Artificial Pinning Centers on MgB2 Superconducting Thin Films Coated by FeO Nanoparticles

MgB₂ thin films were fabricated on MgO (100) single crystal substrates. First, deposition of boron was performed by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850?°C in magnesium vapor. In order to investigate the effect of FeO nanoparticles on magnetic properties of MgB₂ thin films, the films were coated with different concentrations of FeO nanoparticles by spin coating process. The magnetic field dependence of the critical current density $J_{\mathrm{c}}$ was calculated from the M?CH loops and also magnetic field dependence of the pinning force density $f_{\mathrm{p}}(b)$ was determined for the films containing different concentrations of FeO nanoparticles. The values of the critical current density $J_{\mathrm{c}}$ in zero field at 5?K was found to be around 1×10⁶?A/cm² for pure MgB₂ film, 1.4×10⁶ for MgB₂ film coated with 25?%, 7.2×10⁵ for MgB₂ film coated with 33?%, 9.1×10⁵ for MgB₂ film coated with 50?% and 1.1×10⁶?A/cm² for MgB₂ film coated with 100?%. It?was?found that the film coated with 25?% FeO nanoparticles has slightly enhanced critical current density and it can be noted that especially the film coated with 25?% FeO became stronger in the magnetic field. The films coated with FeO were successfully produced and they indicated the presence of artificial pinning centers created by FeO nanoparticles. The superconducting transition temperature of the film coated with 25?% FeO nanoparticles was determined by moment?Ctemperature (M?CT) measurement to be 34?K which is 4?K higher than that of the pure film.     

The role of the substrate surface morphology in enhancing the MgB2 superconducting temperature

We hereby report on the role of the surface morphology of various substrates in the enhancement of the superconducting critical temperature of MgB₂. MgB₂ thin layers were grown by hybrid physical–chemical vapour deposition on silicon carbide SiC substrates/fibers and several other substrates, characterized by diverse surface morphologies. By investigating the structural, morphological and transport properties of MgB₂ thin layers, the presented data show that the superconducting critical temperature T _c exceeds the bulk value only when the MgB₂ films are grown on atomically flat (0001) SiC single crystals and on MgB₂ bottom layers. These results further confirm the interpretation of the coalescence-driven tensile strain mechanism behind the enhancement of superconducting properties in MgB₂ thin films.     

Influence of Interfacial LSMO Nanoparticles/Layer on the Vortex Pinning Properties of YBCO Thin Film

YBa₂Cu₃O_7?δ (YBCO) thin films have been deposited on bare and La_0.67Sr_0.33MnO₃ (LSMO) modified single crystal SrTiO₃ (STO) substrates. The effect of randomly distributed ferromagnetic LSMO nanoparticles and a complete LSMO layer, present at STO/YBCO interface, on the superconducting properties of YBCO thin films has been investigated by temperature dependent magnetization studies. The YBCO thin film on LSMO nanoparticles decorated STO substrate shows significant improvement in the critical current density and pinning force density as compared to the YBCO thin film deposited on bare STO substrate and this improvement is more significant at higher applied magnetic field. However, the LSMO/YBCO bilayer showed the improved flux pinning properties only up to a magnetic field of 1.5 T above which it deteriorates. In the case of LSMO/YBCO bilayer, the underlying LSMO layer gives rise to magnetic inhomogeneities due to domain structure, which leads to improved flux pinning properties limited to lower field. However, in the case of LSMO nanoparticles decorated substrate, the presence of LSMO nanoparticles at YBCO/STO interface seems to introduce magnetic inhomogeneities as well as structural defects, which might be acting as correlated pinning sites leading to improved flux pinning properties of the YBCO thin film over a wide range of applied magnetic field.     

