bluuebunny/crossref_metadata_2025 huggingface dataset:
Inductive energy storage driven vacuum arc thruster
Authors: Schein J. , Qi N. , Binder R. , Krishnan M. , Ziemer J. K. , Polk J. E. , Anders A.
Published: July 2002

A new type of vacuum arc thruster in combination with an innovative power processing unit (PPU) has been developed that promises to be a high efficiency (∼15%), low mass (∼100 g) propulsion system for micro- and nanosatellites. This thruster accelerates a plasma that consists almost exclusively of ions of the cathode material and has been operated with a wide variety of cathodes. The streaming velocity of the plasma exhaust varies with cathode material, from a low of 11 km/s for Ti up to 30 km/s for Al, with a corresponding range of specific impulse from 1100 s for Ta to 3000 s for Al. Initiation of the arc requires only a few hundred volts due to an innovative “triggerless” approach in which a conductive layer between the cathode and the anode produces the initial charge carriers needed for plasma production. The initial starting voltage spike as well as the energy to operate the vacuum arc are generated by a low mass (<300 g) inductive energy storage PPU which is controlled using +5 V level signals. The thrust-to-power ratio has been estimated to reach up to ≈20 μN/W. The vacuum arc thruster was tested at the Jet Propulsion Laboratory using W as cathode material. Experimental results are within 65% of the estimated values.
Weak intrinsic charge transfer complexes: A new route for developing wide spectrum organic photovoltaic cells
Authors: Ruani G. , Fontanini C. , Murgia M. , Taliani C.
Published: July 2002

Solar emission extends in the near IR and one of the main issues in designing organic solar cells resides in extending the response into the near IR. Here we show that this may be achieved by making intimate interpenetrated networks of C60 and Zn–phthalocyanine (Zn–Pc) in the solid. Various spectroscopic investigations of co-sublimated thin films of C60 and Zn–phthalocyanine give indeed ample evidence of the existence of a weak charge transfer (CT) state at 1.4 eV, which quenches the photoluminescence of both molecules. The films produced by co-sublimation undergo to a spinodal decomposition producing domains prevalently constituted by Zn–Pc in contact with domain prevalently of C60. The domains size depends on the deposition conditions (rate, stoichiometry, and substrate temperature) forming a percolating 3D network. The separation in different domains is confirmed by the observation of two overlapping peaks, in the resonant Raman spectrum, that correspond to the Ag(2) pinch mode (C=C double bond stretching) for pristine C60 and for a partially (∼0.25e−) doped one. This indicates that only those donor molecules at the grain boundary, which are in contact with C60, give rise to a renormalized new CT ground state. Photocurrent measurements of interpenetrated networks of C60 and Zn–Pc show a linear dependence with respect to the incident light as a consequence of direct absorption within the CT state. The CT state favors the charge separation between the two components, when it is inserted as interface in the organic photovoltaic p–n junction thus increasing the efficiency of the device.
Time-dependent optical linewidth in fluctuating environments: Stochastic models
Authors: Diezemann Gregor
Published: July 2002

Time-resolved optical line shapes are calculated using a second-order inhomogeneous cumulant expansion. The calculation shows that in the inhomogeneous limit the optical spectra are determined solely by two-time correlation functions. Therefore, measurements of the Stokes-shift correlation function and the inhomogeneous linewidth cannot provide information about the heterogeneity lifetime for systems exhibiting dynamic heterogeneities. The theoretical results are illustrated using a stochastic model for the optical transition frequencies. The model rests on the assumption that the transition frequencies are coupled to the environmental relaxation of the system. The latter is chosen according to a free-energy landscape model for dynamically heterogeneous dynamics. The model calculations show that the available experimental data are fully compatible with a heterogeneity lifetime on the order of the primary relaxation time.
High-level theoretical study of the conformational equilibrium of n-pentane
Authors: Salam A. , Deleuze M. S.
Published: July 2002

An accurate calculation of the energy differences between stationary points on the torsional potential energy surface of n-pentane is performed using ab initio Hartree–Fock theory, advanced many-body methods such as MP2, MP3, CCSD, and CCSD(T), as well as density functional theory, together with basis sets of increasing size. This study focuses on the four conformers of this compound, namely, the all staggered trans–trans (TT), trans–gauche (TG), gauche–gauche (G+G+), and gauche–gauche (G+G−) structures, belonging to the C2v, C1, C2, and C1 symmetry point groups, respectively. A focal point analysis up to 635 basis functions is carried out to determine when the series of relative energies of the four conformers approach convergence. It is found that relative to the minimum energy TT conformer, the energy differences of the TG, G+G+, and G+G− conformers obtained from ab initio methods are 0.621, 1.065, and 2.917 kcal mol−1, respectively. Converged energy differences obtained with three density functionals, B3PW91, B3LYP, and MPW1K, are found to be considerably higher than those computed ab initio. Mole fractions of the various conformers are evaluated at different temperatures from thermostatistical data accounting for vibrational and rotational entropies, as well as zero-point vibrational energies in the rigid rotor-harmonic oscillator approximation.
Coupled strained-layer InGaAs quantum-well improvement of an InAs quantum dot AlGaAs–GaAs–InGaAs–InAs heterostructure laser
Authors: Chung T. , Walter G. , Holonyak N.
Published: July 2002

