Critical Current Density Jc Research Articles

Field, temperature, and angle dependent critical current densityJc(H,T,θ) in coated conductors obtained via contact-free methods

Applications of coated conductors based on high-Tc superconductors often require detailed knowledge of their critical current densityJc as a function of magnetic field orientation as well as field strength and temperature. Thiswork demonstrates experimental methods for obtaining the angularly dependentJc using contact-free magnetic measurements, and qualifies those methods using several welldefined conditions. The studies complement traditional transport techniques and arereadily extended to conditions of field and temperature where the current density is verylarge and transport methods become difficult. Results on representative materials arepresented.

Pinning and irreversibility in superconducting bulkMgB2 with added nanodiamonds

Resistance, R(T), andmagnetization, M(B), studieson superconducting bulk MgB2 samples containing nanodiamonds (ND) as additives (wt% of ND:x = 0%, 1%, 3%, 5%, 7% and 10%) were recently published in two articles (Vajpayee et al 2007Supercond. Sci. Technol. 20 S155, Vajpayee et al 2008 J. Appl. Phys. 103 07C708). Themain observations reported were significant improvements in the critical current densityJc(B), irreversibilityline Birr(T) and uppercritical field Bc2(T) with ND addition. However, a closer look shows that as regards the potential of thistechnologically important material at higher magnetic fields and temperatures,there is still a lot of room for improvement. With that in mind we revisit theR(T) andM(B) data and analyze them, in the present work. We show that, despite ND addition,Jc dependsstrongly on B in the high field region and tends to vanish at irreversibility lines that lie deep, i.e. at around 0.3Bc2(T), inthe B–T phase diagram. The irreversibility lines, determined by in the presence of B, arefound to lie at around 0.5 Bc2(T) in the phase diagram. These results for pinning and irreversibility lines arediscussed in the light of various models such as those of surface sheathsuperconductivity, magnetically introduced percolation in polycrystallineMgB2, thermally assisted flux motion (TAFM) and a modified flux line shear mechanism. Ouranalysis hints at TAFM and weak pinning channels with distributed superconductingproperties percolating in our samples determining the irreversibility and pinningproperties.

Nb/Al-AlOx/Nb junctions with controllable critical current density for qubit application

Nb/Al-AlOx/Nb tunnel junctions with controllable critical current density JC are fabricated using the standard selective Nb etching process. Tunnel barriers are formed in different oxygen exposure conditions (oxygen pressure P and oxidation time t), giving rise to JC ranging from 100 A/cm2 to above 2000 A/cm2. JC shows a familiar linear dependence on P × t in logarithmic scales. We calculate the energy levels of the phase- and flux-type qubits using the achievable junction parameters and show that the fabricated Nb/Al-AlOx/Nb tunnel junctions can be used conveniently for quantum computation applications in the future.

Ultrathin epitaxial MgB2 superconducting films with high critical current density andTc above 33 K

High quality ultrathin MgB2 superconducting films with thickness down to 7.5 nm have been synthesized via hybrid physical–chemicalvapor deposition. The films are phase-pure and oxygen-free. The 7.5 nm thin film shows aTc(0) of 32.8 K and residualresistivity of 120 µΩ cm, and the10 nm thin film shows a Tc(0) of 35.5 K and high critical current densityJc∼106 A cm−2 at 20 K.

The preparation process of plasma-nitrided barriers in NbN Josephson junctions for digitalapplications

This paper describes the preparation process of plasma-nitridedAlNx barriersfor NbN Josephson junctions and the electrical characteristics of the junctions. The plasma-nitridedAlNx barrier is a candidate for the realization of an ultra-thin and homogeneousbarrier layer which is needed to enhance the quality of NbN/AlN/NbNx /NbN junctions. In this work, we have investigated the preparation conditions of plasma-nitridedAlNx barriers by evaluating the electrical characteristics of NbN/AlNx /NbN junctions. By adjusting the Al thickness and nitridation time, we obtained junctions withgood superconductor/insulator/superconductor characteristics at 4.2 K with a quality parameterVm as high as 104 mV and a critical current densityJc as high as 8 kA cm−2. Evaluation of the spreadin Ic showed that theminimum to maximum Ic spread was about ± 3% in a series of 100 junctions with an area of 100 µm2.

