Anomalous sign inversion of spin-orbit torque in ferromagnetic/nonmagnetic bilayer systems due to self-induced spin-orbit torque

TL;DR

This paper reveals that self-induced spin-orbit torques can invert the expected sign of spin-orbit torque in ferromagnetic/nonmagnetic bilayers, affecting the estimation of spin Hall angles and improving magnetization switching efficiency.

Contribution

It introduces a theoretical framework accounting for SI-SOT, explaining the sign inversion and highlighting its impact on spin Hall angle measurements and device performance.

Findings

Sign inversion of SOT observed in experiments.

SI-SOT significantly influences spin Hall angle estimation.

Efficient magnetization switching achieved with aligned SHA signs.

Abstract

Self-induced spin-orbit torques (SI-SOTs) in ferromagnetic (FM) layers have been overlooked when estimating the spin Hall angle (SHA) of adjacent nonmagnetic (NM) layers. In this work, we observe anomalous sign inversion of the total SOT in the spin-torque ferromagnetic resonance due to the enhanced SI-SOT, and successfully rationalize the sign inversion through a theoretical calculation considering the SHE in both the NM and FM layers. The findings show that using an FM layer whose SHA sign is the same as that of the NM achieves efficient SOT-magnetization switching with the assistance of the SI-SOT. The contribution of the SI-SOT becomes salient for a weakly conductive NM layer, and conventional analyses that do not consider the SI-SOT can overestimate the SHA of the NM layer by a factor of more than 150.