Relationship of bias field setting procedure to field stability against external field perturbations for magnetic bubble memory bias field structures

In magnetic bubble memory packages having a bias field HBsupplied by a Ba-ferrite permanent magnet structure, the magnitude of HBmay be appreciably altered after setting by transient exposure to externally applied magnetic fields Hextas much as 10 times smaller than the field used in setting HB. We examine this effect for a particular magnet design having a permalloy yoke, a gap of 0.270 inches, and a saturation field Oe. We find that the magnitude of the effect depends upon the ratio and upon the procedure used in setting HB. After setting to Oe from saturation with a demagnetizing field Oe, a remagnetizing field Oe is required to increase HBby 1%. On the other hand, after setting to Oe with Oe, a remagnetizing field Oe is sufficient to increase HBby 1%. Setting by demagnetizing from saturation yields superior stability to setting by magnetizing from the demagnetized state, and stability of the set magnet may be further improved by demagnetizing with a ringing (alternating) field. This behavior is explained with a simple model and its importance for magnet design is discussed.