bluuebunny/crossref_metadata_2025 huggingface dataset:
V
51
solid-state NMR and density functional theory studies of vanadium environments in V(V)O2 dipicolinic acid complexes
Authors: Bolte Stephanie E. , Ooms Kristopher J. , Polenova Tatyana , Baruah Bharat , Crans Debbie C. , Smee Jason J.
Published: February 2008

V 51 solid-state NMR and density functional theory (DFT) investigations are reported for a series of pentacoordinate dioxovanadium(V)-dipicolinate [V(V)O2-dipicolinate] and heptacoordinate aquahydroxylamidooxovanadium(V)-dipicolinate [V(V)O-dipicolinate] complexes. These compounds are of interest because of their potency as phosphatase inhibitors as well as their insulin enhancing properties and potential for the treatment of diabetes. Experimental solid-state NMR results show that the electric field gradient tensors in the V(V)O2-dipicolinate derivatives are affected significantly by substitution on the dipicolinate ring and range from 5.8to8.3MHz. The chemical shift anisotropies show less dramatic variations with respect to the ligand changes and range between −550 and −600ppm. To gain insights on the origins of the NMR parameters, DFT calculations were conducted for an extensive series of the V(V)O2- and V(V)O-dipicolinate complexes. To assess the level of theory required for the accurate calculation of the V51 NMR parameters, different functionals, basis sets, and structural models were explored in the DFT study. It is shown that the original x-ray crystallographic geometries, including all counterions and solvation water molecules within 5Å of the vanadium, lead to the most accurate results. The choice of the functional and the basis set at a high level of theory has a relatively minor impact on the outcome of the chemical shift anisotropy calculations; however, the use of large basis sets is necessary for accurate calculations of the quadrupole coupling constants for several compounds of the V(V)O2 series. These studies demonstrate that even though the vanadium compounds under investigations exhibit distorted trigonal bipyramidal coordination geometry, they have a “perfect” trigonal bipyramidal electronic environment. This observation could potentially explain why vanadate and vanadium(V) adducts are often recognized as potent transition state analogs.
Reduction of critical current density for domain wall motion in U-shaped magnetic patterns
Authors: Ohshima N. , Numata H. , Suzuki T. , Fukami S. , Nagahara K. , Ishiwata N.
Published: February 2008

The critical current density required for current-induced domain wall motion (Jc) was reduced by decreasing the magnetization (Ms) of the free layer film of a U-shaped pattern for a domain wall motion memory. The Ms was decreased by adding the nonmagnetic elements Ta and Cu to NiFe films. The Jc of the U-shaped patterns with NiFe–Ta and –Cu decreased as the proportion of Ta and Cu in the NiFe increased. The relations between Ms and Jc were examined and it was found that Jc decreased monotonically from 1.2×1012to0.8×1012A∕m2 as Ms decreased from 1.0to0.6T regardless of what other materials were used. It is considered that the decrease in Ms caused a decrease in the hard axis anisotropy of the pattern, and the Jc decreased as predicted by a one-dimensional model.
Chaotic mixing in microfluidic devices driven by oscillatory cross flow
Authors: Phelan Frederick R. , Hughes Nicholas R. , Pathak Jai A.
Published: February 2008

The kinematics of oscillatory cross flow has been studied numerically as a means for generating chaotic mixing in microfluidic devices for both confined and continuous throughput flow configurations. The flow is analyzed using numerical simulation of the unsteady Navier–Stokes equations combined with tracking of single and multispecies passive tracer particles. Two characteristics of chaotic flow are demonstrated: the stretching and folding of material lines leading to particle dispersion and a positive “effective” Lyapunov exponent. The primary mechanism for the generation of chaotic flow is a periodic combination of stretching (which occurs via shear in the channels) and rotation (which occurs via the timing of the oscillations), making these systems effective tendril-whorl type flows. First, the case of confined mixing is studied. It is shown that chaotic flow is generated in a cross-cell device when sinusoidally driven, out-of-phase, perpendicular fluid streams intersect in the flow domain. Calculations indicate that the flow becomes chaotic in the center region starting at a Strouhal number on the order of 1. A degree of mixing based on a relative mixing entropy as high as 91% is obtained. Approximately 10–15 sinusoidal cycles are needed in order to effectively mix different groups of passive tracer particles. In the second phase of the analysis, the cross flow mixing mechanism is utilized in a continuous operation by combining a throughput channel flow with an oscillatory cross flow in a configuration called the star-cell geometry. It is shown that the oscillatory flow remains chaotic even in combination with the throughput flow, and a degree of mixing in the 80%–90% range is obtained for the range of parameters studied here.
Physical properties of disordered double-perovskite Ca2−xLaxFeIrO6
Authors: Bufaiçal L. , Mendonça Ferreira L. , Lora-Serrano R. , Agüero O. , Torriani I. , Granado E. , Pagliuso P. G. , Caytuero A. , Baggio-Saitovich E.
Published: February 2008

