Average Coherence Length Research Articles

Effect of Annealing on Microstructure of Ni80Fe20/Cu Multilayers

[Ni80Fe20/Cu]15 multilayers were fabricated by dc-magnetron sputtering and annealed at 150, 250 and 350℃, respectively. The structures were investigated by low-angle and high-angle X-ray diffraction. It was found that, as the annealing temperature increases, the [111] preferred orientation of superlattices is improved slightly, while the superlattice period, interplane distance, average multilayer coherence length decrease. The interfacial roughness increases with the increase of annealing temperature and/or annealing time, this can be attributed to the Ni80Fe20 and Cu sublayers has been revealed by simulation of high-angle X-ray diffraction, and its thickness increases as the annealing temperature of annealing time increases. The simulation results furthermore showed that the interplane distances of the Ni80Fe20 layer keeps constant, and that the Cu layer decreases slightly as the annealing temperature increases.

Kinetic Instability of Nanocrystalline Aluminum Prepared by Chemical Synthesis; Facile Room-Temperature Grain Growth

Nanocrystalline aluminum (nano-Al) is synthesized by two chemical methods. Method A consists of the following: reaction of LiAlH4 and AlCl3 at 164 °C in 1,3,5-trimethylbenzene produces nano-Al with an average mean coherence length (crystallite size) of 160 ± 50 nm. The byproduct LiCl is removed by washing with MeOH at −25 or 0 °C. Method B consists of the following: nano-Al is produced by decomposition of H3Al(NMe2Et) under reflux in 1,3,5-trimethylbenzene (ca. 100−164 °C), with or without added decomposition catalyst Ti(O-i-Pr)4. Here the mean particle size (40−180 nm) and degree of aggregation of the nano-Al depend on the mole percent of decomposition catalyst used (0−1%). The nano-Al produced by method A contains 3−4 wt % each of C, O, and Cl; whereas that produced by method B contains only ≤0.25 wt % each of C, O, and Cl and is ≥99 wt % Al. nano-Al produced by both methods has been consolidated by uniaxial pressing at 350 MPa for 1 h at 25, 100, or 300 °C. Rapid grain growth is observed at each pressing temperature in method-B powders. The mean grain size doubles at 25 °C within 1 h and continues to increase beyond the nanometer-size regime over longer periods. The low-temperature grain-size instability of the chemically synthesized (method-B) nano-Al contrasts markedly with the high-temperature stability (≥300 °C) of nano-Al prepared by gas condensation or mechanical attrition. Facile grain growth in the chemically synthesized (method-B) nano-Al is attributed to higher grain-boundary purity and to aggregate structures that minimize adventitious oxidation. The results establish that barriers to grain growth in pure nano-Al are intrinsically low.

Neutron irradiation effects on the density, tensile properties and microstructural changes in Hi-Nicalon™ and Sylramic™ SiC fibers

Tensile results are presented for ceramic grade (cg) Nicalon™, Hi-Nicalon™ and Sylramic™ SiC fibers which have all been neutron irradiated in the high flux isotope reactor (HFIR) at damage levels of 0.1, 0.5, 2 and 5 dpa. Single fibers were tensile tested and the results were analyzed using Weibull statistics. Fiber axial displacements were measured with a laser micrometer which allowed for the determination of the tensile moduli. Density changes were measured with a gradient column. Transmission electron microscopy (TEM) was performed to assess microstructural damage and X-ray diffraction (XRD) was used to measure uniform strain, degree of crystallinity, average coherence length and root-mean-square microstrain. The physical and tensile results indicate that cg Nicalon™ and Hi-Nicalon™ are unstable in a neutron field. Both fiber types densify by 3–5% which would be detrimental to a composite's matrix cracking stress due to weakening or debonding of the interface. The Sylramic™ swells which is similar behavior to the monolithic SiC. The failure strength of the Sylramic™ drops by 50% after an irradiation temperature of 500°C which would have little effect on the matrix cracking condition of a composite. The Sylramic™ fiber strength decrease would significantly reduce the ultimate composite strength but the composite strength would remain above the matrix cracking strength such that the fibers may still potentially be viable for fusion applications.

The chemical and magnetic structures of holmium-yttrium and holmium-lutetium superlattices

We present the results of a study of the chemical and magnetic structures of Ho/Y and Ho/Lu superlattices, all grown by molecular beam epitaxy. By combining the results of high-resolution X-ray diffraction with detailed modelling we show that the superlattices have high crystallographic integrity: the average structural coherence length in the growth direction is ~2000 Å, while the interfaces between the two elements are well defined, extending over approximately four lattice planes. The magnetic structures were determined using neutron scattering techniques. In the case of the Ho/Y superlattices, the Ho 3+ moments form a basal-plane helix at all temperatures in the ordered phase, with an apparent suppression of the low-temperature cone phase. Instead spin-slip structures are formed, including some not observed in bulk Ho. For the Ho/Lu superlattices the magnetic structure was found to depend on n Ho, the number of atomic planes in the Ho block. If n Ho > 20, then the Ho 3+ moments form a basal-plane helix down to the lowest temperatures. For n Ho ≤20, the Ho 3+ moments undergo a phase transition at low temperatures from a helix to a state where they are ferromagnetically coupled within one block. In this phase, successive Ho blocks are antiferromagnetically coupled, and there is a small component of the moment along the c axis.

