Theory of Agglomeration of Ferromagnetic Particles in Magnetic Fluids

For some magnetic fluids, large agglomerates of magnetic particles are formed when a weak magnetic field is applied. In this paper we regard this phenomenon as a kind of gas-liquid phase transition induced by the magnetic field, and study the condition for the formation of the agglomerates by a simple mean field theory. It is found that if the particle is small enough, no phase separation occurs even if the magnetic field is infinitely large, while if the particle is large, the liquid phase appears at certain magnetic field. The van der Waals attraction between the particles enhances the agglomeration.