Phase transitions of iron-nitride magnetic fluids.

Phase transitions of magnetic fluids in the zero field were studied from the viewpoint of static susceptibility and typical relaxation time in order to distinguish between the transitions and the blocking phenomena. We used iron-nitride magnetic fluids containing nearly uniform particles so as to remove the effects of particle-size distribution. In the densest sample, we observed the growth of ferromagnetic fluctuations and, in the diluted samples, a temperature-induced first-order phase transition.