Intermittent collective dynamics of domain walls in the creep regime

KOLTON, A.; BUSTINGORRY, S.; ALBORNOZ, L.; MOUGIN, A.; GRASSI, M.; JEUDY, V.; CURIALE, J.

Workshop; Yielding phenomena in disordered systems, the southernmost STATPHYS27 satellite; 2019

[ESTA ENTRADA CORRESPONDE A UNA PRESENTACION ORAL INVITADA INTRODUCIDA EN PARTICIPACION U ORGANIZACION DE EVENTOS, YA QUE AQUI NO SE PUEDE ACLARAR QUE TIPO DE CONTRIBUCION ES Y EN LA OTRA SECCION NO SE PUEDE INCLUIR NI EL TITULO NI EL RESTO DE LOS AUTORES MAS ALLA DEL EXPOSITOR.] Nowadays magnetization reversal and domain wall (DW) dynamics are key topics in the electronic and spintronic field. The control of domain wall short and long-term stability and displacement is critical for potential applications such as those related with the magnetic storage and emerging DW-based spintronic devices. In addition, DWs in ferromagnetic metallic or semiconducting materials are part of a large variety of systems, such as vortex glass in type II superconductor, earthquakes, crack lines, contact lines in wetting..., involving the displacement of an elastic object in a weakly disordered medium. In those systems, the motion results from a competition between driving force, elasticity, collective pinning and thermal noise and proceeds by a succession of avalanches. Even if dramatically di↵erent scales and microscopic physical interactions are at the basis of the motion, those systems present common universal behaviors, which are described in a general statistical physics framework.The nature of the motion depends on the strength of the driving force f. In ferromagnetic systems, the drive can be an external magnetic field or a spin polarized current and the elastic object is a magnetic DW. At finite temperature, there are three main regimes of motion of the elastic object. Below the depinning threshold (f= fd, depinning regime), the velocity presents a power law variation with the drive which is rounded by thermal fluctuations close to fd. At large forces (f> > fd, flow regime), the motion is controlled by dissipation and the velocity varies linearly with the drive. [Jeudy et al. Phys. Rev. B 98, 054406 (2018)]In this talk, I will show results on the ultraslow DW motion, within the creep regime, in thin ferromagnetic films of the archetypical Pt/Co/Pt, driven by a weak magnetic field. Using time-resolved magneto-optical Kerr e↵ect microscopy, we access to the statistics of the intermittent thermally activated domain-wall jumps between deep metastable states. Our observations are consistent with the existence of creep avalanches: roughly independent clusters with broad size and ignition waiting-time distributions, each one composed by a large number of spatiotemporally correlated thermally activated elementary events. This picture, that drastically changes the naive view of creep motion as independent thermally nucleated displacements, is relevant to di↵erent magnetic films and it is likely to emerge in the creep regime of other disordered elastic systems. Moreover, we evidence that the large- scale geometry of domain walls is better described by depinning rather than equilibrium universal exponents. [Grassi et al. Phys. Rev. B 98, 224201 (2018)]