This is a schematic of a graphene transistor showing graphene (red), gold electrodes (yellow), silicon dioxide (clear) and silicon substrate (black). Inset shows the graphene lattice with vacancy defects. Vacancies (missing atoms) are shown surrounded by blue carbon atoms. Credit: Graphic by Jianhao Chen and Michael S. Fuhrer, University of Maryland |
University
of Maryland researchers have discovered a way to control magnetic
properties of graphene that could lead to powerful new applications in
magnetic storage and magnetic random access memory.
The
finding by a team of Maryland researchers, led by Physics Professor
Michael S. Fuhrer of the UMD Center for Nanophysics and Advanced
Materials is the latest of many amazing properties discovered for
graphene.
A
honeycomb sheet of carbon atoms just one atom thick, graphene is the
basic constituent of graphite. Some 200 times stronger than steel, it
conducts electricity at room temperature better than any other known
material (a 2008 discovery by Fuhrer, et. al). Graphene is widely seen
as having great, perhaps even revolutionary, potential for
nanotechnology applications. The 2010 Nobel Prize in physics was awarded
to scientists Konstantin Novoselov and Andre Geim for their 2004
discovery of how to make graphene.
In
their new graphene discovery, Fuhrer and his University of Maryland
colleagues have found that missing atoms in graphene, called vacancies,
act as tiny magnets — they have a “magnetic moment.” Moreover, these
magnetic moments interact strongly with the electrons in graphene which
carry electrical currents, giving rise to a significant extra electrical
resistance at low temperature, known as the Kondo effect. The results
appear in the paper “Tunable Kondo effect in graphene with defects”
published this month in Nature Physics.
The
Kondo effect is typically associated with adding tiny amounts of
magnetic metal atoms, such as iron or nickel, to a non-magnetic metal,
such as gold or copper. Finding the Kondo effect in graphene with
vacancies was surprising for two reasons, according to Fuhrer.
“First,
we were studying a system of nothing but carbon, without adding any
traditionally magnetic impurities. Second, graphene has a very small
electron density, which would be expected to make the Kondo effect
appear only at extremely low temperatures,” he said.
The
team measured the characteristic temperature for the Kondo effect in
graphene with vacancies to be as high as 90 Kelvin, which is comparable
to that seen in metals with very high electron densities. Moreover the
Kondo temperature can be tuned by the voltage on an electrical gate, an
effect not seen in metals. They theorize that the same unusual
properties of that result in graphene’s electrons acting as if they have
no mass also make them interact very strongly with certain kinds of
impurities, such as vacancies, leading to a strong Kondo effect at a
relatively high temperature.
Fuhrer
thinks that if vacancies in graphene could be arranged in just the
right way, ferromagnetism could result. “Individual magnetic moments can
be coupled together through the Kondo effect, forcing them all to line
up in the same direction,” he said.
“The
result would be a ferromagnet, like iron, but instead made only of
carbon. Magnetism in graphene could lead to new types of nanoscale
sensors of magnetic fields. And, when coupled with graphene’s tremendous
electrical properties, magnetism in graphene could also have
interesting applications in the area of spintronics, which uses the
magnetic moment of the electron, instead of its electric charge, to
represent the information in a computer.
“This
opens the possibility of ‘defect engineering’ in graphene – plucking
out atoms in the right places to design the magnetic properties you
want,” said Fuhrer.
This research was supported by grants from the National Science Foundation and the Office of Naval Research.
