Blocking and freezing of magnetic moments for iron nitride fine particle systems

Equilibrium susceptibility <(chi)over bar>(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 <(chi)over bar>(FC) shows a plateau, <(chi)over bar>(eq) of the dense sample is nearly the same as <(chi)over bar>(FC) while <(chi)over bar>(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.