Preparation and Properties of MgB₂ Thin Films

The superconducting transition temperature of 140 nm ± 10 nm thin films on sapphire substrates, which were deposited by magnetron sputtering, was around 36 K. Using a magnetization technique, the film's critical current density was estimated as J_C = 1.8 × 10⁷ /cm² at 10 K, J_C = 8 × 10⁶ /m ² at 20 K in a zero magnetic field B, and J_C = 3 × 10⁶ /m² at 10 K and B = 5 T. The values of the upper critical magnetic field B_c2 and the irreversibility field B_irr estimated using the four-probe technique were B_C2(22 K) = 15 T when Hfilm surface, 11 T when Ḣfilm surface, and B_irr(22 K) = 11 T when Hfilm surface. The X-ray study showed that the microstructure of the film contains only MgB₂ and MgO (in minority). The SEM and EPXMA study and quantitative Auger spectroscopy analysis revealed periodical variations of the film composition on the nanolevel and the presence of (mainly) two intercalated Mg-B-O-C phases of slightly different, especially in oxygen content, and thus with different conductivity and, possibly, with different T_C. The characteristics of superconducting magnesium diboride films make them promising for application in electronic devices, e.g., as high-pass filters.