Theory of the spin Hall effect in metal oxide structures

The spin Hall effect is considered a phenomenon in which a charge current is converted to a spin current due to the spin-orbit coupling (SOC). Recently, large values of the spin Hall angle (SHA), as the conversion efficiency between spin and charge currents, were observed in metal oxide structures in ferromagnetic/metal (FM/M) oxide bilayers, although their underlying mechanisms are ambiguous. The present study aims to analytically indicate that a mixed region of metal and oxide formed at the metal/oxide interface includes the surface oxide charge, which allows for introducing an SOC term in the electron Hamiltonian. Based on the results, the SHA increases by two orders of magnitude in a Cu/oxide thin film through the side jump and skew scattering mechanisms, reaching that of heavy metals. In addition, a comprehensive model is provided for the influence of oxidation on the SHA and spin-orbit torques (SOTs) in FM/M structures. We find that the magnitude of SOTs is greatly dependent on (i) the surface oxidation condition, (ii) the current flow path, and (iii) the electronic interface condition. The model represented in the present study is regarded as a promising model and prediction mechanism which explains recent observations. The findings can be implemented in generating spin current without any use of external magnetic fields and heavy metals in spintronic devices.