Everyone knows that holding two magnets together will produce one of two results: they stick together or they repel each other. From this perspective, magnetism seems simple, but scientists have struggled for decades to really understand how magnetism behaves at the smallest scales. At an almost atomic level, magnetism consists of many continuously changing realms—called magnetic domains—that create the magnetic properties of a material. While scientists know that these domains exist, they are still looking for the reasons for this behavior.
Now a collaboration led by scientists from the US Department of Energy’s Brookhaven National Laboratory, the Helmholtz-Zentrum Berlin (HZB), the Massachusetts Institute of Technology (MIT) and the Max Born Institute (MBI) has published a study in Nature in which they used a new technique for analysis – called coherent correlation imaging (CCI) – to image the evolution of magnetic domains in time and space without any prior knowledge. The scientists could not see the ‘domain dance’ during the measurement, but only afterwards when they used the recorded data to ‘rewind the tape’.
The “movie” of the domains shows how the boundaries of these domains shift back and forth in some areas but remain constant in others. The researchers attribute this behavior to a property of the material called “pinning.” Although pinning is a known property of magnetic materials, the team can directly image for the first time how a network of pinning sites affects the motion of interconnected domain walls.
“Many details of changes in magnetic materials are only available through direct imaging, which we could not do until now. This is actually a dream come true for studying magnetic motion in materials,” said Wen Hu, a scientist at the National Synchrotron Light Source II (NSLS-II) and co-author of the study.
The researchers expect CCI to help unlock other properties of the microcosm of magnetism—such as degrees of freedom or hidden symmetries—that were previously unavailable through other techniques. The utility of CCI also represents a breakthrough beyond magnetic materials, as the technique can be transferred to a variety of measurement techniques and research areas. One area that could benefit most from understanding the motion of magnetic domains at the nanoscale (one nanometer is 0.000000038 inches!) is the new calculus. The new memory technology can use special magnetic domains called “skyrmions”.
“Skyrmions are interesting for artificial intelligence computing because they have a property that is similar to our short-term memory,” said Felix Büttner, group leader at the Helmholtz-Zentrum Berlin, professor at the University of Augsburg and co-correspondent of the study. “In current computer architectures, everything is linear, meaning that memory is decoupled from the processor. This is not a problem for most applications, but for example it makes speech recognition difficult. In speech recognition, the computing part processes only the incoming words, but does not remember what was said before. Also, sending this information back from memory takes a lot of energy. By using skyrmions, we may be able to harness their short-term memory in some way and avoid these questions.”
However, before engineers and scientists can develop technology that exploits this feature, they must first understand how to manipulate skyrmions and other magnetic domains. This was the intention when the collaboration between NSLS-II, Jeffrey Beach’s group at MIT, and MBI was formed. They wanted to study how the skyrmions in their magnetic devices responded to external stimuli, specifically an external magnetic field. HZB joined the collaboration when Büttner moved from MIT to Berlin.
“In 2018, we had time to measure a coherent soft X-ray scattering (CSX) beamline at NSLS-II; however, the experimental camera we wanted to use was not ready. That meant we didn’t have an external magnetic field, but we had a backup plan to study thermal motion,” said Hu, who is part of the CSX beamline team.
Büttner added: “I expected this experiment to be another demonstration experiment, but nothing more. To be honest, I was surprised we saw thermal movement at all. We tested the same device at room temperature and saw almost no thermal movement. This time we studied at 310 Kelvin, which is about 98 Fahrenheit, and we saw a lot more. That was surprising! And that was just 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. The CSX is part of the expanded suite of research tools available at NSLS-II for the study of materials. The research team used the beam line in a holographic setup to make the images. In most holographic experiments, scientists take one image every three to four seconds, but the fast CSX beamline detector allows the team to take up to 100 images per second.
“After the measurement, we started normal data analysis by adding 200 images. After doing this, we realized that the system changed much faster than we expected. The temperature really affected the physics in the sample,” said Dr. Christopher Close, a graduate student at MBI and first author of the study. “This was a real surprise and the beginning of the development of our post-processing technique – coherent correlation imaging (CCI) – so that we could 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 movement were encoded in their data. Although there was no existing data analysis technique that could solve their problem, they were able to find algorithms that could be adapted. Over three years, the team developed the new algorithm that powers the new CCI technique.
“There were many challenges. To develop the CCI, we combined the known correlation function analysis from X-ray photon correlation spectroscopy (XPCS) with holography, which is an imaging technique. One of the problems was that the holographic data was not suitable for XPCS analysis,” Klose said.
When X-rays hit the samples in these experiments, they scatter over both the magnetic domains and the holographic mask that defines the field of view. The detector records all scattered X-rays, regardless of their origin. But the team is only interested in magnetic scattering. So they had to clean the data before they could calculate the correlation functions.
“Once we had the correlation function, we could compare all these frames to each other to find similarities. It also required a new algorithm because we had almost 30,000 frames to sort through,” Close continued.
This challenge requires an algorithm that can catalog the states of the domains for each frame. This algorithm will be a game changer for this task because it will be able to sort these states in ways that no human can achieve.
How pinning shapes the magnetic landscape
After the team sorted their data using CCI, they set about interpretation. The reconstructed images show black and white domains scattered throughout their device. But some of these boundaries, or domain walls, shifted back and forth between frames, while others remained in place. The question: what do the researchers see, and what does it mean for skyrmions and magnetic domains?
“Skyrmions are small spherical objects comparable to balls on a pool table. In our case, the thermal energy causes them to wander around the table. Now, if the pool table has pins, the surface is not smooth, but instead a hilly landscape. We have two types of attachment sites: attractive and repulsive. The former are valleys and the latter are hills. In this case, the skyrmions will rest in the “attractive” valleys. If they want to move around, they will have to overcome the slopes of the ‘repulsive’ hills,” Buettner said.
The researchers found that the domain walls behaved like rubber bands. They can be attached and then oscillate back and forth like a guitar string. While attractive sites can accommodate domain walls, repulsive sites impede the movement of domain walls. A domain wall will need to be erected over the repulsive site. Can’t wander through it. This explains why scientists have seen the walls of some domains move constantly, while others barely move. The latter were surrounded by repulsive places.
“CCI gave us the tool to see that movement over time. Basically, we could make a little movie about how these domains change. This experiment allowed us to see this kind of variable behavior and the cause of it for the first time,” Hu said. “We did not expect that this collaboration would lead to the invention of a new technique that will be of great benefit to other users and researchers studying dynamics.”
Buettner added: “It took us almost a year to fully understand the physics we found and to develop an explanation for the dynamics we saw. In retrospect, the experiment itself was the easiest part of it all. The real work was developing the technique and then the physical explanation.”
The researchers agreed that one key ingredient to this breakthrough was the diverse team of experts they assembled for the task. They hope that many other research groups will benefit from the CCI. While preparing to apply CCI to a wider range of previously inaccessible dynamics, as well as extending the technique to other X-ray sources, they are also working on implementing machine learning to make CCI analysis less manual and more -accessible by an even wider community.
The team for this work consisted of Christopher Close, Michael Schneider, Stefan Eisebit and Bastian Pfau of the Max Born Institute, Felix Buettner and Riccardo Battistelli of the Helmholtz-Center Berlin, Wen Hu, Claudio Mazzoli, Andy Barber and Stuart B. Wilkins of the .. .
Add Comment