Superconducting wires and tapes, consisting of a MgB2 superconducting core surrounded by a ferromagnetic Fe sheath, were investigated. The interaction of the superconductor with the soft magnetic environment was shown experimentally to lead to a significant enhancement of dissipation-free supercurrent densities. The maximum densities of the supercurrents could exceed the critical current densities obtained in the same bare (Fe-sheath removed) wire by more than an order of magnitude under given experimental conditions. This current density enhancement was referred to as the over-critical state, which was observed over a wide range of magnetic fields applied transversely to the wire. No over-critical currents were observed for the longitudinal field orientation. The irreversibility field was shown to be considerably suppressed by the influence of the Fe sheath for both field orientations.

Iron-Sheath Influence on the Superconductivity of MgB2 Core in Wires and Tapes. A.V.Pan, S.Zhou, S.Dou: Superconductor Science and Technology, 2004, 17, S410-4