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

A large unidirectional magnetoresistance (UMR) ratio of UMR/Rₓₓ≈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≈10⁹0.16em0exA/m² (UMR ratio ∝J), a spin-magnon-interaction mechanism regime at intermediate J≈10¹⁰0.16em0exA/m² (UMR ratio ∝J³), and a spin transfer torque (STT) regime at J≈10¹¹0.16em0exA/m² (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.