Modeling Size and Shape Effects on the Order-Disorder Phase-Transition Temperature of CoPt Nanoparticles (original) (raw)

Bimetallic nanoparticles (NPs), such as CoPt, FePt, or FePd with the CuAu-type (L1 0 ) ordered structure, may fi nd applications in next-generation ultrahigh-density magnetic storage media. [ 1 , 2 ] However, as-produced NPs are typically disordered solid solutions with a face-centered cubic (FCC) structure. To recover the ordered structure, an annealing technique must be used. Nevertheless, it is diffi cult to obtain ordered NPs smaller than 5 nm. Recently, Alloyeau et al. have succeeded in preparing ≈ 2-3-nm ordered CoPt NPs with an order-disorder transition temperature ( T C ) of ≈ 773-923 K, which is lower than the 1098 K of bulk CoPt. They reported that a disk-like 0.5-nm-thick CoPt NP always keeps the FCC disordered phase independent of the annealing temperature and its in-plane size and that a 1-nm-thick NP changes from FCC to L1 0 at 1023 K if its in-plane size is larger than 3 nm, showing that T C depression occurs for NPs with in-plane sizes smaller than 5 nm. Further, the authors simulated a nanosolid of 1.5-nm thickness and 4-nm in-plane size and found T C at ≈ 923 K. While these are interesting and signifi cant results, there is, however, no available theory to explain the above experiments. [ 7 , 8 ] In this Communication, we present a model to account for size-and morphology-dependent orderdisorder transition temperatures of bimetallic nanoparticles, aiming to explain the new experimental fi ndings. The free energy difference Δ G between the disordered and ordered phases at a temperature T is given by G (E dis or der − E order ) − T(S dis or der − S order ) , where E disorder and E order are the cohesive energies of the ordered and disordered phases and S disorder and S order are the corresponding entropies, respectively. The volume variation during the solidphase transformation is very small and is neglected. For Δ G > 0, the ordered phase is stable and, for Δ G < 0, the disordered phase is stable, whereas Δ G = 0, denoting the transition point between the ordered and disordered phases, can be used to determine the order-disorder transition temperature, T C .