Superconducting Quantum Interference Device Microscopy Research Articles

SQUID Microscope With Hollow-Structured Cryostat for Magnetic Field Imaging of Room Temperature Samples

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.

Vortex distribution in amorphous Mo80Ge20 plates with artificial pinning center

Vortices in superconductor give rise to a rich variety of phenomena because they interact with shielding currents, temperature gradients, sample defects, boundaries, and other neighboring vortices. It would be very important to understand particular features of vortex states in a downsized system. Our study focuses on vortex states in small star-shaped Mo80Ge20 plates with and without an artificial pin at the plate center. Vortex states are greatly influenced by the sample geometry, the temperature and the magnetic field, and they can be occasionally exotic compared to the bulk case. We use the amorphous Mo80Ge20 films due to the nature of weak pinning in studying vortex configurations. We applied scanning superconducting quantum interference device (SQUID) microscopy because it enables us to see vortex states directly and it is the most sensitive instrument for mapping tiny local current flows or magnetic moments without damaging the sample. We interpreted that vortex configurations had essentially the nature of mirror reflection symmetry in both cases with an artificial pin and without an artificial pin and pinned cases while the influence of disorder was seen in our observation on the specimen without an artificial pin.

Analysis of low-field isotropic vortex glass containing vortex groups in YBa2Cu3O(7-x) thin films visualized by scanning SQUID microscopy.

The glass-like vortex distribution in pulsed laser deposited YBa2Cu3O7 − x thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.

Nonsinusoidal current-phase relationship in Josephson junctions from the 3D topological insulator HgTe.

We use superconducting quantum interference device microscopy to characterize the current-phase relation (CPR) of Josephson junctions from the three-dimensional topological insulator HgTe (3D HgTe). We find clear skewness in the CPRs of HgTe junctions ranging in length from 200 to 600nm. The skewness indicates that the Josephson current is predominantly carried by Andreev bound states with high transmittance, and the fact that the skewness persists in junctions that are longer than the mean free path suggests that the effect may be related to the helical nature of the Andreev bound states in the surface of HgTe. These experimental results suggest that the topological properties of the normal state can be inherited by the induced superconducting state, and that 3D HgTe is a promising material for realizing the many exciting proposals that require a topological superconductor.

SQUID-SIMS is a useful approach to uncover primary signals in the Archean sulfur cycle.

Many aspects of Earth's early sulfur cycle, from the origin of mass-anomalous fractionations to the degree of biological participation, remain poorly understood--in part due to complications from postdepositional diagenetic and metamorphic processes. Using a combination of scanning high-resolution magnetic superconducting quantum interference device (SQUID) microscopy and secondary ion mass spectrometry (SIMS) of sulfur isotopes ((32)S, (33)S, and (34)S), we examined drill core samples from slope and basinal environments adjacent to a major Late Archean (∼2.6-2.5 Ga) marine carbonate platform from South Africa. Coupled with petrography, these techniques can untangle the complex history of mineralization in samples containing diverse sulfur-bearing phases. We focused on pyrite nodules, precipitated in shallow sediments. These textures record systematic spatial differences in both mass-dependent and mass-anomalous sulfur-isotopic composition over length scales of even a few hundred microns. Petrography and magnetic imaging demonstrate that mass-anomalous fractionations were acquired before burial and compaction, but also show evidence of postdepositional alteration 500 million y after deposition. Using magnetic imaging to screen for primary phases, we observed large spatial gradients in Δ(33)S (>4‰) in nodules, pointing to substantial environmental heterogeneity and dynamic mixing of sulfur pools on geologically rapid timescales. In other nodules, large systematic radial δ(34)S gradients (>20‰) were observed, from low values near their centers increasing to high values near their rims. These fractionations support hypotheses that microbial sulfate reduction was an important metabolism in organic-rich Archean environments--even in an Archean ocean basin dominated by iron chemistry.

