Phase diagrams near the Lifshiftz point. II. Systems with cylindrical, hexagonal, and rhombohedral symmetry having an easy plane of magnetization

Paramagnetic (I)-ferromagnetic (II)-helical (III) phase diagrams near a Lifshitz point are studied for systems with cylindrical, hexagonal, and rhombohedral symmetry having an easy plane of magnetization. We show that the II \ensuremath{\rightleftarrows} III phase-transition line in these systems is not tangent to the order-disorder transition line at the Lifshitz point. We also show that the II \ensuremath{\rightleftarrows} III phase transition is second order (but with some peculiar features) in the case of cylindrical symmetry and first order in the case of hexagonal or rhombohedral symmetry. These results differ from those obtained in the uniaxial case and also manifest a dependence on the symmetry of the system. The results for hexagonal systems are compared with experimental data for Gd-Dy, Gd-Sc, and Gd-Y alloys.

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