Showing papers by "Chalk River Laboratories published in 2003"

Dynamic frustrated magnetism in Tb 2 Ti 2 O 7 at 50 mK

Abstract: The low temperature $(Tl1\mathrm{K})$ properties of the cooperative paramagnet ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ have been studied by ac susceptibility, neutron diffraction and neutron spin echo techniques. Like several other frustrated magnets, ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7},$ is believed to remain paramagnetic down to $\ensuremath{\sim}0.07\mathrm{K}.$ However, recent studies [Yasui et al., J. Phys. Soc. Jpn. 71, 599 (2002), for example] suggest that ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ enters an ordered, albeit glassy, state at a relatively high temperature, $\ensuremath{\sim}1.5\mathrm{K}.$ Our results confirm that the majority of the spins in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ fluctuate very rapidly, even at 50 mK and that static, spatial correlations do not develop beyond nearest neighbor at similar temperatures. We suggest that the observation of a partial freezing of this magnetic system, at finite temperature, is a result of a small fraction of spins freezing around defects in the stoichiometric crystal structure.

Dynamical crossover in 'hot' spin ice

Abstract: The magnetic dynamics of the spin ice material Ho2Ti2O7 in its paramagnetic ('hot') phase have been investigated by a combination of neutron spin echo and ac-susceptibility techniques. Relaxation at high temperatures ( 15$>T > 15 K) is proved to occur by a thermally activated single-ion process that is distinct from the process that dominates at lower temperatures (1 K < T < 15 K). It is argued that the low-temperature process must involve quantum mechanical spin tunnelling, as quasi-classical channels of relaxation are exhausted in this temperature range. Our results resolve a mystery in the physics of spin ice: why has a 15 K ac-susceptibility peak been observed in Dy2Ti2O7 but not in Ho2Ti2O7 or Ho2Sn2O7?

Structure of the Ripple Phase of Phospholipid Multibilayers

Abstract: We present electron density maps (EDMs) of the ripple phase formed by phosphorylcholine lipids such as dimyristoyl phosphatidylcholine (DMPC), palmitoyl-oleoyl phosphatidylcholine (POPC), dihexadecyl phosphatidylcholine, and dilauroyl phosphatidylcholine (DLPC) With the exception of DLPC, the rippled bilayers have a sawtooth shape in all the systems, with one arm being almost twice as long as the other For DMPC and POPC bilayers, EDMs have been obtained at different temperatures at a fixed relative humidity, and the overall shape of the ripples and the ratio of the lengths of the two arms are found to be insensitive to temperature EDMs of all the systems with saturated hydrocarbon chains suggest the existence of a mean chain tilt along the ripple wave vector In the literature it is generally assumed that the asymmetry of the rippled bilayers (absence of a mirror plane normal to the ripple wave vector) arises from a sawtoothlike height profile However, in the case of DLPC, the height profile is found to be almost symmetric and the asymmetry results mainly from different bilayer thicknesses in the two arms of the ripple We also present EDMs of the metastable ripple phase of dipalmitoyl phosphatidylcholine, formed on cooling from the L(alpha) phase

Infrared and Raman spectroscopy studies of the α–β phase transition in cristobalite

Abstract: Infrared and Raman spectra of cristobalite are presented as a function of temperature through the phase transition. The modes are assigned and the assignments compared to those of earlier workers. The compatibility of modes at the Γ-point of the α-phase with the X and Γ-points of the β-phase is given. In the transition region of ca. 500–550 K, smooth changes in intensity, frequency and linewidths are seen in many modes, indicative of coexistence of α- and β-forms.

Discontinuous unbinding of lipid multibilayers.

Abstract: We have observed a discontinuous unbinding transition of lipid bilayer stacks composed of phosphatidylethanolamine and phosphatidylglycerol using x-ray diffraction. The unbinding is reversible and coincides with the main (${L}_{\ensuremath{\beta}}\ensuremath{\rightarrow}{L}_{\ensuremath{\alpha}}$) transition of the lipid mixture. Interbilayer interaction potentials deduced from the diffraction data reveal that the bilayers in the ${L}_{\ensuremath{\beta}}$ phase are only weakly bound. The unbinding transition appears to be driven by an abrupt increase in steric repulsion resulting from increased thermal undulations of the bilayers upon entering the fluid ${L}_{\ensuremath{\alpha}}$ phase.

Residual stresses in components formed by the laserengineered net shaping (LENS®) process

Abstract: The residual stress distributions in two laser-engineered net shaping (LENS®) samples were mapped by neutron diffraction. The samples took the form of a thin wall and a pillar of square cross-section. Stresses were measured in the three orthogonal symmetry directions of the parts, parallel and perpendicular to the growth direction. Surprisingly, over most of the bulk of the samples the stress was uniaxial and directed along the growth axis, with compression in the centre of the samples and tension at the edges. The magnitudes of the maximum residual stresses were significant fractions (50 and 80 per cent for the thin wall and pillar respectively) of the 0.2 per cent yield point. The origin of the residual stress distributions is discussed qualitatively in terms of the thermal histories of the samples.

