high-Tc Superconducting Quantum Interference Device Research Articles

Eddy Current Testing Utilizing Cooled Normal Pickup Coil and Superconducting Quantum Interference Device Picovoltmeter: Comparison between Experiment and Analysis

Eddy current testing utilizing a cooled normal pickup coil and a high-Tc superconducting quantum interference device (SQUID) picovoltmeter was performed both experimentally and analytically. In the experiment, we successfully detected a small crack on the back surface of a Cu plate by moving the coil in an unshielded environment. First, we developed a method of avoiding the drift of the detected signal that was caused by the variance of lift-off. Next, we clarified the dependences of the detected signal on the excitation frequency and the thickness of the Cu plate. It was shown that an optimum frequency that maximizes the detected signal exists. Because this frequency changed with the thickness of the Cu plate, the frequency dependence could be used to estimate the depth of the crack from the surface of the Cu plate. The experimental results were analyzed taking into account the phase and amplitude of the signal field caused by the crack. Good agreement was obtained between experiment and analysis.

A highly sensitive YBCO serial SQUID magnetometer with a flux focuser

We have designed and fabricated a low noise, highTc superconducting quantum interference device (SQUID) magnetometer with animproved yield. In order to reduce the field noise level of the magnetometer, weincreased the voltage modulation depth with a series SQUID array and enhanced theeffective area with a flux focuser. Ten washer-type SQUIDs were connected inseries, thereby increasing the voltage flux transfer function by a factor of 10.An enhancement in effective area by a factor of 5 was achieved by coupling the10-SQUID array to a single-layered flux focuser. For the directly coupled SQUIDarray magnetometer, the yield was improved by selecting the best SQUIDs tohave in series in order to reduce the flux noise. The magnetic field sensitivity of40 fT Hz−1/2 in the whiteregime and 100 fT Hz−1/2 at 1 Hz is demonstrated by using a single layer of highTc film. The proposed high yield magnetometers would be suitable for building low noisemulti-channel magnetocardiographs.

High-Tc dc SQUID Cooled by Pulse-Tube Cooler and Corrosion Measurements

We developed an YBCO high-Tc dc superconducting quantum interference device (SQUID) system cooled by a pulse-tube cooler. To stabilize and control the operating temperature of SQUID, a temperature controlling method, using dc SQUID itself as the temperature sensor, was developed. With the temperature controller, the temperature fluctuation could be reduced to about 50 mK and the dc SQUID magnetometer could keep locking for over 8 h. Using this dc SQUID magnetometer, we did some corrosion measurements in shielding. The field produced by the corrosion of aluminum plate with one side immersed in corrosion solution could be successfully detected.

赤外光温度制御装置を用いたＨＴＳ ＳＱＵＩＤ冷凍機冷却システムの検討

A high-Tc superconducting quantum interference device(HTS SQUID)cooling system was developed based on a pulse-tube cryocooler (PTC) and a novel temperature controller. In this system, the temperature controller uses infrared irradiation instead of the commonly employed electric heater to provide accurate and low-noise performances. The infrared light was irradiated via a fiber-optic cable onto a SQUID-mount stage, where a HTS SQUID was cooled to approximately 77 K by the PTC. The output of the irradiation source was controlled with a feedback scheme while monitoring the temperature of the SQUID-mount stage. The temperature of the stage was stabilized at 77 K, and kept within ±0.03 K deviation, for more than 2 hr by the temperature controller. The measured magnetic flux noise of the HTS SQUID magnetometer in the cooling system with the temperature controller showed little magnetic influence due to infrared irradiation.

Impedance magnetocardiography: Experiments and modeling

We report on impedance magnetocardiography studies using high-Tc superconducting quantum interference device (SQUID) magnetometers in a partially shielded environment. The SQUID detects magnetic-field variations due to impedance changes in the thorax at 15kHz. SQUID measurements are compared to impedance magnetocardiograms recorded at 100kHz using a copper pickup coil. An electromagnetic finite element model is used to simulate the distribution of ac during the cardiac cycle.

