Domains in perpendicularly magnetized ultrathin films studied using the magnetic susceptibility

Measurements of the complex, low-frequency ac magnetic susceptibility X-Z o f Fe/2 ML Ni/W(110) films, using a small field applied normal to the film s urface, were used to characterize magnetic domain formation and motion in a perpendicularly magnetized ultrathin-film system. Analysis of the real par t of X shows that a broad peak in the susceptibility roughly divides low- a nd high-temperature regimes, where the domains are pinned and move freely, respectively. At high temperature, the domain density increases exponential ly with temperature, producing an exponential decrease of the susceptibilit y with decay constant K approximate to 0.05 K-1, consistent with theoretica l expectations. At low temperature, domain-wall motion is thermally activat ed, with the activation energy increasing from 2X10(3) K for 3.0 ML Fe to 9 X10(3) K for 1.5 ML Fe. The systematic variation of the activation energy i ndicates an increasing sensitivity to monolayer steps for thinner films, an d yields an average separation of pinning sites of 200+/-30 nearest-neighbo r distances. This is consistent with a 0.2 degrees-0.3 degrees miscut of th e substrate crystal. Films with Fe thickness less than or equal to 2 ML exh ibit an exponential decrease in X-Z up to, and through, the transition to p aramagnetism with no marker for the Curie temperature. Films with Fe thickn ess >2 ML undergo a reorientation of the magnetization from perpendicular t o in plane as the temperature is increased. However, the reorientation prod uces no peak in the susceptibility, giving evidence that it is a discontinu ous transition for these films.