Reduction Of Critical Current Density Research Articles

Structure design of 500 kVA HTS transformer and Joule loss in superconducting windings

HTS transformers can provide lower loss and higher efficiency with smaller size and lighter weight compared with traditional transformers. In this paper, we designed a 500 kVA HTS transformer and developed its simulation using finite element software based on the H-formulation, and the simulation incorporates a heat model and E-J power law. The simulation shows that there is a large radial flux leakage at the end of the low-voltage (LV) windings (superconducting windings), leading to a reduction in critical current density and an associated increase in joule loss in the LV winding. In order to reduce joule loss, various LV winding structures were designed. The results demonstrate that either increasing the axial distance between the coils at the LV winding end or installing a flux diverter outside the LV windings can effectively diminish the radial flux leakage at the LV winding end, consequently reducing joule loss in the LV winding. The impact of the size, position and relative permeability of the flux diverter on radial flux leakage and joule loss are also studied. These structure optimizations and the corresponding effects have important significance to the design of HTS transformers.

Exponential reduction in critical current density in Eu1-xCexBa2Cu3O7-δsuperconductors near critical temperature

We have studied how the current-voltage (IV) characteristic below the critical temperature gets affected by electron doping in Eu1-xCexBa2Cu3O7-δ (ECBCO) superconducting systems. The critical current density as a function of T has been extracted in the vicinity but below the critical temperature. Variation of the low critical current density with temperature in the limited range of T has been explained by using an exponential dependence on T. Possibility of the Ambegaokar-Baratoff (AB) to Ginzburg-Landau (GL) crossover has been explored. An exponential nature of the variation in all electron doped superconductors with large angular separation between grain boundaries is attributed to the existence of the vortex fluid state in a narrow range of T.

The Influence of Wire Bending and Wire Diameter on Transport Critical Current Density in Small MgB2 Superconducting Coils for Applications in Multi-Section Coils

This article presents the impact of MgB2 wire bending and diameter on transport critical current density and irreversible magnetic field of a resultant coil. Unreacted MgB2 wires 500 mm in length and 0.63 or 0.83 mm in diameter have been used in the fabrication of small diameter (14 mm) superconducting coils. The coils were subsequently annealed under isostatic pressure of 1 GPa for 15 min at 700 °C and 725 °C. Our results indicate that larger wire diameter, higher annealing temperature, and bending lead to slight reduction of critical current density and irreversible magnetic field in the coil.

A non-collinear double MgO based perpendicular magnetic tunnel junction

Double MgO based magnetic free layers are state-of-the-art solutions for providing high performance perpendicular spin-transfer torque-magnetic random access memory devices. We provide device measurements showing reduction of switching current in perpendicular magnetic tunnel junctions (p-MTJs) using non-collinear ferromagnets on the double MgO template. This structure is engineered by introducing an in-plane ferromagnetic cap, which produces in-plane stray field effects on the free layer. The non-collinear structure delivers ∼53% reduction in critical current density in STT switching without weakening the thermal stability of the devices. The advantages in device performance using non-collinearity in magnetization is sustained down to ∼20 nm MTJs. Micromagnetic simulations suggest inherent differences in the magnetization reversal process between our proposed non-collinear p-MTJ and a well-known double MgO based p-MTJ.

Thick <formula formulatype="inline"><tex Notation="TeX">$\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta}$ </tex></formula> Superconducting Films by Low-Fluorine Metallorganic Solution Deposition

YBa 2 Cu 3 O 7-δ (YBCO) coated conductor has been a promising candidate for high-current transportation material in industry, while the critical current (I c ) becomes a limitation for large-scale application. An efficient way to improve I c is to increase the thickness of YBCO film without much reduction in critical current density (J c ). Metallorganic solution deposition (MOD) is a traditional method to fabricate YBCO films while it takes time as long as 10h during the pyrolysis process. In comparison, Low-Fluorine MOD (LF-MOD) method is applied to reduce the pyrolysis time to less than 2h. In this paper, we successfully obtain YBCO thick films up to 1.7μm by double dip coating process using low-fluorine MOD method. Though wrinkles are frequently observed on the surface of the first layer after pyrolysis at 400 °C for 0.5h, the surface of the final double coated YBCO thick film is homogeneous and cracks are hardly found. Further tests show relatively good c-axis orientation and J c value of about 10 5 A/cm 2 at 77K. With an optimization for the pyrolysis and crystallization processes, better superconducting properties of YBCO thick film are expected.

