Artificial Pinning Research Articles

Introduction of BaMO3 artificial pins into high mixing entropy FF-MOD REBCO thin films

Abstract We have synthesized middle-entropy type REBCO (ME-REBCO) and high-entropy type REBCO (HE-REBCO) superconducting thin films with BaCeO3 artificial pins by fluorine-free metal organic deposition (FF-MOD) method and evaluated their crystallinity and superconducting properties. The c-axis orientation of crystals was confirmed by XRD analysis for all thin films. The superconducting transition temperatures in all films were over 90 K. STEM observation revealed that the crystal lattice of HE-REBCO was slightly distorted compared to a normal GdBCO film. This distortion is expected to be a “backlash” in the crystal and contribute to an increase in the allowable amounts of artificial pins. The critical current densities and flux pinning force densities at 77.3 K were enhanced by Ce doping, indicating that BaCeO3 artificial pins were introduced into the FF-MOD ME-REBCO and HE-REBCO films.

Critical current of a layered high-temperature superconductor with artificial pinning centers: the Monte Carlo simulation

Current–voltage characteristics (EJ curves) and magnetic field dependences of the critical current have been calculated for a superconductor with artificial pinning in the form of submicron-sized holes and tilted radiation defects. Calculations have been performed within the framework of the three-dimensional model of a layered HTS by means of the Monte Carlo method. S-shaped features of the EJ curves have been observed for a sample with a rectangular lattice of holes. Such features have not occurred in calculations for HTSs with non-magnetic pinning centers before, but they have been observed in experimental studies. In this paper, the features occurred in magnetic fields close to 290 Gs (which is the lower critical field for the Bi2Sr2CaCu2O8-δ superconductor at 1 K) and they were sensitive to the magnitude of the external magnetic field. In addition, the features were more prominent at temperatures below 30 K and in samples with weak intrinsic pinning, and they were connected with matching-like effects in the vortex system (i. e. a certain number of vortices being pinned on each hole, screening new vortices from entering the sample). For samples with tilted radiation defects, decreasing field dependences of the critical current have been obtained, showing weak maxima near the lower critical field of the superconductor. Calculations have shown that, at a fixed value of the external field, the critical current decreases with the increasing tilt angle of the defects.

Probing Evolution of the Flux-Pinning Landscape in REBCO Coated Conductors Caused by Gamma Irradiation Using DC and AC Magnetometry: A Novel Approach to Tokamak Magnet Material Development

Optimisation of REBCO coated conductor tapes specifically for use in nuclear fusion will help improve the magnet component lifetimes in future tokamak reactor power plants. The focus of this work was exploration of a novel approach to irradiation studies on REBCO tapes, utilising multiple magnetic measurements to probe evolution of the REBCO flux-pinning landscape more deeply than reported in other studies, for the purpose of identifying primary limiting factors affecting performance. Gamma irradiation experiments were conducted, and pre-/post-irradiation results from DC and AC magnetic measurements using a Physical Property Measurement System (PPMS) are discussed. Magnetisation critical current density (Jc) decreased in all samples with increasing dose, except for the silver overlayer-only samples which did not contain artificial pinning centres (APCs), where Jc increased with dose. Removal of the copper stabiliser coupled with the presence of APCs allowed gamma irradiation to induce pinning force maximum peak shifts, from above 14 T before irradiation to below 9 T afterwards. Flux creep rate varied with the evolving pinning landscape, and the degree of Jc degradation directly correlated with creep rate fluctuations post-irradiation. Changes in critical temperature and diamagnetic saturation also corresponded with changes in Jc and flux creep rate. The major conclusion from this study was that minimisation of flux creep rate is the key to maintenance of performance under fusion-relevant operating conditions. Flux creep manifests as problematic AC losses in all high-temperature superconducting machines; therefore, future work will focus on reduction/prevention of the phenomenon to enhance longevity of performance in any application.

Relaxation of the trapped magnetic flux in a mesoscopic HTS with artificial pinning in the form of submicron holes

Vortex relaxation has been studied in a microsized HTS with various distributions of artificial pinning in the form of submicro-holes. It has been shown that, in some cases, for a superconductor with dimensions up to several micrometers, the time dependences of the trapped magnetic field do not correspond with the collective creep model. The dependences of the trapped-field relaxation rate and vortex activation energy on the magnetizing field (up to 0.5 T) and temperature (up to 50 K) have been obtained. It has been shown that the presence of submicro-holes slightly increases the relaxation rate, mainly due to the decrease in the number of random nanosized pinning centers.

