Dependence Of Critical Current Density Research Articles

Anisotropic Critical Current Densities in Ba0.6K0.4Fe2As2 with Splayed Columnar Defects

Enhancement and non-monotonic field dependence of critical current density (Jc ), called anomalous peak effect, were observed in Ba1−x K x Fe2As2 with splayed columnar defects. In principle, anisotropy of in-plane Jc should be present in this superconductor since the introduced splayed columnar defects introduce anisotropy in the plane. This study reports in-plane anisotropy of Jc observed in 2.6 GeV U irradiated Ba0.6K0.4Fe2As2 with splay angles of ±15° and ±20°. Results show that Jc component parallel to the splay plane is larger than the perpendicular component, and the parallel component contribute to the anomalous peak effect at around B Φ/3 more.

Peak Effects in 2H-NbSe2 Single Crystals Induced by Particle Irradiations

Effects of three kinds of columnar defects in NbSe2 single crystals introduced by 320 MeV Au irradiation with a dose equivalent matching field B Φ up to 4 T were studied. Through the magnetization measurement using a SQUID magnetometer, two pronounced peak effects in the external field dependence of critical current density (J c) were observed in crystals introduced with splayed and tilted columnar defects. In crystals with splayed columnar defects, J c at the peak field reaches the maximum for splay angle θ CD = ±10°, which is a little different from that found in YBa2Cu3O7 and iron-based superconductors, where optimal θ CD is equal to ±5°.

A New Microscopic Approach To Deal with the Temperature- and Applied Magnetic Field–Dependent Critical Current Densities of Superconductors

Based on the formalism of the Bethe-Salpeter equation, we present here a new approach to deal with the temperature (T)- and the applied magnetic field (H)-dependent critical current densities (jcs) of superconductors (SCs). Saliently, it employs (i) the premise that the chemical potential μ (T, H) subsumes most of the factors to which the widely varying empirical values of jc (T, H) of an SC are conventionally attributed, (ii) the basic equation that defines jc as a product of the electronic charge, the number density of superconducting electrons (ns) and their critical velocity (vc), (iii) a recently derived equation for ns (T, H), and (iv) the empirical values of jc (T, H)—which it is meant to explain—as inputs into one of its equations which, paradoxical though it may seem, is shown to have the virtue of shedding light on several parameters related to jc (T, H), such as μ, ns, vc, m*, nL, and λm, where m* is the effective mass of an electron, nL the number of occupied Landau levels, and λm the magnetic interaction parameter. Remarkably, the approach is found to lead to two branches of μ-dependent solutions with distinctly different values of the parameters corresponding to the same value of jc (T, H). This is reminiscent of the behavior of the chemical potential in SCs with a low density of charge carriers that was reported by Marel [Physica C 165, 35 (1990)] and described as anomalous. Finally, we show that our approach suggests the exciting possibility of fabricating an SC that has a bespoke value of jc at suitably chosen values of T and H. For the sake of concreteness, we employ here some of the available empirical data on the jc (T, H) values of Bi-2212.

Estimation of the Electricity Storage Volume Density of Compact SMESs of a New Concept Based on Si Microfabrication Technologies

A compact superconducting magnetic energy storage system (SMES) produced by Si micro fabrication technologies has been proposed to improve electricity storage volume density, w, in the sub-Wh/L range of conventional SMESs and to produce them at a low cost by mass production. In parallel with the experimental development reported previously, a series of trials was performed to estimate a feasible value of w based on the calculation of the magnetic field generated by the compact SMES by improving the calculation models step by step. In this work, the experimentally obtained magnetic flux density dependence of superconductive critical current density was taken into consideration for the first time in this series of trials, together with the additional improvement of the calculation models. The results of the estimation indicated that a compact SMES produced by the proposed concept can attain a w in the Wh/L range or more, ranking with or surpassing that of presently used capacitors.

Novel sample-thickness-dependent flux pinning behaviors of KFe2As2 intercalations in CaKFe4As4 single crystals

We report the strong dependence of critical current density (J c) on the sample thickness (d) along the crystalline c-axis in CaKFe4As4 single crystals. The J c can be respectively enlarged more than 3 and 50 times in H parallel to c-axis () and ab-plane () at temperature of 5 K and high magnetic fields of 8 T when the d is reduced from 88 to 2.0 . It is surprising to find that, accompanied by the enhanced J c, the fishtail effect fades away with the decreasing d. We believe that both the origins of enhanced J c and the fishtail effect in CaKFe4As4 may have close relationship with the interlayer coupling between CaKFe4As4 and the intercalations of KFe2As2. Our results not only demonstrate an encouraging prospect for application of CaKFe4As4, but also illuminate new light on the origin of fishtail effect in iron-based superconductors.

