Average Ripple-Angle Magnitude, Wall Pinning, and Susceptibility in Thin Permalloy Films

A modified bias-susceptibility apparatus and other instrumentations were used to examine the average ripple-angle magnitude 〈δφ〉av, the ``strip-domain wall pinning'', and the real and imaginary parts of the incremental susceptibility for several 1000-Å 80−20 Permalloy films with various dispersion angles. The measurements were performed under various field conditions and with various field histories. The results of the experiments show that, as suggested by Harte et al., the average ripple-angle magnitude 〈δφ〉av depends primarily on a micromagnetic equilibrium between local perturbing torques and the average field aligning the magnetization. The angles typically ranged from 1° to 15°. Similar average ripple-angle magnitudes were obtained when the Stoner-Wohlfarth field threshold was approached with the magnetization at angles of 0°, 45°, and 90° from the rest direction. However, as discussed by Hoffman, second-order effects produced significant differences in the measurements. Considerable pinning of the walls separating ``strip domains'' occurred. The wall pinning made the measurements sensitive to the magnetic history of a sample and displaced the peaks of the bias permeability loss curves to higher bias field values. Direct measurements indicated that internal demagnetizing fields from pinned walls reduced the real and imaginary parts of the incremental susceptibility in a direction perpendicular to the average magnetization by a factor from 2 to 10.