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松下 太樹*; 小沢 耀弘*; 荒木 康史; 藤本 純治*; 佐藤 昌利*
Physical Review B, 111(24), p.245131_1 - 245131_11, 2025/06
We investigate the intrinsic spin Nernst effect (SNE), a transverse spin current induced by temperature gradients, in topological Dirac semimetals (TDSMs) and magnetic Weyl semimetals (MWSMs) with Ising spin-orbit coupling. The intrinsic SNE is described by the spin Berry curvature, which reflects the geometric nature of TDSMs and MWSMs. We clarified that the intrinsic SNE becomes significant when the Fermi energy is near, but slightly deviates from, the energy of the point nodes. In this situation, Bloch electrons with strong spin Berry curvature contribute to the SNE while avoiding carrier compensation between electrons and holes. We found that in TDSMs with small Fermi surfaces, the spin Nernst angle, which measures the efficiency of the SNE, is larger than that observed in heavy metals. This suggests that TDSMs with small Fermi surfaces can achieve efficient heat-to-spin current conversion. In MWSMs, variation in the magnitude of the exchange coupling with magnetic moments significantly changes the SNE, affecting both the direction and magnitude of the spin Nernst current. This implies that ferromagnetic transitions can be used to reverse the spin Nernst current. These results provide the fundamentals for future topological spin caloritronics.
Sud, A.*; 山本 慧; 飯浜 賢志*; 石橋 一晃*; 深見 俊輔*; 紅林 秀和*; 水上 成美*
Physical Review Letters, 134(24), p.246704_1 - 246704_7, 2025/06
We investigate the nonlinear coupling between acoustic (ac) and optical (op) modes in the coupled magnetization dynamics of synthetic antiferromagnets (SyAFs), utilizing current-driven resonance spectroscopy. A clear spectral splitting is evident in the ac mode when strongly excited by the radio frequency (RF) current at the driving frequency half that of the op mode resonance. The Landau-Lifshitz phenomenology aligns with the experimental observations, affirming the coupling of the RF-excited ac and op modes through three-magnon mixing. The nonlinearity enables a Rabi-like splitting in in-plane magnetized SyAF without breaking its natural symmetries. This discovery holds potential for advancing our understanding of nonlinear antiferromagnetic dynamics.
目黒 智成*; 小沢 耀弘*; 小林 浩二*; 荒木 康史; 野村 健太郎*
Physical Review Research (Internet), 7(2), p.L022065_1 - L022065_8, 2025/06
We theoretically find a topological spin-orbit torque (SOT) in the compensated ferrimagnetic Weyl semimetal Ti
MnAl. The topological SOT arises from all the occupied states, which is free from dissipation. However, it is negligible in metallic magnets and was not reported in Weyl semimetals. We show the enhancement of the topological SOT from the mixed Berry curvature around the Weyl points in TiMnAl. This finding provides a guiding principle for designing highly efficient SOT devices using the topological nature of ferrimagnetic Weyl semimetals.
Go, G.*; Goli, D. P.*; 江崎 蘭世; Tserkovnyak, Y.*; Kim, S. K.*
Physical Review Research (Internet), 7(2), p.L022066_1 - L022066_7, 2025/06
It remains an open question whether or not the scalar spin chirality itself can exhibit a Hall-type transport. In this work, we show that the answer is yes: The scalar spin chirality is Hall-transported in Kagome ferromagnets and antiferromagnets under an external bias, engendering a phenomenon which we dub the scalar spin chirality Nernst effect. Our findings call for the need to lift the conventional assumption that the scalar spin chirality is a static quantity in order to discover the active roles of the scalar spin chirality in transport properties.
Sheng, L.*; 山本 慧; 他18名*
Nature Physics, 8 Pages, 2025/04
被引用回数：1Controlling the spin current lies at the heart of spintronics and its applications. In ferromagnets, the sign of spin currents is fixed once the current direction is determined. However, spin currents in antiferromagnets can possess opposite polarizations, but this requires enormous magnetic fields to lift the degeneracy between the two modes. Therefore, controlling spin currents with opposite polarization is still a challenge. Here we demonstrate the control of spin currents at room temperature by magnon interference in a canted antiferromagnet, namely, haematite that has recently been classified as an altermagnet. Magneto-optical characterization by Brillouin light scattering reveals that the spatial periodicity of the beating patterns is tunable via the microwave frequency. We further observe that the inverse spin Hall voltage changes sign as the frequency is tuned, evincing a frequency-controlled switching of polarization of pure spin currents. Our work highlights the use of antiferromagnetic magnon interference to control spin currents, which substantially extends the horizon for the emerging field of coherent antiferromagnetic spintronics.
