Operation of rf SQUID magnetometers with a multi-turn flux transformer integrated with a superconducting labyrinth resonator

Superconducting quantum interference device (SQUID) magnetometers have been widely used to perform biomagnetic measurements. In biomagnetic measurements such as magnetocardiograms (MCGs) and magnetoencephalograms, a magnetically shielded room (MSR) is used for noise reduction. However, because the MSR is expensive and heavy, the environments in which it can be used are restricted. In addition, MCG measurements obtained outside the MSR have a poor signal-to-noise ratio. We developed a wide dynamic range SQUID magnetometer that uses the flux-quanta counting (FQC) method with direct-feedback noise cancellation for MCG measurements and does not require the MSR. The FQC method is used to expand the dynamic range of the SQUID magnetometers to enable MCG measurements outside the MSR in the presence of large magnetic noise. The noise cancellation system uses two SQUID sensors; one for sensing and another one for reference. Both the sensing and reference signals are fed back to the sensing SQUID. This makes it possible to reduce the magnetic noise. We demonstrated the dynamic range, noise spectrum of the developed SQUID system, and MCG waveforms measured outside the MSR when the noise cancellation was set to ON or OFF. The noise cancellation factor of the developed SQUID system ranged from 10 to 20 dB. In MCG waveforms with noise cancellation, the QRS complex and T wave were clearly observed. The results show that the developed SQUID system clearly measures MCG waveforms in a condition involving large environmental magnetic noise.

We describe a low-noise dc Superconducting QUantum Interference Device (SQUID) magnetometer that is fabricated from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7-x/ on a 2 cm/spl times/2 cm bicrystal substrate. The magnetometer design consists of a single-turn pickup loop that is directly coupled to a low inductance SQUID. Using conventional flux-locked loop electronics with bias current reversal, the white flux noise of several of these magnetometers operated at 77 K is observed to be as low as 2.2 /spl mu//spl Phi//sub 0///spl radic/Hz above 10 kHz, increasing to about 5.7 /spl mu//spl Phi//sub 0///spl radic/Hz at 1 Hz. The field-to-flux conversion efficiency is measured to be 4.6 nT//spl Phi//sub 0/, resulting in a white magnetic field noise of to fT//spl radic/Hz above 10 kHz, increasing to 26 fT//spl radic/Hz at 1 Hz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

We constructed a multichannel superconducting quantum interference device (SQUID) magnetometer system for magnetoencephalogram measurements. The SQUID is based on the double relaxation oscillation SQUID (DROS), which consists of a hysteretic signal SQUID and a reference junction, and shunted by a relaxation circuit of a resistor and an inductor. With the high flux-to-voltage transfers, usually larger than 1 mV//spl Phi//sub 0/, simple flux-locked loop circuits could be used for SQUID operation. The SQUID system consists of 37 integrated magnetometers, distributed on a hemispherical surface, and external feedback scheme was used to eliminate magnetic coupling with the adjacent channels. In addition to the 37 signal channels, 8 reference channels were installed to pickup background noise and to apply software gradiometer. The average noise level of the magnetometers is about 3 fT//spl radic/Hz at 100 Hz, operated inside a magnetically shielded room. The magnetometer system was applied to measure auditory-evoked fields.

The authors assessed the ability of a Superconducting Quantum Interference Device (SQUID) magnetometer to noninvasively detect mesenteric ischemia in a rabbit model. Superconducting Quantum Interference Device magnetometers have been used to detect magnetic fields created by the basic electrical rhythm (BER) and to detect changes in BER of exteriorized bowel of anesthetized rabbits during mesenteric ischemia. The BER of rabbit ileum was noninvasively measured transabdominally using a SQUID magnetometer and compared with the electrical activity recorded with surgically implanted serosal electrodes before, during, and after snare occlusion of the superior mesenteric artery. Transabdominal SQUID recording of BER frequency was highly correlated to the measurements obtained with electrodes (R = 0.91). Basic electrical rhythm frequency decreased from 16.4 +/- 0.8 to 8.3 +/- 0.3 cpm (p < 0.001) after 25 minutes of ischemia. Reperfusion of ischemic bowel resulted in recovery of BER frequency to 14.3 +/- 0.4 cpm 10 minutes after blood flow was restored. A SQUID magnetometer is capable of noninvasively detecting mesenteric ischemia reliably and at an early stage by detecting a significant drop in BER frequency. These positive findings have encouraged the authors to continue development of clinically useful, noninvasive, detection of intestinal magnetic fields using SQUID magnetometers.

