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We investigated the effect of the thickness of superconducting layer on the competition between ferromagnetism and superconductivity. The epitaxial bilayer systems consisting of GdBa2Cu3O7-x (GdBCO) with varying thickness and La0.7Sr0.3MnO3 (LSMO) were grown on (001) SrTiO3 (STO) substrate. From the spectroscopic measurements, we observed an existence of a possible structural coupling between the GdBCO and the LSMO layers. The structural coupling is found to depend on the GdBCO thickness and to induce different types of strain on both layers, leading to the competition between the ferromagnetic and the superconducting orders. With respect to the flux pinning, an additional pinning contribution besides magnetic pinning induced by LSMO is emerged by the structural coupling, which is attributed to local atomic disorder. Controlling the thickness of GdBCO modifies the structural coupling and consequently, the degree of additional pinning contribution aroused by disorder on the CuO2 plane can be adjusted. An appropriate structure with a certain thickness amplifies the hybrid effect of magnetic pinning and additional pinning, resulting in an enhancement of the critical current density at high magnetic fields. Our findings suggest a possibility of controlling complex flux pinning through an optimization of structural coupling between the GdBCO and the LSMO layers.

We have investigated the pressure (P) effect on structural (up to 10 GPa), transport [R(T): up to 10 GPa], and magnetic [(M(T): up to 1 GPa)] properties and analyzed the flux pinning mechanism of the Fe0.99Mn0.01Se0.5Te0.5 superconductor. The maximum superconducting transition temperature (T c) of 22 K with the P coefficient of T c dT c/dP = +2.6 K/GPa up to 3 GPa (dT c/dP = -3.6 K/GPa, 3 ≤ P ≥ 9 GPa) was evidenced from R(T) measurements. The high-pressure diffraction and density functional theory (DFT) calculations reveal structural phase transformation from tetragonal to hexagonal at 5.9 GPa, and a remarkable change in the unit cell volume is observed at ∼3 GPa where the T c starts to decrease, which may be due to the reduction of charge carriers, as evidenced by a reduction in the density of states (DOS) close to the Fermi level. At higher pressures of 7.7 GPa ≤ P ≥ 10.2 GPa, a mixed phase (tetragonal + hexagonal phase) is observed, and the T c completely vanishes at 9 GPa. A significant enhancement in the critical current density (J C) is observed due to the increase of pinning centers induced by external pressure. The field dependence of the critical current density under pressure shows a crossover from the δl pinning mechanism (at 0 GPa) to the δT c pinning mechanism (at 1.2 GPa). The field dependence of the pinning force at ambient condition and under pressure reveals the dense point pinning mechanism of Fe0.99Mn0.01Se0.5Te0.5. Moreover, both upper critical field (H C2) and J C are enhanced significantly by the application of an external P and change over to a high P phase (hexagonal ∼5.9 GPa) faster than a Fe0.99Ni0.01Se0.5Te0.5 (7.7 GPa) superconductor.

MgB2 superconducting materials have important application potential in the 20K temperature region, which is a significant advantage compared with other low-temperature superconductors (such as Nb-Ti and Nb3Sn). However, there is a big gap between its superconducting current-carrying capacity and Ni-Ti and Nb3Sn, which is due to the weak magnetic flux pinning force and the poor connection between the MgB2 grains. Chemical doping is a simple and effective method to improve the performance of MgB2. This paper studies the effect of NaCl doping on the performance of MgB2. It is found that NaCl doping has a positive effect on improving the interconnection of MgB2 grains and refining MgB2 grains, and is a potentially effective method to improve the superconducting properties of MgB2. In addition, all of the samples in this study showed surface pinning characteristics, but the smaller the MgB2 grain size, the better the match with the surface pinning characteristics described by Kramer's theory.

The Monte Carlo method has been used to calculate the trapped magnetic field in superconducting plates with holes. The mechanism of flux pinning on the holes is implemented with special subprocesses added to the algorithm: vortex capture and emission, both occurring on the hole boundaries. Secondary peaks related to the holes emerge on the calculated profiles of the trapped magnetic field. It has been found that these peaks disappear in plates with sufficiently strong pinning or when the computational mesh is coarse (the case corresponding to a low resolution of probes in experiments). The dependence of the trapped field on the hole radius has been analyzed.

Mechanical power transmission (MPT) components are almost indispensable for every engineering equipment with motions. In order to satisfy some rigorous requirements, such as contamination free and zero leakage in the mixing process of biomedical solutions, a contactless MPT mode was proposed in this study based on the high-Tc superconducting flux pinning mechanism. It makes the stirring container with the driven part inside that can be totally isolated from the external environment. The physical principle of superconducting flux pinning effect was discussed firstly to explore a feasible structural scheme, which can completely restrain all the six degrees of freedom (DOFs) by the linkage of magnetic flux lines. Then, a measurement device was established to verify and investigate the proposed contactless MPT mode. The motion can be transferred synchronously from the superconducting driving part to the permanent magnet driven part since they are unified as an integrity through the pinned flux lines. The influence of driving speed, cooling clearance, and magnet arrangement on the transmitted torque was analyzed. The verified contactless MPT mode also has the advantages of self-stability and overload protection, which can avoid the drawbacks of traditional permanent magnetic transmission mode.

YBa2Cu3O7− x (YBCO) is treated as a high-temperature superconductor material for its excellent performance. In this article, we investigated the superconducting properties of YBCO grown on LaAlO3 (LAO) substrate with Eu0.6Sr0.4BiO3 (ESBO) buffer layers. Though the chemical solution-based method, we get ESBO buffer with tetragonal SrO2 particles precipitated on the surface. The cross section of transmission electron microscopy shows more microstructure details of defects of YBCO films introduced by buffer layers and self-assembled SrO2. It is observed that the critical current density Jc of YBCO grown with buffer layers enhanced approximately 40% compared with those on LAO at a low applied magnetic field. The comparison of fitted curves of YBCO/ESBO with different pinning types shows that it is closer to δl pinning and began to lean to δ stress pinning with temperature arise. The values of pinning forces also get improved. Besides, the analysis of normalized pinning force density at 77 K demonstrates the interplay of different flux pinning mechanisms. Finally, all the results show the effectiveness of ESBO buffer layers on enhancing the performance of YBCO.

