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A New Spin on Electronics
25 November 2009 (All day)
You're reading this story on a computer whose chips shift tiny packets of electric charge through circuits etched in the ubiquitous semiconductor silicon. But some physicists aim to develop a whole new technology called "spintronics" that would encode information in the directions in which electrons spin as well. Those efforts could lead to ultra-low-power electronics and even futuristic quantum computers. Now, such technologies may be an important step closer to reality thanks to a group of researchers that has managed to polarize the spinning electrons in silicon, the most common commercial semiconductor.
Such "spin injection" had been achieved before in more exotic semiconductors, such as gallium arsenide and indium arsenide. The trick is to lay a patch of magnetic metal such as nickel iron on top of the semiconductor. The whole reason the metal is magnetic is that more of its electrons' spins point in one way than in the opposite way. So if a current can be driven from the metal into the semiconductor, it should deposit spin-polarized electrons in it. Physicists have shown that those electrons will hold their polarization long enough to flow micrometers through the semiconductor, which should be far enough to use them in circuits.
Incorporating both the magnetic leads and the underlying semiconductor, a spintronics circuit could hold its memory when turned off, as the magnetic elements remain magnetized. Manipulating spin could also require far less power than steering charges does, says Ron Jansen of the University of Twente in Enschede, Netherlands. Some physicists even aspire to create a spooky quantum connection called "entanglement" between spin-polarized currents to make a quantum computer that could crack problems that stymie an ordinary one.
Spin injection had been achieved at room temperature, however, only in materials like gallium arsenide. In silicon it had been done only at temperatures below 150 kelvin. Now Jansen, Saroj Dash, and colleagues at Twente have brought silicon in from the cold, too. To do that, they relied on a simple and elegant scheme to inject and detect the spins with a single nickel iron electrode separated from the silicon with a very even layer of aluminum oxide. As current flowed out of the electrode, it produced a puddle of polarized electrons in the silicon below it. But the researchers could dissipate this polarized puddle by applying a magnetic field, which for a fixed current would cause the voltage across the contact to decrease in a telltale way. And that's exactly what they observed, as the team reports tomorrow in Nature.
To prove that the voltage change wasn't caused by something else, the researchers conducted a control experiment in which they separated the aluminum oxide and the nickel iron with a layer of the metal ytterbium, which destroyed the spin polarization. In that case, the voltage across the contact remained constant even when the magnetic field was ramped up, proving that polarized electrons caused the original effect.
It's a "compelling demonstration," says Paul Crowell, a physicist at the University of Minnesota, Twin Cities. The next step, he says, is to conduct multielectrode measurements that show spin-polarized currents actually flowing through the silicon. The result raises the possibility of quick and simple spintronics applications, he says. "For a chip that needs a relatively simple memory, I think this could be realized in silicon fairly easily." But the grand goals of spintronics, such as ultralow power consumption, remain years away, Crowell says--and Jansen agrees.