  It is known that oscillatory domain-wall (DW) motions in soft magnetic
thin-film nanostripes above the Walker critical field lead to a remarkable
reduction in the average DW velocities. In a much-higher-field region beyond
the velocity-breakdown regime, however, the DW velocities have been found to
increase in response to a further increase of the applied field. We report on
the physical origin and detailed mechanism of this unexpected behavior. We
associate the mechanism with the serial dynamic processes of the nucleation of
vortex-antivortex (V-AV) pairs inside the stripe or at its edges, the
non-linear gyrotropic motions of Vs and AVs, and their annihilation process.
The present results imply that a two-dimensional soliton model is required for
adequate interpretation of DW motions in the linear- and oscillatory-DW-motion
regimes as well as in the beyond-velocity-breakdown regime.
