  The hybridization of magnons (spin waves) with phonons, if sufficiently
strong and comprising long wavelength excitations, may offer a new playground
when manipulating the magnetically ordered systems with light. Applying a
magnetic field to a quasi-2D antiferromagnet, FePS3, we tune the magnon-gap
excitation towards coincidence with the initially lower-in-energy phonon modes.
Hybrid magnon-phonon modes, the magnon polarons are unveiled with demonstration
of a pronounced avoided crossing between the otherwise bare magnon and phonon
excitations. The magnon polarons in FePS3 are primary traced with Raman
scattering experiments, but, as we show, they also couple directly to terahertz
photons, what evokes their further explorations in the domain of
antiferromagnetic optospintronics.
