Local distortions of surface domain walls in cylindrical microwires observed by magneto-optics

Domain wall dynamics in highly magnetostrictive microwires is remarkably sensitive to thermal treatment. One of the reasons is a partial release of the internal stresses that originate during the production process. Another reason is an interaction of the domain wall with local pinning sites, such as surface inhomogeneities, free volumes, or clusters of ferromagnetic atoms, present in as-cast microwires. However, the observation of domain wall shape variations after thermal treatment or following an interaction with a pinning site is challenging. In this paper, we utilize the magneto-optical Kerr effect (MOKE) to locally map the domain wall shape and velocity in cylindrical microwires. The domain wall is trapped in a magnetic potential well, moving back and forth with limited speed. Two complementary techniques are employed in the study: a laser-based setup and time-resolved MOKE microscopy. Different kinds of domain wall propagation are observed. While a planar domain wall propagates synchronously with the potential well, surface pinning gives rise to local jumps and distortions of the domain wall. We relate the first kind of motion to the pinning-free propagation of the domain wall and the second to the propagation of a distorted wall. Finally, local speed and the shapes of the domain wall before and after the introduction of an artificial pinning site are compared.