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This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-SC0015947 (scanning SQUID imaging and implementation of millikelvin microscopes) and the Cornell Center of Materials Research with funding from the NSF MRSEC program under Award No. DMR-1719875 (SQUID and microscope design). Fabrication of the microstructures was supported by the Max Planck Society and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) (Grant No. MO 3077/1-1) and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 715730). We report a scanning superconducting quantum interference device (SQUID) microscope in a cryogen-free dilution refrigerator with a base temperature at the sample stage of at least 30 mK. The microscope is rigidly mounted to the mixing chamber plate to optimize thermal anchoring of the sample. The microscope housing fits into the bore of a superconducting vector magnet, and our design accommodates a large number of wires connecting the sample and sensor. Through a combination of vibration isolation in the cryostat and a rigid microscope housing, we achieve relative vibrations between the SQUID and the sample that allow us to image with micrometer resolution over a 150 µm range while the sample stage temperature remains at base temperature. To demonstrate the capabilities of our system, we show images acquired simultaneously of the static magnetic field, magnetic susceptibility, and magnetic fields produced by a current above a superconducting micrometer-scale device. Publisher PDF

Artificial spin ices are a class of metamaterials consisting of magnetostatically coupled nanomagnets. Their interactions give rise to emergent behavior, which has the potential to be harnessed for the creation of functional materials. Consequently, the ability to map the stray field of such systems can be decisive for gaining an understanding of their properties. Here, we use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image the magnetic stray field distribution of an artificial spin ice system exhibiting structural chirality as a function of applied magnetic fields at 4.2 K. The images reveal that the magnetostatic interaction gives rise to a measurable bending of the magnetization at the edges of the nanomagnets. Micromagnetic simulations predict that, owing to the structural chirality of the system, this edge bending is asymmetric in the presence of an external field and gives rise to a preferred direction for the reversal of the magnetization. This effect is not captured by models assuming a uniform magnetization. Our technique thus provides a promising means for understanding the collective response of artificial spin ices and their interactions.

A single-sample synthesis concept based on multi-element ceramic samples can produce a variety of local products. When applied to cuprate superconductors (SC), statistical modelling predicts the occurrence of possible compounds in a concentration range of ∼50 ppm. In samples with such low concentrations, determining which compositions are superconducting is a challenging task and requires local probes or separation techniques. Here, we report results from samples with seven components: BaO2, CaCO3, SrCO3, La2O3, PbCO3, ZrO2 and CuO oxides and carbonates, starting from different grain sizes. The reacted ceramics show different phases, particular grain growth, as well as variations in homogeneity and superconducting properties. High-Tc superconductivity up to 118 K was found. Powder x-ray diffraction (XRD) in combination with energy-dispersive spectroscopy (EDS), scanning transmission electron microscopy (STEM) can assign “Pb1223” and “(Sr,Ca,Ba)0.7-1.0CuO2” phases in inhomogeneous samples milled with 10 mm ball sizes. Rather uniform samples featuring strong grain growth were obtained with 3 mm ball sizes, resulting in Tc ∼70 K superconductivity of the “La(Ba,Ca)2Cu3Ox” based phase. Scanning SQUID microscopy (SSM) establishes locally formed superconducting areas at a level of a few microns in inhomogeneous superconducting particles captured by a magnetic separation technique. The present results demonstrate a new synthetic approach for attaining high-Tc superconductivity in compounds without Bi, Tl, Hg, or the need for high-pressure synthesis.

