic characteristics of SAW devices and their interaction out-of-resonance and in-resonance with the magnetization in thin films. We describe our own recent results in this research field and closely related works and provide our perspectives moving forward.
山本 慧; Xu, M.*; Puebla, J.*; 大谷 義近*; 前川 禎通*
Journal of Magnetism and Magnetic Materials, 545, p.168672_1 - 168672_10, 2022/03
被引用回数：31 パーセンタイル：76.00(Materials Science, Multidisciplinary)We present a theoretical description of magnon-phonon interactions in a multi-layer structure containing a ferromagnetic thin film. The formalism is applicable to an arbitrary direction of external magnetic field and various types of acoustic waves including Rayleigh and Love surface modes. A particular attention is paid to the spatial profile of the acoustic wave modes and analytical expressions for the effective coupling coefficients are derived taking into account the degree of mode profile overlap between spin waves and acoustic waves. The results are applied to reproduce a strongly anisotropic and non-reciprocal linewidth of acoustic ferromagnetic resonance reported in a recent experiment.
Xu, M.*; 山本 慧; Puebla, J.*; Baumgaertl, K.*; Rana, B.*; 三浦 勝哉*; 高橋 宏昌*; Grundler, D.*; 前川 禎通*; 大谷 義近*
Science Advances (Internet), 6(32), p.eabb1724_1 - eabb1724_4, 2020/08
被引用回数：176 パーセンタイル：98.30(Multidisciplinary Sciences)One of the most fundamental forms of magnon-phonon-interaction is an intrinsic property of magnetic materials, the "magnetoelastic coupling." This particular form of interaction has been the basis for describing magnetic materials and their strain related applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicular anisotropic ultra-thin lim Ta/CoFeB/MgO, which consequently induces nonreciprocal acoustic wave attenuation with an unprecedented ratio up to 100% rectification at a theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling, but also justifies the feasibility of the magneto-rotation coupling based application.
Puebla, J.*; Xu, M.*; Rana, B.*; 山本 慧; 前川 禎通*; 大谷 義近*
Journal of Physics D; Applied Physics, 53(26), p.264002_1 - 264002_7, 2020/06
被引用回数：52 パーセンタイル：87.90(Physics, Applied)Voltage induced magnetization dynamics of magnetic thin films is a valuable tool to study anisotropic fields, exchange couplings, magnetization damping and spin pumping mechanism. A particularly well established technique is the ferromagnetic resonance (FMR) generated by the coupling of microwave photons and magneti- zation eigenmodes in the GHz range. Here we review the basic concepts of the so-called acoustic ferromagnetic resonance technique (a-FMR) induced by the coupling of surface acoustic waves (SAW) and magnetization of thin films. Interestingly, additional to the benefits of the microwave excited FMR technique, the coupling be- tween SAW and magnetization also offers fertile ground to study magnon-phonon and spin rotation couplings. We describe the in-plane magnetic field angle dependence of the a-FMR by measuring the absorption / trans- mission of SAW and the attenuation of SAW in the presence of rotational motion of the lattice, and show the consequent generation of spin current by acoustic spin pumping.
Yagmur, A.*; 軽部 修太郎*; 内田 健一*; 近藤 浩太*; 井口 亮*; 吉川 貴史*; 大谷 義近*; 齊藤 英治
Applied Physics Letters, 108(24), p.242409_1 - 242409_4, 2016/06
被引用回数：9 パーセンタイル：35.03(Physics, Applied)The longitudinal spin Seebeck effect (SSE) in Bi 
O
/Cu/yttrium-iron-garnet (YIG) devices has been investigated. When an out-of-plane temperature gradient is applied to the BiO/Cu/YIG device, a spin current is generated across the Cu/YIG interface via the SSE and then converted into electric voltage due to the spin
O/Cu interface. The sign of the SSE voltage in the BiO/Cu/YIG devices is opposite to that induced by the conventional inverse spin Hall effect in Pt/YIG devices. The SSE voltage in the BiO/Cu/YIG devices disappears in the absence of the BiOlayer and its thermoelectric conversion efficiency is independent of the Cu thickness, indicating the important role of the BiO/Cu interface. This result demonstrates that not only the bulk inverse spin Hall effect but also the spin-orbit coupling near the interface can be used for SSE-based thermoelectric generation.
