A New Spin on Silicon Chips

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A 10-micrometer-thick silicon layer lies on top of the detector, and above that comes the electron-injecting part of the chip. In the injector, highly energized electrons pass through a magnetic iron layer, which filters out all electrons with a spin down. Spin-up electrons pass through the 10-micrometer silicon layer and go to the detector. In the detector, if the nickel-iron layer's magnetic-field direction matches the spin direction, electrons go through to the silicon substrate, leading to a small current. But if the researchers flip the direction of the magnetic field in the nickel-iron layer, there is no current.

The key is the detector's complex layered structure, which the researchers make using a special technique to deposit silicon on top of the nickel-iron layer. "It's a very ingenious scheme to electrically generate and transport spins in silicon, [to] electrically detect the spins, and doing all of this on a chip," says David Awschalom, a physics professor who studies semiconductor spintronics at the University of California, Santa Barbara.

Others believe that the work is an experimental demonstration of a principle but is not very practical. "What this paper shows is that spin can survive 10 microns, which is pretty neat," says Stuart Parkin, director of the spintronics science and application center at IBM's Almaden Research Center in San Jose, CA. "From an application point of view, it doesn't really tell us how to make an interesting, useful device."

One major issue is the tiny current output of the device, Parkin says. The researchers put three milliamperes in, and the output is in picoamperes, which is too small to be useful. Another problem is the special technique that the researchers use to make the device's layered structure. This method is complicated and not at all compatible with current silicon fabrication, says Parkin.

But, says Crowell, this work marks the first time that spin has been measured in silicon, and that's a great start toward silicon-based spintronics.