Enhancement in High-Field J _c Properties and the Flux Pinning Mechanism of ZnO-Buffered MgB₂ Films

We investigated the flux pinning properties in terms of the critical current density (J _c) and pinning force density (F _p) of MgB₂ films with ZnO buffer layers of various thicknesses. At higher thicknesses of the buffer layer, significantly larger J _c values are observed in the high-field region, whereas J _c values in the low- and intermediate-field regions remain largely unaffected. A secondary point-pinning mechanism other than primary grain boundary pinning is observed in the F _p analysis, which depends on the thickness of the ZnO buffer layer. Moreover, a close relationship between the Mg and B bond ordering and the fitting parameter of secondary pinning is obtained, indicating that the local structural distortion of MgB₂ induced by ZnO buffer layers with different thicknesses may contribute to flux-pinning enhancement in the high-field region. Discovering further advantages of ZnO as a buffer layer other than the delamination resistance it provides will help to develop a MgB₂ superconducting cable with a high J _c for power applications.