Scanning Scanning Superconducting Quantum Interference Device Research Articles

Vortex dynamics induced by scanning SQUID susceptometry

We measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a step-like nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

Local imaging of diamagnetism in proximity-coupled niobium nanoisland arrays on gold thin films

In this work we study the effect of engineered disorder on the local magnetic response of proximity coupled superconducting island arrays by comparing scanning Superconducting Quantum Interference Device (SQUID) susceptibility measurements to a model in which we treat the system as a network of one-dimensional (1D) superconductor-normal metal-superconductor (SNS) Josephson junctions, each with a Josephson coupling energy $E_J$ determined by the junction length or distance between islands. We find that the disordered arrays exhibit a spatially inhomogeneous diamagnetic response which, for low local applied magnetic fields, is well described by this junction network model, and we discuss these results as they relate to inhomogeneous two-dimensional (2D) superconductors. Our model of the static magnetic response of the arrays does not fully capture the onset of nonlinearity and dissipation with increasing applied field, as these effects are associated with vortex motion due to the dynamic nature of the scanning SQUID susceptometry measurement. This work demonstrates a model 2D superconducting system with engineered disorder and highlights the impact of dissipation on the local magnetic properties of 2D superconductors and Josephson junction arrays.

Studying Quantum Materials with Scanning SQUID Microscopy

Electronic correlations give rise to fascinating macroscopic phenomena such as superconductivity, magnetism, and topological phases of matter. Although these phenomena manifest themselves macroscopically, fully understanding the underlying microscopic mechanisms often requires probing on multiple length scales. Spatial modulations on the mesoscopic scale are especially challenging to probe, owing to the limited range of suitable experimental techniques. Here, we review recent progress in scanning superconducting quantum interference device (SQUID) microscopy. We demonstrate how scanning SQUID combines unmatched magnetic field sensitivity and highly versatile designs, by surveying discoveries in unconventional superconductivity, exotic magnetism, topological states, and more. Finally, we discuss how SQUID microscopy can be further developed to answer the increasing demand for imaging new quantum materials.

Scanning SQUID microscopy in a cryogen-free dilution refrigerator.

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.

Development of scanning SQUID microscope system and its applications on geological samples: A case study on marine ferromanganese crust

We present developments and applications of a high resolution scanning superconducting quantum interference device (SQUID) microscope for imaging magnetic field of geological samples at room temperature. A directly coupled low-temperature SQUID with a 200 μm × 200 μm pickup loop was mounted on a sapphire rod and separated from room temperature by a sapphire window. The environmental noise of the SQUID was successfully reduced by subtracting the signal of an additional reference SQUID placed inside a cryostat. The resulting system noise level was estimated to be about 50 pT. A geological thin section could be placed on a non-magnetic sample holder with an XYZ stage for scanning in an area of 100 mm × 100 mm. The minimum achievable distance from the SQUID to the sample is measured as ∼200 µm. An application of the SSM to a marine ferromanganese crust successfully provided beautiful stripe patterns in the magnetic images. The patterns could be correlated to the history of geomagnetic field reversals. The boundaries of the magnetic polarity domains were useful guides for the estimation of the deposition age by correlation with the standard geomagnetic polarity timescale. The established age model gave an average growth rate of ∼2.7 mm/Ma, which is consistent with that obtained by radiometric dating using 10Be (∼2.6 mm/Ma).

Ferroelectric Exchange Bias Affects Interfacial Electronic States.

In polar oxide interfaces phenomena such as superconductivity, magnetism, 1D conductivity, and quantum Hall states can emerge at the polar discontinuity. Combining controllable ferroelectricity at such interfaces can affect the superconducting properties and sheds light on the mutual effects between the polar oxide and the ferroelectric oxide. Here, the interface between the polar oxide LaAlO3 and the ferroelectric Ca-doped SrTiO3 is studied by means of electrical transport combined with local imaging of the current flow with the use of scanning a superconducting quantum interference device (SQUID). Anomalous behavior of the interface resistivity is observed at low temperatures. The scanning SQUID maps of the current flow suggest that this behavior originates from an intrinsic bias induced by the polar LaAlO3 layer. Such intrinsic bias combined with ferroelectricity can constrain the possible structural domain tiling near the interface. The use of this intrinsic bias is recommended as a method of controlling and tuning the initial state of ferroelectric materials by the design of the polar structure. The hysteretic dependence of the normal and the superconducting state properties on gate voltage can be utilized in multifaceted controllable memory devices.