The polycrystalline granular BSCCO high Tc superconductors (HTSC) have limitations in various applications. These limitations appear due to the flux pinning's weakness and the weak links between the grains comparatively in high temperatures and high applied magnetic fields. Bi2Te3 nanoparticles are artificially introduced into the Bi-2223 HTCS matrix to be employed as effective flux pinning centers to enhance the flux pinning capability and the critical current density. The effect of the additive of Bi2Te3 nanoparticles on the structural and physical properties of Bi-2223 were investigated for the polycrystalline (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x/(Bi2Te3)x where (x = 0.00,0.01,0.02& 0.03). The phase structure/formation, volume fraction, the lattice constants were described by X-ray powder diffraction (XRD) measurements. Diamagnetic signal has been investigated with two onset temperatures (Tc1&Tc2) for the common BSCCO phases (Bi-2223 and Bi-2212) which confirm the XRD obtained data without any indication for unwanted impurities. The magnetic interactions between Bi2Te3 nanoparticles addition and the superconductor matrix are discussed at 5 and 50 K. The relation between the microstructure, BSCCO phase's contents, the hysteresis loops, the calculated critical current densities, and the flux densities were also reported for the samples. Nano-Bi2Te3 shows a great impact on the BSCCO superconducting properties and influences its flux densities and the flux pinning mechanisms as reported experimentally and theoretically. Consequently, the additive of Bi2Te3 nanoparticles must be carefully controlled to balance the microstructure and superconducting parameters of the BSCCO HTSC.

Internally oxidized Nb3Sn wires with artificial pinning centers (APC) have been developed by manufacturing a multifilament PIT-style wire, each filament consisting of a Nb-Ta-Zr alloy tube filled with a mixture of Sn, Cu, and oxide powders. During heat treatment, the oxide decomposes, and the oxygen goes into solid solution in the Nb alloy. Upon Sn diffusion into the Nb alloy, ZrO2 nanoprecipitates form which serve both to inhibit grain coarsening and directly pin magnetic flux. The finer grain structure and high concentration of non-superconducting precipitates serve as flux pinning sites and enhance high-field J c; direct pinning by the precipitates also shifts the maximum pinning force to higher fields. To distinguish these two effects, an APC wire with a high heat treatment temperature (700 °C) was compared to a conventional PIT wire with very low heat treatment temperature (600 °C), resulting in similar grain size in both samples. The pinning force vs applied field ( F p -B ) curve was deconvoluted into grain boundary and point pinning components. It was found that the grain boundary component of the APC wire was very close to the F p- B curve for the PIT wire, demonstrating that the two pinning components in an APC wire are directly additive. It was then possible to show that, in the 15-20 T regime, direct pinning contributed 45-50% of the total pinning.

Manufacturing of coated conductors can be tailored for future use operating parameters, such as high temperature and low field, mid temperature and mid field and low temperature and high field. Different methods of flux pinning are being tested world-wide to enhance critical currents of high temperature superconductor yttrium barium copper oxide coated conductors for these various operating conditions. Magnetic materials are interesting to consider as flux pinning additions because of their potential for very strong pinning strength. This research describes the study of different magnetic phase additions to superconducting thin films, including: BaFe12O19, La0.67Ca0.33MnO3, Y3Fe5O12, and SrRuO3. Nano-size additions with volume percent ranging from 0.5% to 5% were incorporated by utilizing pulsed laser deposition to form alternating layers of magnetic dopants and yttrium barium copper oxide to minimize degradation of Tc . Experimental critical current density ( Jcm ( H,T )) results for mid temperature (30 K–65 K) / mid field (1 T–5 T) and low temperature ( 5 T) and microstructural TEM are presented. Results illustrate the benefits and interplay of combining magnetic flux pinning and core pinning by the magnetic additions and associated film defects, and suggest applications suitable for each system studied.

In this study, the magnetization measurements have been performed on high-temperature superconductor's single crystals YBa2Cu3O7-δ at large ranges of temperature T (15-85 K) and in magnetic fields up to 6T at different values of the angle θ between the applied magnetic field and c-axis. The critical current density Jc deduced from the magnetic hysteresis loops by the Bean formula for H parallel to the c-axis (θ=0°), our results have shown that the critical current density Jc was strongly dependent on the applied magnetic field. The pinning force Fp=Jc×μ0H was determined from magnetization for H//c, however, a plot of the normalized pinning force density fp= Fp/Fpmax as a function of the reduced magnetic field h= H/Hirr at different temperatures have shown good scaling with the form fp ~hp(1-h)q, where p and q are scaling parameters. We also found that the point pinning is more dominant than surface pinning under high temperatures.

Deposition temperature is an important parameter to control in pulsed laser deposition of thin films, as it affects the ad-atom mobility and diffusion in regards to film growth. Increased growth temperature can also result in increased stacking fault densities in YBa2Cu3O7-δ (YBCO) films. This research investigates the influence of deposition temperature on the critical current density and the microstructure of YBCO thin films double doped with BaHfO3 and Y2O3. A KrF excimer laser was used to produce thin films of YBCO doped with 4 vol. % BaHfO3 and 3 vol. % Y2O3 on LaAlO3 (LAO) substrates at various deposition temperatures from 790–825 °C. The growth temperature influence on the flux pinning landscape and Jc (H,T,θ) properties ( T = 5–77 K, H = 0–9 T, θ = 0–180°) of these films will be presented.

