We have fabricated step edge junctions using MgO substrates and YBCO thin films. By varying the angle of the step edge over a range of angles up to 45", we have obtained 3 distinct step edge morphologies: a deep trench junction, a double junction and a single junction. We found that only the step angle and morphology affected the critical current density (I,) and that the film thickness-tostep height ratio had no effect over the range 0.2-1.1. Noise measurements indicated that the single junction steps had the lowest level of critical current fluctuations and the highest values of dynamic resistance. We have also studied the variation of I, with temperature and found it follows the Ambergaokar-Baratoff model with a lower zero energy gap. We use this information to confirm that the junction parameters are affected by the c-axis tilt and the in-plane orientations proposed by others and consider the transport mechanisms across the junction.
Since December 1992, CSIRO and BHP have been field trialing rf HTS SQUID magnetometers for mineral prospecting applications. Ten field trials in widely varying environments (from -16°C to +4OoC ambient temperatures) in mostly remote locations saw the development of a system which can be operated in many configurations including ground based and airborne Transient ElectroMagnetics (TEM). The magnetometer system has been developed to a point where, at late times in TEM applications, the SQUID system has a higher signal-to-noise level than the competing traditional coil technology. In some trials, a SQUID magnetometer detected anomalies at later times than were observed with the coil system, indicating an enhanced ability to detect highly conductive targets. This paper reviews development of our 3-axis SQUID magnetometer. SQUID systems as B field sensors have advantages over coils which are dB/dt type sensors. We will discuss the importance of these advantages for mineral prospecting in regions with a conducting soil cover or overburden typical of the Australian landscape.
The use of a selective epitaxial growth technique for fabricating YBCO thin-film microstructures is described. No film post-deposition processing is required; hence damage to the structure is minimized. The technique is compatible with a passivation process to protect the structure without exposure to air. The microbridges, Josephson junctions and rf SQUIDs protected by an amorphous YBCO passivation have long lifetime even after severe accelerated aging tests. Rf SQUIDs fabricated by this technique show a significant reduction of low-frequency noise when operating in weak magnetic fields compared with SQUIDs fabricated by the conventional ion beam etching technique.
High-temperature superconducting YBa2Cu3O7- microbridges and step-edge junctions (SEJs) were coated with SiO2 thin film by an rf magnetron sputtering technique. The effect of the coating on critical temperature Tc and critical current Ic was studied. It was found that Tc and Ic of microbridges increased as a result of the coating, by up to 3 K for Tc and up to 23% for Ic. We believe that this is due to the oxygen in the plasma penetrating into YBa2Cu3O7- resulting in an increase in oxygen content and restoration of the degraded microbridge edges. Ic of the SEJ, however, was found to decrease by 20-30% after deposition of SiO2 films despite no degradation in Tc. The results of ageing tests showed that SiO2 film forms a good passivation layer, protecting YBa2Cu3O7- film from reacting with water.
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