Dissipative processes in perpendicularly magnetized ultrathin films studied using the complex AC susceptibility

Domain wall motion in perpendicularly magnetized Fe/2 ML Ni/W(110) films has been studied by measuring the complex magnetic AC susceptibility. The real part of the susceptibility is well-described by the activated motion of domain walls between pinning sites with a characteristic relaxation time τ(T). By contrast, the imaginary, or dissipative, part of the susceptibility departs from the model predictions at low temperature where the dynamic parameter ωτ>1, and does not exhibit the predicted 1/ωτ(T) behaviour. These results appear to be related to an incomplete description of minor hysteresis loops.