Effects of irradiation on superconducting properties of small-grained MgB2 thin films

We investigate the effect of ion irradiation on MgB2 thin films with small grains of approximately 122 nm and 140 nm.The flux pinning by grain boundaries is insignificant in the pristine MgB2 films due to good inter-grain connectivity,but is significantly improved after 120-keV Mn-ion irradiation.The scaling behavior of the flux pinning force density for the ion-irradiated MgB2 thin films with nanoscale grains demonstrates the predominance of pinning by grain boundaries,in contrast to the single-crystalline MgB2 films where normal point pinning was dominant after low-energy ion irradiation.These results suggest that irradiation-induced defects can accumulate near the grain boundaries in metallic MgB2 superconductors.