The nature of A1–L10 ordering transitions in alloy nanoparticles: A Monte Carlo study

Equilibrium A1–L10 ordering transitions in alloy nanoparticles are investigated from Monte Carlo simulations based on a lattice-model framework with parameters chosen to be representative of the FePt system. The present simulations yield results for the effects of size and surface segregation upon equilibrium ordering temperatures in good agreement with previous Monte Carlo studies and the most recent experimental measurements for FePt nanoparticles. Specifically, for nanoparticle diameters in the range of ∼4 nm the calculated equilibrium ordering transition is estimated to lie well above the temperatures commonly employed in the annealing of FePt nanoparticles, and at such annealing temperatures the magnitude of the size-induced reduction in the volume-averaged long-range order parameter is estimated to be of the order of 20%. In this work a detailed analysis of the nature of the L10 ordering transition in confined nanoparticle geometries is presented based upon results obtained for surface concentrations, order parameter profiles and calculated canonical distribution functions. It is shown that the main results obtained from this and previous Monte Carlo simulation studies, namely the size dependence of the volume-averaged long-range order parameters and ordering temperatures, and the continuous nature of the ordering transition, can be understood as being natural manifestations of a disordering mechanism that is surface induced.