Dependence Of Critical Current Density Research Articles

Critical Current Density in the Heterogeneous High-T c Superconductor Ca1−xLa x Fe2As2

We investigate the critical current density of La-doped CaFe2As2 (LaCa122) single crystals via transport and magnetization measurements. A large piece of the LaCa122 shows a sharp superconducting (SC) phase transition at a temperature of T c ~ 45 K with a transition width of ΔT c = 3.3 K. Small pieces exfoliated from the large crystal, however, show multiple SC phase transitions: two disparate SC phases sequentially occurred at ~ 15 (low T c ) and ~ 45 K (high Tc). The temperature dependence of transport critical current density also shows two disparate curves, and a kink appears at ~ 8 K. The magnetization critical current density shows a rapid suppression under an applied magnetic field for T > 10 K. These results indicate that the low-T c phase exists globally in the La-doped CaFe2As2, while the high-T c phase has a small volume fraction and locally resides within small domains. The disparate critical current density determined from transport and magnetization measurements, therefore, reflects the heterogeneous superconducting phases in La-doped CaFe2As2.

Demagnetization induced peak effect for a FeSeTe granular superconductor in ac magnetic field

The possible evidence of a peak effect in the temperature dependent magnetization and in the field dependent critical current density has been found for a FeSeTe superconducting granular sample in presence of an ac field. The existence of the peak effect has been confirmed by the numerical calculation of the inter- and intragranular magnetizations in presence of a demagnetization field with components at the third harmonic frequency generated by the sample magnetization. This suggests that the peak effect could be related to the flux dynamics inside the sample through the demagnetization field third harmonic.

On the Origin of Enhanced Critical Current Density and Thermal-Activated Flux Flow in K x Fe2−y Se2 Superconductors: the Effect of Quenching and Mn Doping

A magnetotransport, vortex pinning, and critical current density study of superconducting K0.8Fe2−z Mn z Se2 (z = 0, 0.02) single crystals is reported. The parameters of upper critical field, coherence length, and anisotropic ratio at 0 K are presented. Based on thermally activated flux flow model, the values of pinning potential, $U_{0}^{\parallel _{c}}(U_{0}^{\parallel _{ab}})$ , at 0.25 T are derived to be as large as 1.3 × 104(1.8 × 104) K for z = 0.02, which shows rather robust and retains still larger than pure ones up to 9 T. Utilizing scanning electron microscope, it is revealed the increased magnitude of magnetic moment in magnetic hysteresis loops correlates well with the crystal morphology variation. The evidence on the inhomogeneous distribution of Mn dopants between the matrix and stripe phase is also shown. Furthermore, the temperature dependence of critical current density is analyzed by different pinning regimes and the model of granular superconductor. It is demonstrated that the Josephson coupling between superconducting grains exists in quenched K0.8Fe2−y Se2. However, for Mn-doped sample, the Mn dopant can induce strong vortex pinning in the form of fine K–Mn–Se inclusions.

Nonlinear effects of the thermal stabilization theory of applied superconducting materials

The stability of dissipative states of a Bi2Sr2CaCu2O8 applied superconducting composite cooled by liquid helium or hydrogen with continuously increasing current-voltage characteristics described by a power equation has been studied. It has been shown that, under intensive cooling conditions, special conditions of thermal stabilization can exist for applied superconducting materials (technical superconductors) if the nonlinear temperature dependences of critical current density and specific resistance of the stabilizing matrix are taken into account. First, the minimum currents of existence and propagation of a normal zone can be absent. Second, the intensive cooling of a technical superconductor substantially increases the range of stable currents in the diapason of supercritical currents. Third, during the irreversible propagation of the thermal instability, the temperature of a technical superconductor can increase under conditions close to adiabatic, despite cooling by liquid coolant. These effects should be taken into account upon determining the conditions for overheating a technical superconductor. The numerical experiments have been compared with the results that follow from the thermal stabilization theory of combined superconductors, which assumes a linear temperature dependence of critical current density of a superconductor and a step-wise transition from the superconducting to normal state.

Use of a Sigmoid Function to Describe Second Peak in Magnetization Loops

Order-disorder transitions of a vortex lattice transfer type-II superconductors from a low critical current state to a high one. The similar transition between different current states can be caused by electromagnetic granularity. A sigmoid curve is proposed to describe the corresponding peak in a field dependence of the macroscopic critical density. Using the extended critical state model, analytic expressions are obtained for the field dependencies of the local critical current density, the depth of equilibrium surface region, and the macroscopic critical current density. The expressions are well fit to published data.

