A form of magnetism emerges from “magnetic graphene”


Image Credit: University of Cambridge

New research shows that under extreme pressure, a 2D graphene-like material can change from an insulator to a conductor.

A deeper understanding of superconductivity may be on the horizon, thanks to the discovery of a new form of magnetism. A team of researchers led by scientists from Cambridge University discovered the still unknown magnetic phenomena in a material called “magnetic graphene” – first synthesized in the 1960s.

Magnetic graphene – formerly known as FePS3 – is similar to traditional graphene in that it is a thin atomic sheet of an allotrope of carbon which is conductive, strong and flexible. The main difference between the two 2D materials is that FePS3 is magnetic, while “traditional” graphene is not.

The Cambridge team discovered that when FePS3 is compressed, it adopts a metallic state and thus, passes from an insulator to a conductor. The team believe this high-pressure magnetic phase is likely to be a precursor to superconductivity, but currently cannot apply enough pressure to induce this new phase shift.

The team’s discoveries, reported in the newspaper Physical examination X¹, could have significant advantages for the IT field. Scientists have long researched a 2D magnetic material that can be integrated with graphene for use in magnetic data storage and spin electronics in semiconductor 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 magnetism-related type of superconductivity in a two-dimensional material, it could give us a chance to solve a problem that dates back decades.

Dr Siddharth Saxena, co-author of the article and group leader of the Cavendish Laboratory

Metal under pressure

Scientists have been aware for some time that matter can change its properties, often quite dramatically when changed dimensionally. An example of this also involves the application of tremendous pressure; Coal and diamond are both made up of carbon atoms, but the different structure and dimensionality between the two results in very different properties.

What the team wanted to know was if magnetism could be added to this list of adjustable properties.

Imagine if you could also change all of these properties by adding magnetism. Mechanically flexible material could form a new type of circuit for storing information and performing calculations. This is why these materials are so interesting, and because they drastically change their properties when put under pressure so that we can control their behavior.

Dr Matthew Coak, first author of the article, based at the University of Warwick and the Cavendish Laboratory, Cambridge

Researchers, including Coak and his Cavendish lab colleague Sebastian Haines, had previously discovered that FePS3 can become metal under high pressure². They had also succeeded in describing the changes that occur in the crystal structure and atomic arrangement of the 2D material during the process.

“The missing piece, however, remained magnetism”, adds Coak. “In the absence of experimental techniques capable of probing the signatures of magnetism in this material at such high pressures, our international team had to develop and test our own new techniques to make this possible.”

A new technique to remove magnetism from FePS3

Get FePS3to adopt magnetic properties, the team had to place the 2D material under artificially induced high pressures that break records using a specially designed “diamond anvil”. The team also used neutrons – neutral particles that lie next to protons in the atomic nucleus – to probe the evolution of material from a metal to a magnetic material.

To the team’s surprise, they discovered that not only did the magnetism survive in the material, it actually appeared to be reinforced in some ways. In the previous study, published in Physical examination letters, the team had discovered that the electrons were indeed frozen in place until they were induced to flow. As it flowed, the interaction between them increased.

“It’s unexpected” Saxena said. “As the newly roving electrons in a newly conductive material can no longer be locked to their parent iron atoms, generating magnetic moments there, unless the conduction comes from an unexpected source.”

As magnetism survives, the team found that it began to take on new quantum properties, essentially becoming an entirely new magnetic material.

The quantum key to magnetic materials

Many properties of materials are determined by the way their electrons move. On the other hand, magnetic qualities are governed by “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, the spin and its manipulation are crucial in advanced magnetic data storage devices. Harnessing the spin and controlling it better is the key to future breakthroughs in computing, especially in information processing.

Better manipulation of 2D materials might depend on understanding both the movement of electrons and their spin and how these qualities can be manipulated – a kind of “quantum multifunctionality”.

The team will now begin to experiment with different chemical compositions, with the goal of reducing the amount of pressure needed to flip the magnetic switch. This could hopefully make the switch to superconductivity feasible.

“We don’t know exactly what’s going on 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 a new door to the properties of quantum information, but we don’t yet know what those properties might be.”

The references

  1. Hates. SRC, Coak. MJ, Wildes. AR, et al, [2019], ‘Evolution of the pressure-induced electronic and structural phase in the van der Waals compound FePS3’ Physical examination letters, [https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.266801]

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