Deterministic Switching of Ferromagnets with Higher-order Spin-orbit Torque in Noncentrosymmetric Weyl Semimetals

The unique topological effect and symmetry breaking of rare earth Weyl semimetals RAlGe (R is rare earth) present substantial potential in spintronics applications. The inversion symmetry breaking allows for the Weyl nodes to be stabilized and also allows direct control of magnetization order via spin-orbit torque mediated by spin-orbit coupling. The crystal's C4z rotational symmetry and two in-plane mirrors restrict the form of spin-orbit torque that can have higher-order angular dependence on magnetization. In Weyl semimetals, the spin-orbit coupling cannot be treated perturbatively near the Weyl points, thus resulting in plausible large higher-order spin-orbit torques. To illustrate the nature of high-order terms, we expand all orders of spin-orbit torque on a vector spherical harmonics basis. We find that deterministic switching is possible when the higher-order terms are comparable to the lower-order terms, which indicates the importance of including higher-order terms to understand the magnetization dynamics in topological materials. We perform first-principles calculations of spin-orbit torque in Weyl ferromagnet PrAlGe and resulting spin dynamics. The combination of these studies results in an understanding of the material's spin-orbit torque response while studying the interplay of topology and magnetization in the presence of spin-orbit torques.