S. Wunderlich

Planar gradiometers with high- DC SQUIDs for non-destructive testing

We investigated planar gradiometers on the basis of galvanically coupled high- DC SQUIDs for application in non-destructive testing (NDT). The stability of the sensor of better than 1% permits NDT investigations in unshielded environments. The layout of the gradiometer sensors and a corresponding determination of their effective areas and baselines will be discussed as well as the properties of the Josephson junctions in the DC SQUID based on step-edge and bicrystal grain boundaries. We show measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as well as the influence of sensor position and experimental environment such as the dewar material on the performance of the sensor and the whole testing system. As an example first investigations of the permanent magnetization of hardened valves will be presented where we look for a correlation between hardening and magnetic field distribution.

High-Tc SQUID systems for practical use

Planar thin film DC-SQUID gradiometers with small base length (4...6 mm) were optimized for application in different measurement systems. The field gradient noise of these planar DC-SQUID gradiometers in unshielded environment is better than 5 pT/cm/spl radic/(Hz) (at 1 Hz, 77 K). Other components of these systems like electronics, cryostats, scanning equipment, and data analysis software are discussed. An industrial system for nondestructive evaluation and a clinical system for bedside investigations on cardiac infarction are demonstrated as examples of starting cooperation between university and small companies interested in application of superconductivity.

Improvement of sensor performance of high-T/sub C/ thin film planar SQUID gradiometers by ion beam etching

The sensor performance of galvanically coupled Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) dc SQUID gradiometers on 24/spl deg/ bicrystal substrates has been improved by thickness reduction in the region of the grain boundary Josephson junctions using ion beam etching. The prepared etching mask allows the reduction of the critical current by more than one order of magnitude while the SQUID inductance is slightly increased. This treatment shifts the SQUID parameter /spl beta//sub L/ from values above 10 to the proposed optimum around 1. The authors observed with decreasing critical current and increasing normal resistance a reduced I/sub C/R/sub N/ product with values between 300 and 400 /spl mu/V at 150-nm film thickness changing to values near 150 /spl mu/V at 50-nm film thickness. Despite this fact, the white flux noise level as well as the low-frequency noise is reduced. With their galvanically coupled 4/spl times/8 mm/sup 2/ dc SQUID gradiometer the authors obtained a white noise level of 4.2 /spl mu//spl Phi//sub 0///spl radic/Hz corresponding to a field gradient sensitivity of 430 fT/cm/spl radic/Hz at 77 K after the trimming process.

High-T/sub c/ DC-SQUID system for nondestructive evaluation

We developed different types of thin film high temperature superconductor (high-T/sub c/) DC-SQUID magnetometers and gradiometers for application in a nondestructive testing system. We used these sensors in a liquid nitrogen dewar mounted above a computer controlled x-y table within a compact /spl mu/-metal shielded chamber. The planar galvanically coupled gradiometers based on step-edge or bicrystal Josephson junctions were investigated in shielded and unshielded environment. We discuss the influence of junction type and SQUID parameters depending on the device geometry on the sensitivity of our complete testing system. Our interest is focused in particular on the noise properties and magnetic field resolution at 77 K. Measurements of ac and dc current distributions will be shown as well as measurements of magnetic moments of different samples.

Development of a heart monitoring system based on thin film high-T/sub c/ DC-SQUIDs

Our aim is to built up an one-channel heart monitoring system to achieve fast and reliable information about the patients condition as well in emergency cases as in serial examinations by a non-invasive and touchless method. Step-edge and bicrystal junctions were used to prepare planar galvanically coupled thin film gradiometers and magnetometers. These sensors were used to carry out measurements in well-shielded and weak disturbed environment and we compared the different noise properties and field gradient resolutions. The influence of antenna configuration, SQUID parameter, cooling conditions, and different kinds of signal analysis will be discussed.