Modulation of the polymorphism of the palmitic acid/ cholesterol system by the pH

Abstract: Differential scanning calorimetry, infrared spectroscopy, solid-state 13C and 2H NMR spectroscopy, and X-ray diffraction were used to characterize hydrated mixtures of palmitic acid (PA) and cholesterol (chol), at low (5.4) and high (8.5−9.0) pH. The pH affects the ionization state of the fatty acid, and it was found to modulate its interaction with cholesterol, and the architecture of the resulting complexes. At low pH, this system undergoes a transition from a phase-separated crystalline state to a lamellar liquid-ordered (lo) phase, in agreement with previous results (Langmuir 2001, 17, 5587−5594). The apparent pKa of PA in these conditions is found to be about 8.2. At high pH, when the fatty acid is deprotonated and consequently negatively charged, both components participate in the formation of an lo lamellar structure that was found to be stable over a large temperature range (20−70 °C). For 50/50 PA/chol mixtures, only this phase, with a d spacing of approximately 49 A, is detected. These fluid bil...

Concentration-independent spontaneously forming biomimetric vesicles.

Abstract: In this Letter we present small-angle neutron scattering data from a biomimetic system composed of the phospholipids dimyristoyl and dihexanoyl phosphorylcholine (DMPC and DHPC, respectively) Doping DMPC-DHPC multilamellar vesicles with either the negatively charged lipid dimyristoyl phosphorylglycerol (DMPG, net charge -1) or the divalent cation, calcium (Ca 2 + ), leads to the spontaneous formation of energetically stabilized monodisperse unilamellar vesicles whose radii are concentration independent and in contrast with previous experimental observations

Molecular dynamics simulations of a mixed DOPC/DOPG bilayer.

Abstract: We have constructed a mixed dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylglycerol bilayer (DOPG) bilayer utilizing MD simulations. The aim was to develop an explicit molecular model of biological membranes as a complementary technique to neutron diffraction studies that are well established within the group. A monolayer was constructed by taking a previously customized PDB file of each molecule and arranging them in a seven rows of ten molecules and duplicated and rotated to form a bilayer. The 140-molecule bilayer contained 98 DOPC molecules and 42 DOPG molecules, in a 7:3 ratio in favour of DOPC. Sodium counter ions were placed near the phosphate moiety of DOPG to counteract the negative charge of DOPG. This was representative of the lipid ratio in a sample used for neutron diffraction. The MD package GROMACS was used for confining the bilayer in a triclinic box, adding Simple Polar Charge water molecules, energy minimization (EM). The bilayer/solvent system was subjected to EM using the steepest descent method to nullify bad contacts and reduce the potential energy of the system. Subsequent MD simulation using an initial NVT (constant number of particles, volume and temperature) for a 20 ps MD run followed by a NPT (constant number of particles, pressure and temperature) was performed. Structural parameters including volume of lipid, area of lipid, order parameter of the fatty acyl carbons and electron density profiles generated by the MD simulation were verified with values obtained from experimental data of DOPC, as there are no comparable experimental data available for the mixed bilayer.

Hydrogen bonding in ikaite, CaCO3.6H2O

Abstract: Ikaite is a metastable hexahydrate of calcium carbonate, forming in aqueous conditions near freezing conditions. Neutron- powder diffraction data of synthetic deuterated ikaite, collected at seven temperatures from 4 to 270 K, were refined. The linear thermal expansion coefficients are quite anisotropic, being smaller in the direction of the C-O bond. A review of the hydrogen-bonding scheme around the (CaCO 0 3 ) ion pair is given and an additional weak potential hydrogen bond is suggested. Temperature-dependent growth of the atomic displacement factors of the carbonate O1 atom agrees with the previous suggestion of a possible low-frequency, hindered librational mode of the carbonate group.

Small-polaron hopping conduction in bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7

Abstract: We report anisotropic resistivity measurements on a ${\mathrm{La}}_{12}{\mathrm{Sr}}_{18}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ single crystal over a temperature T range from 2 to 400 K and in magnetic-fields H up to 14 T For $Tg~218\mathrm{K},$ the temperature dependence of the zero-field in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}(T)$ obeys the adiabatic small polaron hopping mechanism, while the out-of-plane resistivity ${\ensuremath{\rho}}_{c}(T)$ can be ascribed by an Arrhenius law with the same activation energy Considering the magnetic character of the polarons and the close correlation between resistivity and magnetization, we developed a model which allows the determination of ${\ensuremath{\rho}}_{ab,c}(H,T)$ The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites

Kinetic pathway of the bilayered-micelle to perforated-lamellae transition.