Reduction of critical current density in a spin valve with a perpendicular polarizer and a planar biaxial free layer

Ultrafast magnetization switching of a spin valve consisting of a perpendicularly magnetized polarizer and an in-plane magnetized free layer with uniaxial and higher order anisotropy is studied. Here we report the analytical results in which a modified asteroid and current–field state diagram are derived from an effective one-dimensional free energy for higher order anisotropy. Because additional crystalline axes provide an additional nutation channel, the critical current density for the biaxial case is smaller than that commonly observed for quadratic anisotropy. This will be of importance for the design of spin-torque-transfer magnetic random access memories and nano-oscillators.

Low current density spin-transfer torque effect assisted by in-plane microwave field

A strategy is presented to greatly reduce both the critical spin polarized current density and the magnetization switching time in elliptical magnetic spin valve. This method is a combination of microwave field and spin polarized current. The numerical simulation at zero temperature shows that the critical current density and the magnetization switching time are sensitive to the frequency and the amplitude of microwave magnetic field. A 75% reduction in critical current density is observed when the frequency of microwave coincides with the natural ferromagnetic resonance frequency of free layer.

Reduction of intrinsic critical current density for current-induced domain wall motion by using a ferrimagnetic nanowire with perpendicular magnetic anisotropy

We herein discuss the use of a ferrimagnetic nanowire for current-induced domain wall motion in order to satisfy both high perpendicular anisotropy and low saturation magnetization. We demonstrate the current-induced domain wall motion in magnetic nanowires with various saturation magnetizations by using micromagnetic and analytical calculations. The intrinsic critical current velocity was reduced with decrease of saturation magnetization even if the domain wall width was almost constant, and the intrinsic critical current velocity also can be reduced with the optimum wire width. A significant reduction of critical current density can be obtained by decreasing the saturation magnetization even if the spin polarization of ferrimagnetic materials is below 0.1 because the decrease in the hard-axis anisotropy is dominant for current-induced wall motion.

Three-dimensional numerical analysis of magnetic and thermal fields during pulsed field magnetization of bulk superconductors with inhomogeneous superconducting properties

Pulsed field magnetization (PFM) is an effective and practical magnetization method for bulk superconductor. When pulsed magnetic field is applied, however, large shielding current is induced in the direction to prevent magnetic fluxes from penetrating into the bulk superconductor. Trapped magnetic flux characteristics are sensitive to this magnetic shielding effect. In addition, inhomogeneity of superconducting properties influences a lot on the PFM characteristics, because the flux motion induced heat generation, the temperature rise, and the reduction of critical current density are strongly interrelated to one another. Such a complicated problem can be analyzed by three-dimensional finite element method. We have studied magnetic flux motion in bulk superconductor with inhomoneneous critical current distribution during PFM. The results indicate that magnetic flux penetration through the lower critical current density region and the sensitivity of this phenomenon to the assumed analysis model.

Reduction in critical current density by tuning damping constants of CoFeB for spin-torque-transfer switching

Different capping layers were deposited on 2.5 nm Co72Fe8B20 to investigate their effects on damping constants and critical current density (JC0) for spin-torque-transfer switching. The damping constant of CoFeB is affected by the interfacial conditions and the spin-pumping effects. The Cu capping layer significantly suppresses intermixing and possesses the lowest damping constant of 0.009. The micromagnetic simulations reveal that the low damping constant of CoFeB may result in the nucleation of domains at lower current density, and thus reduces JC0. A reduction of 27% in JC0 can be achieved by replacing the conventional Ta capping layer with a Cu layer.

Reduction in critical current density for spin torque transfer switching with composite free layer

A composite free layer (CFL) consisting of a soft layer and a hard layer exchange coupled in parallel is proposed. The experimental results showed that the critical current density (Jc) can be reduced from 7.05×106A∕cm2 of strong coupled CoFeB(12.5Å)∕Ru(4Å)∕NiFe(17.5Å) CFL to 2.65×106A∕cm2 of weak coupled CoFeB(12.5Å)∕Ru(15Å)∕NiFe(17.5Å) CFL. The macrospin simulations showed that the soft layer of CFL can assist the hard layer of CFL to switch at a lower Jc. These results suggest that by using CFL, it is possible to reduce the Jc of spin torque transfer switching without compromising the thermal stability.

