Flux pinning and inhomogeneity in magnetic nanoparticle doped MgB2/Fe wires

The effects of magnetic nanoparticle doping on superconductivity of MgB2/Fe wires have been investigated. Fe2B and SiO2-coated Fe2B particles with average diameters 80 and 150 nm, respectively, were used as dopands. MgB2 wires with different nanoparticle contents (0, 3, 7.5, 12 wt.%) were sintered at temperature 750 degrees C. The magnetoresistivity and critical current density J(c) of wires were measured in the temperature range 2-40 K in magnetic field B <= 16 T. Both transport and magnetic J(c) were determined. Superconducting transition temperature T-c of doped wires decreases quite rapidly with doping level (similar to 0.5 K per wt.%). This results in the reduction of the irreversibility fields B-irr(T) and critical current densities J(c)(B,T) in doped samples (both at low (5 K) and high temperatures (20 K)). Common scaling of J(c)(B,T) curves for doped and undoped wires indicates that the main mechanism of flux pinning is the same in both types of samples. Rather curved Kramer's plots for J(c) of doped wires imply considerable inhomogeneity.