The Current Spin on Spintronics
Spin, which is assigned a value of "up" or "down," is
a quantum-mechanical property of electrons. Like
charge, spin can be encoded with binary data.
and high-tech gurus anticipate that the next big
thing in the electronics industry will be spintronics,
devices based on electron spin — smaller, faster,
and more versatile than today's devices, which are
based on electron charge.
Before the spintronic revolution can begin, however,
scientists will need a much better understanding
of spin currents created by the motion of electrons
through a semiconductor. An important step in this
direction has been taken by a team of scientists
at Lawrence Berkeley National Laboratory and the
University of California at Berkeley, led by Joe
Orenstein, a physicist who holds a joint appointment
with Berkeley Lab's Materials Sciences Division and
UC Berkeley's Physics Department.
By using a unique experimental technique, the research
team was able to demonstrate that contrary to conventional
scientific wisdom, spin current moves through a semiconductor
at a slower rate than does charge current. Depending
on the application, this effect, which is called "spin
Coulomb drag," could prove to be either an advantage
or a disadvantage for future spintronic technologies.
"Spin Coulomb drag results because the motion of
spin through the semiconductor is sensitive to collisions
between electrons, whereas the transport of charge
is not," says Orenstein. "When electrons bump into
one another, the mutual repulsion of their negative
charges creates a drag on their spin current, which
is a relative motion between individual electrons,
but not on their charge current, which is the collective
transport of all the electrons in motion."
The research was done by Orenstein and his graduate
students, Nuh Gedik and Chris Weber, and colleague
Joel Moore, who also holds a joint Berkeley Lab-UC
Berkeley appointment. They worked in collaboration
with Jason Stephens and David Awschalom at UC Santa
Center for Spintronics and Quantum Computation. The team reported their results
in a recent letter to Nature.
MRAM computer chips
use electron spin rather than charge to store bits
of data, which enables them to retain information
even when electrical power is turned off
Spin is a quantum mechanical property that arises
when the rotational momentum of a particle, in this
case an electron, creates a tiny magnetic field.
For the sake of simplicity, spin is given a direction,
either "up" or "down." Just as the positive or negative
values of an electrical charge can be used to encode
data as the 0s and 1s of the binary system, so too
can the up and down values of spin. Unlike charge-based
data storage, however, spin-based storage does not
disappear when the electrical current stops.
Electron spin is already making its mark on the
computer industry with the development of magnetic
random access memory chips, or MRAMs. Computers that
utilize MRAM don't need to be booted up to move hard-drive
data into memory; MRAM can also store data in a much
smaller space and access it much more quickly, while
consuming far less power than today's charge-based
memory. In the future the Holy Grail of the electronics
industry, quantum computing, could be realized through
the utilization of spin.
Since it is the random motion of electrons moving
through a semiconductor that generates a spin current
(just as it generates a current of electrical charge),
scientists had assumed that both currents move at
the same rate of speed. Collisions between individual
electrons were not supposed to affect this rate.
"When talking about the motion of charge current,
you can think of the electrons as acting like a swarm
of bees moving in one direction. Within that swarm,
individual bees might be colliding, but momentum
is conserved with each collision so that the total
motion of the swarm is unaffected," Orenstein explains. "When
talking about the motion of spin current, the electrons
act more like a swarm of honey bees and a swarm of
bumble bees trying to move through one another. As
the bees in these two populations collide, there
is an exchange of momentum that slows the relative
motion of each. Eventually both swarms may move in
a single direction, but the overall effect has been
a drag on their collective motion."
Unknown to Orenstein and his colleagues, the effect
of spin Coulomb drag had first been predicted in
2000 by Giovanni Vignale, a theorist with the University
of Missouri's Physics Department, and his then student,
Irene D'Amico, who is now a faculty member at the
University of York in the United Kingdom. But the
D'Amico-Vignale theory was received with much skepticism,
so entrenched was the belief that spin current was
little more than a tag-along with charge current.
Orenstein, shown here with a grating used in spin
spectroscopy, is an expert at employing electromagnetic
radiation to probe condensed matter systems. (Photo
Roy Kaltschmidt, CSO)
and his colleagues were able to demonstrate spin
Coulomb drag through the use of a technique called
transient grating spectroscopy, which Orenstein adapted
for his spin studies. Conventional transient grating
spectroscopy is well established for measuring the
diffusion of electron charge, among other purposes.
Two beams of laser light on the surface of a semiconductor
create alternating valence and conduction bands,
which can be used to measure the rate at which the
current of charge is flowing. In the variation used
by Orenstein and his colleagues, the two "pump" laser
beams are polarized at a 90-degree angle to one another.
This creates alternating bands of spin up and spin
down electrons that can be used to measure spin current.
"We get a diffusion of spin because we create a
situation in which there will be more spin-up electrons
in one place and fewer in another," Orenstein said.
To speed up the data acquisition process, Orenstein
borrowed from a technique called coherent heterodyne
detection (also called two-dimensional electronic
spectroscopy), used by Graham Fleming, Berkeley Lab's
Deputy Director, to follow the flow of excitation
energy during the process of photosynthesis. Adapting
the technique to transient spin grating enabled Orenstein
and his colleagues to significantly increase the
data acquisition rate.
"Prior to our work, there had been virtually no
measurements of spin diffusion in doped semiconductors," Orenstein
said. "We were able to report more than 100 measurements
carried out on a picosecond timescale."
Orenstein and his colleagues worked with a two-dimensional
electron gas residing in gallium arsenide quantum
wells for the studies they reported on in Nature
. Eventually they plan to turn their attention to
multistructured nanolayers of semiconductors and
to the transition metal oxides, such as the manganites,
that are being used in the development of MRAM.
As for what their findings on spin currents hold
for the future of spintronics, Orenstein says, "Spin
Coulomb drag definitely makes it harder to flow a
pure spin current, but if you have a technology that
calls for separate packets of up and down spin, the
drag effect means those separate packets will have
a longer shelf-life, and that's a good thing. You
could say the impact of spin Coulomb drag depends
on what kind of spin you want to put on it."
Contact: Lynn Yarris, firstname.lastname@example.org
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