Improvement of spatial and field resolution in NDE systems using superconducting sensors

We present a NDE scanning system for industrial purposes in magnetically and electrically unshielded environment. In this scanning system, planar galvanically coupled dc SQUID gradiometers with a field gradient resolution of 300 fT.(cmHz/sup 1/2 /) in the white noise region are used. The spatial resolution is above 1 mm in this case. On the other hand many applications in NDE do not require the extraordinary sensitivity of SQUID based sensors. For those purposes hybrid sensors, in which Hall sensors are connected with high-T/sub c/ superconducting antennas, are an alternative sensor concept less sensitive than SQUID sensors but exhibiting better performance compared to other commercially available magnetic field sensors.

Development of planar thin film HTSC–SQUID gradiometers for different applications

Abstract We investigated the main dependencies of the gradient resolution of planar galvanometer superconducting quantum interference device (SQUID) gradiometers made of YBa 2 Cu 3 O 7-x (YBCO). We focussed especially on the influence of antenna layout and the parameters of the galvanometer SQUID such as effective and parasitic areas on the performance of the gradiometer and the behavior in an unshielded environment. The efficiency (that is, the quotient of effective area to inductance) of different geometries of antennas will be compared. Some aspects of the layout of the galvanometer SQUIDs are discussed in terms of parasitic area and best current resolution. Special problems due to the use of bicrystal Josephson junctions in gradiometers for operation in highly disturbed environment are shown. Step-edge Josephson junctions can offer alternative concepts. Reached gradient sensitivity values in the white noise region are 0.46 pT/(cm/ H z) in the case of bicrystal junctions and 0.69 pT/(cm/ H z) for step-edge junctions.

Superconducting sensors for weak magnetic signals in combination with BiCMOS electronics at 77 K for different applications

Different sensors for weak magnetic signals were realized using thin high temperature superconducting films on different substrates including buffered silicon. Superconducting quantum interference devices (SQUIDs), magnetometers and planar gradiometers based on them as well as a new type of Hall-effect sensor with a superconducting antenna were tested with respect to signal resolution, band width and spatial resolution. To realize adapted systems for biomagnetic research or non-destructive evaluation common room temperature electronics have some disadvantages. Thus we tested discrete elements as well as special adapted integrated BiCMOS circuits placed near the sensors at 77 K. We demonstrate the status of the development of such electronics.

Application of Ion Beam Etching to Investigate the Electrical Behaviour of Josephson Junctions

The investigation of the spatial current distribution in high-Tc thin filin Josephson junctions is a key problem in understanding their electrical behaviour from the point of view of physics as well as for using them in practical applications for example in dc-SQUIDs which became suitable for a wide range of measurement tasks such as nondestructive material testing or in biomagnetism. We prepared different types of Josephson junctions based on YBa2Cu3O7–x (YBCO) thin films on SrTiO3 substrates like step-edge junctions, bicrystal junctions, and ion beam modified microbridges. A special optimized ion beam etching (IBE) process was not only used to prepare the superconducting structures but also to investigate the dependence of electrical junction behaviour on filin thickness by means of thinning the junctions thickness. In this way the ion beam thinning process was used to modify Josephson junctions and especially dc-SQUIDs after their preparation to adjust then to special working conditions.

High-temperature superconducting devices on buffered silicon substrates

The use of silicon as substrate for thin film devices based on high temperature superconducting oxides requires additional buffer layers to prevent interdiffusion, lattice mismatch, and internal stress by different thermal expansion coefficients. We tested different materials like yttrium-stabilized zirconia (YSZ), CeO2, and CoSi2. Laser deposition of a double buffer system YSZ/CeO2 gives best results for silicon substrates up to 2 inch wafers. In this way the superconducting YBa2Cu3O7-x (YBCO) films can reach a zero resistance temperature near 89 K and critical current densities at 77 K of up to 7(DOT)106 A/cm2. Additionally a nonsuperconducting but crystalline phase with the same stoichiometry (YBCO*) is used as passivation layer. Based on this technology we realized and investigated step- edge as well as new silicon bicrystal Josephson junctions, superconducting quantum interference devices (SQUIDs), bolometers using different compensation principles, and a new hybrid magnetometer. The hybrid magnetometer based on a simple Hall sensor was integrated with a superconducting antenna loop on the same chip.