Superconducting properties and growth mechanism of layered structure in MgB₂ bulks with Cu/Y₂O₃ co-doping

Bulk MgB₂ samples containing Cu and Y₂O₃ have been prepared by conventional solid state reaction at 850 °C, and the structure and superconducting properties have been investigated. Differing from the structure in previous studies, a type of novel, layered structure was obtained in Cu/Y₂O₃-doped MgB₂ sample. Furthermore, the critical current density (J_c) at high field (>4 T) was improved compared to undoped, and Cu-doped MgB₂ samples. After analyzing the phase composition, micro-structure, and sintering process, it was found that Y₂O₃ and Cu are independent in providing effective pinning centers, and YB4 impurity should be responsible for the enhancement of J_c as well as the increased irreversible magnetic field. However, J_c at low field was slightly worsened, but still maintaining 10⁵ A cm^-2 at 0 T, since the intercrystalline connectivity was not seriously deteriorated. Finally, a possible growth model was put forward to describe the formation sequences of the layered structure. It was supposed that the formation of steps at low temperature originated from the coherent relationship between Mg and MgB₂, while the steps formed at high temperature were related to the pinning effect of secondary phase during the migration of grain boundary.