bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of surface anisotropy on the magnetization precessional switching in nanoparticles
Authors: Dimian M. , Mayergoyz I. D.
Published: May 2005

A study of surface anisotropy effects on the precessional switching of nanoparticles is presented. Spherical nanoparticles with uniaxial anisotropy in the bulk and radial anisotropy for spins on the boundary are considered. The multispin dynamics is found by using the Landau–Lifshitz equation with the effective field derived from a Heisenberg-type Hamiltonian. The expressions for critical magnetic fields that guarantee the precessional switching are derived analytically for the case of very strong exchange and weak surface anisotropy. These analytical results are also used to test the numerical approach that is applied to the general case of the problem. The distinct features of the precessional switching in nanoparticles are examined and their dependence on various parameters of the problem is discussed.
An inequality for the Gibbs mean number of clusters
Authors: Cammarota Camillo
Published: May 2005

The number of percolation clusters for configurations of the Ising model at zero external field and ferromagnetic first neighbors interaction on a general finite graph is considered. The mean number of clusters with respect to the Gibbs measure at any inverse temperature β is proved to be smaller or equal than the one at β=0.
Oscillatory thickness dependence of the coercive field in three-dimensional anti-dot arrays from self-assembly
Authors: Zhukov A. A. , Goncharov A. V. , de Groot P. A. J. , Ghanem M. A. , El-Hallag I. S. , Bartlett P. N. , Boardman R. , Fangohr H. , Novosad V. , Karapetrov G.
Published: May 2005

We present studies on magnetic anti-dot nano-structures with three-dimensional (3D) architectures, fabricated using a self-assembly template method. We find that patterning transverse to the film plane, which is a unique feature of this method, results in novel magnetic behavior. In particular, one of the key parameters for a magnetic material, the coercive field Bc, was found to demonstrate an oscillatory dependence on film thickness.
Multipole moments of in-plane magnetized disks
Authors: Mikuszeit N. , Vedmedenko E. Y. , Wiesendanger R. , Oepen H. P.
Published: May 2005

The multipole moments of in-plane magnetized disks have been calculated based on the fundamental theory of magnetostatics. Analytical solutions for disks with uniform magnetization or an onion state are given explicitly. It is demonstrated that depending on the polarization configuration, higher-order multipole moments beyond the dipole moment appear. The strength of the multipole moments can be of the same order of magnitude as the dipolar moment. The higher-order moments give rise to an additional energy contribution in arrays of close-packed polarized disks.
A wavelet/moment analysis for the surface morphology of lubricant films
Authors: Guo Qian , Chen Haigang , Smith Brandon C. , Jhon Myung S.
Published: May 2005

Molecular dynamics simulations using a coarse-grained, bead-spring model were performed to generate the morphology of submonolayer and monolayer lubricant films. Different morphological states, such as isolated islands, connected perfluoropolyether bridge network, and monolayers with defects were found for various surface coverage. We introduced the Kohlrausch–Williams–Watts function and the wavelet transform as well as calculated several parameters such as the correlation length, roughness exponent, and wavelet entropy to analyze the surface morphologies. A strong correlation was found between the surface morphologies and transport properties via correlating the parallel component of the self-diffusion coefficient with the wavelet entropy per molecule. Two distinct correlations at low and high surface coverage regions were found where the transition between these two regions depends on the number of beads per molecule and endgroup functionality.
Morphological dynamics of swelling-induced surface patterns in metal-capped polymer bilayer
Authors: Kwon S. Joon , Park Jae-Gwan , Lee Sarng H.
Published: May 2005

We report on the morphological dynamics of surface patterns induced by swelling of metal-capped polymer bilayer on a substrate. When the bilayer is subject to solvent vapor, the strain is generated in the polymer layer that is confined by the substrate and the metal capping layer. An increase in the strain induces the development of the stress in the bilayer to deform the lower polymer layer perpendicularly to the surface of the bilayer. Isotropic surface wave patterns results from the stress relaxation, the wave number of the patterns shows a characteristic temporal dependency on the swelling time, such that km(t)∼t−1/8. This temporal evolution accompanied by the morphological dynamics gives smaller value of the growth rate of the characteristic wavelength than that of the case of swelling of gel.
Magnetization reversal in Co∕Pd nanostructures and films
Authors: Hu G. , Thomson T. , Rettner C. T. , Raoux S. , Terris B. D.
Published: May 2005

The magnetic field required to reverse a Co∕Pd multilayer film with perpendicular anisotropy is greatly increased when the film is deposited onto patterned islands. The variation of the reversal field with Co thickness demonstrates that the island switching field is determined principally by the anisotropy. For sufficiently small islands the reversal field is close to that calculated using the Stoner–Wohlfarth (SW) model and these small islands show the expected SW angle dependence. A similar SW angle dependence is also observed for the larger islands. We propose that for the larger islands reversal is governed by a nucleation event, which has SW characteristics, followed by rapid domain wall motion.
Torque studies of large-area Co arrays fabricated by etched nanosphere lithography
Authors: Weekes S. M. , Ogrin F. Y.
Published: May 2005

