Solve the 40-year riddle behind the mysterious magnetism of iron iodide

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Researcher Xiaojian Bai and his colleagues used neutrons at ORNL’s spallation neutron source to uncover hidden quantum fluctuations in a rather simple iron iodide material discovered in 1929. Research suggests that many magnetic materials similar could have quantum properties waiting to be discovered. Credit: ORNL / Geneviève Martin

Advanced materials with newer properties are almost always developed by adding more items to the ingredient list. But quantum research suggests that some simpler materials may already have advanced properties that scientists simply couldn’t see, until now.

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) discovered almost a century ago. The new research insight into the material’s behavior was made possible through a combination of neutron scattering experiments and theoretical physics calculations at the Oak Ridge National Laboratory (ORNL) of the Department of Energy (DOE) .

The team’s findings — published in the journal Physics of nature– solves a 40-year-old puzzle of the material’s mysterious behavior and could be used as a map to unlock a treasure trove of quantum phenomena in other materials.

“Our discovery was driven in large part by curiosity,” said Xiaojian Bai, the first author of the article. Bai received his doctorate from Georgia Tech and is working as a postdoctoral researcher at ORNL, where he uses neutrons to study magnetic materials. “I came across this iron iodide material in 2019 as part of my doctoral thesis project. I was trying to find compounds with a triangular magnetic lattice arrangement that exhibits what is called “frustrated magnetism”.

Iron-iodide sample

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

In common magnets, like refrigerator magnets, the material’s electrons are arranged in a line like arrows that all point in the same direction – up or down – or they alternate between up and down. The directions pointed by the electrons are called “spins”. But in more complex materials like iron iodide, the electrons are arranged in a triangular grid, in which the magnetic forces between the three magnetic moments are in conflict and do not know which direction to point, hence a “magnetism”. frustrated”.

“Reading all the literature, I noticed this compound, iron iodide, which was discovered in 1929 and was studied quite intensively in the 1970s and 80s,” Bai said. “Back then, they saw peculiarities or unconventional patterns of behavior, but they didn’t really have the resources to fully understand why they were seeing it. So, we knew that there was something strange and interesting unresolved, and compared to forty years ago we have much more powerful experimental tools, so we decided to revisit this problem and hoped to provide new information.

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

“In quantum materials, two things are of great interest: the phases of matter such as liquids, solids and gases, and the excitations of these phases, such as sound waves. Likewise, spin waves are excitations of a magnetic solid material, ”said Martin Mourigal, professor of physics at Georgia Tech. “For a long time our quest in quantum materials has been to find exotic phases, but the question we asked ourselves in this research is, ‘Maybe the phase itself is not apparently exotic, but what if his excitations are? And indeed, this is what we found.

Neutrons are ideal probes for studying magnetism because they themselves act like microscopic magnets and can be used to interact with 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 high flux isotope reactor (HFIR) and spallation neutron source (SNS) neutron scattering at ORNL for several years, using user facilities at the DOE Office of Science to study a wide range of quantum materials and their diverse and bizarre behaviors. .

When Bai and Mourigal exposed the iron iodide material to a neutron beam, they expected to see a particular excitation or band of energy associated with a magnetic moment from a single electron; but instead, they saw not one, but two different quantum fluctuations emanating simultaneously.

“The neutrons allowed us to see this hidden fluctuation very clearly and we were able to measure its full spectrum of excitation, but we still didn’t understand why we were seeing such abnormal behavior in a seemingly classic phase,” Bai said.

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

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

“What the theory predicted and what we were 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.” , said Batista. “When neutrons interact with electron spins, the spins rotate in synchronicity along a certain direction in space. This choreography, triggered by the scattering of neutrons, creates a spin wave.

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

“In a sense, we’re looking for dark particles,” 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 particles that we can see.”

“In quantum mechanics, there is no distinction between waves and particles. We understand the behavior of the particle as a function of wavelength, and this is what neutrons allow us to measure, ”said Bai.

Mourigal compared the way neutrons detect particles to waves crashing around rocks on the ocean surface.

“In calm water, we can’t see the rocks on the ocean floor until a wave moves over them,” Mourigal said. “It was only by creating as many waves as possible with neutrons that, thanks to Cristian’s theory, Xiaojian was able to identify the rocks, or in this case, the interactions that make the hidden fluctuation visible.

The exploitation of quantum magnetic behavior has already led to technological advances such as the MRI machine and magnetic hard disk storage that have catalyzed personal computing. More exotic quantum materials could accelerate the next wave of technology.

In addition to Bai, Mourigal, and Batista, the article’s authors 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 this information to develop and test predictions in a wider set of materials that should yield more promising results.

“As we introduce more ingredients into a material, we also increase potential issues such as clutter and heterogeneities. If we are serious about understanding and creating clean material-based quantum mechanical systems, going back to these simple systems might be more important than we thought, ”Mourigal said.

“So that solves the 40-year-old conundrum of the mysterious excitement in iron iodide,” Bai said. “Today we have the advantage of advancements in large-scale neutron facilities like the SNS that allow us to essentially probe the entire energy and momentum space of a material to see what happening with those exotic excitement.

“Now that we understand how this exotic behavior works in a relatively simple material, we can imagine what we might find in more complicated materials. This new understanding has motivated us and hopefully will motivate the scientific community to further study this type of material, which will surely lead to more interesting physics. “

Reference: “Hybrid quadrupole excitations in the anisotropic spin frustrated magnet FeI2“By Xiaojian Bai, Shang-Shun Zhang, Zhiling Dun, Hao Zhang, Qing Huang, Haidong Zhou, Matthew B. Stone, Alexander I. Kolesnikov, Feng Ye, Cristian D. Batista and Martin Mourigal, January 4, 2021, Physics of nature.
DOI: 10.1038 / s41567-020-01110-1

The research was supported by the DOE Science Office. UT-Battelle LLC manages ORNL for the DOE Office of Science. The Office of Science is the largest proponent of basic research in the physical sciences in the United States and works to address some of the most pressing challenges of our time.


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