  In borocarbide ErNi$_2$B$_2$C the phase transition to the commensurate spin
density wave at 2.3 K leaves 1/20 part of Ising-like Er spins practically free.
Vortices polarize these spins nonuniformly and repolarize them when moving. At
a low spin relaxation rate and at low bias currents vortices carrying magnetic
polarization clouds become polaron-like and their velocities are determined by
the effective drag coefficient which is significantly bigger than the
Bardeen-Stephen (BS) one. As current increases, at a critical current $J_c$
vortices release polarization clouds and the velocity as well as the voltage in
the I-V characteristics jump to values corresponding to the BS drag
coefficient. The nonuniform components of the magnetic field and magnetization
drop as velocity increases resulting in weaker polarization and discontinuous
dynamic dissociation depinning transition. As current decreases, on the way
back, vortices are retrapped by polarization clouds at the current $J_r<J_c$.