Large-area arrays of size-tunable Co nanomagnets have been fabricated using a methodology based on nanosphere lithography. The technique employs a monolayer of latex spheres as an inverse mask for the formation of Co elements by electrodeposition. By tuning the size of the spheres with reactive ion etching, magnetic elements of 310 and 240nm diameter have been obtained. Analysis of the arrays using high-field torque magnetometry and three-dimensional micromagnetic modeling clearly demonstrates a change in anisotropy as the diameter of the elements is reduced. More detailed investigation of the field dependence indicates the presence of magnetic vortices at low fields.
Square-integrable wave packets from the Volkov solutions
Authors: Zakowicz Stephan
Published: May 2005

Rigorous mathematical proofs of some properties of the Volkov solutions are presented, which describe the motion of a relativistic charged Dirac particle in a classical, plane electromagnetic wave. The Volkov solutions are first rewritten in a convenient form, which clearly reveals some of the symmetries of the underlying Dirac equation. Assuming continuity and boundedness of the electromagnetic vector potential, it is shown how one may construct square-integrable wave packets from momentum distributions in the space C0∞(R3)4. If, in addition, the vector potential is C1 and the derivative is bounded, these wave packets decay in space faster than any polynomial and fulfill the Dirac equation. The mapping which takes momentum distributions into wave packets is shown to be isometric with respect to the L2(R3)4 norm and may therefore be continuously extended to a mapping from L2(R3)4. For a momentum function in L1(R3)4∩L2(R3)4, an integral representation of this extension is presented.
Hyperbolic octagons and Teichmüller space in genus 2
Authors: Aigon-Dupuy Aline , Buser Peter , Cibils Michel , Künzle Alfred F. , Steiner Frank
Published: May 2005

In this paper we study different models of the Teichmüller space of compact hyperbolic surfaces with special emphasis on their construction by geodesic octagons. Such surfaces have become increasingly interesting due to their applications to the physical behavior of chaotic quantum systems. They are, on the one hand, complex enough to show the relevant features, and on the other hand, they possess a simple mathematical structure allowing practical implementations. We give a new description of Teichmüller space and the mapping class group in terms of geometric data of the octagons. This provides modellings based on the vertices and also on the generators of the associated isometry group. In addition, we explicitly connect our approach with other models currently used in the literature (Helling matrices, Fenchel–Nielsen parameters). The result of the paper may be considered as a computational tool kit to work with a specific model and to relate it with the others.
A geometric approach to scalar field theories on the supersphere
Authors: Schunck A. F. , Wainwright Chris
Published: May 2005

Following a strictly geometric approach we construct globally supersymmetric scalar field theories on the supersphere, defined as the quotient space S2∣2=UOSp(1∣2)∕U(1). We analyze the superspace geometry of the supersphere, in particular deriving the invariant vielbein and spin connection from a generalization of the left-invariant Maurer–Cartan form for Lie groups. Using this information we proceed to construct a superscalar field action on S2∣2, which can be decomposed in terms of the component fields, yielding a supersymmetric action on the ordinary two-sphere. We are able to derive Lagrange equations and Noether’s theorem for the superscalar field itself.
FeCo nanoparticles assembled film
Authors: Xu Yun-Hao , Qiu Jiao-Ming , Bai Jianmin , Judy Jack H. , Wang Jian-Ping
Published: May 2005

FeCo is a material attracting the interest of many because its very high saturation magnetization (up to 2.45T). In this study, FeCo nanoparticles assembled films with a matrix (carbon) were prepared by an integrated nanocluster deposition system with a gas aggregation cluster source and dc magnetron co-sputtering guns. The nanoparticles are produced from a Fe65Co35 alloy target. The average particle size is controlled to vary from 5to12nm. The size distribution is less than 10%. Transmission electron microscope images show the change of crystallinity upon online heating. FeCo nanoparticles with cubic shape are formed with online heating. Hc is found to increase with the carbon matrix deposited.
Structural and magnetic analysis of the transformation of Sn-doped magnetite to Sn-doped hematite by mechanical milling
Authors: Widatallah H. M. , Gismelseed A. M. , Yousif A. A. , Al-Rawas A. D. , Al-Omari I. A. , Al-Tai S. , Elzain M. E. , Johnson C.
Published: May 2005

Spinel-related Sn-doped Fe3O4 has been ball milled for different times up to 35h. Milling was found to transform the material to corundum-related Sn-doped α-Fe2O3. The influence of the milling time, the crystallite size, and the cationic distribution on transformation process is being analyzed with x-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. The relatively fast spinel-to-corundum structural transformation observed is associated with more Fe3+ ions being reduced to Fe2+ due to doping with Sn4+ ions.
Validity and failure of some entropy inequalities for CAR systems
Authors: Moriya Hajime
Published: May 2005

Basic properties of von Neumann entropy such as the triangle inequality and what we call MONO–SSA are studied for CAR systems. We show that both inequalities hold for every even state by using symmetric purification which is applicable to such a state. We construct a certain class of noneven states giving examples of the nonvalidity of those inequalities.
Enhanced 532-nm emission by frequency-doubling of the one-micron Nd:yttrium vanadate laser in gadolinium calcium oxoborate
Authors: Lupei V. , Aka G.
Published: May 2005

