Graphite magnetism

[Dr Mordrid]

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Graphite mimics iron's magnetism

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.

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.

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.

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.

More information: 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. The paper can be found under DOI 10.1038/NPHYS1399

Source: Eindhoven University of Technology

[Brian Ellis]

Many materials display this kind of effect. What is apparently new here is that it is demonstrable at room temperature as most materials (other than the magnetic metals and ferrites, of course), have subzero Curie temperatures. My textbook in this subject, for those who may want to learn a bit about the theoretical background to understand spintronics is Smart Electronic Materials by Jasprit Singh, Cambridge University Press, ISBN 0-521-85027-4 (2005). Dr Singh is prof. in Applied Physics at the University of Michigan, Ann Arbor, so he's your neighbour! He does not specifically mention graphite as being para- or ferro- magnetic, but he does treat other semiconductors in Chapter 8 of the book, including other elements in Group IVB. However, I doubt whether we are likely to see any practical applications of the phenomenon this side of several decades, if at all. Spintronics has been largely treated in pure, rather than applied, sciences, the theory still being largely under study. I admit that it is a subject way above my head, although I have tried to understand the fundamentals.