Improvement of bulk superconducting current capability of MgB2 films using surface degradation

Abstract The MgB2 films with the degraded MgB2 (SD) surface layer from the Cu ion bombardment show a large improvement in the field performance of critical current density (Jc) and a significant enhancement of the flux pinning force density (Fp). When the SD layer thickness is approximately 11% of the film thickness, the critical field corresponding to Jc = 105 A/cm2, a benchmark for large-scale practical applications, increases from 14.8 kOe at 5 K for the pristine film to 25 kOe. Being markedly different from the widely used chemical doping, the degradation of the small surface region does not lead to the suppression of the superconducting transition temperature (Tc) and low-field Jc. These results demonstrate that the formation of the small active area in MgB2 films is an effective way to improve their superconducting properties, thus paving the way for designing the geometry of flux pinning sites in MgB2.