Bubble memory
The material is arranged to form a series of parallel tracks that the bubbles can move along under the action of an external magnetic field.
The introduction of dramatically faster semiconductor memory chips in the early 1970s pushed bubble into the slow end of the scale and it began to be considered mostly as a replacement for disks.
The equally dramatic improvements in hard-drive capacity through the early 1980s made it uncompetitive in price terms for mass storage.
[1] Bubble memory was used for some time in the 1970s and 1980s in applications where its non-moving nature was desirable for maintenance or shock-proofing reasons.
The introduction of flash storage and similar technologies rendered even this niche uncompetitive, and bubble disappeared entirely by the late 1980s.
The main advantage of twistor is its ability to be assembled by automated machines, as opposed to core, which was almost entirely manual.
If used properly, it allowed the stored bits to be pushed down the tape and pop off the end, forming a type of delay-line memory, but one where the propagation of the fields was under computer control, as opposed to automatically advancing at a set rate defined by the materials used.
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.
These bubbles were much smaller than the domains of normal media like tape, which suggested that very high area densities were possible.
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.
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, called propagation elements.
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.
The windings create a rotating magnetic field parallel to the orientation of the bubble memory, at around 100 to 200 kHz.
This will move or drive the bubbles in the magnetic film in a somewhat circular fashion, guided or restrained by the propagation elements.
For example, the rotating magnetic field can force the bubbles to constantly circulate around loops, which may be elongated and are defined by the locations of the guiding elements.
[3] On top of the substrate is a magnetic film (bubble host or bubble film/layer)[5][4] such as a Gadolinium-containing garnet[5] or more often, single crystal substituted yttrium iron garnet[4] which holds the magnetic bubbles, that is grown epitaxially with liquid-phase epitaxy with lead oxide flux as the liquid with yttrium oxide and other oxides, and then the film is doped with ion-implantation of one or several elements, to reduce undesirable characteristics.
The use of propagation elements formed by ion implantation instead of permalloy, was proposed to increase the capacity of bubble memory to 16 Mbit/cm2.
In 1981 major companies working on the technology closed their bubble memory operations,[16][17] notably Rockwell, National Semiconductor, Texas Instruments and Plessey, leaving a "big five" group of companies still pursuing "second-generation bubble" by 1984: Intel, Motorola, Hitachi, SAGEM and Fujitsu.
This application became obsolete too with the development of flash storage, which also brought performance, density, and cost benefits.
Fujitsu used bubble memory on their FM-8 in 1981 and Sharp used it in their PC 5000 series, a laptop-like portable computer from 1983.
[3] The bubbles are created (the memory is written) with a seed bubble that is constantly split or cut by a hairpin-shaped piece of electrically conductive wire (such as aluminum-copper alloy) using a current strong enough to locally overcome and reverse the magnetic bias field generated by the magnets, thus the hairpin-shaped piece of wire acts as a small electromagnet.
[3] The gadolinium gallium garnet wafers used as substrates for the bubble chips, were 3 inches in diameter and cost $100 each in 1982 as their production required the use of iridium crucibles.
[25] IBM's 2008 work on racetrack memory is essentially a 1-dimensional version of bubble, bearing an even closer relationship to the original serial twistor concept.
