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In this paper, we review the preparation technology, integration in measurement systems and tests of high-Tc superconducting quantum interference devices (SQUIDs) intended for biomagnetic applications. A focus is on developments specific to Forschungszentrum Jülich GmbH, Chalmers University of Technology, MedTech West, and the University of Gothenburg, while placing these results in the perspective of those achieved elsewhere. Sensor fabrication, including the deposition and structuring of epitaxial oxide heterostructures, materials for substrates, epitaxial bilayer buffers, bicrystal and step-edge Josephson junctions, and multilayer flux transformers are detailed. The properties of the epitaxial multilayer high-Tc direct current SQUID sensors, including their integration in measurement systems with special electronics and liquid nitrogen cryostats, are presented in the context of biomagnetic recording. Applications that include magnetic nanoparticle based molecular diagnostics, magnetocardiography, and magnetoencephalography are presented as showcases of high-Tc biomagnetic systems. We conclude by outlining future challenges.

Abdrwt-We have fabricated HTS &SQUID flip-chip the magnetometers could be additionally improved by a sensors with a large area multilayer flux transformers. larger Pickup loop with the flux transformer made on a Different layouts of the flux transformers provide a large largerwafer. variety of magnetometers and planar gradiometers. For the To subtract high magnetic background noise one can use a magnetometers a resolution-6 tT/.\IHz and the planar gradiometric configuration of the pickup coil. Tian et al. [4] gradiometers a resolution of about ,. , 30 fT/cm..\IHz were have achieved a field gradient sensitivity of 73 fT/cm& in routinely obtained at 77 K The noise was nearly white down to the white noise region and 596 fl/cm,,/fi at 1 fi with a frequencies of few Hz. The sensors were vacuum-tight layer gradiometfic flux antenna on a 50 mm si wafer. We have demonstrated [5] a planar HTS flip-chip encapsulated together with a heater and a feedback coil. This makes the handling of the sensors more reproducible and convenient. Production of the magnetometers and gradiometers gradiometer having padometric flux antenna in small series was proven. prepared on a 30 mm wafer. A resolution of-40 ff/cmdHz

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A superconducting integrated receiver (SIR) comprises all of the elements needed for heterodyne detection on a single chip. Light weight and low power consumption combined with nearly quantum-limited sensitivity and a wide tuning range of the superconducting local oscillator make the SIR a perfect candidate for many practical applications. For the first time, we demonstrated the capabilities of the SIR technology for remote operation under harsh environmental conditions and for heterodyne spectroscopy at atmospheric limb sounding on board a high-altitude balloon. Recently, the SIR was successfully implemented for the first spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks (BSCCO mesa) at frequencies up to 750 GHz; linewidth below 10 MHz has been recorded in the high bias regime. The phase-locked SIR has been used for the locking of the BSCCO oscillator under the test. To extend the operation range of the SIR well above 1 THz, a new technique for fabrication of high-quality SIS tunnel junctions with gap voltage Vg up to 5.3 mV has been developed. Integration of a superconducting high-harmonic phase detector with a cryogenic oscillator opens a possibility for efficient phase locking of the sources with free-running linewidth up to 30 MHz that is important both for BSCCO mesa and NbN/MgO/NbN oscillators.

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We have produced submicrometre wide bicrystal Josephson junctions on the basis of YBa 2 Cu 3 O 7−δ (YBCO) films and investigated their microstructure and superconducting properties. A 300 nm thick buffer layer of SrTiO 3 on the SrTiO 3 bicrystal substrates has helped to improve the quality and reproducibility of the junctions. The surface of the films at the grain boundary (GB) was investigated by high-resolution scanning electron microscopy (HRSEM). This has revealed one unit cell steps on the YBCO film surface and meandering of the GB in the YBCO film. Planar transmission electron microscopy investigations of the junction have confirmed that the GB in YBCO had a linear deviation from the underlying GB in the substrate up to ∼0.4 µm. The observed spread of the angles of the deviation of the GB in the film from the direction of the GB in the substrate on the slot-and other defect-free parts of the GB was below 10 • . Due to the d-wave symmetry of the order parameter in YBCO the reduction of the curvature of the GB leads to an improvement of the homogeneity of the current flow and a doubling of the critical current I c of the junction. The observed I c R n product of symmetric 20 • bicrystal junctions was ∼400 µV at 77.4 K and ∼1 mV at 63 K. Integration of such junctions in SQUIDs with a square 16 mm multilayer flux transformer has allowed us, for the first time, to reach a field resolution for the high-T c magnetometers of ∼3.5 fT Hz −1/2 at 77.4 K.