Variation of pinning mechanism and enhancement of critical current density in MgB2 bulk containing self-generated coherent MgB4 impurity

Bulk MgB2, with self-generated MgB4 pinning centers, have experienced two-step sintering process, initially at 750 degrees C and then 900-1000 degrees C. On the contrary to the widely accepted point that MgB4 deteriorates superconductivity, it was found that MgB4 played a significant role in enhancing critical current density. The precipitation pattern of MgB4 was studied from the lattice scale images. It was observed that the initial coherent relation between the MgB4 and the matrix was destroyed to become semi-coherent and even incoherent as the second-step sintering temperature increased. Owing to the lattice distortion caused by the elastic accommodation of the coherent interface, the small-sized MgB4 particles controlled by the sintering temperature, and the fine grain connectivity affected by the porosity, the critical current density was improved over the entire magnetic field. Finally, the dominating pinning mechanism within the crystal was confirmed to be Delta k pinning in the two-step sintered MgB2 sample, where the k is the Ginzburg-Landau parameter, while the mechanism of one-step sintered sample is surface pinning. (C) 2013 AIP Publishing LLC.