In this work we have synthesized polycrystalline samples for the series of double-perovskite Ca2−xLaxFeIrO6. Their structural and magnetic properties were investigated using experiments of magnetic susceptibility and x-ray powder diffraction. The Ca2−xLaxFeIrO6 compounds form in a monoclinic structure, space group P21∕n, with the presence of significant Fe∕Ir cationic disorder. Interestingly, our results indicate a change in the nature of the microscopic magnetic interaction induced by La doping, where the system seems to evolve from antiferromagnetic in the extremities of the series, x=0 (TN≈75K) and x=2.0, to ferrimagnetic in intermediate regions of the series. However, due to significant presence of disorder and strong competition between ferromagnetic and antiferromagnetic interactions, the saturation of the magnetization was not found to be larger than 0.1μB∕Fe.
Quantitative real-time in vivo detection of magnetic nanoparticles by their nonlinear magnetization
Authors: Nikitin M. P. , Torno M. , Chen H. , Rosengart A. , Nikitin P. I.
Published: February 2008

A novel method of highly sensitive quantitative detection of magnetic nanoparticles (MP) in biological tissues and blood system has been realized and tested in real time in vivo experiments. The detection method is based on nonlinear magnetic properties of MP and the related device can record a very small relative variation of nonlinear magnetic susceptibility up to 10−8 at room temperature, providing sensitivity of several nanograms of MP in 0.1ml volume. Real-time quantitative in vivo measurements of dynamics of MP concentration in blood flow have been performed. A catheter that carried the blood flow of a rat passed through the measuring device. After an MP injection, the quantity of MP in the circulating blood was continuously recorded. The method has also been used to evaluate the MP distribution between rat’s organs. Its sensitivity was compared with detection of the radioactive MP based on isotope of Fe59. The comparison of magnetic and radioactive signals in the rat’s blood and organ samples demonstrated similar sensitivity for both methods. However, the proposed magnetic method is much more convenient as it is safe, less expensive, and provides real-time measurements in vivo. Moreover, the sensitivity of the method can be further improved by optimization of the device geometry.
The molecular beam epitaxy growth, structure, and magnetism of Si1−xMnx films
Authors: Chiu S. H. , Hsu H. S. , Huang J. C. A.
Published: February 2008

The type-IV diluted magnetic semiconductor (DMS) [Si(20Å)∕Mn(x)]30 multilayers (MLs) with nominal thickness x=1, 1.5, and 2.0Å have been prepared by molecular beam epitaxy. The structure, magnetism, and electrical property of these MLs were investigated. Above room temperature ferromagnetism has been observed and structure probed by x-ray absorption spectroscopy. The Mn local structure is similar to Si simulation and the Mn chemical valences of the MLs are in the range from zero (Mn foil) to Mn+2. The relative carrier concentration is sensitive to the formation of room temperature ferromagnetism, suggesting that hole mediated mechanism play an important role in Si:Mn DMSs.
Unusual magnetic properties in Pr1−xSrxFe0.8Ni0.2O3−δ (x≤0.3)
Authors: Ruiz de Larramendi I. , López Antón R. , Ruiz de Larramendi J. I. , Rojo T. , Wattiaux A. , Rodríguez Fernández J.
Published: February 2008

Pr 1 − x Sr x Fe 0.8 Ni 0.2 O 3 − δ (x=0.1, 0.2, and 0.3) polycrystalline phases were prepared by combustion synthesis. Their magnetic properties and Mössbauer effect have been examined in the range 5–300 K. Mössbauer spectroscopy shows the existence of charge disproportionation in several of the samples at low temperatures. Meanwhile, the samples present multiple features pointing out a frustrated magnetic order, with competing ferro- and antiferromagnetic interactions. Additionally, exchange bias (EB) phenomena have been observed. Hence, when the samples are cooled in a static magnetic field down to temperatures below 50 K, the hysteresis loops shift in the opposite direction to the applied biasing field, with shifts up to 10 kOe and more, and a small vertical shift of the loops also appears. This behavior seems linked to spin disorder and to the appearance of ferromagnetic (FM) ordered regions at low temperatures when cooling with a high applied field. The growth of these FM regions is likely due to the aligning role of an important cooling magnetic field over Ni-Fe and Fe5+-Fe3+ FM interactions. The higher FM contributions and therefore EB values found in the samples presenting charge disproportionation are related to the formation of FM regions in Fe5+-rich areas.
Magnetic properties of Fe75Si15Al10 crystalline powders and domain structural observations using Lorentz transmission electron microscopy
Authors: Ahn Jae-Pyoung , Yoon Sang-Won , Kim Sang-Woo
Published: February 2008

Fe 75 Si 15 Al 10 flakes with soft magnetism were manufactured by microforging granules and annealing them at temperatures up to 800°C in Ar atmosphere. Spherical granules 50μm in diameter were transferred after milling to a thin plate of 1μm thickness with elongated subgrains. After annealing, the deformed subgrains were recrystallized with equiaxed grains 200nm in size. Lorentz transmission electron microscopy and Foucault imaging revealed the milled and annealed flakes to consist of in-plane spins within a longitudinal double-layered domain structure. The magnetic properties of the Fe75Si15Al10 flakes depend strongly on platelike shape anisotropy and orientation anisotropy due to the {200} texture, which can easily induce magnetization through in-plane spin directions.
Improved fabrication of zero-mode waveguides for single-molecule detection
Authors: Foquet Mathieu , Samiee Kevan T. , Kong Xiangxu , Chauduri Bidhan P. , Lundquist Paul M. , Turner Stephen W. , Freudenthal Jake , Roitman Daniel B.
Published: February 2008

