Graphite mimics the magnetism of iron | Eurek alert!

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image: This is a two-dimensional plane of magnetically coupled defect grains.
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Credit: Kees Flipse, Eindhoven University of Technology

Researchers from Eindhoven University of Technology and 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 be posted on October 4 in Natural Physics.

Graphite is a well-known lubricant and forms the basis of pencils. It is a layered compound with a weak interaction between the individual layers of carbon (graphene). Therefore, this makes graphite a good lubricant.

Unexpected

It is unexpected that graphite is ferromagnetic. Researchers Jiri Cervenka and Kees Flipse (Eindhoven University of Technology) and Mikhail Katsnelson (Radboud University Nijmegen) demonstrated direct evidence for ferromagnetic order and explained the underlying mechanism. In graphite, well-ordered zones of carbon atoms are separated by 2-nanometer wide defect boundaries. Electrons in defect regions (the red/yellow area in image 1) behave differently than in ordered areas (blue in image 1), showing similarities to the electronic behavior of ferromagnetic materials like iron and cobalt.

Debate settled

The researchers found that the grain boundary regions in individual carbon sheets are magnetically coupled, forming two-dimensional networks (image 2). This interlayer coupling was found to explain the permanent magnetic behavior of graphite. The researchers also show experimental evidence to rule out magnetic impurities as the source of ferromagnetism, ending a decade of debate.

Carbon in spintronics

Surprisingly, a material containing only carbon atoms can be a weak ferromagnet. This opens new avenues for spintronics in carbon-based materials. Spins can travel relatively long distances without spin-flip diffusion and they can be reversed by small magnetic fields. Both are important for applications in spintronics. Carbon is biocompatible and the explored magnetic behavior is therefore particularly promising for the development of biosensors.

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Publication Natural Physics

The paper in Natural Physics “Room Temperature Ferromagnetism in Graphite Driven by 2D Lattices of Point Defects” by Jiri Cervenka, Mikhail Katsnelson and Kees Flipse will go live on Sunday, October 4 at 7:00 p.m. CET. The document can be found under DOI 10.1038/NPHYS1399.

The research was funded by Nanoned and FOM.


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