Fig. 3. Evidence for the involvement of the spin–orbit-entangled excitons in the coherent magnon generation.

a Schematic of the experimental setup for the THz emission measurements. b Temporal evolution of the emitted THz electric field (E) at various temperatures showing coherent magnon oscillations. The absorbed pump fluence is 1.6 mJ/cm². c Fourier transform of the traces in (b). A softening of the energy is observed as the temperature approaches T_N. The traces in (b) and (c) are offset vertically for clarity. d Comparison of the magnon energies from the measurements performed in equilibrium (Fig. 1c) and out of equilibrium (c), revealing that the energy of the driven system is redshifted. The error bars for the violet points represent the energy resolution of the time-domain THz experiment, and those for the red points indicate the resolution of the Fourier transform of the THz emission traces. e, f Absorbed pump fluence dependence of the energy of the magnon oscillations at 20 K (e) and 110 K (f) obtained from fits to the THz emission data (red points). The violet points correspond to the magnon energies in equilibrium at these two temperatures. The energy decreases linearly in both cases. The error bars in (e) and (f) denote the 95% confidence interval of the fits.