Irreversible Magnetic Field Research Articles

Superconducting thin films of [formula omitted][formula omitted][formula omitted][formula omitted] with zero-resistance transition temperature close to 100 K

High superconducting transition temperature is favorable for the applications of superconductors. Some cuprate superconductors have the transition temperatures above 100 K, such as the Hg- or Tl-based 1223 and 1234 phases, but many of them contain the toxic elements, like Hg and Tl. Meanwhile, the anisotropy of upper critical field or the effective mass of above mentioned Hg-, Tl-based systems, or the non-toxic Bi2Sr2Ca2Cu3O10 with Tc = 110 K is high, which makes the vortices easy to move and the irreversibility magnetic field is very low in the liquid nitrogen temperature region. Here we report the successful synthesis of the c-axis oriented (Cu,C)Ba2Ca2Cu3O9±δ superconducting thin film with the zero-resistance transition temperature reaching 99.7 K. The superconducting transitions are rather sharp as revealed by both resistivity and magnetization measurements. Temperature dependent resistivity has been measured under different magnetic fields, and the irreversibility lines have been achieved. The resistivity was also measured with the magnetic field rotated in the ac-plane, and the data can be nicely scaled by using the anisotropic Ginzburg-Landau model, yielding a temperature dependent anisotropy which varies from 17 at 110 K to 4 at 77 K. Additionally, the critical current density calculated from the magnetization-hysteresis-loops reaches about 6×105A/cm2 (zero field) at 77 K. Thus the film may be a good candidate for the applications of superconducting cables or high frequency superconducting filters in liquid nitrogen temperature region.

Enhancement of the critical current by surface irregularities in Fe-based superconductors

The critical current Ic of single crystals of the iron pnictide superconductor BaFe2(As 1−x P x )2 has been studied through measurements of magnetic hysteresis cycles. We show that the introduction of micrometer-scale irregularities on the surface significantly increases Ic , primarily near the irreversibility magnetic field Hirr . The observed increase can be attributed to a non-dissipative surface current that arises from the collective bending of the vortex lattice at the sample surface, enabled by the surface irregularities. This mechanism, which is not pinning in the proper sense, has previously been studied in clean, low- Tc , metallic superconductors, but had not been investigated in Fe-based superconductors. The observed increase in Ic is consistent with a theoretical estimate based on the Mathieu-Simon continuum theory of the vortex state.

Advances in (Cu,C)Ba2Ca3Cu4O11+δ Superconductors with High Critical Transition Temperature and High Irreversibility Magnetic Field

Advances in (Cu,C)Ba2Ca3Cu4O11+δ Superconductors with High Critical Transition Temperature and High Irreversibility Magnetic Field

A Quenched Disorder in the Quantum-Critical Superconductor CeCoIn5.

Emergent inhomogeneous electronic phases in metallic quantum systems are crucial for understanding high-Tc superconductivity and other novel quantum states. In particular, spin droplets introduced by nonmagnetic dopants in quantum-critical superconductors (QCSs) can lead to a novel magnetic state in superconducting phases. However, the role of disorders caused by nonmagnetic dopants in quantum-critical regimes and their precise relation with superconductivity remain unclear. Here, the systematic evolution of a strong correlation between superconductive intertwined electronic phases and antiferromagnetism in Cd-doped CeCoIn5 is presented by measuring current-voltage characteristics under an external pressure. In the low-pressure coexisting regime where antiferromagnetic (AFM) and superconducting (SC) orders coexist, the critical current (Ic ) is gradually suppressed by the increasing magnetic field, as in conventional type-II superconductors. At pressures higher than the critical pressure where the AFM order disappears, Ic remarkably shows a sudden spike near the irreversible magnetic field. In addition, at high pressures far from the critical pressure point, the peak effect is not suppressed, but remains robust over the whole superconducting region. These results indicate that magnetic islands are protected around dopant sites despite being suppressed by the increasingly correlated effects under pressure, providing a new perspective on the role of quenched disorders in QCSs.

