Origin of interface magnetism in BiMnO3/SrTiO3 and LaAlO3/SrTiO3 heterostructures

Possible ferromagnetism induced in otherwise non-magnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetic order is stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated to Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+ ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetic moments are quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in the stabilization of interfacial magnetism.Comment: 5 pages, 4 figure

Anisotropy in c-oriented MgB2 thin films grown by Pulsed Laser Deposition

The electronic anisotropy in MgB2, is still a not completely clear topic; high quality c-oriented films are suitable systems to investigate this property. In this work we present our results on MgB2 superconducting thin films grown on MgO and sapphire substrates. The films are deposited in high vacuum, at room temperature, by Laser Ablation, starting from two different targets: pure Boron and stoichiometric MgB2. In both cases, to obtain and crystallize the superconducting phase, an ex-situ annealing in magnesium vapor is needed. The films were characterized by Synchrotron radiation diffraction measurements; the films turned out to be strongly c-oriented, with the c-axis perpendicular to the film surface and an influence of the substrate on crystallographic parameters is observed. Resisivity measurements with the magnetic field perpendicular and parallel directions to the film surface evidenced an anisotropic upper critical field behavior. The Hc2 ratios (h) resulted in the range 1.2-1.8; this difference will be discussed also in comparison with the literature data.Comment: presented at ISS 2001,Kobe,Japan. submitted to Physica

Growth of c-oriented MgB2 thin films by Pulsed Laser Deposition: structural characterization and electronic anisotropy

MgB2 thin films were deposited using Pulsed Laser Deposition (PLD) and ex-situ annealing in Mg atmosphere. The films presented critical temperatures up to 36K and turned out to be preferentially c-oriented both on Al2O3 (r-cut) and MgO(100) substrates. Synchrotron analyses gave also some indications of in plane texturing. The films exhibit very fine grain size (1200angstromin the basal plane and 100angstrom along c-axis) but the general resistivity behavior and the remarkable extension of the irreversible region confirm that the grains boundaries are not barriers for supercurrents. Upper critical field measurements with the magnetic field perpendicular and parallel with respect to the film surface evidenced a field anisotropy ratio of 1.8. The Hc2 values are considerably higher with respect to the bulk ones, namely when the field lies in the basal plane, and the field-temperature phase diagram for the two magnetic field orientations suggest the possibility of strongly enhancing the pinning region by means of texturing.Comment: 20 pages, 6 figure

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