  We investigate the role of domain walls in the ultrafast magnon dynamics of
an antiferromagnetic NiO single crystal in a pump-probe experiment with
variable pump photon energy. Analysing the amplitude of the energy-dependent
photo-induced ultrafast spin dynamics, we detect a yet unreported coupling
between the material's characteristic THz- and a GHz-magnon modes. We explain
this unexpected coupling between two orthogonal eigenstates of the
corresponding Hamiltonian by modelling the magneto-elastic interaction between
spins in different domains. We find that such interaction, in the non-linear
regime, couples the two different magnon modes via the domain walls and it can
be optically exploited via the exciton-magnon resonance.
