Effects of cooling field on magnetic relaxation on an iron-nitride fine particle system

Effects of the external magnetic field Ha applied during cooling on the magnetic relaxation of a fine particle system are studied. We measured the temperature decay of the residual magnetization and the ac susceptibility for frozen iron-nitride magnetic fluids. It was found that the relaxation time distribution of the diluted sample is independent of Ha and coincides with that expected from the particle diameter distribution, as the superparamagnetic model presents. On the other hand, in the concentrated sample, it is clear that the relaxation time in zero field is widely distributed and depends on Ha.