Field-free-switching state diagram of perpendicular magnetization subjected to conventional and unconventional spin-orbit torques

TL;DR

This paper explores how unconventional and conventional spin-orbit torques influence perpendicular magnetization switching, revealing critical parameters and regimes that optimize energy-efficient spintronic device performance.

Contribution

It introduces a comprehensive state diagram for magnetization dynamics under combined spin-orbit torques, highlighting the role of symmetry-broken materials and critical spin Hall angles for deterministic switching.

Findings

Existence of a critical conventional spin Hall angle for switching regimes

Larger unconventional spin Hall angles enhance deterministic switching

Derived an approximate boundary expression for state transitions

Abstract

The lack of certain crystalline symmetries in strong spin-orbit-coupled non-magnetic materials allows for the existence of uncoventional spin Hall responses, with electrically generated transverse spin currents 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 interplay 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 found that there exists a critical conventional spin Hall angle, beyond which deterministic magnetization switching transitions to a precessional or pinned state. Conversely, we showed 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 deterministic 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 opens doors for applications in advanced in-memory computing technologies.