Large Unidirectional Magnetoresistance in Metallic Heterostructures in the Spin Transfer Torque Regime

TL;DR

This study reports a significant unidirectional magnetoresistance in W/CoFeB heterostructures at room temperature, revealing three current-dependent regimes linked to different spin-dependent mechanisms and confirming the role of spin-transfer torque via experiments and simulations.

Contribution

It introduces the first detailed analysis of large UMR in metallic heterostructures, identifying three distinct current regimes and linking UMR to spin-transfer torque effects.

Findings

UMR ratio of approximately 0.36% observed at room temperature

Three regimes identified: spin-dependent scattering, spin-magnon interaction, and spin-transfer torque

Numerical simulations confirm magnetization tilting due to spin Hall effect-induced torques

Abstract

A large unidirectional magnetoresistance (UMR) ratio of UMR/$R_{xx}\sim$ $0.36\%$ is found in W/CoFeB metallic bilayer heterostructures at room temperature. Three different regimes in terms of the current dependence of UMR ratio are identified: A spin-dependent-scattering mechanism regime at small current densities $J \sim$ $10$$^{9}$A/m$^{2}$ (UMR ratio $\propto$ $J$), a spin-magnon-interaction mechanism regime at intermediate $J \sim$ $10$$^{10}$A/m$^{2}$ (UMR ratio $\propto$ $J$$^{3}$), and a spin-transfer torque (STT) regime at $J \sim$ $10$$^{11}$A/m$^{2}$ (UMR ratio independent of $J$). We verify the direct correlation between this large UMR and the transfer of spin angular momentum from the W layer to the CoFeB layer by both field-dependent and current-dependent UMR characterizations. Numerical simulations further confirm that the large STT-UMR stems from the tilting of the magnetization affected by the spin Hall effect-induced spin-transfer torques. An alternative approach to estimate damping-like spin-torque efficiencies from magnetic heterostructures is also proposed.