bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear dynamics of nuclear–electronic spin processes in ferromagnets
Authors: Yukalov V. I. , Cottam M. G. , Singh M. R.
Published: July 2002

Spin dynamics is considered in ferromagnets as consisting of electron and nuclear subsystems interacting with each other through hyperfine forces. In addition, the ferromagnetic sample is coupled with a resonance electric circuit. Under these conditions, spin relaxation from a strongly nonequilibrium initial state displays several peculiarities that are absent in the standard setup for studying spin relaxation. The main feature of the nonlinear spin dynamics considered in this article is the appearance of ultrafast coherent relaxation, with characteristic relaxation times several orders shorter than the transverse relaxation time T2. This type of coherent spin relaxation can be used for extracting additional information on the intrinsic properties of ferromagnetic materials and can also be employed for different technical applications.
Magnetotransport and domain structures in nanoscale NiFe/Cu/Co spin valve
Authors: Kong L. , Pan Q. , Cui B. , Li M. , Chou S. Y.
Published: July 2002

Nanoscale spin-valve structures with a width as small as 70 nm were fabricated using nanoimprint lithography and ion milling or lift off. The spin-valve multilayers consisting of NiFe(10 nm)/Co(1 nm)/Cu(13 nm)/Co(10 nm)/NiFe(2 nm) were deposited using direct current sputtering. The effects of device size, as well as fabrication process on domain structures, switching fields, switching field variation, and giant magnetoresistive ratio were investigated using scanning electron microscopy, atomic force microscopy, magnetic force microscopy, and magnetoresistance measurements.
Fabrication and magnetic force microscopy study of isolated nanoscale nickel balls
Authors: Xu Hai , Li Dan , Shan Han Bao , Wang Fangwei , Fan Shou-shan , Li Qun-qing , Liang Wen-jie , Dang Hai-yan
Published: July 2002

With the carbon nanotube nets being chosen as a substrate, the nanoscale nickel balls have been fabricated by using electroless plating method. In this article, a sample with well dispersed nanoscale nickel balls was prepared by ultrasonically dispersing them in PVK polymer solution and spraying them on a new cleavage of mica. Their images of topography and micromagnetic structure were obtained by transmission electron microscopy, and magnetic force microscopy with the magnetic tips coated with cobalt film. The balls with the diameter ranging from 200 to 400 nm present the same type of magnetic force image pattern, indicating that they are all single crystals with a single domain. Computer simulation was carried out by integrating the interactions between the tip and the balls. The simulated images are consistent with the real patterns.
Kerr effect observations of magnetization reversal process in antiferromagnetically pinned permalloy thin films
Authors: Qian Zhenghong , Kief Mark T. , George Peter K. , Sivertsen John M. , Judy Jack H.
Published: July 2002

In this article, the magnetization reversal process for antiferromagnetic (AFM) materials (NiO, FeMn, and NiMn) coupled to NiFe permalloy thin films have been investigated. The nature of the effect of exchange coupling on the reversal of the AFM-pinned permalloy was observed using the magneto-optic Kerr effect. These studies indicated that the reversal process appears as widespread nucleation of 1–10 μm sized reversal domains over the entire area of the film with subsequent domain expansion and coalescence. Interpretation of the magnetization reversal process requires an understanding of the exchange coupling mechanism. It is believed that the magnetization reversal process observed here is dominated by a nucleation-rotation mechanism in which the interfacial spin system is highly uniform. The exact nature of the spin reversal nucleation process is not fully understood but appears to arise from weak localized pinning states.
H–T
diagrams of Ln1−xCaxMnO3 (x=1/2, 1/3) in pulsed fields up to 50 T
Authors: Garcı́a-Muñoz J. L. , Respaud M. , Frontera C. , Llobet A. , Broto J. M. , Rakoto H. , Goiran C.
Published: July 2002

Well characterized Ln1−xCaxMnO3 manganites (Ln=La, Pr, Nd, Sm) with x=1/2 and x=1/3 have been investigated by magnetization measurements in pulsed magnetic fields up to 50 T. Using such high fields all the CO phase boundaries were identified in the temperature range 4–300 K. The transition fields at 4.2 K have been determined in a series of polycrystalline samples with x=1/2 and different effective bandwidths. They vary from Hc=8 T for La to ≈30 T for Sm. In this article we have focused on selected H–T phase diagrams. In particular, the results on Pr1/2Ca1/2MnO3, with the ideal 1:1 electron/hole ratio, are compared with Pr2/3Ca1/3MnO3 where the carrier concentration has been significantly reduced. The observed anomalies in the Hc(T) curves are discussed in the light on neutron diffraction data.
Imprinting artificial magnetic structures (invited)
Authors: Lohstroh W. , Münzenberg M. , Felsch W. , Fritzsche H. , Maletta H. , Goyette R. , Felcher G. P.
Published: July 2002

Recently we created La/Fe multilayers with a helical magnetic structure imprinted from the conditions of growth rather than by the magnetic interactions between layers. Each sublayer was 30 Å thick, and during deposition the sample was rotated in an external field of 3 Oe, a field strong enough to magnetize the Fe layer being deposited but not sufficient to perturb the magnetization of the Fe layers already grown. As a result adjacent Fe layers formed a helical structure with a chirality and periodicity determined by the rotational direction and speed of the substrate and the rate of deposition. Following this discovery an extensive set of experiments (mainly using Kerr effect magnetometry and polarized neutron reflectivity) was undertaken to ascertain the stability of imprinted magnetic structures, and to understand the onset of magnetization during growth. La/Fe imprinted helical magnetic structures (of different La and Fe thicknesses) were found to be stable in time and to be permanently erased only by magnetic fields larger than 90 Oe.
Impurity model for Mn ions: Possibility of charge order and lattice distortions
Authors: Schlottmann P.
Published: July 2002

