Comparative study of critical current densities and flux pinning among a flux-grown NdBa2Cu3Oy single crystal, melt-textured Nd-Ba-Cu-O, and Y-Ba-Cu-O bulks

We have studied the field and temperature dependence of the critical curren t density J(c)(B,T) for a fluxgrown NdBa2Cu3Oy single crystal (Nd123SC), me lt-textured Nd-Ba-Cu-O (OCMG-Nd-Ba-Cu-O), and Y-Ba-Cu-O (MPMG-Y-Ba-Cu-O) bu lks. In the J(c)-T curves, an exponential-like behavior in a low-temperatur e region could be explained in terms of the collective flux creep model, wh ile J(c)-T curves in a high-temperature region could well be fitted to the Kim-Anderson creep model. For the scaling of J(c)-B curves, power-law relat ion of J(c)proportional to Bgamma-1 is observed for all the samples in a lo w-field region in that the exponent gamma is roughly 1/2 for OCMG-Nd-Ba-Cu- O and MPMG-Y-Ba-Cu-O, and 1 for Nd123SC, corresponding to surface pinning a nd point pinning, respectively. The surface pinning is ascribed to the pres ence of second-phase particles. In an intermediate field region, the gamma values become almost 2 for Nd123SC and OCMG-Nd-Ba-Cu-O, showing that Delta kappa-point pinning is dominant, which presumably originates from the Nd-Ba substituted regions with depressed T-c. On the basis of our J(c)(B,T) anal yses, we could classify the B-T phase diagram into several regimes of diffe rent dominant pinning mechanisms. [S0163-1829(99)04502-6].