In Situ Analysis of Phase and Magnetic Transformation Process in Fe-Pt Nanoparticles
The process of phase transformation in individual Fe-Pt and Fe-Pt-Cu nanoparticles synthesized by the reverse micelle method with chemical homogeneity and mono-dispersion has been investigated by in-situ high-resolution electron microscopy (HREM) observation and in-situ nano-beam diffraction (NBD). The Fe-Pt particles, initially polycrystalline with the chemically disordered fcc (A1) phase, were reconstructed into A1 single crystals between 550 and 650°C, followed by a phase transformation from A1 to the chemically ordered fct (L10) phase between 650 and 680°C. The coalescence began almost concurrently with the phase transformation. They were transformed into round-shaped single-crystalline particles between 680 and 720°C. Similar processes were also observed in the Fe-Pt-Cu nanoparticles. The temperatures at which these processes occurred were substantially lower than those required for the Fe-Pt nanoparticles. We investigated the magnetic-field distribution of a submicron-size island comprising isolated L10 Fe-Pt nanoparticles magnetized along one direction by using in-situ electron holography at elevated temperatures. Although the magnetization decreased between 212 and 412°C to 25% of the strength at 25°C, it recovered 67% of the initial strength during cooling. However, when an island was heated to 512°C, the magnetization diminished and did not recover during cooling. The Curie temperature (Tc) was determined to be 350°C and was in good agreement with the Tc determined by bulk measurements, which was approximately 100°C lower than the Tc for bulk Fe55Pt45.
Young Won Chang, Nack J. Kim and Chong Soo Lee
M. Nakanishi et al., "In Situ Analysis of Phase and Magnetic Transformation Process in Fe-Pt Nanoparticles", Materials Science Forum, Vols. 561-565, pp. 2111-2114, 2007