Intrinsic torques emerging from anomalous velocity in magnetic textures

荒木 康史; 家田 淳一

Physical Review Letters, 127(21), p.277205_1 - 277205_7, 2021/12

https://doi.org/10.1103/PhysRevLett.127.277205

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。

Microscopic theory of electrically induced spin torques in magnetic Weyl semimetals

紅林 大地*; 荒木 康史; 野村 健太郎*

Journal of the Physical Society of Japan, 90(8), p.084702_1 - 084702_9, 2021/08

https://doi.org/10.7566/JPSJ.90.084702

本研究では、磁性ワイル半金属において電流および電圧によって誘起されるスピントルクの理論を導出する。磁性ワイル半金属においては、非自明なトポロジーを持つバンド構造に由来して磁化と電子輸送が結合し、スピントロニクスに適した性質を示す。本論文では、磁化構造の勾配および電流に関する摂動展開に基づき、電流によって誘起されるスピントルクを網羅的に導出する。このトルクはスピン軌道トルク及びスピン移行トルクを内包するものである。更に、電圧(電位差)によって誘起されるトルクも微視的に導出し、このトルクがスピンと軸性カレントの対応関係、及びカイラルアノマリーに由来するものであることを示す。

反強磁性ナノ構造におけるスピントルクと電圧生成

山根 結太*; 家田 淳一

まぐね, 13(5), p.235 - 241, 2018/10

AA2018-0148.pdf:0.6MB

We overview the recent theoretical development on spin-transfer torque in antiferromagnetic nanostructures and related subjects. As antiferromagnetic materials are generating more attention lately due to their potential to play pivotal roles in spintronics applications, demands are rising for reliable methods to control and detect antiferromagnetic textures and their dynamics. While antiferromagnets are largely insensitive to external magnetic fields in general, electrical means are proving to be capable of offering promising ways to access the antiferromagnetic dynamics. We discuss in this article the possibility of manipulation and observation of dynamical antiferromagnetic textures by electrical current and voltage.

Theory of spin-transfer torques in antiferromagnetic textures

家田 淳一; 山根 結太*; Sinova, J.*

no journal, , 

We present a theory of spin-transfer torques in textured antiferromagnets considering the small and large limits of the exchange coupling energy relative to the kinetic energy of the inter-sublattice electron dynamics. Our theory suggests a natural definition of the efficiency of spin-transfer torques in antiferromagnets in terms of well-defined material parameters, revealing that the charge current couples predominantly to the antiferromagnetic order parameter and the sublattice-canting moment in, respectively, the limits of large and small exchange coupling. The effect leads to antiferromagnetic domain wall motion and the characteristic Doppler shift in the antiferromagnetic spin-wave dispersions. The same formalism also predicts a cross effect of magnetic field gradient and charge current on a antiferromagnetic domain wall motion. The findings offer a framework for understanding and designing spin-transfer torques in antiferromagnets belonging to different classes of sublattice structures.

磁気構造中の異常速度に起因した内因性トルク

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.

Intrinsic torques emerging from anomalous velocity in magnetic textures

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.

Intrinsic torques emerging from anomalous velocity in magnetic textures

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.

Theory of spin torques emerging from topological structures of electrons

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.

Intrinsic torques emerging from anomalous velocity in magnetic textures

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.