Ferromagnetic fluids

The equation of state for a system of molecules with exchange interactions in addition to the spin-independent forces is considered, and explicitly evaluated in one and three dimensions for the case of hardsphere particles with weak long-range forces. Depending upon the ratio $R$ of the integrated strength of the exchange interaction to the spin-independent attractive interaction, five topologically different phase diagrams occur for the three-dimensional model. The phases involved are magnetic and nonmagnetic solids, magnetic and nonmagnetic liquids, and nonmagnetic gases. Liquid ferromagnetism occurs if the exchange interaction is sufficiently strong, $R>\mathrm{}0.15\mathrm{}$.