THERMODYNAMIC PROPERTIES OF ISING-LIKE HEISENBERG ANTIFERROMAGNETS WITH ANISOTROPIC COUPLING ON THE HEXAGONAL LATTICE

Nature of ordered phases of Ising-like Heisenberg antiferromagnets on the hexagonal (layered triangular) lattice is investigated by making u se of a Monte Carlo method and mean field analysis. Two phase transiti ons are found, which correspond to the orderings of the z-component an d of the xy-components of the spins, respectively. The uniform magneti zation along the easy axis is found in ordered phases with a concave c urve as a function of the temperature and the spin configuration of th e ordered phases is found to be a three-sublattice ferrimagnetic one. In the low temperature phase, the effect of the nontrivial degeneracy (NTD) of the ground state which is inherent to the present model is in vestigated. NTD causes a nontrivial enhancement of the fluctuation of the uniform magnetization in the hard plane. The magnetization process es in the ground state for fields along the x-axis are also studied.