新見 康洋*; 木俣 基*; 大森 康智*; Gu, B.; Ziman, T.*; 前川 禎通; Fert, A.*; 大谷 義近*
Physical Review Letters, 115(19), p.196602_1 - 196602_5, 2015/11
被引用回数：14 パーセンタイル：62.61(Physics, Multidisciplinary)We have measured spin Hall effects in spin glass metals, CuMnBi alloys, with the spin absorption method in the lateral spin valve structure. Far above the spin glass temperature 
where the magnetic moments of Mn impurities are randomly frozen, the spin Hall angle of a CuMnBi ternary alloy is as large as that of a CuBi binary alloy. Surprisingly, however, it starts to decrease at about 4 and becomes as little as 7 times smaller at 0.5. A similar tendency was also observed in anomalous Hall effects in the ternary alloys. We propose an explanation in terms of a simple model considering the relative dynamics between the localized moment and the conduction electron spin.
井土 宏*; 福間 康裕*; 高橋 三郎*; 前川 禎通; 大谷 義近*
Physical Review B, 89(8), p.081308_1 - 081308_5, 2014/02
被引用回数：26 パーセンタイル：67.18(Materials Science, Multidisciplinary)We have succeeded in fully describing dynamic properties of spin current including the different spin absorption mechanisms for longitudinal and transverse spins in lateral spin valves, which enable one to elucidate intrinsic spin transport and relaxation mechanisms in the nonmagnet. The deduced spin lifetimes are found independent of the contact type. From the transit-time distribution of spin current extracted from the Fourier transform in Hanle measurement data, the velocity of the spin current in Ag with Py/Ag Ohmic contact turns out much faster than that expected from the widely used model.
Wei, D. H.*; 新見 康洋*; Gu, B.; Ziman, T.*; 前川 禎通; 大谷 義近*
Nature Communications (Internet), 3, p.1058_1 - 1058_5, 2012/09
被引用回数：41 パーセンタイル：81.37(Multidisciplinary Sciences)The spin Hall effect (SHE) and its inverse have key roles in spintronic devices as they allow conversion of charge currents to and from spin currents. The conversion efficiency strongly depends on material details, such as the electronic band structure and the nature of impurities. Here we show an anomaly in the inverse SHE in weak ferromagnetic NiPd alloys near their Curie temperatures with a shape independent of material details, such as Ni concentrations. By extending Kondo's model for the anomalous Hall effect (AHE), we explain the observed anomaly as originating from the second-order nonlinear spin fluctuation of Ni moments. This brings to light an essential symmetry difference between the SHE and the AHE, which reflects the first-order nonlinear fluctuations of local moments. Our finding opens up a new application of the SHE, by which a minuscule magnetic moment can be detected.
福間 康裕*; Le, W.*; 井土 宏*; 高橋 三郎; 前川 禎通; 大谷 義近*
Nature Materials, 10(7), p.527 - 531, 2011/07
被引用回数：169 パーセンタイル：96.99(Chemistry, Physical)The non-local spin injection in lateral spin valves is strongly expected to be an effective method to generate a pure spin current for potential spintronic application. However, the spin-valve voltage, which determines the magnitude of the spin current flowing into an additional ferromagnetic wire, is typically of the order of 1
V. Here we show that lateral spin valves with low-resistivity NiFe/MgO/Ag junctions enable efficient spin injection with high applied current density, which leads to the spin-valve voltage increasing 100-fold. Hanle effect measurements demonstrate a long-distance collective 2
spin precession along a 6-m-long Ag wire. These results suggest a route to faster and manipulable spin transport for the development of pure spin-current-based memory, logic and sensing devices.
