Mobility of bubbles with small numbers of Bloch lines

Abstract

The translational and radial mobilities of bubbles with small numbers of Bloch lines are investigated in a garnet film with moderately low damping. Depending on the number of Bloch lines, the bubbles propagate at different angles in an applied field gradient, in agreement with previous theory. However, the dependence of translational speed on Bloch line number, though weak, is an order of magnitude larger than predicted unless one attributes an additional coercivity or viscous damping to each Bloch line. Bubbles whose states have been identified by propagation in a gradient field are investigated by the dynamic bubble collapse technique; the ones which deflect at a particular nonzero angle show the largest dynamic collapse velocities. This experiment gives evidence that these bubbles, rather than those showing no deflection, are the ones without any Bloch lines. Such bubbles are investigated over a large range of radial drive fields; they show an initial high‐mobility region culminating in a sharp peak, followed by a region of lower but appreciable mobility, followed by a velocity saturation. These features are compared to a theory by Slonczewski. In addition it is shown that the dynamic bubble collapse technique can be extended and its analysis simplified by working at very small threshold fields.