Transport and magnetic critical current in superconductingMgB2 wires

Abstract

Direct comparison of the magnetic and transport critical current density(Jc) for the same piecesof copper-sheathed MgB2 wires shows a large discrepancy in magnitude and field dependence of the two. The value of magneticJc can differ from thevalue of transport Jc by a factor of 10 or more. This discrepancy does not occur merely becauseof the difference in the voltage at which the magnetic and transportJc are measured, but mainly because of the specific microstructure ofMgB2. Such microstructure results in superconducting screening on atleast two different length-scales, despite the absence of weak links inMgB2, leading toerroneous magnetic Jc if a simple critical state model is applied to such a system. Consequently, the magneticJc cannot be used for analysis of physical processes where the accurate field dependence ofJc needs to be known, such as vortex pinning. The magneticJc can still be used for qualitative comparison ofMgB2 samples if they are all of the same size and have the samemicrostructure, or if their size is large enough so that the magneticJc does not depend on the sample size.