Effect of Low Annealing Temperature on the Critical-Current Density of 2% C-Doped MgB2 Wires Used in Superconducting Coils with the Wind-and-React (W&R) Method-High-Field and High-Temperature Pinning Centers.

The use of a low annealing temperature during the production of coils made from superconducting materials is very important because it reduces the production costs. In this study, the morphology, transport critical-current density (Jc), irreversible magnetic field (Birr), and critical temperature (Tc) of straight wires and small 2% C-doped MgB2 coils were investigated. The coils were made using the wind-and-react (W&R) method and annealed at various temperatures from 610 °C to 650 °C for 2-12 h. Critical-current measurements were made for both the coils and straight wires at the temperatures of 4.2 K, 20 K, 25 K, and 30 K. During our research study, we determined the process window that provides the best critical parameters of the coils (annealing at a temperature of 650 °C for 6 h). Moreover, we observed that small coils made with unreacted MgB2 wire and then annealed had morphology and critical parameters similar to those of straight 2% C-doped MgB2 wires. Moreover, small-diameter bending of 20 mm and 10 mm did not lead to transverse cracks, which can cause a large reduction in Jc in the coils. This indicates that the processes of optimization of thermal treatment parameters can be carried out on straight MgB2 wires for MgB2 superconducting coils.

Impact of rare earth oxides metal (Dy, Tb, Er, Eu, Sm) admixtures on the structure, morphology, pinning centers and critical parameters of MgB2 materials manufactured by ambient pressure sintering and hot isostatic pressure processes

In our paper, we present the results of the structural, morphological and critical parameters of MgB2 cylindrical materials doped with Dy2O3, Tb2O3, Er2O3, Eu2O3 and Sm2O3. The materials were annealed under low and high isostatic pressures at 800 °C for 1 h in an argon atmosphere. Our results show that the HIP process significantly increases the density of MgB2 materials modified by the RE2O3 additive from 1.05 to 2.4 g/cm3. Scanning electron microscope (SEM) images indicate that the HIP process removes voids from the MgB2 material. The results indicate that the HIP process slightly affects the formation of REB4 and REB6 and does not affect the process of RE substitution for Mg. Further SEM results show that the morphologies of samples annealed at low isostatic pressure with Dy2O3, Tb2O3 and Er2O3 doping are completely different from those of samples with Eu2O3 and Sm2O3 doping. The magnetic measurements indicate that RE2O3 doping and annealing under low isostatic pressure slightly increases the critical temperature (Tc) and decreases the irreversible magnetic field (Birr). On the other hand, thermal treatment under high isostatic pressure with an admixture of RE2O3 lead to a greater increase in Tc and a decrease in Birr. Further magnetic measurements show that RE2O3 doping and annealing under low isostatic pressure mainly increase the magnetic critical current density (Jmc) at 4.2 K–30 K in low magnetic fields. Additional results indicate that high isostatic-pressure annealing of the RE2O3-doped MgB2 material significantly increases Jmc in low magnetic fields and decreases Jmc in high magnetic fields. Our results indicate that the Eu2O3 admixture most effectively increases Tc and Jmc and creates the large amount of the REB6 phase. It is as the only admixture that increases Birr after the HIP process and yields the smallest MgB2 grains.

Influence of bending and heat treatment under high isostatic pressure on synthesis reaction and transport critical current density in isotopic IMD Mg11B2 'wind and react' coils

We show for the first time the results for small coils, which were wound with unreacted magnesium diboride (MgB2) wires with nano-amorphous isotopic boron (11B) by using combined internal magnesium (Mg) diffusion (IMD) and hot isostatic pressing (HIP) techniques. The small coils with a diameter of 20 mm or 10 mm were annealed under low (0.1 MPa) and high (1.1 GPa) isostatic pressure. The 10 mm coils annealed under high isostatic pressure of 1.1 GPa had significantly increased critical temperature (Tc), irreversible magnetic field (Birr) and transport critical current density (Jtc), with significantly accelerated Mg diffusion and improved homogeneity of the Mg11B2 material. Moreover, our findings show for the first time that the bending of the unreacted IMD Mg11B2 wires significantly affects the synthesis reaction under the low and high isostatic pressure. Our findings also show for the first time that diffusion of Mg during heat treatment under high isostatic pressure is completely different than during heating under low isostatic pressure. Mg diffusion under low isostatic pressure annealing is mainly dependent on the heat treatment temperature, bending diameter and 11B layer density. This Mg diffusion is related to the simultaneous formation of the superconducting phase. However, the diffusion of Mg under high isostatic pressure heat treatment is mainly dependent on the isostatic pressure, 11B layer density and Mg state (liquid or solid). Our results indicate that liquid Mg is first pushed into the 11B layer by high isostatic pressure and then a superconducting phase is formed.

