Unidirectional spin Hall magnetoresistance and spin-orbit torques in HM$_1$/Co/HM$_2$ trilayer systems

TL;DR

This study investigates how adding a second heavy metal layer with an opposite spin Hall angle enhances spin-orbit torques and unidirectional spin Hall magnetoresistance in Pt/Co/Ta and Ta/Co/Pt trilayers, showing increased efficiencies and larger effects than bilayers.

Contribution

It demonstrates that trilayer structures with opposite sign heavy metals significantly boost spin-orbit torques and USMR compared to bilayers, with detailed experimental analysis.

Findings

Enhanced SOT efficiencies in trilayers versus bilayers.

Maximum spin Hall angle of 20% in Pt/Co/Ta trilayer.

USMR effect increased by up to 27% in trilayers.

Abstract

We present a detailed analysis of harmonic longitudinal and Hall voltage measurements for in-plane magnetized Pt/Co/Ta and Ta/Co/Pt trilayers in reference to Pt/Co and Ta/Co bilayers. Enhancement of spin-orbit torques (SOTs) and unidirectional spin Hall magnetoresistance (USMR) is achieved by introducing the second heavy metal (HM) with the opposite sign of the spin Hall angle. The extracted SOT efficiencies are larger for the trilayers as compared to the bilayers, confirming the enhanced values reported for the trilayers with perpendicularly magnetized Co. The maximum effective spin Hall angle found for the Pt/Co/Ta trilayer reaches $\theta_{SH}$ = 20%. The USMR of the trilayer yields up to 27% higher effect as for the respective bilayers with the largest effective USMR amplitude of -0.32 $\times$ 10$^{-5}$ for the Pt/Co/Ta trilayer at a charge current density of 10$^{7}$ A/cm$^2$.