Effects of magnetic history on the ac magnetic susceptibility of granular YBa2Cu3O7- delta superconductors in weak fields.

Abstract

We investigate the ac susceptibility of both field-cooled and zero-field-cooled sintered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ granular thin slabs and powders in low magnetic fields. The ac susceptibility of the granular thin slabs appears to depend upon the magnetic history for dc fields larger than a certain value, ${\mathit{H}}_{\mathrm{dc}}^{\mathrm{*}}$, with the existence of the stronger diamagnetic signal (the in-phase part of the ac susceptibility) in the field-cooled case. Our results show that the difference in local flux distributions between the field-cooled case and the zero-field-cooled case exists even for a dc field as low as several Oe, a value much lower than the bulk lower critical field (${\mathit{H}}_{\mathit{c}1}$). The upper limit of a homogeneous superconducting area determined from ${\mathit{H}}_{\mathrm{dc}}^{\mathrm{*}}$ appears comparable to the average grain size in our granular thin slabs. The existence of ${\mathit{H}}_{\mathrm{dc}}^{\mathrm{*}}$ is attributed to the superconducting glass behavior of granular ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with disorder in the intergranular Josephson junctions.