At the smallest scales, magnetism may not work quite the way
scientists expected, according to a recent paper in Physical Review Letters by Rafal Oszwaldowski and Igor Zutic of the
Univ. at Buffalo
and Andre Petukhov of the South Dakota School of Mines and Technology.
The three physicists have proposed that it would be possible to
create a quantum dot that is magnetic under surprising circumstances.
Magnetism is determined by a property all electrons possess:
spin. Individual spins are akin to tiny bar magnets, which have north and south
poles. Electrons can have an “up” or “down” spin, and a
material is magnetic when most of its electrons have the same spin.
Mobile electrons can act as “magnetic messengers,”
using their own spin to align the spins of nearby atoms. If two mobile
electrons with opposite spins are in an area, conventional wisdom says that
their influences should cancel out, leaving a material without magnetic
But the UB-South Dakota team has proposed that at very small
scales, magnetism may be more nuanced than that. It is possible, the physicists
say, to observe a peculiar form of magnetism in quantum dots whose mobile
electrons have opposing spins.
In their Physical Review
Letters article, the researchers describe a
theoretical scenario involving a quantum dot that contains two free-floating,
mobile electrons with opposite spins, along with manganese atoms fixed at
precise locations within the quantum dot.
The quantum dot’s mobile electrons act as “magnetic
messengers,” using their own spins to align the spins of nearby manganese
Under these circumstances, conventional thinking would predict a
stalemate: Each mobile electron exerts an equal influence over spins of
manganese atoms, so neither is able to “win.”
Through complex calculations, however, Oszwaldowski, Zutic, and
Petukhov show that the quantum dot’s two mobile electrons will actually
influence the manganese spins differently.
That’s because while one mobile electron prefers to stay in the
middle of the quantum dot, the other prefers to locate further toward the
edges. As a result, manganese atoms in different parts of the quantum dot
receive different messages about which way to align their spins.
In the “tug-of-war” that ensues, the mobile electron
that interacts more intensely with the manganese atoms “wins,”
aligning more spins and causing the quantum dot, as a whole, to be magnetic.
This prediction, if proven, could “completely alter the
basic notions that we have about magnetic interactions,” Zutic says.
“When you have two mobile electrons with opposite spins,
the assumption is that there is a nice balance to up and down spins, and
therefore, there is no magnetic message, or nothing that could be sent to align
nearby manganese spins,” he says. “But what we are saying is that it
is actually a tug of war. The building blocks of magnetism are still mysterious
and hold many surprises.”
Scientists including UB Professor Athos Petrou, UB College of
Arts and Sciences Dean Bruce McCombe, and UB Vice President for Research
Alexander Cartwright have demonstrated experimentally that in a quantum dot
with just one mobile electron, the mobile electron will act as a magnetic
messenger, robustly aligning the spins of adjacent manganese atoms.
Now, Petrou and his collaborators are interested in taking their
research a step further and testing the tug-of-war prediction for two-electron
quantum dots, Zutic says.
Zutic adds that learning more about magnetism is important as
society continues to find novel uses for magnets, which could advance
technologies including lasers, medical imaging devices and, importantly,
He explains the promise of magnet- or spin-based computing technology—spintronics—by
contrasting it with conventional electronics. Modern, electronic gadgets record
and read data as a blueprint of ones and zeros that are represented, in
circuits, by the presence or absence of electrons. Processing information
requires moving electrons, which consumes energy and produces heat.
Spintronic gadgets, in contrast, store, and process data by
exploiting electrons’ “up” and “down” spins, which can
stand for the ones and zeros devices read. Future energy-saving improvements in
data processing could include devices that process information by
“flipping” spin instead of shuttling electrons around.
Studying how magnetism works on a small scale is particularly
important, Zutic says, because “we would like to pack more information
into less space.”
And, of course, unraveling the mysteries of magnetism is
satisfying for other, simpler reasons.
“Magnets have been fascinating people for thousands of
years,” Zutic says. “Some of this fascination was not always related
to how you can make a better compass or a better computer hard drive. It was
just peculiar that you have materials that attract one another, and you wanted
to know why.”