High Critical Current Densities in MgB2 Films Grown on Hastelloy Tape by Hybrid Physical-Chemical Vapor Deposition

We report on the high critical current densities in MgB₂ films directly grown on Hastelloy tapes without any buffer layer by using the hybrid physical-chemical vapor deposition method. MgB₂ films were formed by reaction of Mg metal vapor with the incoming B₂H₆ gas on the heated substrates. In MgB₂ films grown for 10 min at 500 °C in total working pressure 100 Torr with gas mixing ratio H₂:B₂H₆=70:30, we observed the transport critical current density (J _c) was approximately 10⁶ A/cm² at 4 T and 20 K in magnetic fields applied parallel to the substrate plane. This value is higher than those observed in epitaxial MgB₂ films on sapphire substrates grown by using the same method. Magnetic field dependence of J _c of this sample was well explained by the grain-boundary pinning model. Our result opens up a possibility that the coated conductors made of MgB₂ films have a strong potential for high current applications.     

Comparative Study on Transport and Magnetic <Emphasis Type="Italic">J</Emphasis><Subscript>c</Subscript> of Textured MgB<Subscript>2</Subscript> Thin Film

Critical current densities of c-axis-oriented MgB₂ thin film were measured comparatively by transport and magnetization measurements. Experimental results manifest that transport critical current densities J _ct are quite different from magnetic critical current densities J _cm in both quantity and field dependence. The scaling behaviors of the flux pinning force are also quite different. To explain the inequality of J _cm and J _ct, a microstructure examination has been performed. Based on our results, we believe that J _cm and J _ct correspond to different current paths and different pinning centers. More than two kinds of pinning center are effective in MgB₂ superconductors.     

Improved Critical Current Densities in YBa2Cu3O7?�� Multilayer Films Interspaced with Palladium Nano-dots

Power applications of superconducting coated conductors in high magnetic fields require thick films with high critical current density J _c and strong artificial pinning centers. Here, we report on the artificial pinning centers induced in YBCO quasi-multilayer films interspaced with palladium (Pd) nano-dots. Quasi-multilayered (QM) YBa₂Cu₃O_7??? (YBCO) films composed of YBCO layers interspaced with quasi-layers of palladium nano-dots were grown by pulsed laser deposition on SrTiO₃(100) substrates. DC magnetization and frequency-dependent measurements showed high J _c comparable with best YBCO films in thin quasi-multilayers and significant improvement of J _c in thick quasi-multilayers. TEM study shows regions of planar defects, stacking faults, and pore formations suitable for immobilizing vortices. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.     

Pinning enhancement in MgB2 superconducting thin films by magnetic nanoparticles of Fe2O3

MgB₂ thin films were fabricated on r-plane Al₂O₃ ( ${1} \overline{{1}} {0} {2})$ substrates. First, deposition of boron was performed by rf magnetron sputtering on Al₂O₃ substrates and followed by a post-deposition annealing at 850 °C in magnesium vapour. In order to investigate the effect of Fe₂O₃ nanoparticles on the structural and magnetic properties of films, MgB₂ films were coated with different concentrations of Fe₂O₃ nanoparticles by spin coating process. The magnetic field dependence of the critical current density J _c was calculated from the M–H loops and magnetic field dependence of the pinning force density, f _p(b), was investigated for the films containing different concentrations of Fe₂O₃ nanoparticles. The critical current densities, J _c, in 3T magnetic field at 5 K were found to be around 2·7 × 10⁴ A/cm², 4·3 × 10⁴ A/cm², 1·3 × 10⁵ A/cm² and 5·2 × 10⁴ A/cm² for films with concentrations of 0, 25, 50 and 100% Fe₂O₃, respectively. It was found that the films coated with Fe₂O₃ nanoparticles have significantly enhanced the critical current density. It can be noted that especially the films coated by Fe₂O₃ became stronger in the magnetic field and at higher temperatures. It was believed that coated films indicated the presence of artificial pinning centres created by Fe₂O₃ nanoparticles. The results of AFM indicate that surface roughness of the films significantly decreased with increase in concentration of coating material.     

