STATISTICAL THERMODYNAMICS OF FERROFLUIDS

A ferrofluid is a colloidal suspension of ferromagnetic particles (diameter approximately 100 angstrom) dispersed in a solvent. Its main feature, which is of fundamental and practical interest, is that it is a liquid with strong magnetic properties (magnetic susceptibility chi approximately 0.1 - 1). We propose an approach leading to a statistical theory of ferrofluids. It is shown that a ferrofluid can be treated as a combination of two independent subsystems: (i) magnetic dipoles in an external magnetic field and (ii) molecules of a solvent fluid contained in cells with a potential field. Free energy expansions are derived by means of the thermodynamic perturbation theory for nonideal system. It is shown that magnetostatic properties of a ferrofluid are not of the Langevin type as a result of interparticle interaction. The surface tension is shown to be a slightly increasing function of magnetic field intensity. The coexistence curve and critical parameters are obtained for the first order phase transition leading to an aggregate formation. An estimation of the critical temperature dependence on the field strength is presented; it is shown that for temperatures higher than a certain value there is no phase separation even for infinite values of the magnetic field strength.