Shielding effect of an open-type hybrid magnetic shield with high-Tc superconducting loops

Double-D-shaped high-Tc superconducting loops were used to form an open-ended, cylindrical magnetic shield for biomagnetic measurements. The total magnetic flux through such a superconducting loop stays constant; thus, the loop can shield a cylindrical magnetic shield from an external magnetic field that penetrates into it from its open ends. The high-Tc superconducting loops are made of Bi2Sr2Ca2Cu3Ox tape wire, and the cylindrical magnetic shield is made of flexible magnetic sheets composed of Fe–Cu–Nb–Si–B nanocrystalline alloy. Both axial and transverse magnetic field noise in the shield were measured with high-Tc superconducting quantum interference devices. These measurements indicate that the double-D-shaped high-Tc superconducting loops at each open end of a cylindrical magnetic shield reduce not only axial but also transverse magnetic field noise in the magnetic shield.

A high-Tc SQUID gradiometer with integrated homogeneous field compensation

An integrated gradiometer system is presented, consisting of a single-layer high-temperaturesuperconductor dc superconducting quantum interference device (SQUID) and two coils,realized on printed circuit boards. The coils compensate the homogeneous background field,thus allowing the gradiometer to be freely moved in the Earth’s magnetic field. Thegradiometer signal is not harmed by any additional noise introduced by the homogeneousfield compensation. Therefore, a noise-limited magnetic field gradient resolution of15 pT m−1 Hz−1/2 witha 1/f corner frequency of 2 Hz was achieved. This is preserved even after motion in the Earth’smagnetic field. Because of the small dimensions of the compensation coils, the gradiometersystem can also be used for non-destructive evaluation. Preliminary measurementswere performed by moving the SQUID across a workpiece, without any shielding.

High-Tc and low-Tc dc SQUID electronics

Superconducting quantum interference devices (SQUIDs) are commonly operated in a flux-locked loop (FLL). The SQUID electronics amplifies the small SQUID signal to an acceptable level without adding noise, and it linearizes the transfer function of the SQUID in order to provide sufficient dynamic range. In this paper, the fundamentals of SQUID readout are reviewed including a discussion of preamplifier noise. The basic FLL concepts, direct readout and flux modulation readout, are discussed both with dc bias and bias reversal. Alternative readout concepts such as additional positive feedback (APF), two-stage SQUIDs, SQUID series arrays, relaxation oscillation SQUIDs and digital SQUIDs are briefly described. The FLL dynamics are discussed on the basis of a simple model with finite loop delay. It is shown that with optimized SQUID electronics a system bandwidth of ≈18 MHz and a corresponding slew rate of ≈8 Φ0 µs−1 are possible. A novel FLL scheme involving a Smith predictor is presented which allows one to increase the FLL bandwidth to about 100 MHz. The theoretical predictions are experimentally checked using a high-speed SQUID electronics prototype with a small-signal bandwidth of 300 MHz. Methods for increasing the dynamic range of SQUID systems are described: flux-quanta counting and dynamic field compensation (DFC). With DFC, the residual magnetic field at the SQUID can be kept close to zero even if the device is moved in the Earth's field. Therefore, the noise level of a high-Tc magnetometer measured inside a magnetically shielded room (60 fT Hz−1/2 with a 1/f corner at 2 Hz) remained unchanged after moving the device in the magnetic field outside the room (60 µT dc plus 0.8 µT peak-to-peak power line interference).

High-Tc superconducting quantum interference device observation of heat-affected zone in a spot-welded Fe–Cr–Ni system

A study was carried out to observe a heat-affected zone (HAZ) in a deformed Fe–Cr–Ni system containing α′ martensite using high-Tc superconducting quantum interference device (SQUID) microscopy. Microstructure and remanent magnetization images were studied by an optical microscope and a SQUID microscope, respectively. The HAZ, in which only the face-centered-cubic austenite phase exists, could be visualized by the SQUID microscope. It was also found that the SQUID images were consistent with the results from the microstructural analysis. It was concluded that a simple SQUID measurement may serve as an effective method for a nondestructive evaluation of ferromagnetic steel phases by correlating remanent magnetization images to microstructural characteristics.

Micromachining of SrTiO3 steps for high-Tc step edge junction dc SQUIDs

In this paper, we establish a model of the micromachined SrTiO3 substrate steps for high-Tc direct-current (dc) superconducting quantum interference devices (SQUIDs). The step angle is determined by the local ion milling rate and re-deposition rate, which is caused by back sputtering of ion milling. At dynamic balance, the maximum possible step angle is predicted to be 75° with an ion beam incidence angle of 45°, which agrees well with the measured value of 71°. In order to obtain a better step sidewall profile, we consider the influences of Nb metal mask micromachining. To avoid a rounded angle at the step upper corner, the minimum thickness of the Nb mask should be 440 nm when the desired step height is 300 nm. At optimized process conditions, steps with sharp, steep angles, and flawless profiles have been fabricated. Nine of the twelve dc SQUIDs thus obtained exhibited resistively shunted junction current–voltage behavior and magnetic field modulation at 77 K.

