Pseudodipolar anisotropy in the square rotator model

Abstract

The effect of pseudodipolar anisotropic interactions in a square planar rotator model is investigated. The pseudodipolar anisotropy ranges from the pure dipole spin–spin interaction (η=1) that supports long range order, to the isotropic antiferromagnetic interaction (η=0) decaying as 1/r3 where r is the spin–spin distance for which no long range order exists. At zero temperature a continuous manifold of spin configurations that minimize the energy of the model is found for any η. This implies absence of long range order in the linear spin wave approximation. However order by thermal disorder is restored except for η=0 when thermal fluctuations are accounted for. A rich scenario of configurations is found at low temperature performing both analytic calculations and Monte Carlo simulations.