Prediction of polaron-like vortices and dissociation depinning transition in magnetic superconductors: The example of ErNi2B2C

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 $J_r<J_c$.