An old model for magnetic nematics

A mean-held treatment is given of the off-lattice Krieger-James model of ordered fluids, which reduces to the more familiar Maier-Saupe liquid crystal (Heisenberg fluid) in the absence of ferromagnetic (nematic) interactions. As in the lattice version, isotropic, nematic and ferromagnetic nematic phases are found, but the nematic-ferronematic transition can either change order at a tricritical point, or terminate at a critical end point on the ferronematic-isotropic coexistence curve. In addition it is argued that the sequence of phase diagram topologies, as a function of the relative weights of ferromagnetic and nematic contributions to the free energy, should be similar to that obtained on varying the elongation of dipolar spheroids.