THE AC MAGNETIC RESPONSE OF YBA2CU3O7 WITH Y2BACUO5 INCLUSIONS AND OFSINGLE-CRYSTAL YBA2CU3O7 - A COMPARATIVE-STUDY

We have measured the AC magnetic permeability response function mu=mu' +imu'' in melt-process-melt-growth YBa2Cu3O7 (Y123) with Y2BaCuO5 (Y21 1) inclusions (MPMG) and in single-crystal YBa2Cu3O7 (SC) in applied m agnetic fields up to 8 T, oriented either parallel or perpendicular to the crystalline c-axis. The AC response of the two samples has been m apped out as a function of temperature, AC field amplitude and frequen cy, DC applied field, and field orientation. The peak in the lossy par t of the susceptibility mu'' (H, T) is used to probe the flux dynamics in the vicinity of the ''irreversibility'' line (IL), defined here as the locus of mu'' maxima in the H-T plane. For H parallel-to c, the I L of the MPMG sample exists at higher temperature and field than the S C sample, indicating that the Y211 insulating inclusions are responsib le for additional strong pinning of the vortex structure. For H perpen dicular-to c, the ILs, determined by the peak position of mu'' (H, T), are the same for both samples. However the amount of dissipation meas ured by the temperature width of mu'' (H, T), is greater for the MPMG sample for this orientation implying that the Y211 inclusions reduce t he vortex pinning in this orientation. The precise natures of these ef fects due to the Y211 inclusions are still unknown. The frequency depe ndence and non-linear response differ for the two field orientations. In the range of available parameters and variables we find the system to be in the solid vortex phase for H perpendicular-to c since there i s strong H(AC) dependence and only weak frequency dependence on the te mperature position of the loss peak. For H parallel-to c, however, the loss peak depends only weakly on H(AC) and strongly on the frequency.