Effect of Disorder on Domain Wall Dynamics

The purpose of this work is to study the effects of disorders on domain wall dynamics in perpendicular magnetic anisotropy ultra-thin ferromagnetic materials. It is done, firstly, by experimental study of domain wall dynamics in Ta/CoFeB/MgO material where distributions and strengths of pinning points are controlled by light ion irradiation and secondly, by micromagnetic study of the dynamics and morphology of a bubble domain in a disorder induced PMA material with Dzyaloshinskii-Moriya interaction. By studying the effects of He^{+} ion irradiation on domain wall dynamics, it is found that irradiation influences the distribution of pinning points as well as their strengths, thereby influencing the velocities of domain walls. The velocities are found to be lowest for non-irradiated samples, then it is observed to increase with irradiation and then decrease at higher irradiations suggesting that there is an optimum irradiation where velocity should be maximum. On the other hand, by studying the dynamics of bubble domain using micromagnetic simulations in ultra-thin films with disorder and Dzyaloshinskii-Moriya interaction, it is found, as expected that magnetic bubbles expand asymmetrically along the axis of the in-plane field under the simultaneous application of out-of-plane and in-plane fields. Remarkably, the shape of the bubble was found to have a ripple-like part which caused a kink-like (steep decrease) feature in the velocity versus in-plane field curve. It is shown that these ripples originate due to the nucleation and interaction of vertical Bloch lines. Furthermore, it is also shown that the Dzyaloshinskii-Moriya interaction field is not constant, in contradiction with the results of experiments, but rather depends on the in-plane field.