Structural, electronic and magnetic properties of ConPtM−nConPtM−n, for M=13, 19, and 55, from first principles

• Homotops of CoPt binary clusters with 13, 19 and 55 atoms are relaxed using SIESTA.
• Atomic order and segregation in bimetallic CoPt nanoclusters are analyzed.
• Structure and magnetism are very sensitive to size, composition and local order.
• Interplay between magnetism and spatial order in bimetallic nanoclusters is studied.

The geometrical structure, chemical ordering, electronic and magnetic properties of ConPtM−nConPtM−n, for M=13, 19, and 55, are investigated, within the framework of the density functional theory as implemented in the SIESTA code. For the exchange and correlation potential, we used the Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA). We considered the hexagonal, the cubo-octahedral (CO), the centered pentagonal prism (CPP), and the icosahedral (ICO) geometries. We found that the CO, ICO, and CPP structures have similar energies and the lowest energy configuration changes as a function of the Co concentration and cluster size. In most of the cases, around the equiatomic concentration, the Co and Pt atoms form chemically ordered structures, but most often not the L10 structure stable in bulk. We observed that the Pt atoms are segregated to the surface in all the concentration range. A detailed analysis of the dependence of the magnetic moment of the components, and their average, as a function of structure and of chemical composition is reported. A strong polarization of the Pt atoms by the nearest neighbor Co atoms is observed.