Epitaxial LaNiO 3 thin films: A normal metal barrier for SNS junction
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K. M. Satyalakshmi and R. M. Mallya Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India
M. Rajeswari and H. Zhang Department of Physics, Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742 (Received 8 July 1993; accepted 15 December 1993)
Epitaxial LaNiO 3 thin films have been grown on SrTiO3 and several other substrates by pulsed laser deposition. The films are observed to be metallic down to 15 K, and the temperature dependence of resistivity is similar to that of bulk LaNiO 3 . Epitaxial, c-axis oriented YBa 2 Cu 3 07_ x films with good superconducting properties have been grown on the LaNiO3 (100) films. I-V characteristics of the YBa 2 Cu 3 O 7 -LaNiO 3 junction are linear, indicating ohmic contact between them.
I. INTRODUCTION Metallic oxides having perovskite-related structures are being re-investigated to understand the physics behind their metallic property and to search for superconductivity in the metal-insulator transition region. Semiconducting oxides such as LaCoO 3 showed metallic property by a partial substitution of La 3+ by Sr 2+ , and such a substitution is being investigated as a possible source of hole doping. Since these metallic oxides can form lattice-matched normal metal layers for superconductor-normal metal-superconductor (SNS) type Josephson junctions with high Tc oxide materials, there is special interest in studying them. In addition, they can be good electrodes for superconducting as well as ferroelectric oxides. Thus, LaTiO3, which was believed to be a metallic oxide, has been found to be a semiconductor when the oxygen stoichiometry is 3, and only on doping Sr for La does it become a metal.1 New RNiO3 (R = Pr, Nd, Sm, and Er) compounds have been synthesized, and they showed metal-insulator transitions.2 LaNiO3 was first synthesized by Wold et al.,3 and it has long been known that it is a metallic and Pauli paramagnetic oxide.4 Other related oxide systems are SrRuO 3 , Ln^Sr^MnOs (Ln = lanthanide ion), Lni^Ca x MnO 3 , and Ln^^Sr^CoOs, which are known to be ferromagnetic metals; their electrical and magnetic properties have been discussed by Goodenough and Longo.5 Fabrication of thin films of the metallic oxide materials mentioned above is of current interest. Fabrication and properties of Sri_ x Ca^RuO 3 (Ref. 6) and SrRuO3 (Ref. 7) metallic thin films have been reported recently, and they are used as normal metals for making SNS junctions. In a preliminary report, 898
J. Mater. Res., Vol. 9, No. 4, Apr 1994
we have shown epitaxial growth of metallic LaNiO3 (LNO) films on LaA10 3 (LAO) and also showed c-axis growth of superconducting YBa 2 Cu 3 0 7 (YBCO) on LNO. 8 Sagoi et al.9 have shown that LNO sandwiched between YBCO and Au films formed an SNS junction. Until recently, PrBa 2 Cu 3 0 7 (PBCO) has been the only material lattice matched with YBCO that has been used in the fabrication of Josephson junctions. These junctions have been found to show SNS type behavior even though PBCO showed only semiconducting tr
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