Bubble Lattice Motions Due to Modulated Bias Fields
by B. E. Argyle, J. C. Slonczewski, O. Voegeli
We observe 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," "evaporation," 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 −0.05 × 4πMs applied to a lattice of five-μm bubbles produce a net displacement of up to 1.5 μm/pulse at the rim of a lattice 23 bubbles across and 250 μm 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. The mechanism is strong enough to account for the observed magnitude of rotational frequency and can explain its resonant peaks and sign changes.