Effect of conductor crossings on propagation margins

The effects of conductor delineation technique on magnetic bubble propagation across the conductor edge are described. Propagation margins are obtained for bubble circulation around 18-μm diameter Permalloy discs which cross four edges of an Al-Cu feature. Specifically investigated are isotropic wet etching, anisottopic wet etching to achieve a uniform taper, ion beam milling, and metal lift-off to provide a planar structure. Margins are obtained at ± 40°C, with the most significant degradation observed at the lower temperature. Permalloy magnetic continuity in the crossings can be inferred from hysteresis loop measurements of a Permalloy sheet deposited over a grating pattern formed by the above processing techniques. Although the least anisotropic loops are invariably obtained with smoothly tapered Al-Cu edges under the Permalloy, propagation margins are not maximized with such structures, but rather favor a planar crossing. The results suggest that although patterned stress is still an important concern in functional operation, other geometric effects can be more significant. In particular, poor magnetic step coverage as inferred from loop measurements leads to spurious pole formation from the drive field, while even with adequate step coverage, static bias-field distortions can result because of the component of the field along the step.