It is shown that owing to the improvement of the laser emission parameters at the fundamental frequency and to the reduction of the heat generated by nonradiative processes, the direct pumping into the emitting level of Nd:YVO4 enhances considerably the power emitted at the second harmonic in an intracavity frequency-doubling device. In the case of a laser with an a-cut 0.27at.% Nd:YVO4 crystal frequency-doubled with a GdCa4O(BO3)3 (GdCOB) crystal cut for Type I frequency doubling in the plane XY, an enhancement of the emitted power at 532nm by about 100% was obtained for ∼1100-mW incident Ti:sapphire laser pump at 880nm compared with the traditional pumping at 809nm. It is inferred that besides the improvement of the efficiency, this modality of pumping would enable the scaling of these devices to higher power.
Effect of ultraviolet radiation exposure on room-temperature hydrogen sensitivity of nanocrystalline doped tin oxide sensor incorporated into microelectromechanical systems device
Authors: Shukla Satyajit , Agrawal Rajnikant , Cho Hyoung J. , Seal Sudipta , Ludwig Lawrence , Parish Clyde
Published: May 2005

The effect of ultraviolet (UV) radiation exposure on the room-temperature hydrogen (H2) sensitivity of nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin-film gas sensor is investigated in this article. The present sensor is incorporated into microelectromechanical systems device using sol-gel dip-coating technique. The present sensor exhibits a very high sensitivity, as high as 65 000–110 000, at room temperature, for 900ppm of H2 under the dynamic test condition without UV exposure. The H2 sensitivity is, however, observed to reduce to 200 under UV radiation, which is contrary to the literature data, where an enhanced room-temperature gas sensitivity has been reported under UV radiation. The observed phenomenon is attributed to the reduced surface coverage by the chemisorbed oxygen ions under UV radiation, which is in consonance with the prediction of the constitutive equation, proposed recently by the authors, for the gas sensitivity of nanocrystalline semiconductor oxide thin-film sensors.
Symmetries of distributional domain wall geometries
Authors: Pantoja Nelson , Sanoja Alberto
Published: May 2005

Generalizing the Lie derivative of smooth tensor fields to distribution-valued tensors, we examine the Killing symmetries and the collineations of the curvature tensors of some distributional domain wall geometries. The chosen geometries are rigorously the distributional thin wall limit of self-gravitating scalar field configurations representing thick domain walls and the permanence and/or the rising of symmetries in the limit process is studied. We show that, for all the thin wall space–times considered, the symmetries of the distributional curvature tensors turns out to be the Killing symmetries of the pullback of the metric tensor to the surface where the singular part of these tensors is supported. Remarkably enough, for the nonreflection symmetric domain wall studied, these Killing symmetries are not necessarily symmetries of the ambient space–time on both sides of the wall.
Effects of interparticle dipole interaction on permalloy thin film arrays
Authors: Lai Jun-Yang , Lai Mei-Feng , Chang Ching-Ray , Wei Zung-Hang , Wu J. C. , Lo I. C. , Kuo J. H. , Chang Y. C. , Hsu Jen-Hwa , Huang Jia-Rui
Published: May 2005

The magnetic structures and hysteresis loops of permalloy thin film arrays are investigated here using magnetic force microscopy and vibrating sample magnetometer. The strength of interparticle dipole interaction can be revealed by the number of single-domain pairs with antiparallel magnetizations when the array is relaxed from a strong hard-axis field. Besides, hysteresis loops obtained by vibrating sample magnetometer measurements show that arrays with narrower interparticle spacings have lower coercivities and remanences. The results obtained from vibrating sample magnetometer are in very good agreement with magnetic force microscopy imaging.
Angular dependence of the switching field in patterned magnetic elements
Authors: Dittrich Rok , Hu Guohan , Schrefl Thomas , Thomson Thomas , Suess Dieter , Terris Bruce D. , Fidler Josef
Published: May 2005

Recent experimental studies on the switching behaviour of Co∕Pd multilayer islands with sizes in a range from 20–100 nm shows the classical angular dependence for uniform rotation. We compare measured angular dependence of the switching field with micromagnetic finite element simulations. Simulation results show reversal modes close to uniform rotation for islands smaller than 30 nm. Larger islands (>70nm) reverse by nucleation of a reversed domain followed by domain wall propagation. However, the angular dependence does not change its character with the island size. The nucleation of the reversed domain determines the switching field.
Origin of the anisotropy in soft nanocrystalline FeTaCN films
Authors: Jiang Ruo-Fan , Lai Chih-Huang
Published: May 2005

The magnetic anisotropy of the soft nanocrystalline FeTaCN film was mainly established by the residual stress. When the value of magnetoelastic anisotropy constant (KU) of FeTaCN films was positive and greater than a certain onset value of energy, the film exhibited an in-plane anisotropy and good soft magnetic properties. On the other hand, when the KU was negative, the perpendicular anisotropy appeared, accompanying the presence of stripe domains. The coercivity was reduced to 0.9 Oe after 250 °C annealing. However, the films became magnetically isotropic, which was attributed to the stress release during the annealing process.
Composition and annealing temperature dependent properties of Co1−xPtx(0<x⩽0.2) alloy nanowire arrays
Authors: Li Wuxia , Shen Tiehan H.
Published: May 2005

Arrays of Co1−xPtx(0<x⩽0.2) alloy nanowires have been fabricated by electrochemical deposition of metals into anodic aluminium oxide templates. Compositional and annealing dependent properties of these samples have been investigated by a range of techniques. X-ray diffraction experiments show that the crystal structure of alloy nanowire varies with the chemical composition, ranging from predominantly c axis oriented for pure Co to textured, e.g., Co87Pt13 and to nontextured pure Pt wires. Hysteresis loops, saturation magnetization, squareness ratio (Mr∕Ms), and coercivity (perpendicular and parallel to the wire axis) have all been investigated as a function of the composition. Rapid thermal annealing treatment demonstrates that nanowires with different composition shows different response to the annealing temperatures. The annealing temperature related changes in coercivity, squareness, crystal structure, and ultraviolet-visible spectra are also presented.
Monte Carlo simulation of equilibrium L1 ordering in FePt nanoparticles
Authors: Chepulskii R. V. , Velev J. , Butler W. H.
Published: May 2005

