Intragrain Critical Current Density Research Articles

Larger grains in high-Tc superconductors synthesized by the solid-state reaction route

Solid-state synthesis is widely used in exploratory research to study various structural modifications that affect the properties (critical temperature, critical current density, irreversibility field, etc.) of superconductors. The popularity of this method is due to its relative simplicity and availability of the necessary equipment. Combining solid-state synthesis and top-seeded melt growth allows us to increase the grain size in a Tm- and Nd-based 1-2-3 superconductor. Samples with a grain size up to 0.1 mm have been obtained. X-ray diffraction, scanning electron microscopy and magnetization measurements have been used for investigating this superconducting material. The magnetization width ΔM has increased significantly in the synthesized samples. However the temperature dependence of the intragrain critical current density and the pinning force scaling give evidences that the pinning mechanism in the obtained superconductor is essentially the same as in polycrystalline superconductors synthesized by standard solid-state technology. The increase in grain size in the synthesized samples is the main reason for the high values of ΔM.

Equilibrium magnetic properties of the Fe-doped HTSC YBaCuO

For the polycrystalline high-temperature superconductor Y1 –xFexBa2Cu3Oy (0 ≤ x ≤ 0.05), where a significant increase in the intragrain critical current density has recently been found, the effect of iron doping on the equilibrium magnetic characteristics is studied. The field dependences of the magnetization are measured in the temperature range of 40–85 K and in the field range up to 60 kOe with increasing and decreasing fields. The temperature dependences of the London penetration depth λab, the superconducting carrier density ns, and the thermodynamic critical field Hc are determined from an analysis of the middle lines of the magnetic hysteresis loop. It is shown that the thermodynamic properties of Fe substituted samples do not change at x ≤ 0.03. At a higher Fe content, a slight decrease in Hc and the superconducting transition temperature is observed. However, changes in the condensation energy turn out to be much smaller than values expected due to an increase in the density of effective pinning centres created by substitutional atoms in both the yttrium and copper sublattices. It is concluded that the pinning regions in chain CuO planes near substituted Cu sites are two-dimensional in nature and do not affect the superconducting pairing in CuO2 planes. Another possible pinning mechanism, which does not affect Hc, is the interaction of vortices with the magnetic moments of Fe3+ ions.

Excess conductivity and magnetization of CoFe2O4 combined with Y1Ba2Cu3O7-δ as a superconductor

In this study, we determined the direct current DC isothermal magnetization (M − H and M-T), global critical current density (Jc), intra-grain critical current density (Jcintra), and excess conductivity for samples of CoFe2O4 (CFO; 1.0, 2.0, and 3.0 wt%) combined with Y1Ba2Cu3O7-δ (YBCO). X-ray diffraction analysis and Rietveld refinement suggested that all of the composite samples belonged to the Pmmm space group. No significant variations in the lattice parameters were observed after the inclusion of CFO in the YBCO matrix. The crystallite size decreased after CFO was added to the YBCO matrix according to field emission scanning electron microscopy. The addition of CFO to the YBCO matrix changed the flux pinning ability, thereby reducing the global critical current densities (Jc) and intra-grain critical current density (Jcintra) as the CFO concentration increased, but Jc and Jcintra both increased at the higher concentration., Jc and Jcintra were obtained from DC isothermal magnetization measurements (M − H) using Bean's critical state model. The critical temperature (TC) decreased and the residual resistivity (ρ0) increased as the wt.% of CFO increased, thereby indicating that the trapping of charge carriers occurred as well as grain boundary resistivity enhancement. Excess conductivity analysis was performed to investigate the thermal fluctuations near the critical temperature and various crossover temperatures were obtained using the Aslamazov–Larkin model.

Investigation of the effect of Ag addition on the critical current density of the high-temperature superconductor Nd1.85Ce0.15CuO4

We have synthesized a Nd1.85Ce0.15CuO4 + Ag composite system with the aim of studying the effect of Ag addition in the electron-doped system Nd1.85Ce0.15CuO4 on its superconducting and magnetic properties. Measurements of magnetization using a vibration sample magnetometer indicate a systematic increase in diamagnetic shielding upon Ag addition. A subsequent analysis of the critical current density using the Bean model indicates a small but significant increase in intra-grain critical current density upon Ag addition. From the study of the dependence of electrical resistivity on the temperature and electrical current, an increase in inter-granular critical current has also been evidenced upon Ag addition. These results correlate well with the earlier reports of a similar improvement in the properties on the hole-doped ceramic superconductor/Ag composites. Possible reasons for the enhancement of JC have been described and discussed. Magnetic characterization by AC susceptibility using a SQUID magnetometer has been presented for a representative composition, which enabled delineation of the inter- and intra-granular transitions.

Evaluation of the intragrain critical current density in a multidomain FeSe crystal by means of dc magnetic measurements

The magnetic behavior of an iron-based FeSe crystal sample has been studied by means of dc magnetization measurements as a function of the temperature (T), the dc magnetic field (H) and the time (t). The M(T) curves show a discrepancy in the determination of the onset of the critical temperature TC with respect to what is observed in the superconducting M(H) measurements obtained by subtracting the ferromagnetic background from the curves measured at various temperatures. By using magnetic relaxation measurements M(t), the correct value of TC has been obtained. Moreover, the superconducting M(H) loops show the presence of a noisy signal up to an anomalous ‘peak effect’ only found for positive and negative increasing fields. These features have been analyzed by fitting the temperature dependence of the critical current density Jc(T), extracted from the M(H) loops, with the help of the Jc(T) dependencies governing an S–N–S junction network. This analysis has allowed us to interpret the behavior found in the M(H) loops and to obtain the value of the intrinsic critical current density J0 which is not influenced by the presence of the junctions.

Magnetotransport and thermal properties characterization of 55 K superconductor SmFeAsO0.85F0.15

This report fairly underlines the magneto-transport, thermal properties characterization and bulk superconductivity in the FeAs-based SmFeAsO0.85F0.1. The phase formation and structure are confirmed by Rietveld analysis of room temperature powder X-ray diffraction (XRD) data. Electron microscopy was employed to unravel the micro structural details, such as perfection of the lattice and the grain morphology including size and boundaries. The electrical and magnetic measurements have been carried out to confirm the bulk superconductivity and understand the nature of electrical transport in the normal and superconducting state. The intra-grain critical current density (Jc) with applied magnetic field is calculated from isothermal DC magnetization (MH) plots using conventional Bean critical state model. Superconductivity is observed at transition temperature (Tc) above 55 K without HPHT (high pressure high temperature) synthesis route. The value of Jc is found to be around 5.26 × 104 A/cm2 at 5 K in zero field. The dependence of thermally activated flux flow energy (U/kB) on the applied magnetic field has been observed. AC susceptibility measurements were performed for 55 K superconducting SmFeAsO0.85F0.15 sample at various amplitude of applied AC drive field and its granular nature is confirmed. The parent compound SmFeAsO is found to be magnetic with Fe spin density wave (SDW) like order below 150 K, on the other hand the F doped SmFeAsO0.85F0.15 sample is bulk superconducting at below 55 K. Both Fe (SDW) at 150 K for SmFeAsO and 55 K superconductivity in case of SmFeAsO0.85F0.15 sample has confirmed by Specific heat [Cp(T)] measurement too. Further Sm orders anti-ferro-magnetically at 4.5 K for non-superconducting and at 3.5 K for superconducting samples, also the entropy change is reduced significantly for the later than the former. Summarily complete physical property characterization for both non-superconducting SmFeAsO and 55 K superconductor SmFeAsO0.85F0.15 samples is provided and discussed in the current article.

High field magneto-transport and magnetization study of Y1−xCaxBa2Cu3[formula omitted] (x = 0.00–0.25)

We report DC isothermal magnetization, global critical current density (JC), Intra-grain critical current density (Jcintra) and resistive upper critical field (Bc2) of polycrystalline Y1−xCaxBa2Cu3O7−δ (YBCO); x=0.00, 0.05, 0.10, 0.15, 0.20 and 0.25. Ca doping at Y sites in YBCO superconductor results in flux pinning property, which can enhance the critical current densities and upper critical fields. JC and Jcintra are experimentally calculated through DC isothermal magnetization measurement (M–H), employing the well known Bean’s critical model. It is observed that both type of current densities (global and intra) enhance in lower doped (5%) samples and can be accounted in terms of reaching maximum of pinning centers and improved grain couplings nature. The upper critical field (Bc2) increases with Ca content being consistent with enhanced grain coupling nature of doped samples.

