Role of magnon-magnon interaction in optical excitation of coherent two-magnon modes

Two-magnon modes are terahertz-frequency magnetic excitations in antiferromagnets, governed by exchange interactions and involving magnons from the entire Brillouin zone. The ability to couple to light promotes two-magnon modes as contenders for ultrafast optical manipulation of the magnetic state, beyond conventional zone-center magnonics. While magnon-magnon interactions are known to critically shape the two-magnon line in spontaneous Raman scattering spectra, their role in coherent time-domain excitations remains unexplored. We report a detailed experimental and theoretical study of the influence of magnon-magnon interactions on coherent two-magnon modes in a cubic antiferromagnet excited via Impulsive Stimulated Raman scattering. We reveal the nontrivial evolution of coherent magnetic dynamics in the time domain and the corresponding spectrum and compare it with the spontaneous Raman scattering spectrum. By extending the spin-correlations based theory for two-magnon modes, we derive a unified description of their spectra in Raman Scattering and Impulsive Stimulated Raman Scattering and highlight the role of magnon-magnon interactions.