Bubble memory

Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each storing one bit of data. 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 bubbles are read by moving them to the edge of the material where they can be read by a conventional magnetic pickup, and then rewritten on the far edge to keep the memory cycling through the material. In operation, bubble memories are similar to delay line memory systems. Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each storing one bit of data. 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 bubbles are read by moving them to the edge of the material where they can be read by a conventional magnetic pickup, and then rewritten on the far edge to keep the memory cycling through the material. In operation, bubble memories are similar to delay line memory systems. Bubble memory started out as a promising technology in the 1980s, offering memory density of an order similar to hard drives but performance more comparable to core memory while lacking any moving parts. This led many to consider it a contender for a 'universal memory' that could be used for all storage needs. The introduction of dramatically faster semiconductor memory chips pushed bubble into the slow end of the scale, and equally dramatic improvements in hard drive capacity made it uncompetitive in price terms. Bubble memory was used for some time in the 1970s and 80s where its non-moving nature was desirable for maintenance or shock-proofing reasons. The introduction of Flash RAM and similar technologies rendered even this niche uncompetitive, and bubble disappeared entirely by the late 1980s. Bubble memory is largely the brainchild of a single person, Andrew Bobeck. Bobeck had worked on many kinds of magnetics-related projects through the 1960s, and two of his projects put him in a particularly good position for the development of bubble memory. The first was the development of the first magnetic core memory system driven by a transistor-based controller, and the second was the development of twistor memory. Twistor is essentially a version of core memory that replaces the 'cores' with a piece of magnetic tape. The main advantage of twistor is its ability to be assembled by automated machines, as opposed to core, which was almost entirely manual. AT&T had great hopes for twistor, believing it would greatly reduce the cost of computer memory and put them in an industry leading position. Instead, DRAM memories came onto the market in the early 1970s that rapidly replaced all previous random access memory systems. Twistor ended up being used only in a few applications, many of them AT&T's own computers. One interesting side-effect of the twistor concept was noticed in production; under certain conditions, passing a current through one of the electrical wires running inside the tape would cause the magnetic fields on the tape to move in the direction of the current. 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. However, such a system had few advantages over twistor, especially as it did not allow random access. In 1967, Bobeck joined a team at Bell Labs and started work on improving 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. 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.