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

Abstract

A classical variant of the two-dimensional anisotropic Heisenberg model reproducing inelastic neutron scattering experiments on La_5 Ca_9 Cu_24 O_41 [M. Matsuda et al., Phys.Rev. B 68, 060406(R) (2003)] is analysed using mostly Monte Carlo techniques. Phase diagrams with external fields parallel and perpendicular to the easy axis of the anisotropic interactions are determined, including antiferromagnetic and spin-flop phases. Mobile spinless defects, or holes, are found to form stripes which bunch, debunch and break up at a phase transition. A parallel field can lead to a spin-flop phase.

9 pages, 9 figures; final version as accepted by Phys. Rev. B (Fig. 5 replaced, added remarks in Secs. I, III, and V)