Flux pinning properties in BaPb0.77Bi0.23O[formula omitted] compound

The superconducting properties of BaPb0.77Bi0.23O3−δ compound were tested by transport and magnetization measurements from 2 to 14 K. The R–T curve at various magnetic fields is similar to that for MgB2 superconductors. The temperature dependence of resistance below 10% of normal state resistance follows Arrhenius law and the field dependence of effective flux pinning energy shows an exponential behavior, Ueff=U(0)exp(−0.48H). The Hc–T line down to 2 K displays a power law, Hc(T)∝(1−T/Tc)n. The field dependence of flux pinning force displays two peaks originated from the intragrains and intergrains, respectively. The scaling result of Dew–Hughes model, Fp=Ahp(1−h)q, suggests that the flux pinning in the sample is the collective pinning of normal surface and point centers. The field dependence of critical current density shows a power law in the low magnetic field region and an exponential behavior, Jc=Jc0exp(−βH), in high magnetic field region.

The dependence of critical current density of GdFeCo layer on composition of thermally assisted STT-RAM

Amorphous rare earth–transitional metal (RETM) GdFeCo memory layer with RE- and TM-rich compositions was fabricated in stacks of GdFeCo (10 nm)/Cu (3 nm)/[Co(0.2 nm)/Pd(0.4 nm)]6. Their magnetic properties and spin transfer torque (STT) switching of magnetization were investigated. The maximum magneto-resistance (MR) was around 0.24% for the TM-rich Gd[Formula: see text] (Fe[Formula: see text]Co[Formula: see text])[Formula: see text] memory layer and was −0.03% for the RE-rich Gd[Formula: see text] (Fe[Formula: see text]Co[Formula: see text])[Formula: see text] memory layer. The critical current densities [Formula: see text] to switch the GdFeCo memory layers are in the range of [Formula: see text] A/cm2–[Formula: see text] A/cm2. The dependence of critical current density [Formula: see text] and effective anisotropy constant [Formula: see text] on Gd composition were also investigated. Both [Formula: see text] and [Formula: see text] have maximum values in the Gd composition range from 21–29 at.%, suitable for thermally assisted STT-RAM for storage density exceeding Gb/inch2.

Simulation of Field Dependence of Critical Current Densities of Bulk High Tc Superconducting Materials regarding Thermally Activated Flux Motion

In the upcoming generation, bulk high temperature superconductors (HTS) will play a crucial and a promising role in numerous industrial applications ranging from Maglev trains to magnetic resonance imaging, etc. Especially, the bulk HTS as permanent magnets are suitable due to the fact that they can trap magnetic fields being several orders of magnitude higher than those of the best hard ferromagnets. The bulk HTS LREBa2Cu3O7-δ (LREBCO or LRE-123, LRE: Y, Gd, etc.,) materials could obtain very powerful compact superconducting super-magnets, which can be operated at the cheaper liquid nitrogen temperature or below due to higher critical temperatures (i.e., ∼90 K). As a result, the new advanced technology can be utilized in a more attractive manner for a variety of technological and medical applications which have the capacity to revolutionize the field. An understanding of the magnetic field dependence of the critical current density (Jc(H)) is important to develop better adapted materials. To achieve this goal, a variety of Jc(H) behaviours of bulk LREBCO samples were modelled regarding thermally activated flux motion. In essence, the Jc(H) curves follows a certain criterion where an exponential model is applied. However, to fit the complete Jc(H) curve of the LRE-123 samples an unique model is necessary to explain the behavior at low and high fields. The modelling of the various superconducting materials could be understood in terms of the pinning mechanisms.

Carrier-Doping Dependence of Critical Current Density in Ba1-xKxFe2As2 Single Crystals and Superconducting Wires

The carrier-doping dependence of Jc in Ba1-xKxFe2As2 single crystals and superconducting wires were investigated. Ba1-xKxFe2As2 single crystals (0.24 < x < 0.4) are synthesized by commonly used FeAs self-flux method, while Ba1-xKxFe2As2 wires are fabricated by powder-in-tube (PIT) method and hot isostatic pressing (HIP) technique, using polycrystalline Ba1-xKxFe2As2 (0.25 < x < 0.4) prepared by solid state reaction and silver and copper tubes. Their magnetic Jc was characterized by magnetization measurements. We found that the x-dependence of Jc in single crystals shows the peak in under-doped region around x ~ 0.28 similar to previous study. In the HIP wire, although significant peak is not observed, Jc shows a broad plateau in a wide doping region of 0.3 < x < 0.4. These results indicate that Jc in the under-doped Ba1-xKxFe2As2 HIP wire is controlled by interglanular Jc.

