Atomic ordering and coercivity mechanism in FePt and CoPt polycrystalline thin films

40 nm thick CoPt and FePt films were prepared on oxidized Si substrates with 10 nm MgO underlayers. The maximum coercivity (H_c ) for CoPt films was found to be ~10 kOe after annealing at 700°C for ~20-30 minutes (min). Structural analysis showed a significant amount of FCC phase as well as the ordered L1₀ phase in these films. FePt films showed an abrupt increase of ordered volume fraction and H_c in the initial stage of annealing and predominance of the tetragonal L1₀ phase after 10 min. at 700°C. The maximum H_c reached ~16 kOe after annealing at 700°C for more than 20 minutes. Dark field (DF) images of the annealed CoPt films showed individual grains which exhibited a possibility of several variants or disordered phase with dimensions similar to the exchange correlation length, b_cm. The temperature dependence of H_c seems to indicate a weak pinning mechanism in the highly ordered FePt films. Magnetic force microscopy indicated a complex domain structure consisting of clusters with dimensions of several hundred nanometers