Size dependence, nucleation, and phase transformation of FePt nanoparticles

Monolayers of chemically prepared FePt nanoparticles were deposited on silicon nitride membrane windows, and annealed under different conditions. Hysteresis loops showed a bimodal distribution of switching fields. The size dependence determined from transmission electron microscopy was correlated with the magnetic switching field distribution, assuming that all particles above a threshold size d(*) had high anisotropy. This simplified model yielded a common value of d(*), independent of the annealing conditions. The dependence of d(*) on the initial particle size indicated the importance of grain boundaries as nucleation sites, and supports the hypothesis that the fcc-to-fct phase transformation is kinetically limited. (c) 2005 American Institute of Physics.