Thermal Dance of Magnetic Fields Revealed – ScienceDaily


Everyone knows that holding two magnets together will result in one of two outcomes: they stick together, or they push apart. From this perspective, magnetism seems simple, but scientists have struggled for decades to understand how magnetism behaves on the smallest of scales. At the near atomic level, magnetism is made up of many ever-changing realms — called magnetic domains — that create the magnetic properties of matter. While scientists know these domains exist, they are still researching the reasons behind this behavior.

Now, a collaboration led by scientists from the US Department of Energy’s Brookhaven National Laboratory, Helmholtz-Zentrum Berlin (HZB), Massachusetts Institute of Technology (MIT), and the Max Born Institute (MBI) has published a study in nature In it, they used a new analysis technique — called coherent correlation imaging (CCI) — to depict the evolution of magnetic fields in time and space without any prior knowledge. The scientists could not see the “dance of spheres” during the measurement but only afterwards, when they used the recorded data to “rewind the tape”.

The domains “film” shows how the boundaries of these domains change back and forth in some regions but remain constant in others. Researchers attribute this behavior to a property of matter called “fixation.” While pinning is a known property of magnetic materials, the team can directly depict for the first time how a network of pinning sites affects the motion of interconnected domain walls.

“Many details about changes in magnetic materials can only be accessed through direct imaging, which we haven’t been able to do so far,” said Wen Hu, a scientist at National Synchrotron Light Source. “It’s basically a dream come true for studying magnetic motion in magnetic field.” Materials”. II (NSLS-II) and corresponding co-author of the study.

The researchers expect that CCI will help unlock other properties of the small world of magnetism — such as degrees of freedom or hidden symmetries — that were previously inaccessible through other techniques. The benefit of CCI also represents a breakthrough beyond magnetic materials as the technology can be transferred to different measurement techniques and research areas. One area that would benefit most from understanding the motion of magnetic fields on the nanoscale (one nanometer equals 0.000000038 inch!) is new computing. The new memory technology can take advantage of special magnetic fields called “Skyrmions”.

“Skyrmions are interesting for computing AI because they have a similar property to our short-term memory,” said Felix Buettner, group leader at Helmholtz-Zentrum Berlin, a professor at the University of Augsburg who was involved in the study. “In current computing architectures, everything is linear, which means that the memory is separated from the processor. This is not a problem for most applications but it does, for example, make speech recognition difficult. In speech recognition, only the computing part processes incoming words, but “He doesn’t remember what was said earlier. In addition, it takes a lot of energy to send that information back from memory. By using the heavens, we might be able to somehow harness their short-term memory and avoid these issues.”

However, before engineers and scientists can develop technology that uses this feature, they first need to understand how to manipulate the sky and other magnetic fields. This was the intention when the collaboration between NSLS-II and the Geoffrey Beach Group at MIT and MBI took shape. They wanted to investigate how the heavens in their magnetic devices react to external stimuli, specifically to an external magnetic field. HZB joined the collaboration when Büttner moved from MIT to Berlin.

“In 2018, we had a measurement time at the Coherent Soft X-ray Scattering (CSX) beamline at NSLS-II; however, the experimental room we wanted to use wasn’t ready. That means we didn’t have the external magnetic field, but we did have a plan. back up to study thermal motion,” said Hu, who is part of the CSX beamline team.

Buettner added: “I was expecting this experiment to be another demonstration experiment but nothing more. And, frankly, I was surprised to see thermal motion at all. We studied the same device at room temperature and hardly saw any thermal movement. This time we studied it was at 310 K It’s about 98 Fahrenheit, and we’ve seen so much more than that. It was surprising! And it was only the beginning.”

How a backup plan leads to hidden insights

In their experiment, the team used coherent X-rays from the CSX beamline to take a series of snapshots of the magnetic domains. CSX is part of an advanced suite of research tools available in the NSLS-II Materials Study. The research team used a beamline in a 3D setting to take the images. In most holographic experiments, the scientists take one image every three to four seconds, however, the fast detector at the CSX beamline allowed the team to take up to 100 images per second.

