Domains in perpendicularly magnetized ultrathin films studied using the magnetic susceptibility

Measurements of the complex, low-frequency ac magnetic susceptibility χz of Fe/2 ML Ni/W(110) films, using a small field applied normal to the film surface, were used to characterize magnetic domain formation and motion in a perpendicularly magnetized ultrathin-film system. Analysis of the real part of χ shows that a broad peak in the susceptibility roughly divides low- and high-temperature regimes, where the domains are pinned and move freely, respectively. At high temperature, the domain density increases exponentially with temperature, producing an exponential decrease of the susceptibility with decay constant κ≈0.05 K-1, consistent with theoretical expectations. At low temperature, domain-wall motion is thermally activated, with the activation energy increasing from 2×103 K for 3.0 ML Fe to 9×103 K for 1.5 ML Fe. The systematic variation of the activation energy indicates an increasing sensitivity to monolayer steps for thinner films, and yields an average separation of pinning sites of 200±30 nearest-neighbor distances. This is consistent with a 0.2°–0.3° miscut of the substrate crystal. Films with Fe thickness ⩽2 ML exhibit an exponential decrease in χz up to, and through, the transition to paramagnetism with no marker for the Curie temperature. Films with Fe thickness >2 ML undergo a reorientation of the magnetization from perpendicular to in plane as the temperature is increased. However, the reorientation produces no peak in the susceptibility, giving evidence that it is a discontinuous transition for these films.