Room Temperature Sample Scanning SQUID Microscope for Imaging the Magnetic Fields of Geological Specimens

A microscope to image weak magnetic fields using a low-temperature superconducting quantum interference device (SQUID) had developed with a liquid helium consumption rate of ~0.5L/hour. The gradient pickup coil is made by a low-temperature superconducting niobium wire with a diameter of 66 μm, which is coupled to the input circuit of the SQUID and is then enwound on the sapphire bobbin. Both of the pickup coil and the SQUID sensor are installed in a red copper cold finger, which is thermally anchored to the liquid helium evaporation platform in the vacuum space of the cryostat. To reduce the distance between the pickup coil and sample, a 100 μm thick sapphire window is nestled up to the bottom of the cryostat. A three-dimensional scanning stage platform with a 50 cm Teflon sample rack under the sapphire window had the precision of 10 μm. To test the fidelity of the new facility, the distribution of the magnetic field of basalt slice specimens was determined. Results show that the spatial resolution of the newly-designed facility is 500 μm with a gradient magnetic field sensitivity of 380fT. This opens new opportunities in examining the distribution of magnetic assemblages in samples, which bear great geological and geophysical information.

Locally enhanced conductivity due to the tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces

The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO$_{3}$ and TiO$_{2}$-terminated SrTiO$_{3}$ (STO). Transport and other measurements in this system show a plethora of diverse physical phenomena. To better understand the interface conductivity, we used scanning superconducting quantum interference device microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. The interplay between substrate domains and the interface provides an additional mechanism for understanding and controlling the behaviour of heterostructures.

Optical magnetic imaging of living cells

Magnetic imaging is a powerful tool for probing biological and physical systems. However, existing techniques either have poor spatial resolution compared to optical microscopy and are hence not generally applicable to imaging of sub-cellular structure (e.g., magnetic resonance imaging [MRI]1), or entail operating conditions that preclude application to living biological samples while providing sub-micron resolution (e.g., scanning superconducting quantum interference device [SQUID] microscopy2, electron holography3, and magnetic resonance force microscopy [MRFM]4). Here we demonstrate magnetic imaging of living cells (magnetotactic bacteria) under ambient laboratory conditions and with sub-cellular spatial resolution (400 nm), using an optically-detected magnetic field imaging array consisting of a nanoscale layer of nitrogen-vacancy (NV) colour centres implanted at the surface of a diamond chip. With the bacteria placed on the diamond surface, we optically probe the NV quantum spin states and rapidly reconstruct images of the vector components of the magnetic field created by chains of magnetic nanoparticles (magnetosomes) produced in the bacteria, and spatially correlate these magnetic field maps with optical images acquired in the same apparatus. Wide-field sCMOS acquisition allows parallel optical and magnetic imaging of multiple cells in a population with sub-micron resolution and >100 micron field-of-view. Scanning electron microscope (SEM) images of the bacteria confirm that the correlated optical and magnetic images can be used to locate and characterize the magnetosomes in each bacterium. The results provide a new capability for imaging bio-magnetic structures in living cells under ambient conditions with high spatial resolution, and will enable the mapping of a wide range of magnetic signals within cells and cellular networks5, 6.

Direct Observation of Local Shielding Currents in Superconducting Thin Films under Low Magnetic Field by Scanning Superconducting Quantum Interference Device Microscopy

We have observed local shielding current flows in oxide superconducting thin films under low magnetic field by scanning probe microscopy using a superconducting quantum interference device (SQUID). The strength and direction of current flow were characterized simultaneously and directly by observing magnetic signals around artificial holes. This method allows us to map the current flow without reverse Fourier transform. In this paper, experimental data and analyses of the current flows in oxide superconducting thin films up to 50 µT are shown. Furthermore, results of calculation based on a very simple model are also presented. The possibility of using scanning SQUID microscopy to characterize current flow is presented.

Direct Observation of Local Shielding Currents in Superconducting Thin Films under Low Magnetic Field by Scanning Superconducting Quantum Interference Device Microscopy

We have observed local shielding current flows in oxide superconducting thin films under low magnetic field by scanning probe microscopy using a superconducting quantum interference device (SQUID). The strength and direction of current flow were characterized simultaneously and directly by observing magnetic signals around artificial holes. This method allows us to map the current flow without reverse Fourier transform. In this paper, experimental data and analyses of the current flows in oxide superconducting thin films up to 50 µT are shown. Furthermore, results of calculation based on a very simple model are also presented. The possibility of using scanning SQUID microscopy to characterize current flow is presented.

