New magnetoelectricity based on local and global symmetries

Wider research context Electric and magnetic fields in vacuum are coupled via Maxwell equations. In the matter this coupling, both static and dynamic, is provided by magnetoelectric effect that was discovered nearly a century after Maxwell electrodynamics. The magnetoelectric effect is especially strong in multiferroics that are materials with simultaneous and coupled electric and magnetic orders. Magnetoelectric and multiferroic materials promise a series of applications especially in electronics and memories as they add a new degree of freedom in controlling electricity and magnetism. Recently, new family of magnetoelectric materials based on rare-earth doped langasites (La3 Ga5 SiO14 and related compounds) has been brought into attention due to symmetry-related mechanisms of magnetoelectricity. Crystal symmetry in langasites forbids electric polarization and thus magnetoelectric effect. However, the mechanisms based on local symmetry of the rare earth ions in these structure suggests a way to resolve this problem. Objectives New symmetry-relevant routes to the magnetoelectric effect will be approached and investigated in langasites with different rare-earth substitutions like holmium, neodymium, etc. The interplay between local and global symmetries as well as different crystal field schemes in this material class will allow to find new routes to magnetoelectric effect. Comparison of different substituents in the same material class will help to construct recipes to optimize the values of the polarization and find ways towards possible applications. Methods This project will utilize the combination of crystal growth, static and dynamic experiments, microscopic modelling, and symmetry analysis. The magnetoelectric character of the magnetic and magnetoelectric excitations will be analyzed by polarization technique that allows the detection of dynamic magnetoelectric coupling and of several unusual optical phenomena, like optical activity or directional anisotropy. Degree of innovation The novelty of magnetoelectric effect in rare-earth langasites is due to interplay between the global symmetry of the crystal and the local symmetry of the rare-earth ion. While the former forbids or suppresses the magnetoelectric coupling, the lower local symmetry opens an unusual indirect way to allow the effect via fast saturation of local magnetic moments in external magnetic fields.

Electricity and magnetism are closely linked in nature, but in most materials their interaction is weak. Strengthening and controlling this coupling-known as the magnetoelectric effect-could enable new technologies such as energy-efficient electronics, advanced sensors, and novel communication devices. This project explored an unconventional route to achieving such control. Even when the overall symmetry of a crystal appears to forbid a magnetoelectric response, the local environment of individual atoms can still enable it. By focusing on this subtle interplay between global and local symmetries, we investigated families of magnetoelectric materials such as langasites, borates, and manganites. One of the key achievements was resolving a long-standing puzzle in holmium-based langasites. Earlier experiments had suggested conflicting pictures of how atomic magnetic moments are arranged. By combining complementary measurement techniques and carefully analyzing symmetry effects, we showed that these seemingly contradictory results are in fact consistent. Different local atomic environments can behave in equivalent ways and collectively reproduce the global symmetry of the crystal. This provides a clear and unified understanding of the anisotropic magnetoelectric behavior of these systems. In a related material containing praseodymium, the magnetoelectric effect was found to be significantly stronger. Electric polarization can be generated along multiple directions and controlled over a wide range of conditions. These findings point to practical strategies for enhancing magnetoelectric properties by tailoring the atomic composition of materials. In addition, in magnetoelectric thulium-based aluminoborates we observed a pronounced rotation of the polarization of light and were able to relate it to crystal-field excitations of the rare-earth ions. Beyond these fundamental insights, the project also demonstrated a striking new phenomenon: one-way transparency. In an artificial magnetoelectric material combining a magnetic layer with a specially designed metallic structure, electromagnetic waves can propagate in one direction but are blocked in the opposite direction. Unlike previous realizations, which required extreme conditions such as very low temperatures, this approach offers a more flexible and tunable platform. The effect arises from the dynamic interaction between magnetism and engineered structures. Such directional control of waves is highly relevant for technologies ranging from telecommunications to signal processing, where it is important to guide information without unwanted reflections or losses. The ability to switch and tune this behavior makes the concept particularly attractive for future applications. Overall, the results show how subtle atomic-scale effects can be harnessed to control magnetoelectricity and the properties of light. This work lays the foundation for designing next-generation materials with functionalities that arise from the interplay of local and global symmetries.