Study on the Effect of ELID Grinding on corrosion wear properties of Co-Cr Alloy

Contemporary Co-Cr alloys used in artificial joints can cause health complications in long-term applications. A pin-on-disc friction and wear testing instrument was used to evaluate the corrosion and abrasion of two types of Co-Cr alloys for artificial joints under simulated body fluid conditions after surface electrolytic composite grinding. Co-materials with lower carbon content displayed superior corrosion resistance than those materials with higher carbon content. Moreover, surface processing with electrolytic composite grinding produced a stable surface modification layer, which improved the surface hardness and formed an oxide film that inhibited metal ionization. The formation of precipitates in the corrosive environment is likely to improve the friction and wear properties of the material. As such, a variety of hardness was observed for the artificial joint friction disc and pin, especially when the hardness of the disc was higher than that of the pin, which can improve the friction and wear behavior of joints.

Orthorhombic YBa2Cu3O7−δ Superconductor with TiO2 Nanoparticle Addition: Crystal Structure, Electric Resistivity, and AC Susceptibility

This article reports the effect of a nanoscale addition of TiO2 on the structure and superconducting parameters of the high-temperature superconductor YBa2Cu3O7-δ (Y123). Polycrystalline compounds of Y123 with different percentages of TiO2, x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, were fabricated using the thermal treatment method. An analysis using X-ray diffraction confirmed the formation of Y123 phases for all composite samples. Field-emission scanning electron microscopy (FESEM) analysis revealed the growth of grain size and decrease in porosity, with a sign of partial melting of grains for the samples with TiO2 addition. The magnetic and electric transport properties were investigated using AC susceptibility measurement and the four-probe method, respectively. It was observed that the superconducting transition temperature, Tc-onset, for a pure sample determined by ACS and 4PP was 95.6 K and 95.4 K, respectively. These values were found to decrease with the addition of TiO2, while the superconducting transition (∆Tc) improved with TiO2 addition except for the sample at x = 0.2 wt.%, which showed the broadest transition width. The sharpest superconducting transition (∆Tc) was observed for the sample at x = 1.0 wt.%, indicating that the addition of TiO2 nanoparticles is expected to serve as artificial pinning centres and strengthen the connection among the grains in the Y123 ceramic.

Current-Induced Helicity Switching of Frustrated Skyrmions on a Square-Grid Obstacle Pattern

Topological spin textures on artificial pinning landscape may show unique static and dynamic properties. Here, we computationally show that the helicity of frustrated skyrmions on an artificial square-grid obstacle pattern can be switched by a spin current pulse. The obstacle pattern is formed by defect lines with enhanced perpendicular magnetic anisotropy, which could protect the skyrmion from being annihilated at the sample edge. It is found that the skyrmion driven by a moderate current shows a circular motion guided by the boundary of the obstacle pattern, while it shows an almost straight motion toward the sample edge in the absence of the obstacle pattern. By applying a short pulse current to drive the skyrmion in a sample with the obstacle pattern, we find that the helicity of the skyrmion could be switched between Bloch-type configurations favored by the dipole-dipole interaction. Besides, we demonstrate the possibility of switching the helicity of an array of skyrmions on the square-grid obstacle pattern using the same method. Our results could be useful for the helicity control of topological spin textures, and may provide guidelines for building future helicity-based spintronic functions.

Responses of natural and artificial pin oak (Quercus palustris) reproduction to a sequence of silvicultural release treatments in bottomland hardwood forests in southern Missouri

Developing competitive oak advance reproduction prior to canopy disturbance is understood to be important for oak regeneration success. In the early 2000s, a study was installed in southeastern Missouri to examine the effects of midstory and understory release on natural and artificial sources of pin oak (Quercus palustris Muenchh.) advance reproduction. The findings obtained three years after midstory and understory release indicated photosynthetically active radiation increased from 3 to 15 % and a corresponding increase in density of pin oak reproduction as well as the survival and growth of both natural and artificial pin oak reproduction compared to control. In 2010, eight years after the midstory and understory was removed, three different overstory harvests were applied to release the established pin oak advance reproduction. Here, we investigated the effects of the midstory and understory removal and the subsequent overstory harvests on underplanted and naturally regenerated oak advance reproduction. We also tested effects of a later midstory release on the density of natural pin oak advance reproduction. Results indicate that the survival of bareroot pin oak seedlings increased with increasing initial basal diameter. The survival of Root Production Method (RPM®) container pin oak seedlings, however, was not dependent on the size at the time of planting. The early understory removal with triclopyr herbicide was ineffective in ensuring survival of planted pin oak seedlings. Shelterwood increased the growth of oak advance reproduction but did not eliminate competition from other species. A late release also failed to increase the growth of oak advance reproduction. Given shelterwood also favored oak competitors in our study, multiple applications of midstory and understory competition control treatments following the shelterwood treatment will be imperative for successful oak regeneration.