Some new horizons in magnetic sensing: high-Tc SQUIDs, GMR and GMI materials

Driven by rapid progress in microelectronics and thin film technologies, magnetic sensors development continues to expand. This paper presents issues related to the principles, categorisation and applications of magnetic sensors. Special attention is paid to two types of sensors and their present and future impact: superconducting quantum interference devices (SQUID) magnetometers with their unsurpassed sensitivity, and giant magnetoresistance (GMR)/giant magnetoimpedance (GMI) based sensors as the most promising technology. A review of recent developments in SQUID technology and a discussion of limitations and aspects that could contribute to the wider acceptance of this technology, are presented. The giant magnetoresistance and giant magnetoimpedance effects have already found applications in magnetic sensing and have promise in other applications. Their unique characteristics and miniaturisation potential have contributed to the rapid acceptance of these technologies. The article describes the principles of the GMR and GMI effects along with recent developments in these technologies particularly in manufacturing techniques and materials.

Possibility of new measurement application using high-Tc SQUID (superconducting quantum interference device)

Possibility of new measurement application using high-Tc SQUID (superconducting quantum interference device)

同軸型パルスチューブ冷凍機を用いた低磁気ノイズＳＱＵＩＤ非破壊検査システムの開発

A small, low-magnetic-noise nondestructive inspection (NDI) system that uses a high-Tc superconducting quantum interference device (SQUID) cooled by a coaxial pulse tube cryocooler (PTC) has been developed. The key components for realizing the compact, low-noise system developed in this study, include the following: a high-Tc SQUID gradiometer, the coaxial PTC, and a SQUID-mount stage separated from the cold head of the cryocooler. The high-Tc SQUID gradiometer contributes not only to stable system operation when subjected to the environmental noise without magnetic shielding, but also a decrease in magnetic noise caused by the mechanical vibration of the SQUID, which is transmitted from the cryocooler. By using the coaxial PTC, which was constructed of non-magnetic materials, both magnetic noise and the mechanical vibration are well suppressed compared to conventional two-axial PTCs. In addition, the coaxial PTC is designed to be compact (50 mm diameter, 400 mm height, and 4 kg weight) for practical use. The SQUID-mount stage separation contributes to stable operation by reducing the transmitted vibration to the SQUID, and thermal stability at the SQUID-mount stage. The magnetic flux noise spectrum of the cryo-cooled system was measured and compared with that of a liquid-nitrogen-cooled system using the same SQUID. From the comparison, no increase in magnetic noise due to the cryocooler was observed, although the magnetic flux noise level of the cryo-cooled system was dominated by SQUID operation noise, which was rather high, (i.e., around 100 μΦ0/Hz1/2 at 100 Hz). The relation between the mechanical vibration at the SQUID and magnetic noise is discussed.

Suppression of thermally activated flux entry through a flux dam in high Tc superconducting quantum interference device magnetometer

Thermally activated magnetic-flux entry into a pickup coil through a flux dam in high-Tc superconducting quantum interference device (SQUID) magnetometer is studied. Since the thermally activated flux entry strongly depends on the circulating current in the pickup coil, the behavior of the circulating current is studied with numerical simulation when an external field is applied. It is shown that the circulating current becomes near the critical current of the flux dam after the large external field is applied. In this case, thermally activated flux entry becomes large, and degrades the SQUID performance. In order to suppress the thermal activation, the circulating current must be much below the critical current. For this purpose, two methods are proposed. One is to use a compensation field in the case of the flux dam, and the other is to use a switch instead of the flux dam. Numerical simulation shows that we can rapidly decrease the circulating current much below the critical current, and hence, can suppress the thermally activated flux entry by using these methods. The usefulness of these methods is confirmed experimentally.

High Tc SQUID systems for magnetophysiology

Magnetophysiology is the use of a superconducting quantum interference device (SQUID) based instrument to detect neuromagnetic fields evoked by electrical stimulation of brain tissue slices. In this paper we show that a SQUID based on high temperature superconductors (HTSs) would have considerable advantages over a low T c device in this application. We construct a model of electrical activity in a hippocampal brain slice, which enables the neuromagnetic field to be determined as a function of position and distance from the tissue. We then describe the design of HTS SQUID systems for magnetophysiology and the two styles of system we are developing. Finally we use our model to show that an existing HTS SQUID magnetometer would give a superior signal to noise ratio compared to a low T c system for the hippocampal brain slice preparation at least.