Influence of field-dependent critical current on harmonic AC loss analysis in HTS coils for superconducting transformers supplying non-linear loads

There are two main obstacles in front of the development of high temperature superconducting (HTS) technology for electric power network applications; tape price and cooling cost. In order to reduce cooling cost, it is vital to evaluate AC transport current loss of the tapes precisely and then reduce it by some design innovative approaches. In addition, AC transport current loss in HTS material is a critical design variable for large-scale power network applications such as HTS transformers, superconducting fault current limiters, and power cables, since they are continuously carrying the network/load current during their operating life. In existing power networks, harmonic production sources are commonly used and thus, currents are distorted. Therefore, the effect of nonsinusoidal current on the critical apparatus in the network such as transformer must be studied. In this paper, AC transport current loss of a single-turn 2G YBCO HTS coil was modeled and numerically calculated under nonsinusoidal transport current using finite element method. Furthermore, influence of dependency of critical current density to magnetic field on the AC transport current loss of HTS coil when carries distorted currents was considered. It was observed that nonsinusoidal current causes excessive losses in HTS coil. On the other hand, a case study on an HTS transformer supplying non-linear load was considered to study the loss increment as well as heat load change. It was observed that current harmonics increases the AC loss, and heat load of transformer and decreases the efficiency, consequently.

Finite Element Method Based Estimation of Critical Current Density of NbTi

The purpose of this article is to estimate the magnetic field dependent critical current density scaling relation of NbTi used in a superconducting wire. The estimation problem is set using a finite element method based forward model solving the critical current of a wire in external magnetic fields. A sensitivity analysis is carried out to reveal the uncertainties in the process. It is shown that the methods provide accurate predictions of critical currents in a wire, but there are uncertainties related to the zero-field critical current density of the material.

Cross-Sectional Area Dependence of Tunnel Magnetoresistance, Thermal Stability, and Critical Current Density in MTJ

Tunnel magnetoresistance (TMR), thermal stability, and critical switching current are important metrics of a magnetic tunnel junction (MTJ). In this work, a detailed study of these metrics is conducted for the down-scaling of the transverse dimensions of the MTJ. The quantum transport and the magnetization dynamics simulations are performed using non-equilibrium Green's function in the mode-space approach and object-oriented micromagnetic framework (OOMMF), respectively. The study of areal size quantization effects on the TMR shows that most of the contribution to the TMR comes from lower energy sub-bands and that the TMR saturates for dimensions above 50 nm. An anomalous behavior is observed in the bias dependence of TMR for the lower energy sub-bands and is explained in terms of the modified Slonczewski's analytical model for conductance around zero bias. The study of TMR scaling is extended to consider non-idealities by introducing elastic dephasing into our simulations. It is shown that with down-scaling of diameters, dephasing affects the zero bias TMR predominantly below 20 nm. Furthermore, TMR is also studied in terms of sensitivity to the variations in the interface layer and the asymmetric reduction of TMR with bias and its reversal at higher bias is observed. OOMMF simulations of the larger stack, including the free layer, are carried out to understand the qualitative link between magnet switching behavior, thermal stability, and critical current density with area scaling. It is shown that the area dependence of thermal stability and critical current follows each other qualitatively and the scaling of both these metrics is correlated with different regimes of magnetization switching, such as macrospin behavior or formation of metastable complex textures. The implications of scaling, on the various MTJ metrics, are discussed in terms of the application domain.