Interrelations among zero-field critical current density, irreversibility field and pseudogap in hole doped high-Tc cuprates

Critical current density and the irreversibility magnetic fields are two parameters which control most of the applications of high-temperature superconductors. In this brief communications we have investigated the intrinsic effects that determine the magnitudes of these two parameters. The roles of hole concentration in the CuO2 plane (p) and oxygen deficiency (δ) in the CuO1-δ chains on the zero-field low-temperature critical current density, Jc0, were studied for a series of Y1-xCaxBa2Cu3O7-δ (x = 0.00, 0.05, 0.10, and 0.20) thin films. Jc0 above the optimum hole concentration were found to be high. The magnitude of Jc0 were largely determined by the hole concentration, maximizing at p ∼ 0.185 ± 0.005, regardless of the values of x and δ. This implied that oxygen deficiency/disorder plays a secondary role and the intrinsic Jc0 is governed largely by the number of doped holes in the CuO2 planes of Y1-xCaxBa2Cu3O7-δ. Further support in favor of this finding was garnered from the analysis of the in-plane resistive transitions of thin films of YBa2Cu3O7-δ (YBCO) with magnetic fields (H) applied along the crystallographic c-direction. The characteristic magnetic field (H0), linked to the vortex activation energy and the irreversibility field, exhibits similar p-dependence as shown by Jc0(p). We have explained these observations in terms of the doping dependent pseudogap (PG) energy/temperature scale in the low-energy electronic energy density of states observed in high-Tc cuprates. Both the intrinsic critical current density and the irreversibility field depend directly on the superconducting condensation energy, which in turn is primarily determined by the magnitude of the hole concentration dependent PG in the quasiparticle spectral density.

Review of progress and challenges of key mechanical issues in high-field superconducting magnets.

The development of modern science and technology requires high magnetic fields exceeding 25T. Second-generation high-temperature superconducting wires, i.e. REBCO (REBa2Cu3O7-x, RE refers to Y, Gd, Dy, Eu and other rare-earth elements) coated conductors (CCs), have become the first choice for high-field magnet construction because of their high irreversible magnetic field. The mechanical stresses caused by manufacturing, thermal mismatch and Lorenz forces closely influence electromagnetic performance during operation for REBCO CCs. In addition, the recently studied screen currents have effects on the mechanical characteristics of high-field REBCO magnets. In this review, the experimental and main theoretical works on critical current degradation, delamination and fatigue, and shear investigations on REBCO CCs, are reviewed at first. Then, research progress on the screening-current effect in the development of high-field superconducting magnets is introduced. Finally, the key mechanical problems facing the future development of high-field magnets based on REBCO CCs are prospected.

Effects of Thallium fluoride substitution on the flux pinning energies of (Cu0.5, Tl0.5)Ba2Ca2Cu3O10-δ superconductors

In the present work, the magnetoresistance behavior of (TlF)x-substituted (Cu0.5,Tl0.5)Ba2Ca2Cu3O10−δ superconducting samples, with x = 0.0, 0.1, 0.2, 0.3 and 0.4, was investigated. According to the thermally activated flux creep (TAFC) model, the flux pinning energies U(H, T) were calculated from the resistivity temperature (ρ–T) curves at applied DC magnetic fields ranging from 0.29 to 4.44 kOe. The results indicated that flux pinning energy was increased with increasing the F-substituting up to x = 0.1 and then decreased with increasing the applied field up to 4.44 kOe. Moreover, significant resistive broadening (ΔT) was observed with increasing the values of the applied magnetic fields. Furthermore, the (TlF)x substitution, boosting the transport critical current density Jc(0) and irreversibility magnetic field Hirr(0) up to x = 0.1 and then decreasing for x > 0.1 at various applied magnetic fields, demonstrates strong flux pinning for x = 0.1.

