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Sn thin films by magneto-optical Kerr effect内村 友宏*; Yoon, J.-Y.*; 佐藤 佑磨*; 竹内 祐太郎*; 金井 駿*; 武智 涼太*; 岸 桂輔*; 山根 結太*; DuttaGupta, S.*; 家田 淳一; et al.
Applied Physics Letters, 120(17), p.172405_1 - 172405_5, 2022/04
被引用回数：10 パーセンタイル：83.82(Physics, Applied)We perform a hysteresis-loop measurement and domain imaging for 
-oriented 
-Mn
Sn
thin films using magneto-optical Kerr effect (MOKE) and compare it with the anomalous Hall effect (AHE) measurement. We obtain a large Kerr rotation angle of 10 mdeg., comparable with bulk single-crystal MnSn. The composition 
dependence of AHE and MOKE shows a similar trend, suggesting the same origin, i.e., the non-vanishing Berry curvature in the momentum space. Magnetic domain observation at the saturated state shows that x dependence of AHE and MOKE is explained by an amount of reversible area that crucially depends on the crystalline structure of the film. Furthermore, in-depth observation of the reversal process reveals that the reversal starts with nucleation of sub-micrometer-scale domains dispersed in the film, followed by a domain expansion, where the domain wall preferentially propagates along the ![$$[11bar{2}0]$$](/search/images/ERNTCMK2MJQXE4ZSPUYF0JA.gif)
direction. Our study provides a basic understanding of the spatial evolution of the reversal of chiral-spin structure in non-collinear antiferromagnetic thin films.
荒木 康史; 家田 淳一
Physical Review Letters, 127(21), p.277205_1 - 277205_7, 2021/12
被引用回数：5 パーセンタイル：41.09(Physics, Multidisciplinary)強いスピン-軌道相互作用の下での電子系には、運動量空間内のトポロジーが発現する。この電子系トポロジーは磁性体中の磁気構造に対して、乱れや熱揺らぎに影響されない電場誘起トルクを与える。本研究では現象論的なトルクの分類に基づき、バンドトポロジーとトルクの間の直接的な関係を示す。内因性異常ホール効果と同様に、トルクにもまた、非平衡輸送電流に依らず異常速度に起因した内因的効果が現れる。特に本論文では、磁気構造内でのみ現れる内因的トルクの存在を明らかにし、これを「トポロジカルホールトルク(THT)」と呼ぶ。このTHTはバルク結晶中で現れ、界面や表面といった構造を用いる必要がないものである。数値モデルの計算に基づき、金属強磁性体中での従来のスピン移行トルクと比較して、このTHTは非常に大きなトルクを与えることを明らかにする。特に金属強磁性体
における実験で報告されていた巨大な電流誘起トルクは、このTHTとして理解できることを示す。
竹内 祐太郎*; 山根 結太*; Yoon, J.-Y.*; 伊藤 隆一*; 陣内 佛霖*; 金井 駿*; 家田 淳一; 深見 俊輔*; 大野 英男*
Nature Materials, 20(10), p.1364 - 1370, 2021/10
被引用回数：81 パーセンタイル：98.79(Chemistry, Physical)Electrical manipulation of magnetic materials by current-induced spin torque constitutes the basis of spintronics. Recent studies have demonstrated electrical controls of ferromagnets and collinear antiferromagnets by spin-orbit torque (SOT). Here we show an unconventional response to SOT of a non-collinear antiferromagnet, which has recently attracted great attention owing to large anomalous Hall effect despite vanishingly small net magnetization. In heterostructures with epitaxial non-collinear antiferromagnet MnSn, we observe a characteristic fluctuation of Hall resistance, which is attributed to a persistent rotation of chiral-spin structure of MnSn driven by SOT. We find that level of the fluctuation that varies with sample size represents the number of magnetic domains of Mn
Sn. In addition, MnSn thickness dependence of critical current reveals that SOT generated by small current density below 20 MA cm
effectively acts on the chiral-spin structure even in thick MnSn above 20 nm. The results provide unprecedented pathways of electrical manipulation of magnetic materials, offering new-concept spintronics devices with unconventional functionalities and low-power consumption.
