M.E. McHenry

Magnetic shielding by superconducting Y-Ba-Cu-O hollow cylinders

The authors have measured the magnetic field H at the center of a hollow of sintered Y-Ba-Cu-O superconductor as a function of applied field H, temperature, tube length, and tube wall thickness. The maximum field that can be shielded H/sub sh/ agrees with the value calculated from the critical state model using the measured critical-current density J/sub c/. The maximum trapped field H/sub tr/ in the tube, on decreasing H to zero, exceeds H/sub sh/ by as much as a factor of two, and large enhancements in J/sub c/ are observed in the decreasing field. These phenomena are identified with intragranular flux pinning present only after H has exceeded H/sub c1/ of the grains. Finite tube length does not affect the H/sub tr//H/sub sh/ ratio appreciably. H/sub sh/ depends approximately on the square root of the wall thickness. >

Critical-current anisotropy, intergranular coupling, and effective pinning energy in Bi2Sr2CaCu2O8+δ single crystals and Ag sheathed (Bi,Pb)2Sr2Ca2Cu3O10+δ tapes

Abstract Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals and Ag sheathed (Bi,Pb)2Sr2Ca2Cu3O10+δ (Bi-2223) tapes have been characterized by magnetometry and electron microscopy. Dislocations on the (001) planes appear to be the main defect bserved in these samples and are considered to be important for flux pinning. Their distribution is more homogeneous in the Bi-2223 tape as a result of thermomechanical processing. In Bi-2212 single cyrstals, the measurement of magnetic hysteresis reveals a strong anisotropy in the magnetization critical current density (Jcm). For the two cases H | c and H ⊥ c, the measurable Lorentz forces (FL) that limit Jc are found to be perpendicular to the c-axis; one causing vortices to hop in the ab plane and the other to slide along the ab plane. In contrast to this, we have probed the relative strength ratio of the intrinsic pinning to the extrinsic pinning in Bi-2223 tapes when we compare the results of H | c with H ⊥ c. No significant difference in Jcm is observed, but for the same average J value, the effective pinning energy (Ueff) calculated from the time derivative of the magnetization (M) for H ⊥ c is two orders of magnitude higher than that for H | c. For these two superconductors, their Ueffs follow a power-law expression, Ueff(J, T, H) = (Ui/Hu)[1 −(T/Tx)2]1.5(Ji/J)μ, where Ui is the scale of activation energy, n is of the order of 1, Tx is related to the magnetic irreversibiklity temperature (Trrr), and both Ji and μ depend on the flux-bundle size and the value of J/Jc. A two-dimensional (2D) flux-line lattice (FLL) is clearly identified in the Bi-2212 single crystals; however, imperfect texturing in the Bi-2223 tape smears out to some extent the observed J dependence of Ueff. Consequently, a larger anisotropy in the normalized relaxation rate, S = − d ln M/d ln t, and much higher S values are observed in Bi-2212 than in Bi-2223.

Transport and magnetization currents in BSCCO/Ag tapes

A (Bi,Pb)SrCaCuO-2223/Ag tape has been produced by thermomechanical processing. Transport critical current densities, J/sub c/s, have been measured between H=0 and 7 T and T=7 to 75 K. Magnetization currents have been determined for similar temperatures and fields (0 to 5 T) oriented parallel and orthogonal to the thickness of the tape. Dimensional scaling arguments show that flux penetrates most quickly along the tape thickness direction (for fields parallel to the tape plane and therefore the ab planes of the textured BSCCO grains). This implies that the anisotropy ratio of J/sub c/ and J/sub ab/ is reduced in tapes as compared with single crystals, which is consistent with predictions of the brick wall model. The H dependence of the magnetization currents and effective pinning potentials were also measured for similar field orientations. Fast magnetic relaxation causes an exponential field dependence of transport and magnetization currents. Temperature dependences for H/sub 0/ for transport and magnetization measurements are interpreted in terms of different time scales (or voltage criteria).<<ETX>>

Observation of thermally activated flux motion in weak link structures

Abstract We have measured the time rate of magnetic flux motion through an array of weak links present in the wall of a hollow cylinder of ceramic YBa 2 Cu 3 O 7 . We observe a logarithmic time dependence of the magnetic field in the center of the tube with an applied field of 28 Oe at 75K. Analysis of the logarithmic creep rate with a standard phenomenological model yields an activation energy of ∼ 1.55 eV for the motion of Josephson vortices through the weak links in the structure.

Magnetic hysteresis and flux creep of YBa/sub 2/Cu/sub 3/O/sub x/ grown by the melt-powder-melt-growth (MPMG) process

Magnetic hysteresis and flux creep of melt-powder-melt-growth (MPMG) YBa/sub 2/Cu/sub 3/O/sub x/ containing nominal 0, 25, and 40 mole% Y/sub 2/BaCuO/sub 5/

Annealing of irradiation induced defects in a LaSrCuO crystal

Annealing experiments were performed on a LSCO single crystal irradiated with fast neutrons to a fluence of 2.52/spl times/10/sup 18/ n/cm/sup 2/. This fluence increased the critical current density by a factor of 2-3. The sample was annealed for a total of 4.5 hours at 200/spl deg/C and 8 hours at 300/spl deg/C. It was found that annealing of LSCO has a much lesser effect on its critical current density, particularly along the c axis, compared to YBCO. It is hypothesized that the reduced annealing effect is due to the formation of either a more stable interstitial cluster (compared to the Cu-O cluster in YBCO), or clusters which do not act as pinning sites.