Azimuthal angular rotation (ψ) in domain walls during radial motion of bubble domains

Changes in bubble chirality are investigated using transient photography in order to determine the rotation of the azimuthal angle, either ψ or ψ̄̇, during radial motion. The critical rotation required to change chirality is determined experimentally to be near ψ̄̇=π. The Walker model, which predicts a critical rotation of π/2, cannot be used to describe radial motion. Instead, the horizontal Bloch line model, in which chirality changes occur during HBL punch-through when ψ̄̇ reaches nπ, must be used. The first seven values of the minimum step amplitude, Hn, required to produce exactly n chirality changes during a single bias step are given, within ±0.2 Oe, by Hn=[2vsH′ (γ)−1]1/2(nπ)1/2+Hs, where vs is the constant wall velocity, H′, is an effective field gradient, and Hs is an effective drag field. A 30-nsec pause in the wall motion, observed when ψ̄̇ reaches π, is thought to be due to the large effective gyrotropic field produced during punch-through. Contrary to the HBL stacking mechanism, punch-through occurs each time the HBL reaches the film surfaces, so that the low-frequency oscillations and heavy wall masses observed during radial motion result from the presence of a single HBL. If the effective drive field is small when the HBL reaches the film surface, punch-through can occur nonuniformly around the bubble perimeter, resulting in a 0.8-μm random translation of the bubble.