Reference STATE OF THE ART Stan Augarten

ISBN 0-89919-195-9

Photo of
Storing Data Through Magnetization
Magnetic-Bubble Memories


Compared to other ICs, magnetic-bubble memories are curious devices - crosses between bulk storage systems like magnetic disks and tapes and semiconductor memories like RAMs. Outwardly, magnetic-bubble memories resemble ordinary semiconductor chips, but they don't store information in the form of electrical charges. Instead, they use infinitesimal magnetic fields called domains, which , like bubbles in water, are shifted about in chips made of garnet, an easily magnetized material. (Most ICs are, of course, made of silicon.)

  Domains are fields of energy, not actual particles. They are really cylindrical in shape but look like bubbles when viewed from above through polarized lenses (by means of slow-motion microphotography, the bubbles can be seen moving about). They are generated by electrical circuits in the chip and are set in motion by the tugging of tiny permalloy magnets embedded in the garnet. The magnets are charged and uncharged by built-in electrical circuits. In most bubble memories, a one is represented by the presence of a bubble, a zero by its absence.

  The invention of magnetic-bubble memories in 1966 by Andrew Bobeck, Richard C. Sherwood, Umberto F. Gianola, and William Shockley was quite unexpected. It came at a time when semiconductor memories were still in the experimental stage; the first 1K RAM, for example, didn't arrive until 1970 (p. 26). At first, magnetic-bubble memories seemed to hold great promise as efficient memory chips, but they had several drawbacks that hampered their widespread use.

  Although it's possible to access randomly a given block of data from a magnetic-bubble chip - to go directly to the spot where the information is stored and withdraw it - these ICs are really serial-access devices. That means their contents must be read from front to back, so to speak, before the desired data may be extracted. As a result, their average access times are relatively low, which limits their applications, and they are therefore used when speed is not crucial; the phone company, for instance, employs them to store telephone numbers and recorded messages.

  But bubble memories have certain distinct advantages. They are nonvolatile, which means that they hold on to their contents when the power is shut off (most RAMs, by contrast, are volatile). Also, because domains may be packed quite closely together, bubble memories are capable of enormous feats of data storage. One-million-bit (megabit) bubble memories capable of storing the equivalent of about a hundred double-spaced 8½-by-11-inch typewritten pages are already on the market. By comparison, the most capacious RAM yet constructed can store only 294,912 bits (p. 66).

The early magnetic-bubble memory shown here can store sixteen 12-digit phone numbers. The black bars represent permalloy magnets, the orange ones aluminum conductors. Bubbles are generated at the interconnection of the orange bars and the black squiggles at the right; they are stored in the serrated black columns and are read or erased in the black pyramid. Actual size: 0.115 x 0.180 inches. Photo of


©Copyright Stan Augarten
This book is provided for general reference. The National Museum of American History and the Smithsonian Institution make no claims as to the accuracy or completeness of this work.

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