Reduction of Ordering Temperature in Substituted FePtM (M = Ni,Cu) Nanoparticles Formed by Chemical Synthesis

FePt and CoPt-type nanoparticles made by chemical synthesis have recently become promising candidates for ultra-high density magnetic recording media [1]. However, at high annealing temperatures, the particles sinter together and array formation is lost. Recent studies [2] have reported reduced transformation temperatures with the addition of Ag and Au in the FePt nanoparticles. In this study, we used M = Ni, Cu substitution to reduce the transformation temperature. The as-made FePtNi and FePtCu have the disordered fcc structure with zero coercivity at room temperature. After annealing at temperatures in the range of 300-600 $^{o}$C, the particles become fct with a coercivity of 4 kOe in FePtCu at 400 $^{o}$C and 6 kOe in FePtNi at 500 $^{o}$C. The high coercivity obtained at the lower annealing temperature suggests a lower transformation temperature in both alloys. Preliminary DSC studies showed a reduced transformation temperature in the Ni substituted samples. The structural transformations that occur after annealing and their effects on magnetic properties are currently being investigated by HREM and M\"{o}ssbausser spectroscopy. References [1] S. Sun, et al. Science, 287, 1989-1992 (2000) [2] S. Kang, et al., IEEE T MAGN, 39, 5, 2753 (2003)