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We report on measurements of the ac losses of YBa2Cu3O7−x conductors on technical substrates. The samples are taken from conductors 1 m long produced with an ion beam assisted deposition step in the manufacturing process yielding 1 cm wide conductors with critical currents approaching 100 A at 77.4 K. The losses due to combinations of ac transport currents and ac magnetic fields were measured calorimetrically and the results compared to theoretical estimates. While the results of our measurements are similar to the theoretical estimates, there are some important differences in the qualitative behavior.

The effects of proton-induced linear defects on the dissipationless flow of current in multifilamentary Ag-sheathed tapes of (Bi,Pb)2Sr2Ca2Cu3O10 superconductors in the temperature range of 55–77 K are reported. Randomly oriented columnar defects of estimated areal density 5×1010/cm2 produced by 800 MeV proton-induced fission fragments provide substantial enhancement of critical currents of these tapes. Furthermore, these defects are more isotropic flux pinning centers than high-energy heavy ion irradiation-induced columnar defects. However, there is a tradeoff between enhanced pinning and the deleterious effects of radiation damage at higher proton fluences. It is shown that some of the global damage caused by the products of fission can be repaired by a suitable heat treatment of the samples.

Enhanced in-field critical current densities (Jc’s) have been obtained in epitaxial superconducting (RE1,RE2)Ba2Cu3O7−x (RE1=rare‐earthion 1, and RE2=rare earth ion 2) films grown on both single crystal and buffered metallic substrates. For a constant average RE ionic radius (equal to that of yttrium), there is a systematic dependence of the in-field Jc on the RE ion size variance, with a small, but nonzero, variance being optimum. Compared to the standard YBa2Cu3O7−x composition, a factor of two improvement in Jc(75.5K) is reproducibly observed at 0.2T(‖c) for the composition Dy1∕3Ho2∕3Ba2Cu3O7−x on both single crystal and buffered-metallic substrates. Angular dependent magnetic field studies and transmission electron microscopy indicate the presence of additional pointlike random defects.

We present a comparative study of the angular dependent critical current density in YBa2Cu3O7 films deposited on IBAD MgO and on single crystal MgO and SrTiO3 substrates. We identify three angular regimes where pinning is dominated by different types of correlated and uncorrelated defects. We show that those regimes are present in all cases, indicating that the pinning mechanisms are the same, but their extension and characteristics are sample dependent, reflecting the quantitative differences in texture and defect density. In particular, the more defective nature of the films on IBAD turns into an advantage as it results in stronger vortex pinning, demonstrating that the critical current density of the films on single crystals is not an upper limit for the performance of the IBAD coated conductors.

We report on the structural and transport properties of EuBa2Cu3O7−x (Eu123) films on SrTiO3 substrates. A substrate temperature 100 °C higher than that used to deposit YBa2Cu3O7−x (Y123) is needed to grow high-performance Eu123 films directly on SrTiO3 substrates. However, this high-temperature restriction can be circumvented by inserting a seed layer (∼12 nm) of Y123, Gd123, or Dy123 between the substrate and the Eu123. A structural analysis reveals that a transition region that promotes the growth of the a-axis is first formed on the surface of the substrate if the Eu123 is deposited directly on SrTiO3 at a temperature of less than 800 °C. On the other hand, the use of a seed layer of another superconducting material restructures the Eu123 to the right phase without forming this transition region. Eu123 films with critical temperatures of 94 K and critical current densities of 5.8×106 A/cm2 at 75.5 K have been deposited under standard deposition conditions using such an approach.

Flux pinning in high-temperature superconductors such as YBa2Cu3O7−x (YBCO) in the past has been accomplished by pinning the vortex cores. We demonstrate magnetic-domain-induced flux pinning of the magnetic flux of vortices in a ferromagnet-superconductor bilayer consisting of CoPt grown on YBCO, where the ferromagnet has uniaxial perpendicular magnetic anisotropy and a random domain structure. We observe an improvement of the critical current due to magnetic pinning at temperatures close to the transition temperature.

