Lattice Monte Carlo simulations of FePt nanoparticles: Influence of size, composition, and surface segregation on order-disorder phenomena

Abstract

Order-disorder phenomena in FePt nanoparticles are investigated by lattice Monte Carlo computer simulations. The Metropolis algorithm is applied based on particle exchange and configurational energies that are calculated by an Ising Hamiltonian including nearest- and next-nearest-neighbor interactions. The adjustable parameters were determined from the bulk phase diagram and experiments on surface segregation of FePt thin films. By monitoring the long-range order parameter we study the influence of size, composition and surface segregation on the order-disorder transition. Our results reveal a distinct segregation behavior for nonstoichiometric compositions. Platinum atoms in excess exhibit a clear tendency for complete surface segregation on edge positions and (100) facets. Excess iron atoms, in contrast, tend to preferentially alloy in the bulk and on (100) facets. For a given particle size and variations in composition of $\ifmmode\pm\else\textpm\fi{}4%$ in maximum, the transition temperature is lowered by less than $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but more pronounced for excess Fe than Pt. For a fixed composition and varying size, in contrast, the transition temperature is lowered by $40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}380\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ compared to the bulk value for particle diameters ranging from $8.5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Nevertheless, for temperatures below $1100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the ordered phase is the thermodynamically stable phase for all particle sizes. The most pronounced effect on the ordering behavior is observed, if surface segregation tendencies are systematically modified. If one element is fully occupying the outer surface layer, we observe the formation of internal disorder and the formation of $L{1}_{2}$ domains. By increasing the concentration of the segregating element and thus readjusting the stoichiometric composition in the bulk, the order in the particles can be restored.