Enhancement of spin current by modifying the chemical potential profile in the nanopillar

In this paper, a formalism calculating the spatial (3D) distribution of the electrical chemical potential inside a nanostructure consisting of arbitrary ferromagnetic (FM) and nonmagnetic (NM) layers is developed. The formalism is based on the 3D circuit network of spin-dependent-resistor elements (SDRE) whose electrical response is calculated by Valet-Fert 1D model. With this formalism, nanopillars with various electrode shapes are investigated. The calculated electrochemical potential profiles show the difference between the constriction type and the S-constant and column types. In the constriction type pillar, the top infinite Cu layer is a strong spin-flip scatterer providing large volume for spin current to be scattered. This suppresses the splitting of the electrochemical potential and therefore modify its profile. Based on the numerical analyses, two kinds of nanopillars are fabricated with 15 nm Pt and 20 nm Au as top layers. The spin-flip resistance of Pt is only half of that of Au, indicating that Pt is a stronger spin-flip scatterer than Au. Also, the Pt top layer reduces the critical switching current density J_CS by about 25% compared to Au top layer. The effects of the interfacial perpendicular anisotropy in both cases are found almost the same.