We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.4 nm (25 to 7500 atoms) prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared to previous structural characterizations. The distribution of activation energies was independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant K ef f . We find that K ef f 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 is shown to proceed classically down to the lowest temperature investigated (1.8 K).
By measuring the extraordinary Hall effect on a series of naturally oxidized Pt3 nm/Co90Fe10 0.6 nm/Al tAl samples with 0<tAl<1.2 nm, a first crossover from in-plane to perpendicular magnetic anisotropy is observed when tAl is varied from 0 to 0.2 nm. The CoFe magnetization remains out of plane for 0.2<tAl<0.5 nm. When the Al thickness is further increased, a second crossover back to in plane takes place. In a series of samples in which the Al thickness is kept constant, the same behavior is observed as the time of exposure to an oxygen plasma is varied. The results clearly indicate that the degree of oxidation of Al at the CoFe/AlOx interface has a dramatic effect on the magnetic anisotropy of the transition-metal layer. These sharp crossovers of anisotropy provide a very accurate and convenient way to monitor the oxidation of the tunnel barrier in magnetic tunnel junctions. This technique is also applied to characterize the oxidation kinetics of various ultrathin metallic layers as well as the aging effect in alumina barriers.
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