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Magnetic bubble memory

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Magnetic bubble memory is a type of computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles, which each store a bit.

Magnetic Bubble Memory is largely the brainchild of a single person, Andrew Bobeck. Bobeck had worked on all sorts of magnetics related projects through the 1960s, and two of his projects put him in a particularily good position for the development of bubble memory. The first was the development of the first core memory system driven by a transistor-based controller, and the second was the development of Twistor memory.

Twistor memory was based on magnetostriction, an effect which can be used to move magnetic fields. If you place a pattern on a media, magnetic tape for instance, and then pass a current through the tape, the patterns will slowly be "pushed" down the tape while the patterns themselves will remain unchanged. By placing a detector at some point over the tape, the fields will pass under it in turn without any physical motion. In effect it is a non-moving version of a single track from a drum memory. In the 1960s AT&T had used Twistor in a number of applications.

In 1967 Bobeck joined a team at Bell Labs and started work on improving Twistor. He though that if he could find a material that allowed the movement of the fields easily in only one direction, a 2D Twistor could be constructed. Patterns would be introduced at one edge of the material and pushed along just as in Twistor, but since they could be moved in one direction only, they would naturally form "tracks".

Starting with work on orthoferrite[?] he noticed an additional interesting effect: if an external field was applied to a magnetized patch of the material, the magnetized area would contract into a tiny circle, which he called a bubble. These bubbles were much smaller than the "domains" of normal media like tape, which suggested that very high densities were possible.

It took some time to find it, but eventually garnet turned out to have the right properties. Bubbles would easily form in the material and could be pushed along it fairly easily. The next problem was to make them move to the proper location where they could be read back out – Twistor was a wire and there was only one place to go, but in a 2D sheet things would not be so easy. The solution was to imprint a pattern of tiny magnetic bars onto the surface of the garnet, when a small magnetic field was applied they would become magnetized, and the bubbles would "stick" to one end. By then reversing the field they would be attracted to the far end, and another reversal would pop them to the next bar in line.

A memory is formed by lining up tiny electromagnets[?] at one end with detectors at the other end. Bubbles written in would be slowly pushed to the other, forming a sheet of Twistors lined up beside each other. Attaching the output from the detector back to the electromagnets turns the sheet into a series of loops, which can hold the information as long as you like.

Bubbles were non-volatile. If the power was removed the bubbles remained, just as the patterns did on the surface of a disk drive. Better yet, they needed no moving parts, the field that pushed the bubbles along the surface was generated electrically, whereas media like tape and disk drives had to move the medium under the detectors instead. Finally the density was, in theory, much higher due to the small size of the bubbles. The only downside was speed, you had to wait for the bubbles to cycle to the far end of the sheet before they could be read.

Bobeck's team soon had 1 cm memories that stored 4,096 bits, the same as a then-standard plane of core memory. This sparked considerable interest in the industry. Not only could bubble memories replace core, but it seemed that they could replace tapes and disks as well. In fact it seemed that bubble memory would soon be the only form of memory used in the vast majority of applications, with the high-speed market being the only one they couldn't serve.

By the mid-1970s practically every large electronics company had teams working on bubble memory. By the late 1970s several products were on the market, and Intel released their own 1Mb version, the 7110. Soon, however, bubble memory was a dead end and almost all work on it stopped, as the introduction of larger hard disk systems in the early 1980s pretty much killed bubble memory entirely.

External links:

Bubble Memories (http://www3.sk.sympatico.ca/jbayko/cpuAppendF)



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