Dissipation in perpendicularly magnetized ultrathin films studied using the complex AC susceptibility

Dissipation due to domain wall motion in perpendicularly magnetized ultrathin Fe/2ML Ni/W(110) films has been studied using measurements of the real (reversible) and imaginary (irreversible) components of the magnetic AC susceptibility, χ(T), as a function of frequency and the modulation field amplitude. The additional information afforded by Imχ(T) allows: extraction of the distribution of activation energies for motion of existing domain walls, revealing that it has the form of a narrow, truncated Lorentzian; demonstration of a separate dynamical freezing of the domain density as the temperature is lowered — possibly due to a barrier to domain wall creation; and the quantitative description of the crossover from a metastable, single domain hysteretic state to one with freely moving domains by inclusion of non-linear terms in the susceptibility. Together, these points permit a quantitative, self-consistent description of both components of the measured AC susceptibility.