Magnetic Logic

Electron spins could do cool calculations

By Davide Castelvecchi
From Science News, June 2nd, 2007; Vol.171 #22 (p. 342)

Engineers have proposed a new design for circuits that could process information by using electrons as tiny bar magnets. Such circuits could someday become the building blocks of a new generation of computers.

Computer-chip components called logic gates output a 1 or 0 depending on the configuration of a gate’s inputs. In conventional electronics, such bits of information are represented by electric voltages. Logic gates contain semiconductor-based transistors that switch states in response to those voltages. Such transistors, however, require thin insulating layers that tend to leak electrons and produce excess heat.

In addition to their electric charges, electrons carry a quantum mechanical property called spin. An electron’s spin generates a magnetic field that can point in any direction. In recent years, several research teams have built “spintronics” devices that encode 1s and 0s as spins pointing up or down. Thus, the devices switch states by flipping electron spin rather than changing voltages.

Such systems would reduce the overheating problems that afflict conventional electronics. So far, though, researchers have had to make spintronic gates out of multiple layers of metallic materials, in arrangements that would be difficult to link on a chip, says Hanan Dery, an engineer at the University of California, San Diego.

Dery and his colleagues now propose a spintronic logic gate made mostly of semiconducting materials. The gate would be a horizontal semiconducting bar with five metallic contacts aligned on its surface. Four of these contacts would act as the inputs. Each of them could be magnetized up or down, which would cause electrons with up or down spins to move into the semiconductor. An excess of spin-up electrons would lower the electrical resistance at the boundary of the fifth contact. An excess of spin-down electronic would not. A separate circuit would receive the output, and current would flow only if the resistance had decreased.

Dery’s team also describes in detail how to link such components. Specialized circuits, also made of semiconducting components, would take the output current from one logic gate, amplify it, and use it to magnetize one of the input contacts of another logic gate.

A main advantage of this design, Dery says, is that present-day semiconductor-fabrication techniques could mass-produce the devices. In principle, he adds, engineers could pack these components onto a chip up to 200 times as densely as they can pack conventional gates onto a chip. His team outlines the proposal in the May 31 Nature.

Paul Crowell of the University of Minnesota in Minneapolis says that the new approach is elegant and original, but also untested. “I really cannot say yet whether it is going to be feasible,” he says. Laurens Molenkamp of the University of Würzburg in Germany says that each of the parts of the proposed device is “relatively standard” but adds, “What I like about this proposal is that it’s a full scheme” describing how to link the components in a new way.

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