Rf SQUID Magnetometers Research Articles

Low Noise Active Shield for SQUID-Based Magnetocardiography Systems

A light weight and low-cost active magnetic shield system is proposed for a single channel SQUID-based magnetocardiography system equipped for operating at liquid nitrogen temperature without passive magnetic shield. The developed active shield, using coils around the sensors, is a two-stage magnetic shielding system, each stage of which has been designed to compensate different frequency and dynamic ranges. The sensor part contains two high-Tc rf-SQUID magnetometers, capable of working stably under earth magnetic field, placed in an axial gradiometric arrangement with a baseline of 10 cm. The active shield setup is driven using two correlated proportional-integral-derivative closed loop systems. In the designed system, the coils are in the vicinity of the SQUIDs, resulting in coupling the coils to the SQUIDs as well as to the resonators. Hence, the impedance of the coils along with the phase of the feedback current is affected based on the configuration. In this paper, investigation of the shielding performance of our bi-stage magnetic shield as the active compensation, fed by the optimal feedback control system with all the above considerations, is reported. Using the designed shielding system, the disturbing magnetic signals could be attenuated such that the pico-Tesla signals could be obtained.

Environmental Noise Cancellation for High-TC SQUID-Based Magnetocardiography Systems Using a Bistage Active Shield

An active noise cancellation method is proposed for superconducting quantum interference devices (SQUID)-based magnetocardiography systems working out of magnetically shielded rooms. Using YBCO high-Tc rf-SQUID magnetometers as magnetic field sensors, an active shielding system was implemented based on this method. This method incorporates two different shielding frequency regimes of operation simultaneously. This is because the unwanted background magnetic field signals range from very low frequencies up to high frequencies with a wide range of amplitudes at the upper and lower frequency spectra. Therefore, the shielding system is designed in a bistage configuration, and each stage covers one part of the frequency spectrum. Each shielding stage is based on a coil designed and optimized by finite element method calculations. One of the coils is used for compensating high amplitude and very low frequency (0–0.1 Hz) far-field environmental magnetic field noise, and the second one is used for relatively low amplitude and higher frequency (0.1–100 Hz) magnetic field noise. Because of these frequency and amplitude characteristics, each coil uses independent control circuit. For designing the controllers, methods based on the dynamic model approximation of the first and second orders were examined, which led to optimized proportional-integral-derivative controllers. The shielding efficiency of the proposed bistage active shielding system shows a significant improved effectiveness compared to that of the conventional one stage active shielding systems.

Integrated Monolayer Planar Flux Transformer and Resonator Tank Circuit for High- $T_{c}$ RF-SQUID Magnetometer

The authors propose a new design for monolayer superconducting planar flux transformer integrated with a coplanar resonator serving as a gigahertz range tank circuit for high- Tc rf-SQUID magnetometers. Based on the proposed design, which is optimized using the finite element method, the transformer-resonator configuration is made of 200-nm-thick monolayer YBCO film on a crystalline LaAlO3 substrate. In this optimized design, the SQUID magnetometer is coupled through flip-chip configuration with the configuration providing high coupling coefficient between the devices. The design permits coupling of the rf signals to the SQUID efficiently, whereas the transformer is designed to couple the dc to low-frequency magnetic field signals to the SQUID without disturbing the rf excitation. By means of split-ring resonator structure and also a SrTiO3 layer on the back side of the configuration, tuning the resonance frequency with high precision could be made possible using minimum change in the design. Loaded quality factor and the coupling coefficient of the designed resonator are obtained to be high to meet the SQUID system requirements very well. Also, lower field-to-flux transformation coefficient is achieved by using our single-turn transformer design compared to that of the bare SQUID.

