Browsing by Author "Malisa, Anayesu B."

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    Ca-doped Ba2Cu3O7–δ Bicrystal Junctions Fabricated on Asymmetric SrTiO3 Substrates
    (2005-07) Malisa, Anayesu B.
    Bicrystal grain–boundary Josephson junctions of Ca–doped YBa2Cu3O7–δ that is Y0.7Ca0.3Ba2Cu3O7–δ were fabricated on three bicrystal SrTiO3 (001) substrates with asymmetric 30°, 40° and 45° orientations. An enhancement of the critical current density in these Ca–doped junctions was observed when compared with normal YBCO grain–boundary junctions with similar angular orientations. The observed increase in the critical current density is large for the junctions fabricated on the asymmetric 30° bicrystal substrate and small or negligible, for those on the asymmetric 45° bicrystal substrate. The critical current was modulated by a magnetic field applied in the plane of the junctions. However, the Fraunhofer pattern observed due to the applied magnetic field deviates from the ideal one.
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    Colossal magnetoresistance effect in epitaxially grown La2/3Ca1/3MnO3 perovskite-like manganite thin films
    (2005-02) Malisa, Anayesu B.; Ivanov, Z.
    We report in this work, study on colossal magnetoresistance (CMR) effect in epitaxial La2/3Ca1/3 MnO3 thin films grown on SrTiO3 (0 0 1) substrates by pulsed laser deposition (PLD) technique. The films were grown on as-received SrTiO3 substrates and on SrTiO3 substrates prepared by HF etching (Koster et al., Appl. Phys. Lett. 73 (1998) 2920; V. Leca et al., Wet etching methods for perovskite substrates, University of Twente, MESA+ Research Institute, Low Temperature Division). Two of the samples were annealed in different conditions to investigate the films heat treatment effect on electric and magnetic properties. Electrical resistance was done using the four-probe method at temperatures in the range of 2–375 K without a magnetic field and in an external field of 5 T applied in the film plane. Resistance-magnetic field (R vs. H) at 77 K for the two annealed samples was done in a 5 T sweep magnetic field. The surface morphology and structural information of the films were obtained using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. Secondary ion mass spectroscopy (SIMS) analysis was performed on the annealed samples to investigate any possible chemical reaction between La2/3Ca1/3MnO3 thin films and SrTiO3 substrate.
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    In Situ Superconducting MgB2 Films Grown by Co-evaporation Technique
    (2005) Malisa, Anayesu B.
    Superconducting thin films of MgB2 were grown in situ by co-evaporation of magnesium and boron on MgO (111), SiC (0001) and Al2O3 (0001) substrates at ∼300 °C. The films showed transitions to the superconducting state in the temperature range of 33–37 K. The widths of the superconducting transitions were less than 1 K. The films exhibited growth in the c-axis and absence of other phases. Optimization of the growth parameters enabled films with smooth surface morphology and root-mean-square (RMS) roughness of 2 nm to be grown. The low RMS roughness value indicates that the technique can be used to grow MgB2 films useful for junctions and multilayer devices. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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    Josephson effect in MgB2∕Pd∕Fe∕NbMgB2∕Pd∕Fe∕Nb Josephson junctions
    (2005-11) Malisa, Anayesu B.
    MgB2∕Pd∕Fe∕NbMgB2∕Pd∕Fe∕Nb Josephson junctions were fabricated and characterized. The junctions exhibited the Josephson effect. A supercurrent was observed across the magnetically active layer formed at the Pd and Fe interface and also across the magnetic metal Fe itself. Shapiro steps and fractional Shapiro steps were observed when microwave fields were applied to the junctions. Modulation of the critical currents of the junctions was observed when an external magnetic field was applied in the plane of the junctions. The maxima of the magnetic field dependence of the critical current(Ic−B)(Ic−B) did not occur at zero fields but at small positive fields of about 3.7μT3.7μT. The magnetic field of 3.7μT3.7μT increased the critical current by 100% when it was applied in the plane of the junction or perpendicular to the junction plane. The effect disappeared when the polarity of the field was reversed.
