Self-patterned Nano Structures in Structurally Gradient Epitaxial La 0.5 Ba 0.5 CoO 3 Films Jie He a , Jiechao Jiang a, ⁎, Jian Liu b , Ming Liu b , Greg Collins b , Chunrui Ma b , Chonglin Chen b , Efstathios I. Meletis a a Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, Texas, 76019, USA b Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, 77204, USA abstract article info Article history: Received 17 June 2010 Received in revised form 2 February 2011 Accepted 3 February 2011 Available online 18 February 2011 Keywords: Electron microscopy High-resolution transmission electron microscopy Electron diffraction Epitaxy Oxides Magnetic properties Highly epitaxial La 0.5 Ba 0.5 CoO 3 (LBCO) thin films with sharp interface and a thickness of 200 nm were epitaxially grown on (001) SrTiO 3 substrates using pulsed laser deposition. High-resolution transmission electron microscopy and electron diffraction analysis revealed that the films have a triple-layered structure. The first layer, close to the film/substrate interface, has a thickness of ~ 6 nm and is a defect free single crystal disordered cubic structure (a = 3.882 Å) which has a lattice mismatch of -0.59% with respect to the substrate. The second layer which dominates the film structure has a single crystal disordered cubic structure (a = 3.854 Å) which has a lattice mismatch of -1.31% with respect to the substrate. The third layer located on the top of the film has a thickness of several nanometers and consists of 112-type ordered tetragonal structure. The cubic structures in the first and second layer have an orientation relationship of (001) LBCO //(001) STO and b 100 N LBCO //b 100 N STO with respect to the substrate. Self-patterned 3-dimensional nano structures with a dimension range from 2 to 10 nm were formed in the second and third layers. These nano structures were formed by the enclosure of anti-phase boundary planes which are parallel to the {100} of the cubic structure. Epitaxial LBCO thin films with such nano structures are hard ferromagnetic with a large coercive field value and magnetoresistance effect value (~ 24%), and exhibit semiconductor behavior at temperatures b 300 K. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Perovskite cobaltate LaCoO 3 is a mixed ionic/electronic conducting with high oxygen diffusivity and surface exchange rate and has very interesting electronic structure, phase and magnetic transitions [1–4]. Substitution of La 3+ by A 2+ (A = Sr, Ca, Ba), La 1-x A x CoO 3 , results in the reduction of the valence on doped A-site cations and produces significant changes in their electronic and magnetic properties [5,6]. For example, LaBaCo 2 O 5.5+δ has shown various captivating physical phenomena due to the order–disorder of the A-site cations, various oxidation states (Co 2+ /Co 3+ /Co 4+ ) of cobalt, and different spin state configurations [7,8]. Bulk material of fully oxidized nanoscale-ordered LaBaCo 2 O 6, disordered La 0.5 Ba 0.5 CoO 3 , and the oxygen deficient ordered LaBaCo 2 O 5.5 have shown unusual magnetization and magne- totransport properties associated with the spin state of cobalt at the low temperature and the locking of magnetic spins in the nanodomain structures [9–13]. To better understand the physical phenomena in these complex systems, it is highly desirable to study the single crystalline materials by minimizing the side effects arising from porosity and grain boundaries in the bulk phase. In this paper, we report on the self-patterned 3-dimensional nano structures in structurally gradient La 0.5 Ba 0.5 CoO 3 (LBCO) films epitaxially grown on (001) SrTiO 3 (STO) substrates fabricated using pulsed-laser deposition and the associated physical properties. 2. Experimental details The LBCO thin films were deposited on (001) STO substrate using a KrF excimer Pulsed Laser Deposition (PLD) system with a wavelength of 248 nm. The high density, single phase, stoichiometric LBCO target used in the deposition was purchased from MTI Crystal, Inc. A (001) STO single crystal (a = 3.905 Å) was selected as the substrate for epitaxial growth of LBCO thin film. During the deposition, the energy density was about 2.0 J/cm 2 , the laser repetition rate was 5 Hz, the optimal growth temperature was 850 °C and oxygen pressure was in 1.3–33 Pa. The as-grown films were annealed in 2.67 × 10 4 Pa oxygen for 15 minutes at 850 °C and cool down to room temperature at a rate of 5 °C/min. Cross-section and plan-view transmission electron microscope (TEM) specimens were prepared by the procedure of mechanical grinding, polishing and dimpling and ion milling using a Gatan Precision ion Polishing Systems (PIPS) equipped with a cold stage. During the final ion-polishing of the TEM foils for Figs. 1–4, the cold stage of PIPS was cooled using liquid nitrogen. While the TEM foil for Fig. 5 was prepared with the cold stage of PIPS being not cooled using liquid nitrogen. Electron diffraction patterns (EDP), bright field and high-resolution (HR) TEM images were recorded in a Hitachi H- Thin Solid Films 519 (2011) 4371–4376 ⁎ Corresponding author. Tel.: +1 817 272 0841; fax: +1 817 272 2538. E-mail address: jiang@uta.edu (J. Jiang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.015 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf