bluuebunny/crossref_metadata_2025 huggingface dataset:
Interface structures and magnetoresistance in magnetic tunnel junctions
Authors: Mitsuzuka T. , Matsuda K. , Kamijo A. , Tsuge H.
Published: July 2002

The interface structures of magnetic tunnel junctions were studied using x-ray photoelectron spectroscopy (XPS). The structures were correlated with magnetoresistance (MR) characteristics. For MR measurements, Fe(50 nm)/AlOx/CoFe(30 nm) junctions with an in situ naturally oxidized Al tunnel barrier were fabricated. The thickness of the Al layer, an important parameter in MR characteristics, was varied from 0 to 5 nm. MR curves showed that the largest MR ratio occurred when the Al layers were 2–3 nm in thickness. XPS analysis showed that an Al layer greater than 1 nm thick covers the entire surface of the Fe underlayer. However, if the Al layer is more than 1 nm thick, the unoxidized Al remaining after the oxidation process increases as the thickness is increased. For Al layers that are greater than 3 nm thick, the MR ratio is strongly affected by unoxidized Al, probably due to the decrease in spin polarization at the surface of an Fe/Al electrode. On the other hand, the hysteresis loops indicate that the difference in coercive force between Fe and CoFe layers reduces with decreasing Al thickness for Al layers less than 2.5 nm thick. This means that the antiparallel direction of magnetization in the two layers becomes incomplete due to the gradual increase of the ferromagnetic coupling between them. As a result, the MR ratio decreases, although a 1-nm-thick Al layer seems to be enough to cover the Fe surface.
Anisotropic optical responses of sapphire (α-Al2O3) single crystals
Authors: Yao H. , Yan C. H.
Published: July 2002

Anisotropic dielectric functions of sapphire have been determined by both reflection and transmission variable angle spectroscopic ellipsometry. The measurements were made on a-plane (21̄1̄0) sapphire substrates in the energy range of 0.75–6.5 eV at room temperature. The orientation of the optic axis of the a-plane sapphire sample was determined by polarized Raman scattering based on which the Euler angles are set for the fitting model. Two pairs of Cauchy user-defined functions were constructed to describe the optical constants of both ordinary n⊥(ω) and extraordinary n∥(ω) rays, respectively. As a result the optical constants from the generalized ellipsometry analysis are in good agreement with the reported data. The Kramers–Kronig (KK) relation between the real and the imaginary part have been carefully checked for both ordinary and extraordinary rays. The perfect fitting between the calculated and experimental ε1(ω) function indicates that both functions of ordinary and extraordinary rays are KK consistent. The refractive index difference between the ordinary and extraordinary rays is close to a constant (+0.008), which is determined mainly by the off-diagonal signal of the transmission ellipsometry data. The extinction coefficients are zero below 6 eV, and increase rapidly above 6 eV.
A study on the radiation damage and recovery of neutron irradiated vessel steel using magnetic Barkhausen noise
Authors: Park Duck-Gun , Jeong Hee-Tae , Hong Jun-Hwa
Published: July 2002

The radiation damage and thermal recovery characteristic of neutron irradiated SA508-3 reactor pressure vessel steel specimens have been investigated. Two recovery stages were identified from the results of hardness measurements during isochronal annealing and the mechanism responsible for the two stages was explained by using the results of Barkhausen noise measurement on the basis of the interaction between radiation induced defects and the magnetic domain wall. The coercivity was not changed by neutron irradiation, whereas the maximum magnetic induction increased. Barkhausen noise parameters associated with the domain wall motion were decreased by neutron irradiation and recovered with subsequent heat treatments.
Convergent beam guided mode technique for use in liquid crystal studies
Authors: Smith N. J. , Sambles J. R.
Published: July 2002

The use of a convergent beam to excite guided modes in a liquid crystal layer in order to determine the director profile is described. The technique has two main advantages over the conventional collimated beam procedure. First, the convergent beam guided mode (CBGM) technique uses a highly focused beam spot that simultaneously excites many guided modes and produces reflectivity data over a wide angle range. Such reflectivity can be captured in milliseconds with a charge coupled device array and can therefore potentially be used to study cell dynamics. Second, the CBGM technique removes the need to rotate the liquid crystal cell and consequently the focused beam remains completely stationary on the liquid crystal layer, allowing examination of single pixels. The work presented here is a demonstration of the CBGM technique via a study of the static director profile of a homogeneously aligned nematic (E7) liquid crystal layer.
The significance of oxygen-deficient defects to the photosensitivity of hydrogen-loaded germano-silicate glass
Authors: Heaney A. D. , Erdogan T. , Borrelli N.
Published: July 2002

It is widely believed that oxygen-deficient defects in germano-silicate glass play a crucial role in the photosensitivity of this material to ultraviolet radiation. In this article, it is shown that germano-silicate glass that is defect-free and hydrogen-loaded also exhibits photosensitivity to 248 nm light. In fact, photosensitivity in defect-free germano-silicate glass is shown to require the presence of hydrogen. However, oxygen-deficient defects increase the photosensitivity of germano-silicate glass to light with a wavelength of 248 nm, in both the hydrogen-free and the hydrogen-loaded case. Comparisons are made among four types of otherwise identical germano-silicate glass. Each glass sample studied either does or does not contain oxygen-deficient defects and, each glass sample either does or does not contain hydrogen.
Temperature dependence of the electron affinity difference between Si and SiO2 in polysilicon (n+)–oxide–silicon (p) structures: Effect of the oxide thickness
Authors: Salace G. , Hadjadj A. , Petit C. , Jourdain M.
Published: July 2002

