Flux-pinning and inhomogeneity in MgB2 /Fe wires

Transport critical current densities Jc and irreversibility fields Birr of undoped and nanoparticle doped (10 wt% SiC) Fe-sheathed MgB2 wires were measured from 2 to 40 K in magnetic field B≤16T. For the best segments of wires (≤1 cm) both the magnitude and field variations of Jc and the pinning force density Fp=JcB depend only on the magnitude of Birr, hence the strength of flux-pinning. Birr of doped wire for T≤30K is ∼1.4 times larger than that of undoped and reaches that of NbTi (10 T at 4.2 K) already at 20 K. Accordingly, its high-field Jcs and Fps are large, typically three times larger than the best literature results, and are limited by the porosity and inhomogeneity of the superconducting cores in present-day MgB2 wires.