A Study of the Laser Drilling Processes of Hts Composite Tapes as a Method of Creating the Magnetic Flux Artificial Pinning

A Study of the Laser Drilling Processes of Hts Composite Tapes as a Method of Creating the Magnetic Flux Artificial Pinning

Domain wall pinning through nanoscale interfacial Dzyaloshinskii–Moriya interaction

Neuromorphic computing (NC) has been gaining attention as a potential candidate for artificial intelligence. The building blocks for NC are neurons and synapses. Research studies have indicated that domain wall (DW) devices are one of the most energy-efficient contenders for realizing NC. Moreover, synaptic functions can be achieved by obtaining multi-resistance states in DW devices. However, in DW devices with no artificial pinning, it is difficult to control the DW position, and hence achieving multilevel resistance is difficult. Here, we have proposed the concept of nanoscale interfacial Dzyaloshinskii–Moriya interaction (iDMI) for controllably stopping the DWs at specific positions, and hence, realizing multi-resistance states. We show that the nanoscale iDMI forms an energy barrier (well), which can controllably pin the DWs at the pinning sites. Moreover, a tunable depinning current density was achieved by changing the width and iDMI constant of the confinement region. We have also studied pinning in a device with five successive pinning sites. This feature is a proof-of-concept for realizing multi-resistance states in the proposed concept. Based on these observations, a magnetic tunnel junction—where the free layer is made up of the proposed concept—can be fabricated to achieve synapses for NC applications.

Configurable pixelated skyrmions on nanoscale magnetic grids

Topological spin textures can serve as non-volatile information carriers. Here we study the current-induced dynamics of an isolated magnetic skyrmion on a nanoscale square-grid pinning pattern formed by orthogonal defect lines with reduced magnetic anisotropy. The skyrmion on the square grid can be pixelated with a quantized size of the grid. We demonstrate that the position, size, and shape of skyrmion on the square grid are electrically configurable. The skyrmion center is quantized to be on the grid and the skyrmion may show a hopping motion instead of a continuous motion. We find that the skyrmion Hall effect can be perfectly prohibited due to the pinning effect of the grid. The pixelated skyrmion can be harnessed to build future programmable racetrack memory, multistate memory, and logic computing device. Our results will be a basis for digital information storage and computation based on pixelated topological spin textures on artificial pinning patterns.

Flux pinning characteristics of YBCO: NaNbO3 by introducing artificial pinning centers with different morphology

Introduction of artificial pinning centres in different morphology is a beneficial way to improve the flux pinning properties of a YBCO compound. In the present study, we report the effect of addition of sodium niobate (NaNbO3) nanostructures in two different form i.e. nanorods (NRs) and nanoparticles (NPs) with a varied wt% (0, 0.5, and 1 wt%) on the pinning capabilities of bulk YBCO polycrystalline samples. The orthorhombic crystal structure of YBCO was confirmed by the XRD spectra even in the nanocomposite samples. The existence of NPs or NRs on the surface and between the superconducting grains of YBCO was confirmed by morphological analysis. The critical temperature (Tc) remains almost unchanged as compared to pure YBCO on addition of NaNbO3 nanostructure in YBCO compound upto 1 wt% concentration. Even at higher fields (upto 6 T) and higher temperatures (upto 75 K), the addition of NaNbO3 NPs or NRs in YBCO nanocomposite resulted in a significant increase of ~2.6 times in the critical current density as compared to pure YBCO. In comparison to NRs added YBCO nanocomposite samples, NPs added YBCO nanocomposite samples showed higher improvement. Throughout the entire investigated range of applied field, the maximum pinning force was found for 0.5 wt% NPs added composite sample.

