Magnetic core random access memory. Image courtesy of H. J. Sommer III, Professor of Mechanical Engineering, Penn State University.
Long before the concept of spin had been realized, the phenomenon of magnetism was a source of fascination and curiosity. The first scientific record of magnetism was made by the Greek philosopher Thales of Miletos, who, in the sixth century BC, studied the attraction of materials such as iron to loadstone (magnetite). The first magnetic device was, of course, the compass — probably invented by several cultures independently and first documented in Chinese literature in the eleventh century AD.
Nearly a millennium later, and particularly since the 1950s, devices based on magnetism are once more proving significant in shaping our way of life. The magnetic tape, which was invented in 1878 by Oberlin Smith, was commercialized in the 1930s by AEG and BASF. In later decades, it was developed into, for example, videotape in 1951 and the magnetic stripes on credit cards in the 1960s.
Following the invention of modern computers, technology similar to magnetic tape was the logical choice for long-term data storage. The first hard drive with a moveable head was built into the IBM 305 computer, which shipped in 1956. Its large hard disks — 24 inches in diameter — had a storage density of 2 kilobits per square inch. IBM was also a pioneer in the development of the removable floppy disk. The first floppy disk, which had a diameter of 8 inches and a storage capacity of about 80 kilobytes, dates from 1969. The cumbersome 8-inch format was soon brought down in size: the last popular format was the Sony 3.5-inch floppy.
Magnetism has also been the key to several other, historical, storage techniques. The earliest was the 'drum memory' in the 1950s, which consisted of rotating circular metallic plates coated with a magnetic material. Drum memory was superseded in the 1960s by the 'core memory' — a hand-woven grid of wires with small ferrite rings (the cores) at the intersections. Complex current pulses through the wires were able to read, as well as set, the magnetization of the cores. Despite being an intricate device, a core memory of two cubic feet, with a capacity of 4,096 words, was used in the Apollo guidance computer, onboard the NASA missions to the Moon. Computer memory was miniaturized further in the late 1970s, for example using 'bubble memory' in which data storage is based on small magnetic domains on a thin film. Soon afterwards, hard drives became the dominant data-storage system for computers.
The history of magnetic devices illustrates well how, with a little inventiveness, the macroscopic manifestations of magnetism can be harvested to achieve amazing technological advances. However, it would take a more fundamental understanding of spin physics to achieve the next technological revolution in computing and information storage (Milestone 18).