Non-destructive evaluation of aircraft structures with a multiplexed HTS rf SQUID magnetometer array

Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux sensor known, which is widely used in biomagnetism, low-field nuclear magnetic resonance, geophysics, etc. In this paper, we introduce a high-sensitivity SQUID magnetometer, which consists of an SQUID and a flux transformer. The SQUID is first-order gradiometer configuration, which is insensitive to interference noise. The flux transformer includes a multi-turn spiral input coil and a large-sized pickup coil. And the input coil is inductively coupled to the SQUID through mutual inductance. We present an SQUID magnetometer fabricated with Nb/Al-AlO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;/Nb Josephson junction technology on a 4-inch silicon wafer at our superconducting electronics facilities. We develop a fabrication process based on selective niobium etching process consisting of five mask levels. In the first two mask levels, the trilayer is patterned by a dry etch to define base electrode, contact pads, and interconnects. The shunt resistor and a dielectric insulating layer are then deposited and patterned by using lift-off and dry etchant, respectively. Finally, the niobium wiring layer is deposited and patterned by using reactive ion etching to define input, pickup and feedback coils. The measurement of the SQUID magnetometer is performed inside a magnetically shielded room. The operating temperature is realized by immersing the SQUID into the liquid helium (4.2 K). Moreover, a superconducting niobium tube is employed to protect the SQUID from being disturbed by external environments. A homemade readout electronics instrument with low input voltage noise is used to characterize the SQUID magnetometer. The results of low-temperature measurements indicate that the magnetometer has a magnetic field sensitivity of 0.36 nT/Φ&lt;sub&gt;0&lt;/sub&gt; and a white flux noise of 8 μΦ&lt;sub&gt;0&lt;/sub&gt;/√Hz,corresponding to a white field noise of 2.88 fT/√Hz. This kind of SQUID magnetometer is suitable for multi-channel systems, e.g., magnetocardiography, magnetoencephalography, etc. Although the SQUID process development benefits from the rapid advance of semiconductor process technology, the uniformity of the SQUID on one wafer is fluctuated due to the film deposition. Now, we have realized a best SQUID yield of 50% on a 4-inch wafer. In the future, the SQUID chip yield should be improved by well controlling the optimizing process. The device yield is expected to reach as high as 80%.

A low-noise wideband read-out electronics for dc superconducting quantum interference devices (SQUIDs) is presented. The preamplifier which is directly connected to the SQUID has white voltage and current noise levels of 0.4 nV//spl radic/Hz and 6.2 pA//spl radic/Hz with 1/f corners at 0.2 Hz and 13 Hz, respectively. The SQUID can be operated with both dc bias and ac bias of up to 250 kHz. In the latter case, a special circuit synchronously detects the ac bias component at the preamplifier output and automatically removes it by tuning the bias voltage. An in-system programmable microcontroller is used to control all functions of the SQUID system via a serial RS-485 interface. It also generates the ac bias clock and sets the SQUID working point via low-noise D/A converters. The read-out electronics has been used to operate low-noise SQUID magnetometers. Noise levels down to 35 fT//spl radic/Hz and 0.9 fT//spl radic/Hz with 1/f corners at about 2 Hz have been achieved with thin-film SQUID magnetometers operated at 77 K and 4.2 K, respectively. With static bias a high bandwidth of up to 6 MHz was achieved without affecting the noise level. With 100 kHz bias reversal the rms noise increased by about 3% when increasing the system bandwidth from 100 kHz to 1.4 MHz. The system slew rate was 0.4 /spl Phi//sub 0///spl mu/s to 2.3 /spl Phi//sub 0///spl mu/s. A short integrator reset time of <1 /spl mu/s allows one to increase the dynamic range utilizing the periodicity of the SQUID voltage vs. flux characteristic.

A vibrating normal pick-up coil coupled to a superconducting quantum interference device (SQUID) magnetometer as a nondestructive testing system was studied. The vibrating normal pick-up coil measures the magnetic field gradient at a point. Output of the SQUID magnetometer is lock-in detected at the vibration frequency. The frequency of an excitation magnetic field can be set low to detect deep structural flaws. The use of a normal coil vibrated at 20 kHz with output voltage amplified by a low-noise amplifier enabled detection of a cylindrical defect in an aluminum plate with an excitation magnetic field at 40 Hz.

The magnetic field distributions above the surface of in-situ active corroding 2024-T3 and 7075-T6 aircraft aluminum alloy plates have been measured using a high-resolution superconducting quantum interference device (SQUID) magnetometer. The magnetic field distributions and their variation with time are clearly different for the two aluminum alloys in an identical solution and for 2024-T3 in two different solutions. It is believed that these results demonstrate theability of SQUID to noninvasively detect in-situ active corrosion in aircraft aluminum alloys in a way that present corrosion-detection methods do not allow.

