  Effects of temperature (6--225 K) and magnetic field (0--7 T) on the
low-energy (1.2--5 meV) electrodynamics of the electromagnon, the magnetic
resonance driven by the light electric field, have been investigated for a
hexaferrite magnet Ba$_2$Mg$_2$Fe$_{12}$O$_{22}$ by using terahertz time-domain
spectroscopy. We find the gigantic terahertz magnetochromism via
electromagnons; the magnetochromic change, as defined by the difference of the
absorption intensity with and without magnetic field, exceeds 500% even at 0.6
T. The results arise from the fact that the spectral intensity of the
electromagnon critically depends on the magnetic structure. With changing the
conical spin structures in terms of the conical angle $\theta$ from the proper
screw ($\theta=0^\circ$) to the ferrimagnetic ($\theta=90^\circ$) through the
conical spin-ordered phases ($0^\circ<\theta<90^\circ$) by external magnetic
fields, we identify the maximal magnetochromism around $\theta\approx45^\circ$.
On the contrary, there is no remarkable signature of the electromagnon in the
proper screw and spin-collinear (ferrimagnetic) phases, clearly indicating the
important role of the conical spin order to produce the
magnetically-controllable electromagnons. The possible origin of this
electromagnon is argued in terms of the exchange-striction mechanism.
