Recent data by DeLuca and Malozemoff on the gradient propagation ofS=0 bubbles in an inhyphen;plane field show two peaks in the velocityhyphen;vshyphen;drive curve. In this paper the higherhyphen;drive peak is accounted for in terms of the wellhyphen;known horizontal Blochhyphen;line nucleation and punchhyphen;through effect, while the lowerhyphen;drive peak is interpreted as a novel Blochhyphen;line rotation instability. Such an instability can arise in bubbles with two Bloch lines pinned by the inhyphen;plane fieldHpat opposite sides of the bubble. When the bubble is propagated perpendicular to the inhyphen;plane field, one of the Bloch lines is gyrotropically destabilized and can rotate halfway around the perimeter of the bubble. The critical drive field and velocity for this process are calculated to be agr;Hpfand agr;mgr;Hpf, respectively, where agr; is the Gilbert damping parameter, mgr; is the linear mobility, andfis a factor between 0.63 and 1. As the Bloch line rotates, the velocity is predicted to drop to a minimum of order 8agr;mgr;hminus;1(2A/pgr;)1/2, whereAis the exchange stiffness andhis the film thickness. Eventually a new state is attained with four Bloch lines all of which are gyrotropically and magnetostatically stable.
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