Magnetic shielding properties and transport currents of oxide superconductors

Abstract

We have measured the magnetic flux in YBa 2Cu 3O y superconductors when transport currents were allowed to flow in zero external field at 77 K. The magnetic flux was trapped when the transport current exceeded a value I cs, which was smaller than the transport critical current I ct; typically, 0.4 I ct. We have also measured the critical shielding flux density B o1 as a function of the transport current supplied to the shielding specimens. With increasing transport current, B o1 decreased due to the self-field and diminished to zero at the current I cs. Owing to the difference in superconducting properties of different substructures in the specimen, the shielding critical current I cs is believed to be determined by the part of the substructure with the lowest critical currents, and the transport critical current I ct is thought to be determined by the part of the substructure with the highest critical currents in the sample. The estimated value of B o1 with I ca, derived from the Bean-London critical state model, agreed with the measured value B o1.