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

The microwave surface resistance RS of MgB2 films with the zero-resistance temperature of ∼39 K was measured at 8.0–8.5 GHz. The MgB2 films were prepared by deposition of boron films on c-cut sapphire, followed by annealing in a magnesium vapor environment. The RS appeared significantly reduced by ion milling of the as-grown MgB2 film surface, with the observed RS of ∼0.8 mΩ at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the corresponding as-grown MgB2 film. The reduced RS of the ion-milled MgB2 films is attributed to the effects of the Mg-rich metallic layer existing at the surfaces of the as-grown MgB2 films.