FePt, FePtAu, and FePtCu, synthesized by a chemical method, showed an ordering accompanied by the coalescence of nanoparticles during heat treatment at 750 K for 4h. FePt/SiO2 core/shell nanoparticles that did not coalesce did not show ordering. Only in FePt nanoparticles the heat treated in magnetic field, was the difference in switching field distribution observed to depend on the direction of the applied field. Magnetic measurement showed that this difference was caused by the induced ordered phase with high magnetic anisotropy along the direction of the magnetic field rather than the rotation of the magnetic easy axis. The temperature dependence of magnetization during heat treatment characterized the mechanism of ordering, i.e., the ordering of FePtAu and FePtCu nanoparticles and the coalescence occurred at the same time, in contrast to FePt nanoparticles which did not undergo ordering in complete in the time scale of magnetic measurement during a heating run and slowly underwent ordering over a long period of measurement. This slow ordering process of FePt nanoparticles should play an important role in the growth of magnetic anisotropy of FePt nanoparticle aggregation.
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