DC Shielding Properties of Coaxial MgB2/Fe Cups

We investigated the efficiency in magnetic field mitigation of superimposed MgB2/Fe coaxial cups with cylindrical symmetry, subjected to an applied magnetic field parallel to their axis. The MgB2 cup was grown by a microwave-assisted Mg-liquid infiltration technique in a boron preform. This technique allows obtaining samples with a reduced amount of unreacted magnesium, shape chosen before the growth procedure and easily scalable sizes. Local magnetic induction measurements were carried out by means of a cryogenic Hall probe mounted on a custom-designed stage moveable along the sample axis with micrometric resolution. The measurements were performed at 20 K and 30 K, in magnetic field up to 1.5 T, as a function of the external source field, of the position and of time. At higher magnetic fields the superposition of the Fe cup on the MgB2 one allows obtaining a shielding efficiency 3-4 times higher than that measured with the single superconducting cup. The magnetic induction relaxation rate is also strongly reduced. On the contrary, a decrease of the shielding efficiency of the hybrid system with respect to the MgB2 cup alone turns out at low magnetic field. Numerical simulations indicate a reduction of this worsening by a protrusion of the superconducting cup above the ferromagnetic one.