XMCD study of CoPt nanoparticles embedded in MgO and amorphous carbon matrices

We report the synthesis and characterization of CoPt nanoparticles, using X-ray magnetic circular dichroism (XMCD) at the Co L2,3 edges. Clusters are produced in ultra-high vacuum conditions, following a physical route, and embedded in non-metallic matrices: MgO and amorphous carbon (a-C). In MgO, Co atoms are partially oxidized, which goes with a μL/μS enhancement. On the contrary, a-C appears as a very suitable matrix. In particular, annealing of CoPt cluster embedded in a-C is able to promote L 10 chemical order, without alteration of the sample. This transformation, which has been directly evidenced by transmission electron microscopy observations, is accompanied by a striking augmentation of μS, μL and the μL/μS ratio of Co. The presence of Pt leads to an enhanced Co magnetic moment, as compared to Co bulk, even for the chemically disordered alloy. Moreover, the high value of 1.91μB/at. measured for μS is unusual for Co and must be a signature of chemical order in CoPt alloy nanoparticles.