MIT researchers show how topology can help materials produce magnetism at higher temperatures. For years, researchers have worked to understand the electron arrangement, or topology, and magnetism in certain semimetals, but frustratingly, these materials only display magnetism when cooled to a few degrees above absolute zero.
A new study led by Mingda Li, associate professor in the Department of Nuclear Science and Engineering at MIT, and co-authored by Nathan Drucker, a graduate assistant researcher in the MIT Quantum Measurement Group and a doctoral student in applied physics at Harvard University, and Thanh Nguyen and Phum Siriviboon, graduate students in the MIT Quantum Measurement Group, is challenging this traditional view.
The research, published in the journal Nature Communications, is the first to demonstrate that topology can stabilize magnetic ordering even well above the magnetic transition temperature - the point at which magnetism normally breaks down.
"An analogy I like to use to describe why this works is to imagine a river full of logs, and the logs represent the magnetic moments in the material," said Drucker, the paper's lead author. "For magnetism to work, you need all of those logs to be pointing in the same direction, or some pattern between them. But at high temperatures, where the magnetic moments are all pointing in different directions, like logs in a river, the magnetism breaks down."
He continued: "But what's important about this study is that it's actually the water that's changing. What we've shown is that if you change the properties of the water itself, rather than the properties of the logs, you can change the interaction between the logs to create magnetism."
The role of topology in enhancing magnetism
Essentially, Li said, the paper reveals how a topology known as a Weyl node found in CeAlGe, an exotic semimetal composed of cerium, aluminum and germanium, can significantly increase the operating temperature of magnetic devices, opening the door to a wide range of applications.
While topological materials have been used to make sensors, gyroscopes, and more, they are also widely used in areas such as microelectronics, thermoelectric, and catalytic devices. Nguyen said this study shows a way to maintain magnetism at higher temperatures, opening the door to more possibilities. Many opportunities have been demonstrated in this and other topological materials. This demonstrates a general approach that can significantly increase the operating temperature of these materials.
Linda Ye, assistant professor of physics in Caltech's Division of Physics, Mathematics and Astronomy, added that this "rather surprising and counterintuitive" result will have a significant impact on future work in topological materials.
The research work shows that electronic topological nodes not only play a role in stabilizing the static magnetic order, but more generally they can play a role in the generation of magnetic fluctuations. A natural conclusion to draw from this is that the impact of topological Well states on materials may be far greater than previously thought.
Princeton University physics professor Andre Bonnevig agreed, calling the discovery "puzzling and quite remarkable. It is known that Weyls nodes are topologically protected, but the impact of this protection on the thermodynamic properties of the phase is not entirely clear. The MIT team's paper shows that short-range ordering above the ordering temperature is governed by nested wave vectors between Weyl fermions that appear in the system... This may indicate that the protection of Weyls nodes affects magnetic fluctuations to some extent!"
Unraveling the mystery of magnetism
While these surprising results challenge long-held understandings of magnetism and topology, they are the result of careful experiments and the research team's willingness to explore areas that may have been overlooked.
"Our hypothesis is that there are no new discoveries above the magnetic transition temperature," Li explains. "We used five different experimental methods to create this comprehensive story and piece the puzzle together in a consistent way."
To demonstrate the existence of magnetism at higher temperatures, the researchers first combined cerium, aluminum and germanium in a furnace to form millimeter-sized crystals of the material. The samples were then put through a series of tests, including thermal and electrical conductivity tests, each of which revealed clues to the material's unusual magnetic behavior.
"However, we also used some more exotic methods to test the material," Drucker said. "We hit the material with a beam of X-rays at the same energy level as the cerium in the material and then measured how the beam was scattered. Those tests had to be done in a large facility at a Department of Energy national laboratory. Ultimately, we had to do similar experiments at three different national labs to prove that there was this hidden order there, and that's how we found the strongest evidence."
"Part of the challenge is that it's often very difficult to do these kinds of experiments on topological materials, and often you can only provide indirect evidence," Nguyen said. "What you do in this case is you do multiple experiments with different probes, and when you put them together, they give us a very comprehensive story. In this case, there were five or six different clues, and a whole bunch of instruments and measurements that all played a role in this study."
Impact and future directions
Going forward, the team plans to explore whether the relationship between topology and magnetism can be demonstrated in other materials. They believe this principle is universal. Therefore, this may exist in many other materials, which expands our understanding of the role of topology. We knew it could play a role in increasing electrical conductivity, and now we've shown it can also play a role in magnetism.
Other future work will also address possible applications of topological materials, including their use in thermoelectric devices, which can convert heat into electricity. While such devices are already used to power small devices like watches, they are not efficient enough to power cell phones or other larger devices.
"We have studied many excellent thermoelectric materials, and they are all topological materials," Li said. "If they could show this performance using magnetism... they would release very good thermoelectric properties. This would help them operate at higher temperatures, for example. Right now, many solar cells can only operate at very low temperatures to collect waste heat. A very natural consequence of this is that they will be able to operate at higher temperatures."
This study conclusively shows that while topological semimetallic materials have been studied for many years, relatively little is known about their properties.
"I think our work highlights the fact that when you look at these different scales and use different experiments to study some of these materials, in fact, some very important thermoelectric, electrical and magnetic properties start to show up," Drucker said. "So I think this provides a follow-up to not only how we can use these things for different applications, but also other fundamental research on how we can better understand the effects of these thermal fluctuations."