The Superconducting QUantum Interference Device (SQUID) magnetometer can non-invasively detect the magnetic fields created by the Basic Electrical Rhythm (BER) of the Gastrointestinal Tract (GIT). Using anesthetized adult male New Zealand rabbits the authors recorded signals from two isolated bowel segments at the same time, before and after ischemia was induced in either one or both bowel segments. The dominant frequency peaks for each period of recording were determined using autoregressive (AR) spectral analysis. There was a significant fall in the BER frequency in the ischemic segment from 11.8/spl plusmn/0.9 to 7.8/spl plusmn/0.6 cycles per minute (cpm), while there was no change in the normal bowel. It was possible for two observers (LAB, WOR) who were blinded to the preparation, to identify which bowel segment was ischemic. The results of this experiment demonstrate the ability of the SQUID magnetometer to noninvasively detect and differentiate signals from normal and ischemic bowel sources.

A high-T/sub c/ RF superconducting quantum interference device (SQUID) magnetometer system has been developed. The SQUID sensor is made from YBCO thin films using local oxygen-ion-irradiated microbridges. The SQUID is cooled by liquid nitrogen in an open cryostat. For the scanning process the sample is placed on a nonmagnetic stage inside a magnetic shield. The instrument has a spatial resolution of about one millimeter, which can be further improved. The system is used for nondestructive testing and for detecting the magnetic fields generated by corrosion currents. >

Superconducting quantum interference devices (SQUIDs), as the most sensitive solid‐state magnetic sensors currently available, play a crucial role in both fundamental science and industrial applications. This review systematically examines the research progress in high‐temperature superconducting (HTS) SQUID magnetometers and gradiometers and their possible applications of SQUIDs. By detailing the working principles of DC and RF SQUIDs and reviewing advancements in critical system components—such as sensing elements, cryogenics, and readout electronics—this work lays a solid theoretical and structural foundation for ultra‐sensitive magnetic sensing. Furthermore, by describing the properties of various HTS Josephson junctions, their flexibility, and critical points, we aim at highlighting the uniqueness of certain features and the possibility of tuning a variety of physical processes in these junctions. Additionally, it comprehensively summarizes innovative applications in biomedical imaging, geophysical exploration, and industrial non‐destructive testing. The innovation of this review lies in constructing a developmental framework for HTS SQUID technology from a complete chain perspective of principles‐devices‐fabrication processes‐applications, providing systematic technical references for researchers in related fields, which has important guiding significance for promoting the practical application of quantum sensing technology.

Determination of the number of radiation induced alanine radicals by measuring the magnetic moment using SQUID magnetometer

Superconducting quantum interference devices (SQUIDs) are sensitive detectors of magnetic flux. A SQUID consists of a superconducting loop interrupted by either one or two Josephson junctions for the RF or dc SQUID, respectively. Low transition temperature (T/sub c/) SQUIDs are fabricated from thin films of niobium. Immersed in liquid helium at 4.2 K, their flux noise is typically 10/sup -6//spl Phi//sub 0/ Hz/sup -1/2/, where /spl Phi//sub 0//spl equiv/h/2e is the flux quantum. High-T/sub c/ SQUIDs are fabricated from thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/, and are generally operated in liquid nitrogen at 77 K. Inductively coupled to an appropriate input circuit, SQUIDs measure a variety of physical quantities, including magnetic field, magnetic field gradient, voltage, and magnetic susceptibility. Systems are available for detecting magnetic signals from the brain, measuring the magnetic susceptibility of materials and geophysical core samples, magnetocardiography and nondestructive evaluation. SQUID "microscopes" detect magnetic nanoparticles attached to pathogens in an immunoassay technique and locate faults in semiconductor packages. A SQUID amplifier with an integrated resonant microstrip is within a factor of two of the quantum limit at 0.5 GHz and will be used in a search for axions. High-resolution magnetic resonance images are obtained at frequencies of a few kilohertz with a SQUID-based detector.

Applications of Superconducting Quantum Interference Devices (SQUIDs) usually require high sensitivity at relatively low frequencies, often down to 1 Hz or lower. Excess noise, typically with a spectral density scaling inversely as the frequency, can substantially reduce the sensitivity of SQUIDs at low frequencies. We have studied 1/f noise in niobium rf-SQUIDs in some detail. To reduce any noise contribution of the readout electronics, a cryogenic preamplifier was used. When one measures the signal voltage of the SQUID directly, a pronounced 1/f noise was observed in all samples, and was nearly independent of the bias frequency used. The crossover between 1/f and white noise moved towards higher frequencies as the bias frequency was increased, because of a lower white noise contribution. The 1/f noise scaled approximately as the inductance of the SQUID. When operated in a flux locked loop, however, no 1/f noise could be observed above 0.5 Hz. Operating a rf SQUID in a flux locked loop can thus substantially reduce 1/f noise in rf SQUIDs.