Effect of dipolar interactions on the susceptibility peaks in a solidified ferrofluid

A number of magnetite (Fe3O4) ferrofluids with volume concentrations of Ε = 0.01, 0.02, 0.03, 0.04, 0.06 and 0.08 have been prepared by diluting a stock fluid of M3 = 43 kA/m. These samples were frozen in zero magnetic field, and the variation of the initial susceptibility with temperature was measured for each sample. We find, experimentally, that a maximum occurs at Tp in the initial susceptibility-versus-temperature curve, Tp moves to higher values as the concentration of the ferrofluid increases. A theoretical model derived for a noninteracting fine particle system in a solid matrix along with a semiempirical hypothesis for the effect of the interparticle interaction on the anisotropy constant seems to correctly predict the peak in the susceptibility as well as the effect of the concentration on the temperature of the peak.