Researchers of Eindhoven University
of Technology and the Radboud University Nijmegen in The Netherlands show
for the first time why ordinary graphite is a permanent magnet at room temperature.
The results are promising for new applications in nanotechnology, such as sensors
and detectors. In particular graphite could be a promising candidate for a biosensor
material. The results will appear online on 4 October in Nature Physics.
 | | This is a two-dimensional plane of magnetically coupled grains of defects. Credit: Kees Flipse, Eindhoven University of Technology |
Graphite is a well-known lubricant and forms the basis for pencils. It is a
layered compound with a weak interlayer interaction between the individual carbon
(graphene) sheets. Hence, this makes graphite a good lubricant.
Unexpected
It is unexpected that graphite is ferromagnetic. The researchers Jiri Cervenka
and Kees Flipse (Eindhoven University of Technology) and Mikhail Katsnelson
(Radboud University Nijmegen) demonstrated direct evidence for ferromagnetic
order and explain the underlying mechanism. In graphite well ordered areas of
carbon atoms are separated by 2 nanometer wide boundaries of defects. The electrons
in the defect regions (the red/yellow area in picture 1) behave differently
compared to the ordered areas (blue in picture 1), showing similarities with
the electron behaviour of ferromagnetic materials like iron and cobalt.
Debate settled
The researchers found that the grain boundary regions in the individual carbon
sheets are magnetically coupled, forming 2-dimensional networks (picture 2).
This interlayer coupling was found to explain the permanent magnetic behaviour
of graphite. The researchers also show experimental evidence for excluding magnetic
impurities to be the origin of ferromagnetism, ending ten years of debate.
Carbon in spintronics
Surprisingly, a material containing only carbon atoms can be a weak ferro magnet.
This opens new routes for spintronics in carbon-based materials. Spins can travel
over relative long distances without spin-flip scattering and they can be flipped
by small magnetic fields. Both are important for applications in spintronics.
Carbon is biocompatible and the explored magnetic behaviour is therefore particularly
promising for the development of biosensors.
Publication Nature Physics
The paper in Nature Physics " Room-temperature ferromagnetism in graphite
driven by 2D networks of point defects" by Jiri Cervenka, Mikhail Katsnelson
and Kees Flipse will appear online Sunday 4 October, 7:00 pm CET. The paper
can be found under DOI 10.1038/NPHYS1399.
The research was funded by Nanoned and FOM.
Posted October 4th, 2009
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