bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastic scattering cross sections of protons by copper, molybdenum, silver and tin near the Coulomb barrier
Authors: Nurmela A. , Zazubovich V. , Räisänen J. , Rauhala E. , Lappalainen R.
Published: July 2002

Cross sections for elastic scattering of protons by natural copper, molybdenum, silver and tin have been measured in the energy range of 2.3–6.5 MeV at typical Rutherford backscattering angles of 135° and 165°. The threshold energies have been determined above which the Rutherford cross sections become invalid. The threshold energies obtained are 3.5 and 3.4 MeV for copper, 5.1 and 5.0 MeV for molybdenum, 5.0 and 5.8 MeV for silver, and 6.1 and 6.2 MeV for tin at angles of 135° and 165°, respectively. Resonances or the rapid decrease of the cross sections above the thresholds render practical proton backscattering analysis impossible. A literature survey has been carried out to compile relevant energy threshold data for the present target mass and scattering angle regions. Considerable experimental and theoretical interest has recently been focused at the non-Rutherford cross sections. Comparing the experimental non-Rutherford threshold energies with theoretical calculations, we find the data clearly exceeding the predictions of simple classical formulations, while the optical model approach yields more realistic threshold energy values.
Current snap-back and negative differential resistance in layered PnνN structures
Authors: Trivedi Malay , Shenai Krishna
Published: July 2002

This article reports an anomalous snap-back in the forward I–V characteristics of high power rectifiers having a buffer layer. The buffer layer is located at the rectifying junction. It is shown with the help of theoretical analysis and numerical simulations that the snap-back occurs due to a delay in the onset of high-level injection in the drift region. Conditions are identified for the observation of such an effect. Current density at which the snap-back occurs and the extent of the fall in voltage are shown to be strongly dependent on the doping, width and carrier lifetime of the buffer layer.
Gallium/aluminum interdiffusion between n-GaN and sapphire
Authors: Fung S. , Xiaoliang Xu , Youwen Zhao , Wenhong Sun , Xudong Chen , Niefung Sun , Tongnian Sun , Chunxiang Jiang
Published: July 2002

The distribution profiles of Ga and Al near the interface of the n-GaN/sapphire system were measured by x-ray energy dispersive spectroscopy (XEDS). The results are obtained by the corrected XED spectra. First, the gallium diffusing into the sapphire substrate obeys the law of remainder probability function. The gallium diffusion coefficient DGa=2.30×10−13 cm2 s−1 was calculated by theoretical fitting. Second, the diffusion is associated with the GaN growth process at high temperature. Compared to the diffusion of Ga into the sapphire substrate, much less Al antidiffusion from the substrate to the GaN film, with diffusion coefficient DAl approximately equal to 4.8×10−15 cm2 s−1, was observed in the film.
Calculated cross sections and measured rate coefficients for electron-impact excitation of neutral and singly ionized nitrogen
Authors: Frost R. M. , Awakowicz P. , Summers H. P. , Badnell N. R.
Published: July 2002

R-matrix calculations for neutral (N I) and singly ionized nitrogen (N II) have been carried out in the energy region below 50 eV for N I and 180 eV for N II, respectively. The collision strengths were calculated for optical allowed and forbidden transitions as well as for exchange transitions involving energy levels with principal quantum numbers n⩽3. In both atomic systems, spectroscopic measurements in the visible and vacuum-ultraviolet spectral range have been performed in order to investigate several rate coefficients in the theoretically uncertain near-threshold regions. For this purpose seven N I- and sixteen N II-rate coefficients were determined by means of a 7 mm diameter stationary cascade arc discharge operated at a current of 8 A and different pressures in the range from 1 to 30 hPa. The spectroscopic system for the vacuum-ultraviolet wavelength range was intensity calibrated by means of the continuum radiation of a normal pressure pure helium arc discharge and the calibrated line emission of a hollow cathode lamp. Plasma diagnostics yielded electron densities from 1.8 to 7.0×1013 cm−3. By applying a collisional-radiative model the rate coefficients were obtained from absolute level densities. This collisional-radiative model also yielded the ground and metastable state number densities on the basis of calculated rate coefficients for optical allowed transitions. Variation of the arc pressure resulted in electron temperatures ranging from 4.5 to 8.4 eV. Over this range the calculated rate coefficients have been partly confirmed by experimental ones.
Directional dependence of surface morphological stability of heteroepitaxial layers
Authors: Obayashi Y. , Shintani K.
Published: July 2002

Surface morphological stability in coherent heteroepitaxial layers is analyzed focusing on the directional dependence of surface undulations created by surface diffusion. The critical stability condition is defined in terms of the free energy of the system which is assumed to be the sum of the elastic strain energy and the surface free energy. The displacement and stress fields of the semi-infinite anisotropic solid with the slightly undulating surface are calculated by using the surface admittance tensor and the vector complex potential function. Numerical results for the Si1−xGex/Si systems show that the critical wavelength of the 〈100〉 surface undulations is smaller than that of the 〈110〉 surface undulations, which means that surface undulations are likely to be formed in the 〈100〉 directions. It is also found that the critical wavelength decreases with the increase of Ge fraction. These tendencies are in good agreement with the observations in annealing experiments for the Si1−xGex/Si systems in the literature. If the substrate is assumed to be rigid, the range of layer thickness where the system is absolutely stable against a surface undulation of any wavelength exists. Finally, the growth rate of the amplitude of surface undulations is estimated from an evolution equation for the surface shape. It is shown that even if anisotropy is taken into account, the growth rate of the amplitude takes the maximum value when the wavelength is 4/3 times the critical wavelength, which is the same as in the isotropic approximation.
Changes in the photoelectrical properties and generation of photoinduced defects under light/air exposure of C60 thin films
Authors: Katz E. A. , Faiman D. , Mishori B. , Shapira Yoram , Shames A. I. , Shtutina S. , Goren S.
Published: July 2002

