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The effect of competing crystal-field anisotropies on magnetic order has been investigated in a series of Ho/Er superlattices using neutron and resonant X-ray magnetic diffraction techniques. The neutron diffraction reveals that for temperatures in the interval T-N(Er) less than or equal to T less than or equal to T-N(Ho) the Ho basal-plane order propagates coherently through the paramagnetic Er, and that below T-N(Er) the longitudinal component of the Er moments fail to order across the Ho block. The magnetic superlattice peaks observed in the X-ray scattering display an anomalous energy dependence: a sharp resonance is found at L(III)(Ho), with no resonance visible at L(III)(Er). These results are discussed with reference to models of exchange in metallic superlattices.

The effect of large hydrogen concentrations (\ensuremath{\approx}30% atomic), on the superconducting transition temperature ${T}_{c}$ of Nb films is discussed. The hydrogen is incorporated in solutionlike phases which occur at these concentrations because the (\ensuremath{\alpha} to \ensuremath{\beta}) hydride transition in these films is suppressed. X-ray-diffraction data show an asymmetric expansion of the [110] Nb interplanar spacing. This can be as high as a 6--8 % expansion perpendicular to the plane of the film for the addition of \ensuremath{\sim}70% hydrogen, with only a 1--2 % change in the plane of the film. ${T}_{c}$ is depressed to a value near 50% of that of the undoped film with about 30% atomic hydrogen, and returns to its initial value when the hydrogen is removed. A discussion is given of how both disorder and changes in the electronic structure can affect ${T}_{c}.$

The authors describe synchrotron based X-ray diffraction techniques and issues related to in situ studies of intercalation processes in battery electrodes. They then demonstrate the utility of this technique, through a study of two batches of Li{sub x}Mn{sub 2}O{sub 4} cathode materials. The structural evolution of these spinel materials was monitored in situ during the initial charge of these electrodes in actual battery cells. Significant differences were observed in the two batches, particularly in the intercalation range of x = 0.45 to 0.20. The first-order structural transitions in this region indicated coexistence of two cubic phases in the batch 2 material, whereas the batch 1 material showed suppressed two-phase coexistence. Batch 2 cells also indicated structural evolution in the low-potential region below 3.0 V in contrast to the batch 1 material. Differences in structural evolution between batches of Li{sub x}Mn{sub 2}O{sub 4} could have important ramifications in their cycle life and stability characteristics.

The alignment and the formation of (Bi, Pb)2Sr2Ca2Cu3O10 inside an Ag sheath were investigated for tapes from four different precursor powders. Microstructural characterization along with in situ transmission x-ray diffraction experiments revealed that the kinetics depended strongly on the processing technique and the phase assemblage of the precursors. The alignment process was governed by the preferential grain growth of the Bi-cuprates along the a-b plane and the constraint applied by the sheath. The formation mechanism of (Bi, Pb)2Sr2Ca2Cu3O10 was either the intercalation or the nucleation and growth, depending on whether excessive liquid is adjacent to the platelets.

Hydrogen uptake and discharge by thin metallic films under potentiostatic control was studied using x-ray diffraction at the National Synchrotron Light Source (NSLS). The formation of metal-hydrogen phases in Pd, Pd-capped Nb and Pd/Nb multilayer electrode structures was deduced from x-ray diffraction data and correlated with the cyclic voltammetry (CV) peaks. The x-ray data was also used to construct a plot of the hydrogen concentration as a function of cell potential for a multilayered thin film. {copyright} {ital 1997 Materials Research Society.}

Hard x-rays from a synchrotron source were utilized in diffraction experiments performed at elevated temperatures (up to ˜ 870 °C) on (Bi, Pb)2Sr2Ca2Cu3O10 (Bi-2223) tapes completely encased in silver. The general behavior of the phase and texture development under typical processing conditions was determined, and the effects that several variations in processing conditions had on the phase and texture development were examined. These results and their implications for improving processing conditions are discussed.