Local distortions of surface domain walls in cylindrical microwires observed by magneto-optics

Domain wall dynamics in highly magnetostrictive microwires is remarkably sensitive to thermal treatment. One of the reasons is a partial release of the internal stresses that originate during the production process. Another reason is an interaction of the domain wall with local pinning sites, such as surface inhomogeneities, free volumes, or clusters of ferromagnetic atoms, present in as-cast microwires. However, the observation of domain wall shape variations after thermal treatment or following an interaction with a pinning site is challenging. In this paper, we utilize the magneto-optical Kerr effect (MOKE) to locally map the domain wall shape and velocity in cylindrical microwires. The domain wall is trapped in a magnetic potential well, moving back and forth with limited speed. Two complementary techniques are employed in the study: a laser-based setup and time-resolved MOKE microscopy. Different kinds of domain wall propagation are observed. While a planar domain wall propagates synchronously with the potential well, surface pinning gives rise to local jumps and distortions of the domain wall. We relate the first kind of motion to the pinning-free propagation of the domain wall and the second to the propagation of a distorted wall. Finally, local speed and the shapes of the domain wall before and after the introduction of an artificial pinning site are compared.

Robust edge states in magnetic soliton racetrack

Precise positioning of magnetic solitons requires controllable artificial pinning, while the accurate determination of the pinning profile remains a challenge. Here, we propose a topological solution to this problem. Taking the domain wall (DW) as a representative example, we study the collective dynamics of interacting DWs in a magnetic racetrack with pinning sites of alternate distances. By mapping the governing equations of DW motion to the Su-Schrieffer-Heeger model and evaluating the quantized Zak phase, we predict two topologically distinct phases in the racetrack. A robust edge state emerges at either one or both ends depending on the parity of the DW number and the ratio of alternating intersite lengths. We show that the in-gap DW oscillation frequency has a fixed value which depends only on the geometrical shape of the pinning notch, and is insensitive to device imperfections and inhomogeneities. The spring coefficient of the pinning potential can be quantified as the square of the robust DW frequency multiplied by its constant mass. Our findings pave the way to determining the pinning potential with high accuracy for generic magnetic solitons and suggest as well that the magnetic soliton-based racetrack is a unique playground to study topological phase transitions.

New nanoscale artificial pinning centres for NbTi superconductors

The efficiency of new artificial pinning centres have been investigated in NbTi using the oxide dispersion strengthening (ODS) concept more typically applied to steels and Ni-superalloys. NbTi alloys containing a dispersion of nanoscale Y2Ti2O7 particles have been successfully manufactured through a powder metallurgy route by mechanical alloying Nb, Ti and Y2O3 nanopowders using high energy ball milling and subsequent consolidation and annealing. The microstructure and superconducting properties of the NbTi-Y2O3 have been characterised after each processing step. After 40 h of ball milling, the Y2O3 particles are dissolved into the nanostructured NbTi matrix formed by mechanical alloying of Nb and Ti. On annealing at temperatures above 800 °C the dissolved Y and O react with Ti from the matrix, and fine Y2Ti2O7 particles (<5 nm) are precipitated. These NbTi-Y2Ti2O7 composites showed excellent superconducting performance, with JC of 5.2 kAmm−2 at 4.2 K, 5 T, considerably higher than Y2O3-free bulks manufactured using the same process and NbTi wires manufactured using the conventional thermomechanical process. The results suggest that ODS powder processing is a promising new route to manufacture NbTi alloys containing artificial pinning centres that give superior superconducting properties.

Geometrical surface pinning in the nonlinear AC susceptibility of HTS tapes

The analytic expression for the nonlinear magnetic susceptibility of a thin disk derived from Bean’s model with a uniform J C for predicts a peak-peak ratio of 7.1 (5.4) for the real (imaginary) components of the third harmonic χ 3 susceptibility. Our measurements show a peak-peak ratio closer to 1.1 in the real component and a noise limited lower estimate of ≈ 7 in the imaginary component. The anomalous third harmonic can be explained by the presence of a geometrical surface barrier for flux entry and exit which we have included as a small region of enhanced current density, J C,G, close to the surface of the disk. The geometrical surface current density is predicted to be approximately equal to J C,G ≈ 2B C1/μ 0 d. In 2G HTS tapes with d ≈ 1μm and B C1 ≈ 5mT this is J C,G ≈ 8GAm−2. We have measured both J C,B and J C,G using the nonlinear susceptibility of a Superpower tape without artificial pinning in fields orthogonal to the tape up to 35T. We find a geometrical surface current of J C,G ≈ 10GAm−2 and an average critical current density J C which is in good agreement with transport measurements performed on material from the same reel of tape.

