In the present work, we have performed a detailed comparative study of the irreversibility line (IL) among flux-grown NdBa2Cu3O7-d (Nd123) single crystal, melt-grown Nd-Ba-Cu-O (NdBCO) and melt-grown Y-Ba-Cu-O (YBCO) bulks by d.c. magnetometry and a.c. susceptometry. ILs were determined by measuring d.c. magnetization hysteresis loops, the fundamental χ1 and third harmonic χ3 signals of a.c. susceptibility for the fields parallel to the c-axis of samples (H//c). Among these, melt processed NdBCO exhibited the highest IL. On reduced temperature scale, however, ILs from the χ3 signals for melt-grown NdBCO and YBCO tended to merge together, suggesting that real depinning lines are insensitive to the field-induced flux pinning centers existing in NdBCO, including the fluxgrown sample, but are rather sensitive to the secondary phase inclusions existing in melt-grown samples.
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