Collective transport and phase transitions in two-dimensional magnetic bubble arrays

This thesis presents an experimental investigation of transport phenomena and phase transitions in two-dimensional magnetic bubble arrays. Magnetic bubbles are cylindrical domains in an iron garnet, in which substrate roughness acts as a weak random pinning potential. Artificial pinning is introduced to the system using a lithographically patterned permalloy overlayer. The bubble concentration is controlled by a perpendicular dc magnetic field. Temperature is simulated with an ac field agitation. The system is directly observed using polarization microscopy and computer-video techniques.; We have studied the collective transport of magnetic bubble arrays subject to random substrate pinning. The bubble arrays are triangular arrays with quasi-long-range orientational order and short-range translational order. The bubbles are driven by a gradient dc magnetic field. The velocity-force responses follow power laws v {dollar}propto{dollar} F{dollar}sp{lcub}alpha{rcub}{dollar}, with {dollar}alpha cong{dollar} 3 for the measured ac field range. Spatiotemporally resolved bubble array images and local velocity distributions reveal the collective nature of bubble transport: the array moves forward by correlated clusters of bubbles, and although the instantaneous individual velocity fluctuates, the spatiotemporal averaged velocity remains constant. The measured velocity-velocity correlation length {dollar}xisb{lcub}rm v{rcub}{dollar} increases with the applied force F.; We have also studied the commensurate-incommensurate transitions in magnetic bubble arrays subject to periodic line pinning. Bubble array images and structure function analyses show that the commensurate-incommensurate transitions take place by rotation of array orientations as the bubble concentration decreases. Smooth migration of Bragg peaks in structure function plots indicates a continuous second (or weakly first) order transition. We have identified the commensurate and incommensurate phases that have appeared during the phase transitions, including an incommensurate polycrystalline phase and a commensurate smectic phase. Symmetry breaking phase occurs when the pinning periodicity becomes smaller than the bubble row spacing, as expected from theory.