A. P. Malozemoff

Mobility of bubbles with small numbers of Bloch lines

The translational and radial mobilities of bubbles with small numbers of Bloch lines are investigated in a garnet film with moderately low damping. Depending on the number of Bloch lines, the bubbles propagate at different angles in an applied field gradient, in agreement with previous theory. However, the dependence of translational speed on Bloch line number, though weak, is an order of magnitude larger than predicted unless one attributes an additional coercivity or viscous damping to each Bloch line. Bubbles whose states have been identified by propagation in a gradient field are investigated by the dynamic bubble collapse technique; the ones which deflect at a particular nonzero angle show the largest dynamic collapse velocities. This experiment gives evidence that these bubbles, rather than those showing no deflection, are the ones without any Bloch lines. Such bubbles are investigated over a large range of radial drive fields; they show an initial high‐mobility region culminating in a sharp peak, f...

Effect of misorientation on growth anisotropy in [111]‐oriented garnet films

Phenomenological anisotropy constants have been measured in three garnet bubble films with their surface normal slightly misoriented from the crystallographic [111] direction. A tilted magnetic easy axis is found from measurements of homogeneous nucleation and bubble collapse in the presence of in‐plane fields. An in‐plane anisotropy is also found whose axes do not in general lie either normal to or in the easy‐axis tilt plane. These results are shown to be consistent with a phenomenological growth anisotropy model which includes the film plane misorientation.

Detection of stored momentum in magnetic bubbles by a bias jump effect

A pulse of uniform bias field is applied to an isolated ∼5‐μm bubble in a garnet film. The bubble (S=0) was previously propagated by a strong gradient‐field pulse. The bubble is observed to jump forward in the same direction as the previous gradient propagation, irrespective of the sign of the bias‐field pulse. Subsequent bias pulses may cause further ’’bias jumps’’ of this type until a maximum displacement is reached, which can be many μm in typical cases. Dependence of the bias jumps on the strength and length of the bias‐field pulses and also of the previous gradient‐field pulses is reported. The shape of the bubbles, observed by high‐speed photography during the bias pulse, is elliptical, indicating lower mobility on the sides of the bubble perpendicular to the over‐all direction of motion. These results provide evidence for the existence of unwinding Bloch‐line pairs which remain in the bubble wall at the end of a gradient propagation. These Bloch lines are shown to be tantamount to a stored bubble m...

Brillouin scattering from rapidly quenched ferromagnetic metallic glasses

We report on Brillouin‐scattering measurements of thermally excited surface acoustic phonons, and bulk and surface magnons, in quenched ribbons of ferromagnetic metallic glasses with compositions Fe100−xBx (x=14–28), Fe76Mo4B20, FexNi80−xB20 (x=20–60), and Co75−xFexSi15B10 (x=0–6). Room‐temperature surface‐phonon velocity is found to increase monotonically with increasing B in FeB, and with increasing Fe in FeNiB. The velocity is almost 10% less in Fe76Mo4B20 than in Fe80B20. The phonon velocity is not field dependent, even in compositions where large ΔE effects occur. The g factor, determined from the slope of surface‐magnon frequency with field, is 2.09±0.02 for Fe100−xBx. The intensity of surface‐ and bulk‐magnon scattering is independent of magnetostriction in CoFeSiB.We report on Brillouin‐scattering measurements of thermally excited surface acoustic phonons, and bulk and surface magnons, in quenched ribbons of ferromagnetic metallic glasses with compositions Fe100−xBx (x=14–28), Fe76Mo4B20, FexNi80−xB20 (x=20–60), and Co75−xFexSi15B10 (x=0–6). Room‐temperature surface‐phonon velocity is found to increase monotonically with increasing B in FeB, and with increasing Fe in FeNiB. The velocity is almost 10% less in Fe76Mo4B20 than in Fe80B20. The phonon velocity is not field dependent, even in compositions where large ΔE effects occur. The g factor, determined from the slope of surface‐magnon frequency with field, is 2.09±0.02 for Fe100−xBx. The intensity of surface‐ and bulk‐magnon scattering is independent of magnetostriction in CoFeSiB.

Bias field dependence of domain drag propagation velocities in GdCoAu bubble films

The bias field dependence of the velocity of bubble propagation by the domain drag effect was studied in a GdCoAu amorphous film. Bubbles in close‐packed arrays were propagated by pulsing currents directly through a stripline of the material fabricated by photolithography and ion‐beam milling. As bias field is increased, at given current and pulse width, the velocity decreases, passes through a minimum, then increases again. The domain drag drive field at a given bias is derived both from the critical current at which motion begins and from the slope of velocity with current. A theory for the domain drag drive field Hd of a bubble raft with infinite length along the current direction is derived, giving the simple result Hd /βjD = 1/2(d0/D) −(π/4√3)(d0/D)3, where β is the Hall angle, j is the current density, d0 is the bubble diameter, and D is the nearest‐neighbor spacing. Experimental results from critical current agree approximately with the theory, but results from velocity slope with current are a fac...