We have investigated spatial distribution of local critical current density ( J c) in a Co-doped BaFe2As2 (Ba122:Co) film based on scanning Hall-probe microscopy (SHPM). In-plane J c distribution was derived from the magnetic field distribution. Thanks to the high-resolution characterization, local obstacle can be detected clearly, as well as a local spot with extremely high J c over 7 MA/cm2, which is among the highest value at remanent state reported for this series of films so far. The SHPM allows us to separate the influence of current blocking obstacles and intrinsic flux pinning properties on J c. Validity of the J c analysis was checked by a magnetization measurement based on a SQUID magnetometer. Moreover, we investigated the J c distributions at different temperatures by SHPM and studied their probability distribution functions (PDFs). PDFs are found to be scaled by a Weibull function with the same shape parameter, while J cs are decreasing with higher temperature. These results indicate that the intrinsic in-plane pinning is controlled by the same mechanism.

The flux pinning effect of bulk superconductors facilitates noncontact stable levitation of permanent magnets without any active control. In this study, a levitation system using a bulk superconductor was supplemented with a hybrid stator having a ring-shaped magnet in order to improve the stability of the superconducting levitation. Further, the effect of the size of the inner diameter of the ring-shaped magnet on the levitation stability was elucidated by measuring the levitation characteristics of a levitating magnet located above the hybrid stator to ring-shaped magnets with a variety of inner diameters. The vertical magnetic stiffness was improved only for 25-mm inner diameter, and the stiffness decreased if the diameter was larger or smaller than 25-mm. The horizontal magnetic stiffness decreased at the initial position, but the range expanded for which the restoring force acted. The results show that by the appropriate selection of the inner diameter of the ring-shaped magnet, the performance of the hybrid stator can be maximized.

Angular behaviors of critical current density J c and n -values at 77 K, 1 T were investigated in YBCO films with columnar defects (CDs) tilted at small angles to the ab -plane before and after the additional introduction of CDs at different angles, where CDs were installed by heavy-ion irradiations. The enhanced J c at B || ab by the initial irradiation at θ i = ±85° relative to the c -axis was reduced by the additional irradiation at θ i = ±75°, whereas the J c was increased at the intermediate angle between the two irradiation angles. The addition of CDs at θ i = 0° also lowered the J c around B || ab , which were enhanced by the initial irradiation angles of θ i = ±80°. The n -values, by contrast, were hardly lowered in any direction of magnetic field by the second irradiation: the n -value seems to be determined by a simple sum of the pinning interactions of CDs tilted at different angles. These results suggest that the flux pinning is improved in each direction of the tilted CDs, without impeding the flux pinning in the other CD direction each other. The additional introduction of CDs, however, induces both the reduction of the superconducting percolation path and the irradiation damage to the ab -correlated PCs, resulting in the reduction of J c.

The influence of x wt% C4H6O5 (malic acid) (x = 0.0, 4.0, and 6.0) addition on excess conductivity in bulk MgB2 superconductors were fabricated by two-step solid-state reaction method. Resistivity measurements were carried out as a function of temperature starting from the room temperature at 0 T magnetic field. The electrical resistivity measurements did not show any significant change on Tc with the increasing addition ratio. Estimated α is a coefficient related with the intrinsic electronic interaction was found approximately the same value for added sample but pure sample was found to be lower; indicating increasing electronic interactions with addition. For the analysis of the excess conductivity, Aslamazov and Larkin (AL) model was used and the impacts of C4H6O5 additions on the excess conductivity of MgB2 have been investigated. The samples indicated the existence of three distinct fluctuation regimes above and near to Tc. The samples with x wt% malic acid additions (x = 4.0 and 6.0) showed 2D character such as exhibited by the HTSs, while the pure MgB2 sample exhibited the 3D nature. While the TLD was not observed for pure sample, it was observed for malic acid-added samples. Also, the temperature value increased leading to growing pinning capability with the increasing addition ratio. Flux pinning mechanisms in MgB2 superconductor were analyzed and discussed in detail. Normal point pinning and surface pinning of flux pinning mechanisms were found to be effective in MgB2 superconductors. In addition, while all samples exhibited normal point pinning in the low magnetic field region, it was observed that the pinning mechanism shifted to the surface pinning with the increasing addition rate in the high magnetic field region. In the malic acid-added samples in which the agglomeration occurred, a dominant 2D fluctuation was observed while the 1D fluctuation was observed in the pure sample. Because the agglomeration causes existing of effective pinning centers, 2D fluctuations dominate in the malic acid-added samples.

In the present work, we are reporting the impact of addition of sodium niobate (NaNbO3) nanoparticles (NPs) on the flux pinning properties of YBCO compound with different wt% (0 to 2%) of NaNbO3. The structural analysis reveals that the orthorhombic crystal structure of YBCO remains unchanged in the nanocomposite samples. The presence of NaNbO3 in the YBCO nanocomposites is confirmed by elemental analysis technique. No significant change in the superconducting transition temperature is observed on addition of NaNbO3 NPs in YBCO compound for up to 2 wt% concentration. Magnetization hysteresis loop measurement indicates a significant enhancement in the value of critical current (JC) and pinning force (FP) in the YBCO/x NaNbO3 (NP) composite samples which is attributed to the generation of sufficient no. of defect centers in YBCO matrix due to addition of NPs of NaNbO3. At lower fields and temperature range 10–60 K, the 0.5 wt% NP-added composite sample has shown maximum enhancement in JC whereas at higher temperatures and higher fields, the maximum improvement in the pinning properties is observed for 0.25 wt% NaNbO3-added YBCO sample.

We successfully refined a cheap commercial boron powder by means of high-energy ultra-sonication and utilized it in synthesis of bulk MgB2. Rietveld phase analysis of X-ray diffraction pattern revealed completely formed MgB2 with a low amount of MgO. MgB2 bulk prepared of boron ultra-sonicated in ethanol for 15 min showed self-field Jc of around 300 kA/cm2 at 20 K without any compromise in Tc (~39 K). Pinning analysis based on Dew-Hughes expression showed major pinning contribution from grain-boundary pinning (~95.5%), along with a slight contribution from point pinning (4.5%). The microstructure study detected a system of large MgB2 grains (hundreds nm large) and 10–20 nm sized particles, possibly Mg-B-O, formed at MgB2 grain boundaries.

