RWTH Aachen University: Scientists from Aachen, Kiel and Reykjavik publish research results in the journal “Nature Physics”

Skyrmions are small magnetic vortices that are created by a suitable combination of materials. In data storage, they are regarded as future information carriers. Scientists from RWTH Aachen University, the Christian Albrechts University in Kiel and the University of Reykjavík found out that these so-called magnetic nanoknots can loosen in two ways. With the help of a magnetic field, the probability of node dissolution can be changed by a factor of 10,000. This knowledge could be groundbreaking for information processing with skyrmions. TheThe renowned journal “Nature Physics” published research results on January 4, 2021.

Magnetic nano-nodes encode the information by their presence or absence. The advantage is that the nodes are extremely stable and only a few nanometers in size. They exist at room temperature and can be displaced by the smallest electrical currents. Due to the small currents, the information can be read out particularly energy-efficiently. In principle, skyrmions also perform arithmetic operations so that data storage and processing could be combined in one structure. This makes computers more compact and, above all, more energy efficient. Because of these promising perspectives, work is being carried out worldwide on optimizing the properties of skyrmions. One focus is on stability: While skyrmions are characterized by extremely high stability, the smallest skyrmions disintegrate, which are required for high storage densities, much too quickly at room temperature. With a detailed understanding of the possible disintegration mechanisms, their stability could be significantly improved.

The extraordinary stability of skyrmions results from the knot-like arrangement of the atomic magnets. As with a rope, where the end has to be pulled through the knot hole, the atomic knot structure can only be solved with great effort. With the magnetic nanoknot, however, there is a somewhat easier way: after a single atomic magnet has been turned against the pushing forces of the neighboring atoms, the knot loosens itself. Which of the approximately 100 atomic magnets of the skyrmion is easiest to turn and how this is done in detail happens, but was not previously known.

The international team combined its theoretical and experimental expertise to answer this question. “Which atomic magnet turns around most easily depends on the framework conditions,” explains Florian Muckel from the RWTH Chair for Experimental Physics (Solid State Physics). “By changing a magnetic field that acts on the skyrmions, we can choose between two mechanisms.” With the first mechanism, the skyrmion is initially compressed to a nanometer in order to make it easier to fold down in the center. In the second mechanism, the node center pushes a nanometer outward before one of the atomic magnets can tip over relatively easily. Professor Markus Morgenstern, holder of the RWTH Chair for Experimental Physics (Solid State Physics): “With the help of the two mechanisms we can also control the efficiency of the node resolution. The stability of the skyrmions changes by up to a factor of 10,000, with the most stable configuration withstanding a trillion attempts to untangle the knot before the knot bursts. “

The new understanding of magnetic untying arose from the precise comparison of experiments from Aachen with theoretical work from Kiel and Reykjavik. With atomistic computer simulations, based on many years of development of novel theoretical methods, the movement of each individual atomic magnet can be followed during the process. “Thanks to the use of material-specific parameters from quantum mechanical calculations, the results agree in detail with the innovative experiments,” explains Stefan Heinze, Professor of Theoretical Physics at the Christian Albrechts University in Kiel. In the experiment, individual electrons were introduced into different parts of the skyrmion. For each position it was determined whether the nanoknot remains or whether it unties due to the energy brought by the electron. From this maps of the probability of the skyrmion being untied could be made. “The agreement between experiment and simulation is impressive. It’s fantastic when your own calculations are ‘followed’ so precisely by nature, ”says Stephan von Malottki, who carried out the simulations at Kiel University. “This is a great success of our theoretical method development,” adds Dr. Pavel Bessarab from Reykjavik, who carried out research in the Kiel working group for twelve months as an Alexander von Humboldt fellow in 2019. “The agreement between experiment and simulation is impressive. It’s fantastic when your own calculations are ‘followed’ so precisely by nature, ”says Stephan von Malottki, who carried out the simulations at the University of Kiel. “This is a great success of our theoretical method development,” adds Dr. Pavel Bessarab from Reykjavik, who did research in the Kiel working group for twelve months as an Alexander von Humboldt fellow in 2019. “The agreement between experiment and simulation is impressive. It’s fantastic when your own calculations are ‘followed’ so precisely by nature, ”says Stephan von Malottki, who carried out the simulations at Kiel University. “This is a great success of our theoretical method development,” adds Dr. Pavel Bessarab from Reykjavik, who carried out research in the Kiel working group for twelve months as an Alexander von Humboldt fellow in 2019.

The new findings on the limits of stability should make magnetic nanoknodes even more stable. This will make the use of magnetic nano-nodes in information processing more efficient and nano-nodes could establish themselves in commercial data storage.