A Trinity College research group shows that ordinary carbon can be magnetised, opening up the potential for an entirely new type of magnet. It all started with an extract from a meteorite, writes Dick Ahlstrom
Everyone knows you can't magnetise carbon. Not so, according to a research team from Trinity College Dublin, who measured relatively strong magnetisation in a graphite nodule and opened up the possibility of an entirely new family of magnets.
"This idea of ferromagnetics in graphite is highly controversial," says Prof Michael Coey, of Trinity's Department of Physics - and a Science Foundation Ireland principle investigator. "Magnetism is only found in transition elements like iron. Graphite is just about the other end of the scale."
And yet quite substantial magnetism was found in a graphite sample assessed by the research team, who describe their findings in Nature. Of course, the sample is literally from out of this world.
The graphite nodule studied was extracted from a piece of the meteorite that created Canyon Diablo, in Arizona, in the US. The estimated 50,000 tonne impactor, travelling at 20 kilometres a second, slammed into what is now the Arizona desert 50,000 years ago. It left a suitably large hole - 1.3 km across and half-a-kilometre deep - scattering meteorite debris for kilometres in all directions.
Coey is a leading world researcher in magnetic materials and knew of recent reports of weak ferromagnetism in graphite and buckminsterfullerene, the 60-carbon-atom molecules known affectionately as "buckyballs".
"There had been no idea until recently that there might be the possibility of magnetism in carbon," he says.
The reports prompted him to check out the claims and he knew where to look for the graphite, in a piece of the Canyon Diablo meteorite. "I had it sitting in my drawer for the last 15 years," says Coey. He decided that was as good a source of carbon as any, and began a detailed analysis of the nodule.
Being able to prove magnetism in otherwise non-magnetic carbon would be of considerable interest, he says. "From a chemical point of view it would be weird to find magnetism in carbon."
Much more importantly, if carbon could be magnetised "you would have all the options of organic synthesis of these easily formed carbon-based magnets", he says.
They would be cheap, light and potentially have uses in the latest form of miniature electronics, known as "spin electronics".
This type of electronics is grounded in knowing the spin orientation of electrons as they race around the atoms in a substance. Spin electronics is already found in advanced computer disc "read" heads, he says.
"The next stage is to make more sophisticated spin devices, for example, spin transistors." This would allow much greater miniaturisation of computer devices if it could be achieved.
He began the analysis of the meteorite nodule, which contained graphite, but also iron-rich impurities, magnetite and other minerals. Unlike terrestrial graphite, he knew it had strong, room temperature ferromagnetism, and the team decided to find out why.
They examined 10 Canyon Diablo samples, assessing the magnetic moment of the impurities found within the graphite. They found, however, that these could only account for about two thirds of the total magnetism. "The remainder is somehow associated with graphite," Coey and colleagues report in Nature.
The magnetisation of the graphite assessed by the Trinity team is not insignificant.
"There had been these claims of ferromagnetism in graphite. Mostly they are tiny, tiny effects, perhaps a hundred times smaller than we are seeing," says Coey. In technical terms, the average magnetisation was "0.05 Bohr magnetons per carbon atom", but put another way, the graphite had about a 20th of the magnetic pull of a similar iron magnet.
The big question now is how does this happen? One idea is that meteoritic graphite is somehow different from terrestrial graphite, but Coey favours another. It could be that the magnetic inclusions are able to induce a small but permanent magnetic moment by aligning the electron spin of the adjacent graphite atoms.
If true, this immediately opens up the potential for spin electronics. It might be possible to use magnetite to inject a controlled magnetic spin in the graphite, and this spin in turn could be detected and manipulated, says Coey.
