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 plat
eau, <(chi)over bar>(eq) of the dense sample is nearly the same as <(c
hi)over bar>(FC) while <(chi)over bar>(eq) of the diluted sample incre
ases 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.