Synthesis of Three-Dimensional Carbon Nanosheets and Its Flux Pinning Mechanisms in C-Doped MgB2 Superconductors.

Three-dimensional carbon nanosheets (3D-CNS) were synthesized by salt template spray-drying method in order to solve the agglomeration of 2D nanocarbon by a traditional mixing method. MgB2 bulks doped with 3D-CNS with molar ratio composition of MgB2-x(3D-CNS)x (x = 0, 0.1 and 0.2) have been prepared by in situ sintering process. The microstructure, critical current density and flux pinning of the sintered samples have been investigated. Differing from the structure in previous studies, the 3D-CNS doping is more efficient for the refinement of the MgB2 grains due to the 3D network structures. The results clearly show that more active C releasing from 3D-CNS at high temperature can provide effective flux pinning centers by the substitution of C for B in MgB2 lattice. Furthermore, the lattice distortion and increased grain boundaries should be responsible for the enhancement of critical current density (Jc) at high magnetic fields as well as the increased irreversible magnetic field (Hirr). However, the positive action in Jc at low field has been extremely offset by the concentration of impurities at MgB2 grain boundaries such as released extra C without substitution and MgO, which is considered to further deteriorate the grain connectivity.

The significant influence of packing density of unreacted Mg+2B mixture and heat treatment conditions on some of critical parameters for MgB2/Fe wires

Our research show that the higher packing density of the unreacted material (Mg +2B) by 8%, especially for Mg in the solid state, allows to eliminate the large number of large voids (20 µm), improves the homogeneity and density of the MgB2 superconducting material, allows to obtain a greater number and longer lengths of connections between MgB2 grains, eliminates the formation of the intermetallic phase (iron borides), allows to obtain MgB2 grains of a uniform size and shape, increase irreversible magnetic field (Birr), critical temperature (Tc) and upper magnetic field (Bc2). Research show that spherical grains significantly reduce the number of inter-grain connections in the material with low and high density of unreacted material and allows to obtain the more superconducting phase. Research shows that MgB2 grains with a plate-like and rectangular shape allow to obtain more connections between the grains. Studies show that thermal treatment under high isostatic pressure does not allow to obtain the large amount of superconducting phase for the unreacted material of low density. Moreover, studies show that the high packing density of unreacted material and thermal treatment under high isostatic pressures (0.8 GPa and 1.1 GPa) allow to obtain a large amount of superconducting phase even for Mg in the solid state. Additionally, studies points that heat treatment under medium isostatic pressure (0.3 GPa) significantly reduce Birr and Bc2. Our results are important for long superconducting wires made by using the powder-in-tube (PIT) technique. Because they indicate that the higher packing density of the unreacted material (Mg +2B) will allow to obtain a the superconducting material with greater homogeneity and density, and improve the critical parameters e.g. coils.

Influence of Amorphous Boron Grain Size, High Isostatic Pressure, Annealing Temperature, and Filling Density of Unreacted Material on Structure, Critical Parameters, N-Value, and Engineering Critical Current Density in Mgb2 Wires.

Our results show that a lower density of unreacted Mg + B material during an Mg solid-state synthesis reaction leads to a significant reduction in the quantity of the superconducting phase and lowers the homogeneity of the superconducting material. It also significantly reduces the irreversible magnetic field (Birr), critical temperature (Tc), upper magnetic field (Bc2), engineered critical current density (Jec), and n-value, despite high isostatic pressure (HIP) treatment and the use of nanoboron in the sample. Our measurements show that samples with large boron grains with an 8% higher density of unreacted Mg + B material allow better critical parameters to be achieved. Studies have shown that the density of unreacted material has little effect on Birr, Tc, Bc2, Jec, and the n-value for an Mg liquid-state synthesis reaction. The results show that the critical parameters during an Mg liquid-state synthesis reaction depend mainly on grain size. Nanoboron grains allow for the highest Birr, Tc, Bc2, Jec, and n-values. Scanning electron microscopy (SEM) images taken from the longitudinal sections of the wires show that the samples annealed under low isostatic pressure have a highly heterogeneous structure. High isostatic pressure heat treatment greatly improves the homogeneity of MgB2.

