Polarized-neutron study of the YBa2Cu3O7 system: Granular versus bulk superconductivity

Neutron depolarization measurements have been carried out in order to investigate superconductivity in the YBa2Cu3O/sub 7-//sub x/ system on both powdered and sintered polycrystalline samples. This technique provides indirect evidence of superconductivity through magnetic-flux trapping in low fields as well as vortex-line penetration in higher fields. Powdered and sintered samples show markedly different behavior, probably connected to their granular nature: a sintered sample traps magnetic flux by developing shielding currents when a low magnetic field (<100 Oe) is removed after being applied below the superconducting temperature T/sub c/, whereas a powder does not. T/sub c/ can be well determined (about 90 K) whereas H/sub c/1(T) can only be estimated