Abstract
Upper critical field, irreversibility line, weak superconducting coupling and thermal relaxation are investigated in melt processed YBa2Cu3O7−x in the geometry magnetic field parallel and perpendicular to the a,b plane. Granularity within the highly c-axis oriented grains is excluded from comparison between transport and shielding current if the current flows within the a,b plane. The current along the c-axis is not degraded at the grain boundaries but becomes nonuniform with increasing field and temperature. This information was obtained from a more detailed investigation of magnetization measurements. Oxygen deficiency and inhomogeneous oxygen distribution are proposed as a major reason for this behaviour. Flux creep phenomena were studied by magnetic relaxation and inductive flux profile measurements. Temperature and field dependent activation energies were determined. The increase of the activation energy with temperature is discussed and compared with different explanations. Sweep rate dependent dc and ac measurements which allow access to relaxation in the millisecond regime exclude in the low temperature region a short time decay of the magnetic moment before the relaxation measurements starts.
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