This paper investigates the flux-pinning-induced stress behaviors in a long superconducting slab with a central cuboid hole. The distribution of flux density is analytically derived for the critical current density of Kim model by considering the effects of the hole. The concentration of stress for both of the zero-field cooling (ZFC) and the field cooling (FC) magnetization processes are investigated through numerical simulation by using finite element method. The results indicate the potential failure mode varies with the height-width ratio and the dimensions of the hole. Different to the case of infinite hole height, the stress concentration behavior for the FC process is more critical when the dimensions of the hole are similar. The findings in this paper are useful for understanding of the degrade mechanism of superconductor in service and the design of new system. The flux-pinning-induced stress behaviors in a long superconducting slab with a central cuboid hole was investigated for the critical current density of Kim model by considering the effects of the hole. The results indicate the potential failure mode varies with the height-width ratio and the dimensions of the hole. Different to the case of infinite hole height, the stress concentration behavior for the FC process is more critical when the dimensions of the hole are similar.

Superconducting properties of Cu0.5Tl0.5Ba2Ca2(Cu3−xNix)O10−δ (x = 0, 0.5, 1, 1.5) have been investigated by XRD, dielectric measurements, excess conductivity analyses and resistivity measurements. These samples show orthorhombic crystal structure in which a-axis length suppresses but b-axis length increases with increased Ni concentration. These samples have shown metallic variations of resistivity from room temperature down to onset of superconductivity in which the onset temperature of superconductivity and its zero resistivity critical temperature suppresses with increased Ni concentration. Excess conductivity analyses of conductivity data has shown values of coherence length ξc(0) along c-axis, Fermi velocity of the carriers VF and inter-layer coupling J, suppress with increased Ni-substitution which shows that density of superconducting carriers is also suppressed. Lower values of real part (e′) of dielectric constant has confirmed lower density of carriers in Ni-doped samples which further shows that doped Ni-atoms induce pair breaking effects. The flux pinning characteristics of Ni substituted samples are, however, significantly improved in comparison with parent compound i.e. Cu0.5Tl0.5Ba2Ca2Cu3O10−δ, manifesting that spin carrying Ni-atoms behave like efficient pinning centers.

Polycrystalline (YBa2Cu3Oy-d)1−x/(Dy2O3)x superconductor samples were produced through the solid-state reaction route and by adding amounts of x = 0.1 and 0.5 wt% of Dy2O3 nanoparticles (NP-Dy2O3 with a size of about 10 nm) during the second stage of heat treatment. The structural, microstructural, critical current density and pinning properties were investigated using x-ray diffraction, scanning electron microscope, and DC magnetization at various temperatures ranging from 77 down to 10 K and under an applied magnetic field varying between −6 and +6 Tesla. Both samples crystallize in the orthorhombic structure. It was found that the Dy2O3 nanoparticles reside in the grain boundaries. Although the sample with 0.5 wt% NP-Dy2O3 is characterized by a low zero resistive temperature of about 78 K that is close to the useful temperature for technological applications, it showed the highest Jc values and the excellent flux pinning capacity. The addition of an appropriate amount of NP-Dy2O3 up to 0.5 wt% extends the single bundle pinning regime and retards the liquid vortex regime.

This comprehensive work aims to examine the change in flux pinning mechanism, physical, mechanical, and structural characteristics of pure and Sr-site Nd-substituted Bi1.8Pb0.35Sr1.9−yNdyCa2.2Cu3Ox (Bi-2223) systems. The magnetoresistivity performances for all the samples are carried out by magnetotransport experiments in the existence of external magnetic field strength intervals 0–7 T. It is found that the increment of Nd/Sr substitution amount in bulk Bi-2223 system retrogrades the pinning capability of thermal flux motions for interlayer Josephson junction between the isolated grains. Similarly, the coupling probabilities of copper pairs and potential energy barriers are significantly diminished by increasing Nd impurity. This is in association with the enhancement of permanent structural problems in the crystal structure. Therefore, the excessive Nd inclusions improve the reattached linear/split pancake-like nature. In this regard, the best magnetic performance quantities are obtained for the pure sample. Besides, the SEM images show that the grain connectivity and surface morphology damage significantly with the Nd impurity. Additionally, the experimental microhardness findings conducted at various external loads (0.245–2.940 N) display that the Nd purity in the superconducting system degrades dramatically the key design mechanical features. Besides, we analyze the mechanical characteristic properties founded on the theoretical approaches with the proportional sample resistance, elastic/plastic deformation, and Hays–Kendall methods. The results obtained show that the Nd purity causes the indentation size effect behavior to decrease dramatically for all the samples. Furthermore, the findings of Hays–Kendall method are noticed to much more agree with the real hardness parameters. Thus, the Hays–Kendall model is the best methods to find the load-independent Vickers hardness values for the Sr-site Nd-substituted Bi1.8Pb0.35Sr1.9−yNdyCa2.2Cu3Ox (Bi-2223) systems. Moreover, in the dynamic microhardness measurements, the contact depth (hc), elastic modulus (Er), and load (Pmax) of all the samples are experimentally recorded for the first time. The results reveal that the mechanical properties depend strongly on the load and Nd impurity level.

We investigated the influence of surface damage on the critical current density (Jc) of MgB2 thin films via 140-keV Co-ion irradiation. The Jc(H) of the surface-damaged MgB2 films was remarkably improved in comparison with that of pristine films. The strong enhancement of Jc(H) caused by a surface damage in MgB2 films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (Hirr) at 5 K for surface-damaged MgB2 films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve Jc(H) in a MgB2 superconductor.

