The dependence of nano-contact magnetoresistance on the bulk scattering spin asymmetry in CoFe alloys with oxidation impurities

Nano-contact magnetoresistance (NCMR) spin-valves (SVs) using an AlO_x nano-oxide-layer (NOL) have numerous nanocontacts in the thin AlO_x oxide layer. The NCMR theoretically depends on the bulk scattering spin asymmetry (β) of the ferromagnetic material in the nanocontacts. To determine the relationship between NCMR and β, we investigated the dependence of NCMR on the composition of the ferromagnetic material Co_1-xFe_x. The samples were annealed at 270 °C and 380 °C to enhance the MR ratio. For both annealing temperatures, the magnetorsistance ratio in the low-resistance area product region at less than 1 Ω μm² was maximized for Co_0.5Fe_0.5. To evaluate β exactly, we fabricated current-perpendicular-to-plane giant magnetoresistance SVs with Co_1-xFe_x/Cu/Co_1-xFe_x layers and used Valet and Fert's theory to solve the diffusion equation of the spin accumulation for a ferromagnetic layer/non-ferromagnetic layer of five layers with a finite diffusion length. The evaluated β for Co_1-xFe_x was also maximized for Co_0.5Fe_0.5. Additionally, to determine the difference between the experimental MR ratio of NCMR SVs and the theoretical MR ratio, we fabricated Co_0.5Fe_0.5 with oxygen impurities and estimated the decrease in β with increasing oxygen impurity concentration. Our Co_0.5Fe_0.5 nano-contacts fabricated using ion-assisted oxidation may contain oxygen impurities, and the oxygen impurities might cause a decrease in β and the MR ratio.