Magnetic Flux Transfer in Needle Probe of Scanning SQUID Probe Microscope

A scanning SQUID probe microscope, in which a superconducting quantum interference device (SQUID) is combined with a scanning probe microscope, has been developed using a needle probe with high magnetic permeability. The probe played a key role of a flux guide from the sample to the SQUID. An rf-SQUID magnetometer used with a substrate resonator had been adopted in the initial setup. The magnetic sensitivity was not so high though the spatial resolution was good. In this paper, the flux transfer from the sample to the SQUID through the probe was investigated by using the magnetic field simulations. The result showed that the flux was decreased by a factor of more than 1,000 from the probe tip to the SQUID. The critical drop was occurred around the end of the probe. It suggested the magnetic coupling between the SQUID and the probe should be improved. The additional gap distance was needed in the initial rf-SQUID because of the thickness of the substrate resonator used with the rf-SQUID. Therefore, new rf-SQUIDs, which could be operated without a resonator, were fabricated to reduce the gap distance between the SQUID and the probe. The gap distance affected the quality of the magnetic images.

A simplified HTc rf SQUID to analyze the human cardiac magnetic field

We have developed a four-channel high temperature radio-frequency superconducting quantum interference device (HTc rf SQUID) in a simple magnetically shielded room (MSR) that can be used to analyze the cardiac magnetic field. It is more robust and compact than existing systems. To achieve the high-quality magnetocardiographic signal, we explored new adaptive software gradiometry technology constructed by the first-order axial gradiometer with a baseline of 80mm, which can adjust its performance timely with the surrounding conditions. The magnetic field sensitivity of each channel was less than 100fT/√Hz in the white noise region. Especially, in the analysis of MCG signal data, we proposed the total transient mapping (TTM) technique to visualize current density map (CDM), then we focused to observe the time-varying behavior of excitation propagation and estimated the underlying currents at T wave. According to the clear 3D imaging, isomagnetic field and CDM, the position and distribution of a current source in the heart can be visualized. It is believed that our four-channel HTc rf SQUID magnetometer based on biomagnetic system is available to detect MCG signals with sufficient signal-to-noise (SNR) ratio. In addition, the CDM showed the macroscopic current activation pattern, in a way, it has established strong underpinnings for researching the cardiac microscopic movement mechanism and opening the way for its use in clinical diagnosis.

New magnetically ordered phases in YbRh2Si2

AbstractYbRh2Si2 is a widely studied heavy Fermion system with a variety of interesting properties, among them a quantum critical point at 60 mT for B // a,b‐plane 1. At low temperatures we previously found a new magnetically ordered phase below 2.2 mK 2 taking place in the well known antiferromagnetic phase which starts at 70 mK 3. In addition to this “A” phase, we now observe a second, weaker phase transition below 14 mK and 5 mT which we named B phase. To study these phases of YbRh2Si2 at the lowest possible temperatures, we measured the DC magnetization in magnetic fields up to 60 mT using a home made rf SQUID magnetometer. The A phase is very robust and extends at least up to a magnetic field of 23 mT. Above this field, it is suppressed to below our detection limit of 1 mK. The A transition is characterized by a reduced magnetic susceptibility in the ordered state and there is zero magnetization in the limit B → 0. From these findings, we expect it to be an antiferromagnetic ordered state. The experiments were performed in our low temperature cryostat at the Walther Meissner Institute in Garching which includes a 0.9‐mole PrNi5 nuclear demagnetization stage with a final temperature of 400 µK. The temperatures of the nuclear stage were measured by pulsed NMR on a Cu rod thermally anchored to it. YbRh2Si2 single crystals of very high quality have been grown in an Indium flux at the MPI CPfS Dresden. The samples were clamped in a 5 N Ag rod screwed to the nuclear stage and extending into the pick‐up coil of a conventional flux transformer. Their temperatures were always close to that of the nuclear stage. In addition, we performed specific heat measurements with the semiadiabatic heat‐pulse method.

FRP Dewar for measurements in high pulsed magnetic fields

We describe a liquid helium glass-fibre reinforced plastic (G-FRP) Dewar which we designed, fabricated and tested for excitation spectrum measurements in high pulsed magnetic fields of up to 50 T. The sensitivity of high-resolution magnetic measurements carried out at low temperatures in such high fields is limited inevitably by magnetic and electric properties of the structural Dewar materials involved. Magnetic properties of various G-FRP Dewar-purpose materials are explored with a χ -meter furnished with a RF SQUID magnetometer. The Dewar materials and multilayer insulation effects contributing to the magnetic response signal are analyzed. It has also been discovered that field noise caused by the magnetization of the Dewar materials can be suppressed substantially by using special glass–epoxy technologies. The liquid helium evaporation rate is 3.8 l/day while the hold time is 27 h, the influencing factors are discussed.