We argue that superconductor/ferromagnet multilayers of nanoscale period should exhibit strong pinning of vortices by the magnetic domain structure in magnetic fields below the coercive field when ferromagnetic layers exhibit strong perpendicular magnetic anisotropy. The estimated maximum magnetic pinning energy for single vortex in such a system is about 100 times larger than the pinning energy by columnar defects. This pinning energy may provide critical currents as high as 106−107 A/cm2 at high temperatures (but not very close to Tc) at least in magnetic fields below 0.1 T.

Recent world‐wide progress in the effort to develop long continuous lengths of conductor based on high‐Tc materials with high critical currents in substantial magnetic fields offers promise for a variety of applications. Two major problems, weak links and weak flux pinning, have plagued conductor development efforts from the beginning. Recent advances in mechanical processing of tape conductors based on BSCCO show great promise in overcoming weak links, with low‐temperature Jc values≳105 A/cm2 extending to magnetic fields≳25 T. Unfortunately, these materials continue to exhibit weak flux pinning at moderate temperatures for magnetic fields aligned along the crystalline c axis. This manifests itself in a Jc that declines precipitously with magnetic field for T≳30 K. By contrast, conductors based on YBCO exhibit much stronger pinning at high temperature with smaller sensitivity to magnetic field orientation, but remain severely limited by weak links. The poor performance of BSCCO‐ and TBCCO‐based materials ...

We have studied the microwave properties of 0.4 μm thick YBa2Cu3O7−x (YBCO) films on polycrystalline substrates with ion-beam-assisted-deposited yttria-stabilized zirconia buffer layers using a parallel-plate resonator technique at 10 GHz. The YBCO films with similar in-plane texture grown on both forsterite and Ni-based alloy substrates show similar microwave properties. We measure low-power surface resistance Rs values of about 0.5 mΩ at 76 K and 0.15 mΩ at 4 K for films with an in-plane mosaic spread of about 7°. Single-tone power-dependence measurements show that the surface resistance and the surface reactance increase linearly and by the same amount with increasing microwave field level. At intermediate power levels, the intermodulation measurements show odd-order intermodulation products that increase quadratically with two-tone input power. These results indicate a hysteretic vortex penetration mechanism in the weak links as the most plausible source of the observed nonlinearities in these films.

We show that the in‐plane critical current density of heavily twinned YBa2Cu3O7−δ thick films is anisotropic, depending on the relative orientation between the transport electrical current and the twin boundary mosaic. We demonstrate that the critical current density in an applied magnetic field can be substantially increased by producing conductors with twin boundaries tilted 45° with respect to the electrical current flow. We show that these findings can be readily incorporated to improve the critical currents of biaxially textured YBa2Cu3O7−δ thick films deposited on metallic substrates.

We report superconducting and mechanical properties of YBa2Cu3O7−δ (YBCO) thick films on Ni‐based alloys with a textured yttria‐stabilized zirconia (YSZ) buffer layer. The YBCO and YSZ layers were deposited by pulsed laser deposition and ion beam assisted deposition, respectively. It was found that the transport critical current density (Jc) correlates very well with the YBCO mosaic spread. Jc over 1×10 6 A/cm2 at 75 K and ∼1×107 A/cm2 at 4 K were obtained in the 1‐μm thick YBCO films. Zero field critical current of 120 amps at 75 K was obtained in a 2‐μm‐thick and 1‐cm‐wide YBCO film. Angular dependence measurement revealed Jc peaks for both H∥c and H∥a‐b. The peak for H∥c implies additional pinning due to defects such as small angle grain boundaries or twin boundaries. Bending tests at 75 K showed that the YBCO thick films on the metallic substrates could sustain a strain of 0.4% and over 1% for tension and compression, respectively.

