This year’s Nobel Prize in physics has been given to a pair of researchers who discovered a magnetic property that opened the way for today’s fast and compact hard drives, making possible everything from iPods to the massive data centers that serve as the backbone of the Internet. The discovery has helped improve data storage density by at least an order of magnitude. And it is paving the way for several experimental technologies that could increase it even more.
Albert Fert, scientific director at Unité Mixte de Physique CNRS-Thales in France, and Peter Grünberg, recently retired as a research scientist at the Research Centre Jülich in Germany, independently discovered the property, which Fert called giant magnetoresistance (GMR), in 1988. GMR makes it possible to pack far more information onto a hard disk by significantly increasing the sensitivity of detectors used to read bits of information. Within 10 years of its discovery, hard drives based on the effect were commercialized by IBM.
Before GMR was discovered, hard drives depended on a phenomenon called magnetoresistance, which had been understood for well over 100 years. In magnetoresistance, a magnetic field alters the electrical resistance in a material, causing measurable changes in electrical current. In hard drives, this property was used to detect bits of information–regions on a disk that have magnetized in one of two directions. As the head passes over such a region, its magnetic field changes a current flowing in the head, registering a 1 or a 0. But the technology ran into problems as the density of memory increased and researchers developed ways to write ever smaller bits. “Conventional sensors were finding it harder and harder to detect the magnetic bits stored on a hard drive,” says David Awschalom, a professor of physics at the University of California Santa Barbara. “The industry was facing this brick wall. How do you put more information on a disk and still read it?”
Fert’s and Grünberg’s discovery led to new sensors that show a giant change in electronic resistance when they encounter a magnetic field. This larger change made it possible to detect smaller bits, making it practical to cram far more of them onto a disk. “It’s the reason that a number of years ago all of us saw a very strong increase in the storage density in our hard drives,” Awschalom says. “It’s hit the consumer in a very big way.”
The giant-magnetoresistance effect depends on a quantum-mechanical property of electrons called spin, which has to do with the magnetic properties of a material. An electronic current includes electrons with two types of spin, designated “up” or “down.” Similarly, magnetic materials can be magnetized in different directions, which can also be called up and down. The ease with which an electron can move through a magnetic material depends on its spin. If an electron’s spin is up, it will move freely through an up-oriented magnet but will encounter resistance in a down magnet. The down-spin electron will behave just the opposite way.
Fert and Grünberg exploited this behavior by combining two layers of material, one magnetized up and one down. They then applied a magnetic field that magnetized both in the same direction and observed the effect this had on current running through the layers. They found that when both layers are oriented in the same direction, at least one type of electron can pass freely. But when they are oriented in opposite directions, both types of electrons encounter resistance, causing a large drop in current. Because the effect is large, the magnetic field from even a tiny bit creates a discernible signal, making it possible to detect smaller bits.