Data are presented showing that, besides the improvement in carrier collection, it is advantageous to locate strain-matching auxiliary InGaAs layers [quantum wells (QWs)] within tunneling distance of a single-quantum-dot (QD) layer of an AlGaAs–GaAs–InGaAs–InAs QD heterostructure laser to realize also smaller size QDs of greater density and uniformity. The QD density is changed from 2×1010/cm2 for a 50 Å GaAs coupling barrier (QW to QD) to 3×1010/cm2 for a 5 Å barrier. The improved QD density and uniformity, as well as improved carrier collection, make possible room-temperature continuous-wave (cw) QD+QW laser operation (a single InAs QD layer) at reasonable diode length (∼1 mm), current density 586 A/cm2, and wavelength 1057 nm. The cw 300 K coupled InAs QD and InGaAs QW AlGaAs–GaAs–InGaA–InAs heterostructure lasers are grown by metalorganic chemical vapor deposition.
Electronic structure of poly(9,9-dioctylfluorene) in the pristine and reduced state
Authors: Greczynski G. , Fahlman M. , Salaneck W. R. , Johansson N. , dos Santos D. A. , Dkhissi A. , Brédas J. L.
Published: July 2002

The electronic structure of the conjugated polymer poly(9,9-dioctylfluorene) and the charge storage mechanism upon doping with lithium atoms have been studied using a combined experimental–theoretical approach. The density of states in the valence band region was measured using ultraviolet photoelectron spectroscopy, and the spectra interpreted with the help of the results of quantum-chemical calculations carried out at the Austin Model 1/Valence Effective Hamiltonian level. The valence band spectra as well as the calculated band structure and derived density of valence states of poly(9,9-dioctylfluorene) are compared to those of poly(p-phenylene) and a ladder-type derivative of poly(p-phenylene). The deposition of lithium atoms, in situ, onto the surface of the polymer films resulted in uniform (bulk) diffusion of the lithium atoms, charge transfer to the polymer (resulting in lithium ions), and the appearance of new electronic states in the gap. At low doping levels (up to about 0.1 Li-atom per repeat unit of the polymer), these states are polarons, and, as the doping level increases, they evolve into bipolarons.
Omnidirectional optical mirror in a cladded-superlattice structure
Authors: Bria D. , Djafari-Rouhani B. , El Boudouti E. H. , Mir A. , Akjouj A. , Nougaoui A.
Published: July 2002

In recent years, it has been shown that one-dimensional photonic crystals such as superlattices can exhibit an omnidirectional reflection of light for a given frequency range. However, this property requires that the incident wave be launched from vacuum or from a low refractive index material. In this article, we show that the limitation about the choice of the substrate can be removed by adding a clad layer of low refractive index at one boundary of the superlattice. The additional layer acts like a barrier for the propagation of light. An alternative solution based on a combination of two multilayer structures is also mentioned.
Properties of a hole trap in n-type hexagonal GaN
Authors: Muret P. , Philippe A. , Monroy E. , Muñoz E. , Beaumont B. , Omnès F. , Gibart P.
Published: July 2002

Minority carrier transient spectroscopy is performed in Schottky diodes fabricated on hexagonal n-type GaN grown by metalorganic chemical vapor deposition, either doped with two concentrations of Si or unintentionally doped. Capacitance transients are measured after a light pulse sent through the semitransparent contact which generates electron–hole pairs in the depletion zone. They display the characteristic sign of hole emission. The same deep level is detected in all the samples, independent of the doping level and doping species, with a concentration of some 1015 cm−3, even in the sample prepared by epitaxial lateral overgrowth. The ionization energy and capture cross section deduced from Fourier Transform transient spectroscopy are respectively 0.81±0.03 eV and 2×10−14 cm2. Such a capture cross section for holes indicates an attractive potential and hence a negatively charged center before the hole capture. Hole emission is suppressed by electron–hole recombination when a sufficiently long majority carrier pulse is applied after the light pulse. A single recombination time constant is measured and an electron capture cross section near 10−21 cm2, independent of temperature, is deduced. These facts demonstrate that this deep center is a point defect, still negatively charged after a hole has been captured, since it repels electrons, and hence it is a deep acceptor. All these properties fit very well the theoretical predictions previously published about the isolated gallium vacancy in n-type GaN.
Dielectric and conduction behavior of La-doped SrTiO3 with suppressed quantum-paraelectric background
Authors: Yu Zhi , Ang Chen
Published: July 2002

The dielectric and conduction behavior of La-doped SrTiO3 annealed in different atmospheres was studied. The results showed that the quantum-paraelectric behavior was purely suppressed by La doping. A variable-range-hopping conduction was observed for the samples annealed in a strongly reducing atmosphere, and subsequently an insulator-metal transition occurred. The possible correlation of the singular dielectric behavior and the electrical conduction behavior is pointed out. The validity of the Barrett relation derived from the mean-field theory for quantum paraelectrics is discussed.
Effect of electron density on shock wave propagation in optically pumped plasmas
Authors: White Allen R. , Palm Peter , Plönjes Elke , Subramaniam Vish V. , Adamovich Igor V.
Published: July 2002