The variations with temperature of the Fowler–Nordheim (FN) emission in metal–oxide–semiconductor structures when the injecting electrode is the degenerate polysilicon gate (n+) are investigated. The temperature dependence of the electron affinity difference Φ between Si and SiO2 and of the barrier height Φb for three oxide thicknesses (5, 7, and 12 nm) are analyzed. The results are numerically derived from the exact integral expression of the FN current as functions of temperature varying from 25 to 300 °C. The variation with temperature of both the obtained Φ and dΦ/dT parameters at the polysilicon (n+)–oxide barrier are discussed with respect to the literature data.
Two-dimensional surface band structure of operating light emitting devices
Authors: Shikler R. , Meoded T. , Fried N. , Mishori B. , Rosenwaks Y.
Published: July 2002

We report on measurements of two-dimensional potential distribution with nanometer spatial resolution of operating light emitting diodes. By measuring the contact potential difference between an atomic force microscope tip and the cleaved surface of the light emitting diode, we were able to measure the device surface potential distribution. These measurements enable us to accurately locate the metallurgical junction of the light emitting device, and to measure the dependence of the built-in voltage on applied external bias. As the device is forward biased, the junction built-in voltage decreases up to flat band conditions, and then inverted. It is shown that the potential distribution across the pn junction is governed by self-absorption of the sub-bandgap diode emission.
Brillouin scattering from acoustic excitations in TiN films on high speed steel—A stiffening system
Authors: Pang W. , Every A. G. , Comins J. D. , Stoddart P. R. , Zhang X.
Published: July 2002

Brillouin scattering measurements are presented of surface acoustic waves in TiN films of various thicknesses on high speed steel. Because of its relatively high elastic moduli as compared with those of steel, TiN has a stiffening effect on the surface, causing the surface acoustic wave (SAW) to increase in velocity, merge into the bulk wave continuum, and become a pseudo-SAW. In the limit of large film thickness this pseudo-SAW evolves into the Rayleigh wave for TiN. A Green’s function method, invoking the surface ripple mechanism for the inelastic scattering of light, is used to calculate the Brillouin spectrum for scattering from these surface acoustic modes, and reveals details of the acoustic excitations of stiffening thin films not previously appreciated. A comparison between the measured and calculated dispersion relation for TiN thicknesses ranging from 20 to 4180 nm reveals that the elastic moduli of the thicker films are close to those of bulk TiN, but the effective elastic moduli of the thinner films are found to decrease with reducing film thickness. This conclusion is reinforced by backscattering measurements of Brillouin spectra at incident angles between 50° and 80° for a film thickness of 350 nm. Compositional variations at the interface have been investigated using x-ray photoelectron spectroscopy in an effort to understand this reduction in the elastic constants.
Analysis of high-resolution x-ray diffraction in semiconductor strained layers
Authors: Dunstan D. J. , Colson H. G. , Kimber A. C.
Published: July 2002

The composition and strain of pseudomorphic epitaxial layers is measured by high-resolution x-ray diffraction. Rocking curves or reciprocal space maps are measured from several reflections from sets of lattice planes, and their peak positions combined to yield the physical parameters of interest. We show here that the peak positions are best evaluated as the centres of gravity of the peaks. These data are then analyzed using least-squares multiple linear regression. This gives best estimates of the values and the errors of the physical parameters. The analysis also provides guidance as to the best sets of reflections to measure. Asymmetric reflections are preferable to symmetric reflections; but if symmetric reflections are used then the asymmetric reflections with high angle of incidence are more valuable than with low angle of incidence.
Coercivity extrema in melt-spun CuCo ribbons: Effects of the magnetic moment distribution
Authors: Ferrari E. F. , Nunes W. C. , Novak M. A.
Published: July 2002

Measurements of magnetization loops on melt-spun CuCo ribbons revealed a minimum in the temperature dependence of the coercivity. A coherent interpretation was given through Monte Carlo simulations of a dispersed system of noninteracting, uniaxial magnetic granules embedded in a nonmagnetic matrix. The coercivity is implicitly defined by the balance between the negative magnetization of superparamagnetic granules and the remaining magnetization of blocked granules after saturation in the positive field direction. When the temperature rises in a system made of a large amount of small granules and a small amount of big granules, unblocking predominates over thermal fluctuations and the coercivity decreases until a certain temperature at which most of the small granules are superparamagnetic; above this temperature, thermal fluctuations predominate, and the coercivity increases almost linearly with the temperature until the final unblocking of the big granules.
Stochastic model for magnetic hysteresis
Authors: Magni A. , Beatrice C. , Durin G. , Bertotti G.
Published: July 2002

