Pseudodipolar anisotropy in the square rotator model

The effect of pseudodipolar anisotropic interactions in a square planar rot ator model is investigated. The pseudodipolar anisotropy ranges from the pu re dipole spin-spin interaction (eta=1) that supports long range order, to the isotropic antiferromagnetic interaction (eta=0) decaying as 1/r(3) wher e 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 eta. This implies absence of long ran ge order in the linear spin wave approximation. However order by thermal di sorder is restored except for eta=0 when thermal fluctuations are accounted for. A rich scenario of configurations is found at low temperature perform ing both analytic calculations and Monte Carlo simulations. (C) 2000 Americ an Institute of Physics. [S0021-8979(00)22808-9].