Recent spectroscopic studies at terahertz frequencies for a variety of multiferroics endowed with both ferroelectric and magnetic orders have revealed the possible emergence of a new collective excitation, frequently referred to as electromagnon. It is magnetic in origin, but it becomes active in response to the electric field component of light. Here we give an overview of our recent advance in the terahertz time-domain spectroscopy of electromagnons, or electric-dipole active magnetic resonances, focused on perovskite manganites— (R denotes rare-earth ions). The respective electric and magnetic contributions to the observed magnetic resonance are firmly identified by the measurements of the light-polarization dependence using a complete set of the crystal orientations. We extract general optical features in a variety of the spin-ordered phases, including the A-type antiferromagnetic, collinear spin-ordered phase and the ferroelectric and spiral spin-ordered phases, which are realized by tuning the chemical composition of R, the temperature, and the external magnetic field. In addition to the antiferromagnetic resonances of Mn ions driven by the magnetic field component of light, we clarify that the electromagnon appears only for light that is polarized along the a axis, even in the collinear spin-ordered phase, and it grows in intensity with evolution of the spiral spin order but is independent of the direction of the spiral spin plane ( or ) or, equivalently, the direction of the ferroelectric polarization ( or ). A possible origin of the observed magnetic resonances at terahertz frequencies is discussed by comparing the systematic experimental data presented here with theoretical considerations based on the Heisenberg model.
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