V. Yu. Slobodchikov

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.

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.

High- Dual-SQUIDs With Graphoepitaxial Step-Edge Junctions

The microstructural and noise properties of serially connected high-Tc dc superconducting quantum interferometer devices (high-Tc dc SQUIDs) with step-edge Josephson junctions are studied. It is shown that the implementation of novel graphoepitaxial step-edge Josephson junctions on buffered MgO substrates helps to overcome poor reproducibility of conventional step-edge high-Tc Josephson junctions due to the self-arranged growth of two identical [100]-tilted 45° grain boundaries over a wide range of step heights. The use of such Josephson junctions in two serially connected SQUIDs that are directly coupled to a common pickup loop in a dual-SQUID configuration with current biasing of the individual SQUIDs results in a doubling of voltage swings and an improvement in magnetic field resolution of the sensors.

A high temperature superconductor dc SQUID planar gradiometer measurement system for routine inspections

We have developed a high temperature superconductor (HTS) dc SQUID gradiometric measurement system for routine applications in non-destructive evaluations. Low values of the white flux noise and white field gradient noise were measured for gradiometers operating at 77 K. Low frequency noise was suppressed using an ac bias technique, even in a magnetically unshielded environment. First-order planar dc SQUID flip-chip gradiometers were used in the measurement system for stable operation under electromagnetically noisy conditions. A synchronous filter was employed to remove all harmonic components of parasitic line frequency interference. This improved the resolution of the system in a typical laboratory or industrial environment showing strong magnetic gradients of the line frequency signal due to nearby metallic constructions. Test scans of contaminations with magnetic particles were performed.

Gradiometers based on superconducting quantum interference device for nondestructive testing

A prototype of gradiometer for detection and analysis of magnetic signals that are generated by defects in metal structures and materials in the presence of external magnetic bias is based on dc-current superconducting quantum interference device (SQUID). A prototype of single-channel SQUID gradiometer that contains a fiberglass nonmagnetic cryostat, measurement probe with the SQUID sensor and magnetic flux transformer (second-order axial gradiometer), SQUID electronics, and software for control of SQUID gradiometer is studied. The prototype exhibits stable operation under laboratory conditions in the absence of additional magnetic shielding. Upgrade of the SQUID sensors and remaining elements of the prototype of magnetometer is planned for application in nondestructive testing.

High-sensitivity gradiometer based on three high-Tc SQUID magnetometers

An operating prototype model of an electronic gradiometric system based on three high-sensitivity high-Tc (HTS) SQUID magnetometers is developed, fabricated, and studied. A dynamic range of 140 dB implemented in the system provides its stable operation in an unshielded space with a sensitivity level of about 8 fT/cm2 Hz1/2. The ultimate sensitivity of the system, which can be attained either at large distances from man-made noises or in a weakly shielded space, is 4 fT/cm2 Hz1/2. This value is equivalent to 100 fT/Hz1/2 reduced to the sensitivity of a single SQUID magnetometer. The operating frequency band of the system is from 1 Hz to 15 kHz. The developed system, which is based on three HTS SQUIDs, can be successfully used in its present configuration in geophysical studies performed under field conditions and in magnetometric methods of nondestructive evaluation of materials.

Hysteretic flux dynamics in a detwinned YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// crystal from SQUID picovoltmeter measurements

We report measurements of resistivity hysteresis in both temperature and magnetic field performed on a detwinned YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// single crystal in the region of the vortex lattice melting transition. We have found evidence that the observed hysteresis cannot be explained by current-induced nonequilibrium effects. The measured voltage-current characteristics for the sample in the overheated state also display hysteretic behaviour. This strongly supports the idea that the vortex solid can be melted by transport current.