  Inertia effects in magnetization dynamics are theoretically shown to result
in a different type of spin waves, i.e. nutation surface spin waves, which
propagate at terahertz frequencies in in-plane magnetized ferromagnetic thin
films. Considering the magnetostatic limit, i.e. neglecting exchange coupling,
we calculate dispersion relation and group velocity, which we find to be slower
than the velocity of conventional (precession) spin waves. In addition, we find
that the nutation surface spin waves are backward spin waves. Furthermore, we
show that inertia causes a decrease of the frequency of the precession spin
waves, namely magnetostatic surface spin waves and backward volume
magnetostatic spin waves. The magnitude of the decrease depends on the magnetic
properties of the film and its geometry.
