  We have created a nonequilibrium population of antiferromagnetic spin-waves
in Cr2O3, and characterized its dynamics, using frequency- and time-resolved
nonlinear optical spectroscopy of the exciton-magnon transition. We observe a
time-dependent pump-probe line shape, which results from excitation induced
renormalization of the spin-wave band structure. We present a model that
reproduces the basic characteristics of the data, in which we postulate the
optical nonlinearity to be dominated by interactions with long-wavelength
spin-waves, and the dynamics to be due to spin-wave thermalization.