Metallic subwavelength apertures can be used in epi-illumination fluorescence to achieve focal volume confinement. Because of the near field components inherent to small metallic structures, observation volumes are formed that are much smaller than the conventional diffraction limited volume attainable by high numerical aperture far field optics (circa a femtoliter). Observation volumes in the range of 10−4fl have been reported previously. Such apertures can be used for single-molecule detection at relatively high concentrations (up to 20μM) of fluorophores. Here, we present a novel fabrication of metallic subwavelength apertures in the visible range. Using a new electron beam lithography process, uniform arrays of such apertures can be manufactured efficiently in large numbers with diameters in the range of 60–100nm. The apertures were characterized by scanning electron microscopy, optical microscopy, focused ion beam cross sections/transmission electron microscopy, and fluorescence correlation spectroscopy measurements, which confirmed their geometry and optical confinement. Process throughput can be further increased using deep ultraviolet photolithography to replace electron beam lithography. This enables the production of aperture arrays in a high volume manufacturing environment.
On local equivalence problem of space-times with two orthogonally transitive commuting Killing fields
Authors: Marvan M. , Stolín O.
Published: February 2008

Considered is the problem of local equivalence of generic four-dimensional metrics possessing two commuting and orthogonally transitive Killing vector fields. A sufficient set of eight differential invariants is explicitly constructed, among them four of first order and four of second order in terms of metric coefficients. In vacuum case, the four first-order invariants suffice to distinguish generic metrics.
Symmetry in the design of NMR multiple-pulse sequences
Authors: Levitt Malcolm H.
Published: February 2008

The symmetry principles of NMR pulse-sequence design are summarized. The discussion is guided by an analogy with tiling schemes in the decorative arts. The symmetry operations for NMR pulse sequences are discussed in terms of excitation field modifiers and temporal modifiers. The quantum operators which describe the effect of these modifiers on the excitation field spin Hamiltonian are provided. The symmetry transformations of spin propagators, and the different types of pulse-sequence elements are discussed. The common types of symmetry expansion are treated using the propagator transformations and the Euler angles for the excitation field propagators. The selection rules associated with symmetrical pulse sequences are discussed using average Hamiltonian theory.
Structural, magnetic, and transport properties of a novel class of ferromagnetic semiconductors: SrM2±xRu4±xO11 (M=Fe,Co)
Authors: Shlyk L. , De Long L. E. , Kryukov S. , Schüpp–Niewa B. , Niewa R.
Published: February 2008

A major obstacle to the implementation of spin-polarized semiconductor devices is the current absence of suitable room-temperature, soft ferromagnetic semiconductors (FSs). Dilute magnetic semiconductors are under intense study for applications in spintronics. However, the weak solubility of randomly placed magnetic ions in the semiconductor host makes these materials unsuitable for devices. It is, therefore, crucial to develop a room-temperature FS based on a periodic array of magnetic ions. We have found that ternary ruthenium ferrites exhibit long-range ferromagnetic order well above room temperature, accompanied by narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, and high carrier concentration. The physical properties can be tuned by simple chemical substitution of two elements, Fe and Co, or by varying the relative concentration of 3d and 4d elements within a homogeneity range that we have established. These promising properties—manifest within a single structural family—provide a fertile ground for fundamental studies and open up a host of potential device applications.
X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism investigation of size effects on field-induced Néel-cap reversal
Authors: Cheynis F. , Rougemaille N. , Belkhou R. , Toussaint J.-C. , Fruchart O.
Published: February 2008

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on Néel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of Néel-cap magnetization give clear evidence for the magnetization reversal of the Néel caps around 120mT, with a ±20mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots.
Synthesis, structure, and magnetic studies on self-assembled BiFeO3–CoFe2O4 nanocomposite thin films
Authors: Muralidharan R. , Dix N. , Skumryev V. , Varela M. , Sánchez F. , Fontcuberta J.
Published: February 2008

Self-assembled (0.65)BiFeO3–(0.35)CoFe2O4 (BFO-CFO) nanostructures were deposited on SrTiO3 (001) and (111) substrates by pulsed laser deposition at various temperatures from 500to800°C. The crystal phases and the lattice strain for the two different substrate orientations have been determined and compared. The films grow epitaxial on both substrates but separation of the spinel and perovskite crystallites, without parasitic phases, is only obtained for growth temperatures of around 600–650°C. The BFO crystallites are out-of-plane expanded on STO(001), whereas they are almost relaxed on (111). In contrast, CFO crystallites grow out-of-plane compressed on both substrates. The asymmetric behavior of the cell parameters of CFO and BFO is discussed on the basis of the role of the epitaxial stress caused by the substrate and the spinel-perovskite interfacial stress. It is concluded that interfacial stress dominates the elastic properties of CFO crystallites and thus it may play a fundamental on the interface magnetoelectric coupling in these nanocomposites.
Dependence of switching current distribution on current pulse width of current-induced magnetization switching in MgO-based magnetic tunnel junction
Authors: Morota Misako , Fukushima Akio , Kubota Hitoshi , Yakushiji Kay , Yuasa Shinji , Ando Koji
Published: February 2008