Dependence of the Structural, Electrical and Magnetic Properties of YBa2Cu3O7−δ Bulk Superconductor on the Ag Doping

The effects of Ag dopants on the superconducting properties of YBCO bulk samples, prepared under solid-state reaction method, have been studied by resistivity–temperature (ρ–T), X-ray diffraction (XRD), scanning electron microscope (SEM), bulk density, AC susceptibility, DC magnetization measurements, and theoretical analysis. Small Ag substitutions (x≤0.10) do not effect Tc of pure YBa2Cu3−xAgxO7−δ, while we observed small decreases of 1.5 K for larger doping levels (x≥0.15). AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of DC bias field. The critical current densities (Jc) have been estimated as a function of magnetic field from the magnetization data employing Bean’s critical state model. The increase in Ag amount (x) in YBa2Cu3−xAgxO7−δ (x≥0.10) system effectively decreases the intragrain critical current density, which is attributed to the reduced connectivity between the grain boundaries. In the case of small additions (x=0.05), Ag atoms fill partly the pores, leading better crystallization of the grains.

Enhancement of Intragrain Critical Current Density in Bi-Based Superconductor by Substitutional Structural Defects

We synthesized Ca-rich Bi-based superconducting whiskers by an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -seeded glassy quenched platelet method. The grown whiskers were precisely characterized by synchrotron radiation X-ray photoemission spectroscopy and high-resolution transmission electron microscopy. The Ca-rich Bi-based superconducting whiskers show a high critical current density of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 40 K in self-field. We found that excess Ca <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions substitute for the Sr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> sites and cause nano domains with shorter-period modulation within the conventional 2212 crystal structure. The embedded nano crystalline domains can result in structural distorted defects which work as strong pinning center.

Vortex dynamics and irreversibility line in optimally doped SmFeAsO0.8F0.2from ac susceptibility and magnetization measurements

Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO$_{0.8}$F$_{0.2}$ superconductor. The field - temperature phase diagram of the superconducting state was drawn and, in particular, the features of the flux line lattice derived. The dependence of the intra-grain depinning energy on the magnetic field intensity was derived in the thermally-activated flux creep framework, enlightening a typical 1/H dependence in the high-field regime. The intra-grain critical current density was extrapolated in the zero temperature and zero magnetic field limit, showing a remarkably high value $J_{c0}(0) \sim 2 \cdot 10^{7}$ A/cm$^{2}$, which demonstrates that this material is rather interesting for the potential future technological applications.

Effect of distribution of powder size on estimation of critical current densities in polycrystalline Sr0.6K0.4Fe2As2 superconductors

The effect of distribution of powder size on the estimation of intra-grain critical current density was investigated for polycrystalline samples of Sr0.6K0.4Fe2As2 with and without addition of Ag 20wt.%. It turned out that the value of intra-grain critical current density calculated using the average of powder size Jcl is more overestimated than that calculated taking account of the distribution of powder size JclP as the distribution of powder size is widened, since the magnetic moment drastically increases due to the larger contribution from larger particles. From the distribution of powder size obtained by the microscopic observation using SEM, the factor of overestimation of Jcl/JclP is found to be larger than two in both samples. Moreover, the distribution of powder size in the Ag-added sample is found to be smaller than that of pure sample. Therefore, the factor of overestimation of Jcl is considered to be smaller in the Ag-added sample. It is recommended to take account of the distribution of powder size for precise estimation of intra-grain critical current density.

The magnetization of PrFeAsO 0.60F 0.12 superconductor

The magnetization of the PrFeAsO 0.60F 0.12 polycrystalline sample has been measured as functions of temperature and magnetic field ( H). The observed total magnetization is the sum of a superconducting irreversible magnetization and a paramagnetic magnetization. Analysis of dc susceptibility χ( T) in the normal state shows that the paramagnetic component of magnetization comes from the Pr 3+ magnetic moments. The intragrain critical current density ( J L ) derived from the magnetization data is large. The J L ( H) curve displays a second peak which shifts towards the high-field region with decreasing temperature. In the low-field region, a plateau up to a field H * followed by a power law H −5/8 behavior of J L ( H) is the characteristic of the strong pinning. A vortex phase diagram for the present superconductor has been obtained from the magnetization and resistivity data.

