Phase Transformation and Magnetic Hardening in Isolated FePt Nanoparticles

Isolated monodisperse $L1_{0}$ FePt nanoparticles coated by carbon were obtained by adding enough surfactants that decomposed into carbon after the chemical synthesis and postannealing of the $A$1 FePt nanoparticles. The effect of isolation between FePt nanoparticles on the phase transition temperature and magnetic properties has been studied systematically by thermal, magnetic, and structural characterizations and analyses. It was found that the $A$1 to $L1_{0}$ phase transition temperature is dependent sensitively on the amount of isolation medium. The transition temperature shift reaches 150–200$^{\thinspace\circ}$C from nonisolated particle assemblies to completely isolated particles, which may be attributed to the high activation energy of the phase transformation for the isolated particles.