Vortex dynamics and phase diagram in the electron-doped cuprate superconductor Pr0.87LaCe0.13CuO4

Second magnetization peak (SMP) in hole-doped cuprates and iron pnictide superconductors has been widely explored. However, similar feature in the family of electron-doped cuprates is not common. Here, we report the vortex dynamics study in the single crystal of an electron-doped cuprate Pr$_{0.87}$LaCe$_{0.13}$CuO$_4$ superconductor using dc magnetization measurements. A SMP feature in the isothermal $M(H)$ was observed for $H$$\parallel$$ab$-planes. On the other hand, no such feature was observed for $H$$\parallel$$c$-axis in the crystal. Using magnetic relaxation data, a detailed analysis of activation pinning energy via collective creep theory suggests an elastic to plastic creep crossover across the SMP. Moreover, for $H$$\parallel$$ab$, a peak in the temperature dependence of critical current density is also observed near 7 K, which is likely be related to a dimensional crossover (3D-2D) associated to the emergence of Josephson vortices at low temperatures. The anisotropy parameter obtained $\gamma$ $\approx$ 8-11 indicates the 3D nature of vortex lattice mainly for $H$$\parallel$$c$-axis. The $H$-$T$ phase diagrams for $H$$\parallel$$c$ and $H$$\parallel$$ab$ are presented.

A possible explanation for double-peak structure in strain dependence of critical current density in REBa2Cu3O coated conductors

REBa2Cu3O coated conductors (REBCO-CCs; RE: rare-earth element or Y) have garnered considerable attention in high-field magnet applications. However, the twin structure existing in the ab plane of REBCO complicates the dependence of critical current density on temperature, magnetic field, and uniaxial strain: . In particular, the mechanism for the appearance of anomalous double-peak structure in is unclear.In this study, to understand the complicated strain dependence of critical current density, we measured of REBCO-CCs with c-axis-correlated pins or with random pins as a function of in-plane uniaxial strain and magnetic field applied parallel to the c axis. In the case of (Y,Gd)BCO-CC with BaZrO3 nanorods, we only observed a mountain-shaped response of under the entire magnetic field. However, GdBCO-CC without artificial pinning centers, where random pinning is dominant, exhibited a valley-shaped response at low fields and then a mountain-shaped response at high fields. These results suggest that the variation in the pinning mechanism (correlated- or random pins) plays a crucial role in the absence/presence of double-peak structure in REBCO-CCs.We developed a phenomenological model for in which (i) strain dependence of superconducting properties in two domains of REBCO and (ii) ideal behavior of correlated- or random pinning are considered. Consequently, we observed that the valley-shaped response of can be realized in the case of ideal random pinning with limited parameters, while ideal correlated pinning always exhibits a mountain-shaped response of . Furthermore, a crossover from a valley-shaped response under low fields to a mountain-shaped response under high fields in the strain dependence of is qualitatively consistent with our observations. This qualitative agreement between our model and experimental data validates the efficacy of our model to provide a clear understanding of the strain dependence of in REBCO-CCs, including the anomalous double-peak structure.

Anion height dependence of critical current densities for Fe Te1-x Sex bulk single crystals

We have fabricated FeTe1-xSex high quality bulk single crystals. The crystals with different composition of x = 0.375, 0.4, 0.45, 0.46 and 0.475 were prepared by the melting method with two stage heat treatment. The crystal structure and superconducting properties were evaluated. Temperature dependence of magnetization showed that low-T c region exists inside the crystals. Critical temperature T c was the highest value of 14.3 K for the crystal with x = 0.45. Critical current densities J c under the magnetic fields parallel to the c-axis at 4.2 K achieved 1.52 and 0.72 × 105 A / cm2 at 0 T and 5 T respectively for the crystal with x = 0.4. The crystal lattice parameters were examined by Rietveld refinement. The anion height from Fe layer was calculated using the obtained parameters. We investigated the anion height dependence of T c and J c and mentioned an important role of excess Fe about the magnetic flux pinning.

In-field critical current and pinning mechanisms at 4.2 K of Zr-added REBCO coated conductors

The critical current and pinning mechanisms at 4.2 K have been studied over a magnetic field range of 0–14 T for Zr-added (0, 5 and 15 mol.%) REBa2Cu3O7-x (REBCO and RE = rare earth) coated conductors fabricated by advanced metal organic chemical vapor deposition (A-MOCVD). It is found that the (Ba + Zr)/Cu content in Zr-added (5 and 15 mol.%) REBCO affects the critical current at 77 K, 0 T as well as density, continuity and shape of BaZrO3 (BZO) self-assembled nanocolumns and RE2O3 in-plane precipitates that significantly enhance the pinning force density Fp(H) as well as isotropic pinning landscape at 4.2 K. In addition to bell-shape dependence of critical current density, Jc, at 4.2 K with (Ba + Zr)/Cu content we observed an unusual Fp(H) behavior correlated to particular type of pinning centers, morphology and distribution that have been revealed by TEM microstructure analysis. By fitting the Dew–Hughes equation of the pinning force density Fp(H) at 4.2 K we extract the scaling behaviors of the Fp(H) associated with the competition of pinning mechanisms driven by vertically-aligned BZO nanorods and in-plane RE2O3 pinning defects. This result sheds light on approaches towards interactive control of strong and isotropic pinning centers in Zr-added REBa2Cu3O7-x (REBCO and RE = rare earth) coated conductors, and especially understanding the correlation between microstructural characteristics and vortex pinning mechanisms at 4.2 K in high magnetic fields.

