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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 °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 Jc(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 Birr(t = Tirr(B)/Tirr(0)) of the doped wires was observed in the low field range. Significant enhancement of Jc(B), especially at low temperature (5 K), was observed: at 5 K and 10 T, for both doped wires, Jc is 2.5 times larger than that for the undoped wire.