Microstructure and Magnetic Properties of High-Coercive Fe–Pt Alloy Thin Films

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Abstract
We investigated microstructures and magnetic domain structures of sputtered FePt alloy thin films in order to elucidate the origin of the high coercive force HC. 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 γ 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 HC up to ∼800 kA/m and a decrease in resistivity ρ were observed. The TEM observation indicates that the annealed film consists of an fct γ1 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 HC might be explained as being due to a rotation of magnetization for each grain with a single-domain state. The HC 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 TC of the disordered γ and ordered γ1 phases were evaluated to be 599 K and 739 K, respectively.

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