(with the Laboratory for Neutron and Muon Instrumentation and the Laboratory for Neutron Scattering and Imaging, in NUM)
Multiferroicity has sparked in the last few years a lot of interest in solid state physics. From an application point of view it is desirable to control charges by applying a magnetic field and spins by an electric field. From a physics point of view an important question to answer is how the coupling between ferroelectricity and magnetism in solids is achieved: magnetism is related to an ordering of spins (electrons) in incomplete ionic shells, ferroelectricity mostly results from a relative shift of negative and positive ions. Early attempts to combine ferroelectricity and magnetism proved to be difficult because the microscopic mechanisms of ferroelectricity and magnetism are quite different and do not strongly interfere with each other. Only recently, materials like REMnO3, REMn2O5 (RE: rare earths), Ni3V2O8, CuFeO2 (delafossite), (Co,Ni)Cr2O4 (spinel), MnWO4, and hexagonal ferrite (Ba,Sr)2Zn2Fe12O22 have shown multiferroic properties. Most of these materials are low-dimensional or frustrated magnets with strong magnetic fluctuations. The reason for the sensitivity of the dielectric properties to an applied magnetic field lies in the magnetic origin of their ferroelectricity, which is induced by complex spin structures, characteristic for frustrated magnets. This spin structure can be investigated using neutrons like the low-temperature spiral phase which breaks the inversion symmetry and hence allows an electric polarization .