Scanning SQUID Research Articles

High‐Sensitivity Moment Magnetometry With the Quantum Diamond Microscope

AbstractInterest in magnetic fields on the ancient Earth and other planetary bodies has motivated the paleomagnetic analysis of complex rocks such as meteorites that carry heterogeneous magnetizations at <<1 mm scales. The net magnetic moment of natural remanent magnetization (NRM) in such small samples is often below the detection threshold of common cryogenic magnetometers. The quantum diamond microscope (QDM) is an emerging magnetic imaging technology with ~1 μm resolution and can, in principle, recover magnetizations as weak as 10−17 Am2. However, the typically 1–100 μm sample‐to‐sensor distance of QDM measurements can result in complex (nondipolar) magnetic field maps, from which the net magnetic moment cannot be determined using a simple algorithm. Here we generate synthetic magnetic field maps to quantify the errors introduced by sample nondipolarity and by map processing procedures such as upward continuation. We find that inversions based on least squares dipole fits of upward continued data can recover the net moment of complex samples with <5% to 10% error for maps with signal‐to‐noise ratio (SNR) in the range typical of current generation QDMs. We validate these error estimates experimentally using comparisons between QDM maps and between QDM and SQUID microscope data, concluding that, within the limitations described here, the QDM is a robust technique for recovering the net magnetic moment of weakly magnetized samples. More sophisticated net moment fitting algorithms in the future can be combined with upward continuation methods described here to improve accuracy.

The magnetoelectric effect due to a semispherical capacitor surrounded by a spherical topologically insulating shell

We calculate the magnetic field produced by a capacitor formed by two semispherical perfectly conducting plates of radius a, subjected to a potential difference and surrounded by a spherical shell of a topologically insulating material having internal radius r1 and external radius r2. Fixing r2 = 1 μm and considering the case where the shell touches the metallic plates, i. e. when r1 = a, we find that the maximum magnetic field occurs at the external surface of the shell when r1≲ 0.75 μm. For these cases we examine the effect in the two remaining variables: the thickness of the shell and axial angular dependence of the magnetic field. For r1 = 0.5 μm we find a highly isotropic angular distribution with an average field of magnitude G. The angular anisotropy increases with r1, yielding for r1 = 0.62 μm and for r1 = 0.75 μm. These magnitudes fall within the sensitivities of magnetometers based upon nitrogen-vacancy centers in diamond, as well as of devices using scanning SQUID magnetometry. In the latter case we obtain fluxes in the range of (6 − 10) × 10−10 G cm2 for a pickup loop of radius 10 μm centered at the axial symmetry axis and located parallel to the equatorial plane at distances ranging from 2 to 6 μm from the center of the capacitor.

MoRe/YBCO Josephson junctions and π-loops

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 IcRn 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 Jc = 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.

Skyrmionic chains and lattices in s+id superconductors

We report characteristic vortex configurations in $s+id$ superconductors with time reversal symmetry breaking, exposed to magnetic field. A vortex in the $s+id$ state tends to have an opposite phase winding between $s-$ and $d-$wave condensates. We find that this peculiar feature together with the competition between $s-$ and $d-$wave symmetry results in three distinct classes of vortical configurations. When either $s-$ or $d-$ condensate absolutely dominates, vortices form a conventional lattice. However, when one condensate is relatively dominant, vortices organize in chains that exhibit skyrmionic character, separating the chiral components of the $s \pm id$ order parameter into domains within and outside the chain. Such skyrmionic chains are found stable even at high magnetic field. When $s-$ and $d-$ condensates have a comparable strength, vortices split cores in two chiral components to form full-fledged skyrmions, i.e. coreless topological structures with an integer topological charge, organized in a lattice. We provide characteristic magnetic field distributions of all states, enabling their identification in e.g. scanning Hall probe and scanning SQUID experiments. These unique vortex states are relevant for high-T$_c$ cuprate and iron-based superconductors, where the relative strength of competing pairing symmetries is expected to be tuned by temperature and/or doping level, and can help distinguish $s+is$ and $s+id$ superconducting phases.

