Earthquakelike dynamics in ultrathin magnetic films

We study the motion of a domain wall on an ultrathin magnetic film using the magneto-optical Kerr effect (MOKE). Our measurements were conducted at magnetic fields approximately <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>2</a:mn><a:mo>%</a:mo></a:mrow></a:math> of the depinning field where the wall creeps only via thermal activation over the pinning centers present in the sample. Our results show that this creep dynamics is highly intermittent and correlated. A localized instability triggers a cascade, akin to aftershocks following a large earthquake, where the pinned wall undergoes large reorganizations in a compact active region for a few seconds. The analysis of their scale-free statistics enables us to solve an intricate, long-standing problem: thermal avalanches exist in the creep regime and they are depinning-like even for very low magnetic field. Moreover, we show that their sizes and shapes belong to the depinning quenched Kardar-Parisi-Zhang universality class.