Yu. V. Maslennikov

Dc-SQUID magnetometers and gradiometers on the basis of quasiplanar ramp-type Josephson junctions

Nonaqueous Br-ethanol chemical etching was successfully used for the preparation of the Josephson junctions, vias and crossovers in magnetometers including flux transformers. PrBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films were used for the barrier layer in the Josephson junctions of the SQUIDs and as an insulation in the junctions and in the flux transformers. Dc-SQUID magnetometers with small inductances and even without flux antennas were used for NDE applications which mainly require a high dynamic range. Highly sensitive devices were prepared with flip-chip flux antennas. An ac-bias SQUID electronics significantly improves the sensitivity of the magnetometers at low frequencies.

Operation of high-temperature superconductor magnetometer with submicrometer bicrystal junctions

We investigated the noise properties of dc superconducting quantum interference device flip-chip magnetometers with submicrometer-wide bicrystal junctions operating at 77.4 K. The noise of the magnetometers with electronics was about 6 fT/√Hz at frequencies above 100 Hz increasing up to about 20 fT/√Hz at 1 Hz. The operation of the magnetometers was characterized in an electronic axial first order gradiometer system, which was employed for biomagnetic measurements. The system demonstrated a gradient resolution of about 1 fT/cm√Hz at 77.4 K and stable operation in a standard magnetically shielded room under clinical conditions.

A double dc SQUID based magnetometer

We have developed a double dc SQUID based magnetometer with a direct read-out circuit and measured its signal and noise characteristics. The inductances of the input SQUID and SQUID-preamplifier were 120 and 250 pH. Thin-film flux transformers on the separate substrates had an inductance of the order of 1 /spl mu/H each and a current sensitivity of 0.54 and 0.32 /spl mu/A//spl Phi//sub 0/ for the first and the second SQUIDs, respectively. The intrinsic energy resolution of the fabricated magnetometer for an optimal gain of 58 for the first stage was close to 1.6/spl times/10/sup -32/ J/Hz. The dynamic range and the slew rate were about 126 dB and 4/spl times/10/sup 4/ /spl Phi//sub 0//s.<<ETX>>

Noise in relaxation-oscillation-driven DC SQUIDs

The noise properties of the simple relaxation-oscillation-driven DC SQUIDs (RO-SQUIDs) have been studied. A limitation of its internal energy sensitivity E/sub v/ at a level close to 4*10/sup -31/ J/Hz due to the influence of the Josephson junction plasma oscillation has been found for the 5- mu m-design-rule technology. The signal characteristics with the transfer flux-to-voltage factor up to 2 mV/ Phi /sub 0/ and equivalent noise flux of about 1.3*10/sup -6/ Phi /sub 0//Hz/sup 1/2/ in the new all-niobium version of the balanced RO-SQUID have been measured.

Magnetocardiographic diagnostic complexes based on the MAG-SKAN SQUIDs

A new series of the MAG-SKAN magnetocardiographic (MCG) diagnostic complexes for the measurement and analysis of the biomagnetic signals generated in human heart is developed, produced, and tested. The complexes are based on the magnetometric units that contain superconducting quantum interference devices (SQUIDs) with liquid-helium cooling and 1–36 channels for the detection of the magnetocardiosignals and the original software for the processing and analysis of the MCG data. More than 300 volunteers (a control group of healthy people and a group of patients suffering from cardiovascular diseases) took part in the complex tests. Several qualitative and quantitative MCG parameters that characterize the electric processes in heart and that can be used to identify the patients from the test groups are determined. The diagnostic sensitivity of the parameters ranges from 85 to 97% and the specificity ranges from 80 to 95%.

