Wall oscillation and overdamped motion in magnetic bubble domains

Abstract

Bubble-wall oscillation, in response to a step change in bias field, is reported in four bubble garnet samples with different compositions and characteristics. The wall velocity during oscillatory motion is found to be independent of the instantaneous drive on the domain wall, but does depend on the initial drive on the wall. The experimental relationship between the wall velocity vs and the initial drive Ha is given by vs=0.86 m/sec+(2.4 m/sec Oe) Ha for expanding motion and by vs =1.6+2.6Ha for collapsing motion. The half-period of oscillation, typically between 100 and 300 nsec, increases linearly with initial drive for low drive fields (∼1 Oe) and approaches a constant value at higher fields (≳2 Oe). Wall motion is analyzed in terms of Slonczewski’s formulation of wall mechanics and a velocity-momentum relationship of the form ?=a+b? is found to agree with the observed motion for a=3.2 m/sec and b=1.5 m/sec. Underdamped motion is observed in two ranges of initial drive field separated by a region in which wall motion is overdamped. The lower boundary between underdamped and overdamped motion, near a drive field of 2.3 Oe, is sharply defined, while the upper boundary, near 3.5 Oe, represents a more gradual transition. The transition to overdamped motion is accompanied by a sharp drop in effective domain-wall mobility, and the mobility returns to its original value in the second underdamped region. A comparison between theoretical and experimental parameters indicates that wall oscillation is most likely to be observed in materials with saturation velocities [v0=7.1γA/ h (K)1/2] greater than 10 m/sec and with resonance-damping parameters less than 0.04.