Transport and magnetic critical current in superconducting MgB2wires

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.