Tracks for the guidance of magnetic bubbles propagating in the input and output lines of Vertical-Bloch-Line memory devices can be made in the form of ridges instead of in the traditional form of grooves. The ridge-type tracks offer advantages over the groove-type tracks, as explained below.
A track is formed on a substrate made of suitable magnetic material; namely, a garnet film. A gradient of the vertical magnetic field is associated with a step in the thickness of the film; the field increases as one proceeds from a location where the film is thinner to a location where it is thicker. Thus, in the case of a groove, the field increases as one crosses either wall of the groove from the inside to the outside. Because the stable position of a magnetic bubble lies at a local minimum of the field, a bubble that has been propagating along the groove remains confined in the groove.
The spatial variation of the field is, however, slightly more complex than is the spatial variation in thickness (see figure). The field increases slightly from the wall toward the middle of a groove. Thus, there are shallow local field minima along the sides of a groove. If the groove is wider than about 1.5 bubble diameters, then a bubble tends to move sometimes along one side, sometimes along the other side, moving back and forth in mostly random fashion as it propagates along the groove. If the groove is narrower than about 1.3 bubble diameters, a bubble remains centered in the groove but propagates more slowly than it would if the groove were wider. The customary groove width of 1.5 bubble diameters is a compromise that entails a little of both slowing down and meandering of bubbles.
A ridge-type track is formed by etching wide grooves on both sides of the track. Mirroring the situation in a groove, the magnetic field rises to maxima near the two side walls of the track, while at the middle of the track, the field falls to a local minimum that is isolated from the deeper minima of the adjacent grooves. The confinement gradient of a ridge is weaker than that of a groove, but still adequate for guidance. Unlike a groove track, a ridge track can be made wider than 1.3 bubble diameters without incurring meandering of bubbles; a ridge as wide as 2.5 bubble diameters can provide excellent guidance.
Another advantage of a ridge-type track over a groove-type track arises in connection with the need to expand a bubble into a stripe when it reaches the end of the track, in preparation for detection of the bubble by use of a magnetoresistive strip. In the case of a groove-type track, the bubble expander is a mesa with its top recessed slightly below the surrounding garnet surface. An electric current in a helper loop is needed to provide a momentary additional magnetic field to lift a bubble from the groove, over the potential barrier at the edge of the mesa, so that the bubble can then stripe out on top of the mesa.
In the case of a ridge-type track, the track can simply be terminated in a mesa-type expander of the same height as that of the ridge. There being no step discontinuity in height, there is no need for a helper loop to move the bubble out onto the expander.
This work was done by Udo Lieneweg of Caltech for NASA's Jet PropulsionLaboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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