Annealing mechanisms in the tristable hysteresis loop

By the use of a suitable display circuit, it has been shown that the B-H loops of magnetically annealed cobalt ferrous ferrite toroids exhibit a re-entrant characteristic in which the field required to initiate flux reversal is appreciably higher than the minimum wall coercive field. From the switching behavior, it was concluded that flux reversal in this material was caused by the motion of a single 180° domain wall. By a single-shot pulse technique, the domain wall may be placed within the toroid at a position corresponding to any desired flux level within the limits of saturation. Annealing in the absence of a field has been found to create a third stable state at this flux level. For these tristable hysteresis loops the field Hsrequired to move the domain wall from its stable position gives a useful measure of its stability. In an attempt to clarify the mechanism involved, the influence of various parameters on Hshas been examined. Dependence on temperature and vacancy concentration has been determined for the rates of both the increase of Hsand its decay on the application of a large driving field. By varying the vacancy concentration, specimens may be prepared in which the decay time for Hsat room temperature varies considerably. The effect on Hsof varying the value in the ferrite composition CoxFe1-x²⁺Fe2³⁺O4has also been determined. A simple model of wall stabilization is suggested to explain these observations.