Effects of grain size and disorder on domain wall propagation in CoFeB thin films

Abstract

Micromagnetic simulations are used to investigate the effect of disorder on field-driven domain wall motion in perpendicularly magnetized CoFeB thin films. It is found that some degree of inhomogeneity in the form of an irregular grain structure needs to be introduced in the model in order to account for the domain wall velocities measured experimentally, even for applied fields much larger than the finite propagation field induced by weak disorder in the film. Moreover, the details of this grain structure have a large impact on domain wall motion in this flow regime. In particular, it is found that, for a fixed applied field, domain wall velocity rapidly increases with grain size up to a diameter of 40 nm, above which it slowly decreases. This is explained showing that the grain structure of the material introduces a new form of dissipation of energy via spin wave emission during domain wall propagation. We focus on the relation between grain size and domain wall velocity, finding that the frequency of emission of spin waves packets during domain wall motion depends on the grain size and affects directly the domain wall velocity of propagation.