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We study the magnetic properties of spherical Co clusters with diametersbetween 0.8 nm and 5.4 nm (25 to 7500$ atoms) prepared by sequential sputteringof Co and Al2O3. The particle size distribution has been determined from theequilibrium susceptibility and magnetization data and it is compared toprevious structural characterizations. The distribution of activation energieswas independently obtained from a scaling plot of the ac susceptibility.Combining these two distributions we have accurately determined the effectiveanisotropy constant Keff. We find that Keff is enhanced with respect to thebulk value and that it is dominated by a strong anisotropy induced at thesurface of the clusters. Interactions between the magnetic moments of adjacentlayers are shown to increase the effective activation energy barrier for thereversal of the magnetic moments. Finally, this reversal is shown to proceedclassically down to the lowest temperature investigated (1.8 K).Comment: 13 figures submitted to Phys. Rev.

Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: Influence of the surface and of interparticle interactions