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Fabrication and Characterization of Nanoscale Periodic Ferromagnetic Structures on Superconducting Thin Films
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This research investigates the transport properties of ferromagnetic nanostructures made by nickel or cobalt on superconducting thin films. The superconducting thin films were covered by an insulating layer, and the periodic ferromagnetic nanostructures were fabricated on top of the insulating layer. On separate superconducting thin films, periodic ferromagnetic nanostructures were embedded into the thin films. The photolithography method was used to make contact pads for a four-probe measurement. In order to fabricate the magnetic nanostructures, an Electron Beam Lithography (EBL) system was utilized. The nanoscale patterns were filled with ferromagnetic materials by using a thermal evaporating method or an electroplating technique. The superconducting properties were measured at low temperature (2.0 K<T<8.0 K) for low temperature superconductors and at high temperature (85.0 K<T<90.0 K) for high temperature superconductors with a Physical Property Measurement System (PPMS). Scanning Electron Microscopy (SEM) and an Atomic Force Microscopy (AFM) were used to characterize the nanoscale structures. Magnetic Force Microscopy (MFM) was also used to study their magnetic properties. Superconductivity and ferromagnetism seems mutually exclusive states, however the superconducting and the ferromagnetic states show new and exciting properties when the two states affect each other at the nanoscale: hysteresis and enhanced superconductivity. Both the ferromagnetic nanostructures on the superconducting thin films and the embedded ferromagnetic nanosized arrays in the superconducting thin films exhibited an enhanced critical current density and critical magnetic field. Both of the samples also showed hysteresis and a field matching effect by the artificial ferromagnetic nanostructures when an external magnetic field was applied. The superconducting thin films with the ferromagnetic nanostructures system may be useful for power cable and other low magnetic field applications.
Bang, Wonbae (2015). Fabrication and Characterization of Nanoscale Periodic Ferromagnetic Structures on Superconducting Thin Films. Doctoral dissertation, Texas A & M University. Available electronically from