The AC magnetic susceptibility of high temperature superconductors

This research concerns the development of AC magnetic susceptometers and use of susceptometers in analysing high temperature superconductors. Two of the designs were a differential magnetic susceptometer (DMS) and a double coil screening susceptometer (DCSS) whose descriptions are given in detail including coil design, field measurements, susceptometer operation, experimental instrumentation, phase adjustment, susceptometer calibration and sensitivity for each design. Theoretical details are given regarding each design in order to calculate the complex external and internal susceptibility. Investigation concerning the demagnetisation factor of different geometries, and the significant features and limitation for each design are also provided. The susceptometers were applied to a wide range of YBCO samples, which includes bulk samples with different geometry (i.e. Slabs, disk, powder and thick film) and thin films with different oxygen contents. Several silver sheathed Bi-2223 tapes were also involved. This allowed us to determine the unique effect of geometry and grain size on the sample susceptibility. The AC susceptibility for these samples was measured as a function of parallel and perpendicular magnetic field, frequency and temperature. The AC susceptibility data were then employed to investigate the flux pinning mechanism taking into account the geometrical barrier effect, critical current density, irreversibility line, pinning force and the AC losses for the grains and the intergrains of these samples. Comparing the results for the slab and disk geometries obtained for the parallel and perpendicular fields are presented including the complex AC susceptibility, the contribution of the intergrains and the grains, critical current density, internal field, and magnetisation. Comparison between the slab and disk in terms of flux penetration are also discussed. It was found that isolating the grains of the slab by using powder sample does not change the value o