Superconducting Properties of Spray Deposited TBCCO Films

The present study shows general characteristic features of processes such as spray pyrolysis and heat-treatment, involved in the preparation of Tl-2223/CeO2/Al2O3 superconducting coatings by using aerosol pyrolysis method. The importance of the present study, however, lies in a new process to make a thick Tl-2223 superconducting layer by using spray pyrolysis. In the new method, a relatively thick BaCaCuO precursor film is firstly spray deposited on the buffered alumina substrate and then heat-treated in the presence of thallium source for phase transformation to superconducting phases to assign conductivity to the film. The thickness of the Tl-2223 layer was 2–3 μm. The crystal symmetry was found to be tetragonal with lattice parameters a=3.85, and c=35.7 A. The Tc was found to be 98 K for the samples heat treated at 870 °C for 60 min. Magnetic characterizations were conducted by using a DC-SQUID magnetometer. From the magnetization measurements, the intragrain critical-current density, Jc, was calculated to be 2×104 A/cm2 at 70 K.

The critical current density, irreversibility line, and flux pinning properties of Ba2CaCu2O4(O,F)2 high-Tc superconductor

The high-Tc superconductor of Ba2CaCu2O4(O,F)2 (F-0212) is an appealing material for application as it has a simple crystal structure with a highest Tc of 108 K. We have derived the intragrain critical current density (Jc), irreversibility field (Birr), flux pinning properties, etc., for the polycrystalline samples from under doping (Tc=82 K) to slightly-over doping (Tc=106 K) from the dc magnetization hysteresis loops. The Jc and Birr properties were found to improve rapidly as the doping state changes from under doping to slightly-over doping. Here we show that property of the spacing is crucial to enhance Birr as well as its thickness. An anisotropy factor for under doped Ba2CaCu2O4F2 was reckoned to 118 from a three-dimensional-two-dimensional crossover field of about 0.28 T. The double logarithmic plot of irreversibility field versus [1−(T/Tc)] analysis hints that the flux line melting model is adopted. Analysis of the normalized pinning force reveals that a surface pinning mechanism is dominant and reduced magnetic field bmax=0.2 agree with surface pinning mechanism with closely spaced pins.

Magneto-optical imaging of polycrystalline LaFePO1−xFx

The remanent state field profile has been observed in polycrystalline LaFePO0.94F0.06 by magneto-optical (MO) imaging method. MO images show the absence of magnetic field modulation associated with weak-link nature of grain boundaries, indicating that the inter-grain critical current density Jcinter is comparable to intra-grain critical current density Jcintra in contrast to iron-arsenide superconductors. Jcinter is estimated as 1.9×104A/cm2 at T=0K from magnetization measurement, which is similar to the values in iron-arsenide superconductors. Implication of these Jc behavior is discussed in relation to the possible pairing symmetry.

Magneto-optical imaging of iron-oxypnictide superconductor LaFeAs(O,F)

We report the magneto-optical imaging for polycrystalline LaFeAsO0.95F0.05 and LaFeAsO0.892F0.108. We find a significant magneto-optical contrast with marked inhomogeneities and with a typical superconducting grain size ∼20–40μm. Although intergrain critical current density is negligibly small, intragrain critical current density obtained from magneto-optical image is about 1×105A/cm2 at 5K, which is smaller than other rare-earth-based iron-oxypnictide superconductors but is potentially attractive for technological applications.

Magnetic properties of the TlBa2Ca2Cu3Oy with TC ∼130K

We reported that maximum TC values for Tl system superconductors TlBa2Ca2Cu3Oy (Tl-1223) can be controlled by slightly varying the starting composition. From the DC magnetization measurements, we estimated intragrain critical current densities (JC) and irreversibility fields (Birr) of Tl-1223 samples with different maximum TC values (∼130 K and 120 K). These results were compared with those values for previously reported optimally-doped Hg-1223. It is found that, in the Birr -T curve at temperatures above 77K, Birr values of the Tl-1223 samples with TC∼130K shows slightly higher values than that of the Tl-1223 samples with low TC and Hg-1223 samples.

