Small-angle Neutron Diffraction Research Articles

Neutron scattering and the mixed state in high- Tc superconductors

We review the use of small-angle neutron diffraction to determine the spatial distribution of the magnetic field within the bulk of a superconductor in the mixed state. In high- T c materials, which have long penetration depths, this technique presents some difficulties. However, the high κ, large anisotropy and the effects of temperature and pinning on the flux distribution provide many effects to be explored. Recent neutron observations of the flux-line distributions in high- T c materials are described.

Anomalous region in the magnetic phase diagram of (Fe, Co) Si

The temperature and magnetic field dependence of the magnetic properties of helimagnet Fe0.8Co0.2Si is studied by magnetic measurement and small angle neutron diffraction. It is shown that Fe0.8Coo.2Si exhibits a magnetic phase transition from conical to a new modulated spin state, phase A, just below TN in the field range of 0.3 kOe < H < 0.5 kOe.

Evidence for Antiferromagnetic Coupling between Fe Layers through Cr from Neutron Diffraction

Small-angle neutron diffraction measurements have been made for a multilayer, [Fe(27 A)/Cr(12 A)]×30, at room temperature. It was found that the magnetizations of adjacent Fe layers are coupled in antiparallel. The decrease of antiferromagnetic peak intensity as a function of external field was observed. It is confirmed that the giant magnetoresistance in Fe/Cr multilayers is due to the antiferromagnetic ordering of Fe layers caused by the interlayer coupling across an intervening Cr layer.

Short-range ordering in a three-dimensionally frustrated magnet, Tb2Mo2O7, by wide- and small-angle neutron diffraction

Tb2Mo2O7 is an ordered crystalline oxide with the pyrochlore structure. This material undergoes a magnetic transition at 25 K which is similar to that of a spin glass. Neutron diffraction data indicate the presence of short range magnetic order via broad, diffuse reflections which persist above 25 K. Below 25 K the diffuse peaks increase in intensity but do not narrow. Fourier analysis of the data yield an approximation to the radial distribution function which is interpreted in terms of spin–spin correlations up to fourth neighbors. The first, third, and fourth neighbor interactions are ferromagnetic but the second neighbor interaction is strongly antiferromagnetic and involves only a Tb-Mo coupling. SANS data show magnetic scattering only at large Q values, reenforcing the view that antiferromagnetic interactions dominate. The Q-integrated magnetic SANS intensities show a temperature dependence similar to that for the correlation functions.

Observation by neutron diffraction of the magnetic flux lattice in single-crystal YBa2Cu3O7–δ

THE high-transition-temperature (high-Tc) oxide superconductors are type 2 superconductors: in an applied magnetic field, magnetic flux can enter the superconductor as quantized flux lines. In conventional type 2 superconductors, these lines form a lattice1, but in the new materials the flux lattice seems to be appreciably influenced by thermal fluctuations and may 'melt' at temperatures well below Tc (refs 2–4). Flux lines in high-Tc materials have been imaged by 'decorating' the ends of the lines with tiny magnetic particles5,6, but such experiments are limited to very low fields and indicate only the behaviour at surfaces. Here we report the observation of magnetic flux lines in the bulk of a crystal of YBa2Cu3O7–δ, using small-angle neutron diffraction. The experimental data at low temperatures confirm the existence of a vortex lattice of singly quantized flux lines. The results at higher tema-peratures show that neutron diffraction can be used to investigate the melting of the flux lattice.

Nearest neighbor order in an aligned solution of interacting rod-like micelles

Rod-like micelles of hexadecylpyridiniumsalicylate in aqueous solution were aligned by a shear gradient Γ of up to 40 s−1. Small angle neutron diffraction patterns of a solution of 20 mM/ℓ show a pronounced peak in scattering intensity at a momentum transfer of 0.15 nm−1 due to a correlation of the charged micelles. This correlation peak becomes deformed substantially with increasing Γ. A simple model is presented, taking into account an anisotropic change of the pair correlation function. It turns out that the order is reduced in the direction of the vector of velocity v of the sheared liquid and increases perpendicular to this vector.

