Image Credit: Cambridge University
New research shows that under extreme pressure, a graphene-like 2D material can switch from an insulator to a conductor.
A deeper understanding of superconductivity could be on the horizon, thanks to the discovery of a new form of magnetism. A team of researchers led by scientists from the University of Cambridge found the yet undiscovered magnetic phenomena in a material called ‘magnetic graphene’ — first synthesized in the 1960s.
Magnetic graphene — formally called FePS3 — is similar to traditional graphene in that it is an atom-thin sheet of a carbon allotrope that is conductive, strong, and flexible. The major difference between the two 2D materials is that FePS3 is magnetic, whilst ‘traditional’ graphene isn’t.
The Cambridge team found that when FePS3 is compressed it adopts a metallic state and thus, transitions from an insulator to a conductor. The team believes that this high-pressure magnetic phase is likely to be a precursor to superconductivity, but currently can’t apply enough pressure to induce that further phase shift.
The team’s findings, reported in the journal Physical Review X¹, could have significant benefits for the computing field. Scientists have long been searching for a 2D magnetic material that can be integrated with graphene for use in magnetic data storage and spin electronics in solid-state devices — spintronics. In turn, this could change the way information is processed by computers.
The thing we’re chasing is superconductivity. If we can find a type of superconductivity that’s related to magnetism in a two-dimensional material, it could give us a shot at solving a problem that’s gone back decades.
Dr. Siddharth Saxena, Co-author of the paper and a Cavendish Laboratory group leader
Metal Under Pressure
Scientists have been aware for some time that matter can change its properties, often quite dramatically when it is changed dimensionally. An example of this also involves applying tremendous pressure; both coal and diamond are made of carbon atoms, but the different structure and dimensionality between the two give rise to some very different properties.
What the team wanted to know was if magnetism could be added to this list of adjustable properties.
Imagine if you were also able to change all of these properties by adding magnetism. A material which is mechanically flexible could form a new kind of circuit to store information and perform computation. This is why these materials are so interesting, and because they drastically change their properties when put under pressure so we can control their behavior.
Dr. Matthew Coak, First author of the paper, based at the University of Warwick and the Cavendish lab, Cambridge
Researchers including Coak and his Cavendish Laboratory colleague Sebastian Haines had already found that FePS3 can become a metal under high pressure². They had also managed to describe the changes that occur in the 2D material’s crystal structure and atomic arrangement during the process.
“The missing piece has remained, however, the magnetism,” adds 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.”
A New Technique to Tease Magnetism Out of FePS3
Getting FePS3 to adopt magnetic properties required the team to place the 2D material under artificially induced high pressures that break records using a specially designed ‘diamond anvil.’ The team also employed neutrons — neutral particles that are found alongside protons in the atomic nucleus — to probe the material’s evolution from a metal to a magnetic material.
To the team’s surprise, they discovered that not only did magnetism survived in the material, it actually seemed to be strengthened in some ways. In the previous study, published in Physical Review Letters, the team had found that electrons were in effect, frozen in place until they were induced to flow. Whilst flowing the interaction between them increased.
“This is unexpected,” says Saxena. “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.”
Whilst the magnetism survives, the team found that it begins to adopt new quantum properties, becoming essentially an entirely new magnetic material.
The Quantum Key to Magnetic Materials
Many properties of materials are determined by how their electrons move around. On the other hand, magnetic qualities are governed by the ‘spin’ — a quantum property that has little to do with angular momentum like spin in the macroscopic world — which causes electrons to align in a common direction.
Thus, spin and its manipulation are crucial in advanced magnetic data storage devices. Harnessing spin and better controlling it is the key to future computing breakthroughs, especially when it comes to information processing.
Better manipulation of 2D materials could hinge on understanding both the motion of electrons and their spin and how these qualities can be manipulated — a kind of ‘quantum multi-functionality.’
The team will now begin to experiment with different chemical compositions, with the aim of reducing the amount of pressure needed to flip the magnetic switch. This could hopefully make the switch to superconductivity achievable.
“We don’t know exactly what’s happening at the quantum level, but at the same time, we can manipulate it,” explains Saxena. “It’s like other 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.”
References
- Coak. M. J., Jarvis. D. M., Hamidov. H., et al, [2021], ‘Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3,’ Physical Review X, [https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.011024]
- Haines. C. R. S., Coak. M. J., Wildes. A. R., et al, [2019], ‘Pressure-Induced Electronic and Structural Phase Evolution in the van der Waals Compound FePS3,’ Physical Review Letters, [https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.266801]
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