Order-order and order-disorder transitions in the quantum spin-1 Ising-Heisenberg models

Abstract

The critical properties of a quantum spin-1 ferromagnetic multidimensional Ising-Heisenberg model with uniaxial and single-ion anisotropy are examined in the framework of a mean-field approximation. A Landau free energy expansion is performed to identify critical lines and surfaces. The resulting phase diagram, defined in a three-dimensional space characterized by a single-ion anisotropy variable, a parameter measuring the interaction anisotropy and the temperature, is particularly interesting because it exhibits three different phases, one paramagnetic and two ferromagnetic; the latter two differing for the orientation of the dipolar ordering. The presence of both first order reorientation transitions (order-order transitions) and order-disorder transitions makes the present model promising for describing several physical systems.