AC losses and induced fields in HTS coil wound using two-ply coated conductors

AC losses and screening-current-induced fields (SCFs) in a high temperature superconducting (HTS) inserts for high field magnet are evaluated theoretically. The HTS inserts are composed of stacked pancake coils, which are wound using two-ply conductors, where the superconducting-layer sides of two rare-earth-based coated conductors are attached to each other without electrical insulation. The theoretical formulas of AC loss and SCF in the two-ply conductor are derived for the simultaneous applications of a transport current and an external magnetic field parallel to its broad face. By taking into account the magnetic-field profile in the HTS insert and the magnetic-field dependency of critical current density in the coated conductor, the AC losses and SCFs are estimated using the theoretical formulas for monotonical increase in a central field up to 25.5 T in 60 minutes in combination with low temperature superconducting outsert coils. It is guessed that the AC loss estimated for the HTS insert wound using the two-ply conductor could be cooled by using prepared cryocoolers. The magnitude of SCF in the HTS insert wound using the two-ply conductor becomes smaller than that for the single-tape conductor estimated previously.

Modifying the critical current anisotropy of YBCO films via buffering layers on IBAD-MgO based templates

The role of variations in the buffer layer structure of IBAD-MgO based templates on the critical current anisotropy has been investigated in undoped and BaZrO3 (BZO) doped YBa2Cu3O6+x (YBCO) films. Not only do the natural defects grow distinct within the undoped YBCO lattice but also due to the different lengths of BZO induced nanorods within the YBCO matrix, the flux pinning properties are greatly affected by the underlying layers which in turn has a great impact on the angular dependent critical current density J c(θ). This has been verified by transport measurements where the shape of the J c (θ) varies in accordance with the substrates. Based on our results, the template having a cap layer with the minimum lattice mismatch and a good chemical compatibility with deposited YBCO is proven to be the best for growing both the undoped and BZO doped YBCO films. Furthermore, a model we can present based on the shapes of J c(θ) curves depicts how the formation of nanosized defects affects the flux pinning anisotropy.

Magnetization simulation of ReBCO tape stack with a large number of layers using the ANSYS A-V-A formulation

In this paper we have modeled the magnetization of a disk-shaped ReBCO tape stack by using the ANSYS A-V-A formulation based iterative algorithm method developed at Paul Scherrer Institute (PSI). The tape stack consists of 400 layers of ReBCO coated conductors from SuperOx which have a tape width of 12 mm, a thickness of 46 μm and a field angle dependent critical current density J c( B ). The model of the tape stack is created with real superconducting layers and substrates, without assuming a homogenized J c in the tape or assuming the superconducting layer as 1-D line in the tape ( T-A formulation). The computation time is reduced by using the A-V formulation in superconducting layers and the A -formulation in non-superconducting areas. Simulation studies show that the tape stack is capable of trapping a central magnetic field of 10 T after field cooling (FC) magnetization from 10 T to zero. Further studies are carried out to investigate the possibility of trapping a central magnetic field of 20 T in a tape stack by using wider or thinner ReBCO coated conductors.

TDGL Simulation on Angular Dependence of Critical Current Density in Superconductors with Columnar Defects

We numerically solved the three dimensional time-dependent Ginzburg-Landau (TDGL) equations to visualize the motion of the quantized magnetic flux lines in the superconductor under the transverse magnetic field B. We investigated the angular dependence of the critical current density Jc in the columnar pins using TDGL equations. As a result of simulations, it was confirmed that Jc decreases as increasing the angle θ between the columnar pins and the quantized magnetic flux lines. In particular at B = 0.4, Jc sharply decreased at θ between 20° and 30°. This result was compared with the calculation result of the overlapped volume between the quantized magnetic flux and the pins. The calculated result by TDGL equations was in good agreement with the calculated result by the overlapped volume. In addition, Jc was calculated using various pins such as the sphere, the columnar and the plane pins. It was found that plane pins are more effective than pins of other shapes. In addition, although it is experimentally difficult to fabricate, Jc of the star-shaped pin was obtained by the TDGL simulations. It was found that the Jc of the star-shaped pin shows the highest performance among all the kinds of configurations for the columnar pins in high magnetic fields.

