If you work at a computer, play video games, or listen to music on an iPod, you've benefited directly from the efforts of the winners of the 2007 Nobel Prize in Physics. Albert Fert of France's national research agency, CNRS, in Orsay and Peter Grünberg of the Jülich Research Center in Germany independently discovered an effect known as giant magnetoresistance (GMR) that fueled a dramatic increase in the capacity of computer hard drives. The discovery also laid the cornerstone of a new field known as spintronics, in which researchers try to exploit the fact that electrons spin like little tops to make novel devices.
"It's a physics discovery that has had real consequences," says Robert Buhrman, an applied physicist at Cornell University. "Without GMR, we would not be able to carry our whole life around on a 3-inch hard drive."
Long before the discovery of GMR, researchers knew that applying a magnetic field to a material such as iron or nickel could change its conductivity. Electric current would flow less readily parallel to the direction in which the material was magnetized and more readily across it. Technologists eventually harnessed that effect to make the "read heads" that sensed the setting of the magnetized bits in a computer hard drive. But the effect, known as anisotropic magnetoresistance, was small; the resistance varied by a few percent.
In 1988, Grünberg and Fert found that they could greatly increase the change in resistance if they made layer-cake films with layers of iron separated by layers of nonmagnetic chromium only a few atoms thick. If two adjacent iron layers are magnetized in the same direction, then electrons spinning in one direction will pass along the film readily, whereas electrons spinning in the other direction will not. If, however, the iron layers are magnetized in opposite directions, then all electrons run into greater resistance, regardless of how they are spinning. That makes a GMR film an extremely sensitive magnetic field detector. As a result, all the bits and hardware in a disk drive can be made much smaller.
The basic quantum mechanical concepts behind GMR were understood in the 1970s, but at the time technology was not available to exploit them, Fert says. "I put this idea in the fridge," he says. "Then in the 1980s, it became possible to fabricate these materials." Grünberg could not be reached for comment when ScienceNOW went to press.
Although Fert and Grünberg discovered the effect, Stuart Parkin of IBM's Almaden Research Center in San Jose, California, did much of the work to make GMR technologically useful. Stuart Wolf, a physicist at the University of Virginia, Charlottesville, says he was surprised that Parkin was not honored as well. But Tony Bland of the University of Cambridge, U.K., says that the Nobel committee apparently distinguished between the discovery and its cultivation. "This is properly a physics prize for a truly extraordinary and novel effect."
The advent of GMR helped launch the emerging field of spintronics, Wolf says. "This particular discovery seemed to crystallize a lot of people's interest in working in this area," he says. Their efforts may someday lead to myriad other devices, such as computer memory that can hold information even when it loses power and microchips that exploit spin to perform computations.