In-situ HREM Observation of Phase Transformation Process in FePt and FePtCu Nanoparticles
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In-situ HREM Observation of Phase Transformation Process in FePt and FePtCu Nanoparticles Masatoshi Nakanishi1,2, Gen-ichi Furusawa2, Kokichi Waki2, Yasushi Hattori2, Takeo Kamino3, Katsuhiro Sasaki1, Kotaro Kuroda1, Hiroyasu Saka1 1 Department of Quantum Engineering, Nagoya University, Nagoya, Aichi 464-8603, Japan 2 Advanced Core Technology Laboratories, Fuji Photo Film Co., Ltd., Minamiashigara, Kanagawa 250-0193, Japan 3 Naka Application Center, Hitachi High-Technologies Corporation, Hitachinaka, Ibaraki 312-0057, Japan ABSTRACT The processes of phase transformation in individual nanoparticles of FePt and FePtCu synthesized by the reverse micelle method, which are chemically homogeneous and monodisperse, have been investigated by an in-situ HREM observation in a FE-TEM. Polycrystalline FePt particles, initially of the chemically disordered face-centered cubic phase (A1), were reconstructed into A1 single crystals between 25 and 650 °C, followed by phase transformation from A1 to the chemically ordered face-centered tetragonal phase (L10), which began between 650 and 680 °C. The coalescence began concurrently with phase transformation, i. e., between 650 and 680 °C. Then, they turned to round-shaped L10 particles between 680 and 720 °C. The single crystal formation, the phase transformation from A1 to L10, the coalescence and the round-shaped particle formation were also observed in the FePtCu nanoparticles. The temperatures of single crystal formation, phase transformation (and coalescence) and round-shaped particle formation of the FePtCu nanoparticles were between 25 and 500 °C, between 550 and 600 °C and between 600 and 650 °C, respectively. These temperatures were substantially lower than those for the FePt nanoparticles. INTRODUCTION High-density magnetic recording media have been improved by reducing the grain size of magnetic materials. However when the grain size is smaller than the optimum, the magnetic recording becomes thermally unstable. L10 FePt alloy is expected to be an excellent magnetic material for high-density magnetic recording media because of its high magnetocrystalline anisotropy [1]. Many studies have been carried out to synthesize L10 FePt nanograins. Sun et al. prepared FePt nanoparticles by reduction of Fe(CO)5 and Pt(acac)2 in an organic solution at high temperatures [2]. The crystal structure of the as-prepared nanoparticles was A1, which was transformed to ferromagnetic L10 during high-temperature annealing. They measured the phase transformation temperature of FePt particles from A1 to L10 by annealing in situ in a TEM. They observed coalescence of particles, formation of twin boundaries and faceting of coalesced L10 particles during annealing in TEM. They also observed polycrystalline A1 particles in the as-prepared particles, the occurrence of which decreased after annealing. However, detailed processes of the decrease of polycrystalline A1 particles, phase transformation and coalescence
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are not well understood yet. The effect of Cu addition on t
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