The magnetic relaxation of highly diluted monodisperse iron-based particles was measured between 10−5 and 104 s, for different temperatures and particle sizes. The decay over a very broad range of times indicated a distribution of energy barriers remained despite the narrow range of sizes. Many features of this decay could be explained using a percolation model of particles with weak dipolar interactions. However, this model did not predict the abrupt change in the decay rate at long times, which was observed for samples of both 8.0 and 5.5 nm particles below a threshold temperature.
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Chamberlin et al. (2002) studied this question.
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