Anomalous Spin-Orbit Torques in Magnetic Single-Layer Films

TL;DR

This paper reports the discovery of anomalous spin-orbit torques in single-layer magnetic films, revealing a new mechanism for generating spin currents and challenging existing models of spin-orbit interactions.

Contribution

It introduces the anomalous spin-orbit torque (ASOT) observed in ferromagnetic films, supported by experimental measurements and first principles calculations, expanding understanding of spin transport in magnetic materials.

Findings

ASOT observed in Ni80Fe20, Co, Ni, and Fe.

Spin-Hall-like current efficiency of 0.053+/-0.003 in Ni80Fe20.

Strong transverse spin current exists in ferromagnets despite spin dephasing.

Abstract

Spin-orbit interaction (SOI) couples charge and spin transport, enabling electrical control of magnetization. A quintessential example of SOI-induced transport is the anomalous Hall effect (AHE), first observed in 1880, in which an electric current perpendicular to the magnetization in a magnetic film generates charge accumulation on the surfaces. Here we report the observation of a counterpart of the AHE that we term the anomalous spin-orbit torque (ASOT), wherein an electric current parallel to the magnetization generates opposite spin-orbit torques on the surfaces of the magnetic film. We interpret the ASOT as due to a spin-Hall-like current generated with an efficiency of 0.053+/-0.003 in Ni80Fe20, comparable to the spin Hall angle of Pt. Similar effects are also observed in other common ferromagnetic metals, including Co, Ni, and Fe. First principles calculations corroborate the order of magnitude of the measured values. This work suggests that a strong spin current with spin polarization transverse to magnetization can exist in a ferromagnet, despite spin dephasing. It challenges the current understanding of spin-orbit torque in magnetic/nonmagnetic bilayers, in which the charge-spin conversion in the magnetic layer has been largely neglected.