SQUID microscopy of magnetic field induced in solar cell by laser spot irradiation

A solar cell with surface stripe electrodes was investigated by laser–superconducting quantum interference device microscopy (laser-SQUID microscopy) using two scan methods: the standard method and our new approach. In the standard method, the sample was raster scanned while the positions of the laser irradiation spot and the SQUID were fixed. The resulting magnetic images reflected some defects related to the grain boundaries on the solar cell. Background contrast fluctuations also exist in the images. For a better understanding of these fluctuations, we developed a method to investigate the photocurrent distributions on the solar cell around the laser spot. In this method, the sample was raster scanned with the laser spot fixed to a certain position by means of an optical fiber. We converted the magnetic images of the sample to photocurrent images. The results showed that the anisotropic photocurrent mainly flowed along the electrode near the laser spot rather than in the area around the spot. Therefore, the arrangement of the surface stripe electrodes affected the magnetic images obtained by the standard method in laser-SQUID microscopy.

Domain walls in normal and superconducting states of iron pnictides

The electronic and magnetic structures in the normal and superconducting states of iron pnictides are investigated by solving self-consistently the Bogoliubov-de Gennes equation. It is shown that strong electron correlations can induce domain walls, which separate regions with different spin density wave orders. At zero or low electron doping, $90^\circ$ domain walls are formed while anti-phase domain walls are produced at higher electron doping. On the domain walls, larger electron densities are always present. The results agree qualitatively with recent observations of scanning tunneling microscopy and superconducting quantum interference device microscopy.

Ultrafine-scale magnetostratigraphy of marine ferromanganese crust

Hydrogenetic ferromanganese crusts are iron-manganese oxide chemical precipitates on the seafloor that grow over periods of tens of millions of years. Their secular records of chemical, mineralogical, and textural variations are archives of deep-sea environmental changes. However, environmental reconstruction requires reliable high-resolution age dating. Earlier chronological methods using radiochemical and stable isotopes provided age models for ferromanganese crusts, but have limitations on the millimeter scale. For example, the reliability of 10 Be/ 9 Be chronometry, commonly considered the most reliable technique, depends on the assumption that the production and preservation of 10 Be are constant, and requires accurate knowledge of the 10 Be half-life. To overcome these limitations, we applied an alternative chronometric technique, magnetostratigraphy, to a 50-mm-thick hydrogenetic ferromanganese crust (D96-m4) from the northwest Pacific. Submillimeter-scale magnetic stripes originating from approximately oppositely magnetized regions oriented parallel to bedding were clearly recognized on thin sections of the crust using a high-resolution magnetometry technique called scanning SQUID (superconducting quantum interference device) microscopy. By correlating the boundaries of the magnetic stripes with known geomagnetic reversals, we determined an average growth rate of 5.1 ± 0.2 mm/m.y., which is within 16% of that deduced from the 10 Be/ 9 Be method (6.0 ± 0.2 mm/m.y.). This is the finest-scale magnetostratigraphic study of a geologic sample to date. Ultrafine-scale magnetostratigraphy using SQUID microscopy is a powerful new chronological tool for estimating ages and growth rates for hydrogenetic ferromanganese crusts. It provides chronological constraints with the accuracy promised by the astronomically calibrated magnetostratigraphic time scale (1–40 k.y.).

Measurements of Transmembrane Potential and Magnetic Field at the Apex of the Heart

We studied the transmembrane potential and magnetic fields from electrical activity at the apex of the isolated rabbit heart experimentally using optical mapping and superconducting quantum interference device microscopy, and theoretically using monodomain and bidomain models. The cardiac apex has a complex spiral fiber architecture that plays an important role in the development and propagation of action currents during stimulation at the apex. This spiral fiber orientation contains both radial electric currents that contribute to the electrocardiogram and electrically silent circular currents that cannot be detected by the electrocardiogram but are detectable by their magnetic field, B z . In our experiments, the transmembrane potential, V m , was first measured optically and then B z was measured with a superconducting quantum interference device microscope. Based on a simple model of the spiral structure of the apex, V m was expected to exhibit circular wave front patterns and B z to reflect the circular component of the action currents. Although the circular V m wave fronts were detected, the B z maps were not as simple as expected. However, we observed a pattern consistent with a tilted axis for the apex spiral fiber geometry. We were able to simulate similar patterns in both a monodomain model of a tilted stack of rings of dipole current and a bidomain model of a tilted stack of spiraled cardiac tissue that was stimulated at the apex. The fact that the spatial pattern of the magnetic data was more complex than the simple circles observed for V m suggests that the magnetic data contain information that cannot be found electrically.

