20 February 2009

Bubble memory

The invention: An early nonvolatile medium for storing information on computers. The person behind the invention: Andrew H. Bobeck (1926- ), a Bell Telephone Laboratories scientist Magnetic Technology The fanfare over the commercial prospects of magnetic bubbles was begun on August 8, 1969, by a report appearing in both The New York Times and TheWall Street Journal. The early 1970’s would see the anticipation mount (at least in the computer world) with each prediction of the benefits of this revolution in information storage technology. Although it was not disclosed to the public until August of 1969, magnetic bubble technology had held the interest of a small group of researchers around the world for many years. The organization that probably can claim the greatest research advances with respect to computer applications of magnetic bubbles is Bell Telephone Laboratories (later part of American Telephone and Telegraph). Basic research into the properties of certain ferrimagnetic materials started at Bell Laboratories shortly after the end of World War II (1939-1945). Ferrimagnetic substances are typically magnetic iron oxides. Research into the properties of these and related compounds accelerated after the discovery of ferrimagnetic garnets in 1956 (these are a class of ferrimagnetic oxide materials that have the crystal structure of garnet). Ferrimagnetism is similar to ferromagnetism, the phenomenon that accounts for the strong attraction of one magnetized body for another. The ferromagnetic materials most suited for bubble memories contain, in addition to iron, the element yttrium or a metal from the rare earth series. It was a fruitful collaboration between scientist and engineer, between pure and applied science, that produced this promising breakthrough in data storage technology. In 1966, Bell Laboratories scientist Andrew H. Bobeck and his coworkers were the first to realize the data storage potential offered by the strange behavior of thin slices of magnetic iron oxides under an applied magnetic field. The first U.S. patent for a memory device using magnetic bubbles was filed by Bobeck in the fall of 1966 and issued on August 5, 1969. Bubbles Full of Memories The three basic functional elements of a computer are the central processing unit, the input/output unit, and memory. Most implementations of semiconductor memory require a constant power source to retain the stored data. If the power is turned off, all stored data are lost. Memory with this characteristic is called “volatile.” Disks and tapes, which are typically used for secondary memory, are “nonvolatile.” Nonvolatile memory relies on the orientation of magnetic domains, rather than on electrical currents, to sustain its existence. One can visualize by analogy how this will work by taking a group of permanent bar magnets that are labeled withNfor north at one end and S for south at the other. If an arrow is painted starting from the north end with the tip at the south end on each magnet, an orientation can then be assigned to a magnetic domain (here one whole bar magnet). Data are “stored” with these bar magnets by arranging them in rows, some pointing up, some pointing down. Different arrangements translate to different data. In the binary world of the computer, all information is represented by two states. A stored data item (known as a “bit,” or binary digit) is either on or off, up or down, true or false, depending on the physical representation. The “on” state is commonly labeled with the number 1 and the “off” state with the number 0. This is the principle behind magnetic disk and tape data storage. Now imagine a thin slice of a certain type of magnetic material in the shape of a 3-by-5-inch index card. Under a microscope, using a special source of light, one can see through this thin slice in many regions of the surface. Darker, snakelike regions can also be seen, representing domains of an opposite orientation (polarity) to the transparent regions. If a weak external magnetic field is then applied by placing a permanent magnet of the same shape as the card on the underside of the slice, a strange thing happens to the dark serpentine pattern—the long domains shrink and eventually contract into “bubbles,” tiny magnetized spots. Viewed from the side of the slice, the bubbles are cylindrically shaped domains having a polarity opposite to that of the material on which they rest. The presence or absence of a bubble indicates either a 0 or a 1 bit. Data bits are stored by moving the bubbles in the thin film. As long as the field is applied by the permanent magnet substrate, the data will be retained. The bubble is thus a nonvolatile medium for data storage.Consequences Magnetic bubble memory created quite a stir in 1969 with its splashy public introduction. Most of the manufacturers of computer chips immediately instituted bubble memory development projects. Texas Instruments, Philips, Hitachi, Motorola, Fujitsu, and International Business Machines (IBM) joined the race with Bell Laboratories to mass-produce bubble memory chips. Texas Instruments became the first major chip manufacturer to mass-produce bubble memories in the mid-to-late 1970’s. By 1990, however, almost all the research into magnetic bubble technology had shifted to Japan. Hitachi and Fujitsu began to invest heavily in this area. Mass production proved to be the most difficult task. Although the materials it uses are different, the process of producing magnetic bubble memory chips is similar to the process applied in producing semiconductor-based chips such as those used for random access memory (RAM). It is for this reason that major semiconductor manufacturers and computer companies initially invested in this technology. Lower fabrication yields and reliability issues plagued early production runs, however, and, although these problems have mostly been solved, gains in the performance characteristics of competing conventional memories have limited the impact that magnetic bubble technology has had on the marketplace. The materials used for magnetic bubble memories are costlier and possess more complicated structures than those used for semiconductor or disk memory. Speed and cost of materials are not the only bases for comparison. It is possible to perform some elementary logic with magnetic bubbles. Conventional semiconductor-based memory offers storage only. The capability of performing logic with magnetic bubbles puts bubble technology far ahead of other magnetic technologies with respect to functional versatility. Asmall niche market for bubble memory developed in the 1980’s. Magnetic bubble memory can be found in intelligent terminals, desktop computers, embedded systems, test equipment, and similar microcomputer- based systems.


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