Scanning Hall Probe Research Articles

Topological solitons in charge-[formula omitted] flux quantized state of Kagome superconductors

A recent Little-Parks experiment on the new Kagome superconductor CsV3Sb5 demonstrated resistance oscillation with a period of ϕ0/3=hc/6e. This observation of charge-6e flux quantization is effectively explained by a three-component Ginzburg–Landau (GL) model that incorporates second-order Josephson-type couplings. Here, we numerically solve the GL model to present stable topological solitons. We reveal the structures of these solitons, characterized by closed domain walls with attached vortices. We identify two types of domain walls. These solitons possess multiple flux quanta and exhibit a ringlike geometry. Furthermore, we present the characteristic magnetic field distributions of these solitons, enabling their identification in, e.g., scanning Hall probe and scanning SQUID experiments.

Scanning Raman Spectroscopy Characterization of 1 Meter Long REBCO Coated Conductor

In this work, we evaluate the use of scanning Raman spectroscopy for characterizing long lengths of REBCO coated conductor tapes, as it can provide detailed insight into structure, composition, and local variations arising from defects or strain. We generate 2D maps of Raman wavelength and intensity features over extended 1 meter length of conductor and correlate them to the information collected by reel-to-reel (R2R) 2D X-Ray Diffraction (2D-XRD) and R2R Scanning Hall Probe Microscopy (SHPM). The three methods are compared in terms of depth of information, detectability of variation in features of interest and the potential for evaluating critical current performance over a range of fields and temperatures.

Novel correction procedure for compensating thermal contraction errors in the measurement of the magnetic field of superconducting undulator coils in a liquid helium cryostat.

Superconducting undulators (SCUs) can offer a much higher on-axis undulator field than state-of-the-art cryogenic permanent-magnet undulators with the same period and vacuum gap. The development of shorter-period and high-field SCUs would allow the free-electron laser and synchrotron radiation source community to reduce both the length of undulators and the dimensions of the accelerator. Magnetic measurements are essential for characterizing the magnetic field quality of undulators for operation in a modern light source. Hall probe scanning is so far the most mature technique for local field characterization of undulators. This article focuses on the systematic error caused by thermal contraction that influences Hall probe measurements carried out in a liquid helium cryostat. A novel procedure, based on the redundant measurement of the magnetic field using multiple Hall probes at known relative distance, is introduced for the correction of such systematic error.

Vortex matter and critical magnetic fields in mesoscopic superconducting strips

As compared with bulk materials, superconducting strips with lateral dimensions limited to the mesoscopic region exhibit non-trivial properties, among which the critical magnetic fields related to both vortex penetration and full vortex expulsion still remain elusive. By using low-temperature scanning Hall probe microscope, the critical magnetic fields are studied experimentally in a series of superconducting Pb strips with different widths. It is found that the penetration magnetic field follows a power law dependence with strip width, and vortices can only penetrate the strips when the energy barrier at the edges is completely suppressed. In the field-cooling process, vortices are fully expelled from the strips below , above which the number of trapped vortices linearly increases with magnetic field. Compared with various theoretical predictions, we have found that the experimentally observed corresponds to the field at which the Gibbs free energy in the strip center is minimized. Our study provides important implications for designing nanostructured devices with superconducting strips.

Analysis of inhomogeneities in Nb3Sn wires by combined SEM and SHPM and their impact on J c and T c

We demonstrate the combined use of scanning electron microscopy (SEM) and scanning Hall probe microscopy (SHPM) to analyse inhomogeneities in Nb3Sn wires. Inhomogeneities of the A15 phase in Nb3Sn sub-elements of a Ti-alloyed Restacked Rod Process wire and a Ta-alloyed Powder-In-Tube wire are investigated. Microstructural features are examined by SEM, elemental concentration gradients by energy dispersive x-ray spectroscopy (EDX) and the superconducting properties by SHPM. Correlations between the results are analysed to gain information about the impact of inhomogeneities in the microstructure on the superconducting properties. We find considerable differences in geometry and performance between sub-elements, as well as compositional and geometric inhomogeneities of the A15 phase inside single sub-elements. Additionally, simulations of the influence of Sn concentration gradients on the critical current density J c are performed. We also demonstrate the viability of SHPM and EDX for determining the dependence of the critical temperature T c on the Sn concentration and discuss possible performance gains by a reduction of inhomogeneities in Nb3Sn wires.

