Finite size effects on structure and magnetism in mass-selected CoPt nanoparticles

Abstract

We have studied size effect on the local structure and magnetism in CoPt clusters embedded in carbon matrix upon annealing in the 2–4 nm diameter range. From Co-K and Pt-L edges extended X-ray absorption fine structure (EXAFS) experiments and simulations, we report the refined quantitative non trivial structure of size-selected 3 nm in diameter CoPt magnetic clusters. In agreement with ab-initio simulation calculations, we evidenced an element-specific dependence of the local atomic relaxations in CoPt clusters leading to a strong distortion in pure Co planes. Since pure Co layers do not match the underlying Pt layer in chemically ordered L1₀-like clusters, we claim that a perfect crystal can not exist at nanosize. Such result could explain the low magnetic anisotropy energy measured on chemically ordered CoPt nanomagnets assemblies.