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A surprising twist on skyrmions

2018/06/14

Magnetic tomography has been used to reconstruct a tornado-like 3D magnetic skyrmion structure.

Vortex structures are common in nature, reaching from swirls in our morning coffee to spiral galaxies in the universe. Vortices are been best known from fluid dynamics. Take the example of a tornado. Air circulates around an axis, forming a swirl, and once formed, the twisted air parcels can move, deform, and interact with their environment without disintegrating. A skyrmion is the magnetic version of a tornado which is obtained by replacing the air parcels that make up the tornado by magnetic spins, and by scaling the system down to the nanometre scale. Once formed, the ensemble of twisted spins can also move, deform, and interact with their environment without breaking up ‒ the ideal property for information carriers for memory and logic devices.

What makes a tornado stable is not only coming from its twist, but also due to its three-dimensional properties, i.e., the wind current has extra twist along the column of turbulent flow. This leads to a tight bundling of the vortex sheets at different heights along the tornado column. Similarly, such a 3D structure can also occur in magnetic skyrmions, guaranteeing their topological stability. Up to now, skyrmions have been most commonly treated as two-dimensional objects, and their exciting tornado-like structure remained unexplored. In fact, the 3D characterization of magnetic structures is a rather challenging task. A team of researchers, led by the University of Oxford and Diamond Light Source, have used the energy-dependence of resonant elastic X-ray scattering (REXS) on beamline I10 at Diamond to measure the microscopic depth dependence of ‘skyrmion tornados’ in Cu₂OSeO₃. In their work, published in Proceedings of the National Academy of Sciences, they reveal a continuous change from Néel-type winding at the surface to Bloch-type winding in the bulk with increasing depth. This not only demonstrates the power of REXS for microscopic studies of surface-induced reconstructions of magnetic order, but also reveals the hidden energetics that makes magnetic skyrmions such a stable state – a crucial finding for skyrmion device engineering.

Figure: Illustration of a ‘Skyrmion tornado’. The skyrmion order changes from Néel-type at the surface to Bloch-type deeper in the sample. On the right hand side, the corresponding stereographic projections of these two boundary skyrmion patterns are shown.

Cleaner diesel emissions

2018/03/132018/03/26

More effective control of diesel nitrogen oxides through dosed addition of ammonia

In diesel engines, the burning of the fuel releases nitrogen oxides (NOx), which are harmful to human health. The automobile industry therefore developed a technique that reduces these emissions: Gaseous ammonia is added to the exhaust and, prompted by a catalyst, reacts with the nitrogen oxides to produce harmless nitrogen and water. At low temperatures, however, this process does not yet work optimally. Now, for the first time, scientists at the Paul Scherrer Institute PSI have found a remedy which is based on observations at the molecular level: The precise amount of added ammonia needs to be varied depending on the temperature. With this knowledge, manufacturers can improve the effectiveness of their catalytic converters for diesel vehicles. The researchers have now published their findings in the journal Nature Catalysis.

Image: At the X-ray beam line: Davide Ferri (left) and Maarten Nachtegaal at the SLS experimental station where they studied diesel catalysis.
Photo: Paul Scherrer Institute/Markus Fischer

Atmosphere in X-ray light

2017/09/262018/01/17

Light from the particle accelerator helps to understand ozone decomposition

A new experimental chamber coupled to the Swiss Light Source (SLS), a large-scale research facility of the Paul Scherrer Institute PSI, allows researchers to recreate atmospheric processes in the laboratory through unprecedented precision analysis involving X-rays.

In their first experiments, researchers detailed how bromine molecules are formed in the air. These play an essential role in the decomposition of ozone in the lower layers of the atmosphere. With their results, the researchers have also made an important contribution to models designed to explain and predict changes in climate and air composition. In the future, the experimental setup will be available to researchers in all scientific fields and those particularly concerned with the chemistry of the atmosphere or other topics in energy and environmental research.

>Read More on the PSI website

Image: In the experimental chamber, a very thin vertical jet of water can be seen, which flows downward in the middle of the picture from a small tube. During the experiment, the chamber contains a gas mixture including ozone, which reacts on the surface with bromide in the water and produces bromine. As an intermediate step in the process, a short-lived compound of bromide and ozone is made, which was detected for the first time ever with the help of X-ray light from SLS. For this proof, the X-ray light knocked electrons out of the compound, and these made their way to the detector through an opening in the cone (to the left in this photo). (Photo: Paul Scherrer Institute/Mahir Dzambegovic)