Spin Structures of Quasi-Two-Dimensional Isotropic- and Anisotropic-Antiferromagnetic Heisenberg Models on an hcp Lattice

Abstract

Spin structures of quasi-two-dimensional isotropic and anisotropic antiferromagnetic Heisenberg models on a hexagonal close-packed (hcp) lattice are analyzed by using a mean field theory and studied by using a Monte Carlo method. In anisotropic and an xy-like models, an incommensurate (IC) phase occurs at low temperatures and a phase transition between the IC phase and the paramagnetic phase is of the first order. In an Ising-like model, a special paramagnetic phase occurs at intermediate temperatures where an IC short range order develops much. As the temperature is decreased, an IC long range order occurs when the anisotropy is small, whereas a non-periodic frozen phase occurs when the anisotropy is large. Comments on recent experiments on LiNiO₂ and NaTiO₂ are given.