First, second, and third nearest-neighbor mixing potentials for FePt alloys were calculated from first principles using the Connolly–Williams approach. Using the mixing potentials obtained in this manner, the dependency of equilibrium L10 ordering on temperature was studied for bulk and for a spherical nanoparticle with a 3.5-nm diameter at equiatomic composition by use of Monte Carlo simulation and the analytical ring approximation. The calculated order-disorder temperature for bulk (1495–1514K) was in relatively good agreement (4% error) with the experimental value (1572K). For nanoparticles of finite size, the (long-range) order parameter changed continuously from unity to zero with increasing temperature. Rather than a discontinuity indicative of a phase-transition we obtained an inflection point in the order as a function of temperature. This inflection point occurred at a temperature below the bulk phase-transition temperature and which decreased as the particle size decreased. Our calculations predict that 3.5-nm diameter particles in configurational equilibrium at 600°C (a typical annealing temperature for promoting L10 ordering) have an L10 order parameter of 0.83 (compared to a maximum possible value equal to unity). According to our investigations, the experimental absence of a (relatively) high L10 order in 3.5-nm diameter nanoparticles annealed at 600°C or below is primarily a problem of kinetics rather than equilibrium.
Magnetically induced self-organization
Authors: Jones B. A. , O’Grady K.
Published: May 2005

Self-organized magnetic arrays have received much interest recently. In this work the mechanism of self-ordering is investigated by creating magnetic holes with polystyrene spheres in a thin layer of ferrofluid. When an external field is applied perpendicular to the layer, each hole has an associated magnetic moment due to the Lorentz cavity field. Two sizes of spheres have been measured, 6 and 12μm, and the degree of order was determined by Fourier analysis. The results show that there is a greater ordering with larger spheres; doubling the diameter of the spheres increases the repulsive force by a factor of 30.
Measurement of second order susceptibilities of GaN and AlGaN
Authors: Sanford N. A. , Davydov A. V. , Tsvetkov D. V. , Dmitriev A. V. , Keller S. , Mishra U. K. , DenBaars S. P. , Park S. S. , Han J. Y. , Molnar R. J.
Published: May 2005

Rotational Maker fringes, scaled with respect to χ11(2) of crystalline quartz, were used to determine the second order susceptibilities χ31(2) and χ33(2) for samples of thin AlxGa1−xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064nm. The AlxGa1−xN samples, ranging in thickness from roughly 0.5to4.4μm, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x=0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ∼70μm thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ∼226μm thick removed from its growth substrate, and a crystal ∼160μm thick grown by high-pressure techniques. For the AlxGa1−xN samples, the magnitudes of χ31(2) and χ33(2) decrease roughly linearly with increasing x and extrapolate to ∼0 for x=1. Furthermore, the constraint expected for a perfect wurtzite structure, namely χ33(2)=−2χ31(2), was seldom observed, and the samples with x=0.660 and x=0.666 showed χ31(2) and χ33(2) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample.
Self-organized growth of tetrathiafulvalene–tetracyanoquinodimethane molecular wires using the coevaporation method under a static electric field
Authors: Sakai Masatoshi , Iizuka Masaaki , Nakamura Masakazu , Kudo Kazuhiro
Published: May 2005

Wire-like crystals of tetrathiafulvalene-tetracyanoquinodimethane (TTF–TCNQ) charge-transfer complexes were grown under a static electric field by employing electric-field assisted evaporation. TTF–TCNQ molecular wires grew from the edges of two gold electrodes opposite to each other along the electric lines of force, and finally make a connection at their front end to form a single wire. Self-organization of the wire bridge is derived from a higher local electric field between the tips of the opposing molecular wires. Oriented molecular wires, which have diffuse branches, exhibit the effects of a local electric field. Preferential growth of TCNQ at the tip of the molecular wire during coevaporation of TTF and TCNQ is clearly revealed by microscopic Raman spectroscopy. Asymmetrical growth of coevaporated TTF–TCNQ wire under a static electric field is dominated by the drift motion of TCNQ−.
Spin glass or random anisotropy?: The origin of magnetically glassy behavior in nanostructured GdAl2
Authors: Shand P. M. , Stark C. C. , Williams D. , Morales M. A. , Pekarek T. M. , Leslie-Pelecky D. L.
Published: May 2005

Initially crystalline GdAl2 was mechanically milled for long times to produce a highly chemically disordered phase with approximately 8-nm grains. Analysis of dc magnetization measurements using an Arrott plot and the approach to saturation suggest the presence of significant random anisotropy. ac susceptibility measurements showed that the shift in the peak temperature with frequency usually seen in magnetically glassy and superparamagnetic systems was virtually undetectable in the 10–1000-Hz frequency range. Based on these results, we believe that this material represents an interacting system with random anisotropy, where the anisotropy is the result of surface and interface asymmetries.
Thickness effect of nanosized thin-film iron dot on growth of multiwall carbon nanotubes
Authors: Ma Y.-R. , Wong M. S. , See C.-H. , Cheng K.-W. , Wu J. C. , Liou Y. , Yao Y.-D.
Published: May 2005

