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Polycrystalline MgB 2 films with a thickness of about 1 µm were prepared at ambient pressure. The boron layers were deposited onto single-crystalline and polycrystalline Al 2 O 3 by screen printing, by airbrush techniques or by painting, followed by a reaction with Mg vapour. Grinding, mechanical and electrolytic polishing in non-aqueous dimethyl sulfoxide significantly improved the surface smoothness of the samples. The films had transition temperatures (T c(0) ) of 35 K and irreversibility fields of up to 15 T at 5 K. Transport critical current densities of 3 × 10 8 A m −2 were measured at 20 K and 1 T for MgB 2 films on single-crystalline Al 2 O 3 , whereas 8 × 10 8 A m −2 was found for films on polycrystalline Al 2 O 3 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 MgB 2 films on either single-crystalline or polycrystalline Al 2 O 3 were independent of the orientation of the film with respect to the magnetic field direction.

MgB2 has been doped with 57Fe and 57Co in order to probe the electronic structure of the superconductor by Mössbauer spectroscopy. Simultaneous SEM–EDX studies showed that both Fe and Co were incorporated to some extent into the MgB2 host lattice. Mössbauer spectroscopy indicated that Fe2B and FeB were also formed during the preparation. At higher Fe contents Fe2B appeared unambiguously as a secondary phase in the x-ray diffraction. CoB was detected at substitution levels of 10 mol% and greater. Both Fe and Co doping decreased the Tc(0) modestly. Co and Fe were found to substitute at the Mg site in the MgB2 lattice. The Mössbauer parameters for 57Co and 57Fe were consistent with a metallic environment. The similarity of the isomer shifts in MgB2 and in cuprate superconductors is being discussed.

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