Competitive effects of dipolar interactions and a bias magnetic field on the magnetic relaxation times of Co clusters

We have investigated, by means of ac susceptibility experiments, the magnetic relaxation of layers of Co nanoclusters (D≃2.6 nm) embedded in Al2O3. Superparamagnetic blocking takes place at higher temperatures as the number of layers increases. These results are interpreted using a simple model in which dipole–dipole interactions between nearest neighbor particles increase the relaxation time. The influence of interactions is affected by the application of bias magnetic field H. As the magnetic moments of the particles become polarized by H, the blocking temperature approaches the behavior expected for noninteracting particles.