  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.
