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

Abstract

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...