J.D. McCambridge

Pinning and vortex lattice structure in NbTi alloy multilayers

We made thin film multilayers of Nb/sub 0.37/Ti/sub 0.63//Nb and Nb/sub 0.37/Ti/sub 0.63//Ti (d/sub NiTi/=14-27 nm and d/sub N/=4-11 nm) to examine geometries and materials relevant to flux pinning in commercial NbTi conductors. Samples were characterized by transport measurements between 4.2 K and T/sub c/, in magnetic fields nearly parallel to the layers, up to 6 T. For some multilayers, pinning forces had a large peak at intermediate fields whose onset occurred near /spl sim/0.2 H/sub c2/. We suggest this peak effect is caused by a change in the vortex lattice structure, driven by the strong intrinsic pinning. We have measured the highest pinning force density (113 GN/m/sup 3/ at 4.2K and 5 T) ever achieved in the NbTi system.

Multifilamentary NbTi with artificial pinning centers: the effect of alloy, pin material, and geometry on the superconducting properties

Low temperature superconductors containing artificial pinning centers (APC) in NbTi have shown critical current densities (below 4 T) substantially higher than metallurgically fabricated and optimized NbTi superconductors. Peak current densities are achieved when the pinning center spacing is matched to the spacing of the flux line lattice (FLL) at field of operation. It is shown that the choice of the superconducting alloy as well as the pin material has a marked effect on both the characteristic pinning force, F/sub p/, and the critical magnetic field, H/sub c2/. In addition, the design approach or geometry, such as the size of the artificial superconducting filament, the area of the pinning material, and the position of the pins, can be important factors in determining the strength of the pinning force on the FLL.<<ETX>>

Flux pinning in NbTi/Nb multilayers

We made thin film multilayers of NbTi and Nb (d/sub NbTi/=20 nm and d/sub Nb//spl ap/3-9 nm). Samples were characterized by electrical transport measurements between 4.2 K and T/sub c/, in magnetic fields up to 6 T. We present J/sub c/ as a function of the device geometry and orientation of the field. For some multilayers, J/sub c/ had a large peak whose onset occurs near /spl sim/0.2 H/sub c2/. We suggest this peak effect is caused by a softening of the tilt modulus. Measured critical current densities at 4.2 K of 16 kA/mm/sup 2/ at 3 T and 8 kA/mm/sup 2/ at 5 T are among the highest achieved in the NbTi system.<<ETX>>

Experimental results on Nb 25 wt.% Ta 45 wt.% Ti superconducting wire

Several small billets with NbTi binary and Nb 25 wt.% Ta 45 wt.% Ti ternary alloy filaments were made and fabricated into fine wire after subjecting the alloys to different treatment conditions. J/sub c/ results at 4.2 K and 1.8 K from binary and ternary materials, which were prepared under the identical conditions, are compared. The results of an alpha -Ti precipitation study on the two alloys are also presented. The J/sub c/ results are compared with those from some earlier work on a Nb 15 wt.% Ta 44 wt.% Ti material.<<ETX>>

Flux dynamics in submicron superconducting NbTi wires

Abstract We find that the diameter of a superconducting filament plays a major role in the pinning and depinning of flux lines as the diameter is reduced to the submicron scale. We attribute this behavior to a finite-size effect of flux pinning.

YBa2Cu3O7 films grown by metal cosputtering

Superconducting YBa2Cu3O7 films have been grown in situ by simultaneously sputtering from Y, BaCu, and Cu targets. One advantage of such metal cosputtering is the higher deposition rate compared to oxide target sputtering. Another advantage is the ability to control the individual element rates to vary composition or to substitute for any of the metals without interrupting film growth and without making additional composite targets. To prevent film damage due to oxygen ion bombardment during film growth which was observed when the sputter guns faced the substrate (on‐axis sputtering), an off‐axis geometry was used. One disadvantage we found with in situ metal cosputtering was that reproducibility of stoichiometries was difficult because of the presence of oxygen at the targets. To minimize the oxygen partial pressure at the targets during sputtering, the chamber was differentially pumped. Films grown in the off‐axis geometry with a substrate temperature near 700 °C, a chamber pressure of 7.5 mT, and an O2...

Ferromagnetic Artificial Pinning Centers in Multifilamentary Superconducting Wires

We fabricated lnultifilamentary NbTi wires with ferromagnetic (FM) artificial pinning centers (APCs) to enhance the critical current density (Jc) in magnetic fields. We used a bundle and draw technique to process the APC wires with either Ni or Fe as the pinning centers. Both wires produced higher Jc in the high field range (5–9 T) than previous non-magnetic APC wires similarly processed, even though we have not yet optimized pin percentage. Using a magnetometer we found that the pins remained ferromagnetic for the wires with maximum Jc. However, we did observe a substantial loss of FM material for the wires where the pin diameter approached 3 nm. Thus, we expect further enhancement of Jc with better pin quality.