An experimental investigation of dynamic behavior in FePt systems.

Magnetic relaxation experiments have been used to investigate the non-equilibrium dynamics of FePt nanoparticles. The system exhibits ageing at low temperatures, as well as a narrow energy distribution of the barrier to reversal. These properties were found susceptible to being affected by particle size, matrix and applied field strength. An analysis based on broad rate distributions is presented and compared with results obtained using energy barrier and viscosity interpretations. We find that a single broad distribution of relaxation times suggestive of cooperative effects is sufficient to explain the experimental results.