Imaging anisotropic vortex dynamics in FeSe

Strong vortex pinning in FeSe could be useful for technological applications and could provide clues about the coexistence of superconductivity and nematicity. To characterize the pinning of individual, isolated vortices, we simultaneously apply a local magnetic field and image the vortex motion with scanning SQUID susceptibility. We find that the pinning is highly anisotropic: the vortices move easily along directions that are parallel to the orientations of twin domain walls and pin strongly in a perpendicular direction. These results are consistent with a scenario in which the anisotropy arises from vortex pinning on domain walls and quantify the dynamics of individual vortex pinning in FeSe.

Local spectroscopies across the superconductor-insulator transition

We explore the manifestation of quantum fluctuations across the superconductor-insulator transition (SIT) in three different local measurements: the two-particle density of states $P(\mathbf{r},\omega)$, compressibility $\kappa(\mathbf{r})$, and diamagnetic susceptibility $\chi(\mathbf{r})$. We perform Monte Carlo simulations on the 2D quantum Josephson junction array model and present local maps of these quantities as the system is tuned across the SIT. $P(\mathbf{r},\omega)$, obtained using Maximum Entropy analytic continuation techniques, shows strongly diminished zero-energy spectral weight in nearly-insulating islands, that also correlate with regions of suppressed $\kappa(\mathbf{r})$. We investigate the signatures of quantum fluctuations in the evolution of $\kappa(\mathbf{r})$ and $\chi(\mathbf{r})$ across the SIT, distinct from thermal fluctuations. We discuss the experimental implications of our results for scanning Josephson spectroscopy, compressibility, and scanning SQUID measurements.

Observation of high-Tc superconductivity in inhomogeneous combinatorial ceramics

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.

Confined vortices in de facto mesoscopic Mo80Ge20 disks with sector defects

We introduce a sectoral defect into a disk (creating a Pac-Man™-shaped plate) to investigate effect of the symmetry breaking of a perfect disk on vortex dynamics. The sectoral defect acts as a preferential gateway for vortices to enter the disk. Vortices tend to form a symmetric configuration with respect to the mirror-symmetry line under various different applied magnetic fields. We found that the vortex configuration of the Pac-Man™-shaped disk exhibits an exotic arc structure, in contrast to the well-known shell structure of a disk. A characteristic of the vortex configuration is the critical number of vortices (or ‘magic number’) at which the single-arc structure becomes a multiple-arc structure, which can be tuned by changing the opening angle of the sectoral defect. We also noticed by comparing the free energies of the vortex distributions, that there are several competing metastable states with the same number of vortices when the ratio of the number of vortices in the outer arc to the number in the inner arc approaches two, for which stable triangular clusters might form part of the configurations. We conclude that there are reasonable agreements between the Ginzburg–Landau calculations and experimental scanning SQUID microscope images of vortices in de facto mesoscopic Pac-Man™-shaped Mo80Ge20 thin disks.

Micron-scale measurements of low anisotropic strain response of local Tc in Sr2RuO4

Topological superconductivity is of both fundamental and technological interest as a novel state of matter with potential applications in quantum information processing. Among the most promising materials for exhibiting intrinsic topological superconductivity is Sr${}_{2}$RuO${}_{4}$, which has shown evidence of triplet pairing and time-reversal symmetry breaking. Here, the authors use a scanning SQUID microscope to measure the local change in the critical temperature under application of anisotropic strain to test for a linear cusp at small strains, a key prediction of existing models for the superconducting state. The reported absence of such a cusp constrains future descriptions of this enigmatic material.

Observation of signatures of subresolution defects in two-dimensional superconductors with a scanning SQUID

The diamagnetic susceptibility of a superconductor is directly related to its superfluid density. Mutual inductance is a highly sensitive method for characterizing thin films; however, in traditional mutual inductance measurements, the measured response is a non-trivial average over the area of the mutual inductance coils, which are typically of millimeter size. Here we image localized, isolated features in the diamagnetic susceptibility of {\delta}-doped SrTiO3, the 2-DES at the interface between LaAlO3 and SrTiO3, and Nb superconducting thin film systems using scanning superconducting quantum interference device susceptometry, with spatial resolution as fine as 0.7 {\mu}m. We show that these features can be modeled as locally suppressed superfluid density, with a single parameter that characterizes the strength of each feature. This method provides a systematic means of finding and quantifying submicron defects in two-dimensional superconductors.