This article discusses experimental studies of spark-generated shock wave propagation in CO-laser sustained optically pumped CO–Ar–O2 plasmas. The rotational-translational temperature of the plasma is measured by Fourier transform infrared emission spectroscopy. The electron density in the plasma is determined by microwave attenuation. The line-of-sight averaged density distribution across the propagating shock is detected by photoacoustic deflection (PAD). The measurements show that adding small amounts of oxygen (up to 0.1%) to the baseline optically pumped CO–Ar plasma increases the electron density and the ionization fraction by more than an order of magnitude (up to ne=0.9×1010 cm−3 and ne/N=0.8×10−8, respectively), while the gas temperature remains nearly constant, within 3%–5%. Therefore this approach allows varying the electron density in the plasma nearly independently of the gas temperature. The PAD measurements show considerable apparent weakening and dispersion of a shock wave propagating in the optically pumped plasma with a strong radial temperature gradient, compared to the shock propagating in a cold nonionized gas. However, varying the electron density independently of the gas temperature does not produce any detectable effect on the measured gas density distribution across the propagating shock. It is therefore concluded that the observed shock weakening is entirely due to the radial temperature gradient sustained by resonance absorption of the CO laser radiation near the centerline of the shock tube and is not affected by the presence of the charged species in the plasma.
Reactions of gas-phase H atoms with atomically and molecularly adsorbed oxygen on Pt(111)
Authors: Biener Jürgen , Lang Erwin , Lutterloh Carsten , Küppers Jürgen
Published: July 2002

The interaction of gas-phase H atoms with ordered and disordered adlayers of atomic oxygen, hydroxyl, and molecular oxygen on Pt(111) surfaces was investigated by in situ mass spectrometry and post-reaction TPD (temperature programed desorption). Exposure of oxygen adlayers to gas-phase H atoms at 85 K leads to formation of H2O via two consecutive hydrogenation reactions: H(g)+O(a)→OH(a) followed by H(g)+OH(a)→H2O(g,a). Both reaction steps are highly exothermic, and nascent H2O molecules partially escape into the gas phase before being thermally accommodated on the surface. Empty surface sites and hydrogen bonding promote thermalization of H2O. Separate experiments performed with OH-covered Pt(111) surfaces reveal that the hydrogenation of hydroxyl is a slow reaction compared to the hydrogenation of atomic oxygen; additionally, the abstraction of H from OH by gas-phase D atoms, OH(a)+D(g)→O(a)+HD(g), was detected. Abstraction of H from adsorbed H2O was not observed. Admission of gas-phase H atoms to O2-covered Pt(111) surfaces at 85 K leads to the desorption of O2 and H2O. The thermodynamic stability of the HO2 radical suggests that the reaction is initiated by hydrogenation of molecular oxygen, O2(a)+H(g)→HO2. The intermediate HO2 either decomposes via dissociation of the HO–O bond, HO2→OH(a)+O(a), finally leading to the formation of H2O (∼85%), or via dissociation of the H–O2 bond thus leading to desorption of O2 (∼15%). The whole reaction sequence of formation and decomposition of HO2 is fast compared to the formation of H2O via hydrogenation of atomic oxygen and hydroxyl. The observed coverage dependence of the reaction kinetics indicates the dominance of hot-atom mediated reactions.
Experimental verification of guided modes in 60°-bent defect waveguides in AlGaAs-based air-bridge-type two-dimensional photonic crystal slabs
Authors: Sugimoto Y. , Ikeda N. , Carlsson N. , Asakawa K. , Kawai N. , Inoue K.
Published: July 2002

Sixty-degree-bent defect waveguides (DWGs) in an AlGaAs-based air-bridge-type two-dimensional photonic crystal (2D PC) slab were fabricated, and the resulting optical transmission spectra and the plan-view image of the optical beam propagating along the bent DWG were measured in the wavelength range between 850 and 1100 nm. Using the two output ports of the stripe waveguides, one aligned to and another offset from the output edge of the bent DWG, the guided mode specific to the DWG and the slab mode in the defect-free PC slab were experimentally observed with the same sample. These results were verified using the calculated band structure and transmission spectra.
Conduction and low frequency channel noise of GaAs based pseudomorphic high electron mobility transistors
Authors: Valenza M. , Vildeuil J. C. , Rigaud D.
Published: July 2002

Conduction and low frequency channel noise of gallium–arsenide (GaAs) based pseudomorphic high electron mobility transistors are investigated. The following analysis takes into account both the noise source associated with the intrinsic part of the device and the sources located within the access path. In order to discriminate between these two noise origins, a model of the transistor conduction is proposed using only a few parameters which are easily extracted. It is shown that the intrinsic channel noise agrees with Hooge’s model with αH parameter about 3×10−4 for the studied technology. Moreover, the values of the access resistances are an important parameter to describe correctly the conduction and the noise behaviors.
Second-harmonic generation study of domain walls in xBi2Ti4O11-(1−x)Bi4Ti3O12 films with large dielectric permittivity
Authors: Jiang A. Q. , Wen Weijia , Wong G. K. L. , Chen Z. H. , Zhou Y. L.
Published: July 2002

Phase-sensitive second-harmonic generation (SHG) is employed to investigate domain patterns of c-axis oriented xBi2Ti4O11-(1−x)Bi4Ti3O12 films on SrTiO3 (001) substrates. The film density increases with increasing Bi2Ti4O11 concentration up to x=0.39. No SHG signal was observed on the Bi2Ti4O11 film. Double peaks appear in the curves of the dependence of the SHG signal on the fundamental polarization (0°–180°), which suggests the net polarization of Bi4Ti3O12 domains oriented along SrTiO3 [110] and [11̄0] directions. Under transverse electric field poling along the SrTiO3 [100] direction, the films with x=0.06 and 0.12 behave differently. The double peak intensities both increase monotonically with increasing ±E fields in the film with x=0.06. However, they change reversibly for the film at x=0.12 with increasing voltage from −2.0 to +2.1 kV. Theoretical modeling suggests the possibility of 180° wall motion with an uncorrelated phase relation for the film with x=0.06, but 90° wall motion with a complete phase relation for the film with x=0.16. The large dielectric permittivity for the film with x=0.39 is assumed to come from the 90° wall motion.
Design and performance of a laser-plasma-based compact soft x-ray microscope
Authors: Johansson G. A. , Holmberg A. , Hertz H. M. , Berglund M.
Published: July 2002

