Preparation and characterization of superconducting MgB2 films on alumina

Polycrystalline MgB2 films with a thickness of about 1 µm were prepared at ambient pressure. The boron layers were deposited onto single- and polycrystalline Al2O3 by screen printing, by airbrush techniques or by painting, followed by a reaction with Mg vapor. Grinding, mechanical and electrolytic polishing in non-aqueous dimethyl sulfoxide significantly improved the surface smoothness of the samples. The films had transition temperatures (Tc(0)) of 35 K and irreversibility fields of up to 15 T at 5 K. Transport critical current densities of 3 x 108 A m-2 were measured at 20 K and 1 T for MgB2 films on single-crystalline Al2O3, whereas 8 x 108 A m-2 were found for films on polycrystalline Al2O3 under the same conditions. Magnetization data and magneto-optical imaging indicated instabilities (flux jumps) at 4.2 K, which disappeared at around 10 K. The critical current densities of all MgB2 films on either single- or polycrystalline Al2O3 were independent of the orientation of the film with respect to the magnetic field direction.