Spin-dependent tunneling in Co/insulating granular systems: an AC approach

The spin-dependent tunneling of cobalt clusters embedded in Al₂O₃ or SiO_x has been analyzed as a function of the frequency at room temperature. Two sets of samples, with one or several layers of clusters, have been produced by alternate physical deposition of the metal and the insulator. The impedance versus frequency curves were measured with and without an external magnetic field. The results suggest that when the distance between successive cluster layers is small, some correlations between the cluster positions are present.