PHASE-DIAGRAM OF UNIAXIAL LIQUID-CRYSTALS

The phase diagram of an n-component spin model with dipolar and quadru polar nearest-neighbour interactions is solved exactly in the large-n limit. In d = 3 a tricritical point is found on the line bounding the paramagnetic phase, at which the first-order transition becomes contin uous at decreasing values of the quadrupolar exchange energy. In d = 2 the line of first order transition ends at a critical point. The nema tic-ferromagnetic transition is studied at order 1/n.