Abstract
We report the refined quantitative magnetic moments of size-selected 3 nm in diameter CoPt clusters at both Co and Pt-L2, 3 edges. By combining the high photon flux and the chemical selectivity of the synchrotron radiation delivered on ESRF beam lines, we have studied size effect on the local structure and magnetism in CoPt clusters embedded in carbon matrix upon annealing. On one hand, from Pt-L edges extended X-ray absorption fine structure (EXAFS) experiments and simulations associated to previous Co-K edge ones, we have determined the first-shell Co-Co, Co-Pt and Pt-Pt coordination numbers and mean atomic distances in agreement with recent VASP ab initio calculations obtained for L10-like clusters. On the other hand, X-ray magnetic circular dichroism (XMCD) investigations have revealed a significant increase of both Co and Pt magnetic moments for the chemically ordered CoPt clusters compared to the bulk values while the magnetic anisotropy energy (MAE) of annealed samples is twice that of the as-prepared samples, but one order of magnitude smaller than what is expected for the L10 CoPt bulk.
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