Structures of vortex in Co-doped BaFe2As2 iron superconductors with different doping level by scanning Hall probe microscopy

The 122 iron arsenide unconventional superconductors are part of a new class of iron-based superconductors. Co-doped BaFe2xCoxAs2 (Ba-122) iron superconductors sample have been examine by scanning Hall probe microscopy (SHPM) technique to find out the magnetic properties of Ba-122 . Has been completed the evolution of profiles of vortices which has well isolated and it is as function of temperature, then utilized suitable technique to extract the temperature depending on penetration depth, .So, this allowed to deduce the temperature dependent on density of superfluid and it has been compared with α-model consequences for a (2-band) of superconductor. When the superfluid density for the BaFe2xCoxAs2 (Over D x= 0.113) sample. As result, the two gap -model has been fitted to the data with Δ1=4.25kTc, Δ2=1.92kTc , =0.708, 1=0.293 then a2=1. However, When values of for the BaFe2-xCoxAs2 (Over D x= 0.075) sample. Result of the superfluid density for BaFe2-xCoxAs2 with parameters are (Δ1=3.9k, Δ2=1.6k), =0.615 for Δ1, and =0.237, =1. Suitable parameters produce refer into the symmetry of the order parameter at hole pockets with the electron, and then the relative supports of the bands to the density of superfluid in the iron-based crystals.

Measurement of In-Field E-J Characteristics in Multi-Filamentary Bi-2223 Tapes at Ultra-Low Electric-Field Down to Around 10−13 V/m

We have developed a method to investigate in-field electric-field v.s. current-density (E-J) characteristics at ultra low electric field criterion around 10-13 V/m in a multi filamentary Bi-2223 tape at 5 K based on scanning Hall probe microscopy (SHPM). Bi-2223 is widely used in high field applications, e.g. a 30.5 T NMR magnet system is currently being developed in Japan. Ultra-low electric field is induced in such magnets operating in persistent current mode with extremely steady field. Namely, E-J characteristics need to be clarified around that criterion. E-J characteristics can be estimated from decay of magnetization in the whole sample, assuming the uniformity of J. But for multi filamentary Bi-2223 tape, magnetization current path should be clarified. In this study, we measured local magnetic field profile as a function of time for Bi-2223 tapes using SHPM. Additionally, we carried out flux annealing measurements to realize ultra-low electric field induced in the tape. As a result, two main messages we obtained. Experimental field profile or sheet current density obtained from the inverted Biot-Savart law indicated that filaments remain coupled up to high field, and even at ultra-low E criterion. Moreover, local E-J characteristics around 10-13 V/m are obtained at 5 K and 4.5 T of perpendicular magnetic field, which is comparable perpendicular field component in a Bi-2223 insert coil in the high field NMR magnet system. Ic properties at 10-12 V/m decrease to nearly 50%, in comparison to the transport Ic properties at 10-4 V/m.

Automatic Detection of Local Obstacles in a Long Length RE-123 Coated Conductor by Deep Learning Based Image Classification in Reel-to-Reel Magnetic Microscopy

We have succeeded in detecting local obstacles automatically in a 200-m-long RE-123 coated conductor (CC) by introducing deep-learning based image recognition in reel-to-reel scanning Hall probe microscopy (RTR-SHPM). Longitudinal critical current ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> ) homogeneity in CCs is one of the most important requirements for practical applications. Usually, such properties as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> variation as a function of longitudinal coordinate is characterized by magnetization measurements adopting Hall probe array as a de facto standard characterization method for ensuring uniformity in long CCs. In this measurement, local <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> drop indicates the existence of current blocking obstacles. The group of authors also developed a magnetic microscopy applicable to reel-to-reel continuous measurements, RTR-SHPM, which makes it possible to visualize two-dimensional critical current density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> ) in the tape plane because of its high-resolution imaging along the tape width. As a result, more elaborate defect detection has been enabled. However, in the conventional technique, the observation depends on the human eye in order for characterizing detailed features of the obstacles such as shape and size especially in case with a small <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> drop, therefore, there was a limit to analyze the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> mapping with such detailed information extending to thousands of images in the long tape of several 100 of meters. In this study, the image classification based on the deep learning method was introduced in our magnetic microscopy. The analytical model classifies the input image into the defect position and the normal position, respectively, together with a heat map and a score of confidence in the recognition. As a result, we have succeeded in detecting obstacles automatically from more than 4,000 of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_c</i> images with a high accuracy of 98.5%. Furthermore, we revealed the existence of the obstacles which were not distinguishable by the local <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> criterion. This method allows us to clarify the origin of the instability of long CC wire and will have a strong impact as an evaluation technique for dramatically improving the reliability of the CCs.

