Effects of ion milling on the microwave properties of MgB/sub 2/ films

The new superconductor MgB/sub 2/ may prove useful for microwave applications at intermediate temperatures. MgB/sub 2/ films with the thickness of 300 - 400 nm have surface resistance R/sub S/ less than 10 /spl mu//spl Omega/ at a frequency (/spl omega//2/spl pi/) of 8.5 GHz and 7 K, and /spl sim/ 1.5 m/spl Omega/ at 87 GHz and 4.2 K. The critical temperature (T/sub C/) of these films is /spl sim/ 39 K when they are prepared under optimum conditions. The R/sub S/ appears to scale as /spl omega//sup 2/ up to 30 K. After surface ion-milling, a reduction of the T/sub C/ and an enhanced resistivity /spl rho/(T/sub C/) are observed consistently at 8.5 GHz and 87 GHz along with a reduced R/sub S/ at low temperatures. The observed /spl rho/(T/sub C/) - T/sub C/ relation and the uncorrelated behavior between /spl rho/(T/sub C/) and R/sub S/ values measured at low temperatures are well explained in terms of the two-gap model, with the observed /spl rho/(T/sub C/) - T/sub C/ relation attributed to the properties of the large gap, and the R/sub 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 MgB/sub 2/ films and that the ion-milling process must be performed with great care to preserve the high quality of MgB/sub 2/ films.