  The dependence of the velocity of surface acoustic wave (SAW) as a function
of an external applied magnetic field is investigated in a Fe thin film
epitaxially grown on a piezoelectric GaAs substrate. The SAW velocity is
observed to strongly depend on both the amplitude and direction of the magnetic
field. To interpret the experimental data a phenomenological approach to the
relative change in SAW velocity is implemented. We find that the experimental
velocity variation can be well reproduced provided that the spin wave
dispersion is taken into account. The validity of this phenomenological model
is attested by the comparison with a quasi-exact magnetoelastic one.
