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°C and then 900-1000°C. On the contrary to the widely accepted point that MgB ₄ 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 MgB ₄ 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 Δκ pinning in the two-step sintered MgB₂ sample, where the κ is the Ginzburg-Landau parameter, while the mechanism of one-step sintered sample is surface pinning.