Nanofoam Exhibits Surprising Magnetic Properties
By Ernie Tretkoff
A new form of carbon exhibits surprising magnetic properties that
could make it useful in future spintronics or biomedical
applications, researchers reported at the APS March Meeting. The
material, called carbon nanofoam for its low density and web-like
structure, is the only form of pure carbon known to be
ferromagnetic.
Carbon nanofoam is structurally distinct from the other four
known forms of carbon—graphite, diamond, fullerenes ( buckyballs),
and nanotubes. With a density of about 2 mg/cm3,
comparable to that of aerogel, carbon nanofoam is one of the
lightest known solid substances.
But what's most remarkable about the material, the researchers
said, is that unlike other forms of carbon, the nanofoam is
ferromagnetic, like a refrigerator magnet. However, at room
temperature, the nanofoam's magnetization disappears a few hours
after the material is produced.
A collaboration of researchers from Greece and Australia produced
the carbon nanofoam by shooting a high-powered, ultra-fast laser at
disordered solid carbon in an argon-filled chamber.
By imaging the material using a high-resolution electron
microscope, John Giapintzakis of the University of Crete and
colleagues found that the nanofoam has a sponge-like structure, made
up of carbon clusters a few nanometers in diameter randomly linked
together into a web-like foam.
Because pure carbon is not normally ferromagnetic, the group
tested their sample for impurities that might be causing the
magnetic behavior. Although they did find traces of iron and nickel,
the small amounts of these magnetic elements could not account for
all of the ferromagnetism in the nanofoam. The researchers concluded
that the magnetic properties come from the complex structure of the
nanofoam itself.
David Tománek of Michigan State University, who collaborated with
the group on theoretical interpretation, believes that the carbon
clusters in the foam are made up of nanotubes joined together into
tetrapods. In these four-legged structures, some carbon atoms have a
free electron, one that does not form a chemical bond. These
unpaired electrons carry a magnetic moment that may lead to the
magnetism.
Chemists have long known about such carbon radicals, said
Tománek, but until now they have only been found in carbon connected
to another element. In this case, the structure is entirely carbon.
The researchers have also done some preliminary studies that
suggest that the novel magnetic behavior found in carbon nanofoam
could be present in other nano-structured solids of elements that
are not normally magnetic, including a compound of boron and
nitrogen.
If this behavior turns out to be a general phenomenon,
researchers will have to think more about what makes a material
magnetic, said Tománek. "We need to revisit our magnetic prejudice."
Giapintzakis suggested that carbon nanofoam could be used in
spintronic devices, which are based on a material's magnetic
properties. The unique material may also find uses in biomedicine.
For instance, the tiny ferromagnetic clusters could be injected into
blood vessels to enhance magnetic resonance imaging. The nanofoam
could also be implanted in tumors, where it could turn radio waves
into a source of heat that would destroy the tumor but leave
surrounding tissue unharmed.
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