Non-equilibrium dynamics in fine magnetic particle systems

Extended dynamics (broader than Debye relaxation) occurs in fine magnetic particle systems because of the particle size distribution and the influence of interparticle interaction. In dilute negligibly interacting systems the broadening of the relaxation is governed by the effective anisotropy distribution of the particles. In dense systems, however, interparticle interaction alters the nature of the dynamics and phenomena due to correlation of the particle moments appear. The effects may at a glance only look like a change in the effective anisotropy of the individual particles. However, studies of the dynamics over several decades in observation time reveal that correlation related effects dominate the dynamics on long time scales and extend the maximum relaxation time well beyond the relaxation time of the individual particles. In addition, non-equilibrium dynamics, reminiscent of the ageing phenomenon of spin glasses, appears. Results and analyses of ac susceptibility and magnetic relaxation experiments on various fine magnetic particle systems, ranging from concentrated monodispersed ferrofluids to mechanically alloyed granular materials, are used to illustrate the cooperative nature of the dynamics.