Constructing a Vector Scanning SQUID System

We succeeded in fabricating a vector scanning SQUID sensor, which was optimized to reduce the noise and to enhance the sensitivity by choosing the screening parameter as βL = 1. The sensors were fabricated by a superconductive foundry CRAVITY of Advanced Industrial Science and Technology (AIST) using AIST-STP2 standard process with the critical current density Jc of Josephson junctions (JJs) 320 A/cm2. Under this condition, the critical current Ic becomes 12.5 µA in the areas of 2.2 µm × 2.2 µm of our SQUID. The sensitivity and resolution of sensors were also improved by reducing inner diameter and increasing the number of the pick-up-coil windings with employing of multi-layer technique. We designed the structure of vector pickup coils, of which the same distance is kept between each coil and the sample surface. In addition, the distance between the coils were just set at an integer multiple of a scanning step so that it becomes convenient in reconstructing a magnetic-field vector at a certain position. The fabricated SQUID sensors were operated properly, and characteristics were in good agreement with our design parameters. In order to construct the vector scanning SQUID microscope, we succeeded in connecting our sensor with a commercial flux-locked loop (FLL) readout circuit (Magnicon Inc., Model XXF-1). The spatial resolution of vector SQUID microscopy can be improved by using an XYZ piezo-driven scanner for the precise control of the pick-up-coil positions over the three-dimensional range of 5 mm × 5 mm × 5 mm with a step size of 10 nm at 4K. A Gifford–McMahon cryocooler with an anti-vibration mechanism will be used to conduct long-time measurements.

Modulation of Superconducting Transition Temperature in LaAlO3/SrTiO3 by SrTiO3 Structural Domains

The tetragonal domain structure in SrTiO3 (STO) is known to modulate the normal-state carrier density in LaAlO3/SrTiO3 (LAO/STO) heterostructures, among other electronic properties, but the effect of STO domains on the superconductivity in LAO/STO has not been fully explored. Using a scanning SQUID susceptometer microscope to map the superconducting response as a function of temperature in LAO/STO, we find that the superconducting transition temperature is spatially inhomogeneous and modulated in a pattern that is characteristic of structural domains in the STO.

Scanning SQUID Microscopy for Sensing Vector Magnetic Field

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.

Defect states and room temperature ferromagnetism in cerium oxide nanopowders prepared by decomposition of Ce-propionate

Four batches of cerium oxide powders (with nanocrystallite size of 6.9 nm–572 nm) were prepared from four precursor nanopowders by thermal decomposition of Ce-propionate and annealing in air between 250 °C–1200 °C for 10 min–240 min. Ceria formation reactions, structure, vibrational, luminescence and magnetic properties were investigated by differential scanning calorimetry, x-ray diffraction, electron microscopy, infrared spectroscopy, photoluminescence and SQUID. All the samples exhibit room temperature ferromagnetism, RTFM, (with coercivity, Hc, of 8 Oe - 121 Oe and saturation magnetization, Ms, of up to 6.7*10−3 emu/g) and a broad defect-related photoluminescence, PL, emission in the visible range. The samples derived from the same precursor show Ms proportional to the peak area of defect-related PL emission whereas this is not valid for the samples derived from the different precursors. An improvement of ferromagnetism and intensity of defect-related PL emission was observed when annealing the products in which nanocrystalline cerium oxide coexists with Ce - oxicarbonate traces, Ce2O2CO3. The experimental results were explained based on the following considerations: room temperature ferromagnetism was induced by the defective ceria with high concentration of oxygen vacancies generated by decomposition of Ce-propionate; oxygen vacancies of the starting precursor nanopowders could be redistributed (at the surfaces/grain boundaries, GBs) upon heating under conditions that promote an inert local environment; the decomposition of Ce2O2CO3 residues can provide an excess of oxygen vacancies at the nanoparticles surfaces or GBs, which can induce or enhance ferromagnetism; surfaces/GBs rather than bulk defects appear responsible for RTFM – this can explain the (often reported in literature) inconsistency between oxygen vacancies concentration and Ms.

Scanning SQUID View of Oxide Interfaces

Scanning SQUID View of Oxide Interfaces

Analysing magnetism using scanning SQUID microscopy.