We report on the time development of surface photovoltage (SPV) and electron paramagnetic resonance (EPR) spectra for C60 films irradiated by room light in air. Such exposure is shown to result in the progressive reduction of the SPV signal at 0.9–1.1 eV and in an increase in the signal at approximately 1.3 eV, as well as in the reduction of the signals at about 1.6 and 2.3 eV. A decrease in the dark contact potential difference signal as a result of surface band-bending reduction is also observed after exposure. These results are explained assuming that gap states at Ec−(0.9–1.1) eV in our samples are attributed to nonbonded intercalated O2 in the C60 lattice while other states at Ev+1.3 eV are related to oxygen chemically bonded to the C60 molecules. EPR measurements reveal that light/air exposure causes an increase in the number of C60+ paramagnetic defects. A hypothesis has been suggested that the recombination centers at Ev+1.3 eV and the C60+ paramagnetic centers have the same origin and are attributable to carbon dangling bonds.
Monte Carlo modeling of electron velocity overshoot effect in quantum well infrared photodetectors
Authors: Ryzhii M. , Ryzhii V. , Willander M.
Published: July 2002

Transient response of n-type AlGaAs/GaAs and InP/InGaAs quantum well infrared photodetectors (QWIPs) has been studied using an ensemble Monte Carlo particle method. It has been shown that the photocurrent initiated by an ultrashort pulse of infrared radiation reveals a sharp peak associated with the electron velocity overshoot effect. The existence of the photocurrent peak results in a plateaulike region in the QWIP frequency-dependent responsivity in the terahertz range. This effect can be used for heterodyne detection in QWIP-based mixers at terahertz frequencies and for generation of terahertz radiation. Larger intervalley separation and, consequently, more pronounced electron velocity overshoot in InP-based QWIPs in comparison with QWIPs with AlGaAs barriers lead to higher performance of the former in the terahertz range of spectrum.
Quantum interference in a stub-constriction structure containing an infinite strength potential barrier
Authors: Khatun M. , Joyner P. K. , Cosby R. M. , Joe Y. S.
Published: July 2002

We present a theoretical study of electron conductance in a quantum structure containing an infinite strength rectangular potential barrier. The structure we study consists of a quantum wire connected to a stub and a constriction. Conductance results are obtained with and without the scatterer in the channel. Multiple reflections of the electron waves from the stub and the scatterer produce resonance and antiresonance peaks on conductance plateaus. The modulation in conductance is found to be reduced in the presence of the constriction in the structure. Electron conductance through the stub-constriction structure has also been investigated by scanning the potential barrier across the width of the stub, and it has been found that the conductance is heavily modulated. We describe the resonance and antiresonance oscillatory structures in the conductance as due to quantum interference between the incident wave and the electron waves scattered by the boundaries and a potential scatterer.
In situ laser reflectometry study of the amorphization of silicon carbide by MeV ion implantation
Authors: Henkel T. , Heera V. , Kögler R. , Skorupa W.
Published: July 2002

In situ laser reflectometry and ex situ Rutherford backscattering spectrometry have been used to investigate the ion fluence and temperature dependence of the amorphization process in silicon carbide induced by 3 MeV I2+ irradiation. A comparative study in silicon showed that damage accumulation in silicon carbide proceeds more gradually in the preliminary stage of amorphization. The amorphization fluence depends weakly on temperature below 300 K but strongly above 300 K. Silicon carbide is amorphized more quickly than silicon at elevated temperatures. At very low temperatures a higher ion fluence for the amorphization of silicon carbide is required in comparison to silicon. Owing to this behavior, different mechanisms of damage growth are assumed to be present in these semiconductors. A critical energy density of 5.6×1024 eV/cm3 for the amorphization of the silicon carbide crystal up to the surface has been found at room temperature. Experimental results are compared with predictions of the models proposed by Carter as well as by Morehead and Crowder.
Mechanism of explosive electron emission for dielectric fiber (velvet) cathodes
Authors: Miller R. B.
Published: July 2002

We have investigated the physical mechanisms involved in the explosive electron emission process for dielectric cathode materials such as velvet. We have determined that explosive emission from velvet is initiated by a surface flashover mechanism when the electric field exceeds 16 kV/cm. We have also measured the characteristic polarization time of velvet to be 90 ns. The surface discharge gives rise to a cold, dense plasma/gas column in nearly every fiber tuft on the velvet surface. The corresponding average material erosion rate for most velvets of 3×1015 mole/cm2 limits the velvet lifetime to approximately 105 pulses. We have devised a phenomenological model based on resistive heating of the plasma columns which appears to give a good estimate for the final diode-plasma closure velocity. The theory predicts that the square root of the average electric field in the diode is the most important scaling parameter. We have devised practical guidelines for pulse duration limitations for velvet cathodes arising from gap closure and pulse repetition rate phenomena.
Thermally stimulated current spectroscopy on silicon planar-doped GaAs samples
Authors: Rubinger R. M. , Bezerra J. C. , Chagas E. F. , González J. C. , Rodrigues W. N. , Ribeiro G. M. , Moreira M. V. B. , de Oliveira A. G.
Published: July 2002

