Large unidirectional magnetoresistance in metallic heterostructures in the spin transfer torque regime

A large unidirectional magnetoresistance (UMR) ratio of $\mathrm{UMR}/{R}_xx\ensuremath{\approx}0.36%$ is found in W/CoFeB metallic bilayer heterostructures at room temperature. Three different regimes in terms of the current dependence of the UMR ratio are identified: a spin-dependent-scattering mechanism regime at small current densities $J\ensuremath{\approx}{10}⁹\phantom{\rule{0.16em}{0ex}}\mathrm{A}/{\mathrm{m}}²$ (UMR ratio $\ensuremath{\propto}J$), a spin-magnon-interaction mechanism regime at intermediate $J\ensuremath{\approx}{10}¹⁰\phantom{\rule{0.16em}{0ex}}\mathrm{A}/{\mathrm{m}}²$ (UMR ratio $\ensuremath{\propto}{J}³$), and a spin transfer torque (STT) regime at $J\ensuremath{\approx}{10}¹¹\phantom{\rule{0.16em}{0ex}}\mathrm{A}/{\mathrm{m}}²$ (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 STTs. An alternative approach to estimate dampinglike spin torque efficiencies from magnetic heterostructures is also proposed.