Transport properties of superconducting MgB₂ composites with carbon-encapsulated Fe nanospheres

We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB₂ powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB₂ matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB₂ at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB₂ samples. We have also observed improved current densities, as high as 1100 kA/cm² for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain boundaries and/or on point defects.