Occurrence of liquid-crystalline mesophases in microemulsion dispersions

Although microemulsion dispersions have compositions which are related to lyotropic liquid crystals, their possible structures are not normally considered to be crystalline. As described here, small-angle neutron diffraction spectra have been obtained from some oil–water microemulsion dispersions, and display peaks which correspond to lamellar, hexagonal and cubic arrangements, similar to those observed in lyotropic liquid crystals. The formation of these liquid crystal mesophases (mesomorphic phases) seems to be a general feature of microemulsion compositions and not an isolated example.

Magnetic defects observed by small angle neutron scattering in applied magnetic fields in a weakly frustrated system

Small angle neutron diffraction measurements have been performed on a “reentrant” polycrystalline Ni 1- x Mn x alloy as a function of temperature and applied magnetic field. In applied fields, the scattering I( q) exhibits a peak which moves towards higher value as the field increases, while the total intensity strongly decreases. All these variations take place at nearly constant magnetization. The magnetic scattering I( q) obtained for different field values is shown to obey an approximate scaling law. These results can be interpreted in terms of the existence of rather well defined clusters (magnetic defects). Within this model, these experiments provide the observation of the variation of their size and density with field and temperature. Finally, very similar observations have been made for amorphous (FeMn) 75P 16B 6Al 3.

The shape of cobalt precipitates in copper

Abstract Ferromagnetic single-domain cobalt precipitates have been produced in single-crystal copper spheres with about 1% cobalt by annealing between 5 and 40 hours at 873 K in a magnetic field of 1·5 × 106 Am−1 applied parallel to the [100] direction. Small-angle neutron diffraction studies have been performed on these specimens in the (100) and (0&1bar;1) planes. The size and geometrical shape of the cobalt particles have been found to confirm an earlier assumption; the particles are of octahedral shape with a few per cent deviation from sphericity and with some additional 3% elongation in the direction of the magnetic field which was applied during annealing. Contrary to earlier assumptions, particles annealed in a field parallel to 〈100〉 are not prolate ellipsoids. The deviation of particle shape from sphericity was found to be almost independent of annealing time. Moreover we have observed a pronounced short-range order between the precipitates which could not be deduced from earlier experiments. Th...

Neutron scattering shows that cytochrome b5 penetrates deeply into the lipid bilayer

Cytochrome b5 was asymmetrically reconstituted into small lipid vesicles made of a highly deuterated phospholipid. Small-angle neutron diffraction patterns were collected in a series of H2O-D2O mixtures from vesicles consisting of lipid and native or trypsinized cytochrome b5. The second moment of the radial distribution of scattering density in the vesicles was derived from these data and was compared to values calculated from three proposed models, which differ by the degree that cytochrome b5 penetrates the lipid bilayer. The model in which the hydrophobic domain of the protein is distributed across the bilayer agreed most closely with the data.

Experimental characterization of the flux-line lattice in superconducting V&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Si

Several microscopic properties of the flux-line lattice (FLL) in three separate single crystals of V 3 Si have been investigated by means of small-angle neutron diffraction. These low-field FLL characterizations have been correlated with the following material and superconducting properties: (A) The real crystal symmetry parallel to the applied magnetic field; (B) The micro-structure as determined by TEM; (C) Magnetic irreversibilities in the mixed state; (D) Reversible flux-line motion in ac response; (E) Martensitic structural transformation observed by X-ray diffraction. The three samples, V 3 Si-MP3, -MP4, and -MP5 possessed different defect structures, and this was manifested foremost in the FLL perfection. At low field (B B\gsim0-.5 T, but a highly mosaic, nearly polycrystalline FLL at lower fields. Sample MP4 contained large (500-1000 A) incoherent precipitates, and showed only a polycrystalline FLL at low field. In both MP3 and MP5, distinct anisotropic correlations were observed between the FLL morphology and the real-crystal direction along the applied field. The FLL perfection was strongly dependent on the growth history. The peak width history dependence for two different scattering geometries can be qualitatively modeled by proposed flux-pinning mechanisms. Quantitative comparisons with critical current measurements, however, are not totally reconcilable.

