B. Freitag

Physics of grain boundaries in the colossal magnetoresistance manganites

Abstract The electrical transport properties of grain boundaries in epitaxial thin films of the perovskite manganites have been studied as a function of temperature and applied magnetic field. Below the Curie temperature T C an additional grain boundary resistance, highly nonlinear current–voltage curves, and a large magnetoresistive effect in the whole temperature regime below T C are found. The results can be explained consistently by the presence of a disordered, a few nm wide paramagnetic grain boundary layer that is depleted below T C due to an increase of the workfunction of the ferromagnetic grain material adjacent to this layer. The related band bending and space charge effects are important for the physics of grain boundaries in the manganites.

STRAIN EFFECTS AND MICROSTRUCTURE OF EPITAXIAL MANGANITE THIN FILMS AND HETEROSTRUCTURES

We have grown epitaxial La2/3Sr1/3MnO3 (LSMO) and La2/3Ba1/3MnO3 (LBMO) thin films as well as La2/3Ba1/3MnO3/SrTiO3 heterostructures by pulsed-laser deposition. The microstructure of the films was analyzed by x-ray diffraction and transmission electron microscopy. A significant effect of strain due to lattice mismatch was found. Whereas the thick LBMO films show perfect epitaxy and grow coherently strained over the full film thickness, the LSMO films were found to be composed of two layers separated by an intrinsic interface region containing a high density of defects. The approximately 60 nm thick bottom layer grows coherently on the SrTiO3 (STO) substrate and is highly strained, whereas the top layer is almost strain free. The LBMO/STO heterostructures are coherently strained and show a very low density of defects and sharp interfaces.

Study on the superconducting composite material formation in the system Bi2Sr2CaCu2O8+x/Al-containing phases

Abstract Phase evolution in the Biue5f8Srue5f8Caue5f8Cuue5f8Alue5f8O system was studied. Two Al-containing phases BiSr 1.5 Ca 0.5 Al 2 O z and (Sr 1− x Ca x ) 3 Al 2 O 6 ( x = 0.4 − 0.45) were determined to be chemically compatible with Bi 2.18 Sr 2 CaCu 2 O 8+ x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr 1.5 Ca 0.5 Al 2 O z was found to be in equilibrium only with the melt and the (Sr 1− x Ca x ) 3 Al 2 O 6 phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr 1.5 Ca 0.5 Al 2 O z ↔ (Sr 1− x Ca x ) 3 Al 2 O 6 + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + n BiSr 1.5 Ca 0.5 Al 2 O z and Bi-2212 + m (Sr 1− x Ca x ) 3 Al 2 O 6 were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr 1.5 Ca 0.5 Al 2 O z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr 1− x Ca x ) 3 Al 2 O 6 imbedded in Bi-2212 lamellas. The Bi-2212 + m (Sr 1− x Ca x ) 3 Al 2 O 6 materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content.

High-resolution transmission electron microscopy study on strained epitaxial manganite thin films and heterostructures

Abstract Pulsed laser deposition has been used to grow epitaxial La2/3Sr1/3MnO3 and La2/3Ba1/3MnO3 thin films as well as La2/3Ba1/3MnO3/SrTiO3 heterostructures. The microstructure was studied by four-circle X-ray diffraction and high-resolution transmission electron microscopy. Strain effects due to lattice mismatch between substrate and film, and between different layers in the heterostructures were found to play a key role in determining the thin film properties. La2/3Ba1/3MnO3 films on SrTiO3 substrates show perfect epitaxy and grow coherently strained over the whole film thickness. In contrast, La2/3Sr1/3MnO3 films on SrTiO3 substrates were found to form two layers with different lattice parameters separated by an intrinsic interface region containing a high density of defects. La2/3Ba1/3MnO3/SrTiO3 superlattices are coherently strained and show a very low density of structural defects and sharp interfaces. The observed interface roughness was below 1 nm allowing the controlled growth of continuous two unit-cell thick La2/3Ba1/3MnO3 layers. We also discuss the influence of strain on the electrical transport properties.

Unusual crystal structure of non-superconducting Y1Ba2Cu3O7−x films on buffered silicon substrates

Abstract Superconducting common c -axis oriented YBCO films as well as non-superconducting films epitaxially grown at significantly reduced substrate temperatures were deposited by laser ablation on CeO 2 /YSZ buffered silicon substrates. The crystal structure of the non-superconducting films measured by XRD is collapsed from the original orthorhombic one, whereas the chemical composition detected using RBS and EDX remains identical. Especially the oxygen content in the non-superconducting material is as high as in the common 90 K superconductor. Thus, the change in the electrical behaviour is only due to a structural modification. The crystallographic model of a simple cubic YBCO perovskite unit cell does not represent completely our experimental results measured on the non-superconducting YBCO. A modified model based on detailed TEM and XRD measurements and known crystal defects of orthorhombic YBCO is proposed.