Flux pinning effect of cubic equiaxed morphology and its Ti stabilizing in Nb3Sn superconductors

4ALSTOM, 3 bis, avenue des 3 Chenes, 90018 Belfort, France Two sets of internal-Sn Nb3Sn superconducting strands were fabricated through RRP method, one with 2 wt% of Ti alloyed in Sn core and the other just pure Sn. Four reaction temperatures of 650℃, 675℃, 700℃ and 725℃ and 128 h duration were applied for A15 phase formation heat treatment after Cu-Sn alloying procedure of 210℃/50 h + 340℃/25 h. For the heat-treated coil samples, transport non-Cu JC was examined through standard 4-probe technique and phase microstructure was observed by means of Field Emission Scanning Electronic Microscope (FESEM). The obtained results demonstrate that the transport critical current density JC of Nb3Sn superconductors is more importantly determined by the cubic equiaxed crystalline morphology than by grain size. Ti addition in Sn stabilizes the cubic equiaxed phase at lower temperature so that heat reaction temperature is effectively reduced, the flux pinning performance is largely reinforced and the transport critical current density JC is substantially promoted.

Structural and pinning properties ofY2Ba4CuMOy (M = Nb,Zr)/YBa2Cu3O7−δ quasi-multilayers fabricated by off-axis pulsed laser deposition

Power applications based on YBa2Cu3O7−δ coated conductors demand an enhancement of the critical current densityJc in magnetic fields to be achieved using artificial pinning centres. A well-known approach tostudying pinning by second phase nanoparticles is based on quasi-multilayer deposition ofYBa2Cu3O7−δ (Y123) and a dopant material. In this study we use theY2Ba4CuMOy (M = Nb, Zr; M2411) phase as a dopant material, which proved to be very successful in increasingJc for bulk samples. Although stability of the M2411 phase in bulk Y123 hasbeen reported, experimental evidence of its stability in thin films is stilllacking. We found that during the quasi-multilayer deposition, yttrium dopedBa(M1−xYx)O3 forms instead of the M2411 secondary phase, and this causes an increase ofJc at high magnetic fields as well as an increase in the irreversibility field below 80 K ascompared against pure Y123 thin films.

Superconducting nanowires: Interplay of discrete transverse modes with supercurrent

From a numerical solution of the Bogoliubov-de Gennes equations, we investigate an interplay of the transverse discrete modes with a longitudinal supercurrent in a metallic cylindrical superconducting nanowire. The superconductor-to-normal transition induced by a longitudinal superflow of electrons is found to occur as a cascade of jumps in the order parameter (supercurrent and superfluid density) as a function of the superfluid velocity for diameters $d<10--15\text{ }\text{nm}$ (for Al parameters) and sufficiently low temperatures $T<0.3--0.4{T}_{c}$, with ${T}_{c}$ the critical temperature. When approaching ${T}_{c}$, the jumps are smoothed into steplike but continuous drops. A similar picture occurs for $d>15--20\text{ }\text{nm}$. Only when the diameter exceeds 50--70 nm the quantum-size cascades are fully washed out, and we arrive at the mesoscopic regime. Below this regime the critical current density ${j}_{c}$ exhibits the quantum-size oscillations with pronounced resonant enhancements: the smaller the diameter, the more significant is the enhancement. Thickness fluctuations of real samples will smooth out such oscillations into an overall growth of ${j}_{c}$ with decreasing nanowire diameter.