Stripes of increased diamagnetic susceptibility in underdoped superconductingBa(Fe1−xCox)2As2single crystals: Evidence for an enhanced superfluid density at twin boundaries

Superconducting quantum interference device (SQUID) microscopy shows stripes of increased diamagnetic susceptibility in underdoped, but not overdoped, single crystals of Ba(Fe1-xCox)2As2. These stripes of increased diamagnetic susceptibility are consistent with enhanced superfluid density on twin boundaries. Individual vortices avoid pinning on or crossing the stripes, and prefer to travel parallel to them. These results indicate a relationship between superfluid density, local strain, and frustrated magnetism, and demonstrate two mechanisms for enhancing critical currents.

Meissner response of a bulk superconductor with an embedded sheet of reduced penetration depth

We calculate the change in susceptibility resulting from a thin sheet with reduced penetration depth embedded perpendicular to the surface of an isotropic superconductor, in a geometry applicable to scanning Superconducting QUantum Interference Device (SQUID) microscopy, by numerically solving Maxwell's and London's equations using the finite element method. The predicted stripes in susceptibility agree well in shape with the observations of Kalisky et al. of enhanced susceptibility above twin planes in the underdoped pnictide superconductor Ba(Fe1-xCox)2As2 (Ba-122). By comparing the predicted stripe amplitudes with experiment and using the London relation between penetration depth and superfluid density, we estimate the enhanced Cooper pair density on the twin planes, and the barrier force for a vortex to cross a twin plane. Fits to the observed temperature dependence of the stripe amplitude suggest that the twin planes have a higher critical temperature than the bulk, although stripes are not observed above the bulk critical temperature.

Scanning magnetic microscopy model analysis of circular flaws in thin metallic plates

We studied the eddy currents excited by a time varying external magnetic field in thin metallic plates in the presence of a circular hole piercing the plate. The value of the normal component of the magnetic field over the circular defect is analytically calculated and a complete scanning magnetic operation along a line crossing the defect is simulated. The analytical solution is then tested against a direct numerical simulation with good results. The aim is the reconstruction and interpretation of magnetic signatures due to structural defects in nondestructive evaluation made by superconducting quantum interference device microscopy measurements.

Prospects for imaging magnetic nanoparticles using a scanning SQUID microscope

In this paper I estimate the size of the signals expected for superparamagnetic andferromagnetic nanoparticles using a SQUID (superconducting quantum interference device)microscope with varying loop sizes and loop to sample spacings in the magnetometer andsusceptometer modes and compare these signals to intrinsic noise levels in SQUID sensors.

An Analysis to Improve Spatial Resolution of Scanning Superconducting Quantum Interference Device Microscope

It is shown that the spatial resolution of scanning superconducting quantum interference device (SQUID) microscope, which is considered naively to be limited by the size of the pickup coil (typically grater than of the order of μm), can be improved by applying a mathematical transformation to the obtained data. The resolution of the image is improved at best to the scanning step size (can be less than of the order of μm) even in the presense of the noise. Problems to be solved in applying the method to visualize the fluxons in superconducting network structures are discussed by use of numerical simulations and of real experimental data.

METALLIC CONTAMINANT DETECTION SYSTEM FOR INDUSTRIAL PRODUCTS BY HIGH TC SQUID MAGNETIC SENSOR

High-Tc superconducting quantum interference device (SQUID) system for detection of magnetic foreign matter in industrial products was developed. There is a possibility that ultra-small metallic foreign matter has been accidentally mixed with industrial products, such as lithium ion batteries. The outer dimension of metallic particles less than 100 microns cannot be detected by conventional X-ray imaging. Therefore, we developed a detection system based on high-Tc SQUID microscopes with a high-performance magnetic shield. Using SQUID microscopes with a 0.5 mm-thick vacuum window was proposed. This design enables the SQUID to approach an object to be measured as close as 1 mm and enhances the sensitivity. A new magnetic shield with sleeves was carefully designed and built. As a result, we could successfully measure a small iron particle with 100 μm. This detection level was hard to achieve by conventional X-ray detection methods.