“After the measurement, we started a normal analysis of the data by adding 200 images. Once we did that, we realized that the system changed much faster than we expected. The temperature really affected the physics in the sample,” said Christopher Klose, PhD. Student at MBI and first author of the study. “That was a real surprise and the beginning of our development of a post-processing technology – coherent imaging (CCI) – to be able to resolve this fast motion.”

After this initial realization, the team decided to dig deeper into the data. They knew that the details of the domain’s movements were encrypted in their data. Although no data analysis technology existed to solve their problem, they were able to find algorithms that could be adapted. Over the course of three years, the team developed a new algorithm that underpins the new CCI technology.

“There were a lot of challenges. To develop CCI, we combined correlation function analysis known from X-ray photon correlation spectroscopy (XPCS) with holography, which is an imaging technique. One problem was that the hologram data was not suitable for XPCS analysis,” Klose said.

When X-rays hit the samples in these experiments, they scatter into magnetic fields and the holographic mask that defines the field of view. The detector records all scattered X-rays, regardless of their source. But the team is only interested in magnetic scattering. Therefore, they needed to clean the data before they could compute the correlation functions.

“Once we have the correlation function, we can compare all these frames with each other to find similar ones. This also requires a new algorithm because we have nearly 30,000 frames to sort through,” Klose continued.

This challenge required an algorithm that could index domain states for each frame. This algorithm will be a real game-changer for this task because it will be able to sort out these cases in ways that no human can.

How does installation shape the magnetic landscape?

After the team sorted their data with CCI, they went to work on the interpretation. The reconstructed images showed black and white bands scattered across their devices. But some of these boundaries, or domain walls, went back and forth between frames, while others mostly stayed. Question: “What were the researchers seeing and what does this mean for the sky and magnetic fields?”

“Skyrmions are small spherical objects, comparable to balls on a pool table. In our case, thermal energy makes them travel around the table. Now, if the pool table is anchored, the surface is not smooth, it is a landscape of hills. We have two types of anchor locations: Attractive locations and disgusting locations. The first is valleys, and the second is hills. In this case, the sky will rest in the “attractive” valleys. If they want to move, they’ll need to conquer the “nasty” hillsides, Buettner said.

The researchers found that the domain walls behave like rubber bands. It can be held down and then rocked back and forth like a guitar string. While attractive sites can accommodate domain walls, repulsive sites inhibit movement of domain walls. The domain wall must be raised above the hateful site. Can’t walk through it. This explains why scientists see some domain walls constantly changing, while others barely move. The latter were surrounded by disgusting sites.

“CCI gave us the tool to see this movement over time. Basically, we can make a small movie about how these domains transform. This experiment allowed us to see this kind of volatile behavior and its cause for the first time,” Hu said. “We did not expect that this collaboration would lead to the invention of a new technology that would benefit users and other researchers who study dynamics on a large scale.”

Buettner added, “We needed about a year to fully understand the physics we found and develop an explanation for the dynamics we saw. In hindsight, the experiment itself was the easiest part of it all. The real work was developing the technique and then explaining the physics.”

The researchers agreed that a key component of this breakthrough was the diverse team of experts they had assembled for the task. They hope that many other research groups will benefit from CCI. As they prepare to apply CCI on a broader range of previously inaccessible dynamics as well as extend the technique to other X-ray sources, they are also working on implementing machine learning to make CCI analysis less manual and more accessible to a wider range. social communication.

The team for this work consisted of Christopher Klose, Michael Schneider, Stefan Esbet and Bastien Pfau from the Max Born Institute, Felix Buttner and Riccardo Battistelli from Helmholtz Zentrum Berlin, Wayne Ho, Claudio Mazzoli, Andy Barbour and Stuart P. National Light Source II Synchrotron at Brookhaven National Laboratory, Kay Letzius, Evan Lemich, Jason M. Bartell, Manta Huang and Jeffrey S.D. Piech of MIT, Christian M. Günther of Technische University Berlin.

NSLS-II is a US Department of Energy (DOE) Office of Science user facility located at the US Department of Energy’s Brookhaven National Laboratory.

This work was supported by the Department of Energy’s Office of Science.



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