Unveiling the mechanism of bulk spin-orbit torques within chemically disordered Fe$_x$Pt$_{1-x}$ single layers

TL;DR

This study uncovers the mechanism behind bulk spin-orbit torques in chemically disordered Fe$_x$Pt$_{1-x}$ layers, revealing their dependence on composition gradients and temperature, and suggesting strain engineering for low-power spintronic devices.

Contribution

It demonstrates that bulk SOTs originate from an internal spin Hall effect influenced by strain-induced inversion asymmetry in disordered FePt layers.

Findings

Bulk damping-like SOT arises from an imbalanced internal spin current.

SOT efficiency is insensitive to temperature but reverses with composition gradient orientation.

Strain non-uniformity induced by composition gradient likely causes the observed SOTs.

Abstract

Recent discovery of spin-orbit torques (SOTs) within magnetic single-layers has attracted attention in the field of spintronics. However, it has remained elusive as to how to understand and how to tune the SOTs. Here, utilizing the single layers of chemically disordered Fe$_x$Pt$_{1-x}$, we unveil the mechanism of the "unexpected" bulk SOTs by studying their dependence on the introduction of a controlled vertical composition gradient and on temperature. We find that the bulk damping like SOT arises from an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation. The torque can be strong only in the presence of a vertical composition gradient and the SOT efficiency per electric field is insensitive to temperature but changes sign upon reversal of the orientation of the composition gradient, which are in analogue to behaviors of the strain. From these characteristics we conclude that the imbalanced internal spin current originates from a bulk spin Hall effect and that the associated inversion asymmetry that allows for a non-zero net torque is most likely a strain non-uniformity induced by the composition gradient. The fieldlike SOT is a relatively small bulk effect compared to the dampinglike SOT. This work points to the possibility of developing low-power single-layer SOT devices by strain engineering.