S. Gnanarajan

A SQUID-based metal detector?comparison to coil and x-ray systems

The presence of foreign metal bodies and fragments in foodstuff and pharmaceutical products is of major concern to producers. Further, hidden metal objects can pose threats to security. In particular, stainless steel is difficult to detect by conventional coil metal detectors due to its low conductivity. We have employed an HTS SQUID magnetometer for the detection of stainless steel particles which is based on the measurement of the remanent magnetization of the particle. Our aim was to determine the detection limits of HTS SQUID-based remote magnetometry, especially for food inspection purposes, and to make a comparison of this technique to commonly used eddy current coil and x-ray inspection systems. We show that the SQUID systems sensitivity to stainless steel fragments is significantly higher than that of coil systems if the samples are magnetized in a 100 mT magnetic field prior to detection. Further, it has a higher sensitivity than x-ray systems, depending on the density distribution of the product under inspection. A 0.6 mg piece of grade-316 stainless steel (a fragment of a hypodermic needle 0.5 mm long and 0.65 mm diameter) represents the detection limit of our system with a 150 × 150 mm2 inspection orifice.

Characterization of MgO substrates for growth of epitaxial YBCO thin films

YBCO films were grown on magnesium oxide (MgO) substrates for fabricating step-edge junction SQUIDs and other Josephson junction-based devices. In-plane 45° grain misorientation was frequently observed in films grown on degraded or contaminated MgO substrates. The appearance of these misoriented grains results in a decrease of the thin-film critical-current density and reduces the device yield. In this work, we investigated the chemical properties of MgO substrates with various surface conditions due to different substrate preparation methods and environmental degradation, by using x-ray photoelectron spectroscopy (XPS). The XPS characteristics of the surface are compared before and after a thermal annealing at 760 °C resembling the thin-film deposition heating cycle. The MgO substrates, after lithographic processing or only weeks of exposure to the laboratory environment, showed surface degradation characterized by the presence of hydroxyl groups, carbonate, and other possible carbon compounds such as bicarbonate, alcohols and carboxyl. Heating of the substrates to 760 °C improves the surface quality to a certain degree with the removal of some of the above contaminants, but is not sufficient to recover the MgO surfaces. A final Ar ion-beam etch cleaning process at low ion energy proved to be very effective in refreshing the MgO substrate surface that had been degraded due to lithographic processing or storage. Films grown on MgO with this pre-treatment showed perfect grain alignment and high critical-current densities.

Highly balanced long-baseline axial gradiometer based on high-T/sub c/ superconducting tape

The improving quality of high-T/sub c/ superconducting (HTS) tape with critical current densities larger than 1 MA/cm/sup 2/ creates the possibility to construct high quality flexible superconducting electronics operating at liquid nitrogen temperatures. We patterned a symmetric flux transformer into a 700 nm thick YBa/sub 2/Cu/sub 3/O/sub 7-x/ film on a 70 /spl mu/m thick, 85 mm long Hastelloy tape. The center loop of the transformer was coupled to the pickup loop of a SQUID magnetometer in a flip-chip configuration. The two outer pickup loops of the transformer were bent such that they were facing each other perpendicular to the magnetometer plane. The resulting axial gradiometer has a long baseline of 35 mm and a gradient sensitivity of 7.3 nT/(cm/spl Phi//sub 0/). A superconducting shield was used to reduce uniform magnetic fields applied perpendicular to the magnetometer plane. Common-mode rejection ratios less than 10/sup -4/ were achieved in the best case. The noise-limited gradient field resolution was approximately 330 fT/(cm/spl radic/Hz) at a frequency of 10 Hz. This resolution was mainly limited by the flux noise level of our dc-SQUID magnetometer of approximately 45 /spl mu//spl Phi//sub 0///spl radic/Hz. In an unshielded laboratory environment, external noise contributions at 50 Hz were reduced by a factor of approximately 10/sup 3/.