Superior engineering critical current density obtained via hot isostatic pressing of MgB2 wires manufactured using nano-amorphous isotopic boron

The effects of heat treatment temperature, time and isostatic pressure on the grain connectivity and superconducting properties of Mg11B2 wires prepared using nano-amorphous isotopic boron powder (11B) have been investigated. The article presents detailed studies of the Mg11B2 material structure of the wires using scanning electron microscope (SEM). Heat treatment at low and high temperature for different annealing time and isostatic pressure does not create an internal macro-defect that disrupts connectivity in the Mg11B2 superconducting wires. Our study shows that dense longitudinal superconducting layers are formed at high annealing temperature of 800 °C for 60 min. Formation of dense longitudinal superconducting layers under high isostatic pressure (HIP) allow improved grain connectivity and high density of pinning centers and as a result, high engineering critical current density, Jec, at 4.2 K has been achieved. Also, high upper critical field (Bc2) and high irreversible magnetic field (Birr) were obtained using HIP process while superconducting transition temperature (Tc) was not significantly affected. Better grain connectivity obtained using high annealing temperature and long annealing time under low isotopic pressure leads to Jec enhancement at high magnetic fields at 20 K. Our results show that the annealing temperature of 800 °C for 60 min under isostatic pressure of 1 GPa yields the required Jec of 25 A/mm2 and 50 A/mm2 for ITER applications in magnetic fields of 8 T and 7 T, respectively. The promising results obtained in this work could pave the way for designing low activation superconducting Mg11B2 wires to be considered for next generation fusion magnets.

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (Jc) of MgB2 thin films via 140-keV Co-ion irradiation. The Jc(H) of the surface-damaged MgB2 films was remarkably improved in comparison with that of pristine films. The strong enhancement of Jc(H) caused by a surface damage in MgB2 films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (Hirr) at 5 K for surface-damaged MgB2 films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve Jc(H) in a MgB2 superconductor.

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.

Second magnetization peak effect and the vortex phase diagram of V0.0015NbSe2 single crystal

Effect of the weak point disorder on vortex matter phase diagram is studied by incorporation of V atoms through magnetic and magnetoresistance measurement in layered NbSe2 single crystal. We observe that the point disorder introduces fishtail effect and the second magnetization peak (SMP) in the M-H curve of V0.0015NbSe2 at magnetic field far below upper critical field (Hc2). 3D Collective creep of elastic flux line lattice (FLL) support the vortex motion below the onset of SMP. However, the crossover from collective to plastic deformation of FLL support the occurrence of SMP effect. Presence of the peak effect in pinning force density near irreversible magnetic field (Hirr) indicates the 2D to 3D distortion of FLL due to lattice softening associated with the rapid reduction of elastic modulus. Magneto transport measurement shows the glassy transition near the zero resistance region. In the glassy region, the flux line shows 2D characteristics. Above zero resistive region, thermally activated flux flow region is well described by Arrhenius relation. It is seen that the activation energy is decreased significantly by the incorporation of V atoms in NbSe2. The magnetic field dependence of activation energy follows a power law of U0(H) ~ H-α where the exponent α changed from 0.5 to a 0.9 at a crossover field of Hcr = ~ 0.7 T, indicating the transition from weak plastic deformation of FLL to the strong entangled state in vortex liquid phase. Finally a vortex phase diagram is constructed demonstrating all the different phases of vortex lattice of V0.0015NbSe2 at different magnetic fields and temperatures.

