A contiguous-disk bubble memory without charged walls?

Bubble memory devices based on a contiguous-disk pattern offer a high bit density for a given lithographic capability. A scheme is proposed here for utilizing the attractive contiguous-disk geometry without using charged walls created by ion implantation, but instead by relying on the more familiar NiFe processing techniques. For this purpose the NiFe-garnet spacer thickness outside of the disks is made substantially larger than that inside, and NiFe is deposited simultaneously over both regions. If the geometric dimensions and spacer thicknesses are correctly chosen, bubble propagation around the disks will be determined by the poles from the inner "low" NiFe, while in the cusp regions the surrounding "high" NiFe will take control, thus providing a potential well in the cusps. Samples made with a 2 μm bubble diameter work as anticipated from a crude model; a quasistatic margin of 18 oe at a 40 oe drive field was found. Minimum drive fields less than 40 oe have been observed for frequencies as high as 750 kHz. Such good high-frequency behavior may be associated with smooth bubble motion; a maximum-to-mean bubble velocity ratio of 2.7 has been found in these devices.