Search

CuAl2O4 is a ternary oxide spinel with Cu2+ ions ( s = 1 / 2 ) primarily populating the A-site diamond sublattice. The compound is reported to display evidence of spin glass behavior but possess a non-frozen magnetic ground state below the transition temperature. On the other hand, the spinel CuGa2O4 displays spin glass behavior at ~ 2.5 K with Cu2+ ions more readily tending to the B-site pyrochlore sublattice. Therefore, we investigate the magnetic and structural properties of the solid solution CuAl2(1-x)Ga2xO4 examining the evolution of the magnetic behavior as Al3+ is replaced with a much larger Ga3+ ion. Our results show that the Cu2+ ions tend to migrate from tetrahedral to octahedral sites as the Ga3+ ion concentration increases, resulting in a concomitant change in the glassy magnetic properties of the solution. Results indicate glassy behavior for much of the solution with a general trend towards decreasing magnetic frustration as the Cu2+ ion shifts to the B-site. However, the x = 0.1 and 0.2 members of the system do not show glassy behavior down to our measurement limit (1.9 K) suggesting a delayed spin glass transition. We suggest that these two members are additional candidates for investigation to access highly frustrated exotic quantum states. [ABSTRACT FROM AUTHOR]

The superconducting properties of (Mx/YBa2Cu3O7-δy)N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y2BaCuO5 (211), Y2O3, BaZrO3, CeO2, SmBa2Cu3O7-δ (Sm123), brown-phase La2BaCuO5 (La211), and MgO. Multilayer (Mx/YBa2Cu3O7-δy)N structures were grown by pulsed laser deposition onto SrTiO3 or LaAlO3 single-crystal substrates by alternate ablation of separate YBa2-Cu3O7-δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (Tc), critical current density as a function of applied magnetic field Jc(H), self-field Jc(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO3 at 65 to 77 K, Y2O3 at 65 K, and CeO2 for H < 0.5 T. BaZrO3 had a noticeably different epitaxy forming smaller size nanoparticles ~8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 x 1011 nanoparticles cm-2. To optimize flux pinning and Jc (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field Jc unexpectedly increased with increasing x layer thickness. [ABSTRACT FROM AUTHOR]

Time-resolved magneto-optical imaging measurements show that an ac current enables the vortex matter in YBa2Cu3O7-x thin films to reorganize into two coexisting steady states of driven vortex motion with different characteristics: a quasi-static disordered glassy state in the sample interior and a dynamic state of plastic motion near the edges. Finite-element calculations consistent with the critical state model show good agreement with the measured field profiles in the quasi-static state but predict a larger hysteretic behavior in the dynamic state. [ABSTRACT FROM AUTHOR]

Chemical solution processing of Gd2Zr2O7 (GZO) thin films via sol-gel and metalorganic decomposition (MOD) precursor routes have been studied on textured Ni-based tape substrates. Even though films processed by both techniques showed similar property characteristics, the MOD-derived samples developed a high degree of texture alignment at significantly lower temperatures. Both precursor chemistries resulted in exceptionally dense, pore-free, and smooth microstructures, reflected in the cross-sectional and plan-view high-resolution scanning and transmission electron microscopy studies. On the MOD GZO buffered Ni-3at.% W (Ni-W) substrates with additional CeO2/YSZ sputtered over layers, a 0.8-µm-thick YBa2Cu3O7-8 (YBCO) film, grown by an ex situ metalorganic trifluoroacetate precursor method, yielded critical current, Ic (77 K, self-field), of 100 A/cm width. Furthermore, using pulsed-laser deposited YBCO films, a zero-field superconducting critical current density, Jc (77 K), of 1 x 106 A/cm² was demonstrated on an all-solution, simplified CeO2(MOD)/GZO(MOD)/Ni-W architecture. The present study establishes GZO buffers as a candidate material for low-cost, all-solution coated conductor fabrication. [ABSTRACT FROM AUTHOR]

Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length (~ 2-4?nm for YBCO at temperatures =77?K) and when the areal number density of defects is of the order of (H/2) × 1011?cm-2, where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of ~8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77?K) by a factor of two to three for high magnetic fields. [ABSTRACT FROM AUTHOR]

A controlled introduction of second-phase Y[sub2]BaCuO[sub5] (211) nanoparticles into YBa[sub2]Cu[sub3]O[sub7-δ] (123) thin films was achieved for the first time for the purpose of increasing flux pinning. The island-growth mode of 211 on 123 was utilized to obtain an area particle density >10[sup11] cm&sup-2; of 211 thick-disk-shaped nanoparticles in individual layers. Composite layered structures of (211[suby] nanoparticles/123[subz])xN were deposited by pulsed laser deposition on LaAlO[sub3] substrates, with N bilayers = 24 to 100, y thickness = 1 to 2 nm, and z thickness = 6 to 15 nm (assuming continuous layer coverage). With 211 addition, the critical current densities at 77 K were higher at magnetic fields as low as 0.1 T and increased as much as approximately 300% at 1.5 T. The superconducting transition temperature was reduced by approximately 2 to 4 K for 211 volume fraction <20%. Reinitiation of 123 growth after every 211 layer resulted in a smooth and flat surface finish on the films and also greatly reduced surface particulate formation especially in thicker films (∼1 µm). [ABSTRACT FROM AUTHOR]

Provides information on a study that examined partial-melt processing of Bi[sub2+x]Sr[sub2-x-y]Ca[sub1+y]O[sub8+δ] thick-film conductors with additions of nanophase Al[sub2]O[sub2} for dual purposes of increasing flux pinning and inhibiting strontium-calcium-copper-oxygen phase defect formation. Methodology of the study; Results and discussion on the study.

The status of long length, BiSrCaCuO (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport Js (4.2K, 0 T) of Bi-2212/Ag oxide powder-in-tube singlefilamentary tapes have reached 70,000-80,000, 50,000-60,000, and 30,000-40,000 A/cm for 1, 4-15, and 40-90 meter length tapes, respectively. The decrease in J as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of J to temperature nonuniformities (±3‡C) in the box-type furnace used for annealing. To reduce this problem, a ringtype high-temperature furnace (∼3 meter diameter) was designed and constructed which provides a large-volume (∼13 × 10 × 1000 cm) processing zone with expected excellent temperature uniformity (±0.5‡C). The advantages of the ring-type furnace for processing of kilometer-length conductors are described. [ABSTRACT FROM AUTHOR]

Time-resolved magneto-optical imaging measurements show that an ac current enables the vortex matter in YBa2Cu3O7-x thin films to reorganize into two coexisting steady states of driven vortex motion with different characteristics: a quasi-static disordered glassy state in the sample interior and a dynamic state of plastic motion near the edges. Finite-element calculations consistent with the critical state model show good agreement with the measured field profiles in the quasi-static state but predict a larger hysteretic behavior in the dynamic state. [ABSTRACT FROM AUTHOR]