Electronic structure of CoPt based systems: from bulk to nanoalloys.

An accurate description of the local electronic structure is necessary for guiding the design of materials with targeted properties in a controlled way. For complex materials like nanoalloys, self-consistent tight-binding calculations should be a good alternative to ab initio methods, for handling the most complex and large systems (hundreds to thousands of atoms), provided that these parameterized method is well founded from ab initio ones that they intend to replace. Ab initio calculations (density functional theory) enabled us to derive rules for charge distribution as a function of structural change and alloying effects in Co and Pt based systems, from bulk to nanoalloys. A general local neutrality rule per site, orbital and species was found. Based on it, self-consistent tight-binding calculations could be implemented and applied to CoPt nanoalloys. A very good agreement is obtained between tight-binding and DFT calculations in terms of local electronic structure.

Rules for tight-binding calculations in bi-metallic compounds based on density functional theory: the case of Co-Au.

Even though recent developments in electronic structure calculations based on density functional theory (DFT) allow us to use them for more and more realistic systems, they still remain unsuited for comprehensive studies of complex transition metal compounds involving intricate structural and chemical effects. In that case, the tight-binding approximation (TBA) is a good compromise to get reliable results with only a minimal set of parameters, provided that clear rules enable a proper self-consistent treatment of charge transfers between inequivalent sites. Thus, in the case of the Co-Au system, DFT calculations demonstrate that a local neutrality rule is obeyed per orbital and per chemical species. Shifting the atomic levels accordingly in TBA calculations is then sufficient to accurately determine the local densities of states whatever the chemical configuration. In addition, this also allows us to justify the derivation from TBA of pairwise ordering pair interactions and to determine them self-consistently.

Dominant role of the epitaxial strain in the magnetism of core-shell Co/Au self-organized nanodots.

Self-organized Co nanodots on a Au(111) surface have been surrounded by controlled Au rings that progressively cap the entire dots. The magnetic susceptibility of these dots has been measured in situ as a function of the Au coverage. The blocking temperature increases when the Co bilayer dots are surrounded by the first Au atomic layer and decreases with the subsequent capping. This result cannot be explained by interfacial anisotropy which is generally assumed to be the dominant term in the magnetic anisotropy of nanostructures. Using molecular dynamics simulations, we evidence that the large strain inside the Co clusters is the main driving force for the anisotropy changes during the Au encapsulation.