Local Order and Magnetic Properties in Mass-Selected ${\rm L}1_{0}$ CoPt Nanoparticles

Because of high magnetocrystalline anisotropy in bulk phase, binary nanoalloys from Co 3d and Pt 5d metal are expected to offer the opportunity to obtain room temperature nanomagnets. In this paper, we report a quantitative study combining X-ray absorption and magnetic measurements on well-defined size-selected CoPt clusters assemblies in the 2-4 nm range, diluted in an inert carbon matrix. The particles undergo A1 to L1₀ phase transition without coalescence upon annealing under vacuum. For the as-prepared samples, using a fcc chemically disordered A1 model, we have found a contracted lattice parameter in the whole sizes range, while the cobalt-platinum stacking in L1₀ chemically ordered phase corresponds to a larger tetragonalization at the nanoscale compared to the bulk phase. Such quantitative characterizations will be helpful for future theoretical calculations that may elucidate size and alloy effects on the magnetic behavior at nanoscale.