Transport and inductive critical current densities in superconducting MgB2

The critical current density of four MgB2 samples was measured inductively and for one of them also by transport. Pure phased and dense bulk samples y ield a critical current density which in value, as well as in its magnetic field and temperature dependence, is essentially the same as the intra-gran ular current density measured in a dispersed powder. Also the correspondenc e between the inductive and transport data indicates that the grain boundar ies in the bulk samples are totally transparent for the current. The curren t-voltage relation becomes rapidly shallow in the vicinity of a depinning l ine, well below the second critical field. Between the depinning line and t he second critical field the material is ohmic and shows a marked magneto-r esistivity, indicative of a flux-flow regime. (C) 2001 Elsevier Science BN. All rights reserved.