Hot-filament chemical vapor deposition, using a mixture of methane and hydrogen, is used to grow micrometer-long multiwall carbon nanotubes (MWNTs) on 1-μm-diametric iron dots with thicknesses of 54, 40, and 20nm. Bunches of MWNTs grow on the nanosized thin-film iron dots, and thicker iron dots have more MWNTs, because the nanosized thin-film iron dot behaves as a thin film with many catalyst nanoparticles rather than as a catalyst surface. Therefore, the thickness of the nanosized thin-film iron dot strongly dominates the MWNT growth. However, the MWNTs growing on the iron dots are 1μm long and ∼90nm wide, indicating that the length and diameter of the MWNT are independent of the size and thickness of the iron dot. In addition, the effect of the applied electrical field on the growth of MWNTs is presented.
Improvement of magnetic properties of FePt nanoparticles by adding Mn
Authors: Lai Chih-Huang , Ho Chien-Hsin
Published: May 2005

FePtMn nanoparticles were synthesized by the simultaneous reduction of iron acetylacetonate, platinum acetylacetonate, and manganese acetylacetonate with 1,2-hexadecanediol as the reducing reagent in the polyol process. The coercivity of annealed nanoparticles strongly depends on the amount of Mn addition. A maximum coercivity of 19kOe at room temperature was obtained for (Fe32Pt68)87Mn13 nanoparticles annealed at 700°C for 1h. The presence of Mn in the FePt L10 lattice may induce local strain so the ordering is accelerated. In addition, the high coercivity can presumably be partially related to the local bonding of Pt–Mn (or Fe–Mn) in the L10 structure.
Asymptotic of complex hyperbolic geometry and L2-spectral analysis of Landau-like Hamiltonians
Authors: Ghanmi Allal , Intissar Ahmed
Published: May 2005

In this paper we show that the flat Hermitian complex geometry of Cn, n⩾1, is approximated by the complex hyperbolic geometry of the Bergman complex balls Bρn⊂Cn of radius ρ>0. Furthermore, it will be shown that some elements of the L2-spectral analysis, such as the spectrum, the L2-eigenprojector and the resolvent kernels, associated to the so-called Landau-like Hamiltonian HB,ρ on Bρn give rise to their analogous of the Landau-like Hamiltonian HB,∞ on Cn by letting ρ tend to infinity.
Preparation, structural characterization, and dynamic properties investigation of permalloy antidot arrays
Authors: Vovk Andriy , Malkinski Leszek , Golub Vladimir , Whittenburg Scott , O’Connor Charles , Jung Jin-Seung , Min Suk-Hong
Published: May 2005

Regular nanosized structures are considered to be promising materials for magnetic information storage media with high density of information. Recently attention was paid to static and dynamic magnetic properties arising from dimensional confinement in such nanostructures. Here we present an investigation of permalloy antidot arrays of different thicknesses. Thin permalloy films of thickness ranging from 10to500nm were deposited on nanoporous Al2O3 membranes with a pore size of 100nm. It was found that additional ferromagnetic resonance peaks appear for film thicknesses below 100nm, while films with larger thicknesses show resonance properties similar to continuous films. A comparison between the films deposited onto Si wafers and porous media was done. An evolution of the domain structures observed in MFM experiments was confirmed by micromagnetic calculations.
A novel heat transfer model and its application to information storage systems
Authors: Ghai Sartaj S. , Kim Woo Tae , Escobar Rodrigo A. , Amon Cristina H. , Jhon Myung S.
Published: May 2005

Lattice Boltzmann method (LBM) based on Boltzmann transport equation is developed to simulate the nanoscale heat transport in solids. The LBM can simulate both the metals and semiconductors by properly incorporating the energy carriers. We found that boundary scattering of phonons results in an anisotropic thermal transport in nanoscale solids. The electron-phonon coupling is introduced to accurately describe the thermal behavior of nanoscale confined solids. Our numerical tool will be suitable for simulating complex multiscale systems involving multiple energy carriers with different length and time scales, and is useful in magnetic recording technology when the thermal response plays a crucial role such as for reliability of the head-disk interface and the heat assisted magnetic recording systems.
Memory effects and magnetic interactions in a γ-Fe2O3 nanoparticle system
Authors: Tsoi G. M. , Senaratne U. , Tackett R. J. , Buc E. C. , Naik R. , Vaishnava P. P. , Naik V. M. , Wenger L. E.
Published: May 2005

The low-temperature dynamics of a magnetic nanoparticle system (γ-Fe2O3—alginate nanocomposite with average particle size around 4nm) have been studied by superconducting quantum interference device measurements. Using different temperature and field protocols, memory phenomena in the dc magnetization and magnetic relaxation have been observed at temperatures below its blocking temperature TB=37K. However, aging experiments show an absence of any waiting time dependence in the magnetization relaxation. These observations indicate that the dynamics of this nanoparticle system are governed by a wide distribution of particle relaxation times which arise from the distribution of particle sizes and weak interparticle interactions.
Interparticle interactions and surface contribution to the effective anisotropy in biocompatible iron oxide nanoparticles used for contrast agents
Authors: Arelaro A. D. , Brandl A. L. , Lima E. , Gamarra L. F. , Brito G. E.S. , Pontuschka W. M. , Goya G. F.
Published: May 2005