Evaluation of Layer Thickness Dependence of Critical Current Density using Longitudinal Magnetic Field Effect in Superconducting Coated Conductors

When the critical current density Jc for a superconductor is constant, the critical current Ic increases in proportion to the superconducting cross-sectional area. However, in the case of a superconducting coated conductor, when the thickness of the superconducting layer exceeds about 2 μm, there is no further increase in Ic. This is because Jc decreases due to the structural degradation of the superconducting layer. In this study, Jc for samples with superconducting layers of different thicknesses was measured using the longitudinal magnetic field effect in order to investigate the optimum layer thickness for maximizing Ic. Differences in the Jc characteristics due to the layer thickness appeared more clearly under a longitudinal magnetic field than a transverse magnetic field. This was particularly true at low temperatures, and the optimum layer thickness was found to be approximately 1 μm.

Comparative Investigation on Magnetic Behavior of Partial Rare Earth Element (Re: Lu, Yb, and Dy) Substituted Y0.5Re0.5BCO (123) Superconductors

Partial Re (rare earth elements: Lu, Yb and Dy) substituted YBaCuO (123) superconductors synthesized by solid-state reaction (SSR) procedure have been investigated via x-ray diffraction, scanning electron microscopy, a.c. magnetic susceptibility, and d.c. magnetization, and calculations. The measurements of a.c. magnetic susceptibility have been carried out at constant a.c. field amplitude and constant frequency for the samples studied. The temperature dependence of intergranular critical current density (Jcm) has been estimated from the a.c. magnetic susceptibility data. The d.c. magnetization measurements were performed at the temperatures of 5 K, 20 K, and 77 K separately. The variation of critical current density (Jc) with magnetic field (μ0H) has been evaluated from the d.c. magnetization data by using upper and lower branch of the major hysteresis curve. Lu substituted Y0.5Lu0.5BCO (123) sample shows higher critical current density than Yb and Dy substituted samples at low-temperature (lower than 77 K) regions.

Electromagnetic Properties of Thick Film REBCO Tapes

RE-Ba-Cu-O (REBCO, RE = rare earth) coated conductors are characterized by an anisotropic pinning behavior in applied magnetic fields. Over the years, incorporation of BaZrO3 nanocolumns has been an effective approach to improve the pinning force and reduce the anisotropy in these materials. In this paper, the magnetic field dependence of critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of 4.7 μm thick films grown by advanced metal-organic chemical vapor deposition (A-MOCVD) with 5% and 15% Zr addition has been studied. At 65 K, 1.5 T, the 5% Zr-added thick films manifest a J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at H||c-axis that surpasses 3 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Increasing the Zr content to 15% results in a plateau in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> from 0.4 to 6.5 T and enables higher values of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at fields above 6 T at 65 K. Tapes with 15% of added Zr exhibit a lift factor in critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (Temperature, Applied Magnetic Field) / J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (77 K, 0 T)) exceeding 13 and a J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> exceeding 11 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 30 K, 3 T.

Role of Columnar Defect Size in Angular Dependent Flux Pinning Properties of YBCO Thin Films

The effect of artificially produced BaHfO $_3$ , BaZrO $_3$ , and BaSnO $_3$ nanocolumns within the YBa $_2$ Cu $_3$ O $_{6+{\mathrm{x}}}$ (YBCO) lattice is investigated in terms of flux pinning and vortex dynamics by studying the properties of the angular dependent critical current density. Based on the differences in the growth mechanisms of discussed dopant species, the superconducting properties such as the critical current density, the accommodation field, and the effective pinning force have been observed to be greatly dependent on the size and the distribution of the nanocolumns. Therefore, the mechanisms of the collective individual and multivortex pinning have been observed being in a critical role when interpreting the in-field critical current anisotropy in YBCO films with artificial pinning centers.