Influence of dipolar interaction on magnetic properties of ultrafine ferromagnetic particles

We use Monte Carlo simulations to study the influence of dipolar interactio n and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (F C) simulations we observe that the blocking temperature T-B clearly increas es with increasing strength of interaction, but it is almost not effected b y 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 s pin glass transition at T-B, the relaxational behavior of the magnetic mome nts below T-B is not in accordance with the picture of cooperative freezing .