Most applications of superconducting materials require high critical current density. Melt-powder-melt-grown (MPMG) YBa2Cu 3O7 with Y2BaCuO5 inclusions were found to have the highest critical current in bulk to date. The authors report a systematic study of the irreversible behaviour in the MPMG material. The irreversibility line was measured in external fields up to 8 T using an AC susceptibility technique. Large anisotropy of the irreversibility line for different orientations of the field was observed. The frequency dependence was found to be logarithmic in the range 10 to 105 Hz. In order to study the excitation field dependence in detail, they applied AC fields from 0.01 G up to 30 G.
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