Thermal magnetic noise in a strip wound crystalline ferromagnetic core at 4.2 K

A dc SQUID magnetometer‐based system has been developed and used to measure, in the frequency range 50–2300 Hz, the complex magnetic permeability μr(ν) and the magnetization noise at 4.2 K in a strip wound toroid. This toroidal core has been made of the 3‐μm‐thick ribbon fabricated from a crystalline magnetically soft alloy, Ultraperm. Below 1 kHz a constant value of −arg[μr(ν)]≊2×10−3 and 1/ν shaped noise spectral density have been measured. For frequencies higher than 1 kHz a linear growth of the imaginary part μr and a white noise have been found. The noise due to the sample is found in quantitative agreement with the standard fluctuation‐dissipation formula for the thermal noise, while a comparison of the permeability imaginary part magnitude with the theoretical value has indicated a partially shorted windings in the toroid, which have decreased the toroid roll‐off frequency down to 1 MHz.

Software for the magnetocardiographic complex for the early diagnostics and monitoring of heart diseases

The structure, algorithm, and software are developed for the technical complex that measures, records, and analyzes the magnetic field of human heart. The software is developed as two autonomous subsystems that allow the preprocessing of the magnetic signals and the spatio-temporal analysis of the field characteristics and the field sources. The main functional units are described and the examples of the transformation and presentation of the magnetometric data at various stages of the analysis of the magnetocardiosignal are presented.

Integrated two stage dc SQUID-based amplifier with double transformer coupling scheme

The integrated version of a two stage dc SQUID-based low frequency amplifier has been designed, fabricated and tested. The value of the gain of the applied flux d/spl Phi//sub SQ2//d/spl Phi//sub SQ1/ in the range 30-70 and an overall flux-to-voltage transfer factor d/spl Phi//sub SQ2//d/spl Phi//sub SQ1/ as high as 3 mV//spl Phi//sub 0/ have been obtained. An effective input inductance L/sub IN/ equal to 1.8 /spl mu/H, a current sensitivity 1.33 /spl mu/A//spl Phi//sub 0/, and an effective coupling coefficient k/sup 2//sub IN/ close to 0.1 have been found in the accordance with the design.

DC SQUID Modulation Electronics for Operation With HTS DC SQUID Magnetometers in the Unshielded Environment

The new variant of DC SQUID modulation electronics for functioning with HTS DC SQUID magnetometers in the unshielded environment, was designed, manufactured and tested. The electronics was optimized for operation with new ultrasensitive HTS DC SQUID magnetometer providing magnetic field resolution of about 15 fT/radicHz at frequencies above 10 Hz and 30 fT/radicHz at 1 Hz. The central commutation core of electronics was based on a complex programmable logic device (CPLD). It has allowed to include into the SQUID-electronics the flexible system of a bias reversal which provides a stable magnetometer operation for various configurations of wiring in the cryostat and various HTS DC SQUID topologies. The electronics was manufactured in one compact box with size 110 mm times60 mm times 15 mm. The channel has standard bandwidth of about 100 kHz with greatly increased dynamic range due to high coupling (8 mV/Phi0) of feedback signal with main SQUIDs loop.

Modulation SQUID electronics working with high-Tc SQUIDs in open space

SQUID electronics optimized for operation in unshielded space with dc high-Tc superconducting quantum interference devices (HTS SQUIDs) are developed, manufactured, and studied. The dynamic characteristics of the SQUID electronics are studied with two magnetic-field sensors based on the HTS SQUIDs: a conventional SQUID sensor with a resolution of 100 fT/Hz1/2 and a supersensitive SQUID sensor with a resolution of 15 fT/Hz1/2 at frequencies exceeding 10 Hz and a resolution of 30 fT/Hz1/2 at a frequency of 1 Hz. Stable operation of the magnetometric channel is demonstrated with both SQUID sensors under urban conditions. On the basis of a complex programmable logic device (CPLD), an ac bias can be realized in the SQUID and the modulation signal can be compensated in the feedback, bias-current, and desired-signal circuits. Such a compensation system is the most appropriate and versatile means of providing stable operation of the magnetometric channel in the presence of the SQUID ac bias, regardless of the type of high-temperature sensor and the configuration of the input contacts in the measurement probe.