The particle interaction effects in the field-cooled and zero-field-cooled magnetization processes

The present theories explaining the mechanism of particle interaction within a fine particle system driven by the thermal agitation assign the increase of the interaction strength either to an increase of the particle anisotropy due to the environment reaction to its dipole moment, or to the occurrence of a collective state. The particle interaction effects on the field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves are the anisotropy effect, referring to the increase of the temperature TMAX, corresponding to the ZFC curve maximum, with increasing sample volume concentration, and the mean-field effect, referring to the flattening of both, FC and ZFC, magnetization curves with increasing sample demagnetizing factor, without altering TMAX in the low applied field limit. We demonstrate that the Onsager mean-field model is able to recover an increase of the particle anisotropy with increasing sample volume concentration using a cavity having the shape of an oblate ellipsoid, the eccentricity increasing with increasing sample volume concentration. The proposed explanation is the formation of particle clusters having a uniaxial symmetry in the particle arrangement (chain-of-particles). We show that the anisotropy effect of interactions is due to not only an increase of the particle anisotropy with increasing sample volume concentration, but also to a temperature-dependent interaction field distribution due to the local non-homogeneity of the particle dispersion. The proposed model is able to recover the experimental FC and ZFC initial susceptibility curves for various concentrations of γ-Fe2O3 nanoparticle systems.