Using thermally stimulated current (TSC) spectroscopy we have identified the presence of several deep traps in low temperature grown (LTG) nonintentionally doped bulk molecular beam epitaxy (MBE)-GaAs and silicon planar-doped MBE-GaAs samples. The experiments of TSC spectroscopy were carried out on a LTG MBE-GaAs epilayer grown at 300 °C and the planar-doped layer with a nominal silicon concentration of 3.4×1012 cm−2. The LTG nonintentionally doped bulk MBE-GaAs sample shows three peaks in the TSC spectra but the planar-doped MBE-GaAs sample shows spectra similar to those of bulk samples grown by the liquid-encapsulated Czochralski and vertical gradient freeze methods. The main achievement is the experimental evidence that the potential well present in the planar-doped sample is effective in detecting the presence of different deep traps previously not seen in LTG bulk MBE-GaAs epilayers due to a shorter carrier lifetime (about 10−12 s) in the conduction band which occurs due to EL2-like deep traps recombination. This fact is evidenced by a strong hopping conduction in LTG bulk MBE-GaAs samples at temperatures lower than 300 K, but not in planar-doped MBE-GaAs samples because the two-dimensional electron gas has a higher mobility than lateral LTG bulk MBE-GaAs epilayers.
Highly directional stimulated emission from a polymer waveguide
Authors: Zenz C. , Graupner W. , Tasch S. , Leising G. , Iskra K. , Flieser J. , Neger T.
Published: July 2002

We report on strong amplified spontaneous emission observed both in solution and in neat films of a conjugated polymer. The emission characteristics of a three-layer polymer waveguide are presented. The intense, highly directional, completely linear polarized output emission with a small beam divergence of 3° can be observed for pumping above a threshold of 3 kW/cm2. The properties of the output emission can be explained by leaky modes. These waveguides allow us to observe the efficient optical downconversion due to strong stimulated Raman scattering under appropriate excitation conditions.
Investigation of donor-acceptor pair luminescence from ZnSe:N epilayers
Authors: Moldovan M. , Myers T. H. , Giles N. C.
Published: July 2002

Low-temperature photoluminescence (PL) due to donor-acceptor pair recombination was monitored in a series of nitrogen-doped ZnSe epilayers grown by molecular beam epitaxy on GaAs substrates. Emission energies were measured for incident power densities ranging from 10−4 to 10 W/cm2 in samples having different nitrogen doping levels. An ionization energy of 50 meV for the “deep” donor in a lightly doped ZnSe:N sample is determined using power dependence data. Heavily nitrogen-doped samples (⩾8×1018 cm−3) provided evidence for a second deeper donor with an ionization energy greater than 100 meV. In addition, we show the importance of accounting for interference effects when identifying the emission peaks in the PL spectra from heavily doped ZnSe:N.
Decomposition of magnetization processes in complex permeability spectra of 3% SiFe
Authors: Yoon S. S. , Kim C. G. , Kim I. Y. , Kwon S. D. , Kim H. C.
Published: July 2002

Complex permeability spectra were measured as a function of the ac field amplitude h0 and the cut angle α with respect to the [001] direction in a rectangular 3% SiFe sample. The spectra for α=0° at a relatively low field amplitude showed Debye-type dispersion with a relaxation frequency of 26.3 kHz. However, at a relatively high field amplitude, the spectra changed to a superposition of two dispersions, one with a relaxation frequency at 3.3 kHz and the other with a relaxation frequency at 26.3 kHz. Irreversible and reversible 180° domain wall motions were considered as mechanisms responsible for the h0 dependent relaxation at 3.3 kHz and the h0 independent relaxation at 26.3 kHz, respectively. From the α dependence of the spectra, we obtained another relaxation of the magnetization rotation, with a relaxation frequency of 0.16 MHz, in the original 180° domains for α⩾70°.
Size effects in the magnetization of a superconducting wire
Authors: Venegas Pablo A.
Published: July 2002

The size effects in the magnetization of a long cylindrical wire of circular cross section in the presence of an external magnetic field are investigated. For this study the London theory is used with boundary conditions appropriate for this geometry. Using the Monte Carlo simulated annealing method, the free energy of the mixed state is minimized with respect to the vortex positions. The ground state of the vortex lattice for n=1 up to 18 vortices for a given radius of the cylinder is obtained. It is found that the finite size of the sample provokes a matching effect in the magnetization, as found in experiments with superconducting samples of finite size but different geometry.
The design and construction of a high temperature linear electromagnetic actuator
Authors: Sidell N. , Jewell G. W.
Published: July 2002

This article is concerned with the design and construction of a bidirectional linear actuator capable of operating in ambient temperatures of up to 800 °C with a rated force of 300 N and a stroke of ±0.5 mm. The electromagnetic design synthesis strategy employed is described with particular emphasis on those aspects most markedly affected by the high operating temperature, viz. the selection of an appropriate actuator topology and the relative merits of candidate soft magnetic alloys. The key role of detailed thermal modeling, particularly the importance of radiation as a heat transfer mechanism in such high temperature applications, is illustrated during the design synthesis. The construction of a prototype actuator is discussed, including the use of tape would conductors with ceramic insulation and Inconel 600 disk springs as bearing elements.
Exchange biasing in La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 multilayers
Authors: Panagiotopoulos I. , Christides C. , Moutis N. , Pissas M. , Niarchos D.
Published: July 2002

