Magnetic Graphene

The high-pressure magnetic phase observed in iron trithiohypophosphate, a 2D material that transitions from an insulator to a metal when compressed, likely forms a precursor to superconductivity.

The magnetic structure of FePS₃. Image credit: Cavendish Laboratory.

Iron trithiohypophosphate (FePS₃), or magnetic graphene, belongs to a family of materials known as van der Waals materials.

First synthesized in the 1960s, this 2D material is similar to graphene, and can be ‘exfoliated’ into ultra-thin layers. Unlike graphene however, FePS₃ is magnetic.

In 2018, Dr. Siddharth Saxena from the Cavendish Laboratory and colleagues established that FePS₃ becomes a metal at high pressure, and outlined how the crystal structure and arrangement of atoms in the layers of this material change through the transition.

In the new study, the researchers used new techniques to measure the magnetic structure up to record-breaking high pressures, using specially designed diamond anvils and neutrons to act as the probe of magnetism.

They were then able to follow the evolution of the magnetism into the metallic state.

“To our surprise, we found that the magnetism survives and is in some ways strengthened,” Dr. Saxena said.

“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.”

In their previous study, the scientists showed these electrons were ‘frozen’ in a sense. But when they made them flow or move, they started interacting more and more.

The magnetism survives, but gets modified into new forms, giving rise to new quantum properties in a new type of magnetic metal.

How a material behaves, whether conductor or insulator, is mostly based on how the electrons, or charge, move around. However, the ‘spin’ of the electrons has been shown to be the source of magnetism. Spin makes electrons behave a bit like tiny bar magnets and point a certain way.

“The combination of the two, the charge and the spin, is key to how this material behaves,” said Dr. David Jarvis, a researcher at the Institut Laue-Langevin.

“Finding this sort of quantum multi-functionality is another leap forward in the study of these materials.”

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

“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.”

“Now that we have some idea what happens to this material at high pressure, we can make some predictions about what might happen if we try to tune its properties through adding free electrons by compressing it further,” said Dr. Matthew Coak, a researcher at the Cavendish Laboratory and the University of Warwick.

“The thing we’re chasing is superconductivity,” Dr. Saxena added.

“If we can find a type of superconductivity that’s related to magnetism in a 2D material, it could give us a shot at solving a problem that’s gone back decades.”

The findings were published February 5, 2021 in the journal Physical Review X.

_____

Matthew J. Coak et al. 2021. Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS₃. Phys. Rev. X 11 (1): 011024; doi: 10.1103/PhysRevX.11.011024