A model of ferromagnetic hysteresis is presented, in which the different microscopic magnetization processes are reduced to the motion of a single domain wall in a random energy landscape. The equivalent pinning field acting on the domain wall is assumed to be a space-dependent stochastic process described by a set of coupled Langevin-type equations. The model is inspired by the one proposed by Néel for the description of hysteresis in the Rayleigh region, but it is more general, as it predicts both the static and the dynamic hysteresis loops, as well as their fine structure (Barkhausen effect). The model properties were investigated by computer simulations. In the low field limit, the Rayleigh law is verified, with coefficients depending on the stochastic properties of the pinning field, whereas at high fields the loop behavior is dominated by demagnetizing fields. The results obtained for the energy losses show that the separation of losses into the hysteresis and the dynamic components is a general property of this model. Furthermore, we numerically verified that in our model there is a complete decoupling of the dissipation effects due to the presence of the pinning field (inner disorder) and to the dynamics of the domain wall.
Neutron diffraction studies on (Ho1−xErx)2Fe15Ga2Cy compounds
Authors: Venkatesan M. , Varadaraju U. V. , Rao K. V. S. Rama , Malik S. K. , Luo H. , Yelon W. B.
Published: July 2002

Powder neutron diffraction studies were carried out on (Ho1−xErx)2Fe15Ga2Cy (x=0, 0.5, 1; y=0, 2) with a view to study the effect of substitution of Ga for Fe and the insertion of carbon at the interstitial sites on the structural and magnetic properties. All Ga substituted compounds (y=0) possess hexagonal Th2Ni17 type structure (space group: P63/mmc). However, the carbides adopt rhombohedral Th2Zn17 type structure (Space group: R3̄m). The unit cell volume increases with Ga substitution as well as with interstitial modification by carbon. An increase in the Curie temperature of about 260 K is observed for the compound HoErFe15Ga2C2 vis-a-vis HoErFe17. The increase is attributed to the preferential occupancy of Ga atoms in 18f(12j) and 18h(12k) sites and the presence of interstitial carbon. The magnetization increases with carbon addition. X-ray diffraction studies on magnetically aligned samples and neutron diffraction studies show that the easy magnetization direction lies in the basal plane at room temperature.
Microstructure, magnetoresistance, and magnetic properties of pulsed-laser-deposited external, internal, and mixed-doped lanthanum manganite films
Authors: Pietambaram Srinivas V. , Kumar D. , Singh Rajiv K. , Lee C. B. , Kaushik Vidya S.
Published: July 2002

In this article we report our studies on the microstructure, magnetoresistance (MR) behavior, and magnetic properties of the La1−x−yCaxMnO3 system in thin-film form. By varying the values of x and y in La1−x−yCaxMnO3, we have synthesized an external- (x=0.3, y=0), an internal- (x=0, y=0.3), and a mixed-doped (x=0.2, y=0.1) system with the same Mn3+/Mn4+ ratio. Thin films of these materials have been grown in situ on (100) LaAlO3 substrates using a pulsed-laser-deposition technique. Atomic force microscopy, x-ray diffraction and high-resolution transmission electron microscopy measurements carried out on these films have shown that the films are smooth, highly crystalline, and epitaxial on the (100) LaAlO3 substrates. Electrical resistance and magnetoresistance have been measured in the 10–300 K range in magnetic fields up to 5 T using a superconducting quantum interference device magnetometer. The MR ratios (calculated using the expression, [R0−RH]/RH, where R0 and RH are resistances in zero and applied fields) of the La0.7Ca0.3MnO3 (x=0.3, y=0), La0.7MnO3 (x=0, y=0.3), and La0.7Ca0.2MnO3 (x=0.2, y=0.1) films are found to be 825%, 700%, and 750% at 200, 240, and 220 K, respectively. The MR ratios of these films, calculated using the expression, [R0−RH]/RH, are 91%, 87%, and 88%, respectively, at the same temperatures. The variation in the insulator-to-metal transition and the MR ratio is attributed to internal chemical pressure and vacancy localization effects. Below Tc/2 (Tc is paramagnetic-to-ferromagnetic transition temperature), resistance increases as T2 for La0.7Ca0.3MnO3 and La0.7Ca0.2MnO3 while it increases as T5/2 for La0.7MnO3. The T2 and T5/2 dependence of resistance suggests that the transport is predominantly governed by an electron–electron scattering and a combination of electron–electron, electron–phonon, and electron–magnon scattering, respectively. High-temperature resistance has been observed to be consistent with small polaron hopping conductivity for all three systems. Magnetization measurements carried out on the films show that the films have reasonably square hysteresis loops with sharp Tc’s. Below Tc/2, the magnetization decreases as T2 for La0.7Ca0.2MnO3 and La0.7MnO3, suggesting single-particle excitations in them, while it decreases as T3/2 for La0.7Ca0.3MnO3, representing collective oscillations in this system.
Two true surface acoustic waves and other acoustic modes in (110) plane of Li2B4O7 substrate
Authors: Zhang Victor Y. , Lefebvre J. E. , Gryba T.
Published: July 2002

The surface acoustic waves (SAWs) and other acoustic modes propagating in the (110) plane of Li2B4O7 are investigated by means of the effective surface permittivity (ESP). It is demonstrated that the velocity of all piezoactive SAWs, both true and pseudo, as well as surface skimming bulk waves (SSBWs) can be numerically determined by computing the ESP as a function of acoustic trace slowness. A physical phenomenon not reported has been found for certain propagation directions, namely, simultaneous existence of two true SAWs, both being of the generalized Rayleigh type, together with a pseudo SAW of similar polarization. Propagation velocity, electromechanical coupling coefficient, and decay factor have been verified and confirmed by using two different sets of material constants and two numerical methods. The obtained values and accuracy of SAWs parameters are compared, and the validity conditions discussed. The generalized slowness diagram, plotted for the sagittal plane, enables to determine the total number of SSBW and to interpret the depth penetration properties of SAW. The Nyquist diagram of the ESP is shown to be a more helpful form for identifying a pseudo SAW and for distinguishing it from a SSBW.
Effects of Bi-Pt alloy on electrical characteristics of Pt/SrBi2Ta2O9/CeO2/Si ferroelectric gate structure
Authors: Kim Yong Tae , Shin Dong Suk , Park Young K. , Choi In-Hoon
Published: July 2002

