No abstract

We systematically studied the magnetoresistance effect in a Pt/(CoNi)n multilayer system with perpendicular magnetic anisotropy and the fcc (111) texture. The angular dependence of magnetoresistance, including high-order cosine terms, was observed in a plane perpendicular to the electrical current; this was attributed to the geometricalsize effects caused by crystal symmetry, the ordered arrangement of grains, and the anisotropic interface magnetoresistance effect caused by the breaking of the symmetry at interfaces. Based on the accuracy of our experimental results, the magnitude of spin Hall magnetoresistance (SMR) in Pt/(CoNi)n was expected to be below 1×10 -4 . However, on evaluating the spin Hall angle of  0.07 for Pt using spin-torque ferromagnetic resonance measurements, the theoretical magnitude of SMR in our samples was estimated to exceed 7×10 -4 . This absence of SMR in the experimental results can be explained by the Elliott-Yafet spin relaxation of itinerant electrons in the ferromagnetic metal, which indicates that the boundary conditions of the spin current in the heavy metal/ferromagnetic insulator may not be applicable to all-metallic heterostructures.

We found that the exposure of a Co/Pt bilayer to air will result in a trace amount of oxidation at the Co/Pt interface, while the Pt layer is immune to oxidation. The appearance of CoOx results in a negative spin Hall magnetoresistance and unconventional spin–orbit torques (SOTs), which are observed through temperature-dependent transport and spin-torque ferromagnetic resonance measurements. These results can be understood by considering CoOx as an individual magnetic layer between Pt and Co, with two important characteristics: (1) its magnetization is aligned in the plane that is perpendicular to the magnetization of Co and (2) the spin transparency of CoOx increases with increasing temperature. These results help us understand the features of spin transport at the interface when oxidation occurs and further indicate that trace amounts of oxidation can be a highly effective method to control SOT in magnetic heterostructures.