Enhancing superconducting properties of MgB2 pellets by addition of amorphous magnetic Ni-Co-B nanoparticles

Amorphous magnetic Ni-Co-B nanoparticles with an average size of 5 nm were added to precursor powders of MgB2 superconductor. The preparation procedure for MgB2 pellets was optimized for obtaining the best critical current density ( Jc) at elevated magnetic fields. Addition of Ni-Co-B decreases the Jc for heat treatment of precursor powders at 650 ° C. Heat treatments at 770 ° C and higher improve Jc at 20 and 5 K. This improvement occurs at both temperatures through the increase of the effective connectivity between MgB2 crystals. Vortex pinning was enhanced at 5 K, but not at 20 K. Ni-Co-B nanoparticles reacted with Mg in heat treatments above 730 ° C, forming Mg2Ni and MgCo2 nanoparticles. Ni-Co-B addition was associated with lower oxygen content in MgB2, indicating that reduction of MgO content is the mechanism for improvement of grain connectivity. Decomposition of magnetic Ni-Co-B nanoparticles results mostly in nonmagnetic nanoparticles, so magnetic pinning did not occur in our samples.