In this report, we present the effect of nano-ZnO and nano-Zn0.95Mn0.05O nanoparticles (NPs) and nanobelts (NBs) in polycrystalline superconducting samples of YBa2Cu3Oy (for brevity Y-123). The samples were synthesized using the solid-state reaction. Structural results analyzed via X-ray diffraction showed a similar behavior in all samples while the c parameter increased in the case of samples doped with nanobelts. Measurements of the electrical resistivity’s dependence on temperature revealed that the depression in the superconducting temperature is more pronounced in samples doped with nanoparticles compared to those doped with nanobelts. The excess conductivity in the mean-field region was analyzed using the Aslamazov–Larkin and Lawrence–Doniach models. The effective layer thickness, the zero-temperature coherence length, the wire cross-sectional area, the critical magnetic fields, and the critical current density were also calculated. The Zn0.95Mn0.05O nanobelts are more suitable for enhancing the flux pinning properties of YBa2Cu3Oy.

The purpose of this work is to investigate the change of flux pinning ability, superconducting and physical properties of gold particle addition in the YBa2Cu3O7-x (YBCO) superconducting matrix using temperature dependent resistivity measurements under different applied magnetic fields; namely 0.0, 0.3, 1.0, 2.0, 5.0 and 7.0 T. The room temperature resistivity, transition temperature (Tc), transition width (∆Tc) and mobile hole carrier concentration values for each sample have separately been determined. In addition, we estimated the field-dependent activation energy (U0), irreversibility field (µ0Hirr), upper critical field (µ0Hc2), penetration depth (λ) and coherence length (ξ) with the quantum mechanical approaches related to the fluxoid mechanism for all the samples prepared. It was found that the flux pinning energy values evaluated by Thermally Activated Flux Creep (TAFC) model were determined to be about 11,455 K, 13,413 K, 10,350 K and 9320 K for the undoped, 1 wt%, 5 wt% and 20 wt% Au doped samples, respectively. Moreover, the results obtained showed that the critical temperature, activation energy, irreversibility and upper critical field values were found to depend strongly on the gold content and applied magnetic field strength. The values of µ0Hirr, µ0Hc2, λ and ξ (∆Tc) initially exhibited a slight increment (decrement) followed by a decrease (increase) for the samples with the higher gold-doping concentrations. The possible reasons regarding the observed changes in flux pinning and fundamental superconducting properties due to gold-doping in YBCO superconductors were discussed in detail. We indicated that a certain amount of gold doping has an effective role to form the nucleation centers that serve as the active flux pinning regions in the YBCO system so that the quality of interaction between the superconducting grains and inter-granular coupling strength in the adjacent multilayers can increase remarkably.

The temperature and magnetic field dependence of the critical current density (Jc) and pinning force density (Fp) in Bi1.6Pb0.4Sr2Ca2−xNaxCu3O10+δ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.10) high-temperature superconductors is reported. Polycrystalline samples were prepared by a conventional solid-state reaction route. The influence of Na substitution on Jc and Fp at temperatures ranging between 65 K and 25 K was investigated. The collective pinning model is successfully applied to describe the magnetic field dependence of Jc for all the samples investigated. Magnetic field–temperature phase diagrams were also constructed to illustrate the effects of Na substitution in different field regions. The power-law dependence of the maximum Fp (Fp,max) versus the irreversibility field (Birr) at different temperatures shows nearly unchanged exponent values, suggesting temperature independence of the flux pinning mechanism in the Na-substituted samples. The dominant pinning mechanism is confirmed to be δl pinning, induced by fluctuations in the mean free path of charge carriers.

In applications requiring a large magnetic force, permanent magnets with non-parallel magnetization directions can be assembled in a Halbach array to generate a large gradient of magnetic flux density. The saturation magnetization of permanent magnets, however, brings a fundamental limit on the performance of this configuration. In the present work, we investigate experimentally the assembly of cuboid bulk, large grain melt-textured YBa2Cu3O7−x superconductors (∼14×14×14 mm3) with orthogonal c-axes so as to form a basic unit of Halbach array. The experiments are carried out at 77 K. The experimental distribution of the magnetic flux density above the array of trapped-field superconductors is compared to a similar array made of permanent magnets. A simple analytical model is developed and is shown to accurately reproduce the main experimental observations. The results suggest that a redistribution occurs in the current flowing in the central sample when the distance between the superconductors is reduced, whereas the neighbouring superconductors are unaffected. It is shown that this current redistribution yields a reduced contribution of the central sample to the magnetic flux density above the centre of the array and a new negative contribution associated with stray fields to the magnetic flux density at this location. This interpretation is confirmed by modelling of the distribution of transport currents in the superconductor using a 3D finite element model., Belgian FNRS under grant CDR n° J.0218.20 (35325237). Michel Houbart is recipient of a FRS- FNRS Research Fellow grant.

The effect of Arsenic substitution on the properties of Bi-2212 superconductors was studied. Solid-state reaction technique was used for the synthesis of Bi2−xAsxSr2Ca1.1Cu2.1O8 samples, x = 0, 0.1...

We have studied magnetic behaviors of water-quenched 5 wt.% (Fe, Ti) particle-doped MgB2 compared with those of air-cooled one. Generally, grain refinement is achieved by water quenching, which means increases of grain boundaries in a superconductor. We inspected that the increased grain boundary density of a superconductor influenced the kinds of flux-pinning effects on the field dependence of magnetization. As a result, grain boundaries are served as a pinning centers on a higher magnetic field (2.5–6.0 T) whereas they are served as a pathway to facilitate movements of fluxes pinned on volume defects at a lower field (0.2–2.0 T). As inspecting characteristics of grain boundaries, it is understood that they have a flux-pinning effect as well as a flux-penetrating promotion effect. By using TEM, we confirmed that the width of grain boundary in MgB2 is approximately 1 nm, which is not wide enough to pin several flux quanta across the width. As temperature increases, the flux-pinning effect of water-quenched 5 wt.% (Fe, Ti) particle-doped MgB2 decreases significantly when compared with that of air-cooled one. The behavior was because flux-pinning effect of grain boundaries decreases and flux-penetrating promotion effect of them increases, which are considered to be caused by increased coherence length of the superconductor.

