Microstructures of FePt-Al-O and FePt-Ag nanogranular thin films and theirmagnetic properties

The microstructure and magnetic property relationship in L1(0) ordered FePt -Al-O and FePt-Ag granular thin films have been studied. As-sputtered FePt- Al-O films, composed of isolated fine spherical FePt particles of similar t o2 nm diameter with a disordered face-centered-cubic (fcc) structure, exhib it superparamagnetism. Annealing above 650 degreesC induces a transformatio n from a disordered fcc structure to ordered L1(0) phase and the films beco me magnetically hard. The microstructures of these films change greatly dep ending on the film compositions and annealing conditions, which are correla ted with the magnetic properties. It was found that FePt particles smaller than 5 nm do not order at 500 degreesC, while the continuous FePt film orde rs perfectly at the same temperature, suggesting that the ordering temperat ure, T-c, decreases significantly when the particle size becomes less than 5 nm. In the FePt-Ag granular thin film, when the Ag composition is around 50 at. %, high coercivity (similar to 10 kOe) and fine uniform microstructu re are obtained by annealing the films at 550 degreesC for 1 h. The changes in the magnetic properties of these granular films are discussed based on microstructural observation results. (C) 2001 American Institute of Physics .