Vm. Dubovik et al., THEORY OF THE CURIE-WEISS BEHAVIOR OF AN AGGREGATED MAGNETIC SUSPENSION, Journal of magnetism and magnetic materials, 150(1), 1995, pp. 105-118
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.