Carbon nanotubes are rolled-up sheets of graphite that can act as metals, semiconductors or insulators, depending on their radius and the angle at which the sheets are rolled. If a nanotube is bent so that its ends meet, a nanotorus is produced. The electronic properties of such structures are less well understood, and Liu and co-workers suspected that the magnetic behaviour of nanotori would provide some clues.

The team considered nanotori with various radii, made from different types of metallic nanotube, known as ‘armchair’ or ‘zigzag’. In their calculations, the researchers assume that the nanotori are in a magnetic field of 0.1 tesla, which causes the spins of the electrons to line up, producing a magnetic moment. They also imagined that the electrons were flowing around the rings as an electrical current.

When the researchers calculated the magnetic moments of the nanotori, they found that some of the nanotori made from either ‘armchair’ or ‘zigzag’ metallic nanotubes had magnetic moments thousands of times larger than other tori with similar structures. The team realised that the radius of the torus – and therefore the area it enclosed – affected its magnetic moment. This made the magnetic moment very large for certain ‘magic’ radii.

Liu and colleagues believe that the characteristics of the nanotori arise partly from the unusual behaviour of the electrons in the ring-shaped structures. In a torus made from ‘armchair’ nanotubes, the electrons flow along the circumference of the torus. In a torus made from ‘zigzag’ nanotubes, the electrons follow a spiral path around the circumference of the torus, leading to a different magnetic moment. But the calculations also show that the radius of a torus is an important factor.

Liu and co-workers say their discovery is "a unique example of the unusual magnetic behaviour of carbon-based nanostructures". They also speculate that polygon-shaped rings of nanotubes could have unusual magnetic properties.

Liu’s team now plans to make carbon nanotori in the lab and test their electrical and magnetic properties. “The colossal magnetic moment exhibited by metallic nanotori could be used to fabricate ultra-sensitive magnetic sensors,” Liu told PhysicsWeb.