The anisotropy dependence of bubble dynamics in EuGaYIG films

We have studied the dynamic properties of bubbles in a series of oxygen‐annealed EuGaYIG films which have similar magnetic properties except for decreasing uniaxial anisotropy as a function of anneal time. The translational saturation velocity and skew angle of bubbles translated in a pulsed gradient field experiment increase as the anisotropy decreases. These results are in agreement with theoretical predictions of a Ku−1/2 dependence. We also observe that for a given anisotropy and bubble state (S=1), the skew angle decreases as the drive increases, in agreement with theory which predicts that if the velocity is saturated, drive and angle are inversely related.

Bloch‐line rotation instability during gradient propagation of S=0 bubbles in an in‐plane field

Recent data by DeLuca and Malozemoff on the gradient propagation of S=0 bubbles in an in‐plane field show two peaks in the velocity‐vs‐drive curve. In this paper the higher‐drive peak is accounted for in terms of the well‐known horizontal Bloch‐line nucleation and punch‐through effect, while the lower‐drive peak is interpreted as a novel Bloch‐line rotation instability. Such an instability can arise in bubbles with two Bloch lines pinned by the in‐plane field Hp at opposite sides of the bubble. When the bubble is propagated perpendicular to the in‐plane field, one of the Bloch lines is gyrotropically destabilized and can rotate halfway around the perimeter of the bubble. The critical drive field and velocity for this process are calculated to be αHpf and αμHpf, respectively, where α is the Gilbert damping parameter, μ is the linear mobility, and f is a factor between 0.63 and 1. As the Bloch line rotates, the velocity is predicted to drop to a minimum of order 8αμh−1 (2A/π)1/2, where A is the exchange sti...

The Effects of in‐Plane Fields on Ballistic Overshoot in the Gradient Propagation of Magnetic Bubble Domains

Using high‐speed photography we have studied the effects of an applied in‐plane field Hp on the velocity and ballistic overshoot of S = 0 bubbles propagated by a pulsed field gradient in EuGa, GdTmGa, and SmCaGe garnet films. Peaks appear in the velocity versus drive curve. These peaks shift to higher drives with increasing in‐plane field. At drives below the peaks the velocity is roughly linear with drive with very few state changes and little or no ballistic overshoot. At drives above the peaks the velocity is saturated at a value which increases with in‐plane field. In this range ballistic overshoot and state changes occur quite often, indicating that Bloch lines and Bloch points can be nucleated.

Spin resonance and magnetic susceptibility of amorphous GdxY0.33−xAl0.67 films showing spin‐glass behavior

The effects of dilution of Gd by Y in a series of evaporated amorphous GdxY0.33−xAl0.67 films (x from 0.33 to 0.0027) has been studied by microwave magnetic resonance and SQUID magnetometry. The SQUID measurements show a spin glass freezing temperature Tf of 13.5K for x = 0.33 and 6.5K for x = 0.16, with Tf being less than 1.6K or nonexistent for samples with x<0.04. X‐band microwave measurements reveal a line broadening ΔH and an increasing g‐shift as the temperature T is lowered. These low temperature effects are interpreted in terms of a theory by Salamon which permits determination of the spin‐spin relaxation time τ∝xm/(T−Tf)n. Contrary to earlier experimental or theoretical indications, we find m∼−0.5 and n∼1.5.

Elliptical distortion mechanism for ballistic overshoot of bubbles

A theory for ballistic overshoot of gradient propagated magnetic bubbles is described. The overshoot arises from the relaxation of transient elliptical distortion and is predicted to be of order nμγ−1r−10(r10+r20) where n is the number of vertical Bloch lines on the flanks of the bubble, μ the linear mobility, γ the gyromagnetic ratio, r0 the static bubble radius, r10 the change in average bubble radius at the end of the drive and r20 the elliptical distortion at the end of the drive. If one assumes all Bloch lines punch through to form vertical Bloch lines, the theory can account within 50 percent for the observed ballistic overshoot of many as‐grown films as a function of gradient pulse time,strength and bias compensation. Furthermore new data on the detailed time dependence of bubble behavior during overshoot shows a rough correlation between displacement and elliptical distortion.