A series of [La2/3Ca1/3MnO3/La1/3Ca2/3MnO3]15 multilayers, with bilayer thicknesses between 2 and 32 nm, has been prepared by pulsed laser deposition. The study of their magnetic and magnetotransport properties reveals, for the first time in this category of materials, the presence of an exchange biasing mechanism at low temperatures. Zero-field-cooling and field-cooling magnetic measurements reveal a blocking temperature around 70 K that is independent of the bilayer thickness, whereas the average film magnetization becomes zero at 250 K.
Orientational dependence of the exchange biasing in Ni–Fe/Mn1−xNix (x=0.11, 0.18, 0.23) bilayers
Authors: Konoto Makoto , Tsunoda Masakiyo , Takahashi Migaku
Published: July 2002

The correlation between the exchange anisotropy of ferromagnetic (F)/antiferromagnetic (AF) bilayers and the spin alignment of antiferromagnet at the interface is still under question. In the present study, Mn1−xNix (x=0.11,0.18,0.23) alloys, whose spin structure changes with x, were used for Ni–Fe/AF bilayers. The correlation between AF spin alignment at the interface and exchange anisotropy was investigated by controlling the preferred orientation of the bilayers. The crystal orientation of bilayers was changed by adding nitrogen into the sputtering argon gas only during the deposition of the Ni–Fe layer adjacent to the substrate. The preferred orientation was (111) when the nitrogen partial pressure, PN2=0%–1%; (100) when PN2>1%. When PN2<1%, Jk decreased and Hc increased with increasing PN2, because of the decrease of the grain size of γ-Mn–Ni. When PN2 exceeds 1%, Jk increased discontinuously, accompanied by the orientational change of the bilayers.
Field-induced crossover from cluster-glass to ferromagnetic state in La0.7Sr0.3Mn0.7Co0.3O3
Authors: Li X. -G. , Fan X. J. , Ji G. , Wu W. B. , Wong K. H. , Choy C. L. , Ku H. C.
Published: July 2002

The thermodynamics and kinetics of the magnetic states of the semiconductor-like compound La0.7Sr0.3Mn0.7Co0.3O3 were studied by detailed magnetic measurements. The spontaneous magnetic state was found to be a cluster-glass phase which can cross over to a ferromagnetic state under an external magnetic field H. The defreezing temperature Tf for the cluster-glass phase and the merging temperature Tm for zero field cooled and field cooled magnetizations are related to the field H by Hj=H0j(1−Tj/Tc)nj, where H0j and nj are constants and j=f,m. Codoping tends to destroy the double exchange in Mn4+–O–Mn3+ and broadens the coexistence region of the cluster-glass and ferromagnetic states. The magnetic relaxation was found to deviate from the usual logarithmic time dependence and follow a power law.
Transport and photoluminescence of silicon-doped GaInP grown by a valved phosphorus cracker cell in solid source molecular beam epitaxy
Authors: Yoon S. F. , Mah K. W. , Zheng H. Q.
Published: July 2002

We report the transport and photoluminescence (PL) properties of silicon-doped GaInP layers grown on GaAs (100) substrate using a valved phosphorus cracker cell in solid source molecular beam epitaxy. Within the range of silicon (Si) effusion cell temperature investigated (900–1200 °C), the highest electron concentration obtained was 7.7×1018 and 3.2×1018 cm−3 at room temperature and 77 K, respectively. The concentration decreased with further increase in the silicon cell temperature. The Hall mobility at 300 K varied from 356 to 1720 cm2/V s within the range of electron concentration measured (4.5×1016–7.7×1018 cm−3). Except for the sample grown at the highest silicon cell temperature (1200 °C), the PL spectrum of other samples showed a dominant peak attributed to Si donor-to-band transition (D–B), which shifted to higher energy following an increase in the electron concentration. This phenomenon was attributed to the Burstein–Moss effect. The blueshift of the (D–B) transition peak at increasing temperature was attributed to thermal ionization of the Si donors. The sample grown at the highest Si cell temperature showed a PL peak at ∼1.913 eV which was attributed to transition between the conduction band and Si acceptor (B–A), with an activation energy of ∼57.2 meV as deduced from the PL spectrum. Temperature-dependent Hall measurements confirmed the amphoteric behavior of the Si dopant in this sample. The PL intensity at 10 K decreased and the full width at half maximum increased significantly from ∼8 to ∼32 meV following an increase in the electron concentration from 4.5×1016 to 7.7×1018 cm−3.
In situ
video observation of 180° domain switching in LiTaO3 by electro-optic imaging microscopy
Authors: Gopalan Venkatraman , Mitchell Terence E.
Published: July 2002

We report video observation of 180° domain switching in congruent LiTaO3 under an external electric field by electro-optic imaging microscopy. The sideways wall velocity of independently growing 180° domains in the Z plane of the crystal was measured to be 0.1–0.2 mm/s at a constant field of 21.2 kV/mm. The merging of independently growing domains results in a serrated domain front which displays an order of magnitude increase in the wall velocity to ∼2.3 mm/s. We show that this is directly related to ledge formation upon domain merging where preferential nucleation takes place. These velocities are also an order of magnitude higher than those reported earlier for equivalent switching times that were measured by pulsed voltage application followed by ex situ optical observation.
Influence of growth conditions on the photoluminescence of self-assembled InAs/GaAs quantum dots
Authors: Chu L. , Arzberger M. , Böhm G. , Abstreiter G.
Published: July 2002