We describe the construction and performance of a small‐angle x‐ray scattering (SAXS) instrument which we have used on several beam lines at the National Synchrotron Light Source. The analyzer crystal was a channel cut Si(1,1,1) designed for use at λ=1.54 A with a measured efficiency of 60% and an angular resolution full width at half maximum of 0.001°. In the case of strongly scattering samples (i.e., powders), momentum transfer q between 1×10−4 A

The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x‐ray scattering techniques. The x‐ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c‐axis alignment was measured in ∼100 mono‐ and multifilament samples, and correlations between c‐axis alignment and current carrying capacity at 77 K were found. Multifilament samples generally had better alignment than monofilament samples. The c‐axis alignment along the length of the tapes was uniform, and the superconducting material within ∼1 μm of the Ag was better textured than the bulk of the sample. Intermediate pressings were directly shown to have an adverse affect on c‐axis alignment. Finally, the evolution of texture and phase development was examined in a series of samples annealed for varying times. The 2212 starting material acquired the final c‐axis alignment state after brief heating times, and only after much longer heating times did the 2212 transform into the 2223 phase. These results and their implications for improving processing procedures are discussed.The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x‐ray scattering techniques. The x‐ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c‐axis alignment was measured in ∼100 mono‐ and multifilament samples, and correlations between c‐axis alignment and current carrying capacity at 77 K were found. Multifilament samples gene...

The effect of competing crystal-field anisotropies on magnetic order has been investigated in a series of Ho/Er superlattices using a combination of x-ray and neutron scattering techniques. For temperatures in the interval TN(Er)≤T≤TN(Ho) the Ho basal-plane order propagates coherently through the paramagnetic Er over a typical length scale of 1000 A. At low temperatures the Ho moments retain their bulk-like helical configuration, whereas the magnetic structure in the Er blocks has both basal-plane and c-axis components. Below TN(Er), the coherence length of the basal-plane order decreases on cooling, while the longitudinal component of the Er moments fails to order across the Ho block. It is argued that these results require an extension of current models of indirect exchange in superlattices to explicitly include the superlattice band structure.

Bi-2223 tapes were manufactured from a fine “two-powder” product produced by using an aerosol spray pyrolysis technique. Critical current density of 22000 A/ cm2 at 77K and 0 T was achieved. Nondestructive transmission x-ray diffraction study indicated good alignment of the superconducting grains. The texturing process of the superconducting phase was found to be nearly complete after the first 24 h of heat treatment for the samples studied. Pressing was found to play little role in the texturing process. The texturing can be enhanced by Ag-doping. Jc, however, was not found to be improved significantly, presumably due to the reduced effective cross-sectional area. A new phase, Bi-4435, was identified which may play a significant role in the formation of 2223.

We report the results of resonant x-ray magnetic scattering experiments on bulk and thin-film single crystals of holmium. The scattering at the principal magnetic reflection has been characterized as a function of the temperature in the spiral phases near and below their respective Neel temperatures. The integrated intensity of the principal magnetic peak in both samples shows power-law behavior versus reduced temperature with nearly equal exponents. The exponents for the scattering at the resonant second and third harmonics in the bulk sample are not simple integer multiples of the first, and motivate the consideration of simple scaling corrections to mean-field theory. We also present and compare the results of high-resolution measurements of the temperature dependence of the magnetic wave vectors, [ital c]-axis lattice constants, and correlation lengths of the magnetic scattering of the two samples in their spiral phases. Although the qualitative behavior is similar, systematic differences are found, including uniformly larger magnetic wave vectors and the suppression of the 1/6 phase in the film. The spiral magnetic structure of the film forms a domain state at all temperatures in the ordered phase. The magnetic correlation lengths of both samples are greatest near the Neel temperature, where that of themore » film appears to exceed the translational correlation lengths of the lattice. As the temperature decreases, the magnetic correlation lengths also decrease. These results are discussed in terms of the strain present in the samples.« less

Neutron-scattering measurements have been performed on (Nd,Pr){sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4} (0.0{lt}{ital x}{lt}0.125) single crystals. All samples exhibit three-dimensional (3D) long-range magnetic order; the temperature dependence of the 2D magnetic correlation lengths in all is similar in character to that observed in pure and lightly doped La{sub 2}CuO{sub 4} albeit with renormalized spin stiffness constants {rho}{sub {ital s}}. Thus, in the nonsuperconducting doping regime the hole- and electron-doped systems exhibit analogous magnetic behavior except for the drastically different dopant-concentration scales for the destruction of magnetic order.

