Nanotorus’ nets giant magnetic moment
Carbon nanotubes bent into rings are the latest
nanostructures to display surprising properties, according to new calculations.
Shi-Yu Wu of the
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
behavior 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 realized 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 behavior 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 behavior 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.
Author:
Katie Pennicott is Editor of PhysicsWeb