Current-induced magnetization switching in an MgO-based magnetic tunnel junction (MTJ) was experimentally studied. Probabilistic distribution of a switching current was measured at various current pulse widths from 5μsto100ms. A thermal stability parameter Δ of a free-layer magnetic moment and an intrinsic switching current density Jc0 of the MTJ were evaluated from the switching current distribution using a thermal activation model. The estimated Δ and Jc0 were approximately independent of the pulse width and were in good agreement with the values estimated from the pulse-width dependence of the switching current density, demonstrating the validity of the thermal activation model.
Low temperature vacuum annealing study of (Co∕Pd)n magnetic multilayers
Authors: E Chunsheng , Rantschler James , Zhang Shishan , Khizroev Sakhrat , Lee T. Randall , Litvinov Dmitri
Published: February 2008

The correlation between the magnetic properties and the microstructural and chemical composition modifications of Co∕Pd magnetic multilayers upon annealing in ultrahigh vacuum at 250°C is presented. Magnetic characterization using magnetic sample magnetometer shows the vertical magnetic anisotropy increase and the switching field distribution decrease in the annealed samples. The larger values of magnetic anisotropy in the annealed samples are further shown using the magnetic force microscopy of the ac demagnetized states in the Co∕Pd multilayer films. X-ray diffraction rocking curves show an improvement in the texture and the initial magnetization curve slopes indicate the decreases in defect densities. Overall, vacuum annealing under optimal conditions improves the magnetic properties of Co∕Pd multilayers for applications in ultrahigh density magnetic recording.
Evaluation of thermosensitive magnetic powder coated with Ag paste for cancer therapy
Authors: Takura Tetsuya , Sato Fumihiro , Matsuki Hidetoshi , Sato Tadakuni
Published: February 2008

Soft heating, where thermosensitive magnetic materials inserted into the tumor as a heater produce heat in a high-frequency magnetic field, has been proposed as one method. Although the heater needs to be implanted in the tumor tissue, soft heating has advantages including easy retreatment, localized tumor heating, and a low degree of invasiveness. We created thermosensitive magnetic powder coated with Ag paste as a heater. As a result, this powder produced a greater amount of heat due to losses through hysteresis and inductive current than the magnetic powder alone, and we could also control the temperature of the heater by using the Curie point.
Measurement of dispersive forces between evaporated metal surfaces in the range below 100nm
Authors: van Zwol P. J. , Palasantzas G. , van de Schootbrugge M. , De Hosson J. Th. M.
Published: February 2008

In this work, we describe dispersion force (van der Waals/Casimir) measurements between gold surfaces at separations as close as 12nm. The force measurements were performed in the plane-sphere configuration by atomic force microscopy at distances ranging between 12 and 200nm. This was accomplished by using evaporated smooth metal surfaces for both sphere and plane, and stiff cantilevers to minimize jump-to-contact problems. Finally, it is shown that below separations of 100nm, experiment and theory are found in agreement within 10% by including in the theory the measured optical properties of gold and contribution from surface roughness.
Spin current absorptions and spin Hall effects in ferromagnetic/nonmagnetic hybrid structures (invited)
Authors: Kimura T. , Otani Y. , Vila L.
Published: February 2008

Spin currents absorption effect into a Pt wire is demonstrated by mean of the nonlocal spin valve measurement. Using this absorption effect, an inverse spin Hall effect is successively observed at room temperature. Moreover, the direct spin Hall effect is also demonstrated by a Pt spin current generator with nonlocal detection technique. The spin Hall conductivity in the Pt wire is 260S∕cm at room temperature, which is 104 times larger than that in semiconductor systems. Temperature dependence of the spin Hall conductivity is also investigated.
Dispersed-phase structural anisotropy in homogeneous magnetohydrodynamic turbulence at low magnetic Reynolds number
Authors: Rouson D. W. I. , Kassinos S. C. , Moulitsas I. , Sarris I. E. , Xu X.
Published: February 2008

A new tensor statistic, the dispersed-phase structure dimensionality Dp, is defined to describe the preferred orientation of clusters of discrete bodies. The evolution of Dp is calculated via direct numerical simulations of passive, Stokesian particles driven by initially isotropic, decaying magnetohydrodynamic turbulence. Results are presented for five magnetic field strengths as characterized by magnetic interaction parameters, N, in the range 0–50. Four field strengths are studied at a grid resolution of 1283. The strongest field strength is also studied at 2563 resolution. In each case, the externally applied magnetic field was spatially uniform and followed a step function in time. Particles with initially uniform distributions were tracked through hydrodynamic turbulence for up to 2800 particle response times before the step change in the magnetic field. In the lower resolution simulation, the particle response time, τp, matched the Kolmogorov time scale at the magnetic field application time t0. The higher-resolution simulation tracked ten sets of particles with τp spanning four decades bracketing the Kolmogorov time scale and the Joule time. The results demonstrate that Dp distinguishes between uniformly distributed particles, those organized into randomly oriented clusters, and those organized into two-dimensional sheets everywhere tangent to the magnetic field lines. Lumley triangles are used to demonstrate that the degree of structural anisotropy depends on τp, N, and the time span over which the magnetic field is applied.
Thermally excited magnetic noise from pinned and reference layers in current perpendicular-to-plane structure magnetoresistive heads
Authors: Zhou Yuchen
Published: February 2008

