Microstructure and magnetic properties of high-coercive Fe-Pt alloy thin films

We investigated microstructures and magnetic domain structures of sputtered FePt alloy thin films in order to elucidate the origin of the high coercive force H-c. The FePt alloy thin films were prepared by RF sputtering method on water-cooled glass substrates. Transmission electron microscopy (TEM) shows that the as-deposited film consists of an fcc gamma phase with a grain size of 10 - 20 nm. Rippled domains were observed in the Lorentz micrographs. After annealing at 823 K,a steep increase in H-c up to similar to 800 kA/m and a decrease in resistivity rho were observed. The TEM observation indicates that the annealed film consists of an fct gamma(1) phase with a grain size of 20 - 80 nm. From random patterns of grain size scale in the Lorentz micrograph it is suggested that the high H-c might be explained as being due to a rotation of magnetization for each grain with a single-domain state. The H-c of the annealed samples increases with film thickness up to a thickness of 100 nm. From the thermomagnetic analysis (TMA), it is implied that the order-disorder transformation occurs at T > 620 K; the T-c of the disordered gamma and ordered gamma(1) phases were evaluated to be 599 K and 739 K, respectively.