Magnetic properties of superparamagnetic particles by a Monte Carlo method

We develop and carry out Monte Carlo simulations for an ensemble of superparamagnetic particles uniformly distributed in a nonmagnetic matrix. We find the magnetization below the blocking temperature ${\mathit{T}}_{\mathit{B}}$ when it shows hysteresis and above ${\mathit{T}}_{\mathit{B}}$ in the superparamagnetic region. We determine the blocking temperature for a set of anisotropy strengths from the magnetization and the susceptibility of the particles. A fixed number of Monte Carlo steps with a constrained acceptance rate is shown to be equivalent to an observation time in the simulations that is much shorter than experimental observation times. We show how the blocking temperature obtained in the simulations can be converted into the corresponding experimentally measurable blocking temperature by using this difference in the observation times. This provides a method to compare Monte Carlo simulation results with experiments, such as recent ones on fcc Co particles. © 1996 The American Physical Society.