We have investigated the influence of various growth parameters on the optical properties of self-assembled InAs/GaAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) in Stranski–Krastanov mode. Metastably sized QDs have been observed in photoluminescence measurements, which are fabricated by depositing 2.5 monolayers (ML) InAs at a substrate temperature of 530 °C and As4 pressure of (1.6±0.2)×10−5 mbar. These self-assembled QDs exhibit both good optical quality and a narrow size distribution with a full width at half maximum of ∼35 meV both at room temperature and at 4.2 K. By investigating the dependence of the optical properties on the substrate temperature and arsenic pressure, we show that the diffusion length of the adatoms is responsible for changes in size, density, and quantum efficiency of the QDs beside the amount of InAs deposited. The growth conditions for fabricating QDs are optimized for substrate temperatures Ts=480 and 530 °C. A high QD density (∼1×1011 cm−2) with a broad size distribution and a lower QD density (∼1.2×1010 cm−2) with a narrow size distribution, which are determined by atomic force microscopy have been obtained by MBE growth at Ts=480 and 530 °C, respectively. Stacking several QD layers separated by thin GaAs spacers in order to achieve a higher QD density improves the optical properties of the islands. The thermal stability of the QDs has been tested by annealing the samples at high temperatures in order to determine the highest possible substrate temperature for the following expitaxial layers. The QDs grown at 530 °C can be overgrown at 700–720 °C, while the QDs grown at 480 °C can only be overgrown at 600–620 °C without obvious decrease of the quantum efficiency of the QDs.
Electronic structures and phonon-limited electron mobility of double-gate silicon-on-insulator Si inversion layers
Authors: Shoji Masanari , Horiguchi Seiji
Published: July 2002

Electronic structures and the phonon-limited electron mobility of inversion layers have been studied at 300 K for the thin Si (100) layer of double-gate (DG) silicon-on-insulator (SOI) structures by using a one-dimensional self-consistent calculation and a relaxation time approximation. Both symmetric and asymmetric DG SOI systems have been investigated. The self-consistent calculation presents the electronic structures specific to DG SOI Si inversion layers and the range of the specific electronic structures as functions of Si layer thickness tSi and the vertical effective electric field Eeff. Outside this range, the mobility behavior as a function of Eeff is almost identical to that of bulk Si inversion layers. In this range, however, as tSi decreases, the phonon-limited electron mobility μph increases gradually to a maximum around tSi=10 nm, decreases for tSi=10–5 nm, rises rapidly to another maximum in the vicinity of tSi=3 nm and finally falls. The former gradual increase in the mobility μph results from a reduction of phonon scattering caused by the interaction of upper and lower inversion layers. For tSi of less than approximately 10 nm, the mobility of each subband is reduced by an enhancement of scattering rates due to a confinement effect in general. However, the rapid increase of the fraction of electrons in the lowest energy subband that has a higher mobility than other subbands brings about the latter mobility increase in the vicinity of tSi=3 nm.
High moment soft magnetic FeTiN thin films for recording head materials
Authors: Wang H. Y. , He Y. J. , Ma Z. W. , Jiang E. Y. , Huang H. S. , Mao W. H.
Published: July 2002

Soft magnetic properties and thermal stability of Fe–Ti–N alloys films have been investigated. Thin films with composition in the range of 8–10 at. % Ti, 10.5–14 at. % N, and balance Fe were prepared by reactive sputtering method. The films exhibit good soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., low coercivity Hc∼1.5 Oe, high permeability μ∼3200 at 1 MHz and very high saturation magnetization 4πMs∼24 kG. The thermal stability of these films was also found to be good, e.g., Hc was less than 2 Oe, and 4πMs was higher than 23 kG for the films annealed up to 500 °C. The high electrical resistivity ρ∼100 μ Ω cm of the films can minimize the eddy current loss in the high frequency applications. The combination of high 4πMs and relatively high anisotropy field Ha∼10 Oe in the film is conductive to the suppression of the undesirable ferromagnetic resonance interfere up to the GHz frequency range.
Structural change of polycrystalline silicon films with different deposition temperature
Authors: Hasegawa S. , Sakata M. , Inokuma T. , Kurata Y.
Published: July 2002

Polycrystalline Si films were deposited by changing the deposition temperature (Td=150–750 °C) and film thickness (d=0.05–0.8 μm) using plasma-enhanced chemical vapor deposition. For both the films with Td below 150–250 °C and the films thinner than 0.2 μm with Td=600–650 °C, no crystallization was found. However, the crystallinity for the films with Td=650 °C increased with an increase in d thicker than 0.2 μm. Furthermore, the dominant orientations in thicker films changed in the order of a 〈111〉, 〈110〉, and 〈100〉 texture with increasing Td. It was suggested that less Si dangling bonds in grain boundary regions are formed in strongly uniaxially oriented poly-Si films. Crystallization at a lower Td was strongly influenced by the surface morphology of the substrates, but no crystallization at Td=600–650 °C for films thinner than 0.2 μm may be due to interference among growing grains with different textures, and the crystallization for films thicker than 0.2 μm was suggested to occur as a result of solid-phase crystallization of the underlying 0.2-μm-thick noncrystallized Si layer during film deposition.
Evidence for a thermal mechanism in excimer laser ablation of thin film ZnS on Si
Authors: Sands D. , Wagner F. X. , Key P. H.
Published: July 2002

