Researchers have taken the first step toward building silicon-based computers that use a fraction of the power of today's machines. A team has injected electrons into silicon in such a way that their spins, or magnetic orientations, tend to be aligned in one direction instead of the other.
Although the reported effect is subtle, silicon has never before supported such attempts to implement spintronics—the manipulation of electrons by their spins instead of their charges. "To us this is the holy grail of semiconductor spintronics," says physicist and electrical engineer Ian Appelbaum of the University of Delaware in Newark, who worked on the experiment. "As long as Intel is making CPUs out of silicon, we're going to have to learn how to manipulate spin in silicon."
One promise of spintronics is to slash waste heat in computers. In normal electronics, electric fields propel electrons, which shed heat in the process. In contrast, spins can move around on their own or under a magnetic field without producing much heat. But researchers had only shown they could control electron spin reliably in more niche varieties of semiconductor such as gallium arsenide, which is used in cell phones.
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In the new device, Appelbaum and co-workers inject electrons from a layer of aluminum through a thin layer of ferromagnet (a permanent magnet) and into a pure silicon crystal. Aluminum has a 50–50 mix of spin up and spin down electrons-—the two possible orientations. The ferromagnet, however, blocks electrons of one spin while letting the others flow into the silicon.
The researchers found that their ferromagnet barrier gave silicon a one percent excess of one spin type versus the other, at a temperature of 85 kelvins, they report in a paper published online today in the journal Nature.
A real spintronics device would need to produce a pure stream of one spin type, Appelbaum says, adding that he expects to achieve "vast improvements" on this front in the near future. "Right now we're focused on fundamental materials science," he says. "Integration obviously is a goal, but at the moment it's not on the near term horizon."
The experiment brings the same spintronics tools to silicon that researchers have developed for other materials, says physicist David Awschalom of the University of California, Santa Barbara. In principle, he says, the effect should work at room temperature, which would allow researchers to study silicon spintronics in more realistic conditions. "It's a beautiful piece of work," he says.