Sn thin filmsYoon, J.-Y.*; 竹内 祐太郎*; DuttaGupta, S.*; 山根 結太*; 金井 駿*; 家田 淳一; 大野 英男*; 深見 俊輔*
AIP Advances (Internet), 11(6), p.065318_1 - 065318_6, 2021/06
被引用回数：14 パーセンタイル：79.58(Nanoscience & Nanotechnology)We investigate the relationship between structural parameters, magnetic ordering, and the anomalous Hall effect (AHE) of MnSn (
) thin films annealed at various temperature 
. The crystal structure changes with and and at 
C near the stoichiometric composition (
) epitaxial single-phase 
0
-MnSn(
) is obtained. At room temperature, a larger AHE is obtained when the single-phase epitaxial MnSn with the lattice constant closer to that of bulk is formed. The temperature dependence of the AHE shows different behaviors depending on and can be explained by considering the variation of magnetic ordering. A close inspection into the temperature and composition dependence suggests a variation of magnetic phase transition temperature with composition and/or a possible correlation between the AHE and Fermi level position with respect to the Weyl points. Our comprehensive study would provide the basis for utilizing the unique functionalities of non-collinear antiferromagnetic materials.
Zn
Cr
Se
飯久保 智*; 安井 幸夫*; 小田 啓介*; 大野 陽平*; 小林 義明*; 佐藤 正俊*; 加倉井 和久
Journal of the Physical Society of Japan, 71(11), p.2792 - 2799, 2002/11
被引用回数：11 パーセンタイル：56.91(Physics, Multidisciplinary)スピネル型化合物CuZnCrSeが示す異常ホール効果の特異な振舞とこの系が持つ特異な磁気構造の関連について中性子回折,NMR、及び他の磁気や輸送測定が行われた。その結果この系のコニカルテニウム磁気構造発達と異常ホール係数の関係が明らかになった。
荒木 康史; 家田 淳一
no journal, ,
本講演では、強いスピン-軌道相互作用の存在下で、電流に起因して磁気構造に働くトルクの理論を議論する。スキルミオン等の磁気構造は、実空間内で電子が得るベリー位相に起因して、電子の輸送特性としてトポロジカル・ホール効果に寄与することが知られている。一方でトポロジカル絶縁体・ワイル半金属等の強いスピン-軌道相互作用の下では、運動量空間内に幾何学的構造(ベリー曲率)が強く現れる。運動量空間のベリー曲率は、電場と垂直方向に異常速度を誘起することにより、(内因性)異常ホール効果に寄与する。我々は、このような実空間と運動量空間のトポロジカルな性質が協働し、電流によって誘起される磁気構造のダイナミクスにも寄与することを見出した[1]。まず、強いスピン-軌道相互作用の下で現れるスピン-運動量ロッキング(SML)に基づき、電流誘起トルクのうち磁気構造および異常速度がもたらす寄与を、半古典的な現象論を用いて分類する。その結果として、輸送電流に起因した既知のスピン移行トルク(STT)及びスピン軌道トルク(SOT)の他に、異常速度由来の「内因性」トルクが存在することを示す。これは電子の異常速度が、SMLによりスピン偏極に変換されて現れるトルクである。内因性トルクは電子分布の緩和時間に依存せず、不純物が多い系でも安定して現れることが期待される。特に磁気構造内部では空間反転対称性が破れることにより、異常速度由来のトルクが優位な寄与をすることが期待され、本研究ではこれを「トポロジカル・ホール・トルク(THT)」と呼んで注目する。THTが発現する具体例として、バンド反転した強磁性金属での計算例を示す。図のように磁化に対応して2対のワイル点を持つような系を考え、その中での磁壁に働くTHTを見積もる。その結果として、通常の非断熱的STT(
-項)で~2としたものに相当する、大きなTHTが現れることを示す。これはワイル点近傍のベリー曲率,SML、及びワイル点間に現れるvan Hove特異点に起因したものであり、磁化の温度変化に伴うバンド構造の変調によって到達できることが期待される。[1] Y. Araki and J. Ieda, arXiv:2105.14922.
荒木 康史; 家田 淳一
no journal, ,
We present a theory of current-induced torques on magnetic textures in the presence of strong spin-orbit coupling (SOC). It has been broadly discussed that nontrivial topology in real and momentum spaces contributes to anomalous transport phenomena of electrons. Magnetic textures such as skyrmions give rise to the topological Hall effect in magnetic metals, due to the real-space Berry curvature acting as an effective magnetic field for electrons. The momentum-space Berry curvature, which arises from band inversion by SOC in topological insulators (TIs) and Weyl semimetals (WSMs), leads to the intrinsic anomalous Hall effect (AHE) via the anomalous velocity in response to an applied electric field. Here we propose that the topological characteristics intertwined in real and momentum spaces contribute also to dynamics of magnetic textures driven by electric fields. Based on spin-momentum locking (SML) due to strong SOC, we phenomenologically classify the electrically induced torques, and derive the contributions from magnetic textures and the anomalous velocity. As a result, we find the "intrinsic" torques that arise from the spin polarization converted from the anomalous velocity by SML. The intrinsic torques are distinct from the conventional spin-transfer torque (STT) and the spin-orbit torque (SOT), both of which come from the electron spins in transport current. They are hence independent of the transport relaxation time and are robust against disorder or thermal fluctuation. We especially point out the intrinsic torque arising only in magnetic textures, which we call the "topological Hall torque (THT)". The THT is induced by the cooperation of the real-space magnetic texture and the momentum-space Berry curvature, and can emerge dominantly in bulk crystals. Such an enhanced torque is capable of driving dynamics of the magnetic texture at high speed, which may make the spintronics device highly efficient without building any complex heterostructures.
