Creep motion of a magnetic wall:: Avalanche size divergence

The motion of a magnetic domain wall in a He- ion - irradiated Pt/ Co( 0.5 nm)/ Pt ultrathin film was studied in low applied magnetic fields. The field dependence of the wall velocity displays a creep behaviour, consistently with a critical slowing- down when the field tends to zero. We also show that domain wall motion is induced by large spin reversal avalanches and that the energy barrier increase is directly linked to that of the avalanche size. This observation is in excellent agreement with the theory of elastic manifolds moving in a weakly disordered medium.