Reduction of critical current density for domain wall motion in U-shaped magnetic patterns

The critical current density required for current-induced domain wall motion (Jc) was reduced by decreasing the magnetization (Ms) of the free layer film of a U-shaped pattern for a domain wall motion memory. The Ms was decreased by adding the nonmagnetic elements Ta and Cu to NiFe films. The Jc of the U-shaped patterns with NiFe–Ta and –Cu decreased as the proportion of Ta and Cu in the NiFe increased. The relations between Ms and Jc were examined and it was found that Jc decreased monotonically from 1.2×1012to0.8×1012A∕m2 as Ms decreased from 1.0to0.6T regardless of what other materials were used. It is considered that the decrease in Ms caused a decrease in the hard axis anisotropy of the pattern, and the Jc decreased as predicted by a one-dimensional model.

Significant reduction of coercivity without reduction of tunneling magnetoresistance ratio of CoFeB∕MgO∕CoFeB-based magnetic tunnel junction using sandwich-structured free layer

We report the significant reduction of coercivity (Hc) without huge drop of tunneling magnetoresistance ratio (TMR) in the CoFeB∕MgO∕CoFeB-based magnetic tunnel junction by employing sandwich-structured free layer of CoFeB/metallic spacer/NiFe. Strong magnetostatic coupling without physical contact induces magnetic softness from NiFe to CoFeB and avoids considerable reduction of TMR. This achievement satisfies the requirements given to the magnetic read head in hard disk drive and the spin torque transfer magnetic random access memory for the application to the higher areal density and the reduction of critical current density for current-induced magnetization switching, respectively.

Reduction in critical current density of current-induced magnetization switching

We have investigated the current-induced magnetization switching in an exchange-biased spin valve structure. By using an unpatterned antiferromagnetic layer to pin the fixed Co layer, we obtained a lower switching current density by a factor of 5 than a simple spin valve structure. For the application, it is important to know how to keep the spin polarization when the thicker layer is pinned by an antiferromagnet. The unpatterned pinned ferromagnetic lead can be a good solution for spin-transfer-torque-activated device. The effect of Cu buffer layer on the top of the thin Co and Ru buffer layer under the thick Co layer on the current-induced magnetization switching in cobalt-based trilayer spin valves was also investigated. The experimental results showed that the Ru buffer layer in combination with Cu buffer layer could induce a decrease in the critical switching current by 30%, and an increase in the absolute resistance change by 35%, which is caused by an improvement of a microstructure of a thicker Co polarizer.

Phase formation and thermal treatment optimisation in uranium compound doped (Bi,Pb) 2Sr 2Ca 2Cu 3O x/silver superconducting tapes

A combination of uranium doping with thermal neutron irradiation has been well demonstrated to be one of the most effective means to introduce pinning centres in Bi-2223/Ag tapes. A substantial improvement in flux pinning and reduction of anisotropy in uranium-doped Bi-2223/Ag tapes has been achieved due to thermal neutron irradiation. However, the radioactivity of silver sheath is too high for practical application of this technique. In the current study, a number of U-containing compounds, U 3O 7, UCa 2O 5, and UCa 1.5Sr 1.5O 6 have been used as dopants. The compatibility and interaction between U-compound and Bi-2223 matrix was systematically studied with various thermal processing conditions. UCa 1.5Sr 1.5O 6 shows a remarkable compatibility with Bi-2223 phase, with critical current density reduced by about 15% at a doping level of 1.1 at.%, compared with the 85% reduction in critical current density at the same doping level for pure U-oxide doping. XRD results show that UCa 1.5Sr 1.5O 6 has no effect on Bi-2223 formation while U 3O 7 caused degradation of Bi-2223 phase up to 50% at doping level of 1.1 at.%. The U-compound doping also widens the processing window to 15°C.

