earticle

영어

We investigate the effect of surface disorder on the lower critical field (Hc1) of MgB2 thin films with a thickness of 850 nm, where the disorder on the surface region is produced by the irradiation of 140 keV Co ions with the dose of 1 X 1014 ions/cm2. The thickness of the damaged region by the irradiation is around 143 nm, corresponding to ~17% of the whole thickness of the film, thereby forming the disordered MgB2 overlayer on the pure MgB2 layer. The magnetic field dependence of magnetization, M(H), for the pristine MgB2 thin film and the film with overlayer is measured at various temperatures, and Hc1 is determined from the difference (ΔM) between the Meissner line and magnetization signal with the criterion of M = 10−3 emu. Intriguingly, the film with the disordered overlayer shows a remarkably large Hc1(0) = 108 Oe compared to the Hc1(0) = 84 Oe of pristine film, indicating that the disordered MgB2 overlayer on the pure MgB2 layer serves to prevent the penetration of vortices into the sample. These results provide new ideas for improving the superheating field to design high-performance superconducting radio-frequency cavities.