Lying at the heart of the computer which you are using to read this article is a memory retrieval system based on the discoveries for which the 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg. They discovered, quite independently, a new way of using magnetism to control the flow of electrical current through sandwiches of metals built at the nanotechnology scale.
150 years ago, William Thomson observed very small changes in the electrical properties of metals when they were placed in a magnetic field, a phenomenon he named ‘Magnetoresistance’. In due course, his finding found application, magnetically-induced current fluctuations becoming the underlying principle for reading computer memories. Then, in 1988, Fert and Grünberg, working with specially-constructed stacks made from alternating layers of very thinly-spread iron and chromium, unexpectedly discovered that they could use magnetic fields to evoke much greater increases in electrical resistance than Thomson, or anyone since, had observed. Recognizing the novelty of the effect, Fert named it ‘Giant magnetoresistance’, and it was only a few years before the improvements, and the miniaturization, it offered led to its adoption in favour of classical magnetoresistance.
Giant magnetoresistance is essentially a quantum mechanical effect depending on the property of electron spin. Using an applied magnetic field to cause the electrons belonging to atoms in alternate metal layers to adopt opposite spins results in a reduction in the passage of electric current, in a similar fashion to the way that crossed polarizing filters block the passage of sunlight. When, however, magnetic fields are used to align the electron spins in different layers, current passes more easily, just as light passes through polarizers aligned in the same direction.
The application of this discovery has been rapid and wide-ranging, dramatically improving information storage capacity in many devices, from computers to car brakes. And while quietly pervading the technology behind our daily lives, the principles of giant magnetoresistance are now being used to tackle problems in wider fields, for instance in the selective separation of genetic material.
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