Unusual domain wall motion in the vicinity of the depinning field in a Pt/CoFeB/MgO film

Domain wall (DW) statics and dynamics have been immensely studied for the last few decades in a disordered medium due to rich fundamental physics and spintronics applications. Here, we have studied the DW dynamics in Pt/CoFeB/MgO thin film with perpendicular magnetic anisotropy. We have observed an unusual behaviour in the DW dynamics which cannot be only explained via universal creep or depinning law. At low driving force, the sample follows the universal creep law; however, near the depinning field ( H d ), an excess velocity has been observed in DW dynamics in comparison to the usual creep law. We propose that, due to additional relaxation events near H d , the DW propagation length is increased and hence the DW velocity. The excess velocity has been modelled to a modified creep law.