Influence of Resonances on the Noise Performance of SQUID Susceptometers.

Scanning Superconducting Quantum Interference Device (SQUID) Susceptometry simultaneously images the local magnetic fields and susceptibilities above a sample with sub-micron spatial resolution. Further development of this technique requires a thorough understanding of the current, voltage, and flux () characteristics of scanning SQUID susceptometers. These sensors often have striking anomalies in their current–voltage characteristics, which we believe to be due to electromagnetic resonances. The effect of these resonances on the performance of these SQUIDs is unknown. To explore the origin and impact of the resonances, we develop a model that qualitatively reproduces the experimentally-determined characteristics of our scanning SQUID susceptometers. We use this model to calculate the noise characteristics of SQUIDs of different designs. We find that the calculated ultimate flux noise is better in susceptometers with damping resistors that diminish the resonances than in susceptometers without damping resistors. Such calculations will enable the optimization of the signal-to-noise characteristics of scanning SQUID susceptometers.

Magnetism and Conductivity Along Structural Domain Walls of SrTiO3

The interface between the oxide insulators LaAlO3 and SrTiO3 (LAO/STO) hosts a two-dimensional electron gas. The combination of interfacial conductivity and superconductivity at ultra-low temperatures with the physical phenomena of the oxide parent materials has fueled extensive research in the field since its discovery in 2004. Scanning superconducting quantum interference device (SQUID) measurements have shown that structural domain walls, formed below 105 K, modulate the current flow at the interface and recently revealed weak magnetic signals along the same domain structure. Here we use scanning SQUID to investigate the temperature dependence of different electronic properties of the LAO/STO interface. We find correlation between magnetism and conductivity, which are both spatially modulated on the domain structure. This data suggests a possible relation between the populations of electrons participating in each order.

Scanning SQUID microscopy in a cryogen-free cooler.

Scanning superconducting quantum interference device (SQUID) microscopy is a powerful tool for investigating electronic states at surfaces and interfaces by mapping their magnetic signal. SQUID operation requires cryogenic temperatures, which are typically achieved by immersing the cryostat in liquid helium. Making a transition to cryogen free systems is desirable, but has been challenging, as electric noise and vibrations are increased in such systems. We report on the successful operation of a scanning SQUID microscope in a modified Montana Instruments cryogen-free cooler with a base temperature of 4.3 K. We demonstrate scanning SQUID measurements with flux noise performance comparable to a wet system and correlate the sensor-sample vibrations to the cryocooler operation frequencies. In addition, we demonstrate successful operation in a variety of SQUID operation modes, including mapping static magnetic fields, measurement of local susceptibility, and spatial mapping of current flow distribution.

Scanning SQUID View of Oxide Interfaces.

The emergence of states of matter in low-dimensional systems is one of the most intriguing topics in condensed matter physics. Interfaces between nonmagnetic, insulating oxides are found to give rise to surprising behaviors, such as metallic conductivity, superconductivity, and magnetism. Sensitive, noninvasive local characterization tools are essential for understanding the electronic and magnetic behavior of these systems. Here, the scanning superconducting quantum interference device (SQUID) technique for local magnetic imaging is described and its contribution to the field of oxide interfaces is reviewed.

Scanning SQUID sampler with 40-ps time resolution.

Scanning Superconducting QUantum Interference Device (SQUID) microscopy provides valuable information about magnetic properties of materials and devices. The magnetic flux response of the SQUID is often linearized with a flux-locked feedback loop, which limits the response time to microseconds or longer. In this work, we present the design, fabrication, and characterization of a novel scanning SQUID sampler with a 40-ps time resolution and linearized response to periodically triggered signals. Other design features include a micron-scale pickup loop for the detection of local magnetic flux, a field coil to apply a local magnetic field to the sample, and a modulation coil to operate the SQUID sampler in a flux-locked loop to linearize the flux response. The entire sampler device is fabricated on a 2 mm × 2 mm chip and can be scanned over macroscopic planar samples. The flux noise at 4.2 K with 100 kHz repetition rate and 1 s of averaging is of order 1 mΦ0. This SQUID sampler will be useful for imaging dynamics in magnetic and superconducting materials and devices.

Scanning SQUID Study of Vortex Manipulation by Local Contact.

