  We investigate the critical dynamics of spin superflow in an easy-plane
antiferromagnetic spinor Bose-Einstein condensate. Spin-dipole oscillations are
induced in a trapped condensate by applying a linear magnetic field gradient
and we observe that the damping rate increases rapidly as the field gradient
increases above a certain critical value. The onset of dissipation is found to
be associated with the generation of dark-bright solitons due to the modulation
instability of the counterflow of two spin components. Spin turbulence emerges
as the solitons decay because of their snake instability. We identify another
critical point for spin superflow, in which transverse magnon excitations are
dynamically generated via spin-exchanging collisions, which leads to the
transient formation of axial polar spin domains.