Abstract: Using time-resolved small-angle neutron scattering, we have studied the kinetics of the recently observed bilayered-micelle (or so-called "bicelle") to perforated-lamellar transition in phospholipid mixtures. The data suggest that phase-ordering occurs via the early-time coalescence of bicelles into stacks of lamellae that then swell. Our measurements on this biomimetic system highlight the ubiquitous role of transient metastable states in the phase ordering of complex fluids.

High-Temperature Neutron Diffraction of the AlN–Al2O3–Y2O3 System

Abstract: The importance of aluminum nitride (AlN) stems from its application in microelectronics as a substrate material due to high thermal conductivity, high electrical resistance, mechanical strength and hardness, thermal durability, and chemical stability. Yttria (Y2O3) is the best additive for AlN sintering. AlN densifies by a liquid-phase mechanism, where the surface oxide, Al2O3, reacts with Y2O3 to form an Y-Al-O-N liquid that promotes particle rearrangement and densification. Construction of the phase relations in this multicomponent system is essential for optimizing the properties of AlN. The ternary phase diagram of the AlN–Al2O3–Y2O3 was developed by Gibbs energy minimization using interpolation procedures based on modeling the binary subsystems. This paper aims at testing the resultant understanding experimentally at selected compositions using in situ high-temperature neutron diffractometry. These experimental results agree with the thermodynamic calculations of AlN–Al2O3–Y2O3. The ternary phase diagram has been constructed for the first time in this work. High-temperature neutron diffractometry has permitted real time measurement of the reactions involved in this ternary system, especially to determine the temperature range for each reaction, which would have been difficult to establish by other means.

New magnetic structure study of TbNi5: Evidence of incommensurate structure

Abstract: We have investigated the magnetic structure of TbNi5 using neutron diffraction from 22 to 30 K to find new magnetic satellites below TC = 23 K, which have never been reported before These satellites can be indexed as (0,0,1±τ), (1,0,1±τ), (0,0,2±τ), and (1,0,2±τ), where τ is equal to 00194 rlu (reciprocal lattice units) We have observed these new peaks in addition to magnetic peaks seen in previous measurements We can understand this long-period incommensurate structure in terms of two propagation vectors: k1 = (0,0,0) and k2 = (0,0,τ) From the temperature dependence of the intensity of the commensurate and incommensurate peaks as well as our heat capacity data, we suggest that TbNi5 should have a modulated magnetic structure over the whole temperature range from 23 to 2 K, in contradiction to the long held view that TbNi5 is a collinear ferromagnet below TC = 23 K

Residual stresses in an electron beam weld of Ti-834: Characterization and numerical modeling

Abstract: Neutron and synchrotron X-ray strain scanning techniques are used to characterize the residual stresses in an electron beam weld in the near-alpha titanium alloy Ti-834. Measurements are made using the $$\{ 10\bar 13\} $$ and $$\{ 11\bar 20\} $$ lattice planes in both the longitudinal and transverse directions, i.e., both along and normal to the welding direction. The lattice strains are large and positive (several thousand microstrains) in the longitudinal direction; in the fusion zone, the strains measured on the two planes differ by about 30 pct. The strains in the transverse direction are significantly smaller. A numerical model based upon the finite-element method (FEM) is developed in order to rationalize these results. Account is taken of the transfer of heat to the metal and subsequent conduction into the heat-affected zone; convection and radiation effects are neglected. The residual stresses are then estimated using an elastic-plastic analysis with temperature-dependent properties. Despite the limitations of the modeling approach, there is good agreement between experiment and theory, i.e., the sign and extent of the stress field. The plastically upset region extends about 3.5 mm from the weld centerline. The model is used to examine the adequacies of the experimental approach. It is demonstrated that the difference between the lattice strains measured for the two reflections in the fusion zone occurs as a consequence of the micromechanical response of the material, e.g., the partitioning of stress between the grains and the accumulation of intergranular microstrains.

Anisotropic flux density dependence of magnetic Barkhausen noise in oriented 3% Si-Fe steel laminates

Abstract: This paper compares the anisotropy of surface magnetic Barkhausen noise (MBN) and pulse height distributions between two samples of oriented 3% Si-Fe steel laminate at equivalent flux densities. One sample had lower core loss (0.83 W/kg) than the other (0.97 W/kg). The MBN/sub Energy/ (square of the MBN voltage signal integrated with respect to time) was investigated parallel and perpendicular to the samples' easy axis. The highest levels of MBN/sub Energy/ and magnetization were achieved in the parallel orientation in the low-core-loss sample. The considerable anisotropies observed under similar field conditions were largely removed when the MBN/sub Energy/ was considered as a function of peak flux density. Differences between the two samples were observed in the anisotropic behavior of the MBN/sub Energy/ and pulse height distributions for flux density amplitudes between 1.3 and 1.6 T, the typical operating range of transformer laminates. These were attributed to differences in the number and structure of 180/spl deg/ domain walls. The low-core-loss sample was interpreted as having a finer domain-wall structure that produced abrupt changes in the magnetization projected out of the sample plane, thereby reducing the microscopic eddy-current contribution to losses.

