Critical-current anisotropy, intergranular coupling, and effective pinning energy in Bi 2Sr 2CaCu 2O 8+δ single crystals and Ag sheathed (Bi, Pb) 2Sr 2Ca 2Cu 3O 10+δ tapes

Abstract

Bi 2Sr 2CaCu 2O 8+δ (Bi-2212) single crystals and Ag sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+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 ( J cm). For the two cases H | c and H ⊥ c, the measurable Lorentz forces ( F L) that limit J c 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 J cm is observed, but for the same average J value, the effective pinning energy ( U eff) 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 U eff's follow a power-law expression, U eff( J, T, H) = ( U i/ H u)[1 −( T/ T x) 2] 1.5( J i/ J) μ , where U i is the scale of activation energy, n is of the order of 1, T x is related to the magnetic irreversibiklity temperature ( T rrr ), and both J i and μ depend on the flux-bundle size and the value of J/ J c. 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 U eff. 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.