Magnetic flux noise in copper oxide superconductors

We report on the magnetic flux noise in thin films of YBa₂Cu₃O_7-x (YBCO), Tl₂Ca₂Ba₂Cu₃O_x, and TlCa₂Ba₂Cu₃O_x and in crystals of YBCO and Bi₂Sr₂CaCu₂O_8+x, measured with a Superconducting QUantum Interference Device (SQUID). We ascribe the noise to the motion of flux vortices. In the low magnetic fields in which the experiments are performed the average vortex spacing always exceeds the superconducting penetration depth. The spectral density of the noise usually scales as 1/f (f is frequency) from 1 Hz to 1 kHz and increases with temperature to a peak which is of the same magnitude in all samples, at the transition temperature. Furthermore, the noise power increases with the magnitude of the magnetic field in which the sample is cooled, with a power-law dependence over several decades, whereas a supercurrent well below the critical current density applied to YBCO films suppresses the noise power by an order of magnitude. Most of the measurements were made on YBCO films, and for this set of samples the noise decreases dramatically as the crystalline quality is improved. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, magnetic field, and current. The pinning potential is idealized as an ensemble of symmetrical double wells, each with a different activation energy separating the two states. From the noise measurements, this model yields the distribution of pinning energies, the vortex hopping distance, the number density of mobile vortices, and the restoring force on a vortex at a typical pinning site. The distribution of pinning energies in YBa₂Cu₃O_7-x shows a broad peak below 0.1 eV. Over narrow temperature intervals, most samples exhibit random telegraph signals in which the flux switches between two discrete levels, with activation energies and hopping distances much greater than those deduced from the 1/f noise measurements.     

Vortex Dynamics in Bi2Sr2CaCu2O8 Single Crystal with Low Density Columnar Defects Studied by Magnetic Force Microscope

We have studied vortex dynamics in Bi₂Sr₂CaCu₂O₈ single crystal with low density columnar defects by using a magnetic force microscope. Single crystal Bi₂Sr₂CaCu₂O₈ sample was irradiated by 1.3 GeV uranium ion to form artificial pinning centers along the crystalline c-axis. The irradiation dose corresponded to a matching field of 20 gauss. The radius of an individual vortex is approximately 140 nm, which is close to the penetration depth of this material. Magnetic force microscope (MFM) images show that intrinsic crystalline defects such as stacking fault dislocations are very effective pinning centers for vortices in addition to the pinning centers due to ion bombardment. By counting the number of vortex, we found that the flux trapped at each pinning center is a single flux quantum. At higher magnetic field, the vortex structure showed an Abrikosov lattice disturbed only by immobile vortices located at pinning centers. When increasing or decreasing the external magnetic field, the spatial distribution of vortices showed a Bean model like behavior.     

Synthesis Pressure–Temperature Effect on Pinning in MgB2-Based Superconductors

The volume pinning force, F _p(max), increases with increasing synthesis or sintering pressure (0.1 MPa–2 GPa) in materials prepared at high temperature (1050 °C) while it stays practically unchanged in those prepared at low temperature (800 °C). The position of F _p(max) can be shifted to higher magnetic fields by: (1) increasing the manufacturing pressure or decreasing the temperature (2) additions (Ti, SiC, or C, for example), and (3) in-situ preparation. Grain boundary pinning (GBP) dominates in materials prepared at low temperatures (600–800 °C), while high-temperature preparation induces strong point pinning (PP) or mixed pinning (MP) leading to outstanding properties. In materials produced by spark plasma sintering (SPS), the position of F _p(max) is higher than expected for both grain boundary and point pinning. The distribution of boron and oxygen in MgB₂ based material, which can changed by additions or the preparation conditions, significantly affects the type and strength of pining. Materials prepared under a pressure of 2 GPa with a nominal composition of Mg:7B or Mg:12B consist of 88.5 wt % MgB₁₂, 2.5 wt % MgB₂, 9 wt % MgO or 53 wt % MgB₁₂, 31 wt % MgB₂₀ 16 wt % MgO, respectively. Their magnetic shielding fractions at low temperatures are 10 % and 1.5 %, with a transition temperature, T _c of 37.4–37.6 K. Although their magnetic critical current density at zero field and 20 K was 2–5×10² A/cm², they were found to be insulating on the macroscopic level.     

