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.

• Received 11 January 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.167