Angular-Dependent Vortex Pinning Properties of YBa2Cu3O7−δ /Y2O3 Quasi-Multilayers

A series of quasi-multilayers of YBa2Cu3O7−δ (YBCO)/Y2O3 specifically 70 × (m YBCO/n Y2O3) were prepared on SrTiO3 single crystal using pulsed-laser deposition (PLD) with a controlled deposition pulses of m = 40 and n = 2, 5, and 10 for YBCO and Y2O3, respectively. The x-ray diffraction patterns indicate that all the present quasi-multilayers exhibit good c-axis orientation. The angular dependence of critical current density (J c ) on applied magnetic field directions are systemically measured to study the anisotropic vortex pinning performances for those quasi-multilayers. It is revealed that compared with the pure YBCO films, the quasi-multilayers with n = 2, i.e., a proper constituent pulse of Y2O3, exhibits the enhanced vortex pinning abilities in all angles between c-axis orientation and the applied magnetic field direction. As well, such a quasi-multilayer film (n = 2) shows the higher lift factor J c (Θ)/ J c (90°) and much better vortex pinning properties at high fields and high temperatures, showing promising potential for coated conductor application.

Design and Testing of Coils Wound Using the Conductor-On-Round-Tube (CORT) Cable

Alternative concept for winding of coils from high-temperature superconducting tapes has been investigated. Conductor-On-Round-Tube (CORT) conductors with tapes helically wound on a metallic tube have been manufactured and used to wind single layer coils with the bore diameters between 116 and 325 mm wound and tested in the dc regime. Results were analyzed with the help of numerical calculations considering the critical current density dependence on magnetic field derived from the short sample measurement. We have also modified the standard approach by including the fluctuations of critical current along the tape length into the calculations. Results have shown that the uniformity of critical current is essential for achieving predictable cable and coil performance.

Dynamics of First and Second Switches in Bi$_{2}$ Sr$_{2}$CaCu $_{2}$O$_{8+\delta }$ Intrinsic Josephson Junction Stacks Measured by Specifically Designed Electronics

We measure switching probability distributions for the first and the second switches of stacks of Bi 2 Sr 2 CaCu 2 O 8+δ intrinsic Josephson junctions. To measure switching probability distributions, we design a current source and introduced a microcontroller. The resolution and accuracy are sufficient for measuring switching probability distributions of intrinsic Josephson junctions. For the first switch, crossover temperatures between thermally activated escape and macroscopic quantum tunneling are in the range between 0.8 and 2.2 K. The crossover temperatures of the second switch are approximately 8.0 and 6.8 K for the samples with critical densities of 1.0 and 2.0 kA/cm 2 , respectively. We analyze critical current density dependence of the crossover temperature of the second switch. The crossover temperature decreases with increasing critical current density. This anticorrelation can be explained neither by the single-junction model nor by the heating effect. The anticorrelation is a peculiar phenomenon observed in the second switch. We consider that the anticorrelation is attributed to the capacitive coupling. The strength of the capacitive coupling is weaker in IJJs with higher critical current density because the charge screening length is shorter due to increase of carrier density.

Superconducting properties of nano-sized SiO2 added YBCO thick film on Ag substrate

The microstructure and the flux pinning capability of SiO2-added YBa2Cu3Oy thick films on Ag substrates were investigated. A series of YBa2Cu3Oy thick films with small amounts (0–0.5 wt%) of nano-sized SiO2 particles (12 nm) was prepared. The thicknesses of the prepared thick films was approximately 100 µm. Phase analysis by x-ray diffraction and microstructure examination by scanning electron microscopy were performed and the critical current density dependence on the applied magnetic field Jc(H) and electrical resistivity ρ(T) were investigated. The magnetic field and temperature dependence of the critical current density (Jc) was calculated from magnetization measurements using Bean’s critical state model. The results showed that the addition of a small amount (≤0.02 wt%) of SiO2 was effective in enhancing the critical current densities in the applied magnetic field. The sample with 0.01 wt% of added SiO2 exhibited a superconducting characteristics under an applied magnetic field for a temperature ranging from 10 to 77 K.