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    Josephson Effects in Magnesium Diboride Based Josephson Junctions
    (2004-04) Malisa, Anayesu B.; Valkeapää, Markus; Johansson, Lars G.; Ivanov, Zdravko
    We report on Josephson effects in Josephson junctions fabricated from magnesium diboride (MgB2) thin films using a focused ion beam (FIB) milling technique. The films were deposited on SrTiO3(100) and {\mathrm {Al_2O_3}}(1\bar 102) substrates at room temperature using e-beam evaporation of MgB2 with a post-annealing stage in Mg vapour. Conventional photolithography and Ar-ion-beam milling were used to pattern the films into 4 and 8 µm wide microbridges. A focused ion beam was used to narrow the microbridges to 2 µm. The narrowed microbridges were thinned by making 50 nm wide cuts across them. The depth of each cut was calibrated to remove 75% of the film thickness. A thin MgB2 layer remained on the substrate after this process. The current–voltage (I–V) characteristics of junctions made using this technique show that the junctions carry excess current. The first Shapiro step was observed when one of the junctions was irradiated with a microwave field of frequency f = 8.92 GHz. The Shapiro step appeared at a voltage value V = hf/2e = 18.445 µV.
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    Josephson Junctions Fabricated by Focused Ion Beam from Ex Situ Grown MgB2 Thin Films
    (2004-03) Malisa, Anayesu B.; Valkeapää, M.; Johansson, L. -G; Ivanov, Zdravko
    We prepared MgB2 thin films on SrTiO3 (1 0 0) and Al2O3 (1 1̄ 0 2) substrates by e-beam evaporation of MgB2 pellet. The films were deposited at room temperature and post-annealed at 900 °C in Mg vapour for 5–30 min. Superconducting transition temperatures were observed between 22 and 30 K. Structure and surface morphology of the films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The films grown on Al2O3 substrates are c-axis oriented while a film grown on SrTiO3 substrate is aligned with the (1 0 1) direction normal to the substrate planes. The films have grain sizes of about 70 nm. The films were patterned into 4 and 8 μm wide microbridges. The microbridges were observed to carry large critical current densities of approximately 1 MA/cm2 at 6.7 K. Focused ion beam (FIB) was used on the bridges in order to fabricate Josephson junctions. A cut 50 nm in width was made across the microbridges followed by an in situ platinum (Pt) deposition into the cut made. SNS-like weak-link junctions were formed in the process.
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    Magnesium Diboride Nanobridges Fabricated by Electron-Beam Lithography
    (2005-12) Malisa, Anayesu B.; Charlebois, S.; Lindström, T.
    MgB2MgB2 nanobridges were fabricated by e-beam lithography and Ar-ion beam milling. Nanobridges of widths ranging from 60 nm to 1μm1μm and 3μm3μm in length were realized by Ar-ion beam milling using amorphous carbon as etching mask. The processing did not harm the superconductingproperties appreciably. High values of the critical current density, more than 10MA/cm210MA∕cm2, were measured for bridges with widths down to 60 nm. Current-voltage (I‐V)(I‐V)characteristics showed a behavior typical of a bridge going normal, after the critical current is exceeded, and remaining normal as the current is decreased to a lower switch back value due to Joule heating. We could also observe switching behavior in some bridges indicating formation of normal hotspots in the bridges before they returned to their superconducting state. Alternative explanations may include natural grain boundaries in the film or the movement of Abrikosov vortices. The current-voltage (I‐V)(I‐V)characteristics showing critical current densities up to 5×107A/cm25×107⁢A∕cm2 indicates excellent filmproperties in the nanobridges.
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    Superconductivity in Magnesium Diboride Thin Films
    (2002-04) Ivanova, Z.; Malisa, Anayesu B.; Johansson, L. -G; Komissinski, P. V.
    Mg–B thin films were grown in situ on (1 0 0) SrTiO3 substrates by pulsed laser deposition. Targets pressed from a commercial MgB2 powder had a superconducting transition temperature of 37 K. Films were characterized by atomic force microscopy and susceptibility measurements. First samples showed superconducting onset at 30 K and became superconducting at about 20 K. Films were patterned into microbridges 4–100 μm wide by Ar ion beam milling. Films had large contact resistances, of the order of 500 Ω cm2 in the temperature range 4.2–300 K.