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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. 

Abstract Amongst the rare-earth perovskite nickelates, LaNiO 3 (LNO) is an exception. While the former have insulating and antiferromagnetic ground states, LNO remains metallic and non-magnetic down to the lowest temperatures. It is believed that LNO is a strange metal, on the verge of an antiferromagnetic instability. Our work suggests that LNO is a quantum critical metal, close to an antiferromagnetic quantum critical point (QCP). The QCP behavior in LNO is manifested in epitaxial thin films with unprecedented high purities. We find that the temperature and magnetic field dependences of the resistivity of LNO at low temperatures are consistent with scatterings of charge carriers from weak disorder and quantum fluctuations of an antiferromagnetic nature. Furthermore, we find that the introduction of a small concentration of magnetic impurities qualitatively changes the magnetotransport properties of LNO, resembling that found in some heavy-fermion Kondo lattice systems in the vicinity of an antiferromagnetic QCP.