Magnetically soft, high moment grain-refined Fe films: application to magnetic tunnel junctions

The effect of N-doping on the microstructure and magnetic properties of thin Fe layers has been employed to construct all Fe-electrode magnetic tunnel junctions that displayed the tunneling magnetoresistance (TMR) effect. Using low nitrogen doses, a reduction in coercivity was achieved due to grain refinement, without a concurrent decrease in the saturation magnetization of the Fe films caused by the formation of crystalline iron nitride phases. It was demonstrated that this N-induced grain refinement can be applied beneficially to control the switching field of the "free" layer in magnetic trilayer structures. In general the ability to control magnetic softness without reducing saturation magnetization will prove important for incorporating high spin-polarized materials into spin valves and TMR devices

Magneto-optical characteristics of magnetic nanowire arrays in anodic aluminium oxide templates

Nanocomposite films consisting of regularly ordered iron nanowires embedded in anodic aluminum oxide templates have been fabricated and their magneto-optical properties studied by determining the four Stokes parameters of the transmitted laser beam (λ=670 nm), originally linearly polarized and at normal incidence to the film surfaces. The results of the nanowire arrays are found to be considerably different from that of bulk iron. While an increase in diameter of the nanowire leads to a substantial increase in the values of the Faraday rotation angles per unit length at a fixed value of the magnetic fields, they are substantially less than that of bulk iron, indicating that the effective media theory may not be directly applicable

A Hypothesis-Testing Framework for Studies Investigating Ontogenetic Niche Shifts Using Stable Isotope Ratios

Ontogenetic niche shifts occur across diverse taxonomic groups, and can have critical implications for population dynamics, community structure, and ecosystem function. In this study, we provide a hypothesis-testing framework combining univariate and multivariate analyses to examine ontogenetic niche shifts using stable isotope ratios. This framework is based on three distinct ontogenetic niche shift scenarios, i.e., (1) no niche shift, (2) niche expansion/reduction, and (3) discrete niche shift between size classes. We developed criteria for identifying each scenario, as based on three important resource use characteristics, i.e., niche width, niche position, and niche overlap. We provide an empirical example for each ontogenetic niche shift scenario, illustrating differences in resource use characteristics among different organisms. The present framework provides a foundation for future studies on ontogenetic niche shifts, and also can be applied to examine resource variability among other population sub-groupings (e.g., by sex or phenotype)

A Hypothesis-Testing Framework for Studies Investigating Ontogenetic Niche Shifts Using Stable Isotope Ratios

Ontogenetic niche shifts occur across diverse taxonomic groups, and can have critical implications for population dynamics, community structure, and ecosystem function. In this study, we provide a hypothesis-testing framework combining univariate and multivariate analyses to examine ontogenetic niche shifts using stable isotope ratios. This framework is based on three distinct ontogenetic niche shift scenarios, i.e., (1) no niche shift, (2) niche expansion/reduction, and (3) discrete niche shift between size classes. We developed criteria for identifying each scenario, as based on three important resource use characteristics, i.e., niche width, niche position, and niche overlap. We provide an empirical example for each ontogenetic niche shift scenario, illustrating differences in resource use characteristics among different organisms. The present framework provides a foundation for future studies on ontogenetic niche shifts, and also can be applied to examine resource variability among other population sub-groupings (e.g., by sex or phenotype)

Solution and structural properties of colloidal charged lipid A (diphosphate) dispersions