STM-SQUID Probe Microscope Based on an RF SQUID Magnetometer

A novel STM-SQUID probe hybrid microscope using a high- TC rf SQUID magnetometer was fabricated to improve the spatial resolution and system controllability. We successfully observed topographic and magnetic images of Ni thin film microline or microdot patterns at the same time. The spatial resolution of the magnetic images was found to be about 1 mum.

Magnetization measurements on YbRh2Si2 at very low temperatures

YbRh2Si2, a heavy fermion compound, is in the center of interest for its unconventional behavior around a quantum critical point (QCP) which can be tuned by a magnetic field. To study the system at the lowest possible temperatures, we have measured the dc magnetization in magnetic fields up to 60 mT and down to 1 mK using an rf SQUID magnetometer. Our fields were high enough to reach the QCP in the B ∥ (a, b) crystal orientation. Both field cooled (fc) and zero field cooled (zfc) data were taken. We found a sharp transition to a low magnetization ground state at 2.2 mK and differences between the fc – zfc traces below 11 mK. In this temperature range down to 2.2 mK an additional increase of the magnetization is seen, just before the final drop sets in. These results will be discussed with respect to the nature of the ground state in low magnetic fields and its relation to the QCP.

The set-up of a high temperature superconductor radio-frequency SQUID microscope formagnetic nanoparticle detection

SQUID (superconducting quantum interference device) microscopes are versatileinstruments for biosensing applications, in particular for magnetic nanoparticle detection inimmunoassay experiments. We are developing a SQUID microscope based on an HTS rfSQUID magnetometer sensor with a substrate resonator. For the cryogenic set-up, aconfiguration was realized in which the cryostat is continuously refilled and kept at aconstant liquid nitrogen level by an isolated tube connection to a large liquid nitrogenreservoir. The SQUID is mounted on top of a sapphire finger, connected to theinner vessel of the stainless steel cryostat. The vacuum gap between the coldSQUID and room temperature sample is adjusted by the precise approach of a50 µm thin sapphire window using a single fine thread wheel. We investigated possible sensing tipconfigurations and different sensor integration techniques in order to achieve anoptimized design. A new scheme of coupling the rf SQUID from its back to aSrTiO3 substrate resonator was adopted for the purpose of minimization of the sensor-to-sample spacing.By SQUID substrate thinning and washer size reduction, the optimum coupling conditionsfor back coupling were determined for different rf SQUID magnetometers prepared onLaAlO3 and SrTiO3 substrates. The SQUID microscope system is characterized with respect to its spatialresolution and its magnetic field noise. The SQUID microscope instrument will be used formagnetic nanoparticle marker detection.

Foetal magnetocardiography with a multi-channel HTS rf SQUID gradiometer

Foetal magnetocardiography (fMCG) is a promising technique for prenatal cardiac diagnosis.In this paper, we discuss the special requirements of a system for fMCG measurements. ASQUID system incorporating five HTS rf SQUID magnetometers has been developed andconstructed. Four magnetometers are arranged at the bottom of the cryostat in a4 cm × 4 cm square configuration. One reference magnetometer is located 20 cm above the plane to formfour first-order axial gradiometers with the four bottom magnetometers. Themagnetometer with a 18 mm diameter flux focuser, which reaches a field sensitivity of20–30 fT Hz−1/2, proved sufficient for recording foetal magnetocardiograms in a magneticallyshielded room. The cardiac activities of two foetuses (31st and 33rd weeks ofgestation) were recorded in 90 Hz bandwidth. The foetal QRS peak signals rangedfrom 4 to 7 pT and could easily be identified in the real-time gradiometer outputswith a signal-to-noise ratio of up to five. Furthermore, the averaged fMCG dataenabled the determination of de- and repolarization time intervals. Overall, thefMCG signals proved of sufficient quality to perform foetal heart diagnostics.

