Critical current densities and irreversibility fields of MgB2 bulks

We prepared two MgB2 bulks by applying conventional sintering and high-pres sure sintering methods, and compared the current carrying properties. J(c) obtained by the resistive method was larger than that obtained by the magne tic method. J(c)-B curves obtained by the resistive method showed no histor y effect. These results indicate that most of the superconducting currents flowing in the MgB2 bulks were intergrain (transport) currents and intragra in currents were negligibly small. The high-pressure sintered sample with s maller grain size showed smaller field dependence of J(c) and higher B-irr than the conventionally sintered sample with larger grain size. This behavi or can be explained by the grain boundary flux pinning. (C) 2001 Elsevier S cience B.V. All rights reserved.