It has been possible to prepare electrostatically stabilized aqueous dispersions of lipid A (diphosphate) particles of low polydispersity at low ionic strength (1-10 mM NaCl) over a range of volume fractions of 1.5 × 10-4 < < 5.75 × 10-4 (25 C). These suspensions have been characterized by transmission electron microscopy, light scattering, osmotic pressure measurements, and small-angle X-ray scattering experiments at 25 C. All four measurements yielded independent values for particle sizes, weighted-average molecular weights, number-average molecular weights, and particle surface charge. The mean values obtained are = 37.59 ± 0.75 nm, = 24.89 ± 0.88 nm, = (10.55 ± 0.78) × 106 g/mol, = (9.81 ± 0.90) × 106 g/mol, and the effective surface charge Z* = (756 ± 85). Very good experimental agreement is found for the directly measured osmotic pressure values and those determined from light scattering and small-angle X-ray scattering measurements as a function of volume fraction, , by applying liquid-state theory models. Using the particle parameters for the lipid A (diphosphate) system as determined, the scattering functions and the osmotic pressures can be compared as a function of volume fraction with no adjustable parameters. The ordering of lipid A in solution revealed a body-centered cubic (bcc) type lattice (a = 36.14 nm) at volume fractions of 3.75 × 10-4 < < 4.15 × 10-4, whereas at volume fractions of 4.15 × 10-4 < < 5.75 × 10-4 in the presence of 1.0 mM NaCl a face-centered cubic (fcc) lattice type (a = 57.25 nm) was observed. Small-angle X-ray scattering experiments also indicate the presence of long-ranged order at 1.0 mM or at 10.0 mM NaCl for lipid A dispersions of 3.75 × 10-4 < < 5.75 × 10-4

The liquidlike ordering of lipid A-diphosphate colloidal crystals: the influence of Ca2+, Mg2+, Na+, and K+ on the ordering of colloidal suspensions of lipid A-diphosphate in aqueous solutions

A comprehensive study was performed on electrostatically stabilized aqueous dispersion of lipid A-diphosphate in the presence of bound Ca2+, Mg2+, K+, and Na+ ions at low ionic strength (0.10-10.0-mM NaCl, 25 °C) over a range of volume fraction of 1.0×10-4<=f<=4.95×10-4. These suspensions were characterized by light scattering (LS), quasielastic light scattering, small-angle x-ray scattering, transmission electron microscopy, scanning electron microscopy, conductivity measurements, and acid-base titrations. LS and electron microscopy yielded similar values for particle sizes, particle size distributions, and polydispersity. The measured static structure factor, S(Q), of lipid A-diphosphate was seen to be heavily dependent on the nature and concentration of the counterions, e.g., Ca2+ at 5.0 nM, Mg2+ at 15.0 µM, and K+ at 100.0 µM (25 °C). The magnitude and position of the S(Q) peaks depend not only on the divalent ion concentration (Ca2+ and Mg2+) but also on the order of addition of the counterions to the lipid A-diphosphate suspension in the presence of 0.1-µM NaCl. Significant changes in the rms radii of gyration (RG2)1/2 of the lipid A-diphosphate particles were observed in the presence of Ca2+ (24.8±0.8 nm), Mg2+ (28.5±0.7 nm), and K+ (25.2±0.6 nm), whereas the Na+ salt (29.1±0.8 nm) has a value similar to the one found for the de-ionized lipid A-diphosphate suspensions (29.2±0.8 nm). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, S(Q), to the light-scattering data and they were found to be in the range of Z*=700-750 for the lipid A-diphosphate salts under investigation. The light-scattering data indicated that only a small fraction of the ionizable surface sites (phosphate) of the lipid A-diphosphate was partly dissociated (~30%). It was also discovered that a given amount of Ca2+ (1.0-5.0 nM) or K+ (100 µM) influenced the structure much more than Na+ (0.1-10.0-mM NaCl) or Mg2+ (50 µM). By comparing the heights and positions of the structure factor peaks S(Q) for lipid A-diphosphate-Na+ and lipid A-diphosphate-Ca2+, it was concluded that the structure factor does not depend simply on ionic strength but more importantly on the internal structural arrangements of the lipid A-diphosphate assembly in the presence of the bound cations. The liquidlike interactions revealed a considerable degree of ordering in solution accounting for the primary S(Q) peak and also the secondary minimum at large particle separation. The ordering of lipid A-diphosphate-Ca2+ colloidal crystals in suspension showed six to seven discrete diffraction peaks and revealed a face-centered-cubic (fcc) lattice type (a=56.3 nm) at a volume fraction of 3.2×10-4<=f<=3.9×10-4. The K+ salt also exhibited a fcc lattice (a=55.92 nm) at the same volume fractions, but reveals a different peak intensity distribution, as seen for the lipid A-diphosphate-Ca2+ salt. However, the Mg2+ and the Na+ salts of lipid A-diphosphate showed body-centered-cubic (bcc) lattices with a=45.50 nm and a=41.50 nm, respectively (3.2×10-4<=f<=3.9×10-4), displaying the same intensity distribution with the exception of the (220) diffraction peaks, which differ in intensity for both salts of lipid A-diphosphate

