New Spin for Electronics

Subatomic properties will remake computing.

Imagine a data storage device the size of an atom, working at the speed of light. Imagine a microprocessor whose circuits could be changed on the fly. One minute, it would be optimized for database access, the next for transaction processing and the next for scientific number-crunching.

Finally, imagine a computer memory thousands of times denser and faster than today's memories. And nonvolatile, so it retains its contents when the power is off.

All of these and more are on computing's horizon, thanks to the exploding field of spintronics. Spintronics, from "spin transport electronics," isn't entirely new. The spintronic effect called giant magneto-resistance was introduced by IBM in 1997 in its GMR disk-read head. As a result, disk capacities have jumped by a factor of 100 in the past five years.

Electronic circuits are driven by electron flows, which have a charge that can be measured and controlled. But electrons not only flow; they also spin like tiny bar magnets. Depending on their orientation, the spins are said to be "up" or "down."

This additional variable, or "degree of freedom," means that electrons can do more things and convey more information than they do in conventional electronics. "Spin gives you an additional knob to turn," explains Stuart Wolf, a program manager at the Defense Advanced Research Projects Agency (DARPA), which is funding much of the spintronics research in the U.S.

The most immediate research goal is to produce magnetic random-access memory (MRAM), which stores data using magnetism rather than electrical charges. Unlike the dynamic RAM in your PC, MRAM is nonvolatile.

IBM is working with Munich-based Infineon Technologies AG and says it will have MRAM in production as early as 2005. It will be 50 times faster than DRAM and 10 times denser than static RAM, and it could eventually replace both, says Stuart Parkin, an IBM fellow at the company's Almaden Research Center in San Jose.

Others have even suggested that MRAM might replace disks for data storage. Putting logic and storage in a single chip would eliminate the slow disk I/O that's a bottleneck in most computer processing.

IBM's MRAM will use magnetic tunnel junctions, an application of spintronics in which electrons are allowed to "tunnel" between two ferromagnetic layers based on their spin. Each junction can store one bit. "It promises a sort of universal RAM with very high performance -- high writing and reading speeds -- plus very high density and nonvolatility," Parkin says.

Nuclear Memories

Further out, researchers are working on still more exotic applications of spin. David Awschalom, director of the Center for Spintronics and Quantum Computation at the University of California, Santa Barbara, is looking at what might be done with the spin of an atom's nucleus, a new idea.

"The subatomic part of the atom would store the information, and the electron would act as the bus to carry information in and out of the nuclear subsystem," Awschalom says.

He aims to build an optical-based information processor in which beams of light would transfer information to the nucleus through electrons. Such nuclear memories would be "many orders of magnitude" denser and faster than traditional semiconductor memories, he says.

Indeed, more broadly, the thrust of spintronics research will be to combine electronics and photonics with magnetism -- which traditionally involves metals -- in semiconductor materials. That will enable ultrafast and ultraefficient submicron devices that integrate computing, communications and storage. The slow interfaces between different materials that convert one kind of signal or property into another would be gone, and the latencies that typically slow the movement of data from one processing stage to another would be greatly reduced.

"You'd have everything integrated in a much simpler circuit," says DARPA's Wolf. "They would be much like existing semiconductor devices, except the current is spin-polarized." That would enable, for example, the construction of very fast communication switches. "You could call it spin photonics," he says. "They can easily operate at terahertz speeds."

A semiconductor device can't use spin until a way is found to get spin-polarized electrons into it, and that has proved difficult. But IBM recently demonstrated that it can use magnetic tunnel junctions to inject the current, as they do for MRAM.

IBM's Parkin says spintronic semiconductors could be used to build reconfigurable logic devices. "So maybe your computer could be optimized for certain instructions by rearranging the way [logic] gates are connected, on the fly," he says.

Another tough challenge has been to create magnetic semiconductors that sustain their spin states at room temperature, but physicists, materials scientists and engineers have made tremendous progress on that front just this year. "We are not quite there yet," Awschalom says. "But it's a rapidly moving field. If you'd asked me a year ago where we'd be today, I would have been largely wrong in my assessment."

The rapid development of spintronics seems likely to continue, says Awschalom. "The theory is in quite sound shape. What's exciting about this field is there are no obvious show-stoppers. There are many challenges, though."

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