Short-range exchange and long-range dipole interactions in a triangular planar model

The planar triangular antiferromagnet with nearest neighbor (NN) exchange interaction J<0 exhibits a "chiral" phase transition in addition to the Berezinskii-Kosterlitz-Thouless phase transition. We show that the dipole-dipole interaction which is always present in actual compounds modifies considerably the behavior of the model. Indeed, an analytic low temperature expansion and a Monte Carlo simulation show that a succession of ferromagnetic (F), striped (AF1, AF2), and 120° three-sublattice (T) configurations occur at increasing |J|/ where is the dipole interaction strength. Standard long-range order is present in F, AF1, and AF2 configurations, whereas a chiral order is present in the T configuration. The transition to the paramagnetic (P) phase is continuous for the F phase, and first order for the AF1 and T phases. The T-P chiral phase transition seems to be continuous only for = 0. ©2003 The American Physical Society