Microscopic mechanism of thermomagnetic instabilities in type-II superconducting thin films under AC magnetic fields

Thermomagnetic instability is one of the significant challenges for the application of superconducting devices. In this paper, the microscopic mechanism of thermomagnetic instability in superconducting films subjected to a transient AC magnetic field is numerically investigated by coupling the generalized time dependent Ginzburg–Landau equations and the heat diffusion equation. The influences of magnetic field ramp rate, ambient temperature, and nanometer-sized artificial pinning on the vortex matter are considered in our simulations. It has been found that vortex alignment and repulsion play significant roles in the branching of the penetration trajectories of the magnetic flux. Under fast ramping magnetic fields, the increase in the temperature and instability in the vortex matter are more significant. However, the rising temperature and jump size in the magnetization weaken as the ambient temperature increases. Pronounced hysteresis in the vortex dynamics has been found in the film subjected to AC magnetic fields. As the AC cycle proceeds, the vortex penetration process gets more unstable. We have also found that the nanometer sized pinning strongly modulates the penetration of vortices and the vortex matter is highly correlated with the lattice structure of the pinning sites. Our results provide new insights into vortex dynamics and give a mesoscopic understanding on the channeling and branching in the vortex penetration paths in superconductors under AC magnetic fields.

The magnetic field, temperature, strain and angular dependence of the critical current density for Nb-Ti

A scaling law for J c in commercial Nb-Ti wire is proposed that describes its magnetic field, temperature and strain dependence. The scaling law is used to fit extensive measurements of the total strand critical current density, J c,TS(B, T, ε), with the applied field orthogonal to the axis of the wire. We present critical current density, heat capacity and resistivity measurements to obtain , which shows clear angular anisotropy. At 4.2 K, the resistivity data show . We also discuss whether the fusion community should consider re-optimising standard commercial Nb-Ti wires that were developed for MRI applications at ~ 5 T, to produce higher J c at say 10T, and higher upper critical fields, perhaps using quaternary Nb-Ti alloys with artificial pinning centres.

Angular magnetic-field dependence of vortex matching in pinning lattices fabricated by focused or masked helium ion beam irradiation of superconducting YBa2Cu3O7−δ thin films

The angular dependence of magnetic-field commensurability effects in thin films of the cuprate high-critical-temperature superconductor YBa2Cu3O7−δ (YBCO) with an artificial pinning landscape is investigated. Columns of point defects are fabricated by two different methods of ion irradiation — scanning the focused 30 keV ion beam in a helium ion microscope or employing the wide-field 75 keV He+ beam of an ion implanter through a stencil mask. Simulations of the ion-target interactions and the resulting collision cascades reveal that with both methods square arrays of defect columns with sub-μm spacings can be created. They consist of dense point-defect clusters, which act as pinning centers for Abrikosov vortices. This is verified by the measurement of commensurable peaks of the critical current and related minima of the flux-flow resistance vs magnetic field at the matching fields. In oblique magnetic fields, the matching features are exclusively governed by the component of the magnetic field parallel to the axes of the columnar defects, which confirms that the magnetic flux is penetrated along the defect columns. We demonstrate that the latter dominate the pinning landscape despite of the strong intrinsic pinning in thin YBCO films.

Magnetic particle inspection and EMF Directive 2013/35/EU

EMF Directive 2013/35/EU [1] was transposed into legislation in EU member states in 2016. In the UK, the Health and Safety Executive (HSE) produced the Control of Electromagnetic Fields at Work (CEMFAW) Regulations 2016 [2] . It is the first piece of legislation in the UK that is specific to electromagnetic fields (EMFs) and requires all employers to perform an EMF risk assessment. For many this is a relatively easy process, as most workplaces do not contain equipment capable of producing high fields. However, any piece of equipment that uses a high level of electrical current is capable of generating a high magnetic field and, in the case of magnetic particle inspection (MPI), it is an intended output. As the operator is often required to stand close to the generated field, exposure levels can be relatively high. Magnetic flux density measurements have shown that the action levels (ALs) used as the basis for exposure assessment can be exceeded. In some instances this can be mitigated by some instruction or training, while in others it may be necessary to prove compliance with exposure limit values (ELVs). Proving compliance with ELVs is more complicated than proving compliance with ALs. It is by no means impossible, however, as computer modelling can be employed. The regulations also list categories of worker considered to be at particular risk. Pregnant workers and workers with active medical devices, such as pacemakers, or passive implants, for example artificial joints, pins, plates, screws or stents, are all considered to be at particular risk. To perform a risk assessment for such workers, reference levels contained in EU Council Recommendation 1999/519/EC [3] are used. These are much more stringent than ALs and compliance is more difficult, but a pragmatic approach can be employed to achieve a successful outcome. Fundamentally, the purpose of the regulations is not to place obstacles in the way of industry but to make employers and equipment manufacturers aware of their obligations and the possible routes that can be taken to achieve compliance.