Just like a magnet with a north and a south pole (left), electrons are surrounded by a magnetic field (right). This magnetic momentum, or spin, could be used to store information in more efficient ways. Image: Philippe Jacquod |
In a recent publication in Physical Review Letters, physicists at the Univ. of Arizona
propose a way to translate the elusive magnetic spin of electrons into easily
measurable electric signals. The finding is a key step in the development of
computing based on spintronics, which doesn’t rely on electron charge to
digitize information.
Unlike conventional computing devices, which require
electric charges to flow along a circuit, spintronics harnesses the magnetic
properties of electrons rather than their electric charge to process and store
information.
“Spintronics has the potential to overcome several
shortcomings of conventional, charge-based computing. Microprocessors store
information only as long as they are powered up, which is the reason computers
take time to boot up and lose any data in their working memory if there is a
loss of power,” said Philippe Jacquod, an associate professor with joint
appointments in the College of Optical Sciences and the department of physics
at the College of Science, who published the research together with his
postdoctoral assistant, Peter Stano.
“In addition, charge-based microprocessors are leaky,
meaning they have to run an electric current all the time just to keep the data
in their working memory at their right value,” Jacquod added. “That’s
one reason why laptops get hot while they’re working.”
“Spintronics avoids this because it treats the
electrons as tiny magnets that retain the information they store even when the
device is powered down. That might save a lot of energy.”
To understand the concept of spintronics, it helps to
picture each electron as a tiny magnet, Jacquod explained.
“Every electron has a certain mass, a certain charge
and a certain magnetic moment, or as we physicists call it, a spin,” he
said. “The electron is not physically spinning around, but it has a
magnetic north pole and a magnetic south pole. Its spin depends on which pole
is pointing up.”
Current microprocessors digitize information into bits, or
“zeroes” and “ones,” determined by the absence or presence
of electric charges. “Zero” means very few electronic charges are present;
“one” means there are many of them. In spintronics, only the
orientation of an electron’s magnetic spin determines whether it counts as a
zero or a one.
“You want as many magnetic units as possible, but you
also want to be able to manipulate them to generate, transfer and exchange
information, while making them as small as possible” Jacquod said.
Taking advantage of the magnetic moment of electrons for
information processing requires converting their magnetic spin into an electric
signal. This is commonly achieved using contacts consisting of common iron
magnets or with large magnetic fields. However, iron magnets are too crude to
work at the nanoscale of tomorrow’s microprocessors, while large magnetic
fields disturb the very currents they are supposed to measure.
“Controlling the spin of the electrons is very
difficult because it responds very weakly to external magnetic fields,”
Jacquod explained. “In addition, it is very hard to localize magnetic
fields. Both make it hard to miniaturize this technology.”
“It would be much better if you could read out the spin
by making an electric measurement instead of a magnetic measurement, because
miniaturized electric circuits are already widely available,” he added.
In their research paper, based on theoretical calculations
controlled by numerical simulations, Jacquod and Stano propose a protocol using
existing technology and requiring only small magnetic fields to measure the
spin of electrons.
“We take advantage of a nanoscale structure known as a
quantum point contact, which one can think of as the ultimate bottleneck for
electrons,” Jacquod explained. “As the electrons are flowing through
the circuit, their motion through that bottleneck is constrained by quantum
mechanics. Placing a small magnetic field around that constriction allows us to
measure the spin of the electrons.”
“We can read out the spin of the electrons based on how
the current through the bottleneck changes as we vary the magnetic field around
it. Looking at how the current changes tells us about the spin of the
electrons.”
“Our experience tells us that our protocol has a very
good chance to work in practice because we have done similar calculations of
other phenomena,” Jacquod said. “That gives us the confidence in the
reliability of these results.”
In addition to being able to detect and manipulate the
magnetic spin of the electrons, the work is a step forward in terms of
quantifying it.
“We can measure the average spin of a flow of electrons
passing through the bottleneck,” Jacquod explained. “The electrons
have different spins, but if there is an excess in one direction, for example
ten percent more electrons with an upward spin, we can measure that rather
precisely.”
He said that up until now, researchers could only determine
there was excess, but were not able to quantify it.
“Once you know how to produce the excess spin and know
how to measure it, you could start thinking about doing basic computing
tasks,” he said, adding that in order to transform this work into
applications, some distance has yet to be covered.
“We are hopeful that a fundamental stumbling block will
very soon be removed from the spintronics roadmap,” Stano added.
Spintronics could be a stepping stone for quantum computing,
in which an electron not only encodes zero or one, but many intermediate states
simultaneously. To achieve this, however, this research should be extended to
deal with electrons one-by-one, a feat that has yet to be accomplished.