Speleothems are ideal archives of environmental magnetism and paleomagnetism, since they retain continuous magnetic signals in stable conditions and can be used for reliable radiometric dating using U-series and radiocarbon methods. However, their weak magnetic signals hinder the widespread use of this archive in the field of geoscience. While previous studies successfully reconstructed paleomagnetic signatures and paleoenvironmental changes, the time resolutions presented were insufficient. Recently emerging scanning SQUID microscopy (SSM) in this field can image very weak magnetic fields while maintaining high spatial resolution that could likely overcome this obstacle. In this study, we employed SSM for high spatial resolution magnetic mapping on a stalagmite collected at Anahulu cave in Tongatapu Island, the Kingdom of Tonga. The average measured magnetic field after 5 mT alternating field demagnetization is ca. 0.27 nT with a sensor-to-sample distance of ~ 200 µm. A stronger magnetic field (average: ca. 0.62 nT) was observed above the grayish surface layer compared to that of the white inner part (average: ca. 0.09 nT) associated with the laminated structures of the speleothem at the submillimeter scale, which scanning resolution of the SSM in this study is comparable to the annual growth rates of the speleothem. The magnetization of the speleothem sample calculated from an inversion of isothermal remanent magnetization (IRM) also suggests that the magnetic mineral content in the surface layer is higher than the inner part. This feature was further investigated by low-temperature magnetometry. Our results show that the main magnetic carriers of the speleothem under study are magnetite and maghemite and it can contain hematite or ε-Fe2O3. The first-order reversal curve (FORC) measurements and the decomposition of IRM curves show that this speleothem contains a mixture of magnetic minerals with different coercivities and domain states. The contribution from maghemite to the total magnetization of the grayish surface layer was much higher than the white inner part. Such differences in magnetic mineralogy of the grayish surface layer from that of the inner part suggest that the depositional environment shifted and was likely changed due to the oxidative environment.

The Dirac semimetal PdTe${}_2$ becomes superconducting at a temperature $T_{c}=1.6$~K. Thermodynamic and muon spin rotation experiments support type-I superconductivity, which is unusual for a binary compound. A key property of a type-I superconductor is the intermediate state which presents a coexistence of superconducting and normal domains (flux structures) at magnetic fields lower than the thermodynamic critical field $H_{c}$. By means of scanning SQUID microscopy (SSM) we observe flux structures in the superconducting state of PdTe${}_2$. The flux structures are strongly history dependent with a transition from round shapes to laminar shapes as the magnetic field is more and more increased. The field amplitudes measured at the surface are indicative for the presence of Landau branching. The domain wall width in the intermediate state has been determined., 8 pages 5 figures

This chapter reviews scanning probe microscopy methods that measure the field above a sample contain chiral spin textures. Among those microscopy methods are magnetic force microscopy (MFM), nitrogen vacancy (NV) center microscopy, scanning Hall probe microscopy, and scanning SQUID microscopy. However, only the first two techniques have been applied for the study of chiral spin textures. They can obtain a lateral resolution down to about 10 nm and are sufficiently sensitive to map the stray field from ferromagnetic layer with a thicknesses down to a few atoms. Even submonolayer of spins can be imaged. The information on the magnetic moment distribution inside the sample that can be obtained from the stray field however remains limited because different magnetic moment configurations can generate the same stray field. Thus in most cases the field of candidate magnetization structures is fitted to experimental results. An important advantage of methods imaging the stray field is that these can be applied to sample where the magnetic layer is inside the sample, i.e., below a layer protecting it against oxidation. This makes these experimental methods suitable for the reliable imaging of technologically relevant samples.

Dynamics and textures of magnetic domain walls (DWs) may largely alter the electronic behaviors in a Weyl semimetal system via emergent gauge fields. However, very little is known about even the basic properties of these domain walls in Weyl materials. In this work, we imaged the spontaneous magnetization and magnetic susceptibility of a ferromagnetic (FM) Weyl semimetal CeAlSi using scanning SQUID microscopy. We observed the ferromagnetic DWs lined-up with the [100] direction (or other degenerate directions). We also discovered the coexistence of stable and metastable domain phases, which arise likely due to magnetoelastic and magnetostriction effects and are expected to be highly tunable with small strains. We applied an in-plane external field as the CeAlSi sample was cooled down to below the magnetic phase transition of 8.3K, showing that the pattern of FM domains is strongly correlated with both the amplitude and the orientation of the external field even for weak fields of a few Gausses. The area of stable domains increases with field and reaches maximum when the field is parallel to the main crystallographic axes of the CeAlSi crystal. Our results suggest that the manipulation of these heterogeneous phases can provide a practical way to study the interplay between magnetism and electronic properties in Weyl systems, and that these systems can even serve as a new platform for magnetic sensors.

