We have developed an absolute magnetic field sensor using Superconducting Quantum Interference Filter (SQIF) made of high-Tc grain boundary Josephson junctions. The device shows the typical magnetic field dependent voltage response V (B), which is sharp delta-like dip in the vicinity of zero magnetic field. When the SQIF is cooled with magnetic shield, and then the shield is removed, the presence of the ambient magnetic field induces a shift of the dip position from B0 ≈ 0 to a value B ≈ B1, which is about the average value of the earth magnetic field, at our latitude. When the SQIF is cooled in the ambient field without shielding, the dip is first found at B ≈ B1, and the further shielding of the SQIF results in a shift of the dip towards B0 ≈ 0. The low hysteresis observed in the sequence of experiments (less than 5% of B1) makes SQIFs suitable for high precision measurements of the absolute magnetic field. The experimental results are discussed in view of potential applications of high-Tc SQIFs in magnetometry.

Superconducting Quantum Interference Filters (SQIF's) are multi-loop arrays of Josephson junctions possessing unconventional grating structures. The flux-to-voltage transfer function of SQIF's is unique and has a single delta-peak like characteristics at zero applied magnetic field. The fault tolerance of SQIF's allows them to be realized in standard high-temperature grain boundary Josephson junction technology. We have successfully designed high-temperature YBCO SQIF's with voltage swings of more than 3 mV and a rather large dynamical range. This large dynamical range allows high-Tc SQIF's to be operated in commercial miniature cryocoolers at 50 K-80 K without any significant problems. Typical parasitic oscillations which are induced by the 55 Hz driven compressor of the cryocooler do not degrade the performance of SQIF's. Even if SQIF's are operated fully unshielded no significant degradation of the performance has been observed.

High-performance magnetic field sensor based on superconducting quantum interference filters Appl. Phys. Lett. 85, 1389 (2004); 10.1063/1.1787165Nonperiodic flux to voltage conversion of series arrays of dc superconducting quantum interference devices

We present a theoretical study on the dynamical properties of three-dimensional arrays of Josephson junctions. Our results indicate that such superconducting networks represent highly sensitive 3D-SQUIDs having some major advantages in comparison with conventional planar SQUIDs. The voltage response function of 3D-SQUIDs is directly related to the vector-character of external electromagnetic fields. The theory developed here allows the three-dimensional reconstruction of a detected external field including phase information about the field variables. Applications include the design of novel magnetometers, gradiometers and particle detectors.

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