Researchers present evidence for exotic magnetic phase of matter

Scientists at the U.S. Department of Energy have discovered a long-predicted magnetic state of matter.

Mark Dean, senior author on a paper describing the research just published in Nature Communications, said that this is a novel type of magnet.

The new magnetic state involves strong magnetic attraction between electrons in a material that makes them want to arrange their magnetic moments into a pattern. The idea that antiferromagnetism could be driven by quirks in electrons in a material was first predicted in the 1960s.

Sixty years ago, physicists were just starting to consider how the rules of quantum mechanics apply to the electronic properties of materials. Do you change a simple insulator into a simple metal where the electrons can move freely, or do something more interesting happen?

The prediction was that, under certain conditions, you could get something more interesting.

Why is this material so interesting? Let's look at how this new state of matter forms.

In an antiferromagnet, the electrons on adjacent atoms have their axes of magnetic polarization aligned in alternating directions. On the scale of the entire material, the alternating internal magnetic orientations cancel each other out. The materials can be changed quickly. They are resistant to information being lost due to interference from magnetic fields. Antiferromagnetic materials are attractive for modern communication technologies.

The excitonic is next. Some conditions allow electrons to move around and interact with one another to form bound states. When electrons jump to a different position or energy level in a material, the vacancies left behind can form bound states. The binding is driven by magnetic attractions that are strong enough to overcome the repulsive force between the two particles. In the case of electron-hole interactions, the attraction must be strong enough to overcome the energy gap of the material.

Dean said that an insulator is a material that doesn't conduct electricity. The electrons are stuck in the ground, like people in an amphitheater. To get the electrons to move, you have to give them a boost in energy that is big enough to overcome a characteristic gap between the ground state and a higher energy level.

The energy gain from magnetic electron-hole interactions can be more than the energy cost of electrons jumping across the energy gap.

Physicists can learn how the antiferromagnetic excitonic state emerges thanks to advanced techniques.

A team worked with a material called strontium iridium oxide, which is barely insulated at high temperature. The Advanced Photon Source is a DOE Office of Science user facility. The scientists from the University of Tennessee and the Argonne National Laboratory made important contributions.

The team cooled the material after they started their investigation. The energy gap narrowed with cooling. The antiferromagnetic alignment of adjacent electron spins was triggered by electrons jumping between the magnetic layers of the material. A model was developed using the concept of the predicted antiferromagnetic excitonic insulator, and it explains the results.

The X-rays we used showed that the binding between electrons and holes gives back more energy than when the electron jumps over the band gap. The material looks different from the high-temperature state in terms of the arrangement of electrons and spins after all electrons have accomplished the transition. The new configuration involves the electron spins being ordered in an antiferromagnetic pattern.

The identification of the antiferromagnetic excitonic insulator completes a long journey exploring the fascinating ways electrons choose to arrange themselves in materials. Understanding the connections between spin and charge in materials could lead to new technologies.

More information: D. G. Mazzone et al, Antiferromagnetic excitonic insulator state in Sr3Ir2O7, Nature Communications (2022). DOI: 10.1038/s41467-022-28207-w Journal information: Nature Communications Citation: Researchers present evidence for exotic magnetic phase of matter (2022, February 22) retrieved 22 February 2022 from https://phys.org/news/2022-02-evidence-exotic-magnetic-phase.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.