Weakening of thickness dependence of JC in YBa2Cu3O7-δ films using a titanium-added MOD precursor solution

BaTiO3 (BTO)-doped YBa2Cu3O7-δ (YBCO) films were prepared by the metal-organic deposition method using a precursor solution with titanium ion. These YBCO films were deposited on (00l)-oriented LaAlO3 single-crystal substrates using an automated dip coater. The thickness dependence of critical current density (JC) in the BTO-doped YBCO films was investigated. The JC value of the YBCO composite film was reduced from 6.3 to 4.6 MA/cm2 with increasing film thickness from 450 to 930nm in self-filed at 77K, which arose mainly from degradation of texture and roughening of the film. However, relative to undoped YBCO thick films, JC values of the YBCO composite thick films were greatly improved, and more importantly, the reduction in JC with increasing film thickness was hindered, especially in a high magnetic field. This result indicated that the introduction of BTO nanoparticles as pinning centers into YBCO matrix weakened the thickness dependence of JC.

Numerical simulation on the flux avalanche behaviors of microstructured superconducting thin films

Controlling and suppressing the propagation of magnetic flux avalanches is an important issue for the application of type-II superconductors. The effects of engineered pinning centers (antidots) on the guidance of flux avalanche propagation paths in type-II superconducting thin films are numerically investigated by solving the coupled nonlinear Maxwell's equations and the thermal diffusion equations. The field dependence of critical current density is considered in the simulation in this paper. Dynamic propagations of the thermomagnetic avalanches within the superconducting films patterned with different arrangements of antidots (like random, periodic square, and conformal mapping arrays) are presented. We reveal that presence of the antidots significantly modifies the propagation paths of the avalanches. The flux avalanche patterns of the superconducting films change with the variation of the arrangements of antidots. The patterned antidots in the form of conformal mapping arrays within the superconducting film exhibit strong guidance to the thermomagnetic avalanches. In addition, introducing the antidots in the form of conformal mapping arrays into the superconducting film can effectively lower the magnetic flux jump sizes.

Critical current and flux dynamics in Ag-doped FeSe superconductor

The measurements of DC magnetization as a function of the temperature M(T), magnetic field M(H), and time M(t) have been performed in order to compare the superconducting and pinning properties of an undoped FeSe0.94 sample and a silver doped FeSe0.94 + 6 wt% Ag sample. The M(T) curves indicate an improvement of the superconducting critical temperature and a reduction of the non-superconducting phase Fe7Se8 due to the silver doping. This is confirmed by the field and temperature dependent critical current density Jc(H,T) extracted from the superconducting hysteresis loops at different temperatures within the Bean critical state model. Moreover, the combined analysis of the Jc(T) and of the pinning force Fp(H/Hirr) indicate that the pinning mechanisms in both samples can be described in the framework of the collective pinning theory. The U*(T, J) curves show a pinning crossover from an elastic creep regime of intermediate size flux bundles, for low temperatures, to a plastic creep regime at higher temperatures for both the samples. Finally, the vortex hopping attempt time has been evaluated for both samples and the results are comparable with the values reported in the literature for high Tc materials.

Magnetic proximity effect and superconducting triplet correlations at the cuprate superconductor and oxide spin valve interface

A heterostructure consisting of a cuprate superconductor YBa2Cu3O7-δ and a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve was studied using SQUID magnetometry, ferromagnetic resonance, and neutron reflectometry. It is shown that because of the magnetic proximity effect a magnetic moment is excited in the superconducting portion of the heterostructure, whereas the magnetic moment in the spin valve becomes suppressed. The experimentally obtained value of a typical penetration depth of a magnetic moment into the superconductor is significantly greater than the coherence length of the cuprate superconductor, which indicates that the induced magnetic moment mechanism of Cu atoms is dominant. The mesastructure prepared by adding niobium film as a second superconducting electrode to the existing heterostructure, exhibited a superconducting current (dc Josephson effect) at interlayer thicknesses that are much greater than the coherence length of the ferromagnetic materials. The maximum of the critical current density dependence on the thickness of the spin valve material corresponds to the interlayer coherence length, which agrees with the theoretical predictions associated with spin-triplet pairing. The superconducting current is observed at magnetic fields that are two orders of magnitude greater than the field corresponding to the occurrence of one magnetic flux quantum in the mesastructure. The ratio of the second harmonic of the current-phase dependence of the mesastructure superconducting current to the first, determined according to the dependence of the Shapiro steps on the amplitude of microwave exposure, did not exceed 50%.

