  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 sigma model. We find that two like
vortices scatter at 90 degrees during a head-on collision, whereas a
vortex-antivortex pair is annihilated into spinwave radiation emitted mainly at
90 degrees. 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.
