The effect of nanoscale Fe doping on the superconducting properties of MgB2

Iron is an important sheath material for fabrication of MgB2 wires. However, the effect of Fe doping on the superconducting properties of MgB2 remains controversial. In this work, we present results on nanoscale Fe particle doping in MgB2. The Fe doping experiments were performed using both bulk and thin film forms. Neither free Fe particles nor FeB compound was detected at 1% Fe doping by means of either transmission electron microscopy (TEM) or x-ray diffraction (XRD), suggesting that Fe substituted for Mg in the lattice. The level of Fe substitution for Mg is estimated to occur at a level lower than 1% of Mg, and this substitution is proposed to be responsible for the decrease in transition temperature with Fe doping. Because of the high reactivity of nanoscale Fe particles, Fe doping is largely in the form of FeB at a Fe doping level of 2% while Fe2B was detected at 10% Fe doping by means of both XRD and TEM. The Jc(H) performance was severely depressed at above a 2% doping level. The detrimental effect of nanoscale Fe doping on Jc(H) is attributable to both the Fe substitution for B in the lattice structure and the inclusions of Fe and FeB which act as weak links at grain boundaries.