Magnetic characterization of granular Co/Al2O3 multilayers

We study the magnetic properties of Co clusters with diameters between 0.8 and 3 nm prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data. Comparing this to the distribution of activation energies, obtained from the AC susceptibility, we have determined accurately the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and it increases as the average particle size decreases.