The route that combines with the mechanically alloying (MA) and powder-in-tube (PIT) method is thought as a potential way to make cost-friendly Nb3Al superconducting wire, since the MA could produce the high-quality Nb(Al)ss supersaturated solid solution and thus obtain high-Jc Nb3Al superconductor. In order to know the effect of hot-pressure (HP) sintering on superconducting properties and flux pinning of Nb3Al, a series of Nb3Al superconducting bulks annealed at 950 °C for 3 h with different sintering pressure and ball milling time have been prepared in this work. The results suggest that for the mechanically alloyed Nb3Al superconductor, its critical current density (Jc) and superconducting transition temperature (Tc) can be significantly improved by HP sintering. Comparing with the atmosphere pressure (AP) sintering, the Tc, onset and Jc (8 K, 7 T) of HP Nb3Al sample with a milling time of 2 h increase from 15.1 K and 0.56 × 104 A/cm2 to 15.7 K and 6.03 × 104 A/cm2, respectively. Improvement of grain connectivity and flux pinning properties are thought to be mainly responsible to the enhancement of current-carrying ability of HP Nb3Al superconductor.

The effects of both magnetic field and temperature on the magnetic hysteresis of (Zn0.95Ni0.05O)x/(Cu0.25Tl0.75)Ba2Ca3Cu4O12 − δ, x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 wt.%, were studied. The magnetic parameters such as the lower “Hc1” and the upper “Hc2” critical magnetic fields, the zero temperature coherence length [ξ(0], the coercive field Hcoer., the remanent magnetization Mr, the vertical width Δ M, the magnetic critical current density Jc.mag, and the flux pinning density Fp were extracted from the analysis of the hysteresis loops. The experimental data of the Jc.mag were analyzed according to the power law, Kim model, and the exponential model. The results showed that the exponential model is more convenient in describing the obtained data than the power law and Kim models. In addition, with increasing the applied magnetic field, the samples containing nano-Zn0.95Ni0.05O were degraded less compared with the pure sample (Cu0.25Tl0.75)-1234. Thus, nano-Zn0.95Ni0.05O-addition to (Cu, Tl)-1234 had a significant effect on improving Jc.mag values.

The role of ferromagnetic cobalt tetraoxide (Co3O4) nanoparticles’ addition in (Cu0.5Tl0.5) Ba2Ca2Cu3O10-δ (CuTl-1223) superconductor was investigated via infield transport properties measurements. The solgel method was used for synthesis of Co3O4 nanoparticles and CuTl-1223 superconducting phase was prepared by solid-state reaction method. These Co3O4 nanoparticles were added in appropriate ratio to the precursor of CuTl-1223 superconducting matrix during its synthesis to get (Co3O4)x/CuTl-1223; x = 0, 0.25, 0.50, 1.00 and 2.00 wt% nanoparticle–superconductor composites. The crystal structure of bulk CuTl-1223 superconducting phase was observed not to be changed with the inclusion of Co3O4 nanoparticles, which indicated the dispersion and settlement of these nanoparticles across the grain boundaries of the host CuTl-1223 superconducting matrix. The increase in superconducting transition width and the decrease in activation energy deduced from in field dc-resistivity measurements with the inclusion of Co3O4 nanoparticles were the prominent indicatives of enhancement in flux dynamics. Thus Co3O4 nanoparticles of ferromagnetic nature act as anti-flux pinning agent in the host CuTl-1223 superconducting matrix.

In this study we have successfully fabricated La and Hf co-doped Gd123 thin films on LaAlO3 single crystal substrates by the fluorine-free metal organic deposition method with tetrakishafnium as a new organic metal salt. Hf doping is the effective means to introduce the perovskite nano-structure BaHfO3 as artificial pinning centers (APCs) into REBa2Cu3O y coated conductors. On the other hand, La doping is valid method in promoting high quality epitaxial crystallization of the superconducting phase. By possessing both advantages La 1 mol% and Hf 2 mol% co-doped film achieved high critical current densities of 3.1 MA cm−2 at 0 T, and 0.25 MA cm−2 at 1 T, 77.3 K. The detailed study of the flux pinning revealed that APCs introduced by Hf doping brought about the increase of J c in high fields, and La doping brought about the quality crystallization resulting in the increase of J c in low fields.

Multifilamentary Bi2Sr2Ca2Cu3O10+δ (Bi-2223) high temperature superconducting tapes were fabricated by powder-in-tube process. During the precursor powder packing process, environment atmosphere was changed from pure oxygen to dry air and 7.5% O2 balanced with N2, with the oxygen partial pressure decreasing from 100% to 25% and 7.5%, respectively. The change of packing atmosphere obviously influenced the phase assemblage in the early stage of Bi-2223 phase formation process. Therefore, the chemical composition of obtained Bi-2223 phase as well as the content and distribution of secondary phases changed accordingly. Meanwhile, the changes of intergrain connections and the flux pinning properties were investigated based on the in-field critical current, $I_{c}$ – B measurements. Finally, attributed to the decreased oxygen partial pressure in packing atmosphere, the $I_{c}$ improvements of 25% at self-field and 0.6 T were obtained comparing with the tapes packed under pure oxygen. With further optimization of sintering process, the final $I_{c}$ of 116 A was achieved, which corresponded to the critical current density, $J_{c}$ of ∼21 kAcm−2.

