Abstract
We report the synthesis and characterization of CoPt nanoparticles, using X-ray magnetic circular dichroism (XMCD) at the Co L 2,3 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 μ L / μ S 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 1 0 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 μ S , μ L and the μ L / μ S 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 1.91 μ B /at. measured for μ S is unusual for Co and must be a signature of chemical order in CoPt alloy nanoparticles.
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