Density-functional theory for the freezing of Stockmayer fluids

We examine the freezing of Stockmayer fluids which consist of spherical particles interacting via Lennard-Jones and dipolar forces and thus represent, e.g., models for ferrofluids. The theoretical analysis is based on an appropiate version of density-functional theory which is capable of describing both the freezing transition and the formation of orientationally ordered phases. This allows us to study the variation of the topology of phase dia;:rams as a function of the strength of the dipole moment of the particles. We find an isotropic gas and liquid phase, a ferromagnetic liquid, an orientationally disordered solid, and a ferromagnetic solid. For small dipole moments the formation of the ferromagnetic liquid is preempted by freezing. For the ferromagnetic solid the face-centered-cubic structure turns out to be more stable than the body-centered-tetragonal structure.