Growth, structure and magnetism of self-organized epitaxial nano-alloys on a metallic substrate

The CoPt alloy is one of the most studied bimetallic compounds, due to its potential application for magnetic recording. We report here on Co$_{x}$Pt$_{1-x}$ nano-alloys deposited on the well-known Au(111) reconstructed surface since it has been recognized as a powerful substrate in order to investigate the magnetic properties of metallic nano-clusters [1,2]. The growth of Co$_{x}$Pt$_{1-x}$ clusters on the Au(111) surface observed by STM revealed a morphological transition from single layer to bilayer islands with the Co concentration x. Using molecular dynamics calculations, we show that this transition is driven by the local strain due to Co atoms. These results are interpreted by a competition between the interface energy, the mixing energy and the elastic energy. Using X-ray Magnetic Circular Dichroism, we have studied the magnetic properties of these nano-alloys. The out-of-plane anisotropy of pure Co clusters strongly decreases, until it goes in-plane for 40\% of Pt. This spin reorientation transition is interpreted by a phenomenological pair model for magnetic anisotropy. \textbf{References} [1] N. Weiss et al., Phys. Rev. Lett. 95, 157204 (2005) [2] S. Rohart et al., Phys. Rev Lett. 104, 104, 137202 (2010).