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New Method to Visualize Material Fluctuations

A group of scientists has created a ground-breaking new technique for taking high-resolution pictures of material fluctuations at the nanoscale using powerful and effective X-Ray sources. The team is led by scientists from the Max Born Institute in Berlin and Helmholtz-Zentrum Berlin in Germany, and from Brookhaven National Laboratory and the Massachusetts Institute of Technology in the United States.

The principle of Coherent Correlation Imaging is used to image a random process, such as, figuratively, a coin toss. A single, brief image of the coin may not be sufficiently exposed to clearly identify the image on the coin. However, a new algorithm can sort and combine multiple images to produce clear images of both sides of the coin, which can then be accurately assigned to the moment of exposure. Image Credit: Christopher Klose / MBI

The method, called Coherent Correlation Imaging (CCI), enables the production of sharp, detailed movies without irradiating the sample too much. CCI makes it possible to access data that was previously inaccessible by using an algorithm to find patterns in underexposed images.

The team used samples made of thin magnetic layers to demonstrate CCI, and their findings were published in Nature.

The world at the microscopic scale is constantly in motion and changing. These fluctuations can produce peculiar properties even in solid materials that appear to be static; one example is the lossless transmission of electrical current in high-temperature superconductors.

When a material undergoes a phase transition, such as when it transforms from a solid to a liquid during melting, fluctuations are particularly noticeable.

Additionally, researchers look into a variety of phase transitions, including those from non-conductive to conductive and non-magnetic to magnetic, in addition to modifications to crystal structure. Many of these processes are used in technology and are essential to the operation of living things.

The Problem: Too Much Illumination Might Damage the Sample

It is challenging to investigate these processes in-depth, and it is even harder to film these fluctuation patterns. This is due to the rapid changes at the nanometer scale, or one-millionth of a millimeter.

This quick, erratic motion is too fast for even the most sophisticated high-resolution X-ray and electron microscopes to capture. A photography rule, which states that a certain level of illumination is necessary to capture a clear image of an object, serves as an example of how deeply rooted the issue is.

More light is required to magnify the object, or “zoom in", and even more light is required if there is a need to acquire a fast motion with a brief exposure time. The point at which the object would be harmed or even destroyed by the required illumination is ultimately reached by raising the resolution and lowering the exposure time.

In recent years, science has precisely arrived at this realization: images captured using free-electron lasers, the most powerful X-ray sources currently on the market, unavoidably resulted in the destruction of the sample under study. Due to this, it has been determined that it is impossible to film these random processes in a movie format using multiple images.

New Approach: Using an Algorithm to Detect Patterns in Dimly Lit Pictures

This issue has now been resolved by an international group of scientists. Realizing that material fluctuations are often not completely random was crucial to their solution.

The researchers found that by concentrating on a small portion of the sample, they could see that although the timing and frequency of certain spatial patterns were unpredictable, they did notice that they appeared repeatedly.

Coherent Correlation Imaging (CCI), a novel non-destructive imaging technique, was created by the researchers. They quickly take several snapshots of the sample to make a movie, lowering the illumination just enough to preserve the sample.

However, this causes the fluctuation pattern in the sample to blur in some of the individual images. The images can still be divided into groups because they still have enough information in them.

The team had to first develop a new algorithm that examines the correlations between the images to achieve this, hence the name of the technique. Due to how similar the snapshots in each group are, they probably came from the same particular fluctuation pattern.

A distinct picture of the sample only becomes apparent when all of the shots in a group are viewed at once. The ability to rewind the film has allowed the researchers to clearly picture the state of the sample at each snapshot.

An Example: Filming the “Dance of Domains” in Magnetic Layers

This new technique was developed to address a particular issue in the field of magnetism: microscopic patterns that exist in thin ferromagnetic layers. The magnetization in these layers is split up into areas known as domains, where it points upward or downward.

Modern hard drives employ similar magnetic films, with the two different types of domains encoding bits with “0” or “1”. It was previously thought that these patterns were quite stable. Whether it is actually true is unclear.

The team used the newly created CCI method to examine a sample made up of such a magnetic layer at the National Synchrotron Light Source II on Long Island, which is nearby New York City, to find the answer to this question. The patterns did, in fact, not change at room temperature.

However, at a slightly higher temperature of 37 °C (98 °F), the domains started to shift one another erratically and back and forth. For several hours, the scientists watched this “dance of the domains.”

They then produced a map that showed where the domain boundaries should be placed. Future applications in cutting-edge computer architectures will benefit from a better understanding of the magnetic interactions in the materials as a result of this map and the movie of the movements.

New Opportunities for Materials Research at X-Ray Sources

The next step for the researchers is to apply the cutting-edge imaging technique to free-electron lasers like the European XFEL in Hamburg to gain more insight into even faster processes occurring at the smallest length scales.

They are confident that using this approach will help us better understand how fluctuations and stochastic processes affect the characteristics of modern materials and, as a result, help find new ways to use these phenomena more strategically.

Coherent correlation imaging is a technique used to simulate random processes, such as a coin toss. It might not be possible to clearly identify the image on the coin from a single, brief image. A new algorithm, however, can sort and combine numerous images to create distinct images of both sides of the coin, which can then be precisely dated to the exposure moment.

Journal Reference:

Klose, C., et al. (2023) Coherent correlation imaging for resolving fluctuating states of matter. Nature. doi:10.1038/s41586-022-05537-9.


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