Reduced granularity in BHO-doped YBCO films on RABiTS templates

REBCO based coated conductors (CCs) are a viable alternative to conventional superconductors for many applications, therefore the optimization of their current carrying capacity is an ongoing process. A promising route for the increase in performance is the introduction of artificial pinning centers such as BaHfO3 (BHO) nanoparticles. However, granularity still imposes a substantial performance limitation, especially in REBCO CCs deposited on RABiTS based templates, as the critical current density is severely reduced by moderate misalignment angles of adjacent grains. A combined study of scanning Hall probe microscopy and electron microscopy of undoped and BHO-doped YBa2Cu3O7-δ (YBCO) films on technical templates shows that BHO-doping leads to a denser microstructure of the superconducting layer and higher global and local critical current densities. The statistical evaluation of local current maps allows for a quantification of the magnetic granularity where a reduction of granularity with increasing film thickness, doping and increasing temperature is found. In particular, the dependence of granularity on the film thickness and enhanced film growth through BHO-doping shows the potential for further optimization of YBCO films on RABiTS based templates.

Tunable Noninteger Flux Quantum of Vortices in Superconducting Strips.

Flux quantization has been widely regarded as the hallmark of the macroscopic quantum state of superconductivity. However, practical design of superconductor devices exploiting finite size confinement effects may induce exotic phenomena, including nonquantized vortices. In our research, the magnetic flux of vortices has been studied in a series of superconducting strips as a function of the strip width and the penetration depth. In both circumstances, the observation using scanning Hall probe microscope (SHPM) displays a controlled evolution from singly quantized vortices to nonquantized ones. It is also found that the magnetic flux is immune to the flowing supercurrent. The simulations based on Ginzburg-Landau theory agree well with experimental results. The observed behavior of the vortex flux may open new perspectives for fundamental research and applications based on vortex matter, such as vortex-memory devices and magnetic field traps for ultracold atoms.

Local Quality Control of Helical CORC Cable Winding

Thispaper discusses the possibilities of detecting defects in the helical winding of CORC cables using Hall probe scanning method and a 4-contact measurement method using a “magnetic knife”. The “magnetic knife” technique consists in a local change of the external magnetic field on the investigated part of the cable, which leads to a change in the current-voltage characteristic (I–V curve) of the entire cable. The presence of defects is determined by the appearance of features in the I--V characteristic when the “magnetic knife” moves along the cable. The Hall probe scanning method is sensitive only to the section of the cable located close to the scanning plane, which is not suitable for finding a defect in the coils. And is not suitable for the analysis of deep layers of the tapes, only the signal from the upper tape can be measured. The “magnetic knife” technique makes it possible to detect defects in tapes, the ab-plane of which is located at an angle of less than 45 degrees to the direction of the magnetic field (that is, the ab-plane is almost perpendicular to the direction of the field). We have shown that the technique is well sensitive to local defects, leading to a local decrease in the critical current by 3% of the whole cable. The developed technique is planned to be used to control the quality of winding and laying of CORC cable when creating an inductive energy storage for the NICA accelerator complex.

Magnetic granularity in PLD-grown Fe(Se,Te) films on simple RABiTS templates

Iron-based superconductors are a popular candidate in the search for affordable and simple superconductors for high-field applications. In particular, the relaxed texture requirements fuel hope that films deposited on RABiTS with simple buffer layer architectures could enable cheap coated conductors. We find that a single Yttrium oxide buffer layer can act as a suitable diffusion barrier and epitaxial Fe(Se,Te) thin films were successfully grown by pulsed laser deposition. An analysis of the local current distribution by means of scanning Hall probe microscopy reveals current densities exceeding 1 MA cm−2, however, granularity still seems to be an issue. Transmission electron microscopy images and analysis by transmission Kikuchi diffraction show that the out-of-plane orientation of underlying Ni-W grains in the substrate has a severe impact on the growth of Fe(Se,Te) films.

