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 Céspedes 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 magnetic impurities.
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 electrodes.”
