  Measurements of the ac magnetic susceptibility of perpendicularly magnetized
Fe/2ML Ni/W(110) ultrathin films show a clear signature of the dynamics of
domain growth and domain density changes in the striped domain pattern that
this system supports. The susceptibility peak measured at different constant
heating rates in the range 0.20 K/s < R < 0.70 K/s shifts to higher temperature
as the heating rate is increased. Analysis using a relaxation model
demonstrates quantitatively that the dynamics is driven by a non-equilibrium
domain density at (nearly) zero field (i.e. by dipole interactions), and that
the temperature shift is due to a response time determined by the pinning of
local domain wall segments by structural defects. The fundamental time scale
for relaxation of the domain density driven by dipole interactions is of order
10^5 times slower than the fundamental time scale for an individual Barkhausen
step driven by an applied field. The increase in the fundamental time scale
reflects the relative size of dipole and Zeeman energies, and the need for the
correlated motion of many local domain wall segments to affect domain growth.
