Quantitative structure analysis of L10-ordered FePt nanoparticles by HRTEM

We present a quantitative analysis of the structure and morphology of FePt nanoparticles from the gas phase and a statistical evaluation of the fraction of L10-ordered particles using high-resolution transmission electron microscopy (HRTEM) combined with computer simulations. The particles are generated via inert gas condensation using DC sputtering and annealed during the flight. The nucleation pressure and annealing temperature were varied in the range 0.5 mbar ⩽p⩽1.0 mbar and 873 K ⩽T⩽1273 K, respectively. We observe a decrease of the number of multiply twinned particles with increasing nucleation pressure. Furthermore, at p=1.0 mbar and T=1273 K, about 31% of the particles were found to be simply twinned, and 27% exhibit a single-crystalline structure. The formation of the ordered L10 phase in FePt nanoparticles prepared under these conditions was observed. The number of particles, which had transformed into the L10-state, was evaluated by statistical counting using HRTEM in combination with contrast simulations. We have derived criteria for the visibility of both the L10 super structure and the disordered face-centered cubic (FCC) structure depending on the orientation of the particles. Based upon these criteria, it is shown that 36% of the FePt nanoparticles are in the L10-ordered state.