For the high transition temperature (Tc) and low cost taking both raw materials and fabrication process into account, MgB2 has been a competitive candidate to replace the conventional NiTi superconductor for high-temperature application in fault current limiters, transformers, motors, magnetic resonance imaging, adiabatic demagnetization refrigerators, generators, etc. The carbon-containing materials addition induced high critical current density (Jc) is reviewed based on their influences on the upper critical field (Hc2), flux pinning force, and connectivity. The doping effects were compared in the overview focusing on SiC, organic dopants, and graphene-related dopants. SiC doping is featured for the high-field critical current density, which is caused by the increased Hc2 attributed to the substitution of carbon on boron site and the strong flux pinning force offered by the nanosized secondary phases in the MgB2 matrix. Organic dopants have the advantage over SiC dopant for their relatively homogeneous distribution in the MgB2 matrix based on wet mixing of the organics and the raw boron powders. Low doping level of two-dimensional materials can improve the superconducting properties in all measured fields because of the combined advantages of carbon substitution effect and grain connectivity. MgB2 fabricated with carbon-encapsulated boron also introduces strong flux pinning centers in MgB2, which show weak destruction of the connectivity of the MgB2 grains as reflected by the low-magnetic-supercurrent behavior. High-pressure treatment and diffusion method can fabricate high-density MgB2 superconductors with better connectivity and increase the Jc compared with the in situ and ex situ methods.

We have studied the vortex dynamics of CaKFe4As4 single crystals using a dynamical magnetization relaxation method. A large region of vortex elastic motion is observed indicating strong vortex pinning ability. Temperature dependence of normalized flux pinning energy of g(t) is determined based on the data of transient current density Js and dynamical relaxation rate Q by the generalized inversion scheme (GIS), which suggests that both the δ(Tc)- and δ(l)-pinning may be responsible for the vortex dynamical behaviors in CaKFe4As4 under field parallel to c-axis (H // c). The value of Js for field parallel to ab-plane (H // ab) is obviously larger than that of H // c, this indicates that the flux pinning mechanisms are distinct in different crystal orientations. Under H // ab, a dip structure in the magnetic hysteresis loops (MHLs) is obtained around zero field, which is ascribed to planar defects in ab-plane. We further analyze the flux pinning mechanisms and find that the abnormal flux pinning behaviors should originate from its unique structure in CaKFe4As4.

The effect of Ga substitution on the superconductivity of Tl(BaSr)Ca1-xGaxCu2O7 (x = 0–0.5) is reported. The samples were prepared by solid-state reaction method. XRD patterns showed a single Tl-1212 phase for the x = 0 and 0.1 samples. Unreacted Ga2O3 and other impurity phases were observed in the higher substituted (x = 0.2 to 0.5) samples. The temperature-dependent resistance measurements showed metallic normal state behavior for all samples with onset transition temperature Tc-onset between 94 and 66 K and zero transition temperature Tc-zero between 86 and 55 K. AC susceptibility measurement showed the highest superconducting transition (95 K) and peak temperature (89 K) for non-substituted sample. The intergranular critical current density Jc at Tp for all samples was between 16 and 22 A cm−2. This work showed that the intergranular coupling and flux pinning energy were suppressed with Ga substitution. Some Ga+2 were most likely incorporated at Ca+2 for x = 0.1 and some resided at the grains for x ≥ 0.2 samples. This work showed that a single superconducting Tl-1212 phase was formed for x ≤ 0.1 samples. This work also showed that the internal lattice constrain which is the ratio of the lattice parameter (a/c) is important in improving the transition temperature of the Tl-1212 phase.

Cu0.5Tl0.5Ba2Ca2Cu3O10−δ (undoped), Cu0.5Tl0.5Ba2(CaMg)Cu3O10−δ (Mg-doped), Cu0.5Tl0.5Ba2Ca2Cu1.5Cd1.5O10−δ (Cd-doped), and Cu0.5Tl0.5Ba2(CaMg)Cu1.5Cd1.5O10−δ (Cd/Mg-codoped) samples were synthesized by two-step solid-state reaction at 860°C to investigate the possible role of anharmonic oscillations of atoms in the mechanism of high critical temperature (Tc) superconductivity. The samples showed orthorhombic crystal structure with smaller and larger unit cell volume for the Mg- and Cd/Mg-codoped samples, respectively, compared with the undoped sample. Tc observed by resistivity measurements decreased in the Cd-doped samples, but the maximum suppression was observed in the Cd/Mg-codoped sample showing semiconductor behavior. Fourier-transform infrared (FTIR) absorption spectroscopy revealed hardening of apical oxygen modes on the incorporation of Mg atoms as compared with the undoped sample and similarly with the doping of Mg atoms in the Cd-doped sample, confirming the intrinsic inclusion of these elements in the final compound. Excess conductivity analysis revealed suppression in the coherence length (ξc(0)), interlayer coupling (J), phase relaxation time (τφ) of carriers, Fermi velocity (VF) of carriers, Ginzburg–Landau (GL) parameter κ, breaking energy (Ebreak), and magnetic field penetration depth (λp.d) in Cd-doped samples. The values of Bc0(T), Bc1(T), and Jc(0) increased in the Cd-doped samples, showing enhancement of flux-pinning characteristics. It is proposed that Cd doping at CuO2-planar site induces anharmonic oscillation in the solid-state medium, which suppresses the density of phonons and thereby the density of Cooper pairs and hence superconductivity. These studies emphasize the essential role of electron–phonon interactions in the mechanism of high-Tc superconductivity.

The effect of magnetic flux pinning is investigated in GdBa2Cu3O7 (GdBCO) thin films with two different types of ferromagnetic La0.7Sr0.3MnO3 (LSMO) buffers (nanoparticles and a layer) deposited on an STO substrate. Magnetization analyses reveal the presence of multiple flux pinning mechanisms responsible for the improvement in the critical current density of GdBCO films. While core pinning becomes a dominant pinning mechanism in GdBCO films with LSMO nanoparticles, a hybrid effect of magnetic-volume and core-point pinning is observed in GdBCO films with LSMO layers. Examinations of local structures for both LSMO and GdBCO using extended X-ray absorption fine structure spectroscopy (EXAFS) exhibit a close relation between the parameters in the pinning force scaling and the length ratio of the Mn–O bond to the Cu–O bond. This result implies that the origin of core pinning is probably attributed to epitaxial strain induced by lattice mismatch between LSMO and GdBCO. Therefore, an appropriate strain state of LSMO is required for an effective operation of magnetic pinning.

