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

We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.2 nm (25\char21{}7000 atoms) prepared by sequential sputtering of Co and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared with previous structural characterizations. The distribution of activation energies has been independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant ${K}_{\mathrm{eff}}.$ We find that ${K}_{\mathrm{eff}}$ is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal process is shown to proceed classically down to the lowest temperature investigated (1.8 K).