Effects of ion milling on the microwave properties of MgB2 films

The new superconductor MgB₂ may prove useful for microwave applications at intermediate temperatures. MgB₂ films with the thickness of 300 - 400 nm have surface resistance R_S less than 10 μΩ at a frequency (ω/2π) of 8.5 GHz and 7 K, and ∼ 1.5 mΩ at 87 GHz and 4.2 K. The critical temperature (T_C) of these films is ∼ 39 K when they are prepared under optimum conditions. The R_S appears to scale as ω² up to 30 K. After surface ion-milling, a reduction of the T_C and an enhanced resistivity ρ(T_C) are observed consistently at 8.5 GHz and 87 GHz along with a reduced R_S at low temperatures. The observed ρ(T_C) - T_C relation and the uncorrelated behavior between ρ(T_C) and R_S values measured at low temperatures are well explained in terms of the two-gap model, with the observed ρ(T_C) - T_C relation attributed to the properties of the large gap, and the R_S at lower temperatures reflecting the properties of the small gap, with an enhanced gap energy due to increased interband scattering. This study suggests that the interband scattering should be enhanced to improve the low temperature microwave properties of MgB₂ films and that the ion-milling process must be performed with great care to preserve the high quality of MgB₂ films.