Influence of Local Inhomogeneities in the REBCO Layer on the Mechanism of Quench Onset in 2G HTS Tapes

The resistive-type of superconducting fault current limiters (R-SFCL) using the second generation of high-temperature superconductors (2G HTS) are well adapted to power grid protection. Almost electrically invisible in normal operation, it quenches and becomes highly resistive in the event of a fault. The tape operation at the transition from the superconducting state to the resistive state is the basis of a R-SFCL. In this scope, we studied the onset of a quench occurring in 2G HTS tapes using high-speed imaging to record the bubble generation in liquid nitrogen. In the first milliseconds of operation, the bubble generation appears highly inhomogeneous. Dissipation is initiated on multiple spots over the tape surface, which first expands in the direction of the width of the tape and then along its length. A detailed analysis of the REBCO layer is performed by means of scanning Hall probe microscopy (SHPM) after the optical study of the tapes is completed. Analysis of the local current density distribution demonstrates that the bubble generation occurs at positions of local inhomogeneities in the REBCO layer. These inhomogeneities, well recognized through SHPM, are not reliably described by the critical current as a function of the position derived magnetically (e.g., by traditional TapeStar measurements). Similar results were obtained on samples from Superpower and SuperOx.

Magnetization\u2013structure\u2013composition phase diagram mapping in Co-Fe-Ni alloys using diffusion multiples and scanning Hall probe microscopy

Transition metal alloys are essential for magnetic recording, memory, and new materials-by-design applications. Saturation magnetization in these alloys have previously been measured by conventional techniques, for a limited number of samples with discrete compositions, a laborious and time-consuming effort. Here, we propose a method to construct complete saturation magnetization diagrams for Co–Fe–Ni alloys using scanning Hall probe microscopy (SHPM). A composition gradient was created by the diffusion multiple technique, generating a full combinatorial materials library with an identical thermal history. The composition and crystallographic phases of the alloys were identified by integrated energy dispersive X-ray spectroscopy and electron backscatter diffraction. “Pixel-by-pixel” perpendicular components of the magnetic field were converted into maps of saturation magnetization using the inversion matrix technique. The saturation magnetization dependence for the binary alloys was consistent with the Slater-Pauling behavior. By using a significantly denser data point distribution than previously available, the maximum of the Slater-Pauling curve for the Co–Fe alloys was identified at ~ 32 at% of Co. By mapping the entire ternary diagram of Co–Fe–Ni alloys recorded in a single experiment, we have demonstrated that SHPM—in concert with the combinatorial approach—is a powerful high-throughput characterization tool, providing an effective metrology platform to advance the search for new magnetic materials.

Batch Fabrication of 50 μm-Thick Anisotropic Nd–Fe–B Micro-Magnets

We used standard micro-electro-mechanical systems (MEMS) fabrication techniques (lithography, etching, and sputtering) to the batch fabrication of out-of-plane textured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50~\mu \text{m}$ </tex-math></inline-formula> -thick Nd–Fe–B micro-magnets on Si substrates of 100 mm diameter. Pre-patterning of the Si substrate serves to structure the in-plane dimensions of the micro-magnets and affords stress relief in the magnetic layer. Two protocols were applied to grow the films: a one-step process, in which the film is directly crystallized and a two-step process, where the film is amorphous in the as-deposited state and crystallized during post-deposition annealing. Both processing routes result in micro-magnets with magnetic properties (texture and coercivity) comparable to sintered Nd–Fe–B bulk magnets. We report on how substrate patterning and film thickness affect the very distinctive microstructures of our films. Finally, the stray magnetic field patterns produced above these topographically patterned films were characterized by scanning Hall probe microscopy (SHPM).

Nb3Sn Wires for the Future Circular Collider at CERN: Microstructural Investigation of Different Wire Layouts

In the challenging project concerning the realization of the CERN Future Circular Collider (FCC), Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn represents the best candidate material for the construction of high-field superconducting dipole magnets, since it is able to satisfy the requirements of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (non-Cu) = 1.5 kA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 16 T and 4.2 K. In that context, a cluster layout of prototype internal tin Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires, developed by TVEL and the Bochvar Institute (Russia), was analyzed and compared to a standard layout produced by the same manufacturer. The main reason for dividing the sub-element into clusters is reducing the effective sub-element size (d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ). The microstructural characterization of such a wire layout can provide fundamental contributions to steer the manufacturing processes towards higher performing wires. In particular, since the homogeneity in Sn concentration influences the superconducting properties, the effect of cluster and standard layouts on the Sn concentration gradient over the wire cross-section was evaluated. For this purpose, energy dispersive X-ray (EDX) spectroscopy was employed with both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Finally, scanning Hall probe microscopy (SHPM) measurements were performed to understand how these cluster wire sub-elements, with their specific geometry, influence the local currents flowing through the wire cross-section on a microscopic scale. The comprehension of the correlation between the microstructural characteristics and superconducting performance is crucial for obtaining wires meeting the requirements of FCC dipole magnets.