荒木 康史
no journal, ,
I show from theoretical aspect that the current-induced spin torques in magnetic topological materials are influenced and occasionally enhanced due to their band topology. After reviewing the current status of theoretical and experimental studies on magnetic topological materials, I will introduce some of my recent works. From the general point of view, I first propose that the momentum-space topology gives rise to an "intrinsic" contribution to the spin torques, which is robust against disorder and thermal fluctuation. In particular, I point out the intrinsic torque exerting on magnetic textures, such as domain walls (DWs), which we call the "topological Hall torque (THT)". Experimental measurability of the THT in the metallic ferromagnet SrRuO, in connection with the temperature dependence in its anomalous Hall effect, will also be mentioned.
荒木 康史; 家田 淳一
no journal, ,
We present a theory of current-induced torques on magnetic textures in the presence of strong spin-orbit coupling (SOC). Based on spin-momentum locking (SML) due to strong SOC, we phenomenologically classify the electrically induced torques on the magnetization into the four parts: whether the torque acts on uniform magnetization or nonuniform textures, and whether the torque depends on the transport time or not. From this classification, we point out that the "intrinsic" torques insensitive to the transport time arise from the anomalous velocity and SML. We especially point out an intrinsic torque acting on magnetic textures, which we call the "topological Hall torque (THT)". While the conventional spin-transfer torque (STT) is driven by the transport current and suffers from energy dissipation by the Joule heating, the THT arises from the anomalous velocity from the momentum-space topology and is thus capable of manipulating magnetic textures non-dissipatively, even in centrosymmetric crystals. To illustrate the significance of the THT, we show our model calculation in a metallic ferromagnet. The Weyl points due to the band inversion by SOC gives rise to a large THT acting on a magnetic domain wall therein. Its magnitude becomes compatible to the nonadiabatic STT with the unusual size of the nonadiabaticity parameter 
. We also demonstrate the experimental measurement of the THT from the current-induced domain wall motion in the metallic ferromagnet SrRuO. The idea of the THT may help design the spintronics device highly efficient without building any complex heterostructures.
荒木 康史; 山ノ内 路彦*; 酒井 貴樹*; 植村 哲也*; 家田 淳一
no journal, ,
We present our theoretical and experimental findings of the electric manipulation of magnetic textures enhanced by the electron topology. We first show the theory of the non-dissipative torques acting on magnetic textures, by classifying the electrically-induced spin torques phenomenologically. We propose the "topological Hall torque (THT)", which emerges from the combination of the anomalous velocity from the Berry curvature and the spin-momentum locking structure from the strong spin-orbit coupling (SOC). In contrast to the conventional spin-transfer torque (STT), which is driven by the transport current and suffers from energy dissipation by the Joule heating, the THT is capable of manipulating magnetic textures in a non-dissipative manner. The THT is present even in centrosymmetric crystals, such as the bulk WSMs. The emergence of the THT was verified experimentally, by measuring the current-induced magnetization switching in a ferromagnetic oxide SrRuO (SRO). SRO becomes ferromagnetic below the transition temperature 
147K. After preparing a domain wall (DW) in a film of SRO, we measured the effective magnetic field 
exerted on the DW by a current in a wide temperature range. As a result, the measured revealed a nonmonotonic temperature dependence at low temperature, and a large magnitude compared with that arising from the conventional STT and the spin-orbit torque. Those unconventional behaviors of are successfully described by the THT, in connection with the large Berry curvature of the Weyl fermions present in SRO. The idea of the THT discussed here may help design the spintronics device highly efficient, from the viewpoint of band topology.
