A study was made of nanoscale Fe particle doping in MgB2. The Fe doping experiments were performed by using both bulk and thin-film forms. Neither free Fe particles nor FeB compounds were detected at 1%Fe doping by means of transmission electron microscopy or X-ray diffraction; suggesting that Fe substituted for Mg in the lattice. The level of Fe substitution for Mg was estimated to occur at a level lower than 1%Mg, and this substitution was proposed to be responsible for the decrease in transition temperature with Fe doping. Because of the high reactivity of nanoscale Fe particles, Fe doping was 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 X-ray diffraction and transmission electron microscopy. The JC(H) performance was severely depressed at above a 2% doping level. The detrimental effect of nanoscale Fe doping upon JC(H) was attributed to Fe substitution for B in the lattice structure and to inclusions of Fe and FeB which acted as weak links at grain boundaries.

The Effect of Nanoscale Fe Doping on the Superconducting Properties of MgB2. S.X.Dou, S.Soltanian, Y.Zhao, E.Getin, Z.Chen, O.Shcherbakova, J.Horvat: Superconductor Science and Technology, 2005, 18, 710-5