Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (T(C)(NP)) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4-3-nm particles, T(C)(NP) is found to be 325-175 degrees C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that T(C)(NP) is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of T(C)(NP). This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties.

In order to determine the possibilities to control the chemical configuration of bimetallic nanoparticles, we have considered CuAg nanoparticles synthesized by a physical route as a model in this study. The synthesis was made by pulsed laser deposition under ultra-high vacuum conditions, via a sequential deposition procedure. We show that the temperature of the substrate and the absolute quantity of Ag in a particle are the main parameters that drive the chemical configuration. To explain the transition from a core-shell configuration to a Janus configuration as a function of Ag quantity, we have conducted density-functional theory calculations and atomistic molecular dynamics simulations to investigate the stability of this system. The results are presented together with the experimental observations.