  Local magneto-optical imaging and global magnetization measurement techniques
were used in order to visualize shielding effects in the superconducting core
of MgB_2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can
provide a Meissner-like state in the superconducting core in applied magnetic
fields up to ~1T. The maximum shielding fields are shown to correlate with the
saturation fields of magnetization in Fe-sheaths. The shielding has been found
to facilitate the appearance of an overcritical state, which is capable of
achieving a critical current density (J_c) in the core which is larger than J_c
in the same wire without the sheath by a factor of ~2. Other effects caused by
the magnetic interaction between the sheath and the superconducting core are
discussed.
