R. Baskaran

Development of SQUID-Based System for Nondestructive Evaluation

This article describes the development of a superconducting quantum interference device (SQUID)-based system for nondestructive evaluation. The setup incorporates an in-house developed thin-film-based Nb SQUID with readout flux locked loop electronics and consists of a liquid helium cryostat with adjustable stand-off distance, a precision XY- thetas scanner for studying both flat and cylindrical samples, and a data acquisition system. The system has been used for the detection of artificially engineered subsurface defects in aluminum plates and to track magnetic-to-nonmagnetic phase transformation in stainless steel [grade 316L(N)] weldment specimens subjected to low cycle fatigue deformation.

Increased upper critical field for nanocrystalline MoN thin films deposited on AlN buffered substrates at ambient temperature

Molybdenum nitride (MoN) thin films have been deposited using reactive DC magnetron sputtering on aluminum nitride buffered oxidized silicon substrates at ambient temperature. GIXRD of aluminum nitride (AlN) deposited under similar conditions has revealed the formation of wurtzite phase AlN. GIXRD characterization of molybdenum thin films deposited on AlN buffered oxidized silicon substrates has indicated the formation of nanocrystalline MoN thin films. The electrical resistivity measurements indicate MoN thin films have a superconducting transition temperature of ~8 K. The minimum transition width of the MoN thin film is 0.05 K at 0 T. The inferred upper critical field B c2(0) for these nanocrystalline MoN thin films obtained by fitting the temperature dependence of critical field with Werthamer, Helfand and Hohenberg theory lies in the range of 17–18 T which is the highest reported in literature for MoN thin films.

Aerosol characterization and measurement techniques towards SFR safety studies

An Aerosol Test Facility (ATF) has been designed, fabricated and commissioned at Radiological Safety Division, Safety Group, Indira Gandhi Centre for Atomic Research, INDIA, to carry out safety studies related to aerosols in Sodium cooled Fast Reactor (SFR). The sources of aerosol generation during normal operation of SFR and accidental scenario have been discussed. Aerosol sampling issues to get the representative samples and how they are implemented in ATF are presented. Aerosol measurement techniques and the instruments deployed for the measurement of various aerosol properties have been described briefly. The sodium and fission product aerosols are generated and the sample data analysis is presented. The analysis of data for aerosol characterization include: (i) Initial size distribution of sodium and fission product aerosols, (ii) Chemical speciation of sodium aerosols (iii) Co agglomeration and co deposition of sodium and fission product aerosols and (iv) Enhanced Brownian coagulation of sodium aerosols in the presence of gamma radiation field. Besides, the on going aerosols studies are also presented.

Imaging buried defects in a three dimensional magnetically permeable medium using pseudoinverse technique

Pseudoinverse technique has been used to identify the location and distribution of buried defects inside a magnetically permeable medium by measuring the leakage fields outside the medium. Defects have been modeled as localized antidipoles whose moments and locations have to be determined. Use of pseudoinverse technique has enabled us to separate the moment variables and space variables and factor out the moment variables in the least square minimum norm solution. The leakage fields from two different flaws with an additive noise were simulated and utilized for inversion using this approach. It has been possible to identify the locations of defects and quantify the strength of the moments in the three dimensional medium even in the presence of reasonably large noise. This imaging technique has also been utilized to identify a single defect located 20mm below the top surface in the presence of noise.

High upper critical field in disordered niobium nitride superconductor

Superconducting Niobium Nitride thin films have been deposited on glass, aluminum nitride buffered glass, and oxidized silicon substrates by reactive DC magnetron sputtering at ambient substrate temperatures. The crystal structure of these thin films has been determined to be cubic fcc B1 structure by Glancing Incidence X-Ray Diffraction analysis. The superconducting transition temperatures of the thin films were measured to be greater than 11.6 K with a maximum of 13.4 K. The negative temperature coefficient of resistance observed in these thin films indicates the presence of disorder. Magneto-resistance measurements have been carried out on these thin films patterned into standard four probe geometry upto a maximum magnetic field of 12 T for two films and upto 15 T for the other two films. The dependence of transition temperature on the applied field is analyzed to estimate the upper critical field. The upper critical field for most of the films was estimated to exceed 35 T, while one of the most disordered films had an estimated upper critical field greater than 70 T.

