Intrinsic torques emerging from anomalous velocity in magnetic textures

Araki, Yasufumi   ; Ieda, Junichi   

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.