  The magnetoelectric coupling, i.e., cross-correlation between electric and
magnetic orders, is a very desirable property to combine functionalities of
materials for next-generation switchable devices. Multiferroics with
spin-driven ferroelectricity presents such a mutual interaction concomitant
with magneto- and electro-active excitations called electromagnons. TbMnO3 is a
paradigmatic material in which two electromagnons have been observed in the
cycloidal magnetic phase. However, their observation in TbMnO3 is restricted to
the cycloidal spin phase and magnetic ground states that can support the
electromagnon excitation are still under debate. Here, we show by performing
Raman spectroscopy measurements under pressure that the lower-energy
electromagnon (4 meV) disappears when the ground state enters from a cycloidal
phase to an antiferromagnetic phase (E-type). On the contrary, the
magnetoelectric activity of the higher-energy electromagnon (8 meV) increases
in intensity by one order of magnitude. Using microscopic model calculations,
we demonstrate that the lowerenergy electromagnon, observed in the cycloidal
phase, originates from a higher harmonic of the magnetic cycloid, and we
determine that the symmetric exchange-striction mechanism is at the origin of
the higher-energy electromagnon which survives even in the E-type phase. The
colossal enhancement of the electromagnon activity in TbMnO3 paves the way to
use multiferroics more efficiently for generation, conversion and control of
spin waves in magnonic devices.
