Slow relaxation in ferromagnetic nanoparticles: Indication of spin-glass behavior

We use Monte Carlo simulations to study the influence of dipolar interactions and polydispersion on the magnetic relaxation of single-domain ferromagnetic particles below the blocking temperature. We find the surprising result that for all particle densities the relaxation rate decays by a power law, with a density-dependent exponent n and a temperature-dependent prefactor. Depending on the value of n, the relaxation function shows a simple exponential decay, a stretched exponential decay, or a power-law decay, and seems to approach a finite remanent magnetization at high densities. We interpret the results for intermediate and large densities as indications of spin-glass behavior.