Dynamical signature of a domain phase transition in a perpendicularly magnetized ultrathin film

Domain phases in ultrathin Fe/Ni/W(110) films with perpendicular anisotropy have been studied using the ac magnetic susceptibility. Dynamics on time scales of minutes to hours was probed by quenching the system from high temperature to the stripe phase region, and varying the constant rate of temperature increase as the susceptibility traces were measured. The entire susceptibility peak is observed to relax slowly along the temperature axis, with the peak temperature increasing as the rate of heating is decreased. This is precisely opposite to what would happen if this slow relaxation was driven by changes in the domain density within the stripe phase. The data are instead consistent with a simple model for the removal of a significant density of pattern defects and curvature trapped in the quench from high temperature. A quantitative analysis confirms that the relaxation dynamics are consistent with the mesoscopic rearrangement of domains required to remove pattern defects, and that the experiment constitutes a dynamical observation of the phase transition from a high temperature, positionally disordered domain phase to the low temperature, ordered stripe phase.