Basic quantification of magnetic particles in solid substance and human tissue by the SQUID magnetometer

The present study concerns an rf SQUID magnetometer enabling to measure magnetic fields of order 2 × 10 −14 T Hz −1/2 and its utilization for the detection and quantification of particles with magnetic properties in different materials and human tissue. In the first case, the mass concentration n p of powdered ferro- and ferrimagnetic material (PFM) in a physical model with known shape, volume and PFM content was estimated in order to evaluate the uncertainty of the method. Secondly, the lungs of an arc welder who was long-term exposed to welding fumes were examined. The mean n p of 0.0845 mg cm −3 of ferromagnetic dust with the mass ratio of Fe:Ni materials 99.12%:0.82% and the specific remanent magnetic moment m rs of (0.127 ± 0.076) A m 2 kg −1 was found.

Detection of Magnetic Contaminations in Industrial Products Using HTS SQUIDs

Many products in the pharmaceutical and food industry are packaged in metallized wrappings. With standard high-frequency search coil metal detectors, they can only be tested for metal contaminations before they are wrapped. However, a key requirement of industrial quality control is the inspection of the products at the end of the production line. We have developed an inspection system for detecting the magnetic remanence of the contaminants. The system utilizes two HTS rf SQUID magnetometers with step edge junctions immersed in liquid nitrogen. The SQUIDs are arranged such that they cover the product channel in a rotated planar electronic gradiometer configuration. In order to suppress the low-frequency magnetic disturbances typically found in industrial environment, the product channel and the SQUID system were mounted inside a coaxial three-layer Mumetal shield. In combination with the gradiometric suppression, homogeneous low-frequency disturbance fields were attenuated by a factor of 400,000. The sensitivity of the system for small magnetic particles was determined experimentally, using numerous steel balls and splinters. A stainless steel particle of 3 /spl mu/g, corresponding to a sphere diameter of 0.09 mm, was detected with and without aluminized wrapping.

High-Tc SQUID gradiometer system for magnetocardiography in an unshielded environment

We set up an electronic gradiometer for magnetocardiography (MCG) in an unshielded environment. The electronically balanced gradiometer consists of high-temperature radio-frequency superconducting quantum interference device (rf SQUID) magnetometers. The rf SQUID magnetometers are arranged to form the electronic first-order, four-vector gradiometer, or second-order gradiometer. The output of the MCG signal was filtered by a band pass (1–30 Hz) and power line filters. We can reduce the noise to ∼1 pT Hz−1/2 at 1 Hz for second-order or four-vector gradiometers. Two-dimensional MCG imaging is demonstrated. The results are discussed with data measured in the moderate magnetically shielded environment.

Mobile HTS rf SQUID magnetometer

Using a simple compensation method, we developed a mobile HTS RF SQUID magnetometer. The dc and low frequency (below 20 Hz) magnetic field could be compensated very well. With the compensation, the HTS RF SQUID magnetometer remained locked while being swung at large angles in the Earth's field. For frequencies greater than 20 Hz, there was no increase in the flux noise observed after moving the SQUID system in the Earth's field. Using the mobile HTS RF SQUID magnetometer, we constructed an eddy current NDE system, which demonstrated the possibility of detecting defects in ferromagnetic materials.

Metal detector based on high- Tc RF SQUID

Metal detector has many applications in food industries, wood board plants, safety security and landmine searching. Using eddy-current method, we constructed a metal detector based on high- T c RF SQUID, and aluminum particle as small as 10 mg could be easily detected. A small ferrous particle in food wrapped by aluminum film could also be detected when a lower excitation frequency of 30 Hz was used. We also proved mobile RF SQUID magnetometer was possible and we will use it to detect metal or concrete structures buried in underground.

Magnetic field rectifiers using semicircular Josephson junctions

A novel method for rectifying alternating magnetic fields is demonstrated using fluxons in semicircular Josephson junctions. An external magnetic field applied parallel to the dielectric barrier of the semicircular junction has opposite polarities at the ends of the junction and supports penetration of opposite polarity fluxons into the junction in the presence of a constant dc bias. When the direction of the field is reversed, flux penetration is not possible and a flux-free state exists in the junction. Thus, effective rectification of an alternating magnetic field can be achieved in semicircular Josephson junctions. This unique phenomenon is specific to this geometry and can be employed in rf SQUID magnetometers.

