Ac susceptibility and grain-boundary pinning strengths in YBa2Cu3O7- delta and YBa2Cu2.985Ag0.015O7- delta.

Abstract

Estimation of the relative flux-pinning strengths of the matrix and the grain boundaries in polycrystalline oxide superconductors contributes to the understanding of their superconducting properties. Such understanding is further enhanced if measurements can be compared with those from materials with lattice substitution and doped grain boundaries. The ac susceptibilities of polycrystalline ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{2.985}$${\mathrm{Ag}}_{0.015}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ were measured as functions of temperature, ac-magnetic-field strength, and frequency. The presence of two loss peaks in the imaginary part of the susceptibility, \ensuremath{\chi}'', was confirmed by comparison of ac and dc measurements obtained on materials with different microstructures. One peak in \ensuremath{\chi}'' was found to be associated with the matrix transition and the other, at lower temperatures, with weak-link behavior at grain boundaries. The model by K.-H. M\"uller for the field dependence of the peaks was used to estimate and compare the pinning-force densities in the matrix and at weak links for both pure and Ag-substituted ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. In the temperature range (75--90 K) and field range (1--10 Oe, rms) studied, similar pinning forces, 6\ifmmode\times\else\texttimes\fi{}${10}^{8}$ T A ${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$, for the matrix of the two materials were found, but the pinning-force densities at the weak links were higher for the Ag-substituted material by a factor of 1.5, i.e., ${10}^{4}$ T A ${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$ for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{2.985}$${\mathrm{Ag}}_{0.015}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ as compared to 6\ifmmode\times\else\texttimes\fi{}${10}^{3}$ T A ${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$ for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.