Superconductivity and critical current density in LaFeAsO 1 - x F x compounds M. Shahbazi a , C. Shekhar b , O.N. Srivastava b , D. Attard a , G. Peleckis a , Y. Du a , Z.X. Cheng a , S.X. Dou a , X.L. Wang a, ⁎ a Institute for Superconducting and Electronic Materials, University of Wollongong, Innovation Campus, Australian Institute for Innovative Materials, Squires Way, Fairy Meadow, NSW, 2519, Australia b Centre of Advanced Studies for Physics of Materials, Department of physics, Banaras Hindu University, Varanasi- 221005, India abstract article info Available online 7 April 2010 Keywords: Superconductivity Upper critical field Critical current density Here, we report our studies on the crystal structures, morphologies, and superconductivity in LaFeAsO 1 -x F x compounds which were fabricated by solid state reaction. The crystal structures were refined using Rietveld refinement. Superconducting properties, such as critical temperature, T c , critical current density, J c , and upper critical field, H c2 , were determined using magneto-transport and magnetic measurements over a wide range of temperature below and above T c , and in magnetic fields up to 14 T. A peak effect in the J c versus field is observed at T b 15 K. Upper critical field in the ab and c directions is H ab c2 = 122.8 T and H c c2 = 38.6 T, respectively. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The discovery of the iron based superconductors, LaFeAsO 1 - x F x [1], has attracted enormous attention worldwide. The critical temperature, T c , can be increased by replacing La with other rare earth elements such as Pr, Sm, Nd, and Gd. The T c has reached as high as 43 K for SmO 1 -x F x FeAs [2] and 56 K for Gd 1 - x Th x FeAsO [3]. Two of the most important issues for research work on new superconductors are the upper critical field, H c2 , and the critical current density, J c . The upper critical field, is an intrinsic property, which has been approximated to be higher than 55 or 63–65 T [4] in LaO 0.9 F 0.1 FeAs, 70 T in PrFeAsO 0.85 F 0.15 , over 100 T in SmFeAsO 0.85 F 0.15 [5], and 80– 230 T in high-pressure (HP) fabricated NdFeAsO 0.82 F 0.18 [6]. The F-doped LaO 1 - x F x FeAs compounds have better superconducting properties than the oxygen deficient superconductors. Much research work has been performed on the superconductivity in LaO 1 -x F x FeAs; however, reports on the flux pinning and critical current density of LaO 1 - x F x FeAs compounds are very limited so far. In this paper, we report on our study of the structures, microstructures, flux pinning, J c , and upper critical fields in LaO 1 - x F x FeAs. 2. Experiment details Polycrystalline samples with the nominal composition of LaO 1 - x F x FeAs (x =0.15) was prepared by solid state reaction using La, La 2 O 3 , FeAs, and LaF 3 with purities of 99.9% as the starting materials. The mixture was ground and pressed into pellets, then sintered at 950 °C and finally at 1000 °C for 8 h. The details of the preparation process can be found in Ref. [7]. The X-ray diffraction of samples was performed with Cu-Kα radiation in the 2θ range of 20° to 80°, with a step interval of 0.02°. The microstructures of the as-obtained samples and their morphology were studied using field emission scanning electron microscopy (FE-SEM, JEOL7500), operated at 50 kV. The transport and magnetic properties were measured over a wide range of temperature and magnetic fields up to 13 T, using a physical properties measurement system (PPMS, Quantum Design). Magnetic loops were also measured at the temperatures of 5, 10, 15, 20, 25, and 35 K. The critical current density was calculated using the Bean model. 3. Result and discussions A typical X-ray diffraction (XRD) pattern of LaO 0.85 F 0.15 FeAs is shown in Fig. 1. It can be seen that the as-prepared samples are nearly single phase LaO 0.85 F 0.15 FeAs compound. The Rietveld refinement results show that LaO 0.85 F 0.15 FeAs crystallized in a tetragonal structure with space group P4/nmm and lattice parameters of a = 4.032 Å and c = 8.728 Å. Fig. 2 shows the morphology in a field emission scanning electron micrograph of the fractured surface of the above compound. It is revealed that this sample has a typical layered structure. The macrostructural characterization of LaO 0.8 F 0.2 FeAs sample was conducted by high resolution transmission electron microscopy (HRTEM, FEI, Technai 20G 2 , operated at 200 kV). One outstanding feature, as depicted by the TEM images, is the presence of layered structure. Some nanoclusters are embedded inside grains, as seen in Fig. 3. 5% and 15% doped sample have the similar feature. Thin Solid Films 518 (2010) e42–e45 ⁎ Corresponding author. Tel.: +61 2 42215766; fax: +61 2 42215731. E-mail address: xiaolin@uow.edu.au (X.L. Wang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.129 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf