Neutrons unite a 40-year enigma behind the mysterious magnetism of iron iodide

Researcher Xiaojian Bai and his colleagues used neutrons at ORNL’s Spallation Neutron Source to discover hidden quantum fluctuations in a fairly simple iron-iodide material discovered in 1929. The research suggests that many similar magnetic materials could have quantum properties waiting to be discovered. Credit: ORNL / Genevieve Martin

Advanced materials with newer properties are almost always developed by adding more elements to the list of ingredients. But quantum research suggests that some simpler materials may have already advanced features that scientists simply have not been able to see so far.

Researchers at Georgia Tech and the University of Tennessee – Knoxville have discovered hidden and unexpected quantum behavior in a fairly simple iron-iodide material (FeI2) which was discovered almost a century ago. The new research knowledge on the behavior of the material was made possible by a combination of neutron scattering experiments and theoretical physical calculations at the Department of Energy’s (DOE) Oak Ridge National Laboratory (ORNL).

The results of the team – published in the newspaper Natural Physics—Solves a 40-year riddle about the mysterious behavior of the material and could be used as a map to unlock a treasure trove of quantum phenomena in other materials.

“Our discovery was driven largely by curiosity,” said Xiaojian Bai, the newspaper’s first author. Bai earned his doctorate. at Georgia Tech and works as a postdoctoral researcher at ORNL, where he uses neutrons to study magnetic materials. “I came across this iron-iodine material in 2019 as part of my doctoral thesis project. I was trying to find compounds with a magnetic triangular lattice arrangement that shows what is called ‘frustrated magnetism.’

In common magnets, such as fridge magnets, the electrons of the material are arranged in a line like arrows, which either all point in the same direction – up or down – or they alternate between up and down. The directions shown by the electrons are called “spins”. But in more complex materials such as iron iodide, the electrons are arranged in a triangular grid in which the magnetic forces between the three magnetic moments are conflicting and not sure of which direction to point – hence “frustrated magnetism”.

“As I read through all the literature, I noticed this compound, iron iodide, which was discovered in 1929 and somewhat intensely studied in the 1970s and’ 80s, ”Bai said. “At the time they saw some peculiarity, or unconventional ways of behaving, but they didn’t really have the means to fully understand why they see it. So, we knew there was something unresolved that was strange and interesting, and compared to forty years ago, we have much more powerful experimental tools available, so we decided to revisit this problem and hoped to give new insights. ”

Quantitative materials are often described as systems that exhibit exotic behavior and violate classical laws of physics – as a solid material that behaves like a liquid, with particles that move like water and refuse to freeze or stop their movement even at freezing temperatures. Understanding how these exotic phenomena work, or their underlying mechanisms, is the key to advancing electronics and developing other next-generation technologies.

“In quantum materials, two things are very interesting: phases of matter such as liquids, solids and gases, and excitations of those phases, such as sound waves. Similarly, spin waves are excitations of magnetic solid material,” said Martin Mourigal, a professor of physics at Georgia. Tech. “For a long time our search for quantum materials has been to find exotic phases, but the question we asked ourselves in this research is ‘Maybe the phase itself is not seemingly exotic, but if its excitements are?’ And that’s exactly what we found. ”

Neutrons are ideal probes for studying magnetism because they themselves act as microscopic magnets and can be used to interact and excite other magnetic particles without compromising the atomic structure of a material.

Bai was introduced to neutrons when he was a graduate student of Mourigal at Georgia Tech. Mourigal has been a frequent user of neutron scattering at ORNL’s High Flow Isotope Reactor (HFIR) and Spallation Neutron Source (SNS) for several years, using the DOE Office’s user facilities to study a wide range of quantum materials and their various and bizarre behaviors.

When Bai and Mourigal exposed the iron-iodine material to a beam of neutrons, they expected to see one particular excitation or band of energy associated with a magnetic moment of one electron; but instead they saw not one, but two different quantum fluctuations coming out at once.

“Neutrons allowed us to see this hidden fluctuation very clearly and we were able to measure its entire exciting spectrum, but we still didn’t understand why we saw such abnormal behavior in a seemingly classical phase,” Bai said.

For answers, they turned to theoretical physicist Cristian Batista, a Lincoln professor at the University of Tennessee – Knoxville, and deputy director of ORNL’s Shull Wollan Center – a joint institute for neutron sciences that provides visiting researchers with additional resources and expertise in dissemination. of neutrons.

Neutrons put together a 40-year-old puzzle behind the mysterious magnetism of iron iodide

A small sample of iron iodide held by Bai (above) is mounted and prepared for neutron scattering experiments, which were used to measure the fundamental magnetic excitations of the material. Credit: ORNL / Genevieve Martin

With the help of Batista and his group, the team was able to mathematically model the behavior of the mysterious quantum fluctuation and, after performing additional neutron experiments with the CORELLI and SEQUOIA instruments at SNS, were able to identify the mechanism that caused it to appear.

“What theory has predicted and what we have been able to confirm with neutrons is that this exotic fluctuation occurs when the direction of rotation between two electrons is reversed and their magnetic moments tilt in opposite directions,” Batista said. “When neutrons interact with the spins of the electrons, the spins rotate synchronously in a certain direction in space. This choreography triggered by neutron scattering creates a spin wave.”

He explained that in a variety of materials, electronic spins can take on many different orientations and spin choreographies that create different types of spins. In quantum mechanics, this concept is known as “wave-particle duality”, in which the new waves are viewed as new particles and are usually hidden to neutron scattering under normal conditions.

“In a way we’re looking for dark items,” Batista added. “We can’t see them, but we know they’re there because we can see their effects or the interactions they have with the items we can see.”

“In quantum mechanics, there is no distinction between waves and particles. We understand the behavior of the particle based on the wavelength, and that allows neutrons to measure us,” Bai said.

Mourigal compared the way neutrons detect particles with waves breaking around rocks on the surface of the ocean.

“In still water we can’t see the rocks at the bottom of the ocean until a wave moves over it,” Mourigal said. “Only by creating as many waves with neutrons as possible, with Cristian’s theory, could Xiaojian identify the rocks, or in this case, the interactions that make the hidden fluctuation visible.

Utilizing quantum magnetic behavior has already led to technological advances such as the MRI machine and magnetic hard disk storage, which has catalyzed personal computing. More exotic quantum materials can accelerate the next technological wave.

In addition to Bai, Mourigal and Batista, the authors of the paper include Shang-Shun Zhang, Zhiling Dun, Hao Zhang, Qing Huang, Haidong Zhou, Matthew Stone, Alexander Kolesnikov and Feng Ye.

Since their discovery, the team has used that information to develop and test predictions into a broader material they hope will yield more promising results.

“As we introduce more ingredients into a material, we also increase potential problems such as clutter and heterogeneity. If we really want to understand and create clean quantum mechanical systems based on materials, going back to these simple systems could be more important than we thought., said Mourigal.

“So that solves the 40-year-old puzzle of the mysterious excitement in iron iodide,” Bai said. “We have the advantage today in the advances of large-scale neutron installations like SNS, which allow us to basically probe all the energy and impetuous space of material to see what happens with these exotic excitations.

“Now that we understand how this exotic behavior works in relatively simple material, we can imagine what we might find in more complicated ones. This new understanding has prompted us and hopefully it will encourage the scientific community to explore more such materials. What will certainly lead to more interesting physics. ”

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Additional information:
Xiaojian Bai et al, Hybridized quadrupole excitations in the spin-anisotropic frustrated magnet FeI2, Natural Physics (2021). DOI: 10.1038 / s41567-020-01110-1

Provided by Oak Ridge National Laboratory

Quote: Neutrons unite a 40-year puzzle behind the mysterious magnetism of iron iodide (2021, May 20) taken on May 20, 2021 from iron-iodide .html

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