Anisotropy of flux flow in layered superconductor 2H-NbSe2−x S x

Magnetic-field dependence of the resistivity for various currents in the layered superconductor 2H-NbSe_1.8S_0.2 was investigated. The flux flow was observed for various angles between the magnetic-field direction and the crystal layers near zero-field critical temperature. Anisotropy of the pinning force and the viscosity coefficient was found. The peak effect was observed under a magnetic field perpendicular to the layers, while it was not observed under a magnetic field parallel to the layers. A strong pinning force was found acting only on flux lines parallel to the layers. These results can be explained by the pinning mechanism due to the stacking faults.     

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB₂ thin films were deposited on MgO (100) substrate and r-plane Al₂O₃ $(1\bar{1}02)$ substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB₂ films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density J _c was calculated from M–H loops and also the magnetic field dependence of F _p was compared for the different temperature ranges from 5 to 25 K. The critical current density J _c was found to be around 1.0×10⁶ A/cm² and 1.7×10⁶ A/cm² in zero field at 5 K for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al₂O₃ in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.     

Improvement of the High-Magnetic-Field Critical Current Density of the <Emphasis Type="Italic">Ex-Situ</Emphasis> Annealed MgB<Subscript>2</Subscript> Thick Films by Oxygen Doping

This paper presents a very simple way to synthesis MgB₂ thick films with high critical current density in a magnetic field by ex-situ annealing precursor B films in air with excessive Mg in a sealed quartz tube. The films show a significant improvement of critical current density in a magnetic field compared to the high purity films annealed in vacuum, while its zero-resistance transition temperature T _c ^zero and normal state resistivity still maintain about 38 K and 17 μΩcm. The results demonstrate MgB₂ thick films have great potential applications in superconducting coated conductors.      

Comparison of Flux Pinning Mechanism in Laser Ablated YBCO and YBCO:BaZrO3 Nanocomposite Thin Films

Thin films of YBCO and YBCO:BaZrO₃ (BZO) nanocomposite have been deposited using the pulsed laser deposition technique. Substantial increase in critical current density (J _C ) and pinning force density (F _p ) of the nanocomposite thin films was observed. The possible pinning mechanism in YBCO:BZO nanocomposite thin films has been explored and compared with the pinning mechanism in pure YBCO thin film by studying the variation of J _C with magnetic field (B) and temperature. In the intermediate field regime (0.1–1 T), J _C follows B ^−α with nearly similar values of α for YBCO and YBCO:BZO nanocomposite thin films indicating similar pinning mechanism in both thin films. The variation of J _C with reduced temperature (t=T/T _C ) has been studied for both the films and it was observed that the mechanism of pinning in both YBCO and YBCO:BZO thin films is similar (δT _C pinning). The observed enhanced values of J _C and F _p of the nanocomposite thin film is attributed to the presence of BZO nanoparticles, which induces more defects due to lattice mismatch between YBCO and BZO leading to improved flux pinning properties of the nanocomposite thin film.     

Effect of laser irradiation on superconducting properties of bulk MgB<Subscript>2</Subscript> sintered at different temperatures

The surfaces of MgB₂ bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850?°C, after hot pressing at 200?°C, were irradiated with the same irradiation dose by using an Nd:YVO₄ laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB₂ superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650?°C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850?°C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB₂ in different ways depending on sintering temperature of the superconductor.     

DC electrical resistivity and magnetic studies in Yttrium Barium Copper oxide/barium titanate composite thin films

YBCO + BaTiO₃ composite thin film is synthesized by pulsed laser deposition. Fluctuations on the electrical conductivity were investigated for zero fields. The logarithmic plots of excess conductivity and reduced temperature reveals two distinct regions namely mean field region and short wave fluctuation region. Dimensionality crossover occurs from 3D to 2D at temperature above the transition temperature. The contribution of weak link effect is calculated. The phase formation and grain alignments were analyzed by X-ray diffraction and scanning electron microscopy techniques. Enhancement of flux pinning increases the critical current density in the composite and develops strong pinning force in the material.     