Thermally excited magnetic noise due to weakened antiferromgnetic layer pinning field and weakened coupling field between pinned layer and reference layer is studied with mircomagnetic model. Sensor noise increases with weakened pinning and coupling fields are dependent on sensor aspect ratio. Low stripe height can enhance shape anisotropy that reduces the pinning/coupling field and thus leads to noisier sensor. It is found that weakened pinning field is more effective in producing noise than weakened coupling field. It is also found that noise increases from weakened pinning field and coupling field and shows different trends with applied magnetic field. Experimental method to distinguish pinning and coupling weakness is discussed.
Magnetic properties of Co–Pt∕Co hard/soft stacked dot arrays
Authors: Mitsuzuka K. , Shimatsu T. , Muraoka H. , Aoi H. , Kikuchi N. , Kitakami O.
Published: February 2008

The magnetic properties of Co80Pt20(10nm)∕Co(0,3,5nm) hard/soft stacked dot arrays with dot diameters D of about 50nm were studied. Co–Pt films were deposited on Ru (001) seed layers, resulting in a large uniaxial magnetic anisotropy Ku with the c axis perpendicular to the film plane. Dot patterns were formed using high resolution e-beam lithography and reactive ion etching. Magnetic force microscopy images revealed that all dot arrays showed a single domain state, even after removal of an applied field equal to the coercivity. The remanence coercivity Hr decreased from 9.1to6.9kOe as the Co layer thickness increased from 0to5nm, indicating that the hard/soft stacked structure was effective at reducing the switching field, as theoretically predicted. The applied field angular dependence of Hr for 10-nm-thick Co–Pt dot arrays was in good agreement with calculations based on the Stoner–Wohlfarth model, HS-W indicating coherent rotation of the magnetization during nucleation. Moreover, it was confirmed that the hard/soft stacked structure reduced the angular variation of Hr.
Solid-state nuclear magnetic resonance in the rotating tilted frame
Authors: Trease Nicole M. , Grandinetti Philip J.
Published: February 2008

Recent methodological advances have made it possible to measure fine structure on the order of a few hertz in the nuclear magnetic resonance (NMR) spectra of quadrupolar nuclei in polycrystalline samples. Since quadrupolar couplings are often a significant fraction of the Zeeman coupling, a complete analysis of such experimental spectra requires a theoretical treatment beyond first-order. For multiple pulse NMR experiments, which may include sample rotation, the traditional density matrix approaches for treating higher-order effects suffer from the constraint that undesired fast oscillations (i.e., multiples of the Zeeman frequency), which arise from allowed overtone transitions, can only be eliminated in numerical simulations by employing sampling rates greater than 2I times the Zeeman frequency. Here, we present a general theoretical approach for arbitrary spin I that implements an analytical “filtering” of undesired fast oscillations in the rotating tilted frame, while still performing an exact diagonalization. Alternatively, this approach can be applied using a perturbation expansion for the eigenvalues and eigenstates, such that arbitrary levels of theory can be explored. The only constraint in this approach is that the Zeeman interaction remains the dominant interaction. Using this theoretical framework, numerical simulations can be implemented without the need for a high sampling rate of observables and with significantly reduced computation times. Additionally, this approach provides a general procedure for focusing on the excitation and detection of both fundamental and overtone transitions. Using this approach we explore higher-order effects on a number of sensitivity and resolution issues with NMR of quadrupolar nuclei.
Imaging of isolated magnetic cluster switching in thin CoCrPt films
Authors: Czoschke Peter , Nazarov Alexey V. , McKinlay Shaun E. , Singleton Eric W. , Pant Bharat B.
Published: February 2008

Thin film permanent magnet materials are a vital component of magnetic recording read elements. However, local variations in the magnetic microstructure inherent in such devices can have numerous consequences on the magnetic state of the films. In this study, magnetic force microscopy is used to image the domains in thin nanocrystalline CoCrPt films that are part of a patterned read sensor device. The films were imaged before and after being subjected to stress fields of 1000–2000Oe (less than the sheet-film coercivity of the CoCrPt) transverse to the original magnet set direction. Subtraction of the images reveals that the magnetization of isolated magnetic clusters irreversibly rotates in the film. These data show that the mechanism for net moment rotation in such films is not a uniform grain moment rotation. The change in net magnetization occurs in discrete local moment switching similar to Barkhausen jumps, where moments of weakly coupled grains irreversibly rotate at fields that are lower than the bulk coercive field of the film. This technique yields a two-dimensional map of grain moment rotations from which such weak grains can be identified.
Nonthermal effects on multielectron ionization processes in Lorentzian plasmas
Authors: Na Sang-Chul , Jung Young-Dae
Published: February 2008

The nonthermal effects on multielectron ionization by electron impact are investigated in generalized Lorentzian distribution plasmas. The Born-Bethe-type formula for the multielectron ionization cross section is employed in order to obtain the multiple-ionization rate as a function of the spectral index, ionization degree, and thermal energy. It is found that the ionization rate increases with an increase of the nonthermal character of the plasma. It is also found that the ionization rate increases with an increase of the plasma temperature. In addition, it is shown that the thermalization of the plasma strongly suppresses the multiple-ionization process.
Evolution of granular to particulate structure of (001) FePt on amorphous substrates (invited)
Authors: Wu Yun-Chung , Wang Liang-Wei , Rahman M. Tofizur , Lai Chih-Huang
Published: February 2008

