Phase diagrams of the classical Heisenberg fluid within the extended van der Waals approximation

We use a simple van der Waals theory, suitably extended to the solid phase and to anisotropic interactions, to study the phase behavior of a system of particles with magnetic exchange interactions. A very rich phase behavior is found which indicates, in particular, that the ferromagnetic liquid phase is favored by increasing the range of both the magnetic exchange interactions and of the nonmagnetic interactions. This could well explain why it turns out to be difficult to find such a phase in simulations which use interactions which are cut off at finite range.