一色 弘成; 鈴木 和也; 高梨 弘毅; 今井 正樹; 家田 淳一; 大谷 義近
no journal, ,
The anomalous Nernst effect (ANE) is one of the magneto-thermoelectric effects, where a temperature gradient applied to a ferromagnetic conductor induces an electric field in the direction orthogonal to both the magnetization and the temperature gradient. This effect has attracted considerable attention, as it enables the conversion of heat into electric energy with a simple device structure. To realize higher conversion efficiency, materials with large anomalous Nernst coefficients (SANE [μV/K]) have been explored. Although magnetic topological materials exhibit giant ANE with SANE = 7μV/K, no material has yet been found with sufficient performance to meet the demands of practical heat energy harvesting. Here, we focus on ferromagnetic high-entropy alloys (HEAs) as a new class of materials hosting ANE. These alloys, composed of five or more elements, form stable single-phase solid solutions due to high configurational entropy. While HEAs have been intensively investigated for their mechanical properties, studies on their electronic transport remain limited. In this study, we investigated the anomalous Nernst effect in FeCoNi-based high-entropy alloy thin films. Thin films of FeCoNiCuPd, FeCoNiCuPt, and FeCoNiPdPt with a thickness of 30 nm were deposited on synthetic quartz substrates via sputtering. X-ray analysis confirmed the formation of an fcc structure. First, we changed the Pd composition rate (X) in Fe20Co20Ni20Cu40-xPdx to examine the impact of the heavy element on their ANEs. For the measurement, as illustrated in Fig. 1(a), an in-plane temperature gradient∇xT was applied, and a magnetic field Hz was swept in the out-of-plane direction while the transverse voltage Vy was measured. Figure 1(b)shows the field dependence of Vy for Fe20Co20Ni20Cu40-xPdx with x = 10,20, and 30. In all samples, Vy saturated at approximately 1 T, and the maximum value was nearly constant across compositions, yielding SANE ~ 0.41μV/K. The coefficient can be expressed as SANE =αANEρxx-θAHESSE, where αANE,ρxx,θAHE, and SSE are anomalous Nernst conductivity, longitudinal resistivity, anomalous Hall angle, and Seebeck coefficient, respectively. The first term represents the direct electric field generation from the temperature gradient, and the second term does the contribution from the anomalous Hall effect via the Seebeck effect. Our analysis revealed that, although SANE remained nearly constant, αANE increased by 0.53, 0.61, and 0.74 Am-1K-1 with increasing the Pd ratio x = 10, 20, and 30. This indicates that αANE is enhanced by increasing Pd concentration due to its strong spin-orbit coupling. Subsequently, we examined FeCoNiCuPt and FeCoNiPdPtthin films to further enhance SANE. As shown in Fig. 1(c), the addition of Pt increased SANE to ~0.9μV/K, withαANE reaching 0.96 Am-1K-1. In contrast, the Seebeck coefficient decreased to about two-thirds that of FeCoNiCuPd, limiting the contribution from the term of -θAHESSE to SANE. These results demonstrate that in HEAs,αANE,ρxx,θAHE, and SSE can be tuned by substituting elements and changing the composition rate. Future work will aim to identify the optimal composition to maximize SANE.