Huang, Y.-C.*; 吉川 貴史; 他5名*
Applied Physics Letters, 126(9), p.092403_1 - 092403_6, 2025/03
被引用回数：0 パーセンタイル：0.00(Physics, Applied)The application of spin-current volume effect (SVE) with volume magnetostriction of iron gallium (FeGa) films is examined for micro-diaphragm actuation. A silicon diaphragm measuring 1.5
1.5 mm
is coated with Pt (100-nm-thick) and FeGa (100-nm-thick) thin films. An alternating charge current passed through the FeGa/Pt/Si diaphragm under a magnetic field perpendicular to the charge current generates an alternating spin current via the spin Hall effect in the Pt film, transferring angular momentum from the Pt film to the FeGa film. The injected spin current provides energy and changes the effective temperature, thereby varying the thermal fluctuation of the magnetic moments. In a magnetic material with volume magnetostriction, the thermal fluctuation of the magnetic moments affects its volume. When the spin fluctuations change, the volume magnetostriction induces a corresponding expansion or contraction of the material. Both electrodeposited and ion-beam sputter-deposited FeGa films are investigated, and it is observed that the FeGa film exhibits SVE. The force generated by the SVE is evaluated based on the vibration amplitude at the fundamental resonant mode. This study demonstrates that the force generated by the SVE is correlated with the volume magnetostriction and the deposition process.
杉本 聡志*; 荒木 康史; 高橋 有紀子*; 家田 淳一; 葛西 伸哉*
Communications Physics (Internet), 8, p.100_1 - 100_9, 2025/03
被引用回数：0 パーセンタイル：0.00(Physics, Multidisciplinary)Current-induced spin-orbit torque (SOT) at heavy metal/ferromagnet (HM/FM) interfaces plays a critical role in modern spintronics. The challenges in SOT research are to enhance the conversion efficiency and generate unconventional torque components. Here, we propose a simple protocol to meet such demands by engineering symmetrical structures, specifically by a wedge-shaped topological material interface in R-3m BiTe
(001)/FM heterostructures. The symmetry of the topological interface is manipulated to lead to unconventional emergences of both out-of-plane and in-plane spin polarizations. Torque measurements show that these polarizations have conversion efficiencies up to 10 percent. This considerable improvement in the SOT efficiency is characterized both qualitatively and quantitatively based on the nonreciprocal ferromagnetic resonance (FMR) spectra and the SOT dependence on the magnetic field direction. The observed enhancement in the unconventional SOT is attributed to the wedge-oriented symmetry reduction of the spin-momentum locking structure of interfacial Dirac fermions. This advance opens new avenues for designing geometrically novel spintronics devices, demonstrating the potential of using multidirectional SOTs for forthcoming nanotechnological applications.
Fe
O
Ba, Y.*; 山本 慧; 他6名*
Physical Review B, 111(10), p.104401_1 - 104401_15, 2025/03
被引用回数：0 パーセンタイル：0.00(Materials Science, Multidisciplinary)Ferrimagnet insulator yttrium iron garnet (YFeO, YIG) attracts much interest in magnonics and microwave applications due to its relatively low magnetic damping and long magnon relaxation time. The magnetoelastic interaction between YIG magnetization and surface acoustic waves (SAWs) has been the basis in the development of acoustic-surface-wave isolators. However, the investigation of SAW-driven magnetic resonance in YIG is limited by the low compatibility of YIG and SAW devices. In this work, we demonstrate nonreciprocal resonant SAW absorption in an on-chip YIG-SAW device integrated by the focused ion beam technique. We observed two distinct SAW absorption signals attributed to the perpendicular standing spin waves (PSSW). Furthermore, the large observed nonreciprocity is compared to theoretical predictions incorporating the influence of asymmetric surface conditions on PSSW mode profiles and nonreciprocal magnon-SAW coupling. The strongly nonreciprocal SAW attenuation in YIG offers a promising opportunity for highly efficient rf signal processing technologies.
GeTe
homojunctions小幡 玲二*; 齊藤 英治; 吉川 貴史; 他13名*
Advanced Materials, 37(8), p.2411459_1 - 2411459_11, 2025/02
被引用回数：1 パーセンタイル：0.00(Chemistry, Multidisciplinary)Twistronics, a novel engineering approach involving the alignment of van der Waals (vdW) integrated two-dimensional materials at specific angles, has recently attracted significant attention. Novel nontrivial phenomena have been demonstrated in twisted vdW junctions (the so-called magic angle), such as unconventional superconductivity, topological phases, and magnetism. However, there have been only few reports on integrated vdW layers with large twist angles 
, such as twisted interfacial Josephson junctions using high-temperature superconductors. Herein, vdW homojunctions of the thin-magnetic flakes, FeGeTe (FGT), with large ranging from 0 to 90 deg, without inserting any tunnel barriers are assembled. Nevertheless, these vdW homojunctions exhibit tunnel-magnetoresistance (TMR) like behavior (pseudo-TMR (PTMR) effect) with the ratios highly sensitive to the values, revealing that the vdW gap at the junction interface between the twisted FGT layers behaves like a tunnel barrier and the serves a control parameter for PTMR by drastically varying magnitudes of the lattice-mismatch and the subsequent appearance of antiferromagnetic (AFM) spin alignment. First-principles calculations considering vacuum gaps indicate strong dependence of TMR on the driven by the sixfold screw rotational symmetry of bulk FGT. The present homojunctions hold promise as a platform for novel AFM spin-dependent phenomena and spintronic applications.
