Vacancy effects in an easy-plane Heisenberg model: reduction of T_c and doubly-charged vortices

Magnetic vortices in thermal equilibrium in two-dimensional magnets are studied here under the presence of a low concentration of nonmagnetic impurities (spin vacancies). A nearest-neighbor Heisenberg (XXZ) spin model with easy-plane exchange anisotropy is used to determine static thermodynamic properties and vortex densities via cluster/over-relaxation Monte Carlo. Especially at low temperature, a large fraction of the thermally generated vortices nucleate centered on vacancies, where they have a lower energy of formation. These facts are responsible for the reduction of the vortex-unbinding transition temperature with increasing vacancy concentration, similar to that seen in the planar rotator model. Spin vacancies also present the possibility of a new effect, namely, the appearance of vortices with double topological charges (4 pi change in in-plane spin angle), stable only when centered on vacancies.