  Terahertz magnetic resonance driven by the light electric field, now referred
to as electromagnon, has stimulated interest due to its strong candidate for
future spin-electronics. One unique characteristic of electromagnons is the
terahertz magnetochromism, a color change by external magnetic field, as
recently demonstrated in the hexaferrite. By taking perovskite manganites and
hexaferrite as model cases, the current understating of the electromagnon
activity is discussed in terms of the symmetric exchange mechanism.
