Twisted graphene exhibits previously-unseen form of magnetism
As a flat sheet of carbon atoms arranged in a lattice, graphene is pretty simple, and yet it keeps surprising scientists with new properties. For the latest in a long line of breakthroughs, a team from Stanford has shown that graphene arranged in a specific way can generate a magnetic field. That's surprising enough, but it turns out this particular form of magnetism has previously only been theorized.
Since it's only one atom thick, graphene is effectively two-dimensional. That forces electrons traveling through it to only move along two axes, which in turn creates a host of unusual properties that have given graphene the moniker of a "wonder material."
From that starting point, graphene sheets can be stacked and manipulated in other ways to give it different abilities. In a study last year, an MIT team found that graphene could become a superconductor, meaning electricity passes through it freely with zero resistance. This was done by stacking two sheets and twisting them so their patterns don't quite line up, forming what's called twisted bilayer graphene.
The Stanford team set out to reproduce these results and build on them. In doing so, they accidentally made the graphene exhibit magnetism. They discovered this while sending an electrical current into a graphene sample, when a large voltage was detected perpendicular to the flow of the current. This normally needs a magnetic field to happen, but strangely the voltage stuck around when the external magnetic field was turned off. That means the graphene itself was generating an internal magnetic field.
Weirder still, this wasn't your everyday magnetism. Ferromagnetism is the most common type found in materials, created when the spin states of electrons in the material all sync up. Instead, this appeared to be the result of the electrons' orbital motions lining up, a phenomenon known as orbital ferromagnetism.
"To our knowledge, this is the first known example of orbital ferromagnetism in a material," says David Goldhaber-Gordon, lead researcher on the study. "If the magnetism were due to spin polarization, you wouldn't expect to see a Hall effect. We not only see a Hall effect, but a huge Hall effect."
The strange magnetism came about as the result of two seemingly-minor changes the team made in the manufacturing process. The two graphene layers were sandwiched between thin layers of hexagonal boron nitride, and the team made the choice to rotate one of those layers so it aligned with the twisted bilayer graphene.
The second change was deliberate, as the team rotated the graphene sheets slightly further out of line than the previous study. Rather than being 1.1 degrees off kilter, the Stanford researchers got it to 1.2 degrees. Both of these minor tweaks seemed to contribute to the material's odd magnetism.
The team says the magnetic field of the twisted bilayer graphene is very faint – about a million times weaker than a plain old fridge magnet – but this could be useful for some applications.
"Our magnetic bilayer graphene can be switched on with very low power and can be read electronically very easily," says Goldhaber-Gordon. "The fact that there's not a large magnetic field extending outward from the material means you can pack magnetic bits very close together without worrying about interference."
Graphene has been made magnetic in the past, but it usually requires doping with impurities or combining it with other magnetic materials.
The research was published in the journal Science.
Source: Stanford University