Anisotropic Ising model on the hexagonal-close-packed lattice.

Abstract

The phase diagram of an Ising model on the hcp lattice with antiferromagnetic nearest-neighbor interactions J in the basal plane and coupling D (ferromagnetic or antiferromagnetic) between planes is analyzed. Previously obtained exact results predict the existence of a Lifshitz tricritical point in its phase diagram. In mean-field theory the model is shown to exhibit a twofold Lifshitz point and a phase diagram with paramagnetic, ferromagnetic, and modulated phases. Monte Carlo simulations that clearly indicate the tricritical nature of the Lifshitz point are reported, and series-expansion techniques are used to identify a commensurate phase in a low-temperature portion of the phase diagram.