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 in 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 combined cluster and over-relaxation Monte Carlo simulations. Especially at low temperatures, a large fraction of the thermally generated vortices nucleate centered on vacancies, where they have a lower energy of formation. This fact is 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 the appearance of vortices with double topological charges (±4π change in in-plane spin angle), stable only when centered on vacancies.