We have investigated, experimentally and theoretically, the magnetic flux which threads a YBa 2 Cu 3 O 7− x cylinder sample at 77 K when a weak low frequency ac magnetic field is applied. We explain the highly nonlinear magnetic response in great detail quantitatively, using a critical state model where during each ac cycle, intergranular (Josephson) vortices sweep in and out of the cylinder. The Josephson vortex pinning force is found to be field independent and several orders of magnitude smaller than in conventional type II superconductors. This explains the weak shielding properties and low critical current density found in this material. At magnetic fields greater than about 50 Oe, vortices start to enter the superconducting grains.

Josephson vortices and flux penetration in high temperature superconductors

Non-contact two-coil methods for the low-frequency electrodynamics of superconductors are analysed by representing the superconductor as one or two inductively-coupled loops with various voltage-current characteristics. Various types of behaviour can be explained without specific reference to granular or weak-link effects. For the one-loop model, flux-flow and surface flux-entry barrier characteristics are analysed for the first time. Fissured samples and films of uneven thickness may be modelled by two loops, which can exhibit “overshoots” and double loss-peaks. Attention is paid to the harmonic content and also, in the case of square-wave detection, to the distinction between pick-up coil and Hall-probe data. New YBaCuO ceramic, YBaCuO film and BiSrCaCuO crystal data are analysed.

Superconductor screen viewed as one or two inductive loops

Coils are placed coaxially on either side of a slab of superconductor of thickness 0.05-1 mm. The coil diameters are less than the lateral dimentions of the slab. The voltage in the secondary coil is measured as a function of excitation current and frequency in the primary, and of the temperature and the steady magnetic field provided by an external solenoid. A YBa2Cu3O7 ceramic cooled in zero field screens almost completely, but the greater part of this screening is suppressed by low magnetic fields. The remaining part is shown to be due to isolated superconducting grains. When intergrain superconductivity is partly suppressed the condition is reasonably well described by the Bean critical-state model. Hysteresis of the screening coefficient reflects the known hysteresis of the transport critical current.

AC screening measurement for the characterization of oxide superconductors: I. Application to ceramics

Simple magnetic shielding experiment on high Tc superconducting bulk ceramics and thin films, metals and alloys

The authors have measured a critical shielding flux density B/sub 01/ of YBa/sub 2/Cu/sub 3/O/sub y/ superconducting plates. Sintered specimens were assumed to be bulk superconductors with interconnected sub-structures and were considered to screen the magnetic field completely for applied fields of less than B/sub 01/. The critical shielding flux density was proportional to the thickness d of the specimen, as expected from an equation derived from the Bean-London critical state model. The authors have obtained 16 G/mm as a maximum shielding capacity of B/sub 01//d. The measured flux showed hysteresis when the applied field was raised above B/sub 01//. >

R. Driver

Papers

Electromagnetic shielding properties of yttrium barium cuprate superconductor