Interaction of superconductor with magnetic sheath as a way for improvement of critical current in MgB2/Fe superconductor

Abstract

Magnesium diboride superconducting wires give the largest critical current density (Jc) when produced with iron sheath. Because iron is ferromagnetic, it is expected to improve the field dependence of Jc by shielding of the external field for low magnetic fields. However, transport and magnetic measurements of Jc reveal that Jc in MgB2/Fe is improved far beyond the effect of simple magnetic shielding. The transport measurements in external field show that Jc initially decreases with the field. This is followed by an increase for intermediate fields and again a decrease for high fields, resembling the ?peak effect?. The value of Jc in the field range of this peak effect is higher than the Jc without iron sheath. The field range of improved Jc widens with decreasing the temperature, shifting to the higher values of the field. The explanation of this phenomenon is suggested in terms of a model predicting the occurrence of overcritical state, as a result of interaction between partly vortex filled superconductor and a magnet. In this model, the currents are pushed into vortex-free volume of the superconductor, effectively increasing its value of loss-free current. The occurrence of the overcritical state is supported by magnetic measurements of Jc.