LOW-TEMPERATURE PROPERTIES OF THE QUASI-2-DIMENSIONAL ANTIFERROMAGNETIC HEISENBERG-MODEL

By means of a two-sublattice approach to antiferromagnets, we found that the ground state of a quasi-two-dimensional cubic-lattice antiferromagnet (Jz/Jxy1) is a Néel antiferromagnetic state with a mean magnetic moment of 0.6B(B is the Bohr magneton) and that low-temperature moment decreases with the square of the temperature. The coefficient of the temperature-squared term approaches infinity when Jz/Jxy=0. The z-direction coupling (Jz>0) is essential to keep three-dimensional Néel ordering at nonzero temperature and to obtain a nonzero Néel temperature. The low-temperature specific heat capacity is proportional to the temperature squared. © 1990 The American Physical Society.