Thin film multilayer superconductors and the proximity effect Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r781wj95j

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  • Recent superconducting thin films studies have attempted to create pure metal layer films of Niobium and Titanium with the same properties of superconducting NbTi wire used in industry. These studies have all reported depression of the superconducting properties of the pure metal films which has been attributed to the proximity effect. The purpose of this research project was to construct several NbTi films composed of alloyed layers to overcome the proximity depression of superconducting properties. These films are unique in that they are the only films with both alloyed Nb/Ti superconducting and normal layers reported in the literature. Films with several different compositions and bilayer geometries were designed, constructed and their superconducting properties characterized. The films were created by the RF sputtering of alloyed targets at ambient temperatures. Characterization of the composition of the films was performed by microprobe analysis at two different electron beam voltages. A simulation of the electron beam excitation volume of the microprobe was performed to determine the difference in the two analyses and to determine whether the substrate would be found upon microprobe examination. X-ray diffraction was utilized to determine the bilayer spacing and to give a qualitative understanding of the alignment of the film microstructures. The critical temperature and upper critical magnetic field were measured to determine the superconducting properties and the extent of the proximity effect in the films. The microprobe analysis found most films to be very pure alloys except for films 6001, 6002, 6003(1) in which there were between 1-8 wt% of impurities. All films contained compositional variations on the order of 10 wt% from the design values. X-ray diffraction indicated agreement with the designed bilayer spacing in all films but 6003(2), 6005(1), and 6005(2) which had bilayer periods larger than originally designed. The proximity effect was not observed in any of the film's superconducting properties examined. T[subscript]c and H[subscript]c₂ properties for the films without impurities had properties equal to that of bulk Nb/47wt% Ti. Films 6001, 6003(1) had depressed T[subscript]c and H[subscript]c₂ values which were attributed to their impurity contamination. The low Ti composition in many of the films points to the inaccuracy of the deposition parameters when the films were first processed. The lack of T[subscript]c and H[subscript]c₂ depression normally seen in other film studies with bilayer periods between 10-30 nm demonstrates that alloyed layers should be used to overcome the proximity effect in multilayer thin film superconductor studies. Overcoming the proximity effect should translate into a better understanding of flux pinning mechanisms in the material and increased superconducting critical currents in these films.
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