We have developed planar nanoSQUID with nanobridge-type Josephson junctions based on the oxidation resistant and high H c2 MoRe alloy. The objective of the research was to reduce size of the SQUID loop with the aim being to reduce magnetic flux noise and improve the spatial resolution of the SQUID sensors. Employing RF-magnetron sputtering, electron-beam lithography, and reactive ion etching in CHF3 + O2 plasma using Al hard masks, we have realized nanoSQUIDs with Josephson junctions in the form of 30 − 50 nm wide nanobridges and an effective magnetic flux capture radius of ~ 95 nm. The critical temperature of the fabricated devices was T c = 7.9 K. The I(V)-characteristics demonstrated critical current I 0sime 114 µA at 4.2 K and modulation period in magnetic fields of ~ 700 Oe.

We have developed Josephson junctions between the d-wave superconductor YBa2Cu3O7−x (YBCO) and the s-wave Mo0.6Re0.4 (MoRe) alloy superconductor (ds-JJs). Such ds Josephson junctions are of interest for superconducting electronics making use of incorporated π-phase shifts. The I(V)-characteristics of the ds-JJs demonstrate a twice larger critical current along the [100] axis of the YBCO film compared to similarly-oriented ds-JJs made with a Nb top electrode. The characteristic voltage I c R n of the YBCO–Au–MoRe ds-JJs is 750 μV at 4.2 K. The ds-JJs that are oriented along the [100] or [010] axes of the YBCO film exhibit a 200 times higher critical current than similar ds-JJs oriented along the [110] axis of the same YBCO film. A critical current density J c = 20 kA cm−2 at 4.2 K was achieved. Different layouts of π-loops based on the novel ds-JJs were arranged in various mutual coupling configurations. Spontaneous persistent currents in the π-loops were investigated using scanning SQUID microscopy. Magnetic states of the π-loops were manipulated by currents in integrated bias lines. Higher flux states up to ±2.5Φ0 were induced and stabilized in the π-loops. Crossover temperatures between thermally activated and quantum tunneling switching processes in the ds-JJs were estimated. The demonstrated ability to stabilise and manipulate states of π-loops paves the way towards new computing concepts such as quantum annealing computing.

This chapter focuses on ferromagnetic nanotubes (FNTs) and their experimental investigation by techniques capable of measuring the response of individual specimens. This capability has developed recently and is now having a strong impact on our understanding of the behavior of real FNT structures. Despite the wealth of predictions on the equilibrium configurations and reversal modes of FNTs, direct experimental evidence of their magnetic behavior has emerged only recently. This is partly due to the difficulty of measuring individual magnetic nanostructures with many conventional magnetometry techniques, which do not have the necessary sensitivity to detect the small total magnetic moment of a single nanomagnet. As a result, measurements of their magnetic properties are often carried out on large ensembles, whose constituent nanomagnets have a distribution of size, shape, and orientation and—depending on the density—may interact with each other. These complications conspire to make accurate characterization of the stable magnetization configurations and reversal processes difficult. To obtain a clear understanding of the magnetic properties of FNTs, it is therefore advantageous to investigate individual specimens. We review such efforts, which include measurements of magnetoresistence, torque magnetometry, X-ray microscopy, scanning SQUID microscopy, and magnetic force microscopy.

We report a vector-scanning SQUID microscope for probing a vector magnetic field. We designed and fabricated a vector [three-dimensional (3-D)] SQUID sensor with the aid of Nb multilayer fabrication technology. Our vector SQUID sensor on a single chip is composed of a vector sensor with three orthogonal pick-up coils, which catch the X , Y, and Z magnetic field components. We improved both the sensitivity and the spatial resolution not only by reducing the inner diameter but also by increasing the number of pick-up-coil windings within the framework of the multilayer technique. Our SQUID sensors operated properly, and the characteristics showed a good agreement with our design parameters. Since the sensor is suitable for our SQUID microscope, we are constructing our scanning microscope platform by using a Gifford-McMahon cryocooler for allowing long-time measurements. We also succeeded in connecting our vector SQUID sensor to a commercial flux-locked loop readout circuit (Magnicon Inc., Model XXF-1) to generate the voltage outputs, which are proportional to the XYZ components of the external magnetic field. We also use an XYZ piezo-driven scanner for the precise control of the pick-up-coil positions over the 3-D range of 5 × 5 × 5 mm 3 with a step size of 10 nm at 4 K.

