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

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.

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