Formation of Higher Borides During High-Pressure Synthesis and Sintering of Magnesium Diboride and Their Positive Effect on Pinning and Critical Current Density

Critical current density (jc) of high-pressure (2 GPa) manufactured MgB2-based superconductors depends on the amount and distribution of higher borides (MgB12) in MgB2 matrix, which in turn are determined by the nature of the initial components first of all B or MgB2 and the temperature of sintering or synthesis. Ti and Ta additions can improve jc by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH2 detrimental for jc being formed, which possibly increases the MgB12 nucleation barrier. SiC (0.2-0.8 mum) addition increases jc of MgB2, allowing us to get jc = 106 A/cm2 at 20 K in the 1 T field: pinning is increased by SiC and higher boride grains and there is no notable interaction between SiC and MgB2 . As the synthesis temperature increases from 800 to 1050degC, Ti and SiC additions may affect the oxygen segregation and formation of Mg-B-O inclusions enriched with oxygen as compared to the amount of oxygen in the MgB2 matrix, which can also promote an increase in pinning. Materials high-pressure synthesized from Mg and B taken in 1:4, 1:6, 1:7, 1:8, 1:10, 1:12, 1:20 ratios were superconductive with Tc of about 37 K. High jc (7middot104 - 2middot104 A/cm2 in zero field at 10-30 K, respectively) showed materials with the matrix composition near MgB12 stoichiometry, they have doubled microhardness of MgB2.

Effect of cubic equiaxed grains and its Ti-stabilizing performance inNb3Sn strands**Funds: this research project was supported by the France–China Collaboration Researchcontract CNRS no. 722441 and the SUST doctoral foundation BJ07-07.

Two kinds of multifilament internal-SnNb3Sn superconducting strands were fabricated through the RRP method, one with 2 wt% of Tialloyed in an Sn core and the other just pure Sn. Four reaction temperatures of650,675,700 and725 °C and128 h duration were applied for A15 phase formation heat treatment after a Cu–Sn alloying procedure of210 °C/50 h+340 °C/25 h. Throughthe standard four-probe technique the heat-treated coil samples were examined for the transport non-CuJC variation withapplied field B which was then used to calculate the flux pinning force variationFP–B. The samples’ phase microstructure were also observed by meansof FESEM. The obtained results demonstrate that for fully reactedNb3Sn superconductors the transport critical current densityJC is more importantly affected by the cubic equiaxed morphology than by grain dimension, due toits much stronger flux pinning performance of the morphology. Ti addition in Sn stabilizes thecubic equiaxed grains at lower reaction temperature so that the HT temperature is effectivelyreduced, the flux pinning strength is largely reinforced and thus the transport non-CuJC is substantially promoted.

A high critical current density MOCVD coated conductor with strong vortex pinningcenters suitable for very high field use

We have made extensive low temperature and high field evaluations of a recent2.1 µm thick coated conductor (CC) grown by metal–organic chemical vapor deposition (MOCVD)with a view to its use for high field magnet applications, for which its very strong Hastelloysubstrate makes it very suitable. This conductor contains dense three-dimensional(Y,Sm)2O3 nanoprecipitates, which are self-aligned in planes tilted∼7° from the tape plane. Very strong vortex pinning is evidenced by high critical current densityJc valuesof ∼3.1 MA cm−2 at77 K and ∼43 MA cm−2 at 4.2 K, and by a strongly enhanced irreversibility fieldHirr, whichreaches that of Nb3Sn (∼28 T at 1.5 K) at 60 K, even in the inferior direction of axis. At 4.2 K, Jc valuesare ∼15% of the depairingcurrent density Jd, much the highest of any superconductor suitable for magnet construction.

Reduction of the spin-torque critical current by partially canceling the free layer demagnetization field

We significantly reduce the critical current Ic0 for the onset of spin torque switching of the free layer in nanometer-scale in-plane magnetized spin valves by partially cancelling its intrinsic demagnetization field through the utilization of Co/Ni multilayer free layers. The out-of-plane magnetic anisotropy arising from the Co/Ni interfaces reduces the effective demagnetization field (Heff) while not significantly affecting the thermal stability of the free layer. A zero-thermal-fluctuation critical current density Jc0∼2×106 A/cm2 is determined through both current ramp rate and nanosecond pulse measurements, and comparisons with large Heff control samples confirm that this strategy is efficient in substantially decreasing Ic0.