Local, deterministic manipulation of individual vortices in type 2 superconductors is challenging. The ability to control the position of individual vortices is necessary in order to study how vortices interact with each other, with the lattice, and with other magnetic objects. Here, we present a protocol for vortex manipulation in thin superconducting films by local contact, without applying current or magnetic field. Vortices are imaged using a scanning superconducting quantum interference device (SQUID), and vertical stress is applied to the sample by pushing the tip of a silicon chip into the sample, using a piezoelectric element. Vortices are moved by tapping the sample or sweeping it with the silicon tip. Our method allows for effective manipulation of individual vortices, without damaging the film or affecting its topography. We demonstrate how vortices were relocated to distances of up to 0.8 mm. The vortices remained stable at their new location up to five days. With this method, we can control vortices and move them to form complex configurations. This technique for vortex manipulation could also be implemented in applications such as vortex based logic devices.

Scanning SQUID microscope system for geological samples: system integration and initial evaluation

We have developed a high-resolution scanning superconducting quantum interference device (SQUID) microscope for imaging the magnetic field of geological samples at room temperature. In this paper, we provide details about the scanning SQUID microscope system, including the magnetically shielded box (MSB), the XYZ stage, data acquisition by the system, and initial evaluation of the system. The background noise in a two-layered PC permalloy MSB is approximately 40–50 pT. The long-term drift of the system is approximately ≥1 nT, which can be reduced by drift correction for each measurement line. The stroke of the XYZ stage is 100 mm × 100 mm with an accuracy of ~10 µm, which was confirmed by laser interferometry. A SQUID chip has a pick-up area of 200 μm × 200 μm with an inner hole of 30 μm × 30 μm. The sensitivity is 722.6 nT/V. The flux-locked loop has four gains, i.e., ×1, ×10, ×100, and ×500. An analog-to-digital converter allows analog voltage input in the range of about ±7.5 V in 0.6-mV steps. The maximum dynamic range is approximately ±5400 nT, and the minimum digitizable magnetic field is ~0.9 pT. The sensor-to-sample distance is measured with a precision line current, which gives the minimum of ~200 µm. Considering the size of pick-up coil, sensor-to-sample distance, and the accuracy of XYZ stage, spacial resolution of the system is ~200 µm. We developed the software used to measure the sensor-to-sample distance with line scan data, and the software to acquire data and control the XYZ stage for scanning. We also demonstrate the registration of the magnetic image relative to the optical image by using a pair of point sources placed on the corners of a sample holder outside of a thin section placed in the middle of the sample holder. Considering the minimum noise estimate of the current system, the theoretical detection limit of a single magnetic dipole is ~1 × 10−14 Am2. The new instrument is a powerful tool that could be used in various applications in paleomagnetism such as ultrafine-scale magnetostratigraphy and single-crystal paleomagnetism.

The response of small SQUID pickup loops to magnetic fields

In the past, magnetic images acquired using scanning superconducting quantum interference device (SQUID) microscopy have been interpreted using simple models for the sensor point spread function. However, more complicated modeling is needed when the characteristic dimensions of the field sensitive areas in these sensors become comparable to the London penetration depth. In this paper we calculate the response of SQUIDs with deep sub-micron pickup loops to different sources of magnetic fields by solving coupled London’s and Maxwell’s equations using the full sensor geometry. Tests of these calculations using various field sources are in reasonable agreement with experiments. These calculations allow us to more accurately interpret sub-micron spatial resolution data obtained using scanning SQUID microscopy.

Vector Pickup System Customized for Scanning SQUID Microscopy

We propose a novel system of a 3-D scanning superconducting quantum interference device (SQUID) microscope with high sensitivity for magnetic fields and high spatial resolution. Our 3-D scanning SQUID microscope consists of an XYZ piezodriven scanner for controlling the position of the pickup coils, three SQUID sensors, and the three-channel SQUID electronics at room temperature. Three SQUID sensors are fabricated in the same chip, and each SQUID has a pickup coil for XYZ directions, respectively. The SQUIDs used in our system are reproducibly fabricated using a Nb multilayer process of a fabrication center named CRAVITY. Pickup coils were built either in multiply winding numbers for enhancing sensitivity or in smaller inner diameters for improving spatial resolution. Three pickup coils with readout circuits are configured in orthogonal with each other and are acting as a vector sensor of the local magnetic field. At this moment, we succeeded to design some SQUIDs with 1-D and 3-D pickup coils. The critical current density J c of a Josephson junction (JJ) and the minimum critical current I√ of JJ are 320 A/cm 2 and 12.5 μA, respectively. The size of a chip is 2.9 mm × 2.9 mm, and the inner diameter of the pickup coil can be reduced down to 4 μm. The experimental I-V characteristic of fabricated SQUIDs was suitable for use in a scanning SQUID system. We have also fabricated a SQUID stage for installation in a cryostat for the experiment of 1-D SQUID sensors.

