Physicists Discover a Strange New Form of Magnetism Within ‘Magnetic Graphene’

From childhood, we are taught that the world exists in three physical dimensions. That’s true, for the most part, but it skips over something quite fascinating: the strange two-dimensional world of nanoscale materials, like the ‘wonder material’ graphene.

Graphene and its engineered, single-layer counterparts do in fact exist in three dimensions, albeit just barely – sitting right on the fringe, atomically speaking. That’s because these so-called 2D materials are only one atom thick, embodying an incredible structural thinness that lends them all sorts of weird powers.

We see this in graphene’s formidable strength, and in the way it approaches superconductivity.

Things get even stranger when graphene makes friends: stack sheets of this two-dimensional material into a three-layer, three-atom-high sandwich, and a rare form of magnetism stands revealed.

Now, in a new study led by physicists from the University of Cambridge, scientists have pulled off the same kind of magnetic feat with a different two-dimensional material called iron phosphorus trisulfide (FePS₃).

(University of Cambridge)

Above: Illustration of the magnetic structure of iron phosphorus trisulfide (FePS₃), a two-dimensional material which undergoes a transition from an insulator to a metal when compressed.

FePS₃ isn’t the same thing as graphene – which is composed of a single layer of carbon atoms – but it’s often dubbed ‘magnetic graphene’, due to its mysterious capabilities at ultra-thin, layered dimensions.

In a previous study by some of the same researchers, the team found that when squashed layers of FePS₃ were subjected to high levels of pressure, the material transitioned from being an insulator, impeding the flow of electrons, to a metallic state where it became a conductor.

But researchers still didn’t fully understand what underlies the magnetic behavior of this ‘magnetic graphene’ under pressure, as it was expected that FePS₃ would cease to be magnetic when it enters the metallic state.

“The missing piece has remained however, the magnetism,” says quantum physicist Matthew Coak.

“With no experimental techniques able to probe the signatures of magnetism in this material at pressures this high, our international team had to develop and test our own new techniques to make it possible.”

According to the new research, FePS₃ retains its magnetism under extremely high pressure due to a newly discovered kind of magnetism that still exists during the metallic phase.

“To our surprise, we found that the magnetism survives and is in some ways strengthened,” explains senior researcher and physicist Siddharth Saxena, group leader at Cambridge’s Cavendish Laboratory.

“This is unexpected, as the newly-freely-roaming electrons in a newly conducting material can no longer be locked to their parent iron atoms, generating magnetic moments there – unless the conduction is coming from an unexpected source.”

While we don’t yet have all the answers as to what’s happening here, during compression the ‘spin’ of the electrons in the material seems to be a source of magnetism – and the phenomenon can be tuned depending on how much pressure FePS₃ is subjected to.

While the results contradict previous observations of how this material should behave, the surprises found here suggest we might be able to tweak magnetic graphene and its ilk even further – potentially finding materials that support superconductivity due to these exotic forms of magnetism we don’t yet fully comprehend.

“We don’t know exactly what’s happening at the quantum level, but at the same time, we can manipulate it,” Saxena says.

“It’s like those famous ‘unknown unknowns’: we’ve opened up a new door to properties of quantum information, but we don’t yet know what those properties might be.”

The findings are reported in Physical Review X.