The transport critical current density Jc of oxide‐powder‐in‐tube mono‐ and multifilamentary Bi‐2223/Ag tapes has been determined before and after irradiation by 0.8 GeV protons at fluences up to 7.0×1016 protons/cm2. Proton‐induced fission of the Bi nuclei produced up to 8.6×1013 fissions/cm3, creating long tracks at densities equivalent to matching fields up to 1.1 T. Relative to unirradiated tapes, Jc values at 75 and 64 K show no decrease in self field, indicating no breakdown of intergranular coupling, and show large, dose‐dependent enhancements in magnetic fields oriented parallel to the tape normal.

We have measured the anisotropic transport properties of Bi2223/Ag tapes with and without the silver sheath. The anisotropy ratio of the resistivity along the rolling plane to the resistivity perpendicular to the rolling plane (∼4–10) is orders of magnitude smaller than single‐crystal anisotropy ratios. Results are consistent with current flow along crystallographic ab planes even for transport normal to the tape plane.

The functional dependence of critical current density (Jc) upon defect density is studied with neutron irradiation of YBa2Cu3O7−x. Identical polycrystalline samples of YBa2Cu3O7−x have been irradiated with fast neutrons (E≳0.1 MeV) in eight steps between 0 and 2.1 × 1018 n/cm2. Critical current densities are obtained from magnetic hysteresis measurements and correlated with defect density measurements obtained from transmission electron microscopy. We observe that despite the existence of a significant degree of hysteresis in the as grown material the hysteretic Jc in the irradiated samples appears to be dominated by a power law dependence on neutron fluence with an exponent of approximately 1/3 over a wide range of temperature, field and fluence.

We have prepared epitaxial (100)CeO2 thin films on LaAlO3, sapphire, and yttria‐stabilized zirconia using pulsed laser deposition. It is demonstrated in this letter that the CeO2 films are chemically and structurally compatible to the high‐temperature superconductor YBa2Cu3O7−δ (YBCO). Epitaxial YBCO films on CeO2/LaAlO3 had a zero resistance temperature and critical current density in a zero field of 90 K and 5.9×106 A/cm2 at 75 K, respectively. Furthermore, epitaxial multilayers of CeO2/YBCO were prepared. This work demonstrated that CeO2 is an excellent buffer layer material for the high‐temperature superconductors.

Microstructure and magnetic hysteresis have been compared for two samples of the YBa2Cu3O7−δ (δ∼0) high‐temperature superconductor, one unirradiated, and one irradiated with fast neutrons (E≳0.1 MeV) to a fluence of 3×1018 n/cm2. Notable changes in the microstructure include strain‐induced contrast from regions 2–7 nm in size. An intrinsic critical current density (Jc) of 4.6×106 A/cm2 in zero field at 4 K has been determined from magnetic hysteresis measurements for the irradiated sample while 1.2×106 A/cm2 is noted for the unirradiated sample. We propose that the observed defect structure in the irradiated material is responsible for increased pinning and consequently higher Jc’s.

Remarkable progress has been made in the development of YBa2Cu3O7−δ (YBCO)-based coated conductors, and the problems of continuous processing of commercially viable tape lengths are being rapidly solved by companies around the world. However, the current carried by these tapes is presently limited to about 100A for a 1-cm-wide tape, and this is due to a rapid decrease of critical current density (Jc) as the coating thickness is increased. We have now overcome this problem by separating relatively thin YBCO layers with very thin layers of CeO2. Using this multilayer technology, we have achieved Jc values on metal substrates of up to 4.0MA∕cm2 (75K, self-field) in films as thick as 3.5μm, for an extrapolated current of 1400A∕cm width.

The upper critical field Hc2 and the field‐dependent critical current density Jc(H) have been determined for high‐Tc Nb3Ge prepared by the chemical vapor deposition (CVD) process. Measurements were performed on short samples taken from long lengths (∼10 m) of Nb3Ge deposited under various conditions onto metallic substrates. Using the theory of Hohenberg and Werthamer for dirty type‐II superconductors, values of Hc2(0) were extrapolated from resistive measurements of Hc2 near Tc. Values of Hc2(0) ranged from 32.2 to 33.1 T, the highest upper critical field reported for bulk Nb3Ge. From measurements of Jc(H), pinning force densities Fp were calculated and compared to the Kramer model of flux pinning which predicts certain scaling laws for Fp. We have tested for these scaling laws and have found qualitative agreement with the model of Kramer. This represents the first demonstration of such scaling in bulk high‐Tc Nb3Ge. In addition, values of the flux line lattice shearing parameter Ks have been extracted f...