Application of a single-channel SQUID magnetometer for non-invasive study of cardiac tachyarrhythmias mechanisms

The underlying electrical mechanism associated with self-sustained arrhythmias such as cardiac flutter and fibrillation is not completely elucidated. Most of the evidence points towards a reentry path, but the hypothesis of firing of an ectopic focus with high frequency is still being considered. This paper evaluates the possibility of distinguishing between reentrant movement and focal excitation pulse propagation through non-invasive magnetic measurements using a SQUID system. We present experiments performed on rabbit atrial tissues immersed in a nourishing solution and submitted to different propagation patterns. The magnetic measurements were made inside a shielded chamber, by a single-channel low- T c rf-SQUID magnetometer coupled to a second-order axial gradiometer with 1.5 cm diameter coils and 4 cm baseline. The magnetic signals have been processed to yield amplitude maps, which show distinct behavior for the two different propagation mechanisms. The results obtained agree with the expected magnetic field behavior according to previous simulated studies based on cellular automata models. Therefore, the potential of the non-invasive magnetocardiographic technique for distinguishing between the primary possibilities of propagation mechanisms is corroborated, with implications in electrophysiology and clinical diagnostic studies.

Effects of the step structure on the yield, operating temperature, and the noise in step-edge Josephson junction rf-SQUID magnetometers and gradiometers

Step-edge Josephson junction rf-SQUID magnetometers and gradiometers were made using YBCO films on LaAlO 3(1 0 0) and SrTiO 3(1 0 0) substrates. Designs with 150×150 μm 2 loop and 3.6 mm diameter washer area for the magnetometers (230 pH), and 1.5 mm baseline and 1.5 mm diameter washer areas with a loop of 75×75 μm 2 for the Gradiometers (490 pH) were used. Effects of the step structure on the yield, optimal operating temperature range, and the 1/ f noise of the devices were investigated. The step structure was controlled using different ion beam etching (IBE) processes. The devices on LaAlO 3 showed high sensitivity to the IBE parameters and the step structure while this was much less for the SrTiO 3 substrate samples. This is mainly due to a considerable re-deposition of the substrate material on the step during the IBE process, in particular for LaAlO 3, resulting in very low yield and high 1/ f noise devices. The film structure at the step was also found to be essentially dependent on the step structure strongly affecting the 1/ f noise of the devices. Using an optimized “combinational IBE” process, surface modified sharp steps were prepared resulting in high yield of low 1/ f noise devices when combined with high quality YBCO film. A typical 1/ f noise corner frequency of less than about 10 Hz with a white noise level of about 20 μ Φ o/ Hz at liquid nitrogen temperature was obtained for these devices. The devices have shown stability over many thermal cycles and the time (over half a year since their fabrication) while kept at the room temperature environment. The operating temperature range of the devices was found to be controllable by the step depth and the film thickness for the steps.

1/f noise characteristics of SEJ Y-Ba-Cu-O rf-SQUIDs on LaAlO/sub 3/ substrate and the step structure, film, and temperature dependence

Step edge junction (SEJ) rf-SQUID magnetometers and gradiometers were fabricated using PLD Y-Ba-Cu-O films on LaAlO/sub 3/(100) and SrTiO/sub 3/(100) substrates. Effects of different step structure and the film properties on the yield, optimal operating temperature, and the 1/f noise of the SQUIDs were investigated. The step structure was controlled using various IBE processes. The devices on LaAlO/sub 3/ showed higher sensitivity to the step structure compared to those on SrTiO/sub 3/. This was due to re-deposition of substrate material at the steps prepared using the conventional IBE process resulting in a very low yield of unstable SQUIDs. High yield of low 1/f noise stable SQUIDs was obtained on LaAlO/sub 3/ substrates with sharp steps prepared using an optimized IBE process. A typical 1/f noise corner frequency of about 10 Hz at 77 K with two major temperature dependencies was obtained. The temperature dependencies of the 1/f noise could be correlated to the junction and the film of washer area of the SQUIDs. The white noise of our devices showed a dependency mainly on the amplitude of the flux to voltage transfer function signal. The operating temperature range of the SQUIDs could be controlled by the step structure and narrowed when the optimal operating temperature range was increased. All the measured junctions of our devices on the modified steps showed RSJ type behavior with a moderate decrease of the R/sub N/ versus temperature.