Formation of Bi-2212-based composites with submicrometre-grained (Sr, Ca)SnO3

Phase assemblage was investigated in the Bi-Sr-Ca-Cu-Sn-O system at temperatures of Bi-2212 phase crystallization. It is found that the Sr1-x Cax SnO3 phase (x = 0.1-0.2) is compatible with Bi-2212 as well as with Bi-2201, CuO, alkaline-earth metal cuprates and bismuthates. The solubility of Sn in Bi-2212 is limited to low values. The superconducting composites Bi-2212-(Sr, Ca)SnO3 were prepared by melt crystallization using single precursor and mixed precursor routes. The materials consisted of Bi-2212 matrix with submicron (Sr, Ca)SnO3 grains included in Bi-2212 lamellas and agglomerated in between. The composites exhibited enhanced flux pinning at T = 60 K, while at 5 K the effect of (Sr, Ca)SnO3 addition was not revealed.

Structure and chemical composition of planar defects in Sr0.9La0.1CuO2 infinite-layered superconductors by electron spectroscopic imaging

Abstract The superconducting infinite-layered compound Sr 0.9 La 0.1 CuO 2 was prepared by high-pressure synthesis and characterized using HRTEM and electron spectroscopic imaging (ESI). Defect layers in the a – b plane were observed which show an expansion of the adjacent strontium layers along c , similar as in the calcium-doped SrCuO 2 system. Using high resolution ESI, the planar defects were found to be oxygen deficient, whereas no change in the strontium composition was observed. Removing oxygen, the charge may be compensated by reduction of Cu II to Cu I or by removal of copper ions, and the possible chemical compositions are discussed. Comparison of lattice images with simulated images clearly shows that the copper ions in the defect layer are displaced from the regular positions they occupy in the bulk.

Crystal structure of YBCO thin films grown at substrate temperatures of about 500°C

Abstract Nonsuperconducting YBCO thin films were prepared by laser deposition at 500°C. The crystal structure of the films measured by XRD seems to be cubic: a = b = 3.890A, c = 3.867A. From EDX analysis follows within an accuracy of 1–2% there is no stoichiometric difference between this nonsuperconducting YBCO and the common 90 K superconductor especially regarding the oxygen content. An existing structural model of a simple cubic YBCO perovskite unit cell, however, does not sufficient represent the experimental results. We suggest a modified model which based on known defects of the common c-axis YBCO. This model is discussed in consideration of detailed ED and HTREM investigations.

Phase and microstructure evolution in the process of the composite Bi-2212-(Sr, Ca)3 Al2 O6 glass-ceramics formation

Devitrification of glass with a cation ratio corresponding to the composition {Bi2.1Sr2CaCu2Ox+0.4Sr1.7Ca1.3Al2O6} was studied. At temperatures below 600 °C, the phases crystallized in the order Bi-2201, Cu2O, CaO, (Sr,Ca)3Bi2O6, while Al ions concentrated in the remaining glass. Above 600 °C, Bi-2212 and (Sr,Ca)3Al2O6 formed simultaneously, so that at 750 °C, the two-phase composite Bi-2212-(Sr,Ca)3Al2O6 was obtained that contained grains of the aluminate phase 0.03-0.3 µm in size. The Al-rich phase surplus increased the stability of the glass and accelerated Bi-2212 phase formation, while development of superconductivity in Bi-2212 was slowed down. It was shown that composite glass-ceramics with bulk superconductivity could be obtained at temperatures above 850 °C.

Combined HRTEM and EFTEM Study of Precipitates in Tungsten and Chromium-Containing TiB 2

The structure and chemical composition of two types of precipitates in the system TiB 2 -WB 2 -CrB 2 were studied by means of high-resolution TEM and energy filtering TEM. Type I particles (W 2 B 5 structure) are precipitated at the basal plane of the hexagonal matrix whereas type II precipitates are thin platelets lying parallel to the {1100} prism planes. Lattice imaging yields displacements of the metal positions with respect to the matrix. Information on the chemical composition at high lateral resolution is obtained from elemental maps of all chemical constituents using electron spectroscopic imaging (ESI). The type II precipitates show a decrease in the B and Ti concentration, whereas the tungsten concentration increases and the Cr is homogeneously distributed. The HRTEM results combined with the results of the elemental maps allow to develop a structural model based on the intergrowth of the β-WB structure in the TiB 2 -rich matrix. The two deficient boron layers in W 0.5 Ti 0.5 B with a spacing of 0.38 nm can be used to examine the resolution limit of ESI.