Measurements of 50‐Hz hysteretic losses in Nb3Ge material have been performed in the presence of a dc‐bias magnetic field. The measurements were made by an electrical technique with both the ac and dc magnetic fields parallel to the axis of long Nb3Ge‐coated cylinders. With a fixed ac amplitude h0, the losses are observed to decrease as the dc field H0 is raised from zero, to pass through a minimum, and then to increase monotonically. The position of the minimum is observed to occur when H0=h0−Hs, where Hs is the field at which initial entry of flux into the sample is observed. Minimum depths which reach 30% of the zero‐bias loss are found for samples in which Hs is large (?900 Oe). This ’’minimum’’ effect and other aspects of ac losses in the presence of a dc‐bias field are explained qualitatively by a critical‐state model that takes account of the magnetic field dependences of the critical current density Jc(B) and the surface barrier ΔH (B). The experimental results are also compared with the results o...

We have measured the magnetic hyperfine field Hi at the Fe57 nucleus in a series of MoxNb1−x alloys in which x varies from 0.85 to 1.0. The measurements were taken over a temperature range 0.5⩽T⩽4.0 K and in magnetic fields up to 6.0 T. We fit each spectrum with a four‐site model with relative strengths fixed by the statistical probabilities of the nearest‐neighbor (n.n.) configurations. The results of this analysis indicate the Fe sites with eight Mo n.n. have a moment similar to that of Fe in Mo, but that the Kondo temperature TK increases with the Nb content in the alloy. Fe sites with seven Mo n.n. exhibit a temperature independent Hi about equal to the applied field over 0.5⩽T⩽4 K, indicative of a magnetic site with TK≳4 K. These results support the basic premise of the local environment model, first proposed by Jaccarino and Walker, but indicate a more continuous dependence of magnetic bahavior on n.n. configuration.

Hysteretic losses have been measured on a large number of Nb3Ge samples prepared by the chemical vapor deposition process. These measurements were performed as a function of induced current and temperature on samples which contained differing amounts of second‐phase material, Nb5Ge3. It was found that very low 50‐Hz losses, on the order of 20 μW/cm2 at 500 rms A/cm and at temperatures of up to 12 K, could be attained in as‐deposited material. An apparent correlation between the critical current density and the Nb5Ge3 content was observed, which suggested that enhanced pinning due to the second‐phase precipitates was responsible for these low losses.

A simple cost‐effective method has been developed for the fabrication of low‐resistivity contacts for high‐temperature superconducting ceramics. Surface resistivities have been measured for the contacts in the 10−8 Ω cm2 range at 76 K. The fabrication method involves application of a silver pad to the ceramic by plasma spraying and then attachment of leads by soldering.

We have observed the generation of microwave acoustic pulses in thin films of four of the rare‐earth metals. The pulse—echo technique common to ultrasonic studies in solids was employed. Acoustic generation in Dy, Gd, Ho, and Er has been observed at frequencies between 0.8 and 9.2 GHz at temperatures between 4.2° and 130°K. This effect appears in the antiferromagnetic region only at field values greater than the critical field for ferromagnetic alignment. In Dy, with the magnetic field along the film normal, the echo amplitudes exhibit a monotonic increase with magnetic field up to 20 kOe at 1.0 GHz and 4.2°K. In Gd, however, a resonant peak is seen under the same conditions, similar to that observed in nickel alloys. In Er and Ho acoustic power reaches a plateau at 14 kOe at 1.0 GHz and 4.2°K. The magnetic‐field values at which appreciable acoustic generation is observed are generally much lower than would be necessary for ferromagnetic resonance at the frequencies used here.

The paramagnetic behavior of very dilute (