Saturation of Nuclei Concentration in the Phase Transformation of FePt Nanoparticles

The role of nucleation site densities in the A1 to L1₀ transformation in FePt nanoparticles was studied and quantified. Overcoated FePt nanoparticles were annealed at temperatures from 690 degC to 1100 degC for times of 30 min to 2 h. The concentration of the L1₀ phase in the unsintered particles was found to saturate during annealing. The transformation is shown to be kinetically limited by a slow nucleation rate and to rely on pre-existing nucleation sites. Very few new nucleation sites were created during annealing of separate nanoparticles. Even when particles were allowed to sinter, the rate of forming new nucleation sites was extremely low. Sintering aided the phase transformation because the limited nucleation sites can transform a larger volume fraction after particles sinter together