Resonant Measurement of Non-Reorientable Spin-Orbit Torque from a Ferromagnetic Source Layer Accounting for Dynamic Spin Pumping

TL;DR

This paper experimentally measures the non-reorientable spin-orbit torque in a multilayer structure using ST-FMR, accounting for dynamic spin pumping, and distinguishes the two effects to accurately quantify the torque.

Contribution

It introduces a method to separate spin-orbit torque from dynamic spin pumping effects in ST-FMR measurements of multilayer magnetic structures.

Findings

Measured non-reorientable spin torque ratio $\xi_{Py} = 0.04 \, extpm \, 0.01$

Dynamic spin pumping exerts torques comparable to spin-orbit torques

Frequency-dependent ST-FMR can distinguish between the two effects

Abstract

Using a multilayer structure containing (cobalt detector layer)/(copper spacer)/(Permalloy source layer), we show experimentally how the non-reorientable spin-orbit torque generated by the Permalloy source layer (the component of spin-orbit torque that does not change when the Permalloy magnetization is rotated) can be measured using spin-torque ferromagnetic resonance (ST-FMR) with lineshape analysis. We find that dynamic spin pumping between the magnetic layers exerts torques on the magnetic layers as large or larger than the spin-orbit torques, so that if dynamic spin pumping is neglected the result would be a large overestimate of the spin-orbit torque. Nevertheless, the two effects can be separated by performing ST-FMR as a function of frequency. We measure a non-reorientable spin torque ratio $\xi_{\text{Py}} = 0.04 \pm 0.01$ for spin current flow from Permalloy through an 8 nm Cu spacer to the Co, and a strength of dynamic spin pumping that is consistent with previous measurements by conventional ferromagnetic resonance.