Well-aligned (001)-oriented granular or particulate L10 FePt films on amorphous SiO2 or glass substrates were obtained by depositing atomic-scale Fe∕Pt∕SiO2 multilayers followed by various postannealing conditions. After 350°C annealing, (001) FePt–SiO2 granular films were obtained with an average grain size of 5.14nm, perpendicular coercivity of 7.7kOe, and squareness of 0.95. The diffusion of SiO2 to grain boundaries and free surface of FePt accelerated the diffusion of Fe and Pt atoms to form the L10 phase; therefore, low-temperature ordering of FePt was achieved. By further raising the annealing temperature to 700°C, isolated particles of aligned (001) single crystals were achieved. The nucleation of voids occurred at low temperatures due to the existence of low-surface-energy SiO2, and then an agglomeration process proceeded, leading to a particulate structure. The isolated particles showed a perpendicular coercivity of 60kOe and squareness of unity.
Comparison of electron and hole charge-discharge dynamics in germanium nanocrystal flash memories
Authors: Akca Imran B. , Dâna Aykutlu , Aydinli Atilla , Turan Rasit
Published: February 2008

Electron and hole charge and discharge dynamics are studied on plasma enhanced chemical vapor deposition grown metal-oxide-silicon germanium nanocrystal flash memory devices. Electron and hole charge and discharge currents are observed to differ significantly and depend on annealing conditions chosen for the formation of nanocrystals. At low annealing temperatures, holes are seen to charge slower but to escape faster than electrons. They discharge slower than electrons when annealing temperatures are raised. The results suggest that discharge currents are dominated by the interface layer acting as a quantum well for holes and by direct tunneling for elec-trons.
Differential Galois obstructions for integrability of homogeneous Newton equations
Authors: Przybylska Maria
Published: February 2008

In this paper, we formulate necessary conditions for the integrability in the Jacobi sense of Newton equations q̈=−F(q), where q∊Cn and all components of F are polynomial and homogeneous of the same degree l. These conditions are derived from an analysis of the differential Galois group of variational equations along special particular solutions of the Newton equations. We show that, taking all admissible particular solutions, we restrict considerably the set of Newton’s equations satisfying the necessary conditions for the integrability. Moreover, we apply the obtained conditions for a detailed analysis of the Newton equations with two degrees of freedom (i.e., n=2). We demonstrate the strength of the obtained results analyzing general cases with degFi=l<4. For l=3, we have found an integrable case when the Newton equations have two polynomial first integrals and both of them are of degree 4 in the momenta p1=q̇1 and p2=q̇2. Moreover, for an arbitrary l, we found a particular family of Newton equations depending on one parameter λ. For an arbitrary value of λ, one quadratic in the momenta first integral exists. We distinguished infinitely many values of λ for which the system is integrable or superintegrable with additional polynomial first integrals which seemingly can be of an arbitrarily high degree with respect to the momenta.
Appearance of target pattern and spiral flames in radial microchannels with CH4-air mixtures
Authors: Kumar Sudarshan , Maruta Kaoru , Minaev S. , Fursenko R.
Published: February 2008

This paper presents the experimental evidence of the formation of rotating spiral flames with premixed methane-air mixtures introduced at the center of the two parallel circular quartz plates which are separated by a millimeter scale distance (≤5mm). Both plates are externally heated to create a positive wall temperature gradient in the flow direction to resemble heat recirculation through solid walls, which is a requisite to obtain stabilized combustion in microburners. Contrary to the general perception of a stable premixed flame front at a radial location, a variety of nonstationary flame propagation modes are observed. For lower mixture flow rates and a range of mixture equivalence ratios, a radial flame propagation mode is observed with simultaneous presence of two circular flames at different radial locations. For higher flow rates, a rotating spiral flame propagation mode is observed. In addition to radial and spiral flame propagation modes, random and unsymmetrical flame oscillations are also observed. The rotational rates of the spiral flame fronts were observed to vary from 28 to 83Hz. A simple analysis is carried out to describe the formation of spiral flames from a steady circular flame.
Remanence enhancement in nanoscaled electrodeposited FePt films
Authors: Leistner K. , Schlörb H. , Thomas J. , Schultz L. , Fähler S.
Published: February 2008

L 1 0 FePt films with a small grain size of around 13nm have been prepared by electrodeposition and postannealing at a low annealing temperature of 400°C. A high remanence of up to 0.9T is achieved due to remanence enhancement by exchange coupling between the nanosized grains. Coercivity increases with longer annealing time as the L10 order becomes more complete. The resulting maximum energy product reaches a high value of 70kJ∕m3 after annealing for 120min, exceeding the maximum energy products so far obtained in electrodeposited hard magnetic films.
Standing-wave effects in grazing-incidence x-ray diffraction from polycrystalline multilayers
Authors: Krčmář J. , Holý V. , Horák L. , Metzger T. H. , Sobota J.
Published: February 2008

The intensity of x-ray diffraction from a polycrystalline multilayer in a grazing-incidence scattering geometry is modulated by a standing wave created by the interference of the radiation transmitted through the multilayer stack with the wave field specularly reflected from the superlattice interfaces. Similarly, the radiation being diffracted from the polycrystalline structure is reflected specularly from the interfaces and a standing-wave interference pattern results as well. This effect is demonstrated by a series of diffraction measurements on C∕Ni periodic multilayers; the experimental data have been modeled using the distorted-wave Born approximation and a very good correspondence with the experiments was achieved. The method was used for the study of the profiles of the crystallite sizes and strains in multilayers.
Atomic level mixing induced by Kr irradiation of FeCo∕Cu multilayers
Authors: Graff I. L. , Geshev J. , Teixeira S. R. , Amaral L. , Traverse A.
Published: February 2008

