For the first time, the researchers observed that an extremely thin magnetic material – a so-called two-dimensional van der Waals magnet – ‘stores’ its magnetic state: it responds to a magnetic field and retains some of its magnetisation even when the applied field changes. This ‘memory’ is known as hysteresis and forms the basis of many data storage systems.

The material studied, chromium sulphide bromide (CrSBr), consists of individual layers that can be stacked and are each magnetic. Adjacent layers are magnetised in exactly opposite directions, so that their magnetic fields cancel each other out externally as a so called anti-ferromagnet. The researchers stacked two such layers on top of one another and twisted them by about three degrees relative to each other. This creates a fine structural pattern (moiré pattern) that alters how the two layers interact magnetically.

Using a light-based measurement technique at very low temperatures, the research team observed how the material reacts to an external magnetic field. Unlike untwisted bilayers, the twisted structure exhibited the described hysteresis particularly clearly. In addition, the researchers developed a theoretical model that accurately describes the observed magnetic switching processes. This revealed that, during switching, the twisted bilayer reacts largely like a single, uniform magnet – even though its magnetic properties are not exactly the same throughout the sample. Overall, the results demonstrate that the magnetic properties of such thin materials can be tailored solely through targeted twisting, known as ‘twist engineering’.

This approach has already yielded promising results in the past: for example, the effect of superconductivity – the flow of current with absolutely no resistance – was discovered in such twisted, extremely thin materials. So-called Mott insulators have also been demonstrated. These are materials that block the flow of current, even though, according to classical theory, they should actually conduct it.

The study now published combines basic research with potential practical applications. In the long term, such materials could form the basis for novel data storage devices or for electronic components that can be flexibly reprogrammed. So-called spintronic devices are also conceivable – a new type of electronics that processes and stores information in a particularly energy-efficient manner.

In addition to the Institute of Condensed Matter Physics at TU Darmstadt and the Rhineland-Palatinate Technical University of Kaiserslautern-Landau, the University of Chemistry and Technology in Prague and the National Institute for Materials Science in Tsukuba, Japan, were also involved in the research. The fabrication and characterisation of the CrSBr samples and the magneto-optical measurements were carried out at TU Darmstadt by Priyanka Mondal, Wenze Lan and Lennard Hopf from Professor Bernhard Urbaszek’s Hybrid Quantum Systems research group. The theoretical modelling was carried out in close collaboration.