M.P. Janawadkar

Development of high field SQUID magnetometer for magnetization studies up to 7 T and temperatures in the range from 4.2 to 300 K

We present the design, fabrication, integration, testing, and calibration of a high field superconducting quantum interference device (SQUID) magnetometer. The system is based on dc SQUID sensor with flux locked loop readout electronics. The design is modular and all the subsystems have been fabricated in the form of separate modules in order to simplify the assembly and for ease of maintenance. A novel feature of the system is that the current induced in the pickup loop is distributed as inputs to two different SQUID sensors with different strengths of coupling in order to improve the dynamic range of the system. The SQUID magnetometer has been calibrated with yttrium iron garnet (YIG) sphere as a standard reference material. The calibration factor was determined by fitting the measured flux profile of the YIG sphere to that expected for a point dipole. Gd(2)O(3) was also used as another reference material for the calibration and the effective magnetic moment of the Gd(3+) could be evaluated from the temperature dependent magnetization measurements. The sensitivity of the system has been estimated to be about 10(-7) emu at low magnetic fields and about 10(-5) emu at high magnetic fields ∼7 T.

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.

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.

On the design and implementation of a novel impedance chamber based variable temperature regulator at liquid helium temperatures

A novel variable temperature regulator (VTR) based on the use of a fine impedance capillary to control the flow rate of cold helium gas into the VTR chamber is described. The capillary has a diameter of just 200 microm and the flow rate of cold helium gas through the capillary can be effectively controlled to the desired value by heating the capillary to a preset temperature and by controlling the pressure in the VTR chamber to a preset pressure using automated control circuits. Excellent temperature stability (about +/-1 mK at 10 K and +/-2 mK at 100 K) has been demonstrated in this setup with uniform rates of heating or cooling by an optimal choice of parameters. Compared to the more conventional VTR designs based on the use of mechanical long stem valves in the liquid helium reservoir to control the flow rate of liquid helium into the VTR chamber, and the use of a needle valve at the top of the cryostat to control the exchange gas pressure in the thermal isolation chamber, the present design enables temperature stability at any user desired temperature to be attained with uniform rates of cooling/heating with minimum consumption of liquid helium. The VTR has been successfully incorporated in the high field superconducting quantum interference device magnetometer setup developed in-house. It can also be incorporated in any low temperature physical property measurement system in which the temperature has to be varied in a controlled manner from 4.2 to 300 K and vice versa with uniform rates of heating and cooling.

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 .

Simple insertible high performance variable temperature regulator for measurement of physical properties at low temperature

An impedance capillary based Variable Temperature Regulator (VTR) for regulation of temperature in the range of 4.2 K-300 K, which can be detached and inserted into any experimental setup with a 50 mm diameter top access, has been designed, fabricated, and tested. The VTR may be used as a highly compact probe, which can be readily inserted in any liquid helium dewar or cryostat to realize uniform rates of cooling/heating and to achieve excellent temperature stability of ±1 mK at any temperature between 4.2 K and 300 K. VTR has been subjected to extensive experimental testing to arrive at optimum values of control parameters that are expected to influence its performance. The VTR may be integrated into any experimental setup for measurement of physical properties at low temperatures.

Magnetization and Magnetoresistance Measurements in GdBaCo2−xNixO5.5

The magnetization measurements in GdBaCo2−xNixO5.5. were carried out in a home built SQUID magnetometer developed using a DC SQUID sensor and its associated readout flux locked loop electronics. Magnetotransport measurements were also carried out on this system. The results suggest that TC decreases with Ni substitution and for x 0.3 a FM state is stabilized.

The thermoelectric method for non-destructive evaluation: a finite element study

The thermoelectric method for non-destructive evaluation of inhomogeneous samples is analysed using the finite element technique. As a case study, the magnetic field generated by thermoelectric currents induced in niobium plates with tantalum inclusions is considered. It is shown that this technique can be effectively used to evaluate niobium plates used for superconducting radio frequency resonators. It is further shown that this technique could also be used for non-contact weld inspection and is superior to weld characterization by direct measurement of the thermopower using a reference electrode in contact.