Ising and anisotropic Heisenberg magnets with mobile defects

Holtschneider, M., Leidl, R., Selke, W.
Institut für Theoretische Physik, Technische Hochschule Aachen,
D-52056 Aachen, Germany


Motivated by experiments on (Sr,Ca,La)14 Ca9 Cu24 O41, a two-dimensional Ising model with mobile defects and a two-dimensional anisotropic Heisenberg antiferromagnet have been proposed studied recently. We extend previous investigations by analysing phase diagrams of both models in external fields using mainly Monte Carlo techniques.In the Ising case, the phase transition is due to the thermal instability of defect stripes, in the Heisenberg case additional spin-flop structures pay an essential role..