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 similar to 10 kOe after annealing at 700 degreesC for similar to 20-30 minutes (min). Structural analysis showed a significant amount of FCC phase as well as the ordered L1(0) 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 L1(0) phase after 10 min. at 700 degreesC. The maximum H-c reached similar to 16 kOe after annealing at 700 degreesC 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.