Article ID: MB200826
A realistic lattice gas model with a tetrahedral 4-body interaction is derived for a system composed of Fe, Pt atoms and vacancies on the basis of first-principles calculations. Using this model, we carry out lattice Monte Carlo simulations of order-disorder phase transition in a bulk FePt alloy, aggregation into FePt clusters in vapor, and L10 ordering in FePt clusters. The order-disorder phase transition temperature of a bulk FePt is estimated to be 1970 K, which is slightly higher than the experimental value of 1572 K because of the ignorance of the off-lattice effects. The present model shows inherent atomic cohesion that leads to aggregation into clusters in a simulation starting from a random configuration in vapor. Finally for FePt alloy clusters, we find that the L10 ordered structure is maintained only for those clusters with a size (diameter) greater than 2.5 nm, in accordance with the recent experimental evidence reported by Miyazaki et al.