Interface morphology and electrical properties of Pt/SrBi2Ta2O9(SBT)/CeO2/Si ferroelectric gate structure are characterized by considering the interactions among Bi, O, and Pt atoms during annealing process. It is found that the interfacial roughness of the Pt/SBT might be reduced during the annealing at 800 °C because the bottom side of the Pt electrode reacts with Bi atoms outdiffused from the SBT and the Bi–Pt alloys are molten at 765 °C, and the metallic Bi atoms are consumed by forming Bi oxide. Additionally, the capacitance and memory window of the ferroelectric gate structure annealed at 800 °C decrease to 69% and 80% of those values of the as-deposited gate structure, respectively, due to the additional capacitance and the voltage drop at the low dielectric Bi-oxide capacitor. In contrast, the leakage current characteristics are improved by two orders of magnitude after annealing at 800 °C for 30 min.
Site selectively excited luminescence and energy transfer of X1-Y2SiO5:Eu at nanometric scale
Authors: Yin M. , Duan C. , Zhang W. , Lou L. , Xia S. , Krupa J.-C.
Published: July 2002

Two different types of structure called X1 and X2 are existing in Y2SiO5 at normal conditions. In X1 type, Y3+ ions occupy two sites where they are surrounded respectively by nine and seven oxygen ions, while in X2 structure, only six and seven oxygen ions are involved. Nanometric scale X1-Y2SiO5 crystals were prepared by sol-gel method with particle size around 50 nm. Site selective excitation at low temperature has shown four different luminescent centers. Two of them belong to Eu3+ embedded in X1-Y2SiO5, the other two are attributed to Y2O3:Eu phase and to a particular site on the surface. The occurrence of the latter site may be related to the nanometric size of the sample. A pronounced excitation energy transfer from site 2 to site 1 was also observed on excitation spectra. The energy transfer rate increases rapidly with increasing Eu3+ concentration and for x=0.7 in Y2−xSiO5:Eux, no fluorescence takes place in site 2 at 15 K. The lifetimes of the D05 levels of Eu3+ in the two sites were measured as a function of Eu3+ concentration. The results have shown that the lifetime of the D05 level of Eu3+ in site 2 decreases with increasing Eu3+ concentration. The energy transfer rate dependence upon temperature was studied in detail and compared to a theoretical simulation.
Collective interdiffusion in compositionally modulated multilayers
Authors: Wang Wei-Hua , Bai Hai Yang , Wang W. K.
Published: July 2002

Interdiffusion in Fe–Ti, Ag–Bi, Fe–Mo, Ni–Si, Mo–Si, Nb–Si, and Ag–Si multilayers (MLs) was investigated by an in situ low-angle x-ray diffraction technique. Temperature-dependent interdiffusivities were obtained which can be described as Arrhenius relations. The interdiffusion characteristics of the MLs were summarized. The extremely small values of the prefactor D0 and the marked correlation between the D0 and activation energy He for interdiffusion suggest that a collective atomic jumping mechanism involving 8–15 atoms govern the interdiffusion in the MLs.
Preparation of highly overstoichiometric LaNi5+x (1⩽x⩽4) single-phase films by ion beam sputtering
Authors: Cuevas F. , Hirscher M. , Ludescher B. , Kronmüller H.
Published: July 2002

Single-phase LaNi6.1 and LaNi9.2 films with hexagonal TbCu7 structure have been successfully obtained. Films in the micron scale range were deposited by ion beam sputtering on sapphire and glass substrates heated at 600 K. A two-material La–Ni mosaic target was used for this purpose. X-ray diffraction and transmission electron microscopy investigations show that the lattice parameters of the hexagonal structure follow a linear dependence for Ni concentration. This result relates to a random substitution of La atoms for Ni dumbbell pairs in the ordered CaCu5 structure. The films exhibit a strong preferential orientation, with the c axis parallel to the substrate plane, which seems to be favored by the presence of Ni dumbbell pairs. The expected impact of this phase on the preparation of new hydrogen storage materials is discussed.
Molecular dynamics simulation of silicon surface smoothing by low-energy argon cluster impact
Authors: Kim Chang-Koo , Kubota Alison , Economou Demetre J.
Published: July 2002

The molecular dynamics simulation method was employed to study the mechanism of silicon (001) surface smoothing by impact of Ar16 or Ar40 clusters with energy at or below 20 eV per constituent atom. Smoothing of a pyramid on top of an otherwise “flat” silicon surface was used as a model system to elucidate the mechanism of cluster-substrate interaction. Surface smoothing is achieved by lateral displacement of substrate atoms during cluster impact. There exists an optimum energy of around 4–5 eV per constituent atom of the cluster for efficient surface smoothing; this implies that a proper energy is required for effective lateral displacement. Cluster size also affects surface smoothing because lateral displacement depends on the nonlinear effect of multiple collisions in the near surface region. As anticipated, damage in the substrate increases with cluster energy.
Real-time study of oxygen in c-axis oriented YBa2Cu3O7−δ thin films using in situ spectroscopic ellipsometry
Authors: Gao Y. , Mueller A. H. , Irene E. A. , Auciello O. , Krauss A. R. , Schultz J. A.
Published: July 2002

