A dynamic study of small interacting particles: superparamagnetic model and spin-glass laws

The dynamic properties of a system constituted by small interacting particles (iron grains dispersed in an amorphous alumina matrix) have been studied by AC susceptibility measurements at low frequency ( nu =2*10^-2 and 2*10^-3 Hz). Maxima have been found for the in-phase susceptibility chi ' (at T_B) and for the out-of-phase susceptibility chi ". The frequency dependence of T_B is analysed over a large frequency range, including previous results from AC susceptibility measurements (17<or= nu <or=10⁴ Hz) and Mossbauer spectroscopy. The results are compared with (i) the predictions from a superparamagnetic model, which takes into account the effect of magnetic interactions on the relaxation time, and (ii) the dynamic scaling laws proposed for spin glasses (the Fulcher law, the generalised Arrhenius law (transition at T_c=0) and the power law (transition at T_c not=0)). The results are not compatible with a transition at a finite temperature. They are satisfactorily explained by the authors superparamagnetic model, including inter-particle interactions, which implies a transition at a temperature of 0 K.