  The observation of strongly coupled lattice and spin dynamics in altermagnet
CoF$_2$ is reported. On the background of the expected spin-phonon interaction
leading to renormalization of all phonons at the Neel temperature an additional
strong coupling between one-magnon excitation and the lowest frequency
Raman-active phonon of B1g symmetry was observed and manifested an anomaly in
its energy, full width, and intensity at temperature T*=23 K precisely where
the frequency of the phonon becomes exactly twice the frequency of the magnon.
We assigned this effect to unique magnon-phonon coupling in the form of a
two-magnons-one-phonon interaction. The consistent experimental data clearly
demonstrate that there is an intrinsic coupling that does not require coherent
excitation.