We investigate the superconducting critical current density (Jc), transition temperature (Tc), and flux pinning properties under hydrostatic pressure (P) for Cr0.0009NbSe2 single crystal. The application of P enhances Tc in both electrical resistivity (∼0.38 K GPa−1: 0 ≤ P ≤ 2.5 GPa) and magnetization (∼0.98 K GPa−1: 0 ≤ P ≤ 1 GPa) measurements, which leads to a monotonic increase in Jc and flux pinning properties. The field-dependent Jc at various temperatures under P is analyzed within the collecting pinning theory and it shows that δTc pinning is the crossover to δl pinning above the critical pressure (Pc ∼0.3 GPa). Our systematic analysis of the flux pinning mechanism indicates that both the density of pinning centers and pinning forces greatly increase with the application of P, which leads to an enhancement in the vortex state. Structural studies using synchrotron X-ray diffraction under pressure illustrate a stable hexagonal phase without any significant impurity phase and lattice parameter reduction with P shows highly anisotropic nature.

Chapter 3 covers various electromagnetic phenomena. These include geometrical effects and dynamic phenomena that were not treated in Chap. 2. The rectifying effect in the DC current-voltage characteristics in a superposed AC magnetic field, flux jumps, surface irreversibility, and DC susceptibility in a varying temperature are also included. In addition, it is shown that an abnormal reduction in losses occurs, deviating from the prediction of the critical state model when an AC magnetic field is applied to a superconductor smaller than the pinning correlation length called Campbell’s AC penetration depth. This is attributed to the reversible motion of flux lines limited within pinning potential wells. In high-temperature superconductors the superconducting current sustained by flux pinning appreciably decays with time due to the thermal agitation of flux lines. This phenomenon, which is called flux creep, is also discussed. In extreme cases the critical current density is reduced to zero at some magnetic field called the irreversibility field. The principles used to determine the irreversibility field are described, and the result is applied to high-temperature superconductors in Chap. 8.

Chapter 7 covers the summation theories that predict the global pinning force density as a function of the elementary pinning force and number density of pinning centers. First, the statistical theory of Labusch is discussed. This theory predicted that the pinning force density has non-zero value for the elementary pinning force above the threshold value. This condition is identical with the condition of the hysteresis loss, and this theory is consistent with the dynamic theory. However, this theoretical prediction is significantly contradictory to experiments. Although the fundamental threshold problem was resolved by Larkin and Ovchinnikov, who showed that a long-range order does not exist in the flux line lattice, quantitative disagreements are sometimes found between their theory and experiments. In the coherent potential approximation theory the statistical method is used with taking into account the lack of long range order, and the compatibility of the threshold problem and the instability of flux motion is obtained. A detailed comparison is made between the theories and experiments. The saturation phenomenon observed in metallic superconductors at high fields is also introduced. The pinning potential energy important for the analysis of flux creep is also derived.

Chapter 8 covers the various properties of high-temperature superconductors. These superconductors show significant anisotropy due to the two-dimensional crystal structure composed of superconducting CuO2 layers and insulating block layers. This makes the states of flux lines extremely complicated. Various phase transitions of the flux line system to be pinned and the mechanisms responsible are reviewed. In particular, the transitions in which the pinning plays an important role, i.e., the order-disorder transition associated with the peak effect of critical current density and the glass-liquid transition associated with the irreversibility field, are discussed in detail. The influences on these transitions, not only of the flux pinning strength and the anisotropy of the superconductor, but also of the electric field and the specimen size are discussed. Y-123, Bi-2212, and Bi-2223, which have been developed for applications, are at the focus of the discussion, and their pinning properties and recent progress are introduced. In particular, the effect of nanorods introduced into Y-123 coated conductors as artificial pinning centers is reviewed.

Intermodulation distortion (IMD) of microwave signals incident upon a superconducting YBa2Cu3O7 thin film sample on a LaAlO3 substrate is shown to relax as the sample magnetization also relaxes, revealing the signature of fluxon decay in the microwave distortion. The sample contained a microchannel of columnar defects that was introduced using heavy ion beam irradiation. After exposure to and then removal of a low strength static magnetic field, IMD relaxed due to thermal activation of fluxons over the surface barrier. This process influenced second and third order IMD differently, visible in the differing functional form of the IMD in time and the temperature dependence of the relaxation function. The presence of columnar defects in the test sample suppressed thermal activation by pinning of fluxons, notably with complete suppression of relaxation in third order IMD at low temperatures. Higher order IMD was found to relax with the same functional form as the second and third order IMD of the same parity., Comment: 8 figures, 7 pages

We studied flux pinning in exfoliated FeTe$_{0.6}$Se$_{0.4}$ thin-film devices with a thickness $d$ from 30 to 150 nm by measuring the critical current density $J_{\mathrm{c}}$. In bulk FeTe$_{0.6}$Se$_{0.4}$, the flux pinning has been discussed in the framework of weak collective pinning, while there is little knowledge on the pinning mechanism in the thin-film region. From the thickness $d$ dependence of $J_{\mathrm{c}}$ at a fixed magnetic field of 1 T, we found that the strong pinning is dominant below $d \approx 70$ nm, while the weak collective pinning becomes more important above $d \approx 100$ nm. This crossover thickness can be explained by the theoretical model proposed by van der Beek $\textit{et al}$ [Phys. Rev. B. ${\bf 66}$, 024523 (2002)].