The influence of thermal conduction into the substrate during excimer laser ablation of thin film ZnS on silicon has been investigated. An analytical solution of the heat diffusion equation for a two-layer system has been used to calculate temperature profiles within the film and substrate arising from laser irradiation. The experimentally observed ablation rates are found to correlate with calculated surface temperatures, thereby demonstrating that thermal conduction into the substrate influences the ablation characteristics. Thus we are able to provide a simple predictive model for the ablation rate at any fluence incident on any thin film thickness. We find that for very thin films, optical interference effects reduce the energy coupled into the films and it is necessary to consider these within the model.
Stress dependence of infrared absorption frequencies and oscillator strengths of acceptors confined in GaAs/AlGaAs quantum wells
Authors: Zhao Q. X. , Willander M.
Published: July 2002

Infrared absorption of acceptors confined in a GaAs/AlGaAs quantum well (QW) has been calculated in the presence of an external stress. The absorption frequency and oscillator strength of the dominating infrared transitions of acceptors confined in the QW are examined. The effects of the confined QW potential and the applied stress on the acceptor binding energies are investigated. A simple relation between the acceptor ground state splitting and the first heavy hole-light hole subband splitting is obtained for a GaAs/AlGaAs QW.
Comparison of magnetostatic field calculation methods on two-dimensional square grids as applied to a micromagnetic standard problem
Authors: McMichael R. D. , Donahue M. J. , Porter D. G. , Eicke Jason
Published: July 2002

Magnetization reversal modes and coercivities were calculated for a magnetic particle with thickness : width : length aspect ratios 0.1 : 1 : 5 as a function of the reduced particle width d/lex, where d is the particle width and lex is the intrinsic magnetostatic exchange length. With only exchange energy and magnetostatic energy included, the particle corresponds to μMAG standard problem No. 2. The problem is modeled with two-dimensional grids of three-dimensional spins, and the results are compared for two methods of calculating magnetostatic energies, the “constant magnetization” method and the “constant charge” method. For both magnetostatic computational methods, the coercivity decreases from Hc/Ms=0.06±0.003 to 0.014±0.003 over the range 3<d/lex<80, where the uncertainties reflect the field step size. Also over this interval, as d/lex increases, the magnetization exhibits three modes of reversal: nearly uniform rotation, transverse switching of end domains followed by propagation of head-to-head domain walls from the ends to the center of the particle, and nucleation and propagation of vortices accompanied by more complex domain structures.
A new high density magnetic bar code system
Authors: Watanabe Naoyuki , Sasada Ichiro , Asuke Naoshi
Published: July 2002

Magnetic bar codes can be used in dirty environments instead of optical bar codes or as invisible codes in order to enhance the security of prepaid cards. In this article, we propose a high density magnetic bar code system in which the existence or nonexistence of a bar element is detected by a magnetic bridge. A micro head constituting a magnetic bridge is fabricated with thin amorphous cores and thin copper wire coils. The magnetic bar elements are printed with magnetic ink. The basic characteristics, such as signal separation from neighboring bar elements, are discussed in terms of the element width and the space between two adjacent elements.
Nondispersive magnetostatic forward volume wave propagation in a double-layered yttrium–iron–garnet film structure with dielectric spacer
Authors: Parekh J. P. , Peng Liang-Ping , Tuan H. S.
Published: July 2002

The present article reports a theoretical study of nondispersive magnetostatic forward volume wave (MSFVW) propagation in a double-layered yttrium–iron–garnet (YIG)-film structure consisting of two YIG films separated by a dielectric layer. Numerical computations are presented that show that a significant improvement in bandwidth for nondispersive MSFVW propagation can be obtained in the present separated double-layered YIG-film structure over the previously reported double-layered YIG-film geometry wherein the two YIG films were in direct contact.
Step-induced magnetic anisotropy in Co/stepped Cu(001) as a function of step density and Cu step decoration
Authors: Kawakami R. K. , Bowen M. O. , Choi Hyuk J. , Escorcia-Aparicio Ernesto J. , Qiu Z. Q.
Published: July 2002

The step-induced in-plane uniaxial magnetic anisotropy of fcc Co/stepped Cu(001) was investigated using a curved substrate to provide a continuous range of vicinal angles from 0° to 6°. The anisotropy strength was found to depend linearly on the step density, indicating that the biaxial strain does not make a significant contribution to the step-induced anisotropy. Using a side growth geometry to decorate the Co step edges with Cu adsorbates, we observed that the step-induced anisotropy strength approaches zero at roughly 0.7 atomic rows of Cu, independent of step density.
Magnetic properties of DyAs and DyP (001) films
Authors: Rush E. , Wu L. , Tsui F. , Lee P. P. , Sadwick L. P. , Hwu R. J.
Published: July 2002

