M. Pernet

Magnetic melt texturing combined with hot pressing applied to superconducting (2223) Bi-Pb-Sr-Ca-Cu-O ceramics

We describe a new texturing process for superconducting ceramics which combines the effects of magnetic melt texturing (MMT) and hot pressing (HP), referred to in the following as the MMHPT process. This process essentially involves three steps: (i) a hot pressing at a temperature Tp, (ii) a partial melting in a magnetic field at a slightly higher temperature TM, (iii) a second hot pressing at the same temperature Tp. Optimization of the thermomechanical treatment in a magnetic field applied to the (2223) Bi-Pb-Sr-Ca-Cu-O cuprates led to highly textured and dense samples carrying transport current density Jc of 3800 A cm-2 at 77 K in zero field.

Structural and magnetic properties of alkali-substituted praseodymium manganites Pr1−xAxMnO3 (A=Na, K)

Abstract Structural and magnetic properties of Pr 1− x A x MnO 3 (A=Na, K) are studied. Solid state reactions with alkali metal excess allowed preparation of samples with x up to 0.14 for K and 0.19 for Na. All members of this series are orthorhombic at room temperature, space group Pbnm , with orthorhombic distortion decreasing with increasing alkali metal content. Magnetic studies show a transition from antiferromagnetism in pure PrMnO 3 to canted magnetic arrangement for ∼12% Mn 4+ to ferromagnetism in the range 26–38% Mn 4+ . Curie temperatures strongly depend on substitution: they initially increase with doping level up to an optimum x ≈0.15, corresponding to 30% Mn 4+ , and seem to decrease for higher sodium doping level.

New model for the magnetic structure of the marokite-type oxide CaMn2O4

Abstract This paper reports a neutron powder diffraction (NPD) study of polycrystalline marokite-type oxide CaMn 2 O 4 as a function of temperature in the range 1.5–280 K, together with magnetization measurements up to 8 T and 800 K. The orthorhombic room-temperature structure (space group Pbcm ) is confirmed. Manganese atoms are located in distorted octahedral sites with four short and two long distances at respectively 1.90–1.95 and 2.35–2.44 A, in agreement with the Jahn–Teller character of Mn 3+ ions. Below T N =220 K, this compound orders antiferromagnetically. The magnetic unit cell requires a doubling of the crystallographic a -axis. The magnetic structure has been determined by Rietveld refinement and group theory considerations. The most probable spin arrangement is found to belong to the Γ 2 irreducible representation in the Pbca space group.

Structural and magnetic properties of the CaFe2 − xMnxO4 solid solution

Substitution of Mn for Fe in the orthorhombic compound CaFe2O4 has been achieved by solid-state reactions at 1100 °C in air up to the composition Ca(Fe1.6Mn0.4)O4. The evolution of cell and atomic parameters was determined from X-ray powder data using the Rietveld method. The results show that the presence of the Jahn–Teller ion Mn3+ distorts the Fe1 octahedral site and that Mn most probably occupies the Fe2 site. With increasing Mn content, the magnetism evolves from antiferromagnetic to ferrimagnetic, with Curie temperature (TC) values (194–209 K) well above the Neel temperature of CaFe2O4 (160 K). A correlation between the evolution of TC and the deviation of the Fe2–O–Fe2 angle from 90° is established.

Texture Analysis by the Schulz Reflection Method: Defocalization Corrections for Thin Films

A new method is described for correcting experimental data obtained from the texture analysis of thin films. The analysis employed for correcting the data usually requires the experimental curves of defocalization for a randomly oriented specimen. In view of difficulties in finding non-oriented films, a theoretical method for these corrections is proposed which uses the defocalization evolution for a bulk sample, the film thickness and the penetration depth of the incident beam in the material. This correction method is applied to a film of YBa2Cu307_6 on an SrTiO3 singlecrystal substrate.

Texture influence on critical current density of YBCO films deposited on (100)-MgO substrates

Abstract The texture of two series of YBCO films deposited on (100) MgO substrates by Pulsed Laser Deposition or Metal Organic Chemical Vapour Deposition was analysed using the X-ray pole figure technique. The effect of misorientations on the critical current density is studied.

Texture analysis of Bi-2212 and 2223 tapes and wires by neutron diffraction

Abstract The crystallographic texture of silver sheathed monocore tapes and wires, and multifilamentary tapes containing Bi 2 Sr 2 CaCu 2 O x (Bi-2212) and Bi 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) superconducting oxides was determined by neutron diffraction, using a position sensitive detector. The orientation distribution (OD) was obtained from measurements of 4–6 pole figures. The oxide textures of these tapes and wires displayed axial symmetry, indicative of a grain-shape-controlled orientation mechanism. In monocore and multifilamentary tapes the c -axes of Bi-2212 and Bi-2223 are oriented perpendicular to the rolling plane, in wires they are perpendicular to the wire direction. The multifilamentary tape has the strongest texture with an OD maximum of 46 times that of a random sample. Monocore tapes are 26 and 35 mrd, and wires are 2.1 and 1.5 mrd. For comparison, a sinter-forged Bi-2223 barpowder was analyzed and exhibited c -axes parallel to the compression direction with a maximum of only 3.4 mrd. The texture of the silver sheaths depends on the processing conditions.

Structure and morphology of YBa2Cu3O7−x LPCVD layers

Abstract Thin films of YBa 2 Cu 3 O 7−x have been grown by chemical vapor deposition at 825 °C (5 Torr). The starting materials are tetramethylheptanedionates of Y, Ba and Cu. The superconducting properties of the films obtained on monocrystalline MgO have been studied by AC resistivity, AC susceptibility and magnetization measurements. Superconducting layers with zero resistance at 85 K and a transition width lower than 2 K are obtained. This communication will address more specifically the structural properties as well as the morphology of the layers. The films are highly orientated with their c-axis perpendicular to the substrate plane, and the pole figures show a very good in-plane orientation. The morphology of the layers has been investigated by scanning electron microscopy: we do observe a strong correlation between the roughness of the surface and the composition of the gas phase.

Microstructure of textured YBa2Cu3O7-δ prepared by the melted zone in magnetic field: texturation mechanism

Abstract A zone melting set-up with a magnetic field perpendicular to the temperature gradient is used in order to optimize (a, b) plane alignment and thus critical current densities. Large samples with high Jc values in magnetic fields at 77K are obtained. These studies are carried out by optical microscopy, analytical transmission electron microscopy and X ray diffraction pole figures. Our results indicate that crystallite misorientation within a single domain does not exceed 10°. The application of a magnetic field of 5 T imposes the c-axis orientation during grain growth. Indeed, we observe an intergrain dispersion much smaller than 40° with respect to the magnetic field direction. The 123 matrix contains large amounts of unoriented 211 inclusions which are of different size.

Texture study of MTG produced YBaCuO ceramic using X-ray and neutron diffraction

Abstract The X-ray and neutron diffraction pole figure technics were employed for studying the free surfaces and bulk textures of a MTG YBCO sample. We found a development and an enlargement of the texture from the free surfaces to the entire volume with {113}, {114} or {115} planes parallel to the surface. The 211 phase particulates are randomly oriented.