Overcritical state in superconducting round wires sheathed by iron

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Magnetic measurements carried out on MgB₂ superconducting round wires have shown that the critical current density Jc(Bₐ) in wires sheathed by iron can be significantly higher than that in the same bare (unsheathed) wires over a wide applied magnetic field Bₐ range. The magnetic behavior is, however, strongly dependent on the magnetic history of the sheathed wires, as well as on the wire orientation with respect to the direction of the applied field. The behavior observed can be explained by magnetic interaction between the soft magnetic sheath and superconducting core, which can result in a redistribution of supercurrents in the flux filled superconductor. A phenomenological model explaining the observed behavior is proposed.