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An attractive source for spintronics

Discovery may lead to battery that generates magnetic currents

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3:40pm, October 8, 2008
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Physicists have discovered a phenomenon that may lead to a new type of battery, one that could create magnetic, rather than electric, currents. Magnetic currents are the basis of the emerging field of spintronics, which promises to extend the long run toward computer chip miniaturization by reducing overheating from waste heat.

The new type of battery gives a convenient source of magnetic currents, which were cumbersome to generate in the past, comments physicist Phuan Ong of PrincetonUniversity. “Future generations of physicists can then use it to design devices of their own,” he says.

Eiji Saitoh of KeioUniversity in Yokohama, Japan, and his collaborators found that heating one side of a magnetized nickel-iron rod changes the arrangement of the electrons in the material according to their spins. These spins are the quantum-physics analogs of the south-north magnetic axes in bar magnets.

In the heated rod, electrons with spins that are aligned “up,” or with the material’s magnetic field, tend to prefer the warmer side, while those with spins pointing in the opposite direction, or “down,” tend to prefer the cooler side, the researchers report in the Oct. 9 Nature.

Engineers could harness this spin effect to design new devices for computer chips, Saitoh says. For example, a spintronic battery could produce spin imbalances at its two electrodes, and the chip could use that imbalance, instead of an ordinary electric current, and store information magnetically. Electric currents produce heat, but transferring information by flipping spins does not. Such spintronics devices would then cut down power consumption and operate at faster speeds without overheating. 

The team calls the newly discovered phenomenon the spin Seebeck effect, in analogy with the thermoelectric effect discovered by physicist Thomas Johann Seebeck in the 1800s. In the thermoelectric effect, heating one side of an electrically conducting rod creates a voltage, because electrons at the warmer end become faster as they heat up and thus tend to move toward the cooler end, just like a heated gas tends to expand.

In a magnetized metal, Saitoh explains, electrons throughout the material tend to align their spins up. The team had expected that when one end is warmed, the thermoelectric effect would push more of the spin-up electrons toward the cooler end — just because there are more electrons pointing up than down — reducing the excess of up spins, but only at one edge. Instead, the researchers measured a change in spins along the entire length of each sample, over distances of as many as six millimeters.

“The amount is much more significant than the prediction,” Saitoh says.

The spin difference between the two ends of the device could be tapped to produce magnetic currents in a circuit, he explains, to transfer information into memory storage, for example.

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