Dynamics of bubbles in field access devices studied using a high speed optical sampling microscope

Dynamics of bubbles in field access devices operated at 100 kHz and 200 kHz have been studied using a high speed optical sampling microscope capable of making a 10 nsec single exposure photograph at a known sampling time with respect to a rotating field orientation. By scanning the sampling time through one rotating field cycle, a series of time-sequenced transient domain configurations of bubbles have been recorded on 16 mm film. The devices studied were commonly used permalloy propagating structures, T-bar, T-X, X-bar, and chevron fabricated on an epitaxial garnet film. From each frame of the 16 mm film, plots of the position of the leading and trailing edges of a bubble as a function of rotating field angle were made. These position plots revealed very nonuniform bubble motion in these circuits. The instantaneous velocities of the leading and trailing edges of the bubble were calculated from the position plots. The results show a large velocity variation in all circuits, the ratio of the maximum velocity to the average velocity ranging from 3.5 for the X-bar to 4.8 for the chevron. Maximum vetocities in excess of that predicted on the basis of the critical velocity for dynamic conversion were observed.