Critical current density and pinning potential inYBa2Cu3O7−δ thick filmsablated from a BaZrO3-doped nanocrystalline target

We report on the pulsed laser deposition and characterization of thick(1.27 µm) films ablated from ananocrystalline YBa2Cu3O7−δ (YBCO)target doped with 4 wt% BaZrO3 (BZO), nYBCO/BZO. DC magnetization measurements have shown a four- to five-fold increase in the critical current densityJc and a three- to four-fold increase in the maximum pinning force in comparison with1 µm thick films grown from a commercial microcrystalline YBCO target, at both 77.3and 65 K. We have found that the frequency-dependent critical current densitiesJc(f0/f) estimated from susceptibility measurements are linear in a double-logarithmic plot (wheref is the frequency ofthe AC field and f0 is a macroscopic attempt frequency of about106 Hz), consistent with a logarithmic dependence of the pinningpotential on the current density, and are again several times higher innYBCO/BZO thick films. Fromthe slopes of the Jc(f0/f) dependences we have also estimated the values of pinning potentials in fields between 3 and5 T.

Assessing the spatial and field dependence of the critical current density in YBCO bulksuperconductors by scanning Hall probes

Although the flux density map of a bulk superconductor provides in principle sufficientinformation for calculating the magnitude and the direction of the supercurrent flow,the inversion of the Biot–Savart law is ill conditioned for thick samples, thusrendering this method unsuitable for state of the art bulk superconductors. If a thin(<1 mm) slab is cut from the bulk, the inversion is reasonably well conditioned and the variation of thecritical current density in the sample can be calculated with adequate spatial resolution.Therefore a novel procedure is employed, which exploits the symmetry of the problem andsolves the equations non-iteratively, assuming a planar thickness-independent currentdensity. The calculated current density at a certain position is found to depend on themagnetic induction. In this way the average field dependence of the critical current densityJc(B) is also obtained at low fields, which is not accessible to magnetization measurements due tothe self-field of the sample. It is further shown that an evaluation of magnetization loops,taking the self-field into account, results in a similar dependence in the field rangeaccessible to this experiment.

Magnetic relaxation induced by transverse flux shaking inMgB2 superconductors

We report on measurements and numerical simulations of the behavior ofMgB2 superconductors when magnetic field components are applied along mutuallyperpendicular directions. By closely matching the geometry in simulations andmeasurements, full quantitative agreement is found. The critical state theory and asingle phenomenological law, i.e. the field dependence of the critical current densityJc(B), are sufficient for a full quantitative description of the measurements.These were performed in thick strips of carbon nanotube dopedMgB2 samples. Magnetization was measured in two orthogonal directions using a SQUIDmagnetometer. Magnetic relaxation effects induced by the application of an oscillatoryperpendicular field were observed and simulated numerically. The measurements confirmthe numerical predictions, that two relaxation regimes appear, depending onthe amplitude of the applied magnetic field. The overall agreement constitutes aconvincing validation of the critical state model and the numerical procedures used.

Increased in-field critical current density in neutron-irradiatedMgB2 films

We study the critical current densityJc of aseries of MgB2 epitaxial thin films with disorder introduced in a controlled way by neutron irradiation. We find that thein-field Jc is significantly increased for an intermediate neutron fluence of∼7.6 × 1017 cm−2,reaching ∼1.8 × 105 A cm−2 at μ0H = 4 T, which is larger by more than one order of magnitude than the criticalcurrent density of the pristine sample. Moreover, in the same irradiated sample,Jc(μ0H = 9 T) is still aslarge as 1.1 × 104 A cm−2. By transmission electron microscopy, we find evidence of the presence of amorphousregions created by irradiation whose average size matches the coherence lengthξab. Analysis of the shape of the critical current density and pinning force curves indicatesthat such regions with a suppressed superconducting order parameter behave as effectivepoint-like pinning centers for magnetic flux lines.