Imaging of Strong Nanoscale Vortex Pinning in GdBaCuO High-Temperature Superconducting Tapes.

The high critical current density of second-generation high-temperature superconducting (2G-HTS) tapes is the result of the systematic optimisation of the pinning landscape for superconducting vortices through careful engineering of the size and density of defects and non-superconducting second phases. Here, we use scanning Hall probe microscopy to conduct a vortex-resolved study of commercial GdBaCuO tapes in low fields for the first time and complement this work with “local” magnetisation and transport measurements. Magnetic imaging reveals highly disordered vortex patterns reflecting the presence of strong pinning from a dense distribution of nanoscale Gd2O3 second-phase inclusions in the superconducting film. However, we find that the measured vortex profiles are unexpectedly broad, with full-width-half-maxima typically of 6 μm, and exhibit almost no temperature dependence in the range 10–85 K. Since the lateral displacements of pinned vortex cores are not expected to exceed the superconducting layer thickness, this suggests that the observed broadening is caused by the disruption of the circulating supercurrents due to the high density of nanoscale pinning sites. Deviations of our local magnetisation data from an accepted 2D Bean critical state model also indicate that critical state profiles relax quite rapidly by flux creep. Our measurements provide important information about the role second-phase defects play in enhancing the critical current in these tapes and demonstrate the power of magnetic imaging as a complementary tool in the optimisation of vortex pinning phenomena in 2G-HTS tapes.

Granular Hall Sensors for Scanning Probe Microscopy.

Scanning Hall probe microscopy is attractive for minimally invasive characterization of magnetic thin films and nanostructures by measurement of the emanating magnetic stray field. Established sensor probes operating at room temperature employ highly miniaturized spin-valve elements or semimetals, such as Bi. As the sensor layer structures are fabricated by patterning of planar thin films, their adaption to custom-made sensor probe geometries is highly challenging or impossible. Here we show how nanogranular ferromagnetic Hall devices fabricated by the direct-write method of focused electron beam induced deposition (FEBID) can be tailor-made for any given probe geometry. Furthermore, we demonstrate how the magnetic stray field sensitivity can be optimized in situ directly after direct-write nanofabrication of the sensor element. First proof-of-principle results on the use of this novel scanning Hall sensor are shown.

Peculiarities of the phase transformation dynamics in bulk FeRh based alloys from magnetic and structural measurements

We analyze coexistence of antiferromagnetic and ferromagnetic phases in bulk iron-rhodium and its alloys with palladium, Fe50,4Rh49,6, Fe49,7Rh47,4Pd2,9 and Fe48,3Rh46,8Pd4,9, using neutron diffraction, magnetization and scanning Hall probe imaging. Temperature dependencies of the lattice parameters, AFM and FM phase weight fractions, and Fe magnetic moment values were obtained on cooling and heating across the AFM-FM transition. Substantial thermomagnetic hysteresis for the phases’ weight fractions and a relatively narrow one for the unit cell volume has been observed on cooling-heating. A clear dependence of hysteretic behavior on Pd concentration has been traced. Additional direct magnetic measurements of the spatial distribution of the phase transition are acquired using scanning Hall probe microscopy, which reveals the length scale of the phase coexistence and the spatial progression of the transition in the presence of external magnetic field. Also, the magnetic phase diagram has been constructed for a series of Pd-doped FeRh alloys.

Traceably calibrated scanning Hall probe microscopy at room temperature

Abstract. Fabrication, characterization and comparison of gold and graphene micro- and nanoscale Hall sensors for room temperature scanning magnetic field microscopy applications are presented. The Hall sensors with active areas from 5 µm down to 50 nm were fabricated by electron-beam lithography. The calibration of the Hall sensors in an external magnetic field revealed a sensitivity of 3.2 mV A−1 T−1 ± 0.3 % for gold and 1615 V A−1 T−1 ± 0.5 % for graphene at room temperature. The gold sensors were fabricated on silicon nitride cantilever chips suitable for integration into commercial scanning probe microscopes, allowing scanning Hall microscopy (SHM) under ambient conditions and controlled sensor–sample distance. The height-dependent stray field distribution of a magnetic scale was characterized using a 5 µm gold Hall sensor. The uncertainty of the entire Hall-sensor-based scanning and data acquisition process was analyzed, allowing traceably calibrated SHM measurements. The measurement results show good agreement with numerical simulations within the uncertainty budget.