4 D ec 1 99 6 Vortex dynamics in 2 D antiferromagnets

The dynamics of vortices in a 2D Heisenberg antiferromagnet with an easy-plane anisotropy is studied numerically within the discrete spin model as well as analytically within a continuum approximation based on a suitable extension of the relativistic nonlinear σ model. We find that two like vortices scatter at 90 • during a head-on collision, whereas a vortex-antivortex pair is annihilated into spinwave radiation emitted mainly at 90 •. When a uniform bias field is applied, vortex dynamics is affected rather profoundly and acquires the characteristic features of the Hall effect of electrodynamics or the Magnus effect of fluid dynamics. In particular, a single vortex is always spontaneously pinned, two like vortices form a rotating bound state, and a vortex-antivortex pair undergoes Kelvin motion. Finally, in the presence of a bias field, vortices are shown to be the prominent topological excitations even for an isotropic antiferromagnet.