MAGNETIC FIELDS PRODUCED BY THE COMBUSTION OF METALS IN OXYGEN

Weak magnetic fields produced by the combustion of pure metal and metal/metal-oxide powders in Rowing oxygen were measured by High-Tc Superconducting Quantum Interference Devices (SQUIDs). Two qualitatively different types of magnetic signals were observed for various reactions. The combustion of Nb, Cr, and Co generates a slowly oscillating field, while the combustion of Zr, Hf, Mn, Ti, Mg, and Fe generates a slowly oscillating field on which rapid oscillations are superimposed. The magnetic power spectra for these reactions scales as a power-law up to 100 Hz with an exponent between 0·64 and 0·96. This suggests that the magnetic field oscillations are generated by stochastic current fluctuations over a wide range of time scales. This may result from fluctuations in either the instantaneous shape and/or the velocity of the reaction zone. A simple electromagnetic model is used to predict the qualitative features observed in our experiments.

Properties of a Flux Dam Inserted in the Pickup Coil of a High-Tc Superconducting Quantum Interference Device Magnetometer

The properties of a flux dam inserted in a pickup coil of a high-Tc superconducting quantum interference device (SQUID) are studied. First, the effect of thermal noise at T=77 K on the magnetic flux entry into in the pickup coil through the flux dam is discussed. It is shown that the thermal noise causes the flux entry even when the circulating current in the pickup coil is less than the critical current of the flux dam. The behavior of this thermally activated flux entry is analyzed using the so-called potential-barrier model. It is shown that the flux entry into the coil proceeds logarithmically with time, indicating the necessity of a long waiting time in the use of the flux dam. It is also shown that the critical current of the flux dam must be large enough to obtain a quasistable state, i.e., a state where no thermally activated flux entry occurs during the measurement time. The critical current should be larger than 50 µA in the magnetic field under operation. Next, in order to satisfy this requirement, the magnetic field dependence of the critical current of the flux dam is studied when the flux dam is made of a wide bicrystal junction. It is shown that the magnetic field dependence of the wide junction can be explained reasonably well if we take into account the so-called overlap-type-junction model and the flux-focusing effect of the bicrystal junction. The obtained results will be useful in the design of the flux dam.

Application of high Tc SQUID magnetometer for sentinel-lymph node biopsy

The basic performance of a nanoparticle detection system for lymph node used with a high-Tc superconducting quantum interference device (SQUID) was investigated. Ultra-small iron oxide particles of 360 pg could be detected with spacing of 1 mm between the SQUID magnetometer and the particles. When the space was widened to 40 mm, the detectable weight of the particles was increased and was 1.6 /spl mu/g.

High Tc SQUIDs and eddy-current NDE: a comprehensiveinvestigation from real data to modelling

The interest in magnetometry for eddy-current non-destructivetesting, e.g. of planar conductive structures encountered in the aircraftindustry, using high-temperature superconducting quantum interference devices(SQUIDs) is primarily due to their high sensitivity to magnetic flux even atvery low frequencies. Here it is shown how theoretical, numerical andmeasurement machineries are combined to get reasonable synthetic andexperimental data and to reach a good understanding of the interaction ofdiffusive wavefields with a damaged non-magnetic metal plate (as a first steptowards the retrieval of pertinent features of the defects). The measurementmodalities are considered first. It is illustrated in some detail howlaboratory-controlled experiments are performed by a SQUID-based probedisplaced above artificially damaged plates. Experimental data are thenconfronted with simulation results in order to evaluate the accuracy andreliability of this measurement system. Simulations are carried out by acomputationally fast vector volume integral method dedicated to a planarlayering affected by a volumetric defect, which involves the construction ofthe dyadic Green system of the layering.

High Tc SQUID sensor system for non-destructive evaluation

Magnetic field mapping measurements were performed using commercial high- T c superconducting quantum interference device (SQUID) sensors. The sensors are incorporated in a fully computerized scanning system and were found to be operative only when encased inside a double-layer mu-metal shield. This allowed us to measure signals of the order of 0.5 nT. In all cases, the field map obtained was consistent with the theoretically expected images. In measurements with two samples, the relative orientation and center-to-center separation of the samples could be easily deduced from the field map. Surface flaw detection of unglazed ceramic tiles under an externally applied magnetic field is also discussed.