のワイル電子を活用したスピン制御荒木 康史; 家田 淳一
まぐね, 20(1), p.12 - 17, 2025/02
スピントロニクスにおいて、スピンの電気的操作に際して発生する電力損失は大きな課題である。このジュール発熱を回避する過程として、我々は電子状態のトポロジカル構造を組み込んだ電場誘起スピントルクの理論を構築した。このトポロジカルな寄与は散逸の影響を受けず、従来のスピン移行トルクに比べて単位電流あたりのトルク効率を著しく向上させる。本原理によるスピントルクの発現は、トポロジカルな電子系の典型例である「ワイル電子」を持つ、強磁性酸化物SrRuOにおける磁化反転実験によって検証された。
新居 陽一*; 山本 慧; 菅野 将誠*; 前川 禎通*; 小野瀬 佳文*
Physical Review Letters, 134(2), p.027001_1 - 027001_5, 2025/01
被引用回数：2 パーセンタイル：87.69(Physics, Multidisciplinary)While the rectification phenomenon caused by the simultaneous breaking of time reversal and spatial inversion symmetries has been extended to a wide range of (quasi)particles and waves, the nonreciprocal diffraction, which is the imbalance of upward and downward deflections, was previously observed only for photons and remained to be extended to other (quasi)particles. Here, we present evidence of the nonreciprocal diffraction of surface acoustic wave (SAW) utilizing a magnetoelastic grating on a SAW device. Asymmetric diffraction intensities were observed when the ferromagnetic resonance was acoustically excited. Based on a theoretical model, we ascribe the microscopic origin of this phenomenon to the resonant scattering involving ferromagnetic resonance excitations. The novel property may pave an avenue to further development of SAW devices for various purposes including microwave communications and quantum engineering applications.
堀内 皓斗*; 荒木 康史; 若林 勇希*; 家田 淳一; 山ノ内 路彦*; 他7名*
Advanced Materials, p.2416091_1 - 2416091_9, 2025/00
被引用回数：0Spin Berry curvature characterizes the band topology as the spin counterpart of Berry curvature and is crucial in generating novel spintronics functionalities. By breaking the crystalline inversion symmetry, the spin Berry curvature is expected to be significantly enhanced; this enhancement will increase the intrinsic spin Hall effect in ferromagnetic materials and, thus, the spin-orbit torques (SOTs). However, this intriguing approach is not applied to devices; generally, the spin Hall effect in ferromagnet/heavy-metal bilayer is used for SOT magnetization switching. Here, SOT-induced partial magnetization switching is demonstrated in a single layer of a single-crystalline Weyl oxide SrRuO (SRO) with a small current density of 
. Detailed analysis of the crystal structure in the seemingly perfect periodic lattice of the SRO film reveals barely discernible oxygen octahedral rotations with angles of 
near the interface with a substrate. Tight-binding calculations indicate that a large spin Hall conductivity is induced around small gaps generated at band crossings by the synergy of inherent spin-orbit coupling and band inversion due to the rotations, causing magnetization reversal. The results indicate that a minute atomic displacement in single-crystal films can induce strong intrinsic SOTs that are useful for spin-orbitronics devices.
山根 結太*; 荒木 康史; 深見 俊輔*
Physical Review B, 111(14), p.144412_1 - 144412_9, 2025/00
被引用回数：0 パーセンタイル：0.00(Materials Science, Multidisciplinary)We study the Dzyaloshinskii-Moriya interaction (DMI) appearing at the interface of a nonmagnetic/noncollinear-antiferromagnetic bilayer. The DMI is an antisymmetric exchange interaction between neighboring magnetic spins, arising in the absence of an inversion center between the spins, the explicit expression of which is dictated by system symmetry. We formulate the interfacial DMI for different crystalline orientations of the noncollinear antiferromagnet with a stacked-kagome lattice structure. From this formulation, we show that, when the kagome planes are perpendicular to the sample film plane, the DMI serves as a uniaxial magnetic anisotropy for the antiferromagnetic order parameter. Our findings reveal an important physical manifestation of a DMI, shedding light on microscopic mechanisms of the magnetic anisotropy in noncollinear antiferromagnets.
