The perpendicular magnetic anisotropy (PMA) observed on CoxPt1-x nanostructures makes them good candidates for applications on high density magnetic storage devices. An interesting behaviour has been recently reported on CoPt3(111) grown on WSe2(0001) substrate by molecular beam epitaxy (MBE). In these systems the weak van der Waals interactions between the adatoms and the Se atoms forming the topmost hexagonal plane of the substrate, lead to epitaxially grown nanostructures which depict drastic enhancement of ordering kinetics and perpendicular magnetic anisotropy (PMA) for growth temperatures well below 420K.

Preliminary RHEED (reflection high energy electron diffraction) and XRD (x-ray diffraction) measurements have shown that the nanostructures formed at room temperature have a chemically disordered fcc structure. Raising the deposition temperature (Td) above 400K, the L12-type long range order appears and PMA vanishes.

Polarized XAFS measurements allow probing the local structural anisotropy related to the out-of-plane easy magnetization axis responsible for the PMA. In particular, it has been possible distinguishing the structural features parallel and perpendicular to the substrate plane by accurate Co K-edge XAFS data analysis, performed including single and multiple scattering contributions up to the fourth neighbour shells. A short range structural anisotropy has been observed by XAFS in the sample prepared at room temperature, in contrast with the disordered long range structure found by RHEED and XRD. This anisotropy, characterized by preferential Co-Co correlations parallel to the film plane, disappears in the samples deposited at higher temperatures Td=700K.

To understand the reason of such Co segregation, CoPt3 nanostructures were grown on the low energy surface, NaCl(001). In these samples the local (XAFS) as well long range (XRD) structural features are definitively isotropic demonstrating the relevance of the substrate in determining the structural anisotropy then the magnetic response of these nanoparticles.