A comparative study of microwave power handling capabilities of Tl-1212 and Hg-1212superconducting microstrip lines

The microwave power handing capabilityPc of the microstrip lines of the isomorphic ‘1212’ pair is studied for the superconductorsTlBa2CaCu2O7 andHgBa2CaCu2O6+δ at≥77 K.The Pc for the pair follows the same temperature dependence at a reduced temperature scale(t = T/Tc). A comparison of the dc critical current densityJc and theradio-frequency one Jrf derived from the Pc of the ‘1212’ pair reveals a similar reduced temperature dependence, suggesting that the depinningof the magnetic vortices, which is dictated by the ‘1212’ structure, pays a similar role in limitingJc andJrf in theproximity of Tc.

Macroscopic quantum tunneling and qubit design parameters ofBi2Sr2CaCu2O8+δ surface intrinsic Josephson junctions

Macroscopic quantum tunneling (MQT) has been demonstrated recently in aBi2Sr2CaCu2O8+δ surface intrinsic Josephson junction (SIJJ) with its critical current densityJc below100 A cm−2 and itssize below 1 µm. In this work, we present a study of the switching currentdistributions of SIJJs fabricated on the same crystal, withJc>500 A cm−2 and size of0.8 and 1.6 µm. MQT is clearly observed, and the crossover from MQT to thermal activation(TA) is seen. Our analysis shows that the data agree well with the theoreticalpredictions of MQT and TA for different-sized SIJJs when parameters that roughlyscale with the SIJJ size are used. In the crossover regime, the data are foundto be better fitted by considering quantum corrections to TA. We discussthe realistic design of phase- and flux-type qubits using the experimentallyattainable SIJJ parameters, which shows that the SIJJs, with their controllableJc and size (or junction capacitance), are feasible for qubit applications in the future.

Strong flux pinning in YBa2Cu3O7−δ thin films due to nanometer-sized precipitates

The magnetic field angle dependence of the critical current densityJc(H,θ) was measuredin epitaxial YBa2Cu3O7−δ (YBCO) thin films with strong flux pinning(Jc>25 GA m−2 at 77 K). The YBCO films were classified into two categories: (1) films that showedJc(θ) peaks around with the shape of a stratovolcano (i.e., like Mount Fuji) and (2) films that showed high, broadJc(θ) peaks centered at in addition to less prominent peaks. Transmission electron microscope observations revealed that the filmsin category 1 contained a high density of very small precipitates, most ofwhich were less than 7 nm, and that the films in category 2 contained a highdensity of precipitates whose typical diameters ranged from 5 to 25 nm. TheJc(H,θ) data were analyzed based on the angular-dependent coherence lengthξ(θ) within an anisotropic Ginzburg–Landau approximation. The pinning of the filmsin category 1 can be described by a direct summation of the core pinninginteraction that is due to small point defects whose diameters are less than2ξ. The high,broad Jc(θ) peaks centered at in the films in category 2 were due to a high density of larger precipitates, and they can alsobe explained by a similar analysis for spherical pinning centers whose diameters are larger than2ξ.

High quality MgB2 thick films and large-area films fabricated by hybrid physical–chemical vapor depositionwith a pocket heater

A hybrid physical–chemical vapor deposition process using a pocketheater was developed for the growth of high quality epitaxial large-areaMgB2 thin filmsand c-axistextured MgB2 thick films. This technique is able to independently control the substrate and Mg source temperaturesand maintain sufficient Mg overpressure to ensure phase stability. The two-inch large-areaMgB2 thinfilms showed uniform superconducting properties with the superconducting transition temperatureTc of about 40 K, residual resistivity ratio (RRR) of about 10, and critical current densityJc of about107 A cm−2 (0 T, 5 K). Thethick films (∼10 µm) on sapphire substrates showed a maximumTc of 40 K and RRRof 15, and a Jc of 1.6 × 106 A cm−2 at low applied magnetic fields even at 20 K. High quality thick films also have beenobtained on metal substrates.