Direct Measurement of Current-Phase Relations in Superconductor/Topological Insulator/Superconductor Junctions

Proximity to a superconductor is predicted to induce exotic quantum phases in topological insulators. Here, scanning superconducting quantum interference device (SQUID) microscopy reveals that aluminum superconducting rings with topologically insulating Bi2Se3 junctions exhibit a conventional, nearly sinusoidal 2π-periodic current-phase relations. Pearl vortices occur in longer junctions, indicating suppressed superconductivity in aluminum, probably due to a proximity effect. Our observations establish scanning SQUID as a general tool for characterizing proximity effects and for measuring current-phase relations in new materials systems.

Detecting fatigue damage in 2.25Cr–1Mo steel with scanning SQUID gradiometry

In order to develop a new method for detecting fatigue damage nondestructively in 2.25Cr–1Mo steel, which is used in high temperature steam piping of fossil power plants and petroleum chemical facilities, fractured and interrupted samples in low-cycle fatigue (LCF) tests were investigated using scanning superconducting quantum interference device (SQUID) gradiometer. High temperature LCF experiments on 2.25Cr–1Mo steel were conducted to obtain various fatigue damage ratio, D f . Simultaneously, we also examined the fatigued samples by optical microscopy, electron back scatter diffraction pattern, magnetic force microscopy and hardness measurements to analyze the damage level. Except for a couple of samples, SQUID integrated intensity decreased with increasing D f , suggesting the potential of SQUID to evaluate fatigue damage in 2.25Cr–1Mo steel.

Ferromagnetism at the surface of aLaCoO3single crystal observed using scanning SQUID microscopy

Evidence for ferromagnetism at the surface of a $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ single crystal is reported using a scanning superconducting quantum interference device (SQUID) microscope. Stray magnetic flux detected with the scanning SQUID shows typical ferromagnetic behavior in $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ below ${T}_{c}\ensuremath{\sim}85\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in agreement with previous work on $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ particles. Analysis of the magnetization of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ particle samples clearly shows that the magnetization is inversely proportional to the particle radius, giving the information that the ferromagnetism is restricted within a few unit cell layers from the surface. X-ray photoemission spectroscopy also indicates that the ferromagnetism likely originates from the metallic surface due to hole doping with oxygen chemisorption.

Scanning SQUID microscopy study of electron-doped high- Tc superconductors

Local magnetic images of electron-doped Nd 2− x Ce x CuO 4 (NCCO) superconducting single crystals above and below the critical temperature T c were obtained using scanning superconducting quantum interference device (SQUID) microscopy. Surface scanning of the ab-plane of an NCCO ( x = 0.15) crystal cooled to 3 K under a low magnetic field (∼1 μT) revealed vortex patterns clearly. These vortex patterns totally disappeared at temperatures higher than T c. In contrast, when the crystal is cooled below T c under an external magnetic field (i.e., additional magnetic field controlled by a coil or plating sample stage), an unusual ‘stripe-shaped’ magnetic image is observed due to alignment of the vortices in a quasi-1D manner.

Practical Scanning SQUID System for Nondestructive Evaluation

An advanced scanning Superconducting Quantum Interference Device (SQUID) system called a "Traveling SQUID-NDE" was developed for nondestructive evaluation (NDE). The traveling SQUID-NDE can continuously "travel" back and forth over the surface of an object during testing without the need for magnetic shielding. Such movement by the SQUID sensor itself enables the SQUID-NDE to scan objects of unlimited size. In this work, the traveling SQUID-NDE system was combined with a manipulator used by conventional robots (either rectangular-type or articulated-type) used in industry, to manipulate the SQUID sensor for travel across an object. The effectiveness of this robotic traveling SQUID-NDE system was demonstrated by using it to detect artificial damage in stainless steel samples. This robotic traveling SQUID-NDE system proved useful for measuring two-dimensional magnetic images of fixed objects.