A group of scientists at the Institute of Molecular Science (ICMol) of the University of Valencia has taken an essential step in the field of "graphene twistronics" by demonstrating, in a pioneering scientific paper published in Nature Materials that two-dimensional materials other than graphene can also exhibit emerging and unexplored properties when two atomically thick monolayers of these nanomaterials are stacked and rotated at a certain angle.
The study reveales for the first time that a magnetic bilayer twisted at a 90° angle exhibits spectacular magneto-transport properties, such as the emergence of magnetic multi-states with memory effects. This pioneering result bodes well for the future of "spin twistronics" as a platform for designing new materials with "on-demand" properties that could be of interest in emerging technologies such as spintronics, magnonics or quantum technologies.
The discovery of graphene - a two-dimensional (2D) semiconducting material consisting of a one-atom-thick layer of carbon atoms laid out in a honeycomb pattern - is revolutionising condensed matter physics and materials science today. One of the most spectacular results has been the observation of superconductivity in twisted bilayers of graphene with a small angle of 1.4°, reported by Pablo Jarillo's team at MIT in 2019.
This milestone was made possible by the fact that, like cards in a deck of cards, two layers of graphene can be stacked and rotated at will to form an angle between them. This possibility - unique for two-dimensional materials - has opened up a new field of research called twistronics that provides an ideal platform for designing an infinite number of materials with new properties, simply by rotating the layers of 2D material that make them up.
This new field called twistronics has now been extended to 2D materials other than graphene, including magnetic materials, but has always been limited to layers rotated at small angles (less than 5°).
The contribution of the ICMol research team extends twistronics to ferromagnetic bilayers in order to achieve emergent magnetic properties by means of a 90º rotation of one layer relative to the other. In these artificial magnetic bilayers, the chosen material is formulated as CrSBr and consists of layers of chromium atoms linked through sulphur and bromide ions. In each of the layers, the magnetic moments of the chromium are oriented along one of the two axes defining the layer. When rotated, these orthogonal layers show frustration phenomena when an external magnetic field is applied.
As a consequence of this rotation, and in contrast to the natural (unspun) bilayer, spectacular properties are observed in the artificial bilayer, such as the appearance of magnetic field-induced magnetic multi-states that store information even in the absence of a magnetic field. All this demonstrates that twistronics is a simple and efficient technique to design "tailor-made" magnets, simply by changing the spin angle or the nature of the starting magnetic monolayers. The new direction that is opening up in magnetism with so-called spin twistronics (or magnetic twistronics) is likely to have a significant impact on emerging technologies such as spintronics, magnonics or quantum technologies.
The ICMol scientific team is formed by the doctoral researcher Carla Boix-Constant, Samuel Mañas-Valero, currently a Marie Curie postdoctoral researcher at the Delft University of Technology (Netherlands) and Professor Eugenio Coronado, director of the research group and of ICMol.
Link to Nature Materials article: https://www.nature.com/articles/s41563-023-01735-6