Field-Free-Switching State Diagram of Perpendicular Magnetization Subjected to Conventional and Unconventional Spin-Orbit Torques

The lack of certain crystalline symmetries in strong spin-orbit-coupled nonmagnetic materials allows for the existence of unconventional spin Hall responses, with electrically generated transverse spin cur-rents possessing collinear flow and spin directions. The injection of such spin currents into an adjacent ferromagnetic layer can excite magnetization dynamics via unconventional spin-orbit torques, leading to deterministic switching in ferromagnets with perpendicular magnetic anisotropy. We study the inter-play between conventional and unconventional spin-orbit torques on the magnetization dynamics of a perpendicular ferromagnet in the small intrinsic damping limit, and identify a rich set of dynamical regimes that includes deterministic and probabilistic switching, precessional and pinning states. Contrary to common belief, we find that there exists a critical conventional spin Hall angle, beyond which deter-ministic magnetization switching transitions to a precessional or pinned state. Conversely, we show that larger unconventional spin Hall angle is generally beneficial for deterministic switching. We derive an approximate expression that qualitatively describes the state-diagram boundary between the full deter-ministic switching and precessional states and discuss a criterion for searching symmetry-broken spin Hall materials in order to maximize switching efficiency. Our work offers a roadmap towards energy -efficient spintronic devices, which might open the door for applications in advanced in-memory computing technologies.

Automated summary

The lack of certain crystalline symmetries in nonmagnetic materials with strong spin-orbit coupling allows for unconventional spin Hall responses, which can be used to excite magnetization dynamics in ferromagnetic materials. In this study, we investigate the interplay between conventional and unconventional spin-orbit torques on the magnetization dynamics of a perpendicular ferromagnet and identify different dynamical regimes. Our findings challenge common beliefs about the role of spin Hall angles in magnetization switching and provide insights into maximizing switching efficiency.