Operation and failure of a hairpin nucleate generator on a bubble memory chip

Abstract

The dynamic operation as well as the failure modes of a hairpin nucleate bubble generator operating at 125 kHz has been studied using a high-speed optical sampling system. The hairpin generator loop consists of a 6.8-μm wide 4.2-μm gap conductor located under a 20-μm period 110° chevron propagation pattern. In stable operation in the center of the operating range, bubbles are nucleated along the hairpin conductor preserving a U shape that reflects the shape of the hairpin conductor. After nucleation, the gap of the conductor fills with a long wide domain. This long domain then shrinks to a stable position under the propagation elements with a wall velocity of 40 m/s. All velocities observed are consistent with velocities observed on free bubbles using radial expansion, except for the growth in length after nucleation during the generate pulse, where the domain grows about three times faster than would be expected. This difference is attributed to the in-plane component of the generate current field that is very nonuniform through the thickness of the sample. The failure mode for a low or short generate pulse is multibubble generation caused by nonuniform nucleation along the length of the conductors of the hairpin loop. For certain pulse characteristics at this low end of the operating range, a stable U-shaped domain is nucleated only to be cut into two domains at the end of the generate pulse as the drive field reverses the poles on the nearest chevron stack from what it was during the pulse. The failure mode at the upper end of the operating range also results in excess bubbles. Here the mechanism can be identified as the development of a wavy wall as it moves rapidly under the conductor element in the presence of a large in-plane field caused by the generate current. This distortion then increases as the domain shrinks, resulting in nobs that are pinched off to create excess bubbles. The distortion is similar to that seen during the collapse of highly expanded stripes. This failure can be cured by increasing the fall time of the generate pulse to a least 100 ns.

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