Vortex-antivortex dynamics and field-polarity-dependent flux creep in   hybrid superconductor/ferromagnet nanostructures
  Vortex-antivortex arrays that are created in a type-II superconducting film by lattices of ferromagnetic dots with perpendicular anisotropy have been investigated. The highest critical current is shifted to a nonzero value of the applied field, and current-voltage characteristics show different regimes of vortex motion even in zero-applied field due to the presence of the vortex-antivortex arrays. Creep of interstitial vortices is observed at low driving forces. This flux creep is strongly field-polarity dependent. 
M. Lange, M.J. Van Bael, A.V. Silhanek, V.V. Moshchalkov