However, researchers in The Netherlands [Flipse et al., Nature Phys. (2009) DOI: 10.1038/NPHYS1399] have now obtained direct evidence for ferromagnetism in pyrolytic graphite. Their discovery could have applications in sensor technology and ultimately, spintronics.
The idea that graphite might display permanent magnetism, or ferromagnetism, has remained controversial for at least a decade. Although several independent teams have hinted at the possibility through their experiments, it is well known that graphite possesses only weakly magnetic sp electrons and its Curie point is above room temperature so ferromagnetism seemed to be precluded from its repertoire of properties. The earlier research hinted, however, that imperfections in the graphite crystal lattice might underpin any demonstration of ferromagnetism.
Researchers at Eindhoven University of Technology and Radboud University of Nijmegen point out that besides the experiments with graphite, some degree of ferromagnetism has also been demonstrated in polymerized fullerenes, carbon nano foams, and proton-irradiated thin carbon films as well as nanoscopic diamonds implanted with nitrogen and carbon ions. These experiments imply that carbon could display inherent ferromagnetic behaviour despite the controversy.
The team has studied a particular form of carbon, highly oriented pyrolytic graphite (HOPG), using bulk magnetization measurements at room temperature in conjunction with magnetic force microscopy (MFM). The MFM tip itself is magnetized in two opposite directions and so disturbances caused by it being in close proximity to another magnetic material can be detected.
The researchers have now uncovered ferromagnetic order at the points where defect structures are present in the HOPG. The ferromagnetism arises solely because of the way the electrons become arranged at the boundaries between crystal grains, they say. Two-dimensional arrays of point defects between individual carbon sheets become magnetically coupled giving rise to the permanent magnetic behaviour of the bulk graphite. They emphasise that they also have evidence to show that the ferromagnetism is not due to magnetic impurities in the HOPG.
The team's theoretical analysis of the system corroborates their experimental results, offering a value for the magnetic measurements close to that seen in the experiments. The results end a decade or more of debate about graphite's magnetic properties and could open up ferromagnetic graphite for use in biosensors and spintronics.