A two‐step process is proposed for the formation of c‐axis aligned (Bi,Pb)2Sr2Ca2Cu3O10+δ [Bi(2:2:2:3)] platelets in a silver sheath. The process involves: (1) the formation of c‐axis aligned (Bi,Pb)2Sr2CaCu2O8+δ at early stage of heat treatment and (2) the subsequent intercalation of Ca–Cu–O layers to form Bi(2:2:2:3). This is based on the measurements of (1) the rocking curves for c‐axis alignment and the two theta scans for the Bi(2:2:2:3) conversion ratio, both by a transmission x‐ray technique, and (2) a quantitative study of the phase conversion due to intercalation of Ca–Cu–O layers into existing Bi(2:2:1:2) by transmission electron microscopy.

Hard x rays from a synchrotron source were utilized in diffraction experiments which probed the bulk of electrode materials while they were operating in situ in battery cells. Two technologically relevant electrode materials were examined; an AB2‐type anode in a nickel–metal–hydride cell and a LiMn2O4 cathode in a Li‐ion ‘‘rocking chair’’ cell. Structural features such as lattice expansions and contractions, phase transitions, and the formation of multiple phases were easily observed as either hydrogen or lithium was electrochemically intercalated in and out of the electrode materials. The relevance of this technique for future studies of battery electrode materials is discussed.

Improved strain sensitivity of critical currents Ic of the multicored over the monocored (Bi,Pb)2Sr2Ca2Cu3O10–Ag tapes is shown for bending strains, but not for axial tensile strains. The improved mechanical tolerance is due, in part, to a better alignment of the Bi‐cuprate platelets achieved in the multicored tapes. Furthermore, the difference in the response to these tapes to axial tensile and bending strains is due to the difference in the precise manner in which the strains are applied to the tapes.

The purpose of this CRADA was to characterize the structural properties of the superconductor material within the wires in order to determine which processing procedures produce the best superconductor texture and phase development, and hence the best ultimate current carrying capacity.

In this paper, we discuss changes in the phase diagram of hydrogen in both bilayer (i.e. 200–2000 Å Nb/ 100 Å Pd on glass) and multilayer configurations. In fact, it is the changes in this phase diagram, in addition to the Pd coating, that makes theses bilayer systems useful for hydrogen gas sensors. Comparison of x-ray diffraction, electrical resistivity and volumetric measurements of the films before and after hydrogen charging indicate that the phase equilibria between a correlated (high concentration) and a dilute phase of hydrogen in Nb is not sensitive to the number of layers in the films. On the other hand, the experimental methods show different behavior for 200 Å thick Nb films and thicker (>400 Å) Nb layers. Diffraction results showing unisotropic lattice expansion will also be discussed..

Resonant magnetic X-ray scattering from UAs using the Miv uranium absorption edge allows a detailed examination of the first order antiferromagnetic phase transition occuring at 122 K which appears to proceed via the formation of antiferromagnetically coupled surface layers.

Among Cu/sup 2+/ compounds, cupric oxide (CuO) has unusual magnetic properties, some of which are shared by the superconducting copper oxides. In order to study these properties further, neutron scattering experiments on a CuO single crystal have been performed. Two magnetic phase transitions at T/sub N//sub 1/ = 231 K and T/sub N//sub 2/ = 212.5 K have been observed, with the magnetic structure below T/sub N//sub 2/ antiferromagnetic and the structure between T/sub N//sub 1/ and T/sub N//sub 2/ helical in nature. The dynamics of the magnetism was also studied. As in La/sub 2/CuO/sub 4/, the dispersion of the low-temperature spin-wave excitations is unusually steep (dE/dk = 250 +- 75 meV A/sup -1/), indicating strong magnetic interactions. Inelastic magnetic scattering has also been measured above T/sub N//sub 1/. At temperatures slightly above the 231-K phase transition, dynamic critical slowing has been observed.

Using neutron-scattering techniques, we have studied the temperature dependence of the magnetic excitations at varied energies in a high-quality single crystal of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$. This crystal exhibits a sharp superconducting transition at 33 K with nearly 100% flux exclusion. For magnetic excitation energies of 9 meV and lower, the intensity drops dramatically with decreasing temperature for temperatures below \ensuremath{\sim}150 K. However, at 12 MeV the intensity is independent of temperature between 300 and 5 K.

We review the results of recent neutron-scattering studies of the spin fluctuations in samples of La 1.89 Sr 0.11 CuO 4 which are ∼80% superconducting with T c = 10 K. The structure factor, S(Q), reflects three-dimensional modulated spin correlations with an in-plane correlation length of order 18 ± 6 A. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasi-elastic (|ΔE| < 0.5 meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

The structural phase transition from the tetragonal to the orthorhombic phase of doped and undoped samples of La2−xSrxCuO4 has been investigated by using inelastic neutron scattering techniques. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity.