We describe the design of a user-friendly compact water-window x-ray microscope. The microscope is based on a λ=3.37 nm liquid-jet-target laser-plasma source in combination with a normal-incidence multilayer condenser mirror and high-resolution diffractive optics for the imaging. With its high mechanical and thermal stability, the instrument demonstrates enhanced resolution and potential for compact x-ray imaging with the quality of synchrotron-based microscopes. Furthermore, a new sample handling system, computer control, and other improvements facilitate application-oriented x-ray microscopy outside the synchrotron laboratory.
Optical behavior of reactive sputtered carbon nitride films
Authors: Wei J. , Hing P.
Published: July 2002

Carbon nitride films with β-C3N4 crystals were grown on Si(100) substrates using reactive sputtering. The deposition was achieved using a graphite target in an argon/nitrogen plasma. Different nitrogen (N2) fractions and substrate biases were used while the other parameters remained fixed. Atomic force microscopy (AFM) was used to measure the surface roughness and surface morphology. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements were carried out to analyze nitrogen content, chemical bonding state, and optical properties. AFM measurement indicated the surface roughness ranged from 0.2 to 2.5 nm. From XPS data, maximum N/C ratio of 0.5 was achieved in the films. The XPS C 1s spectrum for C–N bond is at 287.32 eV while the N 1s spectrum has a corresponding peak of C–N bond at 398.46. At N2 fraction from 0.6–0.8 and bias from −120 to −200 V, high sp3/sp2 ratio and more β-C3N4 crystals were obtained. Consequently, the films grown at these conditions had high optical band gap. The optical band gap ranged from 1.35 to 2.5 eV.
Configurational anisotropy in square lattices of interacting cobalt dots
Authors: Natali M. , Lebib A. , Chen Y. , Prejbeanu I. L. , Ounadjela K.
Published: July 2002

Magnetostatic coupling effects are investigated in square lattices of polycrystalline circular Co dots as a function of dot diameter, spacing, and thickness. We observe a quadratic anisotropy for closely spaced dots when the dot thickness exceeds 15 nm, correlated to a change in the magnetization reversal process from coherent rotation to vortex nucleation/annihilation. To understand the observed anisotropy micromagnetic simulations have been performed which show that vortex nucleation is strongly dependent on the micromagnetic configurations.
Exchange bias in standard spin valves after different thermal processes
Authors: Li Y. F. , Xiao John Q. , Dimitrov D. V.
Published: July 2002

dc, ac field cool, and their various combinations were applied to standard top and bottom spin valves. While dc field cool from the above blocking temperatures helps to align the antiferromagnetic (AFM) spins, therefore introducing exchange bias, ac field cool statistically randomizes the AFM spins, thus effectively erasing it. The various combinations of these two processes help to separate the contribution to the total exchange bias from different AFM grain sizes. It is found that, compared to the smaller grains, the local exchange bias for larger AFM grains is smaller, but with better temperature stability. More detailed analysis of the temperature dependence of the exchange bias reveals that it is influenced not only by the exchange coupling between FM and AFM layers, but also the interaction among different AFM grains. Such interactions include the direct antiferromagnetic interactions among AFM grains, and the indirect ferromagnetic interaction through the adjacent FM layer. The net interaction is the result of the competition of both, which can be either antiferromagnetic or ferromagnetic.
Spin injection across a hybrid heterojunction: Theoretical understanding and experimental approach (invited)
Authors: Hu C.-M. , Nitta J. , Jensen A. , Hansen J. B. , Takayanagi H. , Matsuyama T. , Heitmann D. , Merkt U.
Published: July 2002

Spin injection across a hybrid ferromagnet/semiconductor junction has proven to be difficult, unlike in an all-metal junction used in giant magnetoresistance devices. The difference responsible is highlighted in a simple model. We perform spin-injection-detection experiments on devices with two ferromagnetic contacts on a two-dimensional electron gas confined in an InAs quantum well. We demonstrate that spin injection allows the hybrid device to combine both the advantage of the ferromagnet as well as that of the semiconductor.
Resonant electromagnetic field cavity between scanning tunneling microscope tips and substrate
Authors: André P. , Charra F. , Pileni M. P.
Published: July 2002

Confined electromagnetic fields are created at the surface of various substrates such as indium-tin-oxide (ITO) and gold films. Two scanning tunneling microscope tips (Pt–Ir and W) are used to create a localized perturbation. With ITO as a substrate, an evanescent field is observed without a tip-substrate interaction. Conversely, with a gold film surface formation of “gap modes,” the particle-substrate cavity is seen. Gap modes at the interface of a metallic film are involved essentially when the modulation amplitude of the particle is below 100 nm. In the context of apertureless near-field microscopy, this demonstrates the influence of tip-surface coupling in scanning plasmon near-field microscope (SPNM) signals. The strong interaction of the tip with the metal substrate, through its surface plasmon, when combined with SPNM, may result in inaccuracies in the claimed chemical identification or intrinsic optical properties of the particle.
Periodic magnetic nanostructures on self-assembled surfaces by ion beam bombardment
Authors: Chen Y. J. , Wang J. P. , Soo E. W. , Wu L. , Chong T. C.
Published: July 2002

