Current-Access Magnetic Bubble Circuits

We have been investigating the potential of conductor-access bubble circuitry 1 to meet the need for a high-density, high-capacity, low-cost, nonvolatile memory device. There are two basic ways to move bubbles: field-access" and current-access (conductor-access). Field-access de1453 vices, the only type in use at this time, require a pair of orthogonal drive coils to provide an in-plane magnetic field which, coupled to a bubble data stream via structured permalloy or ion-implanted features,' provides the necessary bubble drive forces. It is difficult, however, to take advantage of the data rates intrinsic in field-access bubble devices as they are scaled down in size. In particular, the volt-ampere product increases with increasing frequency, making economical coil drive circuits difficult to design. Present field-access devices are limited to about a 250-kHz rotating field rate and are thus too slow for applications such as television frame storage. A bit rate per chip of 1 Mb/s or greater would make bubbles attractive to a wider range of potential customers. In traditional current-access bubble circuitry, meandering conductor strips of relatively high resistance, rather than drive coils, make it possible to provide the necessary currents to move bubbles at stepping rates of 1 MHz and greater with relative ease but give rise to a concern that current-access chips would dissipate excessive power, especially at the higher bit storage densities such as 10' b/cm 2 . The disadvantage of the current-access circuits disclosed to date has been the complexity of the conductor patterns themselves.