Researchers have identified a new form of magnetism in so-called magnetic graphene, which may point the way to the understanding of 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 (FePS).3), a two-dimensional material that undergoes a transition from an insulator to a metal when compressed. This class of magnetic materials offers new routes 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 the unconventional metal state, which is only realized under ultra-high pressure conditions. When the material becomes metal, it remains magnetic, which is inconsistent with the previous results and gives clues as to how the electrical conductivity works in the metal phase. The newly discovered magnetic phase under high pressure probably forms a precursor to superconductivity, so it is important to understand its mechanisms.
Their results, published in the journal Physical overview X, also suggests a way in which new materials can be designed to have conductive and magnetic properties, which can be useful for the development of new technologies such as spintronics, which can transform the way in which computers process information.
Illustration of the magnetic structure of iron thiophosphate (FePS3), a two-dimensional material that is transferred from an insulator to a metal when compressed. Credit: University of Cambridge
Properties of matter can change dramatically with changing dimensionality. For example, graphene, carbon nanotubes, graphite and diamond are all made of carbon atoms but have very different properties due to their different structures and dimensions.
“But imagine that you could change all of these properties by adding magnetism,” said the first author, Dr Matthew Coak, who worked together in Cambridge’s Cavendish Laboratory and the University of Warwick. A material that can be mechanically flexible and form a new circuit to store information and perform 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. ”
In a previous study by Sebastian Haines of the Cavendish Laboratory and the Department of Earth Sciences, researchers determined that the material becomes a metal under high pressure, and explained how the crystal structure and arrangement of atoms in the layers of this 2D material through the transition.
“The missing piece, however, remained, the magnetism,” Coak said. “With no experimental techniques capable of examining the signature of magnetism in this material at such a high pressure, our international team had to develop and test our own new techniques to make it possible.”
The researchers used new techniques to measure the magnetic structure to record-breaking high pressure, using specially designed diamond amps and neutrons to serve as the probe of magnetism. They could then follow the evolution of magnetism in the metal state.
“To our surprise, we found that the magnetism survives and is amplified in some ways,” said co-author, dr. Siddharth Saxena, group leader at the Cavendish Laboratory. “This is unexpected, since the newly-free-floating electrons in a newly conductive material can no longer attach to their older iron atoms, and can generate magnetic moments there, unless the conduction comes from an unexpected source.”
In their previous paper, the researchers showed that these electrons were ‘frozen’ in a sense. But as they let them flow or move, they began to interact more and more. The magnetism survives, but is changed into new shapes, giving rise to new quantum properties in a new kind of magnetic metal.
How a material behaves, whether conductor or insulator, is mostly based on how the electrons, or charge, move. However, the ‘spin’ of the electrons is the source of magnetism. Rotation allows electrons to behave like small bar magnets and point in a certain way. Magnetism through the arrangement of electron pins is used in most memory devices: it is important to develop new technologies such as spin electronics, and it can change the way computers process information.
“The combination of the two, the charge and the twist, is the key to the behavior of this material,” said co-author, Dr. David Jarvis, of the Institut Laue-Langevin, France. at the Cavendish Laboratory. “It’s another leap forward in the study of this material to find this kind of quantum multifunctionality.”
“We do not know exactly what is happening at the quantum level, but at the same time we can manipulate it,” Saxena said. “It’s like the famous ‘unknown unknowns’: we’ve opened a new door for quantum information properties, but we do not yet know what the properties might be.”
There are more possible chemical compounds to synthesize than could ever be fully investigated and characterized. But by carefully selecting and adjusting materials with special properties, it is possible to indicate the way to the creation of connections and systems, but without having to exert great pressure.
Gaining fundamental understanding of phenomena such as low-dimensional magnetism and superconductivity, also enables researchers to make the next leaps in materials science and engineering, with particular potential in energy efficiency, generation and storage.
As for the case of magnetic graphene, the researchers plan to continue the search for superconductivity within this unique material. “Now that we have an idea of what happens to high pressure with this material, we can predict what might happen if we try to set its properties by adding free electrons by further compressing them,” Coak said.
“The thing we are chasing is superconductivity,” Saxena said. “If we find a type of superconductivity related to magnetism in a two-dimensional material, it can give us a chance to solve a problem that dates back decades.”
Reference: “Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3”By 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, 5 February 2021, Physical overview X.
DOI: 10.1103 / PhysRevX.11.011024