Ta doping effect in Co60C40 thin films

(Co₆₀C₄₀)_1-xTa_x films (0≤x≤-10%) with thickness of 30 nm were cosputtered onto C-buffered glass substrates at room temperature. The as-deposited films are structurally amorphous and nonferromagnetic. The annealed films (400°C for 30 min) become magnetically hard with H_c ranging from 480 to 780 Oe and M_s changing from 480 to 600 emu/cm³. Both H_c and M_s increase with Ta concentration as x<3%, show peaks at x=3%, and then decrease with Ta concentration. This is because a small amount of Ta doping might enhance the diffusion of carbon in the films and thus form more cobalt grains upon annealing. However, doping more Ta (x>3%) destroys the Co structure and this causes the drop in both H_c and M_s. After annealing at 300°C, H_c of (Co₆₀C₄₀)₉₇Ta₃ films increases from 0 to about 380 Oe, while almost no changes are observed for Co₆₀C₄₀ films. Doping in a Co-C system with a larger atom such as Ta may be a way to enhance the segregation of Co and C and, therefore, reduce phase segregation temperature.