一色 弘成; 鈴木 和也; 今井 正樹; 中堂 博之; 大谷 義近; 高梨 弘毅; 家田 淳一
no journal, ,
ハイエントロピー合金は、優れた機械的性質から構造材料として研究されてきたが、最近、その電子物性にも注目が集まっている。本研究では、FeCoNi系の強磁性ハイエントロピー合金薄膜を作製し、異常ネルンスト効果を室温で測定した。合金中の重元素の含有量が増えることで、異常ネルンスト伝導度が上昇することがわかった。
一色 弘成; 鈴木 和也; 今井 正樹; 小野 凌太; 森 道康; 中堂 博之; 大谷 義近; 高梨 弘毅; 家田 淳一
no journal, ,
Metallic thermoelectric materials generally exhibit smaller Seebeck coefficients S than semiconductors; however, owing to their high electrical conductivity, they can achieve large power factors PF. It is well known that the Seebeck coefficient S strongly depends on the energy derivative of the density of states (DOS) near the Fermi level. Indeed, NiFe alloys with appropriate compositions have been reported to exhibit very large S and PF due to the formation of a steep DOS originating from the Ni 3d orbitals near the Fermi level. However, in many cases, such binary alloys suffer from solid-solubility limits, which restrict the freedom of materials design. Therefore, we focused on high-entropy alloys (HEAs) based on the 3d transition metals Fe, Co, and Ni. HEAs consist of five or more elements and form single-phase solid solutions owing to the configurational entropy effect. This enables highly flexible design of thermoelectric materials, as elemental selection and continuous compositional variation allow systematic tuning of multiple transport parameters-including the Seebeck coefficient, electrical conductivity, and thermal conductivity-thereby enabling performance optimization. Furthermore, because Fe, Co, and Ni are ferromagnetic, magnetic transport phenomena can emerge, making these materials promising for spintronic applications. In the experiment, 30-nm-thick films of FeCoNi-based HEAs were fabricated on synthetic quartz substrates by sputtering. X-ray diffraction confirmed the formation of an fcc structure. First, in Fe
CoNiCu
Pd
, the Pd composition (x) was varied to investigate the effect of heavy elements on the anomalous Nernst effect (ANE). The coefficient of the anomalous Nernst effect SANE was nearly independent of the composition ratio and remained SANE approx. -0.41 uV k. In contrast, the anomalous Nernst conductivity aANE was estimated to be 0.53, 0.61, and 0.74 A m-1 k for x = 10, 20 and 30, respectively, indicating that aANE is enhanced with increasing Pd concentration as Pd possesses strong spin-orbit coupling. In FeCoNiPdPt, where Cu was replaced by Pt, SANE increased to approximately 0.9 uV k and aANE reached 0.96 A m-1 k. These results suggest that the anomalous Nernst effect can be systematically enhanced in HEAs by adding heavy elements. The Seebeck coefficients S of FeCoNi, FeCoNiCuPd, and FeCoNiPdPt were -30, -18, and -12 uV k, respectively. Interestingly, the magnitude of S decreased as the concentration of heavy elements increased. First-principles calculations based on the Coherent Potential Approximation (CPA) method were performed to evaluate the DOS. The results showed that bands derived from Ni 3d, Pd 4d, and Pt 5d orbitals accumulate near the Fermi level, which flattens the DOS and consequently reduces S. These insights will be useful for the selection of appropriate elements and compositions to enhance both the Seebeck and anomalous Nernst effects.
SiTeの擬一次元電子状態一色 弘成; 木俣 基; 大谷 義近
no journal, ,
TaSiTeは、室温以下で極めて大きな熱電パワーファクターを示す擬一次元ファンデルワールス物質である。しかし、試料がウィスカー状結晶としてのみ得られるため、この大きな熱電性能の背後にある異方的な電子状態の実験的検証は困難であった。本研究では、マイクロスポット角度分解光電子分光(micro-ARPES)と、集束イオンビーム(FIB)加工微小デバイスを用いた電気輸送測定を組み合わせ、TaSiTeの電子構造と輸送の異方性を定量的に評価した。ARPES測定により、結晶c軸方向に急峻な分散とそれに垂直な方向に平坦な分散を有する異方的なバンド構造を観測した。輸送測定では200Kにおいて、c軸平行・垂直方向の電気抵抗率の比は約5倍であることを明らかにした。この結果は、TaSiTeが適度な輸送異方性を有する弱い一次元電子系であることを示している。また、ホール効果測定の解析からおよそ0.14eVのバンドギャップが見積もられた。第一原理計算では、ディラック的な分散がスピン-軌道相互作用によって開いた結果ナローギャップが生じることが報告されており、本実験結果はこれとよく一致している。本実験で明らかになった電子構造が、TaSiTeの高移動度と大きなゼーベック係数の両立を可能にし、巨大熱電性能の発現を支えていると考えられる。
