Influence of Dipolar Interaction on Magnetic Properties of Ultrafine Ferromagnetic Particles

We use Monte Carlo simulations to study the influence of dipolar interaction and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (FC) simulations we observe that the blocking temperature $T_{\mathrm{B}}$ clearly increases with increasing strength of interaction, but it is almost not effected by a broadening of the distribution of particle sizes. While the dependence of the ZFC/FC curves on interaction and cooling rate are reminiscent of a spin glass transition at $T_{\mathrm{B}}$, the relaxational behavior of the magnetic moments below $T_{\mathrm{B}}$ is not in accordance with the picture of cooperative freezing.