The Feasibility of Neutron Moderation Imaging for Land Mine Detection

Abstract: Neutron moderation land mine detection involves irradiating the ground with fast neutrons and subsequently detecting the thermalized neutrons which return This technique has been studied since the 1950s, but only using non-imaging detectors Without imaging, natural variations in moisture content, surface irregularities, and sensor height variations produce sufficient false alarms to render the method impractical in all but the driest conditions This paper describes research to design and build a prototype land mine detector based on neutron moderation imaging After reviewing various neutron detector technologies, a design concept was developed It consists of a novel thermal neutron imaging system, a unique neutron source to uniformly irradiate the underlying ground, and hardware and software for image generation and enhancement A proof-of-principle imager has been built, but with a point source offset from the detector to roughly approximate a very weak uniform source at the detector plane Imagery from the detector of mine surrogates is presented Realistic Monte Carlo simulations were performed using the same two dimensional neutron imaging geometry as the detector in order to assess its performance The target-to-background contrast was calculated for various soil types and moisture contents, explosive types and sizes, burial depths, detector standoffs, and ground height variations The simulations showed that the neutron moderation imager is feasible as a land mine detector in a slow scanning or confirmation role and that image quality should be sufficient to significantly improve detector performance and reduce false alarm rates compared to non-imaging albedo detection, particularly in moist soils, where surface irregularities exist and when the sensor height is uncertain Performance capability, including spatial resolution and detection times, was estimated

Magnetic structures of Laves phase superlattices

Abstract: We present a determination of the magnetic structures of three Laves phase superlattice samples of structure [70/30]60, [150/100]50, and [50/70]60, where the structure is given as [t1 A DyFe2/t2 A YFe2]N, grown by molecular beam epitaxy. The experiments were performed using magnetization measurements and neutron scattering measurements at the NRU reactor at Chalk River in Canada. For the [70/30]60 sample, a magnetic field parallel to the scattering vector, Q, was applied to determine the nuclear component of the scattered intensity. The magnetic structure with the field aligned along the [001] had all the moments aligned along the field direction and the magnitude of the moment on the iron site was temperature independent with a value of 2.3(0.3) μB. The moment on the dysprosium site was found to decrease with temperature from about 10 μB to a value of 6(0.5) μB at 300 K. When the field was applied in the [10] direction the magnetic moments were found to rotate out of the (110) epitaxial plane towards the [110] direction by an angle ψ = 10° at 300 K, which increased at 4 K to an angle of ψ = 40° close to the [100] out-of-plane direction. For the [150/100]50 sample, when a 6 T magnetic field was applied along the [10] direction, the main peak of the magnetically sensitive (11) reflection was found to decrease in intensity while the scattering at the satellite peaks increased. This change in intensity is due to the formation of magnetic exchange springs in the 'soft' YFe2 layers of the superlattice. Detailed measurements around the (11) reflection and calculations for an exchange spring model give excellent agreement between the model and the experiment. Finally, the [50/70]60 sample showed unexpected behaviour because the moments aligned largely perpendicular to an applied field. This is similar to a spin-flop phase of an antiferromagnet and it is argued that this occurs because the net moments on the DyFe2 and YFe2 layers are nearly equal.

Suppression of incommensurate spin-density waves in thin epitaxial Cr(110) layers of a V/Cr multilayer

Abstract: We observed a complete suppression of the incommensurate spin-density wave in thin Cr layers of a V/Cr multilayer in a temperature range from 550 K down to 2 K. The (110)-oriented V/Cr multilayer consisting of 30 nm thick Cr layers and 5 nm thick V layers was investigated by neutron and X-ray diffraction (XRD). From the XRD experiments we were able to determine that the epitaxial strain of the Cr layers in the V/Cr multilayer is about 90% larger than in earlier studied Fe/Cr(110) multilayers. That leads to a completely different magnetic phase diagram as revealed by neutron diffraction experiments. The existence of the commensurate antiferromagnetic structure in the Cr layers can be observed in the whole temperature range without a phase transition to an incommensurate spin-density wave at lower temperatures. In order to elucidate the proximity effects further we also performed experiments in an external magnetic field. Up to a field of 4 T we found no change in the magnetic structure of the Cr films whereas in earlier experiments on Fe/Cr(110) multilayers we could observe a strong perpendicular pinning of the Cr polarization to the Fe magnetization.