We have carried out elastic, quasielastic (..integral.. dE), and inelastic neutron-scattering studies of the antiferromagnetic spin correlations in La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ with x varying between 0.02 and 0.18. The crystals, which were grown in three different laboratories, exhibit behavior that varies smoothly with x. In all cases, antiferromagnetic correlations with a scattering amplitude corresponding to a fully occupied Cu/sup 2+/ square lattice are observed. However, the Neel state is destroyed by the doping and spin-spin correlation lenght xi is quite short, varying from approx.35 to approx.8 A as x varies between 0.02 and 0.18; the local order is, however, the same as in pure La/sub 2/CuO/sub 4/. The fluctuations are dynamic in character as in La/sub 2/CuO/sub 4/ above the Neel temperature T/sub N/. To a first approximation, xi = 3.8/ ..sqrt..x A, the average separation between the holes introduced by the Sr/sup 2+/ doping. The x = 0.08 sample exhibits superconductivity with T/sub c/ = 10 K and with a Meissner fraction exceeding 15% at 5 K; no important differences in the magnetic scattering are observed in the normal and superconducting states. In an appendix we present additional data on the spin dynamics in pure La/sub 2/CuO/submore » 4/ at T = 300 K in a sample with T/sub N/ = 235 K.« less

The resistivity of nonmetallic La/sub 2-//sub y/Sr/sub y/Cu/sub 1-//sub x/Li/sub x/O/sub 4-//sub delta/ single crystals and ceramics accurately follows the functional form exp((T/sub 0//T)/sup 1/4/), characteristic of variable-range hopping. For each of the crystals grown from Li-containing flux and CuO flux, pure reduced ceramics, and ceramics containing 0.025 to 0.2 mol % Li, the values of T/sub 0/ are in the range 0.3--7 x 10/sup 6/ K. The hopping conductivity shows that the crystals, which also manifest the two-dimensional quantum spin fluid state, antiferromagnetism, and the tetragonal-to-orthorhombic transition, are nonmetallic because the electronic states at the Fermi energy are localized. No evidence of a large gap is observed, and all samples, including the reduced ceramic, are p type, leading to the suggestion that in the nonmetallic state as well as the superconductor, the Fermi energy lies near the top of the band of singly occupied states (lower Hubbard band). The suppression of superconductivity by Li impurities is discussed.

Neutron-scattering studies provide direct evidence for a field-driven magnetic transition which originates from the canting of spins out of the Cu${\mathrm{O}}_{2}$ planes: At the transition between antiferroand ferromagnetic ordering of the canted component of the spins in the layers, the (100) Bragg peak vanishes while the (201) peak appears. Detailed measurements and analysis suggest that the phase transition can be described by the mean-field theory of Thio et al. only close to the N\'eel temperature.

The soft optical phonons associated with the tetragonal-to-orthorhombic structural phase transition in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ have been investigated using inelastic-neutron-scattering techniques. Single crystals with Sr concentration of x = 0, 0.08, and 0.14 were studied. Both branches of the doubly degenerate X-point phonon which go soft at the tetragonal-to-orthorhombic transition have been observed in the orthorhombic phase. In all three samples one branch hardens, while the other remains relatively soft with behavior indicative of a further incipient structural phase transition. We identify the hard and soft branches of the orthorhombic phase as zone center and zone-boundary phonons, respectively. The zone-center mode has A/sub g/ symmetry and is, consequently, Raman active. The relation of these measurements to other structural properties and to the superconductivity in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ is discussed.

The phase diagram of the quasi-two-dimensional disordered antiferromagnet, Rb2Mn0.7Mg0.3F4 in a uniform magnetic field along the unique c-axis has been measured using neutron scattering techniques. There are three distinct phases ; at high temperatures the material is paramagnetic, and on cooling in fields above 1.7 T, it undergoes a transition to a spin-flopped phase with long range order perpendicular to the c-axis. The transition temperature decreases with decreasing magnetic field and is below 2 K at 1.75 T. At lower fields there is a phase in which different domain sizes are metastable and the metastability boundary decreases in temperature with increasing field and appears to meet the phase boundary for the spin-flopped phase at T = 0.