Microstructural characterization and superconducting properties of YBa 2 Cu 3 O x films on nano-undulated Nd 1/3 Eu 1/3 Gd 1/3 Ba 2 Cu 3 O x sublayer Irene Rusakova a, * , Ruling Meng a , Tom H. Johansen b , Duc Pham a , Feng Chen a , Zhenyu Zuo a , Andrei Baikalov a , Yanyi Sun a , C.W. Chu a,c a Department of Physics and the Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002, USA b Department of Physics, University of Oslo, P.O. Box 1048, Blindern, N-0316 Oslo, Norway c Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Available online 24 March 2007 Abstract Properties of YBa 2 Cu 3 O x (YBCO) thin films deposited on Nd 1/3 Eu 1/3 Gd 1/3 Ba 2 Cu 3 O x (NEG) sublayers with a nano-undulated surface morphology were investigated. We found a strong influence of the NEG sublayer on the microstructure and superconducting properties of such YBCO films. The correlation between microstructural features and superconducting properties is discussed. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Flux-pinning; Critical current density; TEM; YBCO film 1. Introduction Flux-pinning enhancement of type-II superconductors with inherent defects has been widely studied and many novel techniques have been introduced to create artificial pinning centers. In these approaches, substantial lattice mismatch or chemical poisoning was used to create the desired flux-pinning defects [1–4]. We have applied a simple and easily controllable method of forming self-organized nano-corrugations on the surface of Nd 1/3 Eu 1/3 Gd 1/3 - Ba 2 Cu 3 O x (NEG) superconducting sublayer on which YBCO film is deposited. This process led to a 50% increase in j c in bilayer superconducting YBCO/NEG thin films. In this paper we report on the microstructure aspects of these bilayer films, aiming to understand the origin of the large flux-pinning enhancement. 2. Results and discussion The films were deposited on SrTiO 3 (1 0 0) single crystal substrates by pulse laser deposition. The space-resolved magnetic behavior of the samples was investigated by mag- neto optical (MO) imaging. The critical current density was determined by using the in states of partial flux penetra- tion. Using the procedure described above, we find that in the bilayer configuration the YBCO film alone has a critical current density as high as j c = 7.2 · 10 7 A/cm 2 at 5 K. As T increases the value drops to j c (60 K) = 1.8 · 10 7 A/cm 2 and j c (77 K) = 4.6 · 10 6 A/cm 2 . These numbers are compared with the critical current density of a reference sample of YBCO prepared under the same conditions. For such a film we find j c (5 K) = 4.7 · 10 7 A/cm 2 , j c (60 K) = 1.2 · 10 7 A/cm 2 and j c (77 K) = 2.4 · 10 6 A/cm 2 , showing consistently that YBCO on NEG has a 50–100% enhance- ment in j c between 5 and 77 K. Conventional and high resolution transmission electron microscopy (HR TEM) reveals atomically smooth interface of NEG film and STO substrate while the top surface of 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.057 * Corresponding author. Tel.: +1 713 743 8286; fax: +1 713 743 8201. E-mail address: rusakova@uh.edu (I. Rusakova). www.elsevier.com/locate/physc Physica C 460–462 (2007) 384–385