The oxygen transport process in c-axis oriented YBa2Cu3O7−δ thin films was investigated in real time during deposition and postdeposition oxidation using in situ spectroscopic ellipsometry (SE). Two transport regimes dominated by oxygen out- and in-diffusion have been observed during deposition by ion beam sputtering at 700 °C. The effect of oxygen partial pressure on the extent of oxidation of as-deposited films has also been studied during the postdeposition cooling process. The thermodynamic stability of the grown films was examined by real time SE during the postannealing process. The relaxation time for oxygen diffusion has been found to decrease with higher oxygen partial pressures. The imaginary part of the pseudodielectric function 〈ε2〉 at an absorption peak was quantitatively correlated to the oxygen concentration. The pseudodielectric functions of oxygen deficient YBa2Cu3O6 films in the temperature range from 27 to 700 °C have been modeled using Lorentz oscillators.
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reactive gas effect on rf magnetron sputtered Pb(Zr0.52Ti0.48)O3 thin films
Authors: Kim Tae Song , Kim Dong Joo , Jung Hyung Jin
Published: July 2002

In depositing Pb(Zr0.52Ti0.48)O3 thin films by using rf magnetron sputtering process, N2O gas was used instead of oxygen as a reactive gas. In order to investigate an effect of this N2O gas on the electrical and structural properties of the thin films, a various range of thicknesses from 1000 to 4000 Å was deposited on the Pt/Ti/Si(100) substrate at a substrate temperature of 520 °C, and then annealed in the range of 500–700 °C for enhancement of the crystallinity. The ratio of Ar and O2(N2O) gas was 9:1. There were no apparent differences in crystallographic orientation between N2O and oxygen as reactive gases. However, the denser films were fabricated by using N2O gas, and the electrical properties, i.e., remanent polarization, leakage currents and tan δ  values were improved in the thinner films (1000 Å).
Polarization dependence of electroluminescence in Sb-based mid-infrared interband cascade emitters
Authors: Dupont E. , Liu H. C. , Yang Rui Q.
Published: July 2002

We report an experimental investigation of polarization dependence of mid-infrared electroluminescence from type-II InAs/GaInSb interband cascade devices. The in-plane polarized emission was found to be dominant from a laser structure, supporting the assertion based on the conventional k⋅p theory. The observed electroluminescences from the light-emitting diodes were found to be nearly independent of polarization. These experimental results have been discussed along with theoretical calculations based the conventional k⋅p theory.
Spectroellipsometric characterization of plasma-deposited Au/SiO2 nanocomposite films
Authors: Dalacu Dan , Martinu Ludvik
Published: July 2002

Nanocomposite films consisting of metal nanoparticles embedded in a dielectric matrix were fabricated by simultaneous sputtering of a gold target and plasma-enhanced chemical vapor deposition of hydrogenated SiO2. The optical constants of the films were determined from spectroscopic ellipsometry measurements and were modeled using the Maxwell–Garnett effective medium theory. The particle size dependence of the free electron absorption was included according to the limited electron mean free path effect using a broadening parameter A=0.16 determined from the comparison of the measured spectra with transmission electron microscopy micrographs. Using bulk interband optical constants for gold, very good agreement was obtained between the model and the measured spectra but only in the narrow particle size range ∼10–20 nm, the latter of which marks the onset of phase retardation effects. For smaller particles, the energy of the surface plasmon resonance was progressively blueshifted with respect to the predicted value. This was interpreted by a size dependence of the interband transitions as a result of strain-induced variation of the lattice constant within the particle.
Effects of rapid thermal annealing on the optical properties of GaNxAs1−x/GaAs single quantum well structure grown by molecular beam epitaxy
Authors: Li L. H. , Pan Z. , Zhang W. , Lin Y. W. , Zhou Z. Q. , Wu R. H.
Published: July 2002

The effect of rapid thermal annealing (RTA) on the optical properties of GaNxAs1−x/GaAs strained single quantum well (SQW) was studied by low-temperature photoluminescence (PL). The GaNxAs1−x/GaAs SQW structures were prepared by dc active nitrogen plasma assisted molecular beam epitaxy. PL measurements on a series of samples with different well widths and nitrogen compositions were used to evaluate the effects of RTA. The annealing temperature and time were varied from 650 to 850 °C and 30 s to 15 min, respectively. Remarkable improvements of the optical properties of the samples were observed after RTA under optimum conditions. The interdiffusion constants have been calculated by taking into account error function diffusion and solving the Schrodinger equation. The estimated interdiffusion constants D are 10−17–10−16 cm2/s for the earlier annealing conditions. Activation energies of 6–7 eV are obtained by fitting the temperature dependence of the interdiffusion constants.
Approximate description of the three dimensional director and electric field in a liquid crystal display at a high voltage
Authors: Panasyuk G. , Allender D. W.
Published: July 2002

