Bubble Memory - Magnetic Bubbles

Magnetic Bubbles

In 1967, Bobeck joined a team at Bell Labs and started work on improving Twistor. He thought that, if he could find a material that allowed the movement of the fields easily in only one direction, a strip of such material could have a number of read/write heads positioned along its edge instead of only one. 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" across the surface, increasing the areal density. This would produce a sort of "2D Twistor".

Paul Charles Michaelis working with permalloy magnetic thin films discovered that it was possible to propagate magnetic domains in orthogonal directions within the film. This seminal work led to a patent application. The memory device and method of propagation were described in a paper presented at the 13th Annual Conference on Magnetism and Magnetic Materials, Boston, Massachusetts, September 15, 1967. The device used anisotropic thin magnetic films that required different magnetic pulse combinations for orthogonal propagation directions. The propagation velocity was also dependent on the hard and easy magnetic axes. This difference suggested that an isotropic magnetic medium would be desirable.

Starting work extending this concept using orthoferrite, Bobeck noticed an additional interesting effect. With the magnetic tape materials used in Twistor the data had to be stored on relatively large patches known as "domains". Attempts to magnetize smaller areas would fail. With orthoferrite, if the patch was written and then a magnetic field was applied to the entire material, the patch would shrink down 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 area densities were possible.

Five significant discoveries took place at Bell Labs:

1. The controlled two-dimensional motion of single wall domains in permalloy films
2. The application of orthoferrites
3. The discovery of the stable cylindrical domain
4. The invention of the field access mode of operation
5. The discovery of growth-induced uniaxial anisotropy in the garnet system and the realization that garnets would be a practical material

The bubble system cannot be described by any single invention, but in terms of the above discoveries. Andy Bobeck was the sole discoverer of (4) and (5) and co-discoverer of (2) and (3); (1) was performed in P. Bonyhard's group. At one point, over 60 scientists were working on the project at Bell Labs, many of whom have earned recognition in this field. For instance, in September 1974, H.E.D. Scovil, P.C. Michaelis and Bobeck were awarded the IEEE Morris N. Liebmann Memorial Award by the IEEE with the following citation: For the concept and development of single-walled magnetic domains (magnetic bubbles), and for recognition of their importance to memory technology.

It took some time to find the perfect material, but they discovered that 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. Unlike the original experiments, the garnet did not constrain the bubbles to move only in one direction, but its bubble properties were too advantageous to ignore.

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, moving down the surface. Another reversal would pop them off the end of the bar to the next bar in the line.

A memory device 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 needed.

Bubble memory is a non-volatile memory. Even when power was removed, the bubbles remained, just as the patterns do on the surface of a disk drive. Better yet, bubble memory devices needed no moving parts: the field that pushed the bubbles along the surface was generated electrically, whereas media like tape and disk drives required mechanical movement. Finally, because of the small size of the bubbles, the density was in theory much higher than existing magnetic storage devices. The only downside was performance; the bubbles had to cycle to the far end of the sheet before they could be read.