A promising direction to improve the sensitivity of a SQUID is to increase its junction's normal resistance value, R n , as the SQUID modulation voltage scales linearly with R n . As a first step to develop highly sensitive single layer SQUID, submicron scale YBCO grain boundary step edge junctions and SQUIDs with large R n were fabricated and studied. The step-edge junctions were reduced to submicron scale to increase their R n values using focus ion beam, FIB and the measurement of transport properties were performed from 4.3 to 77 K. The FIB induced deposition layer proves to be effective to minimize the Ga ion contamination during the FIB milling process. The critical current–normal resistance value of submicron junction at 4.3 K was found to be 1–3 mV, comparable to the value of the same type of junction in micron scale. The submicron junction R n value is in the range of 35–100 Ω, resulting a large SQUID modulation voltage in a wide temperature range. This performance promotes further investigation of cryogen-free, high field sensitivity SQUID applications at medium low temperature, e.g. at 40–60 K.

We have fabricated YBa2Cu3O7 (YBCO) dc SQUIDs containing nanobridges across twin boundaries of LaAlO3 (LAO) substrates as Josephson elements by using a focused ion beam (FIB) etching method and measured their transport properties. The beam energy was 30 keV and the current was 1.5 pA for the nanobridge pattern. Each bridge with a nominal width of 200 nm crossed a twin boundary in the (100) direction. The SQUID loop had a 10 μm × 10 μm hole with a 5.7 μm average linewidth. The SQUID voltage showed modulations in response to the external flux with a maximum modulation depth of 350 μV at 77.0 K. HR-XRD spectra showed that the epitaxially grown YBCO film was twinned in commensurate with the twinning of the LAO substrate. Tilting of the c-axis of YBCO across the twin boundary is believed to play a role as a tunnel barrier.

Superconducting quantum arrays (SQAs) capable of providing highly linear voltage response to magnetic signal and high dynamic range have been suggested and developed. Base elements of the arrays, quantum cells, were devised and studied in detail. Using niobium process, SQAs with different number of the cells and prototypes of the SQA-based broadband active electrically small antennas were fabricated and tested.

Ferromanganese crusts record long-term deep-sea environmental changes. Thus, providing their reliable high-resolution age models is important. We applied a magnetostratigraphic technique to estimate the growth rate of a ferromanganese crust using scanning SQUID (superconducting quantum interference device) microscope (SSM). SSM is designed to map the magnetic field across thins sections with sub-millimeter resolution. The crust sample was taken from the Takuyo–Daigo Seamount, northwestern Pacific, and recorded a limited supply of dust and sediment from continents. After drift correction and removal of spike noises, the magnetic field values were stacked within the areas of high signal-to-noise ratios. By correlating the obtained profiles with a standard geomagnetic polarity timescale, we obtained an average growth rate of 3.37 ± 0.06 mm/m.y., which is consistent with that obtained by 10Be/9Be geochronology (2.93 ± 0.15 mm/m.y.). S-ratio mapping shows low values after ~3 Ma, associated with voids between columnar structures.

We have developed an all-NbN-integrated dc SQUID magnetometer for future applications in the higher operating temperature by taking advantage of NbN films’ higher critical temperature about 16 K. Our NbN dc SQUID magnetometer is fabricated on a single crystal MgO (100) substrate, consisting of epitaxial NbN/AlN/NbN Josephson junctions, Ti/Pd shunt resistors, and a top 400-nm-thick NbN wiring layer. The critical current, inductance, and shunt resistance of NbN SQUID are set at 20 μA, 240 pH, and 1 Ω, respectively. We measured the current–voltage characteristics, voltage–flux characteristics and flux noise of the NbN SQUID magnetometer in a superconductive niobium shield at 4.2 K. In a flux locked loop operation mode, the NbN SQUID magnetometer exhibits excellent performances with a flux noise level of 6 μΦ0 /√Hz in the white region. The corner frequency is only 10 Hz. Based on the calibrated magnetic field sensitivity of 1.7 nT/Φ0 at magnetically shielded room, the magnetic field resolution of 10 fT/√Hz is achieved.

