Spin anomalous-Hall unidirectional magnetoresistance

Abstract

We predict an anomalous-Hall unidirectional magnetoresistance (AH-UMR) in conducting bilayers composed of a ferromagnetic layer and a nonmagnetic layer, which does not rely on the spin Hall effect in the normal metal layer-in stark contrast to the well-studied unidirectional spin-Hall magnetoresistance-but, instead, arises from the anomalous Hall effect in the ferromagnetic layer. Physically, it is the charge-spin conversion induced by the anomalous Hall effect that conspires with the structural inversion asymmetry to generate a net nonequilibrium spin density in the ferromagnetic layer, which, in turn, modulates the resistance of the bilayer when the direction of the applied current or the magnetization is reversed. The dependences of the AH-UMR effect on materials and geometric parameters are analyzed and compared with other nonlinear magnetoresistances. In particular, we show that, in magnetic bilayers where anomalous Hall and spin Hall effects are comparable, the overall UMR may undergo a sign change when the thickness of either layer is varied, suggesting a new scheme to quantify the spin Hall or anomalous Hall angle via a nonlinear transport measurement.