An approximate analytical approach for describing properties of a liquid crystal display associated with a homeotropic to multidomainlike transition for a liquid crystal with a positive dielectric anisotropy was developed. The electrode and surface coating of the display are prepared in such a way that in the absence of an electric field (dark state) the liquid crystal alignment is homeotropic. The bright state corresponds to a situation when a high voltage is applied to the interdigitated electrodes located on both substrates, which causes a director deformation of a multidomain type. Approximate solutions for the three dimensional electric field and director configuration in the case of strong homeotropic anchoring were obtained to describe the bright state. It was found that two different defect structures are possible for the director configuration: one corresponds to a wall defect and the other to two disclination lines. A first order phase transition between the two structures is predicted at a particular value of the voltage. An estimate of this critical voltage is obtained.
Influence of charged defects on the dielectric response of incipient ferroelectrics
Authors: Vendik Orest G. , Ter-Martirosyan Leon T.
Published: July 2002

Charged grain boundaries between crystalline blocks in bulk and thin film samples of incipient ferroelectrics are considered as charged defects producing a built-in electric field which is treated as a statistical dispersion of the biasing field. The normalized statistic dispersion of a biasing field, denoted as ξS, is used as one of the parameters of a phenomenological model of the dielectric response of incipient ferroelectrics. The surface charge density on the grain boundary is quantitatively estimated on the basis of a multiple-scattering analysis of spatially resolved electron-energy-loss spectra of the boundary between crystalline blocks in SrTiO3. The built-in electric field in the incipient ferroelectrics is quantitatively estimated by the parameter ξS which determines some specific features of the dielectric response of the incipient ferroelectric single crystal and thin film samples.
Internal friction and Young’s modulus of SrBi2Ta2O9 ceramics
Authors: Yan Feng , Chen Xiaobing , Bao Peng , Wang Yening , Liu Jianshe
Published: July 2002

The internal friction (IF) and the Young’s modulus of SrBi2Ta2O9 (SBT) ceramics were measured by using the reed vibration method in the temperature range from 100 to 600 K with kilo-hertz frequencies. A high IF peak associated with a modulus defect appeared around 500 K, which was assumed to be due to the migration of oxygen vacancies with the activation energy U of about 0.95 eV. The mechanism of the IF peak was discussed in detail. At 570 K, an IF peak due to the viscous motion of domain walls near the Curie temperature was found. Below room temperature, a low IF peak with a modulus defect was found with peak temperature of 200–250 K, which varied for tens of degrees in different samples. This peak was due to the depinning process of 90° domain walls from oxygen vacancies. These results can be helpful in explaining the excellent fatigue resistance property of SBT thin films at room temperature.
Molecular diffusion in periodic porous media
Authors: Hizi Uzi , Bergman David J.
Published: July 2002

The diffusion of fluid through a porous medium was modeled for a periodic array. The time dependent bulk effective diffusion coefficient D(t) was calculated using Bloch type eigenfunctions. The calculations were made for periodic media composed of octahedron-like obstacles which ranged from completely separated to highly overlapping obstacles (porosity of 3%). The calculation of D(t) involves evaluation of the nuclear magnetic resonance pulsed field gradient spin echo amplitude M(k,t), which is a measurable quantity. The method, which is most effective for long times, produces results which also agree with short time asymptotics. This method is applicable for any periodic microgeometry and virtually any porosity.
Laser-induced thermal effects on the optical properties of free-standing porous silicon films
Authors: Koyama Hideki , Fauchet Philippe M.
Published: July 2002

A detailed study of the unique optical properties of free-standing oxidized porous silicon films has been performed. Under continuous-wave laser irradiation, a strongly superlinear light emission and a very large laser-induced absorption are observed. The nonlinear emission is characterized by a sharp intensity increase that is in proportion to at least the eighth power of the excitation intensity. This emission has a broad peak (600–1300 nm), slow time constant (⩾10 ms), and very low polarization memory (⩽0.01 near the emission peak). The induced absorption increases linearly with the pump laser intensity and can be as large as several times the linear absorption. The increase in the normalized absorption coefficient is almost independent of the oxidation temperature and emission wavelength, with essentially no polarization dependence. These experimental results are discussed in terms of laser-induced thermal effects. An evaluation of the temperature rise under the laser irradiation is performed both experimentally and theoretically. A remarkably high-temperature rise of ⩾700 °C is estimated for a moderate excitation intensity of 20 W/cm2.
Interference effects in anisotropic optoelectronic devices
Authors: Wan W. M. V. , Greenham N. C. , Friend R. H.
Published: July 2002

In this article, we develop a model which calculates the effects of optical interference in polymer devices. In this model, we include the effects of optical birefringence in the layers making up the devices, as well as the variation of quantum efficiency across the active layer. We also extend the plane wave emission modification to relate to the emission power measured outside the device. Experimental measurements of photoluminescence emission from poly(p-phenylene vinylene) (PPV) devices were obtained. Comparison between the experimental and calculated angular distribution of emission is made, and good agreement is found. This illustrates that the model is sufficient and that optical birefringence must be taken into account. Electroluminescence emission from poly(p-phenylene vinylene) PPV devices was also measured. By comparing with the calculated spectra from our model, we demonstrate that the model can be used as a tool to locate the position of the recombination zones in electroluminescent devices.
Multiple scattering effects of thermal waves by two subsurface cylinders
Authors: Terrón J. M. , Sánchez-Lavega A. , Salazar A.
Published: July 2002