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 trapping is a very serious problem in SQUID-based instrumentation and superconducting electronics. A critical consideration for avoiding flux trapping is a prevention of SQUID sensors or circuits from being exposed to magnetic fields. Unfortunately, this measure cannot be applied, if sensors must be exposed to a strong magnetic field or transients. For example, unavoidable exposures take place in ultralow field magnetic resonance imaging and superparamagnetic relaxation measurements using unshielded thin-film planar SQUID sensors. SQUID sensors stop working after being exposed to magnetic fields of only a few mT in strength. Earlier, we reported successful results of defluxing of planar LTS SQUID-gradiometers using strong decaying ac magnetic pulses. In this paper, we present further experimental results and discuss a possible mechanism to explain the observed defluxing effects. The new ac defluxing technique is much faster and dissipates much less energy than a conventional thermal cycling.

Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current–voltage characteristics in the absence of any shunting resistance.

We report the application of a scanning high- $T_{{\rm{c}}}$ SQUID (superconducting quantum interference device) microscope with a ferromagnetic flux guide for the nondestructive evaluation of weld seams and wear tracks and scars on austenitic stainless steel plates as well as measurement of magnetic stray fields distribution above patterned by electron lithography 30-nm-thick cobalt films. A soft magnetic amorphous Vitrovac foil was used to guide the flux from the samples, which were held at room temperature, to the liquid-nitrogen-cooled SQUID-sensor and back. The flux guide passes through a hole in the $\text{1 mm}\times \text{2.5 mm}$ pick-up loop of the high- $T_{{\rm{c}}}$ SQUID sensor, thereby providing improved coupling of the magnetic flux from the object to the SQUID. In order to avoid the influence of the SQUID biasing the magnetic field on the object under investigation, a modulation and feedback coil was coupled to the pick-up loop of the SQUID directly and beyond the ferromagnetic flux guide. Such decoupling of feedback coil from soft magnetic flux antenna ensures that the high- $T_{{\rm{c}}}$ SQUID microscope does not disturb the sample magnetization during image recording. The SQUID microscope can be used to measure the spatial distribution of the z -component of the stray field above a specimen without mechanical contact to it.

We designed a digital superconducting quantum interference device (SQUID) system that contains a double relaxation oscillation SQUID (DROS) as a magnetic flux comparator that is connected to an additional relaxation oscillation (RO) circuit with mutually coupled inductances. In the designed digital SQUID, the RO circuit works as a resonator and increases the relaxation oscillation frequency without decreasing the input current of dc/SFQ converters. Numerical simulation shows that the digital SQUID system can operate up to 7 GHz. A fabricated digital SQUID using a DROS comparator with an RO circuit provides correct digital SQUID operation. For a magnetometer using the digital SQUID system, we can realize a magnetic field noise less than 1 pT/ $\sqrt{\mathrm{Hz}}$ with a small signal bandwidth of more than 10 MHz.

We developed a metallic contaminant detector using a high-Tc rf-SQUID with a Cu pickup coil for industrial products. For manufacturers producing industrial products, problems arising from metallic contaminants are critical issues. A detection system using a SQUID is a powerful tool for sensitive inspections. However, since the detection width is dependent on the size of the SQUID, multiple sensors are required for practical use. However, such devices are costly and complicated. This is the reason as to why the SQUID system has not been widely used in the field. Therefore, we have recently proposed a new detection system using Faraday's law of electromagnetic induction. The detection section consists of a pair of permanent magnets surrounded by copper wound pickup coils. The output of the pickup coil is remotely connected to a copper wound input coil. The input coil couples magnetically with a SQUID. In this case, the width of the pickup coil can be widened as to be as large as 27 mm in diameter. As compared with 1-ch SQUID direct detection, the width of the new method is three times larger. In this paper, we describe the performance of the system with a conveyor, which uses an LC resonant circuit. By applying the LC resonance, the amplitude increased and the signal to noise ratio ( SNR ) was improved by a factor of two.

