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

We investigate the ac susceptibility of both field-cooled and zero-field-cooled sintered YBa2Cu3O7-δ 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, Hdc*, 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 (Hc1). The upper limit of a homogeneous superconducting area determined from Hdc* appears comparable to the average grain size in our granular thin slabs. The existence of Hdc* is attributed to the superconducting glass behavior of granular YBa2Cu3O7-δ with disorder in the intergranular Josephson junctions.