Theory and observation of topological Hall torque emerging from band topology

バンドトポロジーに由来したトポロジカル・ホール・トルクの理論及び測定

荒木 康史   ; 山ノ内 路彦*; 酒井 貴樹*; 植村 哲也*; 家田 淳一   

Araki, Yasufumi; Yamanouchi, Michihiko*; Sakai, Takaki*; Uemura, Tetsuya*; Ieda, Junichi

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.