  The infrared and Terahertz properties of GdMnO_3 have been investigated as
function of temperature and magnetic field, with special emphasis on the phase
boundary between the incommensurate and the canted antiferromagnetic
structures. The heterogeneous incommensurate phase reveals strong
magnetodielectric effects, characterized by significant magnetoelectric
contributions to the static dielectric permittivity and by the existence of
electrically excited magnons (electromagnons). In the commensurate canted
antiferromagnetic phase the magnetoelectric contributions to the dielectric
constant and electromagnons are suppressed. The corresponding spectral weight
is transferred to the lowest lattice vibration demonstrating the strong
coupling of phonons with electromagnons.