We demonstrate that periodic magnetic nanostructures can be fabricated by depositing perpendicular magnetic films on self-assembled surfaces by ion beam bombardment. Similar stripe domain patterns and reduced coercivity are observed for the as-deposited sample as compared with those for a reference sample with unbombarded substrate. Further processing steps such as etching and polishing will be needed to isolate the magnetic elements from each other, in order to make ultrahigh density magnetic storage media.
Structure and energetics of He2* bubble-states in superfluid 4He
Authors: Eloranta J. , Schwentner N. , Apkarian V. A.
Published: July 2002

Structure and energetics of solvation of the triplet Rydberg states of the He2* excimer in liquid He4 (LHe) are analyzed using ab initio potentials and density functional methods. The results are used to interpret the known spectroscopy. Having established the reliability of the various semiempirical functionals, interfacial properties of the superfluid on molecular scales are discussed. Due to its spherical electron density, the a(Σu3) state solvates in a spherical bubble of 7 Å radius in which the excimer freely rotates. This explains the observed rotationally resolved b3←a3 and c3←a3 absorption spectra. A deep potential minimum occurs at the equatorial node of the a(Σu3) state, where a radially frozen belt of six He atoms can be sustained at R=2.3 Å, inside an ellipsoidal cavity with major axis of 8 Å and a more diffuse minor axis of 6 Å. Despite the absence of a potential energy barrier, or a many-body interfacial tension preventing the wetting of the belt, the bare c3 state is observed in emission. It is argued that in the superfluid, wetting is prevented by the hindered rotation of the excimer, hence the sensitivity of the c3→a3 emission to pressure induced quenching. The nodal plane in the b(3Πg3) state passes through the molecular axis, as such, rotation cannot provide protection against wetting and subsequent quenching of the b3 state via the He3* manifold; hence the absence of b3→a3 emission despite its large transition dipole. In its global minimum, the d3 excimer sustains a shell of 16 He atoms, localized at the radial node of its Rydberg electron, at R∼2.5 Å. The shell, in turn, is contained in a nearly spherical bubble held at a radius of 13 Å by the extra-nodal electron density. The repulsion between extra-nodal electron density and LHe provides a barrier to filling of the deep nodal well, ensuring the stability of the bare d3 excimer in a large spherical bubble. This explains the free-rotor envelopes of the d3→b3 and d3→c3 emissions, and their negligible spectral shifts relative to the gas phase. The predicted minimum energy structures, the belted c3 state and the crusted d3 state, if formed, should be metastable.
Magnetic and thermal characteristics of a magnetic shape memory alloy (Fe–Cr–Ni–Mn–Si–Co)
Authors: Enokizono M. , Todaka T. , Szpryngacz M. , Kanada T. , Watanabe Y.
Published: July 2002

The shape memory alloy made by melt-spinning method was heat treated in argon in order to investigate the influence of heat treatment on its magnetic property. The specimens after heat treatment were set into the vibration sample magnetometer and the hysteresis loops and the Curie point temperature were measured. It can be shown that temperature as was expected has a significant influence on the magnetic and a shape memory property of investigated material.
Electronic spectroscopy of intermediates involved in the conversion of methane to methanol by FeO+
Authors: Aguirre Fernando , Husband John , Thompson Christopher J. , Stringer Kay L. , Metz Ricardo B.
Published: July 2002

Specific ion–molecule reactions are used to prepare two intermediates of the FeO++CH4 reaction, and photodissociation of the jet-cooled intermediates is examined in the visible and near-ultraviolet using time-of-flight mass spectrometry. The photodissociation spectrum of the aquo iron carbene complex [H2C=Fe–OH2]+ shows transitions to at least four excited electronic states in the FeCH2+ chromophore, with broad vibrational structure. Photoexcitation of the insertion intermediate [HO–Fe–CH3]+ leads to formation of FeOH++CH3 and also triggers the reaction to produce Fe++CH3OH. The photodissociation spectrum of [HO–Fe–CH3]+ presents a vibrationally resolved band involving progressions in the excited state Fe–C stretch, Fe–O stretch, and O–Fe–C bend. The change in the Fe–C bond length in [HO–Fe–CH3]+ and [H2C=Fe–OH2]+ upon photoexcitation is calculated from a Franck–Condon analysis of the vibronic features observed. The analysis of the experimental results is aided by hybrid Hartree–Fock/density-functional (B3LYP) calculations on [HO–Fe–CH3]+ and [H2C=Fe–OH2]+ performed to determine molecular parameters, and time-dependent density functional theory (TD-DFT) calculations on FeCH2+ to predict excited electronic states.
Excimer laser crystallization of amorphous silicon on molybdenum coated glass substrates
Authors: Brendel K. , Lengsfeld P. , Sieber I. , Schöpke A. , Nickel N. H. , Fuhs W. , Nerding M. , Strunk H. P.
Published: July 2002

Hydrogenated amorphous silicon (a-Si:H) films on molybdenum coated glass substrates were crystallized using a XeCl excimer laser. Structural information on the resulting polycrystalline silicon (poly-Si) films was obtained from scanning electron microscopy and electron backscattering diffraction measurements. The average grain size varies with laser fluence. The maximum average grain size in the super lateral growth energy–density range is considerably smaller for poly-Si on Mo coated substrates than for poly-Si on quartz. In addition, the metal layer affects the laser fluence necessary to achieve super lateral growth. Samples crystallized under super lateral growth conditions show a preferential surface orientation along the {111} direction. Intermixing of Mo and silicon is not observed.
Long-range order and short-range order study on CoCrPt/Ti films by synchrotron x-ray scattering and extended x-ray absorption fine structure spectroscopy
Authors: Sun C. J. , Chow G. M. , Wang J. P. , Soo E. W. , Hwu Y. K. , Je J. H. , Cho T. S. , Lee H. H. , Noh D. Y.
Published: July 2002

