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25 Jan 1988

Volume 52, Issue 4, pp. 249-336

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Chemical durability of high‐temperature superconductor YBa2Cu3O7−x in aqueous environments

Narottam P. Bansal and Ann L. Sandkuhl

Appl. Phys. Lett. 52, 323 (1988); http://dx.doi.org/10.1063/1.99481 (3 pages) | Cited 70 times

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The stability of YBa2Cu3O7−x in water and 100% humidity has been investigated at three temperatures using pH measurements, x‐ray diffraction, and scanning electron microscopy. The oxide‐ceramic superconductor is highly unstable and reacts rapidly with water and also degrades in moisture. Dissolution of the oxide perovskite in water is highly incongruent. The corrosion products are found to be BaCO3, CuO, O2, etc. Barium hydroxide is first formed which further reacts with atmospheric CO2 to form needle‐shaped crystals of BaCO3. For any practical applications, devices made from these materials would have to be protected with an impermeable coating to prevent deterioration from atmosphere.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.65.-b Surface treatments
74.70.-b Superconducting materials other than cuprates

Decoration of flux pinning positions in YBa2Cu3O7−δ superconductors

C. J. Jou, E. R. Weber, J. Washburn, and W. A. Soffa

Appl. Phys. Lett. 52, 326 (1988); http://dx.doi.org/10.1063/1.99650 (2 pages) | Cited 9 times

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Flux pinning in the sintered high Tc oxide superconductors, YBa2Cu3O7−δ, in the vortex state has been demonstrated for the first time using the Bitter pattern technique and scanning electron microscopy. External magnetic fields of 150 or 800 G perpendicular to the specimen surfaces were applied during the evaporation of cobalt after the specimens cooled with liquid nitrogen had reached a temperature below Tc. All specimens thus decorated with cobalt show nonuniformly distributed flux pinning positions as would be expected due to the porosity and multiphase character of the specimens as well as the relatively low field applied.
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74.70.-b Superconducting materials other than cuprates
81.40.Rs Electrical and magnetic properties related to treatment conditions
75.50.-y Studies of specific magnetic materials
75.30.Cr Saturation moments and magnetic susceptibilities

Persistent currents in ceramic and evaporated thin‐film toroids of Ba2YCu3O7

E. M. Gyorgy, G. S. Grader, D. W. Johnson, L. C. Feldman, D. W. Murphy, W. W. Rhodes, R. E. Howard, P. M. Mankiewich, and W. J. Skocpol

Appl. Phys. Lett. 52, 328 (1988); http://dx.doi.org/10.1063/1.99454 (3 pages) | Cited 18 times

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We have demonstrated the existence of circumferential persistent currents in Ba2YCu3O7 ceramic toroids at 77 and 4.2 K and in thin films at 77 K. The field dependence of the critical current densities obtained from these persistent current measurements at 77 K is in fair agreement with that obtained from transport measurements. The persistent critical current densities for fields less than 10 G are 150 A/cm2 for the ceramic at 77 K and 105 A/cm2 for the thin film. A long term experiment showed that the resistivity is less than 3×10−18 Ω cm at 77 K.
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74.25.Sv Critical currents
74.78.-w Superconducting films and low-dimensional structures
74.70.-b Superconducting materials other than cuprates

Method for making low‐resistivity contacts to high Tc superconductors

J. W. Ekin, A. J. Panson, and B. A. Blankenship

Appl. Phys. Lett. 52, 331 (1988); http://dx.doi.org/10.1063/1.99455 (3 pages) | Cited 65 times

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A method for making low‐resistivity contacts to high Tc superconductors has been developed, which has achieved contact surface resistivities less than 10 μΩ cm2 at 76 K and does not require sample heating above ∼150 °C. This is an upper limit for the contact resistivity obtained at high current densities up to 102–103 A/cm2 across the contact interface. At lower measuring current densities the contact resistivities were lower and the voltage‐current curve was nonlinear, having a superconducting transition character. On cooling from 295 to 76 K, the contact resistivity decreased several times, in contrast to indium solder contacts where the resistivity increased on cooling. The contacts showed consistently low resistivity and little degradation when exposed to dry air over a four‐month period and when repeatedly cycled between room temperature and 76 K. The contacts are formed by sputter depositing a layer of a noble metal−silver and gold were used−on a clean superconductor surface to protect the surface and serve as a contact pad. External connections to the contact pads have been made using both solder and wire‐bonding techniques.
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73.40.Cg Contact resistance, contact potential
74.70.-b Superconducting materials other than cuprates
73.40.Ns Metal-nonmetal contacts

Amorphous GdCo: Effect of surface segregation on reading of the magnetization by spin‐polarized photoemission

G. L. Bona, F. Meier, H. C. Siegmann, and R. F. Gambino

Appl. Phys. Lett. 52, 334 (1988); http://dx.doi.org/10.1063/1.99456 (3 pages) | Cited 2 times

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Spin‐polarized photoemission can be used to read the surface magnetization direction of perpendicularly magnetized amorphous GdCo films. Effects specific of the photoemission process are observed: (1) surface depolarization by disordered Gd moments in the outermost segregated layers of the film, (2) change of the polarization upon varying the probing depth of the photoelectrons. Both effects are consequences of the inhomogeneous surface composition of amorphous GdCo due to segregation.
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75.70.Rf Surface magnetism
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.70.Ak Magnetic properties of monolayers and thin films
79.60.-i Photoemission and photoelectron spectra
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