  Hybridized nuclear and electron spin excitation in a MnCO$_3$ crystal, a
weakly-anisotropic antiferromagnet, has been investigated. In this material,
the hyperfine interaction is strong enough to form a nuclear spin wave. We
measure the microwave absorption by a bulk MnCO$_3$ and observe the dispersion
representing strong frequency repulsion between electron and nuclear modes due
to their hybridization, the signature of nuclear spin wave. Additionally, we
observe that the nuclear spin resonance enters a nonlinear regime above a
certain excitation power, attributed to the excitation of finite wavenumber
nuclear spin waves.
