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Fe100-xPtx (x = 20, 25, and 30) thin films with nominal thickness 50 nm were prepared by radio-frequency (RF) magnetron sputtering onto glass substrates. Crystallographic structure and magnetic properties of such films were investigated by varying Fe content, annealing temperature (400-700degC), and heating process. Crystallographic structure obtained by X-ray diffraction (XRD) indicated that ordering of L12 Fe3Pt phase takes place for Fe75Pt25 postannealed above 400degC. Ordering parameter of the Fe75Pt25 sample increases from 0.29 to 0.94 as annealing temperature rises from 400degC to 700degC. Saturation magnetization 1270 emu/cm3 was obtained in Fe75Pt25 films annealed at 700degC. Fe70Pt30 films annealed at 700degC show similar characteristics with those of Fe75Pt25, while Fe80Pt20 films exhibit paramagnetism due to disordered Fe-rich face-centered cubic (fcc) structure. Postannealing seems necessary for formation of pure L12 Fe3Pt phase while in situ annealing leads to formation of a metastable tetragonal Fe3Pt phase, as confirmed by XRD.