Influence of the lamella structure and high isostatic pressure on the critical current density in in situ MgB2 wires without a barrier

In this article, the significant impact of a lamellar (layered) structure and a high isostatic pressure on the normal state resistance (Rn), critical temperature (Tc), irreversible magnetic field (Birr), upper magnetic field (Bc2) and critical current densities (Jc) at 4.2 K and 20 K was presented. Our research showed that annealing at temperatures in the range of 630 °C–680 °C (above the melting point of Mg) at atmospheric pressure (0.1 MPa) did not create a lamellar (layered) structure. This led to low Tc, Jc and Birr and high Rn. The analysis, made by using scanning electron microscopy (SEM), showed that the annealing temperature increased up to 700 °C under a pressure of 0.1 MPa, which created a lamellar structure. This led to significant growth of Tc, Jc and Birr and a slight increase of Rn. Moreover, the measurements showed that annealing at temperatures from 630 °C to 700 °C did not change the Bc2 value. In comparison to pressureless heat treatment, annealing under the high isostatic pressure of 1.1 GPa obtained a lamellar structure with layers of lower thickness and higher density. This led to significant increases in Jc and Birr and a visible reduction of Rn. SEM analysis showed that the increase of isostatic pressure up to 0.3 GPa created a lamellar structure with thicker layers and lower density. This microstructure led to lower Jc and Birr and significantly higher Rn. On the other hand, the SEM analysis showed that annealing under 0.8 GPa did not cause the formation of a layered structure, and as a result, it led to significant reductions in Birr and Jc (4.2 K and 20 K) and higher Rn. The increase of the isostatic pressure from 0.1 MPa to 1.1 GPa did not affect Tc (B = 0 T) and Bc2. The results indicated that the layered structure obtained a high density of pinning centers, which were particularly effective at higher magnetic fields. Jc of 100 A/mm2 in 8 T at 4.2 K was obtained in in situ undoped MgB2 wires after annealing at 700 °C for 40 min under an isostatic pressure of 1.1 GPa.

A Comparative Study of Ge-Based Organometallic Additions to MgB2

Four Ge-based organometallic (OM = C 6 H 10 Ge 2 O 7 ) polymers with similar chemical composition were used as additions for MgB 2 . MgB 2 (OM) 0.0014 dense samples with a relative density higher than 97% were obtained by ex situ spark plasma sintering. The critical current density and the irreversibility magnetic field of the added samples are slightly improved at high magnetic fields by repagermanium (Alfa Aesar) addition when compared to the pristine reference sample. Addition of Ge-straight-chain polymer shows some improvement of the maximum volume pinning force, Fp max . There is no significant Ge substitution in the crystal lattice of MgB 2 , while C substitutes for B. Pinning mechanism is mainly of a point pinning type, with the grain boundary pinning mechanism strengthening at lower temperatures and when additives are not used. A relatively high amount of carbon in the samples washes out the Ge effects on pinning-force-related parameters. The structure and morphology of the polymer additive play an important role in dispersion when mixing, impacting the quality of the superconducting properties. The nanosize dimension of the pristine MgB 2 powder shows also some drawbacks in mixing and sintering.

Critical Properties of Bulk-Doped BaFe2As2 Pnictides for Magnet Design

A comprehensive study of upper critical ( Hc 2) and irreversibility magnetic fields ( H irr) in (Ba0.6K0.4)Fe2As2 (critical temperature $T_{c} = \text{38.3 K}$ ), Ba(Fe 0.95Ni0.05)2As2 ( Tc = 19.2 K), Ba(Fe0.94 Ni0.06)2As2 ( Tc = 18.5 K), Ba(Fe0.92 Co0.08)2As2 ( Tc = 23.2 K), and Ba(Fe0.91 Co0.09)2As2 ( Tc = 25.3 K) polycrystalline bulk pnictide superconductors with different average grain sizes was made in pulsed fields at the Los Alamos National Laboratory. The magnetic field–temperature ( Hc 2– T ) phase diagrams with Hc 2 as high as 65 T at 28 K for the K-doped samples and critical current density $(J_{c})$ measurements as high as 105 A/cm2 for the smallest, submicrometer grain size samples were obtained. The high Hc 2, H irr, and $J_{c}$ data show the suitability of these materials for magnet design as their mechanical strength and random grain alignment show promise in the manufacturing of next-generation magnets.