Superconducting behaviour of Nb-Fe multilayers

Abstract Superconductor–ferromagnetic multilayer structures offer an opportunity to investigate the superconducting behaviour in the presence of interesting effects such as inelastic electron scattering from virtual bound states, magnetic spin flip scattering and the magnetic exchange potential etc. Wong et al. [J. Low Temp. Phys. 63 (1986) 307] first reported a non-monotonic variation of T c as a function of layer thickness of magnetic material in V–Fe multilayers. Subsequently, similar behaviour has also been reported in such multilayers by several groups. While non-monotonic variation of T c has been interpreted by a few groups as arising from the stabilisation of the π phase in specific thickness regimes, this is highly controversial. In view of the current interest in the superconducting behaviour of such structures, we have carried out studies on superconducting behaviour of Nb–Fe multilayers deposited by the RF magnetron sputtering technique. Multilayers were photolithographically patterned into a geometry suitable for current-in-plane (CIP) measurements. Each multilayer consisted of seven layers of Nb and seven layers of Fe. In different multilayers, layer thickness of Nb ( d Nb ) was held constant at 450 A, while the thickness of the magnetic Fe layer ( d Fe ) was progressively varied from 4.5 to 18 A. Another set of Nb–Fe multilayers was also prepared by changing the Nb layer thickness to 200 A. In the multilayers with d Nb =450 A, our preliminary results show a non-monotonic variation of T c with d Fe .

Proximity effect controlled superconductivity in Y1Ba2Cu3O7 − x

Abstract The superconducting transition curves measured by magnetic methods show considerable broadening for the high temperature oxide superconductors. This has been generally attributed to the presence of microscopic inhomogeneities, experimental evidence for which exists in the literature. This paper attempts to explain the origin of the broad transition to the proximity effect induced depression of Tc of the host superconductor by these microscopic inhomogeneities. The results of the calculations are in qualitative agreement with experimental features observed by us on samples of Y1Ba2Cu3O7 −x.

Development of SQUID based TDEM system for geophysical applications

Superconducting Quantum Interference Devices (SQUID) are highly sensitive sensors for magnetic flux. These sensors have been recently utilized for Time Domain Electromagnetic (TDEM) systems for geophysical applications. In view of this, a program has been initiated at IGCAR to develop SQUID based TDEM system suitable for geophysical explorations. For this work, SQUID sensors with fast FLL system, transmitter, transmitter controller and data acquisition system have been integrated. A metal object has been detected by using the SQUID based TDEM system by recording the flux profile as the object is moved relative to the SQUID system. This paper describes the development, characterization and performance testing of the system.

Magnetoresistance measurements of superconducting molybdenum nitride thin films

Molybdenum nitride thin films have been deposited on aluminum nitride buffered glass substrates by reactive DC sputtering. GIXRD measurements indicate formation of nano-crystalline molybdenum nitride thin films. The transition temperature of MoN thin film is 7.52 K. The transition width is less than 0.1 K. The upper critical field Bc2(0), calculated using GLAG theory is 12.52 T. The transition width for 400 µA current increased initially upto 3 T and then decreased, while that for 100 µA current transition width did not decrease.

Development of planar first order gradiometer coupled to HTS SQUID sensor

The HTS SQUID sensors coupled to an on-chip integrated planar gradiometric pick-up loop were fabricated using highly reliable advanced micro-fabrication techniques such as deposition of superconducting thin films by Pulsed Laser Deposition (PLD), UV photolithography and RF ion beam etching. It may be noted that use of wire wound gradiometers and approaches based on electronic subtraction of sensor outputs to realize synthetic gradiometers have not been entirely satisfactory. We have designed a first order planar gradiometer with two planar loops in opposition with the Josephson weak links in the form of microbridges located at the centre. Bicrystal SrTiO3 substrates with a 24° misorientation angle were used for realizing the Josephson weak links using YBCO thin films, as well as HTS planar gradiometer coupled to a HTS SQUID sensor. The performance of these devices are evaluated.