Irreversibility line and flux pinning properties in a multilayered cuprate superconductor ofBa2Ca3Cu4O8(O,F)2 (Tc = 105 K)

Irreversibility line (IL) and flux pinning properties were investigated for aBa2Ca3Cu4O8(O,F)2 (F-0234) multilayered cuprate superconductor with aTc of 105 K. The intragrain critical current density(Jc) andirreversibility field (Birr) were determined by using Bean’s critical state model for the grain-aligned sample (nominal compositionBa2Ca3Cu4O8.7F1.3). The irreversibility line (IL) of F-0234 is much lower than that of(Cu,C)Ba2Ca3Cu4Oy ((Cu,C)-1234) and HgBa2Ca3Cu4Oy (Hg-1234) in spite of the spacing between the superconducting blocksof F-0234 (7.3 Å) being much thinner. The double logarithmic plot ofBirr fieldversus [1−(T/Tc) ] analysis hints that the flux line melting model has been adopted. An anisotropy factor of65 was calculated from a 3D to 2D crossover field of about 0.95 T. Due to the highanisotropy of this system, a low IL has resulted. The flux pinning force densityFp (≈JcB) exhibits scaling behaviour when the magnetic fieldB is normalizedby the Birr field. Analysis of the normalized pinning force reveals that a surfacepinning mechanism is dominant and the reduced magnetic fieldbmax = 0.2 agrees with surface pinning mechanism with closely spaced pins.

Critical current densities and irreversibility field of high-Tc Ba2Ca3Cu4O(O,F)2 superconductor

The superconductivity in apical fluorine system of Ba2Ca3Cu4O10-yFy: the fourth member of Ba2Can-1CunO2n(O,F)2 homologous series is significant to study as it deals with the novel phenomenon of superconductivity like co-existence of superconductivity and antiferromagnetism, in-equivalent and non-similar charge distribution in the CuO2 planes in a unit cell etc. The polycrystalline samples of Ba2Ca3Cu4O8(O,F)2 (F-0234) were synthesized under high pressure and showed sharp superconducting transitions temperature at 105 K. From the DA magnetization hysteresis loops, we determined the intragrain critical current density (Jc) and irreversibility field (Birr) using Bean's critical state model. The irreversibility line(IL) of F-0234 is higher than that for optimally-doped Bi-2212. However, IL much lower than that of Cu-1212 (Y-123) although the spacing of F-0234 (7.3 Å) is much thinner than that of Cu-1212.

High critical current density and improved flux pinning in bulk MgB2 synthesized by Ag addition

In the present investigation, we report a systematic study of Ag admixing in MgB2 prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural∕ microstructural characterization employing x-ray diffraction and transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by physical property measurement system. The TEM investigations reveal the formation of MgAg nanoparticles in Ag admixed samples. These nanoparticles may enhance critical current density due to their size (∼5–20nm) which is compatible with the coherence length of MgB2 (∼5–6nm). In order to study the flux pinning effect of Ag admixing in MgB2, the evaluation of intragrain critical current density (Jc) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on intragrain Jc is obtained for 10at.% Ag admixed sample at 5K. This corresponds to ∼9.23×107A∕cm2 in self-field, ∼5.82×107A∕cm2 at 1T, ∼4.24×106A∕cm2 at 3.6T, and ∼1.52×105A∕cm2 at 5T. However, intragrain Jc values for MgB2 sample without Ag admixing are ∼2.59×106, ∼1.09×106, ∼4.53×104, and 2.91×103A∕cm2 at 5K in self-field, 1T, 3.6T, and 5T, respectively. The high value of intragrain Jc for Ag admixed MgB2 superconductor has been attributed to the inclusion of MgAg nanoparticles into the crystal matrix of MgB2, which are capable of providing effective flux pinning centers. A feasible correlation between microstructural features and superconducting properties has been put forward.