Effect of twin planes in the magnetization hysteresis loops ofNdBa2Cu3O7single crystals

The angular dependence of the critical currents is investigated in a moderately twinned ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (NdBCO) single crystal in the temperature range $30<~T<~90\mathrm{K}.$ In intermediate fields around the fishtail peak a reduction of critical current density due to twin structure is observed up to 90 K. This effect implies that vortices retain their three-dimensional character practically up to ${T}_{c}.$ The effect of twin planes disappeared at external fields declined more than $\ensuremath{\theta}\ensuremath{\approx}15\ifmmode^\circ\else\textdegree\fi{}$ from the $c$ axis (twin planes). In this case, the hysteresis loops exhibited a clear fishtail shape. The intermediate peak observed previously in NdBCO single crystal at $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}$ is identified with the low-field limit of the twin plane activity.

Effect of BTA on electrochemical corrosion and stress corrosion cracking behaviour of type 304 stainless steel in 1 M HCl

Effect of benzotriazole (BTA) on polarization and stress corrosion cracking (SCC) behaviour of type 304 stainless steel in 1 M HCl was investigated. The anodic polarization curves showed that with BTA additions the anodic polarization kinetics in the active region was not affected, though a reduction in critical current density, icrit, and passive current density, ip, was observed. However, BTA was found to influence significantly the cathodic reaction kinetics. SCC results using smooth tensile test specimens showed an increase in time-to-failure, tf, with BTA additions. Crack growth rate studies using single-edge notched (SEN) specimens showed an increase in threshold stress intensity for SCC, KISCC, and a decrease in crack growth rate, da/dt, with BTA additions. While the adsorption isotherms derived from weight loss data followed a Langmuir adsorption isotherm signifying a monolayer adsorption, the adsorption isotherms derived from SCC test data deviated from this at higher BTA concentrations. The paper discusses the possible reason for this deviation.

YBa2Cu3O7/noble metal composite thin films for applications in fluxonic and flux-flow devices

This research involves the study of thin films for use in three-terminal, non-linear two-terminal, and photodetector superconducting devices. The morphological and electrical properties of composite films fabricated from YBCO and noble metals have been investigated for use in these devices. Films of Ag and Au, when heated to YBCO deposition temperatures, coalesce into small, approximately micron-sized islands. We take advantage of this property to form a composite film with distinct YBCO and noble metal regions by depositing a layer of YBCO on the array of noble metal islands. With certain compositions the films exhibit reduction of critical current density by a factor of 65 over YBCO and show no significant degradation of critical temperature. Also, the current-voltage characteristics of the composites are sensitive to small, applied magnetic fields. These properties make the YBCO/noble metal composites potentially well suited for the fabrication of devices whose transport is dependent upon vortex motion. Photoresponse measurements taken on a 1 mm × 1 mm YBCO/Au bridge irradiated by red laser light showed a possible non-bolometric response below the composite superconducting transition temperature.

GENERIC FIELD PROBLEMS IN DESIGN STUDIES OF HIGH TEMPERATURE SUPERCONDUCTING TRANSFORMERS

A feasibility study for a large power transformer reveals several key design issues, each of which is expressible in terms of simple, fundamental field effects. Six generic effects are briefly reviewed and their importance explained. In all cases but one their influence can be estimated, to an accuracy adequate for feasibility purposes, without computation or detailed analysis. They are: mechanical stress due to magnetic fields; electrical breakdown caused by winding capacitance distribution; dielectric breakdown caused by the mixing of different materials, including liquid nitrogen; reduction of critical current density due to internal magnetic fields; the need, or otherwise, for conductor transposition; modelling of field penetration and ac losses in HTc superconductors.

Transport characteristics of Tl2Ba2CaCu2O8 bicrystal grain boundary junctions at 77 K

We report the fabrication and characterization of thin-film grain boundary junctions in Tl-based superconductors on bicrystal substrates. Post-deposition processed Tl2Ba2CaCu2O8 films were grown on bicrystal SrTiO3 substrates, with varying degrees of misorientation angle θ. Critical current densities and current-voltage curves had a strong dependence on θ. For θ≥10°, there was a two-to-three order of magnitude reduction in critical current density, and for θ≥20°, the current-voltage curves displayed resistively shunted junction behavior. These high angle grain boundary junctions have features at 77 K that are attractive for device applications, including sharp voltage onsets, well-behaved dc magnetic and rf field dependence, IcRn products as large as 300 μV, and low 1/f noise. Simple dc superconducting quantum interference devices fabricated with these junctions exhibited transfer functions of up to 30 μV/Φ0 at 77 K.