We present a magnetic detection system based on superconducting gradiometric sensors (SQUID gradiometers). The
system provides a unique fast mapping of large areas with a high resolution of the magnetic field gradient as well as the
local position. A main part of this work is the localization and classification of magnetic objects in the ground by
automatic interpretation of geomagnetic field gradients, measured by the SQUID system. In accordance with specific
features the field is decomposed into segments, which allow inferences to possible objects in the ground. The global
consideration of object describing properties and their optimization using error minimization methods allows the
reconstruction of superimposed features and detection of buried objects. The analysis system of measured geomagnetic
fields works fully automatically. By a given surface of area-measured gradients the algorithm determines within
numerical limits the absolute position of objects including depth with sub-pixel accuracy and allows an arbitrary position
and attitude of sources. Several SQUID gradiometer data sets were used to show the applicability of the analysis
Thin films of YBa2Cu3O7-x (YBCO) were prepared on different kinds of substrates (SrTiO3, LaAlO3, sapphire or silicon) with critical current densities jc (77K) > 2 (DOT) 106 A/cm2 and critical temperatures (zero resistance) of up to 90 K. We used laser deposition techniques for film preparation on SrTiO3-substrates with a maximum substrate size of 10 X 10 mm2. Applying bicrystallin substrates made from this material a pronounced RSJ-behavior of the grain boundary Josephson junctions was observed with ICRN-products of 410 (mu) V and critical current density of 1.2(DOT)105 A/cm2 in maximum on 24 degree grain boundaries. On that basis planar galvanically coupled dc-SQUID-gradiometers were patterned on this limited substrate size with a field gradient resolution of 308 fT/(cm(root)Hz) in the white noise level and 2 pT/(cm(root)Hz) at 1 Hz in electrically and magnetically unshielded environment. This extraordinary field gradient resolution even in unshielded environment enables this kind of dc-SQUID sensor for measurements of the magneto-cardiogram (MCG) of the human heart as well as for investigations in non-destructive testing (NDT). The layout of the dc-SQUID as well as the antenna layout of the gradiometer antennas were varied in order to determine their influence on the whole sensor performance. For the dc-SQUID layout itself we present a gradiometric scheme with decreased parasitic area of the dc-SQUID in the gradiometer. Furthermore the additional use of buffer layers to prevent interdiffusion, lattice mismatch and internal stress by different thermal expansion coefficients enables the use of silicon substrates for YBCO thin film deposition. Gradiometric flip-chip- antennas were patterned on 2'-silicon-substrates and combined with dc-SQUID-gradiometers explained above. With this sensor concept the resolution was increased by a factor of 6 in shielded environment thus also the human MCG was measured. By the integration of superconducting antennas with Hall-effect sensors on the same substrate a hybrid sensor concept is introduced having less sensitivity compared to SQUID-based sensors but improved dynamic range enabling their application in a NDT-measurement system in highly disturbed environment.
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.
Using high temperature grain boundary Josephson junctions (GBJJs) made of YBa2Cu3O7-(delta ) (YBCO) deposited across silicon bicrystal boundary, we successfully demonstrated direct detection at wavelength as short as 118.8 micrometer (frequency of 2.525THz) and the operation temperature up to 70 K. Radiation from a far infrared (FIR) laser was coupled to the junction, via a TPX plano convex lens and a high resistivity Si hyperhemispherical lens. The response at wavelength of 183.4 micrometer was obtained for the YBCO GBJJs on MgO bicrystal substrates. Also, investigated are the effects of response on external DC magnetic fields and polarization of electromagnetic waves as well as the harmonic mixing properties.