The long-range order (LRO) and short-range order (SRO) of CoCrPt/Ti/NiP perpendicular media films were studied by synchrotron x-ray scattering and extended x-ray absorption fine structure (EXAFS) spectroscopy in order to correlate their effects with the magnetic properties. The thickness of the magnetic media films and the NiP seed layers were kept constant, whereas the Ti underlayer thickness was varied. The increase of out-of-plane coercivity and squareness was related to the increase in the crystallinity and texture of CoCrPt (002), which first increased and then decreased with increasing Ti thickness. The best magnetic properties were observed for the magnetic film grown on the 30 nm Ti underlayer. The improvement was associated with the local atomic environment in which a relatively ordered Co, and disordered Cr and Pt existed. This suggests that a higher degree of Co and Cr phase segregation contributed to the enhancement of the magnetic properties. The LRO and SRO results are consistent with the magnetic properties.
Effect of deposition sequence on interface intermixing in Cu/Co/Ru and Ru/Co/Cu multilayers studied by NMR
Authors: Je̢dryka E. , Wójcik M. , Nadolski S. , Kubinski D. , Parsons M. , Holloway H.
Published: July 2002

Co 59 NMR and magnetization study has been performed in two series of multilayer stacks: [Ru(2.5 nm)/Co(x)/Cu(2.5)]10 and [Cu(2.5 nm)/Co(x)/Ru(2.5)]10. The amount of Co involved in the interfaces of the two respective series was found to be 0.6 nm and 0.75 nm per Co layer. This asymmetry of intermixing as a function of deposition sequence is related to differences in spreading energy between Co on Ru and Ru on Co. The resulting difference in the effective thickness of the Co layer was shown to affect the magnetoresistive properties of the mixed Co/Cu/Co/Ru multilayers.
Magnetization process in ferromagnetic bilayer with exchange coupling through a pinhole
Authors: Saitoh E. , Matumoto H. , Nakamura T. , Miyajima H.
Published: July 2002

The magnetization process has been investigated for SmCo5/SiO2/Fe19Ni81 trilayer films, in which the exchange coupling between hard-ferromagnetic SmCo5 and soft-ferromagnetic Fe19Ni81 layers is controlled by the size (1–100 μm in diameter) of a pinhole penetrating the SiO2 layer. The observed magnetization curve indicates that the magnetization reversal in the SmCo5 layer consists of a nucleation around the pinhole and the propagation of the domain wall. This can be responsible for the suppression of the coercivity induced by the coupling through the pinhole. Variation of the magnetization curve with the size of the pinhole can be argued from the viewpoint of the simple domain nucleation model.
Investigations of lines in domain walls of yttrium orthoferrite
Authors: Didosyan Y. S. , Hauser H. , Papanicolaou N. , Reider G. A.
Published: July 2002

We report on the visualization of domain walls (DWs) by means of the Faraday effect for the case when the direction of light has a nonzero projection on the easy axis. Observations of individual DWs between crossed polarizers reveal segments of different brightness. Under the influence of an external magnetic field, the boundaries between these segments move along the DWs. The positions of the boundaries coincide with those observed by a dark field method and indicate locations of lines of zero magnetization. Evidence is provided that the DW contrast, that depends on the polarization of the incident light, is associated with the DW inclination and crystallographic birefringence.
Efficient quantum well to quantum dot tunneling: Analytical solutions
Authors: Chuang S. L. , Holonyak N.
Published: July 2002

Tunneling transfer of carriers from a quantum well (collector) into a quantum dot is investigated theoretically using the transition-probability approach of Bardeen. The approach is attractive since it allows analysis of the tunneling rate using the wave function of the initial state in the quantum well and that of the final state in the quantum dot. We derive an analytical tunneling formula for the transition rate. We show that the quantum well to quantum dot tunneling can be extremely fast when the well, barrier, and dot potentials are properly designed. These calculations show that, in the recent experiment of a quantum-dot laser using an auxiliary InGaP quantum well for carrier collection and lateral transport followed by tunneling to the quantum dot layer via a thin coupling barrier, the added requirement of tunneling does not impede the population of the dot states.
Local reactivity index defined through the density of states describes the basicity of alkaline-exchanged zeolites
Authors: Santos Juan C. , Contreras Renato , Chamorro Eduardo , Fuentealba Patricio
Published: July 2002

The basicity of alkaline-exchanged zeolites (Li+, Na+, K+, Rb+, and Cs+) has been studied using a local descriptor of reactivity defined in terms of the density of states for an arbitrary bandwidth. This is the first report where a local reactivity index is evaluated through a scheme of the density of states in a cluster model of exchanged zeolite. We have found that this empirical model assesses well the experimental basicity trend of zeolites, and it gives a better description of basicity than the frontier-orbital-based approximations for the Fukui function and local softness or Mulliken population analysis. We discuss the relationship between this empirical index and the local softness defined as the local density of states evaluated at the Fermi level introduced by Yang and Parr [Proc. Natl. Acad. Sci. U.S.A. 82, 6723 (1985)].
Theory of adsorption in a polydisperse templated porous material: Hard sphere systems
Authors: Rżysko Wojciech , Sokołowski Stefan , Pizio Orest
Published: July 2002