We have investigated the dynamic magnetic properties of dextran-coated magnetite (Fe3O4) nanoparticles in the form of (a) particles suspended in a carrier liquid and (b) concentrated powder obtained from lyophilization. The blocking temperature was found to increase from TB=42(2)to52(2)K (@μ0H=10mT) after lyophilization, showing the effects of dipolar interactions in samples with identical size distributions. The temperature dependence of the hyperfine field Bhyp(T) reveals the effects of collective magnetic excitations at low temperature, and allowed us to obtain the magnetic anisotropy energy Ea=3.6×10−21J for noninteracting particles. The obtained values can be understood assuming only magnetocrystalline anisotropy, without any additional contributions from surface, shape, or exchange origin. Moreover, a magnetocrystalline anisotropy constant value K1=10kJ∕m3 was obtained by assuming the cubic phase with easy magnetic direction [111] of the bulk material above the Verwey transition, supporting the idea that the Verwey transition is absent in nanosized particles. Accordingly, no indication of magnetic transition at TV could be observed in our measurements. From the dynamical parameters of ac susceptibility χ(f,T) curves, the contribution of the dipolar interactions to the total anisotropy energy barrier could be estimated to be Ω=4.5×10−21J, larger than the single-particle value.
Broad distributions of relaxation times in FePt nanoparticles
Authors: Fuller R. O. , Koutsantonis G. A. , Stamps R. L. , Bardou F.
Published: May 2005

Results from magnetic viscosity experiments on chemically synthesized magnetic nanoparticles are presented. Our synthesized Fe20Pt80 particles are distinguished by a narrow size distribution but display features associated with broad distributions of activation rates. Results are presented from viscosity measurements in which the remnant magnetization is measured at different times after the removal of a static field, and studied as a function of temperature. We discuss how these results may be analyzed in terms of activation rate distributions for thermal activated reversal processes after the application and removal of a field. A method for the extraction of energy barrier distributions from directly measured time dependent data is presented.
Modulation of magneto-resistance with measurement current in patterned Ni80Fe20 wires
Authors: Lepadatu S. , Xu Y. B. , Ahmad E.
Published: May 2005

The dependence of magnetoresistance traces on measurement current in Ni80Fe20 wires patterned with a nanoconstriction is studied for samples with constriction width in the range 50–350nm. An enhancement of resistance is observed for samples with constriction width in the range 50–250nm and it is shown to be due to the additional resistance of a trapped domain wall at the constriction, nucleated after reversal from saturation at measurement current densities below 107A∕cm2. Increasing the measurement current above a critical current density of 107A∕cm2 removes the domain wall from the constriction due to current-induced domain wall movement. This enables the control of the magnetoresistance response with the applied current, showing an enhancement at low fields, within ±20Oe of the zero point, for measurement current densities below 107A∕cm2 and no enhancement for greater current densities.
Elastic properties of nc-TiN∕a-Si3N4 and nc-TiN∕a-BN nanocomposite films by surface Brillouin scattering
Authors: Manghnani Murli H. , Tkachev Sergey N. , Zinin Pavel V. , Glorieoux Christ , Karvankova Pavla , Veprek Stan
Published: May 2005

The hardness of nanocomposite (nc) films developed recently appears to reach the hardness of diamond. High hardness is commonly attributed to the granular structure of nanocomposites (Hall–Petch effect) [E. O. Hall, Proc. Phys. Soc. Lond. B 64, 747 (1951); N. J. Petch, J. Iron Steel Inst. 174, 25 (1953)]. However, grain size in nanocomposites is generally small (5–15nm) and falls in the region where the Hall–Petch effect does not apply. The objective of the present study is to report the elastic properties of the superhard nanocomposites determined by means of surface Brillouin scattering (SBS), and to compare the results with those obtained by nanoindentation. Two types of nanocomposite films were studied: nc-TiN∕a-Si3N4 and nc-TiN∕a-BN. The SBS measurements presented yield values of Young’s modulus significantly larger than those obtained from the slope of unloading indentation curve. This discrepancy is attributed to the lack of the validity of the assumptions behind the Sneddon’s derivation of the formula used for the calculation of the Young’s modulus from the indentation data.
The role of nitrogen-related defects in high-k dielectric oxides: Density-functional studies
Authors: Gavartin J. L. , Shluger A. L. , Foster A. S. , Bersuker G. I.
Published: May 2005

Using ab initio density-functional total energy and molecular-dynamics simulations, we study the effects of various forms of nitrogen postdeposition anneal (PDA) on the electric properties of hafnia in the context of its application as a gate dielectric in field-effect transistors. We consider the atomic structure and energetics of nitrogen-containing defects which can be formed during PDA in various N-based ambients: N2, N2+, N, NH3, NO, and N2O. We analyze the role of such defects in fixed charge accumulation, electron trapping, and in the growth of the interface SiO2 layer. We find that nitrogen anneal of the oxides leads to an effective immobilization of native defects such as oxygen vacancies and interstitial oxygen ions, which may inhibit the growth of a silica layer. However, nitrogen in any form is unlikely to significantly reduce the fixed charge in the dielectric.
Electron coherence and magnetic structure in a nanostructured ferromagnetic ring
Authors: Saitoh E. , Kasai S. , Miyajima H. , Yamaoka T.
Published: May 2005