The effects of Kr ion irradiation of FexCo1−x∕Cu multilayers are investigated by means of magnetic and x-ray measurements. The irradiation was performed at room temperature with 600keV of Kr, and the fluences were ranged from 1×1015to5×1015ions∕cm2. X-ray diffraction patterns show that the irradiation was able to produce a significant grain growth and, at the same time, it triggered a strain release. The x-ray absorption measurements around Fe K edge have shown that the FeCo environment changed from bcc, for the as-deposited multilayer, to fcc after appropriate fluence, depending on the Cu thickness and on the Fe∕Co content. For 50Å of Cu, the phase transformation occurs even for the lowest fluence, regardless the Fe∕Co concentration. For 25Å of Cu, it depends on the Fe∕Co concentration. After irradiation, the saturation magnetization suffers a strong decrease, especially for 50Å of Cu. The temperature for the onset of irreversibility also decreases with irradiation, again being the effect stronger for 50Å of Cu. These data indicate that Kr irradiation induced mixing between Cu and FeCo, even though Cu is immiscible with Fe and Co in the equilibrium state.
Electron capture and emission properties of interface states in thermally oxidized and NO-annealed SiO2/4H-SiC
Authors: Chen X. D. , Dhar S. , Isaacs-Smith T. , Williams J. R. , Feldman L. C. , Mooney P. M.
Published: February 2008

Postoxidation annealing in nitric oxide (NO) results in a significant reduction of electronic states at SiO2/4H-SiC interfaces. Measurements of electron trapping dynamics at interface states in both thermally oxidized and NO annealed SiO2/4H-SiC interfaces were performed using constant-capacitance deep level transient spectroscopy (CCDLTS) and double-CCDLTS. We show that the interface state density in as-oxidized samples consists of overlapping distributions of electron traps that have distinctly different capture cross sections. The dominant trap distributions, centered at Ec−0.24 eV with σ∼7×10−19 cm2, and at Ec−0.46 eV with σ∼4×10−17 cm2 are passivated by NO annealing. The remaining interface states all have capture cross sections in the range 10−19−10−21 cm2.
X-ray diffraction, μ-Raman spectroscopic studies on CeO2−RE2O3 (RE=Ho, Er) systems: Observation of parasitic phases
Authors: Mandal B. P. , Roy M. , Grover V. , Tyagi A. K.
Published: February 2008

The phase relations in CeO2−Ho2O3 and CeO2−Er2O3 systems have been established under the slow-cooled conditions. As per x-ray diffraction (XRD), in both the series a single-phasic solid solution forms up to the nominal composition Ce0.6RE0.4O1.8 (RE=Ho, Er) retaining the F-type structure of parent ceria. In Ce1−xErxO2−x/2 system the presence of microdomains of C-type phase have been revealed by Raman spectroscopy for composition x=0.4, which has been identified as single phasic by XRD. Photoluminescence studies also show that biphasic region commences from x=0.4 for Ce1−xHoxO2−x/2 series. The biphasicity continues until x=0.7 for both the series. From x=0.8 the solid solutions exist as C-type single phasic, which is isotypic to another end member RE2O3 (RE=Ho, Er) and as revealed by both XRD and Raman spectroscopy. High temperature XRD studies show that no temperature induced phase change has been observed in either of the series until 1273 K. In this work photoluminescence data was used to delineate the phase boundary.
Tunability of wire-grid metamaterial immersed into nematic liquid crystal
Authors: Gorkunov M. V. , Osipov M. A.
Published: February 2008

We propose electrically tunable hybrid metamaterial consisting of special wire grid immersed into nematic liquid crystal. The plasmalike permittivity of the structure can be substantially varied due to switching of the liquid crystal alignment by external voltages applied to the wires. Depending on the scale of the structure, the effect is available for both microwave and optical frequency ranges.
The principle of equivalence and cosmological metrics
Authors: Hall G. S. , Lonie D. P.
Published: February 2008

This paper is concerned with the extent to which the geodesics of space-time (that is, the experimental consequences of the principle of equivalence) determine the metric in general relativity theory and, in particular, in Friedmann–Robertson–Walker–Lemaitre (FRWL) space-times. Thus it discusses projective structure in these space-times. The approach will be from a geometrical point of view and it is shown that if two space-time metrics share the same (unparametrized) geodesics and one is a (generic) FRWL metric then so is the other and that each is a member of a well defined family of projectively related (FRWL) metrics. Similar techniques are then applied to study the existence and properties of symmetries of the Weyl projective tensor and projective symmetries in FRWL space-times.
Transport measurements in silicon-on-insulator films: Comparison of Hall effect, mobility spectrum, and pseudo-metal-oxide-semiconductor-field-effect-transistor techniques
Authors: Chandrasekhar Rao T. V. , Antoszewski J. , Faraone L. , Cristoloveanu S. , Nguyen T. , Gentil P. , Bresson N. , Allibert F.
Published: February 2008