A theoretical description of adsorption in a templated porous material, formed by an equilibrium quench of a polydisperse fluid composed of matrix and template particles and subsequent removal of the template particles is presented. The approach is based on the solution of the replica Ornstein–Zernike equations with Percus–Yevick and hypernetted chain closures. The method of solution uses expansions of size-dependent correlation functions into Fourier series, as described by Lado [J. Chem. Phys. 108, 6441 (1998)]. Specific calculations have been carried out for model systems, composed of hard spheres.
Magnetocaloric effect in the Laves phase pseudobinary Er1−cYcCo2
Authors: de Oliveira N. A. , von Ranke P. J. , Costa M. V. Tovar , Troper A.
Published: July 2002

In this article we study the magnetocaloric effect in the pseudobinary Er1−cYcCo2 using a theoretical model, in which the localized 4f spins are immersed in an effective subsystem of itinerant electrons. The interaction between the localized 4f spins is treated in the molecular field approximation and the disorder entering in the Er site is considered in the virtual crystal approximation. Our theoretical results for the adiabatic temperature change and for the isothermal magnetic entropy change with magnetic field are in good agreement with experimental data.
Influences of granular microstructure on the reversal mechanism of Co nanoelements
Authors: Spargo A. W. , Ridley P. H. W. , Roberts G. W. , Chantrell R. W.
Published: July 2002

A two-dimensional micromagnetic simulation is developed using a spatial finite element method, with the dynamic Gilbert equations discretized by a Galerkin method. An algorithm is presented for the generation of granular microstructures via the Voronoi tessellation and rectangular nanoelements of 200×40×20 nm3 are constructed. Hysteresis simulations are performed on isolated nanoelements in order to determine the extent to which experimentally observed variations in coercivity are attributable to the microstructural properties rather than magnetostatic interactions with neighboring members of an array. Dependence of coercivity on grain size is examined, as well as the role of grain structure in the reversal process. Mean grain size is shown to dictate the mode of reversal, whereas grain irregularity is observed to influence specific intermediate magnetization configurations and therefore specific coercivity values. Low correlation between grain irregularity and coercivity indicates that the magnetization dynamics depend on a combination of physical microstructure and easy-axis distribution unique to each nanoelement.
Structure and tunnel magnetoresistance in Fe/MgF2/Co junctions with an oxide seed layer on an Fe bottom electrode
Authors: Mitani S. , Moriyama T. , Takanashi K.
Published: July 2002

Fe/MgF 2 / Co magnetic tunnel junctions (MTJs) with an oxide seed layer (MgO, Fe–O) inserted between an Fe bottom electrode and a MgF2 barrier layer have been prepared through the use of molecular beam epitaxy, and the effects of the seed layers on structure of subsequently deposited MgF2 barrier layers and tunnel magnetoresistance (TMR) have been investigated. The crystallographic orientation of MgF2 layers depends significantly on the seed layers, and furthermore, it has been found that the surface roughness of MgF2 layer is reduced by inserting a MgO seed layer. In Fe/MgF2 (3–12 nm)/Co MTJs without any seed layer, top Fe and bottom Co electrodes contact each other through pin holes in MgF2 layers. On the other hand, however, Fe and Co electrodes are separated both magnetically and electrically in Fe/MgO (0.3–0.5 nm)/MgF2 (2–5 nm)/Co MTJs, i.e., MgF2-based MTJs have successfully been prepared by inserting a thin MgO seed layer, resulting in TMR of about 10% observed at 4.2 K.
Surface and bulk spin waves in magnetic superlattices with biquadratic exchange coupling
Authors: Bezerra C. G. , Cottam M. G.
Published: July 2002

A study is made of the bulk and surface spin wave modes propagating in magnetic superlattices that are composed of two simple cubic ferromagnetic materials which interact by bilinear and biquadratic exchange couplings at the interfaces. A transfer matrix treatment is employed with the calculations carried out for the exchange dominated regime within the framework of the Heisenberg model and taking into account the random phase approximation. The numerical results obtained show that from a certain value of the ratio R between the biquadratic and interfacial bilinear exchange couplings the surface acoustic modes are suppressed and only the optical modes are present.
FeCoC film as the soft magnetic underlayer for perpendicular media
Authors: Soo E. W. , Zhou T. J. , Wang J. P.
Published: July 2002

FeCoC granular thin films are prepared and studied as the soft magnetic layer in order to obtain a low noise double-layered perpendicular recording medium. FeCoC granular soft magnetic underlayer prepared by cosputtering at room temperature showed good soft magnetic properties; with coercivity of a few oersteds and Bs of 1.4–1.7 T. Using a spin-stand and a ring-type head, sharper transitions can be obtained in the [Co/Pd]n media with an FeCoC soft underlayer at higher recording density compared to media without a soft magnetic layer. No spike noise was observed in the read back signals for [Co/Pd]n media with an FeCoC granular film as the soft magnetic underlayer. Preparing the [FeCo/C]n laminated films by alternately sputtering FeCo and C layers allows the properties of the films to be flexibly adjusted. A Bs of 1.9 T with relative permeability of 100–400 and coercivity lower than 40 Oe can be obtained with laminated [FeCo/C]n (210 nm) multilayer thin films.
Experimental observations of thermally excited ferromagnetic resonance and mag-noise spectra in spin valve heads
Authors: Zhou Yuchen , Roesler Alex , Zhu Jian-Gang
Published: July 2002

