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.
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.
The electron density of states on a grain boundary of defects. The arrows (pointing in the reader’s direction) indicate the direction of the magnetic moments. Credit: Kees Flipse, Eindhoven University of Technology
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 2) behave differently compared to the ordered areas (blue in picture
2), 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 1). 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
|
This is a two-dimensional plane of magnetically coupled grains of defects. Credit: Kees Flipse, Eindhoven University of Technology
|
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.
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
Eindhoven University of
Technology