We report on the nature of electrical transport in silicon-on-insulator layers, investigated using several techniques: the standard single magnetic field Hall effect, mobility spectrum analysis of the magnetic field-dependent Hall effect, and the pseudo-metal-oxide-semiconductor-field-effecttransistor technique. For moderate and strong inversion, electrical transport in the temperature range 77–300 K is dominated by a lone electron species with a mobility of 500−1000 cm2/Vs. A good correlation is noted between these methods.
Passive states for essential observers
Authors: Strich Robert
Published: February 2008

The aim of this note is to present a unified approach to the results given by Borchers and Buchholz [“Global properties of vacuum states in de Sitter space,” Ann. Inst. Henri Poincare, Sect. A. 70, 23–40 (1999)] and by Buchholz and Summers [“Stable quantum systems in anti-de Sitter space: Causality, independence and spectral properties,” J. Math. Phys. 45, 4810–4831 (2004)] which also covers examples of models not presented in these two papers (e.g., d-dimensional Minkowski space-time for d⩾3). Assuming that a state is passive for an observer traveling along certain (essential) worldlines, we show that this state is invariant under the isometry group, is a temperature equilibrium state for the observer at a temperature uniquely determined by the structure constants of the Lie algebra involved, and fulfills (a variant of) the Reeh-Schlieder property. Also, the modular objects associated with such a state and the observable algebra of an observer are computed and a version of weak locality is examined.
Spins as qubits: Quantum information processing by nuclear magnetic resonance
Authors: Suter Dieter , Mahesh T. S.
Published: February 2008

Storing information in quantum mechanical degrees of freedom and processing it by unitary transformation promises a new class of computers that can efficiently solve problems for which no efficient classical algorithms are known. The most straightforward implementation of this type of information processing uses nuclear spins to store the information and nuclear magnetic resonance for processing it. We discuss the basics of quantum information processing by NMR, with an emphasis on two fields of research: the design and implementation of robust logical gate operations and the loss of quantum information, which is known as decoherence.
Conformational distributions of unfolded polypeptides from novel NMR techniques
Authors: Meier Sebastian , Blackledge Martin , Grzesiek Stephan
Published: February 2008

How the information content of an unfolded polypeptide sequence directs a protein towards a well-formed three-dimensional structure during protein folding remains one of the fundamental questions in structural biology. Unfolded proteins have recently attracted further interest due to their surprising prevalence in the cellular milieu, where they fulfill not only central regulatory functions, but also are implicated in diseases involving protein aggregation. The understanding of both the protein folding transition and these often natively unfolded proteins hinges on a more detailed experimental characterization of the conformations and conformational transitions in the unfolded state. This description is intrinsically very difficult due to the very large size of the conformational space. In principle, solution NMR can monitor unfolded polypeptide conformations and their transitions at atomic resolution. However, traditional NMR parameters such as chemical shifts, J couplings, and nuclear Overhauser enhancements yield only rather limited and often qualitative descriptions. This situation has changed in recent years by the introduction of residual dipolar couplings and paramagnetic relaxation enhancements, which yield a high number of well-defined, quantitative parameters reporting on the averages of local conformations and long-range interactions even under strongly denaturing conditions. This information has been used to obtain plausible all-atom models of the unfolded state at increasing accuracy. Currently, the best working model is the coil model, which derives amino acid specific local conformations from the distribution of amino acid torsion angles in the nonsecondary structure conformations of the protein data bank. Deviations from the predictions of such models can often be interpreted as increased order resulting from long-range contacts within the unfolded ensemble.
Optimization of a single defect photonic crystal laser cavity
Authors: Frei Walter R. , Johnson H. T. , Choquette Kent D.
Published: February 2008

Using nonlinear programing and the geometry projection method, the quality factor of the monopole mode of a single defect photonic crystal laser cavity is improved from 38 000 to 87 000. Beginning with a design that considers only round air holes shifted away from the cavity, the radius of the nearest neighbor and of the surrounding air holes are optimized while satisfying a constraint on the resonant frequency. The total reflectivity of the photonic crystal laser structure is then defined, and it is shown that this quantity correlates strongly to the total quality factor. The reflectivity of the structure is improved by altering the shape of the holes immediately surrounding the cavity, thus leading to an improvement in quality factor. The geometry projection method is used to define the shape of the holes and the finite element and adjoint methods are used to compute the objective function and sensitivities required by the optimizer. This work demonstrates one way to optimize the Q factor of a photonic crystal laser by altering the hole shape.
One-dimensional birefringent photonic crystal laser
Authors: Kim Sun Woong , Oh Seung Seok , Park Jin Ho , Choi Eun Ha , Seo Yoon Ho , Cho Guang Sup , Park Byoungchoo
Published: February 2008

We studied the polarized laser emission from an anisotropic one-dimensional (1D) birefringent photonic crystal (PC) laser. An active medium layer, which consisted of an epoxy resin doped with fluorescent dye, was sandwiched between two anisotropic 1D PC films. It was shown that efficient laser emissions were generated by optical pumping at relatively low lasing thresholds. The wavelengths of the emitted lasers were 611 and 618 nm, which correspond to the two split eigenmodes at the low-energy band edges that are due to the anisotropy of the PCs. We also demonstrated that the polarization of the lasing emission can be controlled by adjusting the birefringence of the anisotropic PCs.