Magnon polarons in the van der Waals antiferromagnet Fe PS 3

The hybridization of magnons (spin waves) with phonons, if sufficiently strong and comprising of long wavelength excitations, may offer a new playground when manipulating the magnetically ordered systems with light. Applying a magnetic field to a quasi-two-dimensional antiferromagnet, $\mathrm{Fe}{\mathrm{PS}}_{3}$, we tune the magnon-gap excitation to coincide with the initially lower-in-energy phonon modes. Hybrid magnon-phonon modes, the magnon polarons are unveiled with the demonstration of a pronounced avoided crossing between the otherwise bare magnon and phonon excitations. The magnon polarons in $\mathrm{Fe}{\mathrm{PS}}_{3}$ are traced with Raman scattering experiments. However, as we show, they also couple directly to terahertz photons, evoking their further explorations in the domain of antiferromagnetic optospintronics. The magnon-phonon coupling is also discussed as a possible reason of the magnon mode splitting observed in the absence of a magnetic field.