The magnetic properties of two single crystal (001) films of DyP and DyAs have been investigated at temperatures between 2 and 300 K and at fields up to 5 T using SQUID magnetometry. The films were grown epitaxially in the simple NaCl structure on (001) GaAs substrates using molecular beam epitaxy techniques. The phosphide compound exhibits ferromagnetic-like order below ∼9 K. The ordered phase appears to have 90° spin alignments, a suppressed saturation moment (∼1.0μB), and a large magnetic anisotropy. In contrast, the arsenide develops an antiferromagnetic-like correlation below ∼6 K, with a much lower anisotropy and a higher moment (∼2.8μB) than that of the phosphide. Above the ordering transitions, the two samples exhibit paramagnetism with paramagnetic moments of 4.5 and 7.8μB for the DyP and the DyAs sample, respectively. The observed behavior is consistent with the presence of strong crystal field interactions. Experiments have been proposed to further explore these effects.
Effect of angular biasing on the magnetic performance of spin-valve devices
Authors: Lederman M. , Tong H. C.
Published: July 2002

In spin-valve devices, hard magnets are placed at both ends of the sensor to suppress Barkhausen noise. As a consequence, the signal amplitude decreases. We have shown that, by initializing the magnets in a direction other than along the free layer easy axis, it is possible to increase the magnetoresistance ratio by more than a factor of 2. The ΔR/R of similar devices without permanent magnets cannot be reached by this method. This angular initialization may have technological applications.
Effects of orientation on the time decay of magnetization for cobalt–alloy thin film media
Authors: Wang J. P. , Alex Michael , Tan L. P. , Yan M. L.
Published: July 2002

The dependence of the time decay of magnetization on orientation ratio was investigated for longitudinal Co–alloy thin film media. The coercivity orientation ratio was controlled by the degree of mechanical texture. For oriented samples, it was found that the remanent magnetization along the circumferential direction decayed faster with time than that along the radial direction when the applied reverse magnetic field was near the remanent coercivity. However, the remanent magnetization along the circumferential direction decayed more slowly with time than that along the radial direction when the applied reverse magnetic field was less than roughly half the remanent coercivity. Anisotropic interactions and magnetic anisotropy distributions appear to be the cause for the different time decay of magnetization along the circumferential and radial directions for oriented media.
Magnetostriction of single crystal and polycrystalline Tb0.60Dy0.40 at cryogenic temperatures
Authors: Dooley J. A. , Lindensmith C. A. , Chave R. G. , Good N. , Graetz J. , Fultz B.
Published: July 2002

At cryogenic temperatures, single crystals of TbDy alloys exhibit giant magnetostrictions of nearly 9000 ppm, making these materials promising for engineering service in cryogenic actuators, valves, and positioners. The preparation of single crystals is difficult and costly. Preliminary results on the magnetostriction of textured polycrystalline materials are presented here. For instance, polycrystalline Tb0.60Dy0.40, plane-rolled (one direction of applied stress) to induce crystallographic texture, has shown magnetostrictions at 77 K of 3000 ppm for an applied field of 4.5 kOe and an applied load of 23 MPa, or 48% that of a single crystal under similar conditions. Comparisons are presented between the magnetostrictive response of plane- and form-rolled (two orthogonal directions of applied stress) polycrystalline Tb0.60Dy0.40 at 10 and 77 K. It is reported that at 10 K plane-rolled Tb0.60Dy0.40 exhibits 1600 ppm magnetostriction at an applied field of 4.4 kOe with a minimal applied load of 0.28 MPa. An observed restoration of the initial unstrained state may be a useful feature of polycrystalline materials for engineering service. Finally it is reported that thermal expansion measurements provide a measure of crystallographic texture for comparison with the magnetostriction.
Effects of laser mode and scanning direction on melt pool shape
Authors: Farooq K. , Kar A.
Published: July 2002

A three-dimensional quasi-steady-state mathematical model is presented for laser heating with a multimode beam. The effects of laser beam scanning direction on the melt pool shape are investigated through the conduction analysis without phase change. The maximum temperature is found to be behind the center of the focal spot due to advection. The isotherms ahead of the focal spot bunch together and spread apart behind the focal spot on the surface of the workpiece due to the same effect. The temperature profile shows four distinct peaks as a result of four intensity peaks. Scanning direction affects the melt pool shape which needs to be considered for high precision cutting applications such as integrated circuit cutting. The melt pool shape is found to be symmetric about the x axis in the x-y plane and z axis in the y-z plane for the scanning angles φs=0° and −90°, however it is asymmetric for other scanning angles.
Effect of dipolar molecules on carrier mobilities in photorefractive polymers
Authors: Goonesekera Arosha , Ducharme Stephen
Published: July 2002

The grating formation speed in photorefractive polymers is greatly reduced by highly polar molecules incorporated by necessity in large concentrations to produce large diffraction efficiency and two-beam energy coupling gain. The random electric fields generated by these dipoles interfere with charge transport by increasing the width of the hopping site energy distribution and thus greatly reducing the carrier mobility and the photorefractive speed. We conducted transport studies of several model systems consisting of combinations of two polymer binders, six charge transport agents (four for holes and two for electrons), and varying concentrations of two highly polar electro-optic chromophores. The results confirm that carrier mobility is greatly reduced in the presence of polar molecules in accordance with the predictions of models of hopping transport in the presence of dipolar disorder. The randomly positioned and oriented dipoles increase the width of the hopping site energy distribution by an amount proportional to the square root of the dipole concentration and to the strength of the dipole moment. The results also show that transport agents with smaller dipole moments reduce the sensitivity to the dipolar effect. The photorefractive speed may therefore be increased by using transport agents with small dipole moments.
Modification of metal/GaN contacts with GaAs interlayers
Authors: Horiba K. , Ono K. , Fujioka H. , Oshima M. , Miki H. , Fukizawa A. , Okuyama M. , Watanabe Y.
Published: July 2002

