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.

It is shown that the region of the phase diagram where the ferromagnetic liquid phase of a Heisenberg fluid (with long-ranged exchange interactions) is thermodynamically stable (relative to the solid phases) remains confined to a fairly small temperature–density domain also in the presence of nematic interactions. The present analysis is carried out on the basis of a van der Waals theory for both translationally and/or orientationally ordered and disordered phases.

We develop the calculation of free energy in a nematic phase for a model of spherical particles with the long-range anisotropic interaction from the van der Waals theory. We map the gas-liquid equilibrium, which is coupled to a first-order isotropic-nematic transition. We discus how the topology of the phase diagrams changes upon varying the strengths of the isotropic and nematic interactions.

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We use a simple van der Waals theory, suitably extended to the solid phase and to anisotropic interactions, to study the phase behaviour of a system of particles with nematic interactions. Very rich phase behaviour is found which indicates, in particular, that the nematic liquid is stable only for large values of the strength of the nematic interactions. The isotropic liquid-nematic liquid and the isotropic solid-nematic solid transitions are always first-order. Additionally, we have found that the nematic liquid is thermodynamically stable only in a small domain of the temperature-density plane contained between two triple lines. Keywords:  Phase diagrams; Nematic interactions; Free energy; Transitions. © 2010 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved. DOI: 10.3329/jsr.v2i3.4438                 J. Sci. Res. 2 (3), 419-432 (2010)