Phase Decomposition in Cu-Ti Metallic Glass

ABSTRACTA metallic glass ribbon of Cu55Ti45 prepared by melt spinning was examined by x-ray, neutron, and electron diffraction, by small angle neutron diffraction (SANS), transmission electron microscopy (TEM), and by differential thermal analysis (DTA). In the liquid quenched condition large angle diffraction data (both x-ray and neutron) show the broad banded structure typical of the amorphous state. The SANS data, however, exhibit highly anisotropic patterns arising from the phase decomposition during solidification. Ribbons annealed below the glass transition temperature (Tg ) produced neutron diffraction patterns of materials with the same amorphous structure combined with a new short range order; and the SANS patterns retained the asymmetry of the as-quenched material. Ribbons annealed above the crystallization temperature (Tc) show both isotropic and anisotropic contributions to the SANS patterns. Formation of the equilibrium TiCu phase occurs directly from the metallic glass at Tc. The equilibrium Ti3Cu4 phase, however, forms from the TiCu phase at slightly higher temperatures.

Short-range clustering and long-range periodic decomposition of an electron irradiated Ni-Cu alloy

Diffuse and small-angle neutron diffraction studies of the null matrix 62Ni-41 at.% 65Cu have been performed after thermal treatment and after irradiation with 3 MeV electrons at temperatures between 373K and 640K. Besides an increase in the short-range clustering, a periodic decomposition with a wavelength around 45 AA is observed after irradiation at temperatures between 373K and 480K. The decomposition structure was found to be stable against thermal annealing and irradiation, at least up to 510K. Upon thermal treatment above 600K dissolution of the long range decomposition takes place. By extrapolation of the short-range clustering data from high temperatures, the critical temperature of the coherent miscibility gap is obtained at 527K and that of the incoherent gap at 575K. The decomposition kinetics within the gap can be described satisfactorily by Cook's clustering model (1969, 1970) for large scattering wavevectors, i.e. for the short-range clustering. The small-angle peak kinetics show several specific deviations from the predictions of the common spinodal decomposition theories. The possibility cannot be excluded that the observed long-range decomposition is a radiation-induced phase transformation.

Direct Observation of the Equilibrium Misalignment between Fluxoids and an Applied Magnetic Field Due to Anisotropy Effects in a Type-II Superconductor

The anisotropic interaction between the crystal lattice and the flux-line lattice results in a misalignment between the applied magnetic field and the fluxoids in a type-II superconductor. Small-angle neutron diffraction has been used to provide the first direct observations of this phenomenon. It is shown that the misalignment angle in the intermediate mixed state is related to the relative anisotropy in the lower critical field ${H}_{c1}$, and this relationship is tested for a spherical sample of pure niobium.

Structure of polymeric glasses and melts

In order to describe the structure of polymers in the molten or vitreous state four different kinds od static correlation functions should be studied experimentally: (i) the pair distribution function of the monomer units, (ii) the distance distribution function of the repeating units of a single chain, (iii) the orientation correlation function of the bond vectors, and (iv) the density fluctuation correlation function of the bulk sample. The appropriate methods include: X-ray wide-angle scattering and electron diffraction, small-angle neutron diffraction of deuterium tagged molecules, depolarized light scattering and magnetic birefringence, small-angle X-ray and polarized light scattering. A review of the results is presented with special emphasis on the differentation between short range order, as e.g. revealed from the pair distribution function, and long range correlations as proposed for the so-called “bundle” models. It is shown that there exists no experimental evidence for such a structure besides the “grains” in electron micrographs.