Optically transparent and electrically conducting epitaxial Ta2O films

An optically transparent and electrically conducting oxide Ta2O epitaxial thin film has been fabricated and characterized. In our experiments, we grew epitaxial Ta films on r-cut sapphire crystal substrates, which were subsequently oxidized into Ta2O films. The x-ray θ-2θ scan peaks match the American Society for Testing and Materials x-ray powder data card for Ta2O (18-1302) with a cubic structure. The x-ray photoelectron spectroscopic measurements indicated Ta 4f peak energy values between those for Ta and TaO whereas the O∕Ta atomic ratio is between 0.5 and 1. Resistivity-temperature measurements showed a metallic behavior with resistivity values in the range of (1.5–3.3)×10−4Ωcm at 293K. Optical measurements on 25–100nm thick films indicated that they are transparent in the range of 400–2000nm and have an energy band gap between 2.04 and 2.17eV. The Hall mobility and carrier density of the films were in the range of 62–110cm2V−1s−1 and (1.9–3.0)×1020cm−3, respectively.

YBCO/YSZ/hastelloy superconducting tapes by IBAD magnetron deposition

Superconducting YBCO/YSZ/hastelloy tapes were fabricated by depositing epitaxial YBCO films on biaxially aligned YSZ layers on polished hastelloy substrates. YSZ buffer layers were deposited by ion beam assisted magnetron deposition. The degree of biaxial alignment in YSZ and YBCO films was determined by x-ray pole figures and /spl phi/-scans. YSZ layers of different thicknesses reveal biaxial alignment develops at a thickness as small as 100 nm and enhanced biaxial alignment was observed in homoepitaxial YSZ layers. The best YBCO films had biaxial alignment phi scan full width half maximum (FWHM) of /spl sim/ 9/spl deg/. Tapes were fabricated up to 20 cm in length with critical current density of more than 0.5 MA/cm/sup 2/ at 77 K in most parts of the tape. Bending measurements on 3 cm length tapes indicate it retains 80% of the critical current for bending diameter up to 20 mm.

Coexistence of epitaxial Ta(111) and Ta(110) oriented magnetron sputtered thin film on c-cut sapphire

Epitaxial Ta films were grown on c-cut and r-cut sapphire substrates using magnetron sputtering. X-ray diffraction measurements indicate that the Ta films grown on c-cut sapphire substrates have two different crystal orientations. Low vacuum base pressure produced films with the (111) plane parallel to the basal plane after an initial (110) plane growth phase, whereas high base pressure resulted in the growth of (110) plane parallel to the basal plane with three azimuthally oriented domains coexisting. The superconducting critical temperatures Tc of the films grown on c-cut substrates were found to be slightly higher than those for the films grown on r-cut substrates. The x-ray photoelectron spectroscopic data show oxygen content in the high vacuum base pressure deposited films and a difference in the valance band with different Ta surface planes.

Hysteretic behaviour of nanoSQUIDs—prospective application as trapped-vortex memory

We have studied the critical current–magnetic field dependence, Ic(B), of nanoSQUIDs. The vortex penetration field of a Nb/Au bilayer film was determined from the hysteretic behaviour of the nanoSQUIDs Ic(B) pattern. The current–voltage characteristics of Nb/Au bilayer microbridges were also studied. The vortex penetration field was found to be much lower than the Nb single-layer counterpart. These properties suggest that a nanoSQUID may be a potential trapped-vortex memory device, which is discussed.

Transport properties of Nb/yttria-stabilized-zirconia/Nb Josephson junctions

Nb/yttria-stabilized zirconia (YSZ)/Nb Josephson junctions have been fabricated. The YSZ interlayer resistivity was found to have values of /spl sim/1 m/spl Omega/ - m and has a weak temperature dependence. The current-voltage characteristics at temperatures from 4.2 K to values close to the Nb critical temperature of /spl sim/8.0 K have been studied. The junction critical current was found to have an exponential relationship with both the YSZ interlayer length and the temperature. These results are consistent with the predicted behavior of a superconductor-normal metal-superconductor junction due to the proximity effect.