Blocking and Freezing of Magnetic Moments for Iron Nitride Fine Particle Systems

Equilibrium susceptibility ${\overline{\chi }}_{\mathrm{eq}}$ of frozen iron nitride magnetic fluids is estimated as the convergent value of the relaxation curves for various initial states. In the lower temperature range in which the field cooled susceptibility ${\overline{\chi }}_{\mathrm{FC}}$ shows a plateau, ${\overline{\chi }}_{\mathrm{eq}}$ of the dense sample is nearly the same as ${\overline{\chi }}_{\mathrm{FC}}$, while ${\overline{\chi }}_{\mathrm{eq}}$ of the diluted sample increases with decreasing temperature. These indicate that blocked moments are observed for the isolated particles owing to a finite measurement time and that the magnetic moments of the interacting particles freeze cooperatively as seen in a spin glass.