荒木 康史
no journal, ,
We present our theoretical and experimental findings of the electric manipulation of magnetic textures enhanced by the electron topology. We first show the theory of the non-dissipative torques acting on magnetic textures, by classifying the electrically-induced spin torques phenomenologically. We propose the "topological Hall torque (THT)", which emerges from the combination of the anomalous velocity from the Berry curvature and the spin-momentum locking structure from the strong spin-orbit coupling (SOC). In contrast to the conventional spin-transfer torque (STT), which is driven by the transport current and suffers from energy dissipation by the Joule heating, the THT is capable of manipulating magnetic textures in a non-dissipative manner. The THT is present even in centrosymmetric crystals, such as the bulk WSMs. The emergence of the THT was verified experimentally, by measuring the current-induced magnetization switching in a ferromagnetic oxide SrRuO (SRO). SRO becomes ferromagnetic below the transition temperature 147K. After preparing a domain wall (DW) in a film of SRO, we measured the effective magnetic field exerted on the DW by a current in a wide temperature range. As a result, the measured revealed a nonmonotonic temperature dependence at low temperature, and a large magnitude compared with that arising from the conventional STT and the spin-orbit torque. Those unconventional behaviors of are successfully described by the THT, in connection with the large Berry curvature of the Weyl fermions present in SRO. The idea of the THT discussed here may help design the spintronics device highly efficient, from the viewpoint of band topology.
荒木 康史
no journal, ,
「トポロジカル絶縁体」、「ワイル半金属」等のトポロジカル物質を用いたスピントロニクス研究は、理論・実験とも近年めざましい勢いで発展している。その一方、「トポロジー」という概念は抽象的なものであり、スピントロニクス・デバイスの構築、及びその構成要素となる物理現象を探求していくにあたって、トポロジカル物質が具体的にどのような恩恵をもたらすかは直感的に理解しにくい。本講演では、物質中の「トポロジー」が、スピントロニクスの省電力化に貢献しうる可能性を持つことを、基礎的な理論から出発して段階的に解説する。最初に物質中の電子において「トポロジー」がどうやって特徴づけられるかを解説し、どのような物質においてトポロジカルな構造が現れるかを説明する。次に、電子のトポロジーによって誘起される性質として「異常速度」を導入し、これがスピントロニクスの省電力化に重要な役割を果たすことを説明する。最後に講演者の研究成果の一つとして、異常速度によって実現される省電力の磁気制御メカニズム「トポロジカル・ホール・トルク」の理論及び実験測定について、簡潔に解説する。
荒木 康史; 家田 淳一
no journal, ,
We present our theoretical finding of the topological enhancement of current-induced torques on magnetic textures, in the presence of strong spin-orbit coupling (SOC). Based on spin-momentum locking (SML) around the band inversion, we phenomenologically classify the electrically induced torques on the magnetization into the four parts. From this classification, we point out that the "intrinsic" torques insensitive to the transport time arise from the anomalous velocity due to the momentum-space Berry curvature. We especially point out an intrinsic torque acting on magnetic textures, which we call the "topological Hall torque (THT)". While the conventional spin-transfer torque (STT) is driven by the transport current and suffers from energy dissipation by the Joule heating, the THT arises from the anomalous velocity and hence is capable of manipulating magnetic textures non-dissipatively. Its magnitude per current can exceed that of the conventional STT with the unusual size of the nonadiabaticity parameter 
. We also present our experimental verification of the THT in a ferromagnetic oxide SrRuO (SRO). We measured the current-induced torque exerted on a magnetic domain wall prepared in a film of SRO, which revealed a nonmonotonic temperature dependence and a large magnitude that cannot be described by the conventional STT. Those unconventional behaviors are successfully described by the THT, in connection with the large Berry curvature of the Weyl fermions present in SRO.
荒木 康史
no journal, ,
「トポロジカル絶縁体」、「ワイル半金属」等のトポロジカル物質を用いたスピントロニクス研究は、理論・実験とも近年めざましい勢いで発展している。その一方、「トポロジー」という概念は抽象的なものであり、スピントロニクス・デバイスの構築、及びその構成要素となる物理現象を探求していくにあたって、どのような恩恵をもたらすかは直感的に理解しにくい。本講演では、物質中の「トポロジー」がどのようにスピントロニクス関連現象に寄与していくかを、理論の詳細には立ち入らず、初学者向けに平易な解説を行う。最初に、物質中の電子において「トポロジー」がどうやって特徴づけられるかを解説し、トポロジカルな構造が現れるために必要な要素を説明する。次に、トポロジカル物質が持つ利点として、「強いスピン軌道相互作用」「トポロジカル表面状態」「内因性効果」の3点について解説する。講演の後半では、トポロジカル物質の利点を活用して、実現されるスピントロニクス機能性について解説する。特にトポロジカル物質を用いた磁化制御過程(スピントルク)について、講演者の最近の研究成果も含めて紹介する。
