Enhancement in High-Field J c Properties and the Flux Pinning Mechanism of ZnO-Buffered MgB 2 Films.

We investigated the flux pinning properties in terms of the critical current density ( J c ) and pinning force density ( F p ) of MgB 2 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 MgB 2 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 MgB 2 superconducting cable with a high J c for power applications.