Modeling the size- and shape-dependent surface order-disorder transition of Fe0.5Pt0.5 nanoparticles

On the basis of our previous work, the Debye model has been generalized for the size- and shape-dependent order-disorder transition and surface order- disorder transition of FePt nanoparticle (NPs) by considering the configuration entropy. The ordering temperature of FePt NPs decreases with a decrease of the particle size for a specific shape and decreases with a decrease of the shape factor at fixed size. The particle size is the dominant factor while the shape is the secondary one in affecting the transition. The ordering process starts from the surface and then propagates to the core, indicating the order-disorder transition is a surface-dominant process. To induce ordering, the annealing temperature should be lower than the surface ordering temperature, which suggests our calculation can be used to determine the highest annealing temperature. The calculation results also suggest that, at a small size and low temperature, the ordered NPs are stable but, at a large size and high temperature, the disordered NPs are stable. The present predictions agree well with the available literature data.