The external-magnetic-field dependence of the Aharonov–Bohm oscillation in a ferromagnetic Fe19Ni81 nanoring was measured. The amplitude of the Aharonov–Bohm oscillation is suppressed above the magnetization saturation, which can be argued in terms of the magnetic flux density in the ring body. In the vicinity of the magnetization saturation, the oscillation almost vanishes, which is relevant to the magnetic fluctuation affected by the competition between the shape magnetic anisotropy and the external magnetic field.
Domain structures and magnetic ice-order in NiFe nano-network with honeycomb structure
Authors: Tanaka M. , Saitoh E. , Miyajima H. , Yamaoka T. , Iye Y.
Published: May 2005

The magnetic domain configurations and the magnetization processes in a permalloy wire-based honeycomb nano-network have been investigated by means of magnetic-force microscopy and magnetoresistance measurement. The magnetic structure is mainly governed by the magnetic interaction among the magnetic pole on the vertices, being similar to the so-called “ice-rule.” The magnetization vector in a wire behaves coherently. The present results seem to give a direct analogy between the honeycomb network and an Ising system on a kagomé lattice. The ice-rule type interaction, however, disappears with reducing magnetic energy at the vertices.
Photoreflectance and photoluminescence investigations of a step-like GaInNAsSb∕GaAsN∕GaAs quantum well tailored at 1.5μm: The energy level structure and the Stokes shift
Authors: Kudrawiec R. , Yuen H. B. , Ryczko K. , Misiewicz J. , Bank S. R. , Wistey M. A. , Bae H. P. , Harris James S.
Published: May 2005

The energy level structure of a step-like GaInNAsSb∕GaNAs∕GaAs quantum well (QW) has been investigated by photoreflectance (PR) spectroscopy and was analyzed by theoretical calculations. In the active region of this structure, i.e., GaInNAsSb∕GaNAs QW, we have observed PR resonances related higher order QW transitions in addition to the ground state transition. Based on calculations from experimental data, we have found that the electron effective mass in the active QW is 0.12m0 and the conduction band offset for GaInNAsSb∕GaAs interface is about 0.85. The emission observed from this structure at 10K has a small Stokes shift (i.e., 6meV and <2meV for as-grown and annealed structures, respectively) and is without the exponential-like tail at the low-energy side. Hence, we conclude that the incorporation of Sb atoms into GaInNAs alloy helps to achieve QW structures which emit light at longer wavelength and have quite narrow emission line and small Stokes shift.
Interparticle interaction effects in the magnetic properties of NiO nanorods
Authors: Seehra M. S. , Shim H. , Dutta P , Manivannan A. , Bonevich J.
Published: May 2005

Magnetic properties of nanorods of NiO (diameter ≈5nm and average aspect ratio ≈12), with and without oleic acid coating, are reported. Temperature variations (5 to 300 K) of the magnetization and electron magnetic resonance (EMR) spectra were measured to determine the respective blocking temperatures Tb(m) and Tb(EMR). The following differences are noted: (1) Tb(m) is reduced from 230 K (uncoated) to 85 K (coated) for H=25Oe(2.5mT); (2) Decrease of Tb(m) with H is weaker and the ratio Tb(EMR)∕Tb(m) is smaller for the uncoated particles. These differences are due to stronger interparticle interaction present in the uncoated particles.
Chaotic system for self-synchronizing Doppler measurement
Authors: Carroll Thomas L.
Published: May 2005

In a radar system, it is necessary to measure both range and velocity of a target. The movement of the target causes a Doppler shift of the radar signal, and the size of the Doppler shift is used to measure the velocity of the target. In this work, a chaotic drive-response system is simulated that detects a Doppler shift in a chaotic signal. The response system can detect Doppler shifts in more than one signal at a time.
Spin-dependent transport in Fe∕GaAs Schottky diodes at non-normal incidence: A study with circularly polarized light above and below the band gap
Authors: Liu Yanwei , Shen Tiehan H. , Li Wuxia , Liang W. Y.
Published: May 2005

A circularly polarized light with photon energy either above (1.85eV) or below (1.17eV) the band gap of GaAs is used to illuminate the Fe∕GaAs Schottky diodes at non-normal incidence to investigate the magnetic asymmetry of the photocurrents at zero bias in a small magnetic field. A strong influence of the magnetization on the chirality-dependent photocurrent is observed for both cases. The results suggest that the optically excited spin injection could take place in this non-normal incident geometry, even though the combined efficiency is much lower compared with the case at normal incidence. A higher asymmetry value using the lower-energy photons is attributed to the elimination of interband transitions in the semiconductor substrates.
Direct synthesis and easy axis alignment of L1-FePt nanoparticles
Authors: Kang Shishou , Jia Zhiyong , Shi Shifan , Nikles David E. , Harrell J. W.
Published: May 2005

