Alloying Effect in CoPt Nanoparticles Probed by X-ray Photoemission Spectroscopy: Validity of the Bulk Phase Diagram

CoPt organized cluster assemblies are developed to produce ultra-high-density magnetic media that need the smallest sizes for recording particles. At the nanometer scale (3−4 nm in diameter in our case), surface atoms represent around 50% of the total number of atoms and the segregation effect could dominate the alloying effect. Our purpose is to describe the competition between segregation and alloying effects in as-deposited or embedded bimetallic CoPt clusters preformed in the gas phase. The segregation is studied by X-ray photoemission spectroscopy. First of all, it was found that satellite peaks at high energy disappear when particles are fully embedded in a matrix. Moreover, CoPt clusters exhibit a partial core−shell structure when the clusters are not chemically bonded to the matrix. Conversely, an alloying effect is reported when CoPt particles are embedded in a silicon matrix. These effects are analyzed on the basis of both the phase diagrams and the enthalpy formation in the bulk phase. The bulk phase diagram remains valid for the nanometric scale, taking into account a scaling factor. Furthermore, the asymmetry of the photoemission peaks is carefully analyzed in terms of intrinsic and extrinsic energy losses.