Transport and magnetic critical current in superconducting MgB2 wires

Direct comparison of the magnetic and transport critical current density (J(c)) for the same pieces of copper-sheathed MgB(2) wires shows a large discrepancy in magnitude and field dependence of the two. The value of magnetic J(c) can differ from the value of transport J(c) by a factor of 10 or more. This discrepancy does not occur merely because of the difference in the voltage at which the magnetic and transport J(c) are measured, but mainly because of the specific microstructure of MgB(2). Such microstructure results in superconducting screening on at least two different length-scales, despite the absence of weak links in MgB(2), leading to erroneous magnetic J(c) if a simple critical state model is applied to such a system. Consequently, the magnetic J(c) cannot be used for analysis of physical processes where the accurate field dependence of J(c) needs to be known, such as vortex pinning. The magnetic J(c) can still be used for qualitative comparison of MgB(2) samples if they are all of the same size and have the same microstructure, or if their size is large enough so that the magnetic J(c) does not depend on the sample size.