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 H
B
supplied by a Ba-ferrite permanent magnet structure, the magnitude of H
B
may be appreciably altered after setting by transient exposure to externally applied magnetic fields H
ext
as much as 10 times smaller than the field used in setting H
B
. We examine this effect for a particular magnet design having a permalloy yoke, a gap of 0.270 inches, and a saturation field
H_{sat} \simeq 240
Oe. We find that the magnitude of the effect depends upon the ratio
H_{B}/H_{sat}
and upon the procedure used in setting H
B
. After setting to
H_{B} = 200
Oe from saturation with a demagnetizing field
H_{ext} = -1400
Oe, a remagnetizing field
H_{ext} \simeq 1800
Oe is required to increase H
B
by 1%. On the other hand, after setting to
H_{B} = 100
Oe with
H_{ext} = -2300
Oe, a remagnetizing field
H_{ext} \simeq 400
Oe is sufficient to increase H
B
by 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.