THz magneto-optical study of multiferroic compound TbMnO$_3$*

We present results of magneto-optical absorption measurements in the THz region on multiferroic TbMnO$_3$ in magnetic fields up to 12\,T. The temperature range of our studies covers the paramagnetic phase ($T>42$\,K), the collinear incommensurate spin density wave phase of Mn$^{3+}$ $S=2$ spins (28\,K$<42$\,K), the ferro-electric phase with incommensurate elliptic spin order ($T<28$\,K) and the phase where the magnetic moments of Tb are ordered, $T<7$\,K. It is known that in the FE phase the magnetic field ${\mathbf B_0}\parallel {\mathbf a}$ equal to 10.5\,T flops the electric polarization from ${\mathbf P}\parallel {\mathbf c}$ to ${\mathbf P}\parallel {\mathbf a}$. The polarization ${\mathbf P}$ is in the plane of elliptical spiral and perpendicular to spiral order vector, ${\mathbf k}\parallel {\mathbf b}$ in TbMnO$_3$. It is expected that the selection rule for the electric-dipole active spin excitation in the spiral phase changes from ${\mathbf E_1}\parallel {\mathbf a}$ to ${\mathbf E_1}\parallel {\mathbf c }$. Polarization-sensitive absorption measurements are performed to distinguish between magnetic- or electric-dipole active spin excitations, i.e. magnons or electromagnons, respectively, in the whole temperature and magnetic field range.