Significant reduction of the microwave surface resistance of MgB2 films by surface ion milling

The microwave surface resistance $R_S$ of ${\mathrm{MgB}}_2$ films with the zero-resistance temperature of ∼39 K was measured at 8.0–8.5 GHz. The ${\mathrm{MgB}}_2$ films were prepared by deposition of boron films on $c$ -cut sapphire, followed by annealing in a magnesium vapor environment. The $R_S$ appeared significantly reduced by ion milling of the as-grown ${\mathrm{MgB}}_2$ film surface, with the observed $R_S$ of ∼0.8 mΩ at 24 K for an ion-milled ${\mathrm{MgB}}_2$ film as small as 1/15 of the value for the corresponding as-grown ${\mathrm{MgB}}_2$ film. The reduced $R_S$ of the ion-milled ${\mathrm{MgB}}_2$ films is attributed to the effects of the Mg-rich metallic layer existing at the surfaces of the as-grown ${\mathrm{MgB}}_2$ films.