Amorphous intermixing of noble and magnetic metals in thin film-based nanostructures

In nanostructures made of a mixture of bulk-immiscible metallic species, the alloy formation down to the atomic scale is a crucial and debated point. We report on the first experimental evidence of an amorphous metallic phase in Au-Co thin films and 2D array of nanostructures, that is constituted by a fine mixing of single-metal (sub)-nm domains, as shown by experiments coupling short- and long range- order characterization techniques, as X-ray Absorption Spectroscopy-XAS, X-ray Diffraction-XRD, Diffraction Anomalous Fine Structure-DAFS. Despite the mixing does not reach the atomic scale, the extended Au-Co interface can entail about half of atoms, and is responsible for the previously measured magnetic moment of Au in these systems. This amorphous nanomixed phase coexists with a minor fraction of fcc AuxCo1-x nanocrystals, preferentially oriented with the 111 crystallographic planes parallel to the film surface. 2D patterned Au-Co films with very similar structure can be easily obtained, but with smaller and randomly oriented nanocrystals. The thermal stability of the system (amorphous and crystalline) is limited to below 250 °C. At higher temperatures an extended decomposition occurs and Au and fcc Co nanocrystals coexist.