We propose a novel type of magnetic scanning probe sensor, based on a single planar Josephson junction with a magnetic barrier. The planar geometry together with high magnetic permeability of the barrier helps to focus flux in the junction and thus enhance the sensitivity of the sensor. As a result, it may outperform equally sized SQUID both in terms of the magnetic field sensitivity and the spatial resolution in one scanning direction. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic CuNi and a ferromagnetic Ni barrier. We demonstrate that the planar geometry allows easy miniaturization to nm-scale, facilitates an effective utilization of the self-field phenomenon for amplification of sensitivity and a simple implementation of a control line for feed-back operation in a broad dynamic range., Comment: 5 pages, 2 figures

A prototype of gradiometer for detection and analysis of magnetic signals that are generated by defects in metal structures and materials in the presence of external magnetic bias is based on dc-current superconducting quantum interference device (SQUID). A prototype of single-channel SQUID gradiometer that contains a fiberglass nonmagnetic cryostat, measurement probe with the SQUID sensor and magnetic flux transformer (second-order axial gradiometer), SQUID electronics, and software for control of SQUID gradiometer is studied. The prototype exhibits stable operation under laboratory conditions in the absence of additional magnetic shielding. Upgrade of the SQUID sensors and remaining elements of the prototype of magnetometer is planned for application in nondestructive testing.

Exploration of areas with thick low resistivity overburden is still a challenge for time domain transient electromagnetic method (TEM). We report modeling of a sandwich-layered earth by simulating the B field response with different conductive target layer thicknesses, thus obtaining a relationship between the resolution of the B field and the exploration depth. A low temperature Superconducting Quantum Interference Device (SQUID) is an ideal sensor for measuring the secondary magnetic field B in TEM measurements, because its sensitivity of several fT/√Hz is independent of frequency. In our TEM experiments, we utilized two different coils as receivers, a simple SQUID system, and a large transmitter loop of 200 × 200 m2 to compare the detected decay curves. At some measurement points, a decay signal of more than 300 ms duration was obtained by using the SQUID. Apparent resistivity profiles of about 9 km length are presented.

A superconducting quantum interference device (SQUID) miniaturized into the nanoscale is promising in the inductive detection of a single electron spin. A nano-SQUID with a strong spin coupling coefficient, a low flux noise, and a wide working magnetic field range is highly desired in a single spin resonance measurement. Nano-SQUIDs with Dayem bridge junctions excel in a high working field range and in the direct coupling from spins to the bridge. However, the common planar structure of nano-SQUIDs is known for problems such as a shallow flux modulation depth and a troublesome hysteresis in current–voltage curves. Here, we developed a fabrication process for creating three-dimensional (3-D) niobium (Nb) nano-SQUIDs with nanobridge junctions that can be tuned independently. Characterization of the device shows up to 45.9% modulation depth with a reversible current–voltage curve. Owning to the large modulation depth, the measured flux noise is as low as 0.34 μΦ0/Hz1/2. The working field range of the SQUID i...

A topology of thin-film SQUID sensors that are based on Nb/AlO x /Nb tunnel junctions has been developed and optimized for nondestructive testing of materials and for other systems with a magnetic field sensitivity of

Measurement methods with SQUID can accurately detect small magnetic metal contaminants based on their magnetic remanence. But, a high-frequency excitation is necessary to detect nonmagnetic metals, on the base of contrasts in electric conductivity. In this work, an open loop technique is introduced to facilitate this. The SQUID is negative feedback controlled (flux locked loop (FLL) operation) for the low frequency range, which includes significant noise due to the movement of the magnetic body or the change of the ambient magnetic field composed of the geomagnetic field and technical signals, and it operates in an open loop configuration for the high frequency range. When using the open loop technique, negative feedback is not applied to the high frequency range. Consequently, the V–Φ characteristic changes due to various causes, which leads to variations in the conversion factor between the SQUID output voltage and the magnetic field. In this study, conversion techniques for the magnetic field for open loop operation of SQUID in the high frequency range are examined.

We have developed a high-spatial-resolution superconducting quantum interference device (SQUID) microscope for magnetic field imaging of rock samples at room temperature, using a hollow-structured cryostat. A directly coupled low-temperature SQUID with a $200\ \mu\text{m} \times 200\ \mu\text{m}$ pickup loop, which is mounted on a sapphire conical rod, is separated from room temperature by a thin sapphire window. Precise and repeatable adjustment of the vacuum gap between the SQUID and the sapphire window is performed by rotating a micrometer spindle connected to the sapphire rod through the hollow portion of the cryostat. A spacing of 230 $\mu\text{m}$ between the SQUID and a sample has been achieved. We demonstrated the imaging of the magnetic field on a zircon containing magnetic grains.