Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurements of Spin-Orbit Torques

TL;DR

This paper introduces a transverse measurement approach in spin-torque ferromagnetic resonance experiments to better distinguish spin-orbit torque signals from artifacts like spin pumping and heating effects, improving measurement accuracy.

Contribution

It demonstrates that measuring in a Hall geometry allows separation of SOT signals from artifacts, enhancing the precision of SOT measurements in bilayer structures.

Findings

Artifact contributions are small in thin CoFeB layers but significant in thicker layers.

A sign change in artifact voltage with CoFeB thickness suggests competing effects.

Transverse measurements improve artifact discrimination in SOT experiments.

Abstract

Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to measure spin-orbit torques (SOTs) in heavy metal/ferromagnet bilayer structures. In the course of a measurement, other resonant processes such as spin pumping (SP) and heating can cause spin current or heat flows between the layers, inducing additional resonant voltage signals via the inverse spin Hall effect (ISHE) and Nernst effects (NE). In the standard ST-FMR geometry, these extra artifacts exhibit a dependence on the angle of an in-plane magnetic field that is identical to the rectification signal from the SOTs. We show experimentally that the rectification and artifact voltages can be quantified separately by measuring the ST-FMR signal transverse to the applied current (i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We find that in Pt (6 nm)/CoFeB samples the contribution from the artifacts is small compared to the SOT rectification signal for CoFeB layers thinner than 6 nm, but can be significant for thicker magnetic layers. We observe a sign change in the artifact voltage as a function of CoFeB thickness that we suggest may be due to a competition between a resonant heating effect and the SP/ISHE contribution.