We present a theoretical model and experimental measurements of the temperature field produced by the multiple scattering of thermal waves between two close subsurface cylinders and the material surface. The thermal waves are excited at the surface of an opaque material by an extended modulated optical beam. The cylinders are taken as thermal insulating holes and the model is run for a variety of modulation frequencies, thermal properties of the sample, and geometric properties of the cylinders (sizes, depths, mutual separation). The photothermal signal obtained using an infrared radiometry technique on calibrated samples validate our model predictions. The model is used to address the question of the spatial resolving power in photothermal wave experiments and introduce a novel resolution criterion.
Origin of the magnetic moments in La0.65Pb0.35MnO3 epitaxial thin films
Authors: Borca C. N. , Cheng R. H. , Xu Q. L. , Liou S. H. , Stadler Shane , Idzerda Y. U. , Dowben P. A.
Published: July 2002

Crystalline films of La0.65Pb0.35MnO3 grown on (100) LaAlO 3 substrates by rf sputtering have been investigated using magnetic circular dichroism and inverse photoemission spectroscopy. We find evidence for strong hybridization between unoccupied levels associated with Mn 3d and O 2p states. The oxygen atoms “pick-up” a small magnetic moment through hybridization with Mn.
Finite size effects in patterned magnetic permalloy films
Authors: Gubbiotti G. , Albini L. , Carlotti G. , De Crescenzi M. , Di Fabrizio E. , Gerardino A. , Donzelli O. , Nizzoli F. , Koo H. , Gomez R. D.
Published: July 2002

Electron beam lithography has been used to prepare 250-Å-thick square-shaped permalloy dots whose width and separation are in the range between 1 and 4 μm. The static and dynamic magnetic properties of these structures have been investigated by complementary techniques such as magneto-optic Kerr effect (MOKE), magnetic force microscopy (MFM), and Brillouin light scattering (BLS). Longitudinal MOKE enabled us to compare the hysteresis loops of the specimens with different dot size and interdot separation, showing a marked influence of the demagnetizing field inside the dots. MFM images recorded at zero applied field showed that, depending on the interdot spacing, there is a prevalence of either four- or seven-domain patterns together with a minority of nonsolenoidal patterns that possess a net magnetic moment. BLS from thermally activated spin waves were then used to determine the intrinsic magnetic parameters of the permalloy films and to show evidence of the discretization of the spin-wave peak due to the wave vector resonance condition within each dot.
The effect of field cooling and field orientation on the martensitic phase transformation in a Ni2MnGa single crystal
Authors: Chu S.-Y. , Cramb A. , De Graef M. , Laughlin D. , McHenry M. E.
Published: July 2002

The temperature and field dependence of the magnetization in a Ni2MnGa single crystal was measured using a magnetometer with an applied field oriented in the [001] and [011] directions, respectively, of the parent cubic phase. It was found that the magnetic field magnitude and direction could be used to determine the magnetization of the sample during a thermal transformation from the austenitic phase to the martensite phase. This is explained in terms of a magnetic field induced growth of the twin variant having a favorable orientation to the external magnetic field. A model to interpret the magnetic response in terms of aligned twin variants in the shape memory material is discussed.
Ferromagnetic resonance frequency of a thin Mo-permalloy film
Authors: Klemmer T. J. , Ellis K. A. , van Dover B.
Published: July 2002

There is great interest both in the magnetic recording and wireless phone industries in the development of magnetic films with permeabilities that reach out into the GHz frequencies. In order to obtain such films the understanding of the frequency response of the ferromagnetic resonance of basic materials is needed. In this study the permeability of a thin film of Mo permalloy is measured up to 3 GHz utilizing a strip loop device. The effect of an externally applied field (Hdc) along the easy axis and hard axis on the transverse bias permeability as a function of frequency is measured through resonance. The full width at half maximum (Δf ) of the imaginary part of the permeability at resonance which is related to resonance damping is also measured. It is found that in the vicinity of Hdc-Hk near zero, Δf increases with decreasing dc field. This phenomenon is explained in terms of the Hoffmann ripple theory.
Magnetic moment distributions in laser-ablated metallic glass thin films
Authors: Sorescu Monica
Published: July 2002

Using amorphous Fe81B13.5Si3.5C2 as a model system, we apply the laser-Mössbauer method in order to obtain the properties of metallic glasses. As shown by transmission Mössbauer spectroscopy, the bulk magnetic texture was modified in a rational manner by applying a varying number of laser pulses per spot. This process left the surface magnetic texture unchanged and induced a distribution of magnetic-moment directions through the thickness of the foil. When thin films of amorphous Fe81B13.5Si3.5C2 were obtained by pulsed-laser deposition and characterized by conversion electron Mössbauer spectroscopy, the hyperfine magnetic-field distributions exhibited new shapes as a function of laser fluence. Moreover, the surface magnetic texture was found to be controlled by the values of the laser fluence employed.
Mechanism of field emission from chemical vapor deposited undoped polycrystalline diamond films
Authors: Shim Jae Yeob , Baik Hong Koo , Song Kie Moon
Published: July 2002

Field emission characteristics of undoped polycrystalline diamond films with different structural properties have been investigated. By introducing positive bias voltage and/or increasing CH4/H2 ratio, the film quality is significantly deteriorated together with the increase of nondiamond carbon component and the surface morphologies of the films lost their unique facet shape. The reason for the increase of nondiamond carbon content is described in terms of both the increase of substrate temperature and the excessive generation of CHn radicals. It is confirmed that an increase in the nondiamond carbon content markedly enhances field emission properties of diamond films. From the spatial distribution of emission sites, it is suggested that the transport path of field-emitted electrons depends on the nondiamond carbon content: for the film with a large amount of nondiamond carbon, electrons transport preferentially through the conducting mediums such as grain boundaries while for the film with a relatively small amount of nondiamond carbon, electron transport occurs mainly through the diamond surface.
Metal ion implantation using a filtered cathodic vacuum arc
Authors: Bilek M. M. M. , Evans P. , Mckenzie D. R. , McCulloch D. G. , Zreiqat H. , Howlett C. R.
Published: July 2002

