Intrinsic torques emerging from anomalous velocity in magnetic textures

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   

Araki, Yasufumi; Ieda, Junichi

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.