Scientists identify a long-sought magnetic state for nearly 60 years.
Scientists at the US Department of Energy’s Brookhaven National Laboratory have discovered a long-anticipated magnetic state of the material called an “antimagnetic excitonic insulator.”
Mark Dean, a physicist at Brookhaven Lab and senior author of a paper describing the research just published in Nature Communications. “With magnetic materials at the core of so many technologies around us, new types of magnets are fundamentally fascinating and promising for future applications.”
The new magnetic state involves a strong magnetic attraction between electrons in a layered material that causes the electrons to want to arrange their magnetic moments, or “spin,” in a regular “antimagnetic” pattern from top to bottom. The idea of such antiferromagnetism was first predicted by twisted electron coupling in an insulator in the 1960s when physicists discovered the different properties of metals, semiconductors, and insulators.
An artist’s impression of how the team identified this historical phase of the material. The researchers used X-rays to measure how the spinules (blue arrows) move when they are turbulent and were able to show that they oscillate in length in the pattern shown above. This particular behavior occurs because the amount of electric charge at each location (shown as yellow discs) can also vary and is the fingerprint used to determine the new behavior. Credit: Brookhaven National Laboratory
“Sixty years ago, physicists were just beginning to look at how to apply the rules of quantum mechanics to the electronic properties of materials,” said Daniel Mazon, the former Brookhaven Laboratory physicist who led the study and is now at the Paul Scherer Institute in Switzerland. “They were trying to figure out what happens when you make the electronic ‘energy gap’ between an insulator and conductor smaller and smaller. Do you just change a simple insulator to a simple metal where the electrons can move freely, or something more interesting happens?”
The expectation was that, under certain conditions, you could get something more interesting: the “antiferromagnetic excitation” just discovered by Brookhaven’s team.
Why is this article so weird and interesting? To understand it, let’s dive into these terms and explore how this new state of matter is formed.
In an antiferromagnet, the electrons on neighboring atoms have axes of magnetic polarization (spin) in alternating directions: up, down, up, down, and so on. On the scale of the whole material, these alternating internal magnetic directions cancel each other out, resulting in no net magnetism of the aggregate material. These materials can be quickly swapped between different states. It also resists information loss due to interference from external magnetic fields. These properties make antimagnetic materials attractive to modern communication technologies.
Research team members include: Daniel Mazon (formerly of Brookhaven Lab, now at the Paul Scherrer Institute in Switzerland), Yao Shen (Brookhaven Lab), Gilberto Fabbris (Argonne National Laboratory), Hidemaro Suwa (University of Tokyo and University of Tennessee), Ho Miu (Oak Ridge National Laboratory-ORNL), Jennifer Sears* (Brookhaven Lab), Jian Liu (U TN), Christian Batista (U TN and ORNL), and Mark Dean (Brookhaven Lab). Credit: Various sources including *DESY, Marta Meyer
Next we have the excitonic. Excitons arise when certain conditions allow electrons to move around and interact strongly with each other to form bound states. Electrons can also form states associated with “holes,” which are vacancies left when electrons jump to a different position or energy level in a material. In the case of electron-electron interactions, the bonding is driven by magnetic attractions strong enough to overcome the repulsive force between the two like particles. In the case of electron-hole interactions, the attraction must be strong enough to overcome the “energy gap” in the material, a characteristic of an insulator.
“An insulator is the opposite of a metal; it is a material that does not conduct electricity,” Dean said. “The electrons in the material generally remain in a low-energy, or “ground” state. “All the electrons are crammed into place, like people in a full amphitheater; He said. To make the electrons move, you have to give them an energy boost large enough to overcome the characteristic gap between the ground state and a higher energy level.
In very special circumstances, the energy gain from magnetic electron-hole interactions can outweigh the energy cost of electrons jumping through the energy hole.
Now, thanks to advanced technologies, physicists can explore those special conditions to see how the state of the antiferromagnetic axitonic insulator might appear.
A collaborative team worked using a material called strontium iridium oxide (Sr .).3Infrared2a7), which is hardly an insulator at high temperature. Daniel Mazzoni, Yao Shen (Brookhaven Laboratory), Gilberto Fabrice (Argonne National Laboratory), and Jennifer Sears (Brookhaven Laboratory) used X-rays at the Advanced Photon Source—a user facility of the Department of Energy’s Office of Science at Argonne National Laboratory—to measure magnetic interactions and energy costs. associated with moving electrons. Jian Liu and Johnny Yang of the University of Tennessee and Argonne scholars Mary Upton and Diego Casa also made important contributions.
The team started their investigations at a high temperature and gradually cooled the material. With cooling, the power gap gradually narrowed. at 285 K (about 53 degrees[{” attribute=””>Fahrenheit), electrons started jumping between the magnetic layers of the material but immediately formed bound pairs with the holes they’d left behind, simultaneously triggering the antiferromagnetic alignment of adjacent electron spins. Hidemaro Suwa and Christian Batista of the University of Tennessee performed calculations to develop a model using the concept of the predicted antiferromagnetic excitonic insulator, and showed that this model comprehensively explains the experimental results.
“Using x-rays we observed that the binding triggered by the attraction between electrons and holes actually gives back more energy than when the electron jumped over the band gap,” explained Yao Shen. “Because energy is saved by this process, all the electrons want to do this. Then, after all electrons have accomplished the transition, the material looks different from the high-temperature state in terms of the overall arrangement of electrons and spins. The new configuration involves the electron spins being ordered in an antiferromagnetic pattern while the bound pairs create a ‘locked-in’ insulating state.”
The identification of the antiferromagnetic excitonic insulator completes a long journey exploring the fascinating ways electrons choose to arrange themselves in materials. In the future, understanding the connections between spin and charge in such materials could have potential for realizing new technologies.
Brookhaven Lab’s role in this research was funded by the DOE Office of Science, with collaborators receiving funding from a range of additional sources noted in the paper. The scientists also used computational resources of the Oak Ridge Leadership Computing Facility, a DOE Office of Science user facility at Oak Ridge National Laboratory.
Reference: “Antiferromagnetic Excitonic Insulator State in Sr3Ir2O7” by D. G. Mazzone, Y. Shen, H. Suwa, G. Fabbris, J. Yang, S.-S. Zhang, H. Miao, J. Sears, Ke Jia, Y. G. Shi, M. H. Upton, D. M. Casa, X. Liu, Jian Liu, C. D. Batista and M. P. M. Dean, 17 February 2022, Nature Communications.
DOI: 10.1038/s41467-022-28207-w