Thermally excited ferromagnetic resonance (FMR) gives rise to so-called mag-noise in the spin valve read heads used in disk drives. In this article, an experimental method of measuring the mag-noise power spectrum density (PSD) in a frequency range of 2.5–6 GHz is presented. The thermally excited FMR modes in the spin valve and the corresponding resonant frequencies have been observed. It is found that the resonance frequencies occur in the range from 3 to 5 GHz in today’s spin valve heads at the quiescent state. Multiple resonance peaks were observed for some spin valve read heads while others only show a single resonance peak. An external field was applied to the spin valve heads to study the field dependence of the FMR spectrum. The mag-noise level at low frequencies is at the similar level of the head’s Johnson noise for heads of track width W=0.3 μm and 50 Ω resistance. It is suggested that the thermally excited FMR PSD could be used as a rapid diagnostic tool for spin valve heads.
Analysis of magnetic noise in lead overlaid giant magnetoresistive read heads
Authors: Zhang J. , Huai Y. , Lederman M.
Published: July 2002

Thermally induced magnetic noise in giant magnetoresistive (GMR) read heads was characterized with both contiguous junction and lead overlaid design using an integrated spectral method. At 0.25 μm magnetic read width and 4 mA bias current, magnetic noise is twice as large as Johnson noise for lead overlaid design, while it is comparable with Johnson noise for contiguous junction design. The head signal-to-noise ratio gain of 2–3 dB with lead overlaid design is less than an amplitude gain of 50%–100% since it is limited by increased magnetic noise, which roughly scales with amplitude. Magnetic noise increases with bias current at a much faster rate than Johnson noise. Higher magnetic noise is attributed to weaker longitudinal bias field associated with the lead overlaid design. Fortunately, magnetic noise does not limit system level signal to noise ratio since media transition noise is dominant. However, it is projected that magnetic noise will account for an increased portion of total noise power at higher areal densities.
Exchange tab readback transducer using reactive ion etching to define the trackwidth
Authors: Seigler M. A. , Mack A. M. , Subramanian K. , Pust L.
Published: July 2002

IrMn was used to stabilize the free layer in exchange tab type magnetic recording readback heads. These heads were fabricated using either reactive ion etching (RIE) or argon ion beam etching (IBE) to define the trackwidth. The two processes were used to eliminate the coupling between the exchange tab material (IrMn) and the free layer (NiFe), which creates a sensitive, nonhysteretic, linear, stable free layer. Wafer level test structures were measured during the head build process and results for the RIE and IBE processes are compared. Transmission electron micrographs were taken. Undercutting of the electrodes can be seen on the RIE processed heads. Spin-stand data for the read heads were compared and the RIE processed heads had a larger low frequency amplitude and sensitivity. Micro-track profiles and tack scans were also taken using the RIE processed heads. The increased process latitude that the RIE process has over the IBE process leads to, on average, overall better performing heads.
Thermal magnetization fluctuations in CoFe spin-valve devices (invited)
Authors: Smith Neil , Synogatch Valeri , Mauri Danielle , Katine J. A. , Cyrille Marie-Claire
Published: July 2002

Thermally induced magnetization fluctuations in the Co86Fe14 free (sense) layer of micron-sized, photolithographically defined giant magetoresistive spin-valve devices are measured electrically, by passing a dc current through the devices and measuring the current-dependent part of the voltage noise power spectrum. Using fluctuation–dissipation relations, the effective Gilbert damping parameter α for 1.2, 1.8, and 2.4 nm thick free layers is estimated from either the low-frequency white-noise tail, or independently from the observed thermally excited ferromagnetic resonance peaks in the noise power spectrum, as a function of applied field. The geometry, field, and frequency dependence of the measured noise are found to be reasonably consistent with fluctuation–dissipation predictions based on a quasianalytical eigenmode model to describe the spatial dependence for the magnetization fluctuations. The extracted effective damping constant α≈0.06 found for the 1.2 nm free layer was close to 3× larger than that measured in either the 1.8 or 2.4 films, which has potentially serious implications for the future scaling down of spin-valve read heads.
Thermal fluctuation effects on quasistatic magnetic switching of patterned Ni81Fe19 elements (invited)
Authors: Shi Jing , Li Jian , Tehrani Saied
Published: July 2002

We have studied quasistatic magnetic switching of electron-beam patterned Ni81Fe19 (NiFe) elements over a wide range of temperatures. Switching properties depend on both lateral and thickness dimensions of the patterned structures. In large aspect-ratio elements, the switching between the two bistable states occurs through thermally activated nucleation and the switching field depends linearly on temperature. As the aspect ratio decreases, a third stable remanent state-trapped vortex state develops, but the switching fields between different states also linearly depend on temperature. As the film thickness decreases, the trapped vortex population clearly becomes thermally activated.
Optical control of magnetic order in molecule-based magnet Mn(TCNE)x⋅y(CH2Cl2)
Authors: Pejaković Dušan A. , Kitamura Chitoshi , Miller Joel S. , Epstein Arthur J.
Published: July 2002

Studies of photoinduced magnetization and photoinduced absorption in molecule-based magnet Mn(TCNE)x⋅y(CH2Cl2) (x≈2, y∼0.8, TCNE=tetracyanoethylene) are reported. Optical excitation in the blue region of spectrum leads to increased magnetization, accompanied by changes in the electronic spectrum. These effects are preserved long after illumination, indicating formation of a highly metastable electronic state. Partial reversal of the photoinduced effects is obtained by excitation with lower energy visible light. The results suggest that the photoinduced effects are controlled by two distinct electronic transitions.