Recent experiments have revealed nonlinear features of the magnetoresistancein metallic bilayers consisting of a heavy-metal (HM) and a ferromagnetic metal(FM). A small change in the lon- gitudinal resistance of the bilayer has beenobserved when reversing the direction of either the applied in-plane current orthe magnetization. We attribute such nonlinear transport behavior to thespin-polarization dependence of the electron mobility in the FM layer acting inconcert with the spin accumulation induced in that layer by the spin Hallcurrent originating in the bulk of the HM layer. An explicit expression for thenonlinear magnetoresistance is derived based on a simple drift-diffusion model,which shows that the nonlinear magnetoresistance appears at the first order ofspin Hall angle (SHA), and changes sign when the current is reversed, inagreement with the experimental observations. We also discuss possible ways tocontrol the sign of the nonlinear magnetoresistance and to enhance themagnitude of effect.
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