Graphite Proves Ferromagnetic

In what could be promising results for new applications in nanotechnology, such as biosensors and detectors, researchers at Eindhoven University of Technology show for the first time why ordinary graphite is ferromagnetic, meaning it is a permanent magnet at room temperature.

According to researchers Jiri Cervenka and Kees Flipse (Eindhoven University of Technology) and Mikhail Katsnelson (Radboud University Nijmegen), finding graphite to be ferromagnetic was quite unexpected.

Shown here is the electron density of states on a grain boundary of defects. The arrows indicate the direction of the magnetic moments. (Image: Eindhoven University of Technology)

Graphite is layered compound with a weak interlayer interaction between the individual carbon (graphene) sheets that are separated by 2 nanometer wide boundaries of defects. The electrons in the defect regions (seen in the red/yellow) behave differently compared to the ordered areas (seen in blue), showing similarities with the electron behavior of ferromagnetic materials like iron and cobalt.

The researchers found that the grain boundary regions in the individual carbon sheets are magnetically coupled, forming two-dimensional networks. This interlayer coupling was found to explain the permanent magnetic behavior of graphite. The researchers also show experimental evidence for excluding magnetic impurities to be the origin of ferromagnetism, ending ten years of debate.

Because carbon is biocompatible, the explored magnetic behavior is therefore particularly promising for the development of biosensors.

To the surprise of the researchers, a material containing only carbon atoms can be a weak ferro magnet. This opens new routes for spintronics (spin transport electronics) 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 important for applications in spintronics.

The research was funded by Nanoned and FOM, and was published online in Nature Physics.

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