Critical properties of the mixed spin-1 and spin- anisotropic Heisenberg model in the Oguchi approximation

The effects of both an exchange anisotropy and a single-ion anisotropy on the phase diagram of the mixed spin-1 and spin- Heisenberg model are investigated by the use of an Oguchi approximation. Particular emphasis is given to the simple cubic lattice with coordination number z=6 for which the phase diagram and magnetizations are determined numerically. Besides second-order transitions, lines of first-order transitions terminating either at a tricritical point or an isolated critical point, are determined. The origin and nature of the new ordered phase in the quantum mixed-spin system at the low temperature region is also discussed.