Evidence for Domain-Wall Compressibility in Permalloy Wire

Abstract

This paper describes dynamic magnetization experiments in hard-drawn niobium-silver Permalloy wire of the type used in domain-wall shift registers. In particular, it reports evidence for the compression, under the influence of an applied bucking field, of the transition region separating two domains of antiparallel longitudinal magnetization. In the absence of external field, the transition region is characterized by a domain wall roughly conical in shape, with a minimum length of about 1 cm for wire of 0.005-cm diameter. The length of the transition region is inversely proportional to the longitudinal derivative of flux, which may also be referred to as magnetic pseudocharge. The latter quantity was measured by a bifilar pickup coil wound around the wire, and subsequent electronic integration. Dynamic experiments employing ac bucking fields with or without an additional homogeneous field result in hysteresis loops of particularly simple and predictable types. Identifiable individual processes within the cycle are nucleation, compression and expansion of the transition region, and wall motion. A compression of the longitudinal dimension of the domain wall by a factor four is observed with a bucking field of 17 Oe. The curved portion of the loops is, with good accuracy, described by a parabolic dependence. This shape is expected from magnetostatic arguments which predict a domain-wall length proportional to (HB±HW)−½, where HB is the applied bucking field and HW is the wall-motion threshold.