When plasma immersion ion implantation is performed in the condensable plasma stream produced by a cathodic vacuum arc, deposition as well as implantation usually occurs. In this article we describe a method of achieving pure implantation by orienting the substrate so that it is shadowed from the plasma beam. Implantation depth profiles measured in glassy carbon and CR39 polymer using Rutherford backscattering are compared to illustrate the effectiveness of the technique for conducting and insulating substrates. Charging of the insulating substrate was found to cause a reduction in implantation depth compared to a conducting substrate. The depth profiles in glassy carbon were comparable to those achieved by conventional extracted ion beam implantation. Implantation of magnesium into hydroxyapatite and alumina was carried out to improve the bone cell adhesion onto these materials for prosthetic applications.
The effect of interface on the electrical properties of (Ba, Sr)TiO3 adopting the perovskite electrodes
Authors: Kim Boum-Seock , Oh Se-Hoon , Son Seung-Young , Park Kyung-Woong , Choi Duck-Kyun , Dai Z. R. , Ohuchi Fumio S.
Published: July 2002

Perovskite type conducting materials (Ca, Sr)RuO3 and (Ba, Sr)RuO3 were prepared by rf magnetron sputtering as bottom electrodes for (Ba, Sr)TiO3 thin film capacitors. The crystallinity, interface property, and electrical properties of the (Ba, Sr)TiO3 capacitor applying the perovskite oxide electrodes were investigated. The interface between the dielectric and the electrode were analyzed by high-resolution electron microscopy and Auger electron spectroscopy. The interface had a marked effect on the electrical properties of the (Ba, Sr)TiO3 thin films. Dielectric constant for the (Ba, Sr)TiO3 had considerable dependence on the thickness of interfacial layer. The leakage current density of the capacitor was sensitive not only to the structural match but also to the chemical match between (Ba, Sr)TiO3 and the electrode.
Exchange biasing in sputtered NiFe/PtMn bilayers
Authors: Xi Haiwen , Bian Bo , Laughlin David E. , White Robert M.
Published: July 2002

The exchange coupling between ferromagnetic Ni81Fe19 and antiferromagnetic (AF) PtxMn1−x films prepared by rf and dc magnetron sputtering has been investigated. The Pt content in the PtxMn1−x film is in the range of 0 at. %<x<20 at. %. The exchange field and coercivity were found to depend strongly on the deposition conditions and the AF film composition. X-ray diffraction measurements and transmission electron microscopy measurement showed a γ-PtMn phase with a disordered fcc structure when the PtxMn1−x was deposited on top of the Ni81Fe19 layer. The exchange bias was found to depend on the texture and film composition of the γ-PtMn layers.
Experimental study of interactions in the nanostructured Ni pillar arrays
Authors: Bae Jungmok , Kim Sang-gook , Mondol Mark , Farhoud Maya , Hwang Minha , Youcef-Toumi Kamal
Published: July 2002

In this work, the experimental way of measuring the interaction fields in nanoscale single domain magnet arrays is presented. This method uses a magnetic force microscopy (MFM) and an in situ electromagnet. The experiments were performed on the arrays of nanoscale nickel pillars with a period, ranging from 200 to 300 nm and a diameter of 90 nm. The up and down switching fields of the pillar were measured by applying a known field to the target pillar and raising the field until the pillar reverses the magnetization state. We used a scheme in which the MFM tip stray field and the bias field from the electromagnet are used to magnetize the individual pillars. The exact strength of the field concentrated near the tip was found experimentally by comparing the switching fields measured with and without the tip in contact with the pillars, having different switching fields. The numerical results were compared with the experimental data.
Effects of the substitution of Cu or Nb for Fe on the structure and magnetic properties of Nd8Fe84Ti8 alloys prepared by mechanical alloying
Authors: Cui B. Z. , Sun X. K. , Liu W. , Geng D. Y. , Zhao X. G. , Zhang Z. D.
Published: July 2002

The effects of a partial substitution of Cu or Nb for Fe on the structure and magnetic properties of mechanically alloyed Nd8Fe84Ti8 alloys were studied. Compared with Nd8Fe84Ti8 alloys prepared under the same conditions, the temperature necessary for the formation of the Nd(Fe, Ti)12 phase decreases from 900 to 750 °C with the addition of 3 at. % Cu. The addition of Cu hinders the nitrogenation of the Nd(Fe, Ti)12 phase. The pretreatment of hydrogenation on the Cu-added alloys at 300 °C facilitates the formation of nitrides. With increasing annealing temperature, the intrinsic coercivity of the nitrides increases until it attains a maximum at 850 °C. The addition of Nb is harmful to the formation of the perfect Nd(Fe, Ti)12 phase. For the 3 at. % Nb-containing alloys, annealing at 960 °C makes a metastable phase Nd(Fe, Ti)7 be formed. The addition of Nb is unfavorable for the intrinsic coercivity and the maximum magnetic energy product of the nitrides.