Editing Bubble memory (section)

=== Development ===
[[file:CMOS magnetische Blasendomänen.jpg|thumb|upright=.8|Bubble domain visualization by using CMOS-MagView]]
[[File:Bubble memory driver coils and guides.png|thumb|Bubble memory driver coils/windings/field coils and guides (T bar guides in this case); the guides or propagation elements, are on top of a magnetic film, which is on top of a substrate chip. This is mounted to a PCB (not shown) and then surrounded by two windings shown in yellow and blue.]]

In 1967, Bobeck joined a team at [[Bell Labs]] and started work on improving [[Twistor_memory|twistor]]. The [[memory density]] of twistor was a function of the size of the wires; the length of any one wire determined how many bits it held, and many such wires were laid side-by-side to produce a larger memory system.

Conventional magnetic materials, like the magnetic tape used in twistor, allowed the magnetic signal to be placed at any location and to move in any direction. [[Paul Charles Michaelis]] working with [[permalloy]] magnetic thin films discovered that it was possible to move magnetic signals in [[orthogonal]] directions within the film. This seminal work led to a patent application.<ref>{{cite patent |country=US |number=3,454,939 |status=patent |gdate=1969-07-08 }}</ref> 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, 15 September 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.

This led to the possibility of making a memory system similar to the moving-domain twistor concept, but using a single block of magnetic material instead of many twistor wires. 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:

# The controlled two-dimensional motion of single wall domains in permalloy films
# The application of orthoferrites
# The discovery of the stable cylindrical domain
# The invention of the field access mode of operation
# 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 by P. Michaelis 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 it was discovered that some [[garnet]]s had the correct 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, called propagation elements. 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, and so on, controlling or guiding the direction of travel of the bubbles. T bars/guides, shaped like the letters, were used in early bubble memory designs, but were later replaced by other shapes such as asymmetrical chevrons.<ref name="intel"/> In practice the magnetic field rotates and is provided by a pair of coils, that produce a rotating magnetic field in the X and Z axes, it is this rotating magnetic field that moves the bubbles in the memory.

Amorphous magnetic films were also considered as they had greater potential for improvement of bubble memories vs garnet magnetic films, however the existing experience with garnet films meant that they did not gain a foothold. Garnet films have the same or better magnetic properties than orthoferrite films which were considered less promising by comparison. Garnet materials (as films on top of a substrate) could allow for higher propagation speeds of the bubbles (bubble speed) than orthoferrites.
Hard bubbles are slower and more erratic than normal bubbles, a problem that is often overcome by ion-implantation of the garnet magnetic film with neon,<ref name="donald"/> and can also be done by coating the garnet magnetic film with permalloy.<ref name="nist"/>

A memory device is formed by lining up tiny [[electromagnet]]s 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.<ref name="intel"/>

A bubble memory device consists of a case, that houses a PCB with connections to one or more bubble memory chips which may be translucent. The area around the chips on the PCB is surrounded by two windings made of copper wire or other electrically conductive material, that mostly wrap the area, leaving some space for the PCB to pass through the windings and connect to the chips. The windings are wound in directions opposite to each other, for example one winding has wires oriented along the X axis and the other winding has wires along the Z axis. The windings, in turn, are surrounded by two permanent magnets, one below and another above the windings. This forms an assembly that is housed inside the case which acts as a magnetic shield and forms a magnetic return path for the magnetic field from the magnets. The permanent magnets are critical; they create a static (DC, direct current) magnetic field, used as a bias field that enables the contents of the memory to be retained, in other words they allow bubble memories to be non-volatile. If the magnets are removed, all bubbles will disappear and thus all contents will be deleted. The windings create a rotating magnetic field parallel to the orientation of the bubble memory, at around 100 to 200&nbsp;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.<ref name="intel"/><ref>Intel magnetics. 1 mega bit Bubble Memory
Design Handbook. 1979. <!--https://www.lo-tech.co.uk/downloads/manuals/intel/BubbleMemoryDesignHandbook_text.pdf--></ref>

To allow the bubbles to move around the bubble chips and to guide them through the chip, the chips have some sort of pattern made of ferromagnetic metal that can include for example asymmetrical chevrons.<ref name="intel"/> For example, the bubbles can move around the edges of the chevrons. The patterns can be called propagation elements as they allow the bubbles to move or propagate across it. They define pathways for the bubbles to be stored and retrieved for reading and the rotating magnetic field moves the bubbles along these paths. For bubble memory, a material like Gadolinium Gallium Garnet (GGG) is used as the substrate in the chips.<ref name="intel"/> On top of the substrate is a magnetic film (bubble host or bubble film/layer)<ref name="nist"/><ref name="donald"/> such as a Gadolinium-containing garnet<ref name="nist"/> or more often, single crystal substituted yttrium iron garnet<ref name="donald"/> 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.<ref name="nist">FOREIGN AND DOMESTIC ACCOMPLISHMENTS IN MAGNETIC BUBBLE DEVICE TECHNOLOGY. National Bureau of Standards. 1977. https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nbsspecialpublication500-1.pdf</ref><ref name="intel"/> The epitaxy process would be carried out with a platinum crucible and wafer holder.<ref name="donald"/> The chevrons and other parts are built on top of the film.<ref name="intel"/> The propagation elements, including the chevrons, can be made of a material such as Nickel-Iron permalloy. The materials in bubble memories are chosen mainly for their magnetic properties.<ref name="intel">Intel Memory Components Handbook. 1984. <!--http://www.nj7p.org/Manuals/PDFs/Intel/1984_Intel_Memory_Components_Handbook.pdf--></ref> Gadolinium Gallium Garnet is used as a substrate because it can support the epitaxial growth of magnetic garnet films, and is nonmagnetic,<ref name="donald"/> although some bubble memories used Nickel-Cobalt substrates instead.

The use of propagation elements formed by ion implantation instead of permalloy, was proposed to increase the capacity of bubble memory to 16&nbsp;Mbit/cm<sup>2</sup>.<ref name="donald"/>