Magnetization processes in permalloy overlays on magnetic bubble devices

Using colloid techniques domain structure has been studied in several overlay components used in contemporary bubble devices. In isolated elements the demagnetized state is generally simple, containing a small number of domains. The influence of anisotropy on domain structure is demonstrated. Elements initially respond to applied fields by reversible domain boundary movement but in each case it has been found that partial saturation and hysteresis occur once the applied field exceeds a critical value, H(_s). This causes the formation of remanent states with 'magnetization buckling' similar to that found in larger samples of thin-film permalloy. The relationship between and element geometry and thickness and the formation of buckled states by a rotating field were investigated. Such states may adversely affect the operation of a bubble device. The approach to partial saturation in a simple bar has been modelled on the basis of a curved domain wall and approximate values for the saturation field calculated. The external field profile of the bar has also been obtained. Domain structure in various connected chevron columns (bubble detectors) was also studied. In contrast to isolated elements the initial 'zero-field' state in these components is generally one of saturation. This state can be broken by components of applied field parallel or perpendicular to the column and again magnetization buckling is involved. Magnetoresistance changes related to the buckled state were measured and found to be consistent with the colloid observations. These observations can be used to explain the characteristic magnetoresistance signal of a chevron column in a rotating field.