[meteorite-list] Carbon Nanotubes Go Magnetic
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:47 2004
Carbon nanotubes go magnetic
8 March 2004
Physicists have shown that carbon nanotubes can become magnetized
when they are placed in contact with a magnetic material. Michael
Coey of Trinity College in Dublin and colleagues believe the
mechanism relies on the transfer of spin - carried by electrons -
from the magnetic substrate to the nanotube (O CÚspedes et al.
2004 J. Phys.: CM 16 L155).
It is widely believed that graphite and other forms of carbon can
have ferromagnetic properties, but the effects are so weak that
physicists are not sure if the magnetism is due to tiny amounts of
iron-rich impurities, or if it is an intrinsic property of the
carbon. In 2002 Coey's group measured the magnetic properties of a
meteorite sample and found that only two-thirds of the magnetization
could be accounted for by magnetic minerals present in the sample.
The rest, they argued, must come from the carbon. In particular,
they proposed that ferromagnetic nanocrystals in the sample
induced a magnetic moment in the carbon via proximity effects.
Subsequent theoretical work by Mauro Ferreira and Stefano Sanvito
showed that a measurable magnetic moment could be produced in carbon
nanostructures if they were placed close to a ferromagnetic surface.
Now, the team has confirmed these predictions in experiments with
multi-walled carbon nanotubes that have been shown to be free from
Coey says that the main challenge in his experiment was to measure
the tiny magnetic moment of the nanotubes over the large background
magnetic moment coming from the magnetic material. To overcome this,
the team placed the nanotubes onto ferromagnetic substrates that had
been uniformly magnetized in one direction. This ensured that no
stray fields were produced by the substrate. However, the nanotubes
did produce a sizeable stray field when placed on the surface, and
the Dublin team was able to measure this with a magnetic force
microscope. Moreover, nanotubes placed on non-magnetic substrates,
such as silicon or gold, showed no magnetization.
The group calculated the average room temperature magnetization in
the nanotubes to be 0.1 Bohr magnetons per carbon atom. By
comparison, the figure for iron is 2.2. "This work opens new
avenues for magneto-electronics," Coey told PhysicsWeb.
"For instance, one can foresee devices where the magnetic and
electrical contacts are separated. The magnetic contact could be
used to magnetically polarize the nanotubes - and to manipulate the
spins - while the non-magnetic contacts are used as current/voltage
Received on Mon 08 Mar 2004 11:35:59 AM PST