Discovering a new form of magnetism in “magnetic graphene”


AZoNano talks to Dr Matthew Coak from the University of Warwick and the University of Cambridge, leader of an international team that has identified a new form of magnetism in so-called magnetic graphene – it could lead to new ways to understand superconductivity in this unusual type of material.

Can you give our readers a summary of your recent research?

We have measured the magnetic properties of a laminated material under extreme pressure conditions. We performed the first high-pressure neutron diffraction study on the TMPS3 family of van-der-Waals materials (TM = a transition metal), compounds currently under intense research worldwide.

They are layered honeycomb antiferromagnetic insulators, long studied as near-ideal 2D magnetic systems with a rich landscape of competing magnetic interactions and a variety of magnetism across the family. We use pressure, generated by compressing liquids between opposing anvils which can then be cooled to low temperatures, to adjust these 2D materials to become normal 3D systems by squeezing their crystal planes together – like turning graphene into graphite. … or finally in diamonds?

Our previous work has identified structural changes and a shift from insulator to metallic conduction in FePS3 as pressure is applied – this new breakthrough should provide an understanding of what happens to magnetism as the crystal structure evolves and becomes more 3D, and in particular as the material becomes metallic. The magnetism in an insulator comes from localized electrons attached to individual atoms in the crystal and how these interact – when such a material becomes a metal and the electrons are suddenly free to move around, you would expect certainly to great changes in magnetism.

We wanted to understand exactly what is going on in FePS3. Unfortunately, when we looked at this problem and how to answer this question, there was simply no way to perform this measurement with existing experimental technology – we had to assemble a team of experts from around the world (UK United, Korea, France, Uzbekistan, Russia, USA, Czech Republic, Vietnam) to push the boundaries of what was feasible and make measurements possible so that we could answer the physics questions we had.

Image credit: University of Cambridge

What were the experimental techniques involved in the research?

We previously explored the crystal structure, interaction with light, and electrical conduction properties of FePS3 – the remaining elephant in the room was magnetism. We hadn’t measured this because it just wasn’t possible! You can measure basic magnetic properties in the lab using a magnetometer, but for antiferromagnets with small magnetic moments like FePS3 and therefore the detection of tiny signals is very difficult.

Combine that with the body signal from a pressure cell, and these measurements become insurmountable. Even then, the maximum pressure achievable by existing laboratory methods is only a fraction of what is needed to observe evolution over a broad pressure regime.

So we turned to a non-lab based option and the most powerful tool for looking at magnetism, which is neutron scattering – neutrons bouncing from a nuclear reactor onto your material, their angle of deflection being controlled by the arrangement of the magnetic moments within it. Since neutrons only interact weakly with matter, a lot of them are needed – and a lot of samples. This again makes it difficult to generate high pressure, because pressure = force/area and we have to enlarge the area – so we have to use a 200 ton press (which terrifies me for one!) to generate the necessary loads.

We used a new pressure cell design with sintered diamond anvils machined into a very precise double toroidal shape to generate pressures of 20 GPa – twice what was previously achievable, in neutron source experiments of the Institut Laue Langevin (ILL) in Grenoble, France.

What are the main differences between magnetic graphene and traditional graphene?

What we nostalgically call “magnetic graphene” is definitely not graphene! Graphene is a sheet of carbon atoms purely in honeycomb layers, FePS3 and the related compounds are honeycombs of metal atoms with clusters of phosphorus and sulfur between them which space the layers.

Magnetism comes from metal sites – and changing the metal to another, like nickel or manganese, radically alters the magnetic behavior, giving us a nice playground for the fundamental physics of magnetism and for finding applications in science. technology. What they have in common, however, is that in both cases the atoms are organized in a honeycomb and can be peeled off into a single layer of atoms.

Graphene is a very exciting material and is rightly getting a lot of attention right now. What it and other highly researched 2D materials lack is magnetism. How does the addition of magnetism, a very complex and rich phenomenon, in these low-dimensional systems modify their behavior? What does magnetism really look like in a 2D material? And what kind of exciting new technology can we build with these “magnetic graphenes”?

They can be separated into single atomic layers with a simple tape like graphene, then stacked and combined together to form exotic composite materials and exciting new alternatives to silicon-chip electronics.

What benefits are the results likely to have for the IT field?

For a long time, advances in computing hardware have boiled down to making the functionality of silicon chips smaller and smaller – but we’re getting closer to the hard limits set by the sheer size of the silicon atoms themselves, the quantum limit.

New quantum technologies and devices are needed to address the emerging challenges of conventional electronics reaching its limits and the search for more energy-efficient alternatives. 2D materials offer a promising candidate, thanks to new transistor designs and spintronics, which uses magnetic currents instead of electricity to transfer information. To design and use these systems at the application level, it is essential that the underlying physics, including the inherent limitations and possibilities of the materials, are fundamentally understood and tested, and that the synthesis routes for new materials of design are discovered.

At what stage did the team use neutrons in this research?

Neutrons were the tool we used to “image” magnetism everywhere. This work is almost entirely a report of a collection of neutron scattering experiments.

Could you tell our readers more about the previous discovery than FePS3 can become a metal under high pressure?

It was a fascinating discovery – we showed that the transition from matter to metal was simultaneous with the transition to 3D – that is, there were no longer large empty spaces between its layers of atoms. This is precisely the type of dimensionality control and adjustment that we seek to achieve with our high pressure techniques.

Who is the team behind the research and what is the next step for you?

I am most proud of this work because it represents the assembling of a global team of experts in their technique or field, to tackle a seemingly insurmountable problem. The team goes far beyond Cambridge (for my part, I am based in Warwick), to France, Russia, the United States, Uzbekistan, South Korea and beyond.

For our plans, now that we have unlocked and activated these measurement techniques, they can become routine and accessible to all ILL users. That’s what’s really exciting to me – there will be so much science that will now be achievable with these achievable pressures. I have great respect and gratitude for the Sample Environment team at ILL, and for Thomas Hansen and Stefan Klotz, the team’s high-pressure neutron experts. They did the real drought work to make this happen – I just got the team together and led the science and analysis. In the immediate future, for us, there remain many other compounds of the TMPS3 family which we wish to study, and we want to be interested in the question of superconductivity.

Where can readers find more information?

Here are some links where readers can find more information:

About Dr. Matthew Coak

I am a postdoctoral researcher at the University of Warwick and a visiting researcher at the University of Cambridge, doing basic research in condensed matter physics. I use extreme pressures, magnetic fields and low temperatures to tune and study low-dimensional magnetic materials and superconductors. I got my undergraduate degree in physics from Oxford University and my PhD. at the University of Cambridge in 2017.

My specialization was in the use of high pressure to tune quantum criticality in ferroelectric systems and Mott transitions in 2D insulating antferromagnetics. My first postdoctoral position was in the Emerging Phenomena Group of the IBS Center for Correlated Electronic Systems at Seoul National University, under the supervision of Prof. Je-Geun Park. Here I led the van-der-Waals materials team and continued my research on Mott transitions and the tuning of low-dimensional magnetism with pressure, including synchrotron and neutron scattering studies .

I then joined the Superconductivity and Magnetism group at Warwick in March 2019. I was very grateful to be named one of the 2019 Institute of Physics Emerging Leaders – “the most exciting researchers of their generation, with the potential to revolutionize their fields”.

My current work focuses on tuning low-dimensional quantum magnet systems and materials that exhibit unconventional superconductivity through hydrostatic pressure, low temperatures, and ultra-high magnetic fields. I am motivated by the search for new multifunctional materials and combinations of materials for energy saving and computational breakthroughs through a fundamental understanding of the underlying physics.

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