Wall Transitions in Coupled Films

Abstract

Néel walls have been generally observed in Permalloy films below a certain thickness whereas thicker films seem to support Bloch walls or cross-tie walls. Two coupled Permalloy films of 81% Ni-19% Fe composition separated by a layer of silicon monoxide were used to study domain wall interactions. Néel walls with no Bloch lines were observed in 900-Å coupled film segments with 300-Å film separation. In a single layer of Permalloy of this thickness, Bloch walls predominate. Since the separating layer is an insulator and nonferromagnetic, the interaction is thought to be magnetostatic. The films were prepared in a step fashion on a single substrate so that uncoupled and coupled portions of a film, prepared in the same evaporation, could be observed simultaneously using Bitter powder pattern technique. Multiple-step coupled structures of varying film thickness with constant separation and varying separation with constant film thickness were prepared with thicknesses chosen in the region of Néel-Bloch transition. The coercive force of the coupled film segments is lower than that of similar uncoupled film segments. The coercive force starts to decrease at film separations of approximately 1100 Å.