We have investigated feasibility of GaAs interlayers for the metal/GaN interface with synchrotron-radiation photoelectron spectroscopy. We have found that the use of piranha/HCl solutions is effective as a surface cleaning technique for GaN. We have confirmed that (111) GaAs grows epitaxially on a (0001) GaN substrate. Pd/GaAs/GaN sandwich structures have been successfully fabricated with molecular beam epitaxy. We have confirmed the GaAs interlayer modifies the band diagram at the metal/GaN interface.
Photoluminescence excitation study of nitrogen-doped zinc selenide epilayers
Authors: Moldovan M. , Giles N. C.
Published: July 2002

Photoluminescence excitation (PLE) spectroscopy at liquid-helium temperature is used to study six ZnSe:N epilayers grown by molecular beam epitaxy. These samples represent nitrogen-doping levels ranging from less than 1017 to 3×1019 cm−3. The luminescence emission bands from the heavily doped samples exhibit peak energies varying from 2.45 to 2.61 eV. The energy range over which the PLE signal intensity decays and the energy difference between the onset of this decay and the PL peak energy are essentially the same for all the heavy-doped samples. A model is proposed to explain the PL and PLE results for a semiconductor in the presence of potential fluctuations.
Time-resolved magnetization modulation spectroscopy: A new probe of ultrafast spin dynamics
Authors: Koopmans B. , Haverkort J. E. M. , de Jonge W. J. M. , Karczewski G.
Published: July 2002

Time-resolved magnetization modulation spectroscopy (TIMMS) is proposed as a new probe of the ultrafast spin dynamics in a wide range of magnetic media. This pump–probe technique offers a high magnetic sensitivity by imposing a high frequency modulation to the circular polarization of the pump beam. The resulting modulated magnetization is reflected in an oscillating magneto-optical signal, and is picked up using a lock-in scheme. Experimental details are discussed and a theoretical modeling of the detected signals in a TIMMS experiment is presented. We applied TIMMS to the spin dynamics of CdTe/CdMnTe quantum well structures at room temperature. A spin scattering time of 10–15 ps is found, almost independent of the photon energy, whereas additional time scales (<5 ps) are found if a short period superlattice is grown in the barriers.
Radio-frequency properties of stacked long Josephson junctions with nonuniform bias current distribution
Authors: Filatrella G. , Pedersen N. F.
Published: July 2002

We have numerically investigated the behavior of stacks of long Josephson junctions considering a nonuniform bias profile. In the presence of a microwave field the nonuniform bias, which favors the formation of fluxons, can give rise to a change of the sequence of radio-frequency induced steps. The amplitude of the steps is enhanced when the external frequency matches the fluxon shuttling regime.
Itinerant to localized electron transition in ferromagnetic (Ca, Sr)1−xNaxRuO3
Authors: Alexander C. S. , Cao G. , McCall S. , Crow J. E.
Published: July 2002

SrRuO 3 is an itinerant ferromagnet with TC=165 K and CaRuO3 is a metallic paramagnet down to at least 30 mK. These materials are known to be narrow d-band metals, the physical properties of which are highly sensitive to small perturbations such as chemical doping. In this article, we report results of resistivity, specific heat, and magnetic susceptibility measurements on Na doped SrRuO3 and CaRuO3. Resistivity measurements show a change from metallic to nonmetallic behavior for Na concentration of 12% with the ferromagnetic ordering temperature depressed in Sr1−xNaxRuO3. The nonmetallic behavior in the x=0.12 sample is supported by specific heat measurements which are suggestive of a partial opening of an energy gap and brings into question the role of itineracy in the magnetic ground state. Ca1−xNaxRuO3, with x=0.00, 0.05, and 0.09, remains metallic and shows no evidence of long range magnetic order for all x.
In-plane [111] crystallographic texture in Terfenol-D films through magnetic annealing
Authors: Loveless M. , Guruswamy S.
Published: July 2002

The annealing conditions that favor the formation of RFe2 phase in amorphous Terfenol-D films include preventing preferential Tb loss during annealing and a narrow annealing temperature window around 600 °C. The amorphous Terfenol films were obtained by magnetron sputter deposition using alloy targets on glass, Si and Terbium substrates. Use of Tb substrate provided a nearly stress free film after annealing due to a good thermal expansion match. The presence of magnetic field (2000 Oe) parallel to the film plane during annealing of samples on Tb substrates promotes in-plane orientation of {220} planes of RFe2 phase indicating preferred in-plane orientation of 〈111〉 directions.
Anisotropic magnetic systems with competing interlayer interactions
Authors: Tzavellas A. , Trohidou K. N.
Published: July 2002

The magnetic ordering of the compounds RBaCuTO5+y where R is a rare earth (Y or Pr) and T is a transition metal (Fe or Co) and 0<y<0.5 has been studied by molecular field theory and the Monte Carlo simulation technique. We introduce a model based on competition between weak ferromagnetic interlayer coupling and strong antiferromagnetic coupling induced by the presence of the oxygen defects. We show that even for small y, three-dimensional long range order is rapidly destroyed.