Electromagnetic behaviour of bulk MgB2 determined by numerical modelling using regional supercurrent properties

Previous studies in the literature have shown experimentally that the critical current density in bulk MgB2 depends on the position at which it is measured within the sample, and that the trapped magnetic field saturates above large values of sample diameter. To better understand this behaviour, we have carried out a detailed study of the trapped magnetic field and local critical current distribution in bulk MgB2 using numerical modelling with H-formulation. The properties of bulk MgB2 samples with radius-independent (i.e. uniform) and radius-dependent critical current were modelled, based on experimentally-determined parameters obtained from the literature. Radius-independent samples took properties originally derived from either the central or edge region of the original samples. In our study it was determined that the peak trapped magnetic field value of the sample with the central region properties was higher by 22.4% and 9.20% than the samples with edge region properties and that with radius dependent parameter, respectively. Additionally, to understand why the trapped magnetic field saturates as the diameter of sample increases, the trapped magnetic field was modelled using the radius dependent critical current density Jc (B,r) as input data rather than using a constant bulk critical current density. The peak trapped magnetic field values increase by 8.6% and 6.2% respectively for the central region radius-independent and radius-dependent property bulk MgB2 samples as the bulk diameter increases from 25 mm to 35 mm. This tendency to saturation in the peak trapped field indicates that increasing the bulk diameter alone does not have a significant effect on the value of the trapped magnetic field, unless the bulk superconducting current density is improved uniformly throughout the MgB2 bulk. These results enable us to understand how we can experimentally enhance the trapped magnetic field in bulk MgB2 by producing samples with uniform high critical current density distribution using fabrication methods such as graded doping.

Experimental research of high field pinning centers in 2% C doped MgB2 wires at 20 K and 25 K

High field pinning centers in MgB2 doped with 2 at. % carbon under a low and a high hot isostatic pressures have been investigated by transport measurements. The field dependence of the transport critical current density was analyzed within the different pinning mechanisms: surface pinning, point pinning, and pinning due to spatial variation in the Ginzburg-Landau parameter (Δκ pinning). Research indicates that a pressure of 1 GPa allows similar pinning centers to Δκ pinning centers to be obtained. This pinning is very important, because it makes it possible to increase the critical current density in high magnetic fields at 20 K and 25 K. Our results indicate that the δTc and δl pinning mechanisms, which are due to a spatial variation in the critical temperature (Tc) and the mean free path, l, respectively, create dislocations. The high density of dislocations with inhomogeneous distribution in the structure of the superconducting material creates the δl pinning mechanism. The low density of dislocations with inhomogeneous distribution creates the δTc pinning mechanism. Research indicates that the hot isostatic pressure process makes it possible to obtain a high dislocation density with a homogeneous distribution. This allows us to obtain the δTc pinning mechanism in MgB2 wires. In addition, a high pressure increases the crossover field from the single vortex to the small vortex bundle regime (Bsb) and improves the δTc pinning mechanism. Our research has proved that a high pressure significantly increases the crossover field from the small bundle to the thermal regime (Bth), with only a modest decrease in Tc of 1.5 K, decreases the thermal fluctuations, increases the irreversibility magnetic field (Birr) and the upper critical field (Bc2) in the temperature range from 4.2 K to 25 K, and reduces Birr and Bc2 above 25 K.

Superconductor-ferromagnet-superconductor nanojunctions from perovskite materials

The lateral superconductor-ferromagnet–superconductor (SFS) nanojunctions based on high critical temperature superconductor YBa2Cu3Ox (YBCO) and half-metallic ferromagnet La0.67Sr0.33MnO3 (LSMO) thin films were prepared to investigate a possible presence of long range triplet component (LRTC) of Cooper pairs in the LSMO. We applied Ga3+ focused ion beam patterning to create YBCO/LSMO/YBCO lateral type nanojunctions with LSMO length as small as 40nm. The resistivity vs. temperature, critical current density vs. temperature and resistance vs. magnetic field dependence were studied to recognize the LRTC of Cooper pairs in the LSMO. A non-monotonic temperature dependence of junction critical current density and a decrease of the SFS nanojunction resistance in increased magnetic field were observed. Only weak manifestations of LRTC and some qualitative agreement with theory were found out in SFS nanojunctions realized from the perovskite materials. The presence of equal-spin triplet component of Cooper pairs in half-metallic LSMO ferromagnet is not such apparent as in SFS junctions prepared from low temperature superconductors NbTiN and half-metallic ferromagnet CrO2.