家田 淳一; 深田 幸正; 福田 竜生
日本原子力学会誌ATOMO
, 66(10), p.521 - 524, 2024/10
放射性廃棄物に含まれる放射性同位体(RI)は、見方を変えれば長期間安定したエネルギー源と考えられる。主にアルファ線による熱エネルギーをスピントロニクス技術により電力変換する「スピン熱電発電」、そして主にガンマ線の光電変換を用いた「ガンマ線発電」のための素子開発を進めることで、RIのエネルギーを再資源化する事を目指している。
山本 慧
日本磁気学会研究会資料, 250, p.7 - 11, 2024/10
この講演では、まず表面音波が三つの異なる意味での回転と関係し得ることを解説する。次にこのうち変位ベクトルの回転が回転磁場を生じることを指摘し、スピン波共鳴を介して表面音波の非相反性として観測できることを、実験結果を交えて紹介する。
小沢 耀弘*; 荒木 康史; 野村 健太郎*
Journal of the Physical Society of Japan, 93(9), p.094704_1 - 094704_9, 2024/09
被引用回数：0 パーセンタイル：0.00(Physics, Multidisciplinary)磁性ワイル半金属における磁化の変調は、運動量空間におけるワイル点位置のシフトを引き起こし、ワイルフェルミオンに対して実効的なカイラリティ依存のゲージ場として働く。本研究では、完全スピン偏極したワイル強磁性体CoSnSにおいて、このような磁化誘起カイラルゲージ場の効果を取り扱う。磁化と結合した積層カゴメ格子上で定義したCoSnSの強束縛模型を用いて、運動量空間におけるワイル点の位置(カイラルゲージ場に相当)の磁化方向依存性を計算した。これを用いて、磁壁構造が存在する場合に、カイラルゲージ場の空間構造から生じるカイラル磁場を評価した。その結果磁壁存在下では、ワイルフェルミオンに対してランダウ量子化を強く引き起こすほどの、数百テスラという巨大なカイラル磁場が生じることが導出された。このような巨大カイラル磁場は、従来の磁性体スピントロニクスにおけるスピン起電力に相当する、磁壁の運動による電荷ポンピングを誘起することも予想される。
Fe
O/Pt bilayer diskEmdi, G.*; 日置 友智*; 巻内 崇彦*; 齊藤 英治
Physical Review Applied (Internet), 22(3), p.L031002_1 - L031002_6, 2024/09
被引用回数：0 パーセンタイル：0.00(Physics, Applied)Sigmoidal curve behavior of 0 and n state probability in a magnon parametron, using yttrium iron garnet (YFeO) thin disk, is systematically investigated. We demonstrate that the probability distribution can be tuned by bias and pump microwave power. Our numerical calculation that considers increasing damping due to four-magnon scattering processes reproduces the results well.
Josephson junctions and synchronized precession仲田 光樹; Zou, J.*; Klinovaja, J.*; Loss, D.*
Physical Review Research (Internet), 6(3), p.033207_1 - 033207_11, 2024/08
We uncover theoretically that a surprisingly rich dynamics can emerge in magnetic junctions due to intrinsic non-local damping, using analytical and numerical methods. In particular, under microwave pumping, we show that coherent spin precession in the right and left insulating ferromagnet (FM) of the junction becomes synchronized by non-local damping and thereby a magnonic analog of the Josephson junction emerges, where stands here for the relative precession phase of right and left FM in the stationary limit. Remarkably, decreases monotonically from 
to 
as the magnon-magnon interaction, arising from spin anisotropies, increases. Moreover, we also find a magnonic diode effect giving rise to rectification of magnon currents. Our predictions are readily testable with current device and measurement technologies at room temperatures.
星 幸治郎*; 日置 友智*; 齊藤 英治
Physical Review Applied (Internet), 21(6), p.064057_1 - 064057_6, 2024/07
被引用回数：0 パーセンタイル：0.00(Physics, Applied)Spin-motive force induced by standing spin waves under nonlinear magnon scattering in a permalloy strip is investigated numerically. When the power of an applied microwave field is increased above a threshold under ferromagnetic resonance, spin waves with finite wave numbers are excited via fourmagnon scattering, leading to standing spin waves in real space. We calculate the Spin-motive force induced by the spin waves with finite wave numbers in a permalloy strip and estimate the electric voltage due to the Spin-motive force. We demonstrate the increase of electric voltage as a result of steep magnetization modulation of the nonlinearly excited spin waves and the nonlinear spin waves produced.
細川 海慈*; 山 正樹*; 松尾 衛; 加藤 岳生*
Physical Review B, 110(3), p.035309_1 - 035309_12, 2024/07
被引用回数：0 パーセンタイル：0.00(Materials Science, Multidisciplinary)Our result provides an important foundation for an accurate analysis of the inverse Rashba-Edelstein effect induced by thermal spin injection.
