V. V. Khanin

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.

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.

ANOMALOUS MAGNETIC-PROPERTIES OF Y2CU2O5 COMPOUND IN A WEAK MAGNETIC-FIELD

Measurements of magnetic susceptibility χ(T, H0) of the single phase Y2Cu2O5 compound in weak H0=(0.05~2 Oe) and moderate (H0≤500 Oe) magnetic fields are reported. Quite surprisingly, in weak magnetic field H0=0.05 Oe, a diamagnetic χ(T) response of about 2% of χideal(-1/4π) is observed below T≈150 K. At temperatures T~13 K the Y2Cu2O5 diamagnetism is strongly enhanced up to χ(T=10 K, H0=0.05 K)=26%χideal. The sharp diamagnetic χ(T) minimum at T=5~13 K is suppressed by the external magnetic field, and for H≥0.4 Oe this minimum is transformed into a paramagnetic χ(T) maximum. In moderate magnetic field H0=500 Oe, a cusp on the χ(T, H0) curve is observed. This peak corresponds to the transition from the paramagnetic Curie-Weiss χ(T)~1/(T+θ) to a simple paramagnetic Curie χ(T, H0)~1/T behavior.

A wideband device for measurement of the magnetic properties of materials

We have designed and tested the first version of the wideband SQUID-based universal instrument intended to study magnetic properties of matter within the frequency range 0-1 KHz, in the magnetic fields up to 0.15 T and the temperature range 4.2 - 330 K with the available sample volume ∼ 0.5 cm3. In the present device the spectral density of noise reduced to magnetic moment of the sample was close to 10-10Axm2/Hz1/2at frequencies higher than 1 Hz, for the applied field 0.05 T and sample temperature 170 K. An analysis of the noise spectra for various applied magnetic fields has enabled us to indicate a probable main source of this noise and to outline ways to reduce the total noise, hopefully to the SQUID-determined values.

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.

Detection of the proximity effect in Au/YBCO bilayers at 77 K with the aid of a low-temperature scanning tunneling microscope

The characteristic features of the conductance of a tunneling contact, formed by a microscope tip and an Au/YBCO sandwich, are investigated with the aid of a scanning tunneling microscope. It is shown that there exist three types of dependences of the conductance of the structure on the applied voltage, which are distinguished by the position of the characteristic features on the voltage axis. It is established that this difference is due to the local characteristics of the proximity effect in gold films on a high-Tc superconductor surface. It is concluded that the transmittance of the YBCO/Au boundary is spatially strongly nonuniform.