Relaxation mechanisms in perpendicularly-magnetized ultrathin Fe/ 2 ML Ni(111)/ W(110) films, with thickness between 1.25 and 2.00 ML, have been studied using the ac magnetic susceptibility as a function of temperature and/or time. Different time scales were probed by varying the constant rate of temperature variation, R as the susceptibility was measured. After quenching the film from high temperature, the susceptibility curve was found to relax through a shift in the peak position along the temperature axis and through changes in shape, as a function of time. In general, two opposing behaviors were found; for small R (≤0.30 K/s) the susceptibility peak temperature decreases as R increases, for large R (≥ 0.30 K/s) the peak temperature increases with R. The first behavior is understood as a "dynamical observation" of a domain phase transformation. The density of topological defects in the quenched high temperature delocalized phase undergoes an activated relaxation as low temperature ordered stripe phase is established. The fundamental time scale (𝜏_0R) of this process is in the order of 1.0 s. These findings complement the results of numerical simulation [24, 26, 27] and quantify the important dynamical barriers involved in the geometrical rearrangement of domains in moving from a delocalized phase to the ordered stripe phase. The experiments at large R are sensitive to a much shorter time scale over which the domain density equilibrates when temperature is changed. This process causes an increase in the peak temperature with R that depends linearly on R over the range of values of R accessible in this study. / Thesis / Doctor of Philosophy (PhD)