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.