Dynamics of domain growth driven by dipolar interactions in a perpendicularly magnetized ultrathin film

Measurements of the ac magnetic susceptibility of perpendicularly magnetized Fe/2 ML Ni/W(110) ultrathin films show a clear signature of the dynamics of domain growth and domain density changes in the domain pattern that this system supports. The susceptibility peak measured at different constant heating rates in the range $0.20\text{ }\text{K}/\text{s}\ensuremath{\le}R\ensuremath{\le}0.70\text{ }\text{K}/\text{s}$ shifts to higher temperature as the heating rate is increased. Analysis using a relaxation model demonstrates quantitatively that the dynamics is driven by a nonequilibrium 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 individual domain wall steps required to affect domain growth.