Myriad of terahertz magnons with all-optical magnetoelectric functionality for efficient spin-wave computing in the honeycomb magnet Co 4 Ta 2 O 9

Terahertz (THz) magnonics represents the notion of mathematical algebraic operations of magnons such as addition and subtraction in the THz regime---an emergent dissipationless ultrafast alternative to existing data processing technologies. Spin waves on antiferromagnets with a twist in spin-order host, such magnons in the THz regime, possess advantages of higher processing speeds, additional polarization degrees of freedom, and longer propagation lengths compared to gigahertz magnons in ferromagnets. While interactions among THz magnons are the crux of algebra operations, these require magnetic orders with closely spaced magnon modes for easier experimental realization of their interactions. Herein, a rich wealth of magnons spanning a narrow energy range of 0.4--10 meV is unraveled in ${\mathrm{Co}}_{4}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ using magneto-THz spectroscopy. A rare multitude of ten excitation modes, either of magnons or hybrid magnon-phonon modes, is presented. Among other attributes, spin-lattice interactions suggest a correlation among spin and local lattice distortion, magnetostriction, and magnetic exchange interactions, signifying a THz magnetoelectric effect. This unification of structural, magnetic, and dielectric facets, and their magnetic field control in a narrow spectrum unravels the mechanism underneath the system's complexity, while the manifestation of a multitude of spin-excitation modes is a potential source to design multiple channels in spin-wave computing-based devices.