  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.
