Enhancement of the critical current density in MgB2 wires doped with Ni nanoparticles

Iron-sheathed MgB2 wires doped with 0, 1.3 and 2.52 wt% carbon protected nickel superparamagnetic nanoparticles (the average diameter of the particles is 20 nm) and sintered at 650 degrees C were prepared. X-ray diffraction patterns and magnetization measurements showed that neither substitution of C for B nor substitution of Ni for Mg occurred during the synthesis process. Scanning electron microscopy imaging of the doped sample revealed a homogeneous distribution of nickel particles within the MgB2 matrix. Transport (magnetoresistivity R(T, B) and critical current density J(c)(B) in the temperature range 1.5-40 K) and magnetic measurements (magnetic hysteresis loops at temperatures below and above the superconducting transition temperature) were performed on Fe-sheathed wires and the superconducting cores of these wires. A small enhancement of the irreversibility field B-irr(t = T-irr(B)/T-irr(0)) of the doped wires was observed in the low field range. Significant enhancement of J(c)(B), especially at low temperature (5 K), was observed: at 5 K and 10 T, for both doped wires, J(c) is 2.5 times larger than that for the undoped wire.