Magnetic Imaging of Micrometer and Nanometer-size Magnetic Structures and Their Flux-Pinning Effects on Superconducting Thin Films

In this work the interactions between neighboring superconducting thin film and ferromagnetic structures, i.e. superconductor-ferromagnet hybrid systems, were studied. A type-II superconducting thin film (Pb82Bi12), was deposited in close proximity to various ferromagnetic structures. These magnetic structures include: (i) alternating iron-brass shims of 275 mu m period, (ii) an array of 4 mu m wide Co stripes with smaller period (9 mu m), (iii) a square array of 50nm diameter, high aspect ratio (5-7) Ni rods with 250nm period. Measurements of critical transport current (IC), resistance (RH(T)) and second critical field (HC2) are reported. A variety of novel effects (enhancement of (IC) and (HC2), matching field effect, field compensation effect, and large hysteresis) are also reported. Using measurements on thin superconducting films atop a Co stripe array with a 9 mu m period, a superconductor-ferromagnet hybrid device (a mechanical superconducting persistent switch) is proposed. In addition, scanning Hall probe microscopy (SHPM) and other imaging techniques were used to characterize the magnetic properties of the systems mentioned. The SHPM was also used to acquire B-H and M-H curves. An additional sharp magnetic needle and electromagnetic coil assembly intended for micromanipulation of small magnetic particles and individual cells was also characterized.