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 p hase 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 ferro magnetic liquid phase is favored by increasing the range of both the m agnetic exchange interactions and of the nonmagnetic interactions. Thi s could well explain why it turns out to be difficult to find such a p hase in simulations which use interactions which are cut off at finite range.