  We report on the magnetic properties of Co$\_x$Pt$\_{1-x}$ clusters embedded
in various matrices. Using a careful analysis of magnetization curves and ZFC
susceptibility measurements, we determine the clusters magnetic anisotropy
energy (MAE) and separate the surface and volume contributions. By comparing
different chemical compositions, we show that a small amount of Pt (15%)
induces an important increase in the volume anisotropy with respect to pure Co
clusters, even in chemically disordered fcc clusters. Comparing the
measurements of clusters embedded in Nb and MgO matrices, we show that the
oxide matrix induces an important increase of the surface MAE attributed to the
formation of an antiferromagnetic CoO shell around the clusters.
