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

Abstract: Field-free deterministic switching of perpendicular ferromagnets is a central challenge for spintronics applications, typically requiring explicit symmetry breaking. Here we show that deterministic switching can instead be achieved through higher-order (in magnetization angles) spin-orbit torques, even in systems that preserve in-plane mirror symmetry. Using a vector spherical harmonics expansion, we demonstrate that higher-order torque terms naturally give rise to additional out-of-equator fixed points, enabling reliable magnetization reversal when their magnitude is comparable to conventional lowest-order torques. We illustrate this mechanism with first-principles calculations on the noncentrosymmetric Weyl ferromagnet PrAlGe, where the combination of Weyl-node band topology and strong spin-orbit coupling produces sizable higher-order torque components. Because the Fermi surface is small, the conventional lowest-order torques are relatively weak, allowing the higher-order harmonics to compete on equal footing and strongly reshape the magnetization dynamics. The resulting spin dynamics confirm deterministic switching without additional symmetry breaking. Our results establish higher-order spin-orbit torque as a key ingredient for understanding and controlling magnetization dynamics in topological and spintronic materials.