THE EFFECT OF A HARD AXIS BIAS FIELD ON THE SWITCHING FIELD IN SINGLE-LAYER AND BILAYER THIN PERMALLOY-FILMS

A sensitive method for the measurement of individual Barkhausen events has been developed which enables the reconstruction of hysteresis loo ps to be performed with great accuracy. The examination of such loops reveals that for Bloch wall transitions a hard axis bias field, H-b, h as little effect until Hb > 0.5 H-k. At this point, the Bloch walls gi ve way to Neel type transitions, which are energetically more favourab le. 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 decre ase in energy as a bias field is applied. In this case, the wall energ y is proportional to the field required to cause rotation in the plane of the material, and thus a linear decrease in switching field with b ias field is also observed. Where Neel walls provide the rotation mech anism the loop is also observed to shear towards the saturation direct ion, causing a decrease in the permeability of the material. This is t hought to be due to an increase in the wall stiffness; i.e.; a relucta nce for the magnetisation in the centre of the walls to rotate away fr om the bias field direction.