A new type of magnetism formed by “magnetic graphene” – could reveal the secrets of superconductivity

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Researchers have identified a new form of magnetism in what is called graphenewhich could open the way to understanding superconductivity in this unusual type of material.

The researchers, led by the University of Cambridge, were able to control the conductivity and magnetism of iron thiophosphate (FePS3), a two-dimensional material that undergoes a transition from an insulator to a metal when compressed. This class of magnetic materials offers new ways to understand the physics of new magnetic states and superconductivity.

Using new high-pressure techniques, the researchers showed what happens to magnetic graphene during the transition from insulator to conductor and into its unconventional metallic state, achieved only under ultra-high pressure conditions. When the material becomes metallic, it remains magnetic, which is contrary to previous results and provides clues to how electrical conduction works in the metallic phase. The recently discovered high-pressure magnetic phase likely forms a precursor to superconductivity, so understanding its mechanisms is vital.

Their results, published in the journal Physical examination Xalso suggest a way in which new materials could be engineered to have combined conductive and magnetic properties, which could be useful in the development of new technologies such as spintronics, which could transform the way computers process information.

Magnetic graphene structure

Illustration of the magnetic structure of iron thiophosphate (FePS3), a two-dimensional material that undergoes a transition from an insulator to a metal when compressed. Credit: University of Cambridge

The properties of matter can change drastically with the change in dimensionality. For example, graphene, carbon nanotubes, graphite, and diamond all consist of carbon atoms, but have very different properties due to their different structure and dimensionality.

“But imagine if you could also change all of these properties by adding magnetism,” said first author Dr. Matthew Coak, who is jointly based at Cambridge’s Cavendish Laboratory and the University of Warwick. “A material that could be mechanically flexible and form a new type of circuit to store information and perform calculations. That’s why these materials are so interesting, and because they drastically change their properties when put under pressure so that we can control their behavior.

In a previous study by Sebastian Haines of the Cavendish Laboratory and the Department of Earth Sciences, researchers established that the material becomes a metal at high pressure and described how the crystal structure and arrangement of atoms in the layers of this material 2D change through the transition.

“The missing piece remained though, the magnetism,” Coak said. “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.”

The researchers used new techniques to measure the magnetic structure up to record high pressures, using diamond anvils and specially designed neutrons to act as probes for magnetism. They were then able to follow the evolution of magnetism towards the metallic state.

“To our surprise, we found that the magnetism survives and is in some ways reinforced,” co-author Dr. Siddharth Saxena, group leader at the Cavendish Laboratory. “This is unexpected, because newly free electrons in a newly conducting material can no longer be locked onto their parent iron atoms, generating magnetic moments there – unless the conduction comes from an unexpected source.”

In their previous paper, the researchers showed that these electrons were “frozen” in one direction. But when they made them sink or move, they started to interact more and more. The magnetism survives, but changes into new forms, giving rise to new quantum properties in a new type of magnetic metal.

The behavior of a material, whether conductive or insulative, is primarily based on how electrons, or charge, move around. However, the “spin” of electrons has been shown to be the source of magnetism. Spin causes electrons to behave a bit like tiny magnetic rods and point in a certain direction. Magnetism resulting from the arrangement of electronic spins is used in most memories: its mastery and control are important for developing new technologies such as spintronics, which could transform the way computers process information.

“The combination of the two, charge and spin, is key to the behavior of this material,” said co-author Dr. David Jarvis of Institut Laue-Langevin, France, who carried out this work as a basis. of his doctoral studies. at the Cavendish Laboratory. “Finding this kind of quantum multifunctionality is another step forward in the study of these materials.”

“We don’t know exactly what’s going on at the quantum level, but at the same time we can manipulate it,” Saxena said. “It’s like those 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.”

There are more potential chemical compounds to synthesize than are ever possible to fully explore and characterize. But by carefully selecting and adjusting materials with special properties, it is possible to lead the way in creating compounds and systems, but without having to apply huge amounts of pressure.

Additionally, gaining a fundamental understanding of phenomena such as low-dimensional magnetism and superconductivity enables researchers to take a new step forward in materials science and engineering, with particular potential for efficiency. energy, generation and storage.

As for the case of magnetic graphene, the researchers then plan to continue the search for superconductivity within this unique material. “Now that we have an idea of ​​what happens to this material at high pressure, we can make predictions about what might happen if we try to tweak its properties by adding free electrons by compressing it further,” Coak said. .

“What we’re looking for is superconductivity,” Saxena said. “If we can find a type of magnetism-related superconductivity in a two-dimensional material, it could give us a chance to solve a problem that dates back decades.”

Reference: “Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3by Matthew J. Coak, David M. Jarvis, Hayrullo Hamidov, Andrew R. Wildes, Joseph AM Paddison, Cheng Liu, Charles RS Haines, Ngoc T. Dang, Sergey E. Kichanov, Boris N. Savenko, Sungmin Lee, Marie Kratochvílová, Stefan Klotz, Thomas C. Hansen, Denis P. Kozlenko, Je-Geun Park and Siddharth S. Saxena, February 5, 2021, Physical examination X.
DOI: 10.1103/PhysRevX.11.011024

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