  We study magneto-elastic effects in crystals of magnetic molecules. Coupled
equations of motion for spins and sound are derived and the possibility of
strong resonant magneto-acoustic coupling is demonstrated. Dispersion laws for
interacting linear sound and spin excitations are obtained for bulk and surface
acoustic waves. We show that ultrasound can generate inverse population of spin
levels. Alternatively, the decay of the inverse population of spin levels can
generate ultrasound. Possibility of solitary waves of the magnetization
accompanied by the elastic twists is demonstrated.
