THEORY OF THE CURIE-WEISS BEHAVIOR OF AN AGGREGATED MAGNETIC SUSPENSION

We develop the aggregate mechanism of interaction between particles in a ferrofluid. As the mechanism of Curie-Weiss behavior of a magnetic liquid seems to be the formation of aggregates of magnetic particles, further information on the formation of aggregates can be obtained by measuring the toroidal susceptibility of the aggregate suspension. We consider the model of rigid (unchangeable in form) aggregates consisti ng of superparamagnetic particles. It is assumed that the magnetic str ucture of dipoles inside the aggregate arises owing to their interacti ons with each other when the temperature is sufficiently reduced. A ge neral expression is derived for initial susceptibilities of the suspen sion (magnetic, toroidal and cross) for a rigid aggregate of arbitrary form, and it is shown that the temperature dependence of the suscepti bilities is of a more general form than in the simple Curie-Weiss law. These susceptibilities are calculated for the best known aggregates, two-particle aggregates. It is shown that the main contribution of the se aggregates to the magnetic susceptibilities is ferromagnetic in nat ure, which corresponds to the type of the ordered state of these aggre gates. We also present the results of a numerical calculation of the t emperature dependence of the susceptibilities for certain types of agg regate including densely packed ones.