The effect of a hard axis bias field on the switching field in single layer and bi-layer thin permalloy films

A sensitive method for the measurement of individual Barkhausen events has been developed which enables the reconstruction of hysteresis loops to be performed with great accuracy. The examination of such loops reveals that for Bloch wall transitions a hard axis bias field, H_b , has little effect until H_b>0.5 H_k. At this point, the Bloch walls give way to Neel type transitions, which are energetically more favourable. There is a decrease in the field required to cause nucleation and the loop narrows. A model of the Neel wall energy shows a linear decrease in energy as a bias field is applied. In this case, the wall energy is proportional to the field required to cause rotation in the plane of the material, and thus a linear decrease in switching field with bias field is also observed. Where Neel walls provide the rotation mechanism the loop is also observed to shear towards the saturation direction, causing a decrease in the permeability of the material. This is thought to be due to an increase in the wall stiffness, i.e. a reluctance for the magnetisation in the centre of the walls to rotate away from the bias field direction