Partially ordered Fe53Pt47 nanoparticles with size around 8nm were prepared by the simultaneous decomposition of iron pentacarbonyl and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. The reflux temperature of the solution could exceed 360°C, where disordered FePt particles could be partially transformed into the ordered L10 phase. A nonmagnetic mechanical stirrer was used in order to avoid agglomeration of the fct-FePt particles during synthesis. The particles were dispersed in toluene and films of the particles were cast onto silicon wafers from the solution. X-ray diffraction patterns of as-made samples showed weak superlattice peaks, indicating partial chemical ordering of the Fe53Pt47 particles. The room-temperature hysteresis loop of the as-made sample reveals a small coercivity (∼600Oe) because of thermal fluctuations; however, the loop is wide open and hard to saturate. The remanence coercivity from the dcd curve is about 2.5kOe, which is four times larger than the hysteresis coercivity. The large remanent to hysteresis coercivity ratio and the shapes of the hysteresis loop and dcd curve suggest a broad distribution of anisotropies in the partially ordered particles. By coating the ordered nanoparticles with a polymer binder, the easy axis of the particles could be aligned under an external field.
Toward the direct deposition of L1 FePt nanoparticles
Authors: Qiu Jiao-Ming , Judy Jack H. , Weller Dieter , Wang Jian-Ping
Published: May 2005

In this paper we report a technique that can directly fabricate L10 phase FePt nanoparticles. FePt nanoparticles were generated through gas-phase aggregation using a magnetron-sputtering-based nanocluster source. Following the source chamber, an online halogen-lamp heater was used for the L10 phase formation during the particles’ flight in vacuum. Transmission electron microscopy and vibrating-sample magnetometer data verified the successful fabrication of the L10 phase FePt nanoparticles. The coercivity value at 300K is 1100Oe for the nanoparticles with online heating. Neon carrier gas was applied to manipulate FePt nanoparticle size and to enhance particle size uniformity. The size dependence of nanoparticle ordering was investigated.
Monitoring simultaneously the growth of nanoparticles and aggregates by in situ ultra-small-angle x-ray scattering
Authors: Kammler Hendrik K. , Beaucage Gregory , Kohls Douglas J. , Agashe Nikhil , Ilavsky Jan
Published: May 2005

Ultra-small-angle x-ray scattering can provide information about primary particles and aggregates from a single scattering experiment. This technique is applied in situ to flame aerosol reactors for monitoring simultaneously the primary particle and aggregate growth dynamics of oxide nanoparticles in a flame. This was enabled through the use of a third generation synchrotron source (Advanced Photon Source, Argonne IL, USA) using specialized scattering instrumentation at the UNICAT facility which is capable of simultaneously measuring nanoscales to microscales (1nmto1μm). More specifically, the evolution of primary-particle diameter, mass-fractal dimension, geometric standard deviation, silica volume fraction, number concentration, radius of gyration of the aggregate, and number of primary particles per aggregate are measured along the flame axis for two different premixed flames. All these particle characteristics were derived from a single and nonintrusive measurement technique. Flame temperature profiles were measured in the presence of particles by in situ Fourier transform infrared spectroscopy and thermophoretic sampling was used to visualize particle growth with height above the burner as well as in the radial direction.
Study of in situ magnetization reversal processes for nanoscale Co rings using off-axis electron holography
Authors: Hu H. , Wang H. , McCartney M. R. , Smith David J.
Published: May 2005

We report a study of the magnetic switching behavior of nanoscale Co rings using off-axis electron holography. Arrays of 10nm thick polycrystalline Co rings with 400nm outer diameter (OD) and different inner diameter (ID) were fabricated by electron-beam lithography. The switching behavior of the rings was studied for different OD∕ID ratios, and two kinds of reversal mechanism were identified. For OD∕ID of 400nm∕250nm and 400nm∕50nm, the reversal started from the so-called onion (bidomain) state, proceeding to a stable vortex state, and finally to the reversed onion state. For intermediate OD∕ID of 400nm∕150nm, the reversal was instead accomplished via rotation of head-to-head domain walls around the rings to the reversed onion state without formation of a vortex state. The OD∕ID ratio of the rings thus played the most important role in determining the switching process. Irrespective of the reversal mechanism, the coercive field of the rings and the range of the field needed to reverse their magnetization, both increased as the inner ring diameter was increased (i.e., narrower ring). The significance of different contributions to the total energy in causing these differences in switching behavior is briefly discussed.
Simulation studies of bus electrode effect on discharge and luminous characteristics of plasma display panels
Authors: Lee Insook , Choi K. Y.
Published: May 2005

We have investigated how bus electrodes affect discharge and luminous characteristics of a discharge cell. Three-dimensional simulations have been performed in a coplanar structured alternating current plasma display panel cell, with phosphor saturation effect taken into account. There exists the optimal position of bus electrodes for high luminance and luminous efficiency. Considering bus electrode thickness, we have found that in-bus structure is a promising candidate for high luminance, high luminous efficiency, and fast operation plasma display panels. Our simulation results would be very useful to understand the influence of bus electrode on characteristics of a plasma display panel cell, and would also provide a general guidance to improve its display performances.
Suppression of the superconducting critical current of Nb in bilayers of Nb∕SrRuO3
Authors: Feigenson M. , Klein L. , Karpovski M. , Reiner J. W. , Beasley M. R.
Published: May 2005

In bilayers consisting of ferromagnetic and superconducting films, the ferromagnetic film in its domain state induces inhomogeneous distribution of magnetic fields in the superconducting film. When the ferromagnetic film has bubble magnetic domains in a labyrinth structure, it has been found that the pinning of the vortices increases; hence, the critical current of the superconducting film becomes larger. Here we study the effect of parallel ferromagnetic domain structure in Nb∕SrRuO3 on the critical current of Nb with current flowing perpendicularly to the domains and find that in this case the ferromagnetic domain structure decreases the critical current.