Friday, May 18, 2007

Magnetic graphite made using high energy proton beam?

A "Stanford Report" titled Carbon joins magnetic club: Element holds promise for information technology discusses (ferro)magnetic carbon. In context, the material is graphite which has been bombarded by a high energy proton beam.

The band structure of graphite is complicated, and the electronic conductivity of graphite is that of a semimetal.

The Stanford Report included text:

Carbon's possible magnetic identity first emerged when meteorites were found containing bits of the magnetized element, but those flecks of carbon were packed in close proximity to nickel, leading to the suspicion that the observed magnetism came from the nickel. But until now, attempts to prove that pure carbon can be magnetized have remained unconvincing.

"With carbon, we know how to make things very small," said Ohldag. "On the other hand, we know a lot about how to process and store information using magnetism. This opens up the door for future studies that will lead to improved magnetism in carbon, which could one day lead to combining the 'magnetic' and the 'carbon' world."

Harnessing the magnetic properties of carbon could one day revolutionize a range of fields from nanotechnology to electronics. Carbon nanodevices could be built one atom at a time, leading to miniaturized machines and lightweight electronics. Magnetism, which forms the basis of information storage and processing in computer hard drives, could be employed in novel ways in tomorrow's electronic devices.

According to Ohldag, magnetism is an "ordering phenomenon." All atoms behave like tiny magnets because of the spin orientation of electrons, he says. When enough of those tiny magnetic spins, or "moments," align, the material emanates a measurable magnetic field. The electron spins of iron align readily, even at high temperatures, making it an ideal magnetic material.

One notes that it is possible to have graphite intercalation compounds with magnetic properties distinct from graphite. For example, "C14CrO3" is paragmagnetic. Also, one can make (poorly characterized) materials in the reductive destruction of things like graphite/FeCl3. Separately, it was once a big deal to have a paramagnetic superconductor, achieved with an intercalated disulfide; the trick being that the paramagnetic species is separate from the conduction electrons.

IPBiz hasn't looked at the May 4 PRL paper by Ohldag and co-workers yet.

See also:

**Separately, one notes that proton bombardment to produce magnetic graphite has been claimed previously (in fact in PRL):

Magnetic Graphite. Physicists at the University of Leipzig have irradiated graphite with protons to produce a lightweight, pure-carbon, metal-free, room temperature magnet. Pure carbon comes in several notable solid forms - graphite (powdery because with its two dimensional planes of atoms are so loosely bound--hence the use of graphite as a lubricant or pencil lead), diamond (hard because its constituents are well connected to atoms in all 3 dimensions), buckyballs (60-atom soccerballs), and nanotubes. All have important electrical properties, but in general they are not magnetic. Until now no pure-carbon sample was known to be magnetic, except when doped and held at temperatures close to absolute zero. In the Leipzig experiment, the protons were supplied by a nearby accelerator, and their presence in the sample in small amounts was just enough to inspire a small magnetic ordering among the carbon atoms. The magnetism was then measured by sensitive SQUID detectors and magnetic force microscopy at the surface. According to one of the researchers, Pablo Esquinazi< (+49-341-9732751), room-temperature magnetic graphite might have interesting applications in spintronics (some theoretical work suggests that atoms in a 2-dimensional graphite layer sprinkled with protons might be 100% spin polarizable) or as a data storage medium in which magnetic bits could be inscribed in a pure carbon film rather than in metal or metal-semiconductor films. Weak magnetism in graphite might also have implications for the study of biomolecules, which are rich in carbon-hydrogen bonds, or for astronomy since space is rich in carbon-filled gas clouds undergoing irradiation. (Esquinazi /et al./ //<, /Physical Review Letters/, 28 November 2003)

**One IPBiz reader wrote-->

Protons hitting graphite would probably displace carbon atoms out of the sheet. This is a well known phenomena in graphite exposed to nuclear reactors! (where neutrons are the principle culprits). The graphite swells, with these interstitial carbons. I would expect these interstitial carbons to be paramagnetic. Could you get enough, close together for an "ordering"??? Boy, at that point, I would hesitate to call it "graphite". There is also a RKKY interaction, whereby conduction electrons are spin polarized near a spin....and thus the conduction electrons can mediate the "ordering" from one spin to another. (The distance between carbons, determines if the ordering is ferromag or antiferromag.)
I suspect what the Stanford folks have are very very localized region with these interstitials....and they can pick up any ordering with their SQUIDS. I don't think there will be any "utility" for these materials....and if you get too many, the swelled material will break, split, ... As I recall in the nuclear reactor game, one allows exposure for the graphite for only so long, then you remove them. There is a build up of strain energy with these interstitials....which can lead to either breakage or a large heat release if the interstitial goes back into place (upon heating, as I recall).


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