MgB2 Thin Films on Metal Substrates for Superconducting RF Cavity Applications

Magnesium diboride is a promising material for superconducting RF (SRF) cavity applications. Compared to the currently used superconductor for SRF cavities Nb, MgB₂ has the potential to achieve lower RF loss and higher acceleration field due to its higher critical temperature and thermodynamic critical magnetic field. Since the RF field only penetrates a few penetration depths into a superconductor, a superconducting coating of several hundred nanometers on a metal cavity is sufficient for superb SRF cavity performances. In this work, we report the properties of MgB₂ thin films deposited by the hybrid physical–chemical vapor deposition (HPCVD) technique on different metal substrates including Nb, Mo, Ta, and stainless steel. All the films were polycrystalline, as indicated by X-ray diffractometry and scanning electron microscopy, and showed T _c ～39 K, determined by resistance versus temperature, magnetic susceptibility, and dielectric resonator measurements. MgB₂ films deposited on Nb substrates polished to various degrees of smoothness exhibit similar T _c . The result is a promising step in the investigation of using MgB₂ as an alternative to Nb for SRF cavities.     

Interaction between the Ferromagnetic Dots and Vortices: Numerical Calculation and Experimental Results

Enhancing the pinning force in high-T _c superconductors can be achieved by externally introduced periodic magnetic dots. We numerically calculate the interaction between ferromagnetic dots and vortices in high-T _c superconductors. The London equation is used to generate two-dimensional vortex lattice. In the matching condition, we calculate the attraction force between magnetic dots and vortices. It is found that in an ideal condition, the pinning force of the magnetic dot reaches 2.5×10⁻¹¹ N that is more than one order magnitude stronger than the intrinsic pinning force in YBa₂Cu₃O₇ thin films. In the experimental side, we use a novel nano-technique to deposit periodic submicron Ni dots on YBa₂Cu₃O₇ thin films. The current versus voltage characteristics of an YBa₂Cu₃O₇ thin film strip with uniform Ni dots are measured at various temperatures and magnetic fields. They are compared with the current versus voltage characteristics of a bare YBa₂Cu₃O₇ thin film strip without magnetic dots. It is found the critical current value of the strip with Ni dots reduces with a much slower pace as the magnetic field strength increases in comparison with the value of the bare sample.     

Flux Pinning in YBCO Single Crystal Grown on Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Layer

In this study, single-crystal Y123 samples were grown by a cold top-seeding method by using Nd123 seed, and the effect of Y₂O₃ buffer layer was investigated. The upper critical magnetic field and coherence length were established as 110 T and 17.3 ?, respectively. The dependence of the effective activation energy U of the flux pinning on the magnetic field and temperature of the sample were determined using the Arrhenius activation energy law from the resistivity curves. It was found that the deduced value of the activation energy for a Y123 sample is in good agreement with the corresponding values in YBCO samples. The maximum activation energy value was approximately 0.9 eV in the zero magnetic field. In order to examine the homogeneity of the pinning properties of different layers, rectangular specimens were cut from the sample. AC susceptibility measurement was performed, and it was found that the shifting of the peak temperature (T _p) with an AC magnetic field is small, indicating good pinning properties. The normalized pinning force density versus the reduced field was examined at different temperatures to determine the pinning mechanism. It was found that normal core-type pinning was effective, and in low fields, pinning was only due to Y211 particles.     

Computer Simulation of Flux Pinning in the High-Temperature Superconductors Bi-2212

The magnitude of critical current density J _c (or critical magnetic induction B _c), is closely related to the flux pinning property of the high-temperature superconductors (HTSC) materials. So, the investigation on flux pinning mechanism is an important research field in the high-temperature superconductors. There are different pinning centers by analyzing the flux pinning force of the HTSC. In this study, based on Sun and Zou et al.’s previous works, we studied the flux pinning mechanism in Bi-2212 single crystals with an improved scaling function of pinning density, which is simplified and possesses determined physical meaning. Then, a computer simulation was conducted to determine the category of flux pinning in Bi-2212 according to Sun’s experimental data. It is revealed that the simulation result is consistent with the experiment.