Phase Diagram of Ising-Like Heisenberg Model on the Triangular Lattice with Degenerate Orbital Effect

Abstract

Ising-like Heisenberg model with a degree of freedom of degenerate orbits at each site is investigated. The orbital variable is two-fold and coupling constants between spin variables depend on the orbital configuration. The interactions between spin variables, however, are always fully frustrated. The model was originally proposed to explain magnetic properties of LiNiO 2 ( S =1/2) by Kitaoka. We investigate thermodynamic properties of a generalized model with orbital degeneracy in the classical spin model mainly by a Monte Carlo method. This model is found to have various kinds of orders such as the sublattice ordering of spins, the orbital ordering and in particular, a new type of ordering where a product of the orbital variable and the spin variable becomes ordered while each of the variables is not ordered. Changing the dependence of the coupling constant of the spin variables on the orbital configuration, a rich structure of phase diagram is found.