BUBBLE LATTICE MOTIONS DUE TO MODULATED BIAS FIELDS

This paper investigates collective bubble motions that occur in low-damping garnet films by excitation involving homogeneous or nearly homogeneous fields, e. g. , bias-field modulations. Their gradients, if significant, are in any case orthogonal to the motion studied. These various motions belong to a new phenomenon called bubble automotion because the self-propulsion results from coupling of the principal translational degree of freedom of the bubble either with the internal degrees or with the orthogonal component of translation. It is observed that periodic variations of bias field can couple to a close-packed lattice of magnetic bubbles to produce a steady rotation of the bubble lattice (RBL). Pulsed fields excite various other many-body phases as well. The physical motions of such bubble arrays can be described by ″lattice melting″ , and ″rotating galaxies″ . The RBL phase is stable over wide ranges of pulse width and amplitude when the film is thick and the lattice is confined either by a circular ion-milled groove or by radially symmetric inhomogeneous fields from the excitation coil itself. Microsecond pulsed fields of minus 0. 05 multiplied by 4 pi M//s applied to a lattice of five- mu m bubbles produce a net displacement of up to 1. 5 mu m/pulse at the rim of a lattice 23 bubbles across and 250 mu m in diameter. Sinusoidal bias modulation in the range 1 to 30 MHz produces a spectrum of lattice rotational velocities vs frequency having both signs. At frequencies near the low end of the spectrum both the magnitude and the sign of the rotation are sensitive to drive amplitude. A tentative theory attributes lattice rotation to nonlinearities involving the bubble-deflection effect.