Determination of low-field critical parameters of superconducting niobium by small-angle neutron diffraction

The perfect double-crystal small-angle diffraction technique enables measurement of scattering angles to within 0.3 arc sec. accuracy. At a wavelength of 2.55 A, this provides a resolution of 3 x 10/sup -6/ A/sup -1/ in the scattering vector. This technique has been used to study the anisotropic behavior of the critical parameters B/sub 0/ and H/sub c1/, characteristic of the first-order magnetic phase transition which occurs in low-kappa type-II superconductors. Magnetic fields were applied parallel to several crystal axes of a large single-crystal sphere of pure niobium, resulting in well-defined flux-line lattices (FLL). Measurement of the FLL cell area in the intermediate mixed state field region gives the equilibrium flux density B/sub 0/, which results from an attractive interaction between fluxoids. In addition, field variation of the scattered neutron intensity allows measurement of the transition field between the mixed state and intermediate mixed state. This transition field is related to the lower critical field H/sub c1/ and enables its determination to a precision 0.2%. Data at T = 4.3 K display a small anisotropic effect of about 2% in B/sub 0/ and 1% in H/sub c1/. Although orientation effects of this magnitude are difficult to resolve by bulk measurements, themore » neutron data are in accord with magnetization data. Observations regarding the temperature dependence of these parameters also will be presented, and comparisons made with current theoretical models.« less

Small-angle X-ray and neutron diffraction from corneal stroma

The corneal stroma is a connective tissue which combines the usual properties of strength and resilience with one property, transparency, unusual in such tissues. Several models have been put forward to account for this transparency, these differ in detail but in almost all cases the regularity of arrangement of the collagen fibrils is taken to be an important parameter of the model (Maurice, 1957; Farrell & Hart, 1969; Benedek, 1971). So far the only experimental evidence about this arrangement comes from electron microscopy, and we have therefore applied small-angle diffraction techniques because they involve less preparative procedures which might cause artifacts.

Anisotropy in the flux-line lattice symmetry of a low-κ Type II superconductor

Correlation between the symmetry of the two-dimensional flux-line lattice (FLL) and the real crystal lattice (CL) has been studied in a superconducting niobium sphere by means of small-angle neutron diffraction. A double-perfect-silicon-crystal diffractometer enabled precise determination of the three interfluxoid distances corresponding to the FLL basic cell. A systematic study of the anisotropic behavior was made as a function of temperature and magnetic field amplitude for fields parallel to a few high-symmetry CL axes in the (1{\bar 1}0) plane. In addition, at T = 4.30 K progressive deformation of the FLL was studied as the sample was rotated in the (1{\bar 1}0) and (100) planes. The FLL was found to be hexagonal only for fields parallel to the threefold CL axis. Twofold symmetry prevailed for other CL directions in these planes except near the fourfold axis, where either of two distorted triangular lattices existed, preserving the reflection symmetry in composite, but not individually. When compared to current models for fluxoid-CL interactions, the present results show that no theory predicts the observed behavior quantitatively under general conditions, but some models agree well for certain high-symmetry CL axes.

Structure of collagen in cartilage of intervertebral disk.

Small-angle x-ray and neutron diffraction patterns have been obtained from the annulus fibrosus of porcine intervertebral disk. These show that the collagen in this tissue is modified compared with that in tendon.

Short Range Order in Liquid Lithium-Sodium Alloys

Abstract The short range order of a liquid lithium sodium alloy with nearly critical composition has been studied by small-angle and high-angle neutron diffraction at 10 and 145 K above the critical temperature. In this temperature range the small-angle scattering yields linear Ornstein-Zernike plots from which formally critical exponents and amplitudes could be determined. The atoms prefer like nearest neighbors and the extent of this preference varies only slowly with temperature.