Over the last decade,
spintronic technology
has found its way into
consumer electronics,
used in devices like cell
phones, hard drives,
and RAM. Now, researchers at the University of Delaware have opened a door
to making spintronics even more ubiquitous. They've demonstrated how to control electron spin in silicon, today's semiconductor of choice ().
"Silicon forms the basis of the microelectronics industry," said Ian Appelbaum,
professor of electrical engineering and
lead author of the study (). "We can
make complex devices like CPUs if we can
harness… extra information encoded in
spin and apply it in silicon."
Spintronics involves harnessing an
electron's spin—a quantum property with
either an "up" or "down" orientation—to
encode and process data. While electronics uses electrical fields to push
electrons (read: information) in a current,
spintronics uses magnetic fields to move
electrons along.
To work, spintronics requires all electron spins to be oriented in the same
direction. To achieve this in silicon,
Appelbaum's team injected electrons
from aluminum—which, like silicon, is
non-magnetic and therefore has the
same amount of spin-down electrons as
spin-up electrons—through a ferromagnetic alloy of cobalt and iron. Electrons in
the ferromagnet have only a spin-down
orientation.
When Appelbaum applied voltage to
the device, spin-down electrons
remained with the ferromagnet while
spin-up electrons passed through the silicon and precessed (or gyrated) along
the same spin orientation. "At this point,
we're just demonstrating potential,"
Appelbaum said, explaining that previously, nobody knew how to manipulate
electron spin in silicon. "We've finally
shown one way of doing it."
Researchers have been able to control
spin in other semiconductors like gallium
arsenide, which is used in cell phones.
But those methodologies, Appelbaum
said, would not work in silicon. And like
other spintronic devices, silicon spintronics could reduce power consumption,
reduce circuit scaling limitations, and
slash waste heat.
"One limitation to scaling electronic
circuits is that you can't get the heat out
fast enough," Appelbaum said. "If you can figure out how to isolate spin current, you won't dissipate heat."
Currently, spin current is coupled with
charge current. Other researchers at the
University of Delaware will be investigating ways to capture a "pure" spin current
at the university's up-and-coming Center
for Spintronics and Biodetection. The U.S. Department of Energy recently
awarded the university a $1.9 million
grant for the establishment of the center.
John Xiao, professor of physics and
astronomy, will lead the center, which will focus on spintronics in metals, not
semiconductors. It is easier to generate
a spin current in metals, Xiao said, which
is ideal for the center's aim of generating
a type of spin "battery."
"We are more concentrated on developing a source… that does not source an
electric current, but sources a spin current," Xiao said. This research, he added,
can eventually be applied to semiconductor spintronics.
While the "most visible impact of spintronics in consumer electronics today is
entirely metals-based," Appelbaum said,
semiconductor spintronics are superior
because spin lifetime in semiconductors
is orders of magnitude longer than in metals.
"That's not to overstate the impact [of
silicon spintronics] on consumer electronics," he said. "But I think now it's
clear that the potential is there and
there's a lot of work to be done. We've
opened the door, but we're taking just
one step in the direction."
Ultimately, Appelbaum hopes to see
his research applied to quantum computing. "It may take my lifetime to do it," he
says, "but that's where it's going."