Liquid-like ordered colloidal suspensions of lipid A: the influence of lipid A particle concentration

Electrostatically stabilized aqueous dispersions of nm-sized free lipid A particles at low volume fractions (1.0×10-4<=θ<=3.5×10-4) in the presence of 1.0-10.0 mM NaCl (25 °C) have been characterized by static and quasielastic light scattering (QELS) techniques, electron microscopy (SEM and TEM), conductivity measurements, and acid-base titrations. QELS and electron microscopy (<overbar>ρTEM=8.0±0.6%) yield similar values for the particle size and particle size distribution (<overbar>ρQELS=10.9±0.75 %), whereas conductivity and acid-base titrations estimate surface chemical parameters (dissociation constant, ionizable sites, and Stern capacitance). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, <overbar>S(Q), to the light scattering data. Using the particle properties as determined from these experiments, the polydisperse structure factor, <overbar>S(Q), was calculated as a function of volume fraction, θ, which was found to be consistent with a <overbar>S(Q) dependence on the number particle density. It can be concluded that, at low volume fractions and low ionic strength, the light scattering data are well represented by a Poisson-Boltzmann model (PBC) of fluid-like ordering of free lipid A in aqueous solution. We find that the light scattering data of this dispersion are best described by a model where only a small fraction of the ionizable phosphate groups is dissociated at neutral pH. Finally, light scattering studies of lipid A dispersions of volume fractions of 3.9×10-4<=θ<=4.9×10-4 indicate the presence of long-range order, resulting in distinct peaks which can be assigned either to a face-centered cubic (fcc) lattice (a=51.7 nm) or a body-centered cubic (bcc) lattice (a=41.5 nm), respectively

The formation of colloidal crystals of lipid A diphosphate: evidence for the formation of nanocrystals at low ionic strength

Dilute electrostatically stabilized aqueous solutions of hexa-acylated (C14) lipid A diphosphate from Escherichia coli form stable and regularly shaped colloidal crystals in a size range of approximately 50-1000 nm in width and 50-100 nm in thickness. The formation of these nanocrystals occurs over a range of volume fractions between 3.5 × 10-3 and 1.2 × 10-2 and at a low ionic strength, ~10-5. The shape of these crystals appears to be cubic or rhombohedral, and when exposed to the electron beam, these fragile nanocrystals are easily damaged. Electron diffraction patterns obtained from single particles reveal that they are orientated (001) crystals that conform to a trigonal or hexagonal unit cell (a = 3.65 ± 0.07 nm and c = 1.97 ± 0.04 nm), revealing crystal-like pore walls that exhibit structural periodicity with a spacing of 0.65 nm and are at least four times the size of the unit cell adopted by lipid A diphosphate

Magnetic anisotropy and the shape memory phase transition in Ni52Mn26Ga22

Torque measurements of magnetocrystalline anisotropy energies, made on (1 0 0) and (1 1 0) oriented planar disc samples of the shape memory alloy Ni52Mn26Ga22 as a function of temperature and applied directional field cooling are reported. The results confirm small values for the anisotropy constants in the austenite phase (Phys. Rev. B 65 (2002) 134422). Optical observations reveal the influence of applied stresses and magnetic fields on the mobility of the phase transition and twin boundaries. Lorentz mode TEM observations from a (1 1 0) planar foil reveal the interactions between magnetic and anti-phase domain boundaries as the specimen is thermally cycled through the transition temperature