Anisotropic Heisenberg model for the mixed spin-2 and spin-1/2 in the Oguchi approximation on the simple cubic lattice

The mixed spin-2 and spin-1/2 Heisenberg model is studied on a simple cubic lattice by using the Oguchi approximation to examine the effects of exchange anisotropy and crystal field. The thermal variations of the order-parameters, i.e. magnetization and quadrupole moment, are investigated to obtain the possible phase diagrams of the model. The detailed phase diagrams are obtained on the (D∕J,kBT∕J) and (Δ,kBT∕J) planes for given values of Δ and D∕J, respectively. In the phase diagrams all possible solutions are illustrated whether stable or not. The model yields second and first-order phase transitions, in addition to the tricritical and isolated critical points. The model also gives two compensation temperatures for given system parameters, therefore, they exhibit reentrant behavior. The low temperature behavior of the model is very complicated, therefore, the reliability of the Oguchi approximation should be investigated further.