Graphite magnetism

http://www.physorg.com/news173881546.html

Graphite mimics iron's magnetism

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

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.

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Graphite is a well-known lubricant and forms the basis for pencils. It is a layered compound with a weak interlayer interaction between the individualcarbon (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.

October 4th, 2009

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

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

http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1399.html

Article abstract

Nature Physics

Published online: 4 October 2009 | doi:10.1038/nphys1399

Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects

ervenka¹^,3, M. I. Katsnelson² & C. F. J. Flipse¹

Abstract

Understanding the mechanism of ferromagnetism in carbon-based materials, which contain only s and p electrons in contrast to traditional ferromagnets based on 3d or 4f electrons, is challenging. Here, we demonstrate direct evidence for ferromagnetic order locally at defect structures in highly oriented pyrolytic graphite (HOPG) with magnetic force microscopy and in bulk magnetization measurements at room temperature. Magnetic impurities have been excluded as the origin of the magnetic signal. The observed ferromagnetism has been attributed to originate from localized electron states at grain boundaries of HOPG, forming two-dimensional arrays of point defects. The theoretical value of the magnetic ordering temperature based on weak interlayer coupling and/or magnetic anisotropy is comparable to the experimental value. The unusual chemical environment of defects bonded in graphitic networks can reveal the role of the s and p electrons, creating new routes for spin transport in carbon-based materials.

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Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
Institute of Molecules and Materials, Radboud University of Nijmegen, NL-6525 ED Nijmegen, The Netherlands
Present address: Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-162 53 Prague 6, Czech Republic

Correspondence to: C. F. J. Flipse¹ e-mail: C.F.J.Flipse@tue.nl

http://arxiv.org/abs/0910.2130

Condensed Matter > Materials Science

Room-temperature ferromagnetism in graphite driven by 2D networks of point defects

J. Cervenka, M. I. Katsnelson, C. F. J. Flipse

(Submitted on 12 Oct 2009)

Ferromagnetism in carbon-based materials is appealing for both applications and fundamental science purposes because carbon is a light and bio-compatible material that contains only s and p electrons in contrast to traditional ferromagnets based on 3d or 4f electrons. Here we demonstrate direct evidence for ferromagnetic order locally at defect structures in highly oriented pyrolytic graphite (HOPG) with magnetic force microscopy and in bulk magnetization measurements at room temperature. Magnetic impurities have been excluded as the origin of the magnetic signal after careful analysis supporting an intrinsic magnetic behavior of carbon. The observed ferromagnetism has been attributed to originate from unpaired electron spins localized at grain boundaries of HOPG. Grain boundaries form two-dimensional arrays of point defects, where their spacing depends on the mutual orientation of two grains. Depending on the distance between these point defects, scanning tunneling spectroscopy of grain boundaries showed two intense split localized states for small distances between defects (< 4 nm) and one localized state at the Fermi level for large distances between defects (> 4 nm).

Submission history

From: Jiri Cervenka [view email]

[v1] Mon, 12 Oct 2009 10:23:42 GMT (1302kb)

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