Phase diagrams of a two-dimensional Heisenberg antiferromagnet with single-ion anisotropy

Abstract

A two-dimensional Heisenberg antiferromagnet with a single-ion anisotropy is studied in the presence of a uniform magnetic field along the easy axis. The concept of effective field-dependent anisotropy is applied to the theoretical description of the phase diagrams. We perform calculations using the self-consistent harmonic approximation to treat the XY phase and the Green function technique for the Ising phase. Monte Carlo simulations on L× L square lattices, with L=8,16,32 and 64, and with periodic boundary conditions were performed for the classical model. In order to test the validity of the effective anisotropy concept we compare our theoretical calculation with simulation data. We also calculate theoretically the phase diagram of the compound Rb 2MnF 4, a quasi-two-dimensional spin 5/2 antiferromagnet, finding good agreement with experimental data. Our estimates for the transition temperature of the compounds K 2MnF 4 and Mn(HCOO) 2·2H 2O, in zero magnetic field, are also in agreement with the experimental values.