Reduction of strain in high temperature superconductor thin film devices M.I. Faley * , S.B. Mi, A. Petraru, C.L. Jia, U. Poppe, K. Urban IFF, Forschungszentrum Ju ¨ lich GmbH, D-52425 Ju ¨ lich, Germany Available online 6 April 2007 Abstract Multilayer buffer layers containing BaZrO 3 thin films on the bottom and SrTiO 3 thin films on the top were successfully applied to improve the epitaxial growth of the YBa 2 Cu 3 O 7x (YBCO) films on the MgO substrates, which have the best correspondence of the thermal expansion coefficient with one of YBCO. The HRTEM and X-ray diffraction studies demonstrated pure c-axis orientation and absence of the in-plane misoriented grains in the YBCO films grown on the buffered MgO substrates. High superconducting param- eters and no cracks were observed in such YBCO films even at their thicknesses 1 lm. The multilayer flux transformers having total thickness 2-lm were prepared on the buffered MgO substrates and demonstrated an improved insulation between the superconducting layers and an increased dynamic range compared to flux transformers on SrTiO 3 substrates. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Buffered MgO; Epitaxial passivation; HTS heterostructures; Strain; Cracks 1. Introduction The epitaxial films of high-temperature superconductor (HTS) YBa 2 Cu 3 O 7x (YBCO) are typically prepared at substrate temperatures 700–900 °C higher than their stor- age and operation temperatures. Due to the difference in the thermal expansion coefficients of the substrates and films an essential tensile strain in the YBCO films appears, which degrades their superconducting properties and can even crack the films when their thickness exceeds some crit- ical value. Thick multilayer HTS thin film structures like multiturn flux transformers are, for example, required for the production of sensitive SQUID magnetometers [1]. MgO substrates have similar thermal expansion coeffi- cients to that of YBCO. The main disadvantage of MgO substrates is the degradation of the hygroscopic surface in the air. These substrates also have a lattice mismatch of 9% with YBCO and a different crystal structure. These properties usually lead to a significant amount of in-plane 45° misoriented grains in the YBCO films deposited on MgO substrates, which significantly suppresses the average critical current density of the HTS films. Deposition of buf- fer layer materials like SrTiO 3 (STO) or BaZrO 3 (BZO) have been suggested in order to improve the epitaxial growth of YBCO films on MgO substrates (see, e.g., [2]). The buffer layer can preserve the hygroscopic surface of the MgO substrates from degradation in air and/or during the lithographic procedures. At least two buffer layers are required to deposit YBCO on MgO: the first one should provide the epitaxial growth of films with perovskite struc- ture on the rock-salt structure of MgO, while the second buffer layer should match the lattice constants. In the present work, we describe a multilayer thin film buffer, which provided an epitaxial passivation of the sur- face of the single crystal MgO substrates and allowed a sig- nificant improvement of the epitaxial growth of the relatively thick crack-free YBCO films on it. 2. Experimental results High-oxygen-pressure sputtering was used to deposit the thin film buffer layers and YBCO films on STO, LaAlO 3 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.006 * Corresponding author. Tel.: +49 2461 61 4366; fax: +49 2461 61 6444. E-mail address: m.faley@fz-juelich.de (M.I. Faley). www.elsevier.com/locate/physc Physica C 460–462 (2007) 1371–1372