  Coherent oscillations associated with spin precessions were observed in
ultrafast optical experiments on ferromagnetic (Ga,Mn)As films. Using a
complete theoretical description of the processes by which light couples to the
magnetization, values for the exchange and anisotropy constants were obtained
from the field-dependence of the magnon frequencies and the
oscillation-amplitude ratios. Results reveal a relatively large negative
contribution to the energy due to surface anisotropy leading to excitations
that are a mixture of bulk waves and surface modes.
