Transient shapes of the magnetic bubble domains during gradient propagation and overshoot

Characteristic transient shapes were observed for free magnetic bubble domains during gradient translation and overshoot using high speed photography with 10 nsec laser pulse illumination. The two flanks of the bubble start to expand transversely to the driven motion direction at about .5\mu sec after the onset of a 7.4 Oe/rad, 2 μsec gradient field pulse. They expand transversely at 2.6 m/sec and continue for 3 μsec after the termination of the gradient pulse. During this expansion, the tip of the flanks move longitudinally at 2.6 m/sec lagging behind the bubble center moving at 5.3 m/sec. Considerable scatter exists in the transverse motion after the driven phase especially in the recovery where some bubbles require as long as 15μsec to regain their round shape. The transverse expansion was found to be independent of the bias compensation applied; however, the transient distortion during the driven phase and the recovery time from the transverse expansion were sensitive to the compensation. This transverse motion as well as the transient shape of the bubble can be explained qualitatively on the basis of a model involving opposite winding vertical Bloch lines.