Magnetic hysteresis dynamics of thin Co films on Cu(001)

Hysteresis properties of ultrathin (2-4 monolayers) epitaxial Co films grown on Cu(001) surfaces are studied as a function of film thickness, temperature and the strength (H-0), in-plane direction, and frequency (Omega) of applied sinusoidal magnetic field. Scaling of the hysteresis loop area (power loss) of the form A = A(0) +H(0)(alpha)Omega(beta)zeta(H, Omega) where zeta is a scaling function is explored. All films exhibit a threshold field (H-t) where switching between equivalent magnetized states is initiated. Hysteresis loop areas measured over five decades in frequency exhibit very weak power-law dependence (beta similar to 0.02). No evidence of a dynamic phase transition is observed and no indication of a low-frequency (Omega(0) similar to 10(2) Hz) characteristic resonance is apparent over the drive frequency range covered. The observed weak power-law scaling does not support results of prior experiments that have been interpreted as corroborating the mean-field Ising model (alpha = beta= 2/3) and continuum spin models of thin-him hysteresis energy-loss scaling. The measured frequency and applied field-dependent scaling of the dynamic coercive force (H-c*) also appears to be inconsistent with recent phenomenological models of hysteresis behavior based on domain-wall motion that predict that H-c* scales as ln(H) over dot. The results of this study of Co on Cu(001) and a corresponding study of Fe on W(110) suggest that the dynamics of magnetization reversal in real ultrathin film systems do not exhibit universal behavior in the low-field low-frequency limit. Recent theoretical results based on a more realistic model that accounts for thermal noise and spatial fluctuations in the dynamics yield logarithmic scaling at low Omega and effective exponents beta that are compatible with the experiments. A simple physical picture of low drive-frequency energy-loss scaling is described that accounts for the experimental observations. [S0163-1829(99)03505-5].