Angular evolution of thickness-related unidirectional magnetoresistance in Co/Pt multilayers

We report the magnetoresistance of Co/Pt superlattices having thickness gradients at different orientations relative to an applied current. We measure the magnetoresistance at a fixed field as a function of the out-of-plane field angle, and find a unidirectional magnetoresistance (UMR) in addition to the expected anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR). Specifically, the UMR signal is the difference in resistance between the (out-of-plane) +z and −z field orientation. The magnitude of UMR is minimized when the thickness gradient is parallel to the applied current and maximized when the gradient is nearly perpendicular to the current. The results imply the possibility of an alternative source of UMR in Co/Pt multilayers in addition to the previously considered anomalous Hall effect.