J.S. Satchell

Effect of small changes in composition on the electrical and structural properties of YBa2Cu3O7 thin films

Epitaxial thin films of YBa2Cu3O7 have been grown in situ by evaporation onto (001) MgO substrates. The composition was varied systematically to investigate the effects of changes in Cu content and Ba/Y ratio on the film properties. The results demonstrate that deviations from stoichiometry at the limit of resolution of most analytic techniques can have a large effect on structural and transport properties, as well as causing marked changes in surface morphology. The best properties (Jc≳3×106 A/cm2 at 77 K) are only found for a narrow range of compositions, which can be readily identified from the surface morphology.

YBa2Cu3O7 thin‐film step junctions on MgO substrates

Josephson junctions have been made by the evaporation of epitaxial YBa2Cu3O7 over steps etched in (001) MgO. Such junctions differ from those grown on perovskite substrates in that the c‐axis of the film grows approximately normal to the local substrate surface, so the junction comprises two tilt grain boundaries. Results are reported for junctions formed by this process which showed Josephson currents at temperatures up to 89 K and resistively shunted junctionlike current‐voltage curves. A weak magnetic field applied normal to the substrate gave Fraunhofer patterns indicative of good current uniformity across the junction. The dc SQUIDs show deep modulation, and promising noise performance.

Orientation control of YBa2Cu3O7 thin films on MgO for epitaxial junctions

A low‐temperature argon ion milling process has been used to induce growth of c‐oriented YBa2Cu3O7 on (001) MgO substrates with the in‐plane film axes rotated 45° with respect to those of the substrate, and with respect to those of films grown on untreated regions of the substrate. This process is compatible with standard photolithographic processing, and has been used to produce defined areas of the two orientations on the same substrate. Josephson junctions and dc SQUIDs have been fabricated using the resulting grain boundaries.

Low‐noise YBa2Cu3O7 ‐PrBa2Cu3O7 multiturn flux transformers

Epitaxial thin‐film YBa2Cu3O7 flux transformers with multiturn input coils have been constructed on 1 cm2 MgO substrates using PrBa2Cu3O7 as an insulator. By coupling these to high‐temperature superconducting quantum interference devices (SQUIDs) using a flip‐chip technology, several low‐noise magnetometers and a gradiometer have been constructed. The yield of fully working flux transformers on 1 cm2 substrates exceeds 80%, and field sensitivities at 77 K of (520±30)fT/√Hz at 1 Hz and (165±8)fT/√Hz above 10 Hz have been achieved. No evidence for excess low‐frequency noise above that of the uncoupled SQUID has been found in the flip‐chip magnetometers.

Narrow tracks in YBa2Cu3O7 thin films defined by laser ablation

The use of cw laser ablation for defining narrow tracks in YBa2Cu3O7 thin films on (001) SrTiO3 substrates is described. Critical current densities have been observed up to 5×105 A/cm2 in 4‐μm‐wide tracks at 77 K. For these films, it is found that the value of critical current density calculated is insensitive to the length of track and the voltage criterion used.

OPTIMISATION OF YBa2Cu3O7 THIN FILMS FOR MULTILAYERS

Abstract Thin films of YBa2Cu3O7 have been grown “in situ” using e-beam evaporation. The electrical and structural properties of the films have been correlated with composition, oxygen environment, temperature and substrate type and orientation. When the composition is controlled with sufficient precision ( 90K, Jc > 3 106 A/cm2 at 77K) and smooth surfaces are reproducibly obtained. Fully epitaxial SIS trilayers of YBa 2 Cu 3 O 7 Y 2 O 3 have been grown.

Critical currents and rapid measurements of magnetic relaxation in superconducting thin films

Abstract A fast DC magnetisation technique using a Hall effect probe is described. It is used for measuring thin film critical currents and their relaxation as functions of temperature. At fixed temperature it has been used to take current relaxation data over 6 decades of timescale in under 3 h, much faster than standard magnetisation techniques. Using a thin film ring geometry we deduce E − J characteristics extending down to 10 −14 V/cm. Magnetic relaxation measurements made on over 300 films consistently reveal a sharp divergence near to T c . At lower temperatures the relaxation rate is found to vary with film growth temperature and annealing conditions. The effect of field line curvature has been measured and its significance for our results is discussed.

End point detection in ion beam milling of YBa2Cu3O7 thin films

In the processing of high temperature superconductor thin films into devices, the use of calibrated milling rates has proved unreliable as a means of determining the milled depth. This problem has been solved using in situ secondary ion mass spectrometry for identifying interfaces in multilayers based on YBa2Cu3O7. The depth resolution obtained during routine patterning of typical multilayers was ∼4 nm. Accurate termination of milling at an insulator/superconductor interface has been demonstrated. The presence of water vapor during milling has been shown to affect the proportions of the different secondary ion species, but has only a slight affect on the milling rate.

High-temperature superconducting gradiometric SQUIDs and flux transformers

Abstract Gradiometric DC superconducting quantum interference devices (SQUIDs) have been constructed from thin-film YBa 2 Cu 3 O 7− x on MgO substrates. The SQUIDs were fabricated as forst-order gradiometers with PrBa 2 Cu 3 O 7− x insulator providing electrical isolation at the crossovers. Highly symmetrical first-order gradiometer flux transformers have also been fabricated. These couple to the SQUIDs and enhance the sensitivity to magnetic-field gradients. Each flux transformer has two 20 turn flat spiral SQUID input coils and a pair of larger-area balanced pickup coils. Flux transformers with both parallel and series configured pickup loops have been fabricated. When coupled to a SQUID as a flip chip, one gradiometer was operated at 77 K with a gradient sensitivity of (0.79±0.03) nT/m/ √Hz at 10 Hz and (2.0±0.1) nT/m/ √Hz at 1 Hz.

Strongly coupled high Tc edge junctions

Abstract We report the fabrication of high T c edge junctions with high critical current densities. They carry Josephson currents up to 83.5K, just below the T c of the electrodes. The magnetic field dependence of the critical current shows conventional sinc function like modulation just below T c . At lower temperatures the junctions are so strongly coupled that they are large compared to the Josephson penetration depth, and show the long junction behaviour seen previously in low T c superconductors.