Specific heat of the 90-K superconductor Tl 2Ba 2CuO 6 (“2201”) prepared in high pressure Ar or He gas

We present high resolution heat capacity data in the temperature range 30–300 K for several homogeneous samples of the superconductor Tl 2Ba 2CuO 6 (“2201”) with transition temperatures in the vicinity of 90 K. The specific heat peaks that appear close to the superconducting transition in some samples are identified as being due to gases trapped in the pores of the ceramics. The very small intrinsic anomaly at T c, of the order of 1% of the total specific heat, is best observed in samples prepared under He atmosphere. The data are analysed using a model that includes a parametrized phonon spectrum, a mean-field jump at T c, and a fluctuation contribution of the Lawrence-Doniach type. The fit yields an average coherence length of 7.5 Å (sample with T c =92 K) and 8.2 Å (sample with T c=87 K), and a vanishingly small mean-field jump. The homogeneity of the electronic subsystem is questioned. The data are compared with new results for Bi 2Sr 2CaCu 2O 8 (“2212”, T c=93 K) and YBa 2Cu 3O 7 (“123”, T c=91 K). The fluctuation component is similar for all three, but YBa 2Cu 3O 7 stands out with its relatively large mean-field jump.

Effect of microstructure on coercivity of c-axis in-plane-oriented Co/Cr multilayered films

The effect of the insertion of nonmagnetic Cr planes on coercivities of thin Co films has been studied in Co/Cr multilayered films. The in-plane coercivity Hc∥ can be as high as 1800 Oe and can be adjusted easily, ranging from 650 to 1800 Oe. The large Hc∥ is related to the Co(110) in-plane orientation which is induced by a Cr(100) in-plane-oriented underlayer. In addition, it appears that a decreasing saturation magnetization Ms, a decreasing average structural coherence length D in the direction perpendicular to the film, and a narrowing distribution of D are correlated to the increasing in Hc∥. The effects of texture, Cr interlayers, D and its distribution, and the Cr underlayer on the magnetic properties are discussed.

Theory of beam coupling and pulse shaping of mode-locked laser pulses in a photorefractive crystal

We describe how optical beams from a mode-locked laser will couple in a photorefractive crystal. We show that the two-beam-coupling gain coefficient is proportional to the square of the electric field correlation of the incoming light pulses. Consequently we show that two-beam-coupling experiments can measure the average coherence length of mode-locked laser pulses. We also describe how the temporal envelopes of the mode-locked optical pulses distort as they couple and propagate through the photorefractive crystal, and we give examples of how pulses can be shaped by using photorefractive coupling.

Lateral density fluctuations in the chain-melting phase transition of lipid monolayers

A detailed statistical mechanical study has been carried out on a microscopic interaction model of lipid monolayers on the air—water interface. The model is Pink's ten-state model which accounts for the internal conformational states of the lipid acyl chains and their statistics and mutual interactions. Particular attention is paid to the chain-melting phase transition of saturated lecithin monolayers (e.g., DPPC) and the microscopic phenomena which accompany this phase transition. By means of computer-simulation techniques we have calculated the equilibrium and nonequilibrium isotherms and obtained quantitative measures of the lateral density fluctuations in the transition region in terms of the size distribution of lipid domains and the average coherence length of the conformational order of the acyl chains. Indications of the presence of a critical point are obtained. The results are compared with experimental observations of DPPC lipid monolayer phase behavior with special emphasis on an understanding of the origin of the nonhorizontal experimental isotherms.

Coherent energy migration in solids: Determination of the average coherence length in one-dimensional systems using tunable dye lasers

The coherent nature of energy propagation in solids at low temperatures was established from the time resolved response of the crystal to short optical pulses obtained from a dye laser (pumped by a nitrogen gas laser). The trapping and detrapping of the energy by shallow defects (x traps) was evident in the spectra and enabled us to extract the coherence length: l≳700 ?=186 molecules for the one-dimensional triplet excitons of 1,2,4,5-tetrachlorobenzene crystals at T<4.2 °K. This length which clearly exceeds the stochastic random walk limit is related to the thermalization mechanisms in this coupled exciton–trap system, and its magnitude supports the notion that exciton–phonon coupling is responsible for the loss of coherence on very long molecular chains (trap concentration is 1/256 000).