Dynamic magnetoelectric phenomena with electromagnons in rare-earth borate multiferroics

Electroactive spin excitations (electromagnons), an analog of the low-frequency quasiferromagnetic antiferromagnetic resonance (AFMR) mode in Fe subsystem, are observed in multiferroic rare-earth ferroborates specifically in SmFe(BO) in the frequency range of 40–150 GHz, which are shown to contribute dominantly to the giant static (quasistatic) magnetodielectric effect and which produce two types of dynamic magnetoelectric effects: (a) giant optical activity, which occurs for the wave vector k parallel to the crystallographic -axis, , in a transverse magnetic field and is accompanied by polarization-plane rotation by more than 70 deg mm in the resonance, and (b) directional birefringence and dichroism in a transverse magnetic field -axis, which show up in transmission asymmetry between the forward () and backward () directions, being equivalent to the sign change of the magnetic field, . A theory is developed which explains the observed dynamic magnetoelectric phenomena quantitatively by taking into consideration different symmetries of the tensors of magnetic, magnetoelectric and dielectric susceptibilities for and .