Scanning superconducting quantum interference device microscopy (SSM) is a scanning probe technique that images local magnetic flux, which allows for mapping of magnetic fields with high field and spatial accuracy. Many studies involving SSM have been published in the last few decades, using SSM to make qualitative statements about magnetism. However, quantitative analysis using SSM has received less attention. In this work, we discuss several aspects of interpreting SSM images and methods to improve quantitative analysis. First, we analyse the spatial resolution and how it depends on several factors. Second, we discuss the analysis of SSM scans and the information obtained from the SSM data. Using simulations, we show how signals evolve as a function of changing scan height, SQUID loop size, magnetization strength, and orientation. We also investigated 2-dimensional autocorrelation analysis to extract information about the size, shape, and symmetry of magnetic features. Finally, we provide an outlook on possible future applications and improvements.

Powder processing and characterisation of a quinary Ni-Mn-Co-Sn-Cu Heusler alloy

Ni-Co-Mn-Sn-Cu alloys were sintered from elemental powders at a temperature of 1050°C for 24h. The resulting microstructures and physical properties were characterised by differential scanning calorimetry, X-ray diffraction, optical/electron microscopy and SQUID magnetometry. The use of the alloys in energy conversion was satisfactorily tested. The new alloys exhibited a distinct martensitic transformation when cooled to room temperature. The presence of Cu in the powders moved the transformation temperature toward a higher value. The findings of this study demonstrate that the powder processing could be used to replace time-consuming arc/induction melting process in the development of high performance Heusler alloys at a lower cost.

Observation of local magnetic distribution in ramp-edge Josephson junctions

The observation of magnetic images for ramp-edge Josephson junctions (JJ) by scanning SQUID microscopy is reported. The dependence of the trapped vortex at a JJ on the external magnetic field was investigated, and it was found to be consistent with the modulation of the maximum Josephson current, showing that the vortex was not continuously changing, but that the change was quantized. A novel vortex trapped at the junction under a relatively large field is observable, which increases in quantized units as the external field is increased. Three different types of ramp-edge junctions were also investigated, showing that the difference in the magnetic penetration around the junctions reflects the junction property. These results provide us easy and useful information and an evaluation method of the junctions and the superconductivity of the films with no lead line.

NanoSQUIDs: Basics & recent advances

Abstract Superconducting Quantum Interference Devices (SQUIDs) are one of the most popular devices in superconducting electronics. They combine the Josephson effect with the quantization of magnetic flux in superconductors. This gives rise to one of the most beautiful manifestations of macroscopic quantum coherence in the solid state. In addition, SQUIDs are extremely sensitive sensors allowing us to transduce magnetic flux into measurable electric signals. As a consequence, any physical observable that can be converted into magnetic flux, e.g., current, magnetization, magnetic field or position, becomes easily accessible to SQUID sensors. In the late 1980s it became clear that downsizing the dimensions of SQUIDs to the nanometric scale would encompass an enormous increase of their sensitivity to localized tiny magnetic signals. Indeed, nanoSQUIDs opened the way to the investigation of, e.g., individual magnetic nanoparticles or surface magnetic states with unprecedented sensitivities. The purpose of this chapter is to present a detailed survey of microscopic and nanoscopic SQUID sensors. We will start by discussing the principle of operation of SQUIDs, placing the emphasis on their application as ultrasensitive detectors for small localized magnetic signals. We will continue by reviewing a number of existing devices based on different kinds of Josephson junctions and materials, focusing on their advantages and drawbacks. The last sections are left for applications of nanoSQUIDs in the fields of scanning SQUID microscopy and magnetic particle characterization, placing special stress on the investigation of individual magnetic nanoparticles.

Numerical restoration of surface vortices in Nb films measured by a scanning SQUID microscope

In the present work, we investigated a vortex profile appeared on a pure Nb film (500 nm in thickness, 10 mm x 10 mm) by using a scanning SQUID microscope. We found that the local magnetic distribution thus observed is broadened compared to a true vortex profile in the superconducting film. We therefore applied the numerical method to improve a spatial resolution of the scanning SQUID microscope. The method is based on the inverse Biot-Savart law and the Fourier transformation to recover a real-space image. We found that the numerical analyses give a smaller vortex than the raw vortex profile observed by the scanning microscope.