Electric control of the optical magnetoelectric effect in multiferroics

Multiferroics are novel materials with coexisting ferroelectric and magnetic order. Recently, the dynamic magnetoelectric effect in multiferroics has attracted much interest due to several unusual optical phenomena like non-reciprocal light propagation or giant polarization rotation. These phenomena are essential especially for multiferroics because of strong cross-coupling between electric and magnetic dipole excitations. While the modulation of the intensity and polarization of light by external magnetic fields have been demonstrated in multiferroics, their electric field control, which is highly desirable for real applications, remains an experimental challenge. Another advantage of multiferroics is that the characteristic frequencies of their unconventional magnetoelectric excitations lie in the terahertz spectral range, i.e. in the expected working frequency range of future electronic devices. Therefore, dynamic magnetoelectric phenomena provide a fundamentally new concept to improve existing technologies and to develop novel devices for microelectronics and light control. In this project, we aim to investigate the electric field effect on terahertz excitations in several multiferroic systems. We are going to study the voltage-control of the propagation and the polarization of electromagnetic radiation via the magnetoelectric coupling. These experiments will demonstrate several non-trivial optical effects, like asymmetric forward/backward propagation of light, and can pave the way for applications of multiferroics in microwave and THz photonics.

In this project we investigated the possibilities of light control and of unusual light propagation using dynamic magnetoelectric effect. Magnetoelectric susceptibility is the coupling constant between electric and magnetic properties of the material. Similarly, the light is an electromagnetic wave having both electric and magnetic component. Therefore, magnetoelectricity can be effectively used to manipulate light especially in such materials like multiferroics, i.e. systems with simultaneous electric and magnetic ordering. - In multiferroic borates and manganites we were able to identify the propagating light modes that are sensitive to static magnetic field and to applied electric voltage. For these modes it is thus possible to have the control on light propagation using static parameters. These effects were demonstrated experimentally in the terahertz frequency range. Theoretical explanation utilizes the fact that static electric and magnetic moments can be modified by external fields. On their part, the modification of the static structures changes the dynamic properties and, therefore, influence the light propagation. - In various magnetically-ordered materials we observe the effect of non-reciprocal propagation, i.e. roughly these systems are transparent for one direction of light and opaque for the opposite. We could show that this asymmetry is directly related to specific symmetry breaking in the material itself. Theoretically, such effects can be reduced to direction-dependent terms in the propagation constant basically due to magnetoelectric susceptibility. - We could demonstrate and explain the new symmetry-related rotation of light polarization, called gyrotropic birefringence. This effect turns to reverse sign both, after time inversion and after space inversion. This is in contrast to conventional rotation phenomena like optical activity or Faraday rotation that are sensitive to one type of inversion only. - We have found that in a collinear miltiferroic manganite GdMn2O5 an unusual sequence of magneto-electric states can be observed in some special geometries. The application and subsequent removal of a magnetic field reverses the electric polarization of the material and appears together with an unusual 4-state hysteresis cycle. In this cycle half of the magnetic moments undergo a rotation of about 90o each time the magnetic field is ramped from zero to maximum, leading to a full-circle rotation when applying and removing a magnetic field two times in a sequence. So GdMn2O5 converts the back and forth motion of the magnetic field into a circular spin motion making it a magnetic crankshaft. Effects like this may open new technical opportunities and could improve the energy efficiency of magnetic memory and data processing devices. Finally, application of electric voltage revealed an additional channel to switch between the four states in this system.