Mixed valent manganese ions play an important role in the magnetoresistance of LaMnO3-based systems. We consider a Mn impurity with the Mn4+ represented by a spin S=3/2 (three localized 3d-t2g electrons) and the Mn3+ configuration having an additional localized 3d-eg electron to form a total spin (S+1/2). The eg electron hybridizes with conduction electrons (representing the crystal), giving rise to a quadrupolar Kondo effect. The formation of the quadrupolar singlet interferes with the usual spin Kondo effect. Using the noncrossing diagram approximation, we calculate the ground state energy, valence, charge susceptibility, and the quadrupolar susceptibility as a function of the energy of the eg level for H→0 and compare it to results for H=0 and the ferromagnetic lattice. Both, the charge and quadrupolar susceptibilities, are larger in the spin-polarized limit, indicating that this state is more favorable for charge order and lattice distortions.
Hydrostatic pressure effect in La1−xCaxMnO3
Authors: Liu J. W. , Zheng Z. , Zheng Q. Q. , Lin H. Q. , Wong H. K.
Published: July 2002

The pressure effect is studied for the doped compound La1−xCaxMnO3 (x=1/3) using the first-principles spin-polarized discrete variational method, in the framework of density functional theory with local density approximation. The pressure dependence of both effective electron hopping integral t̃ and Hund’s coupling strength JH is investigated systematically, which is essential to explain the qualitative relations between the pressure and properties such as colossal magnetoresistance and the Curie temperature Tc of this material.
Polarized neutron diffraction study of the extended honeycomb molecular network d20−P(C6D5)4MnFe(C2O4)3
Authors: Visser D. , Carling S. G. , Watts I. D. , Day P. , Andersen K. H.
Published: July 2002

The MII and MIII magnetic ions in the extended molecular network P(C6D5)4MnFe(C2O4)3 form a two-dimensional honeycomb magnetic lattice. The Mn2+ and Fe3+ ions alternate in the extended network which is formed by the oxalate (C2O4) ligands. These hexagonal layers are separated and charge compensated by large [P(C6D5)4]+ ions, positioned in between the honeycomb layers. P(C6D5)4MnFe(C2O4)3 orders magnetically at TN=27(1) K. A full neutron spin polarization study of the neutron scattering cross section has been carried out which allows the unambigious separation of the magnetic cross section from the total diffraction process. The magnetic structure can be described with the magnetic Shubnikov group R3c. The magnetic moments are antiferromagnetically aligned along the c axis while the Mn2+ and Fe3+ ions form an antiferromagnetic alignment on the honeycomb lattice.
Effects of background n- and p-type doping on Zn diffusion in GaAs/AlGaAs multiple-quantum-well structures
Authors: Ky Nguyen Hong , Reinhart F. K. , Ganière J. D. , Deveaud B. , Blanchard B.
Published: July 2002

The effects of background n- and p-type doping on Zn diffusion in GaAs/AlGaAs multilayered structures are investigated by secondary-ion-mass spectrometry and photoluminescence measurements. Zn diffusions are performed at 575 °C into Si-doped, Be-doped, and Si/Be-codoped identical GaAs/Al0.2Ga0.8As multiple-quantum-well structures. The results obtained by secondary-ion-mass spectrometry show that the Zn diffusion induces an enhancement of Be out-diffusion and the disordering of all structures. The effective Zn diffusivity and the disordering rate are increased by Be doping and reduced by Si doping. Photoluminescence measurements give information about the reactions of different point defects during the diffusion process. Before Zn diffusion, the Si-doped structures contain a high concentration of column-III vacancies, whereas As vacancies are the dominant defects in the Be-doped structures. After Zn diffusion, we observe a reduction of column-III vacancy concentration in Si-doped structures and an increase of column-III interstitial concentration in Be-doped structures. A model based on the “kick-out” mechanism of Zn diffusion is proposed to explain our observations. The supersaturation of column-III interstitials behind the Zn diffusion front is responsible for the enhancements of Al–Ga interdiffusion and Be out-diffusion. The effective Zn diffusivity is controlled by the background donor or acceptor concentration ahead of the Zn diffusion front and by the concentration of column-III interstitials behind the Zn diffusion front. For Be-doped structures, the increase in the background acceptor concentration and the supersaturation of column-III interstitials in the Zn-diffused region results in an enhancement of the Zn diffusivity. For Si-doped structures, the effective Zn diffusivity decreases with increasing background donor concentration. Moreover, the concentrations of column-III interstitials and column-III vacancies in the Zn-diffused region are reduced due to their mutual annihilation, leading to a retardation of Zn diffusion.
Positron annihilation spectroscopy of sandstone and carbonate rocks
Authors: Urban-Klaehn J. M. , Quarles C. A.
Published: July 2002

The techniques of positron annihilation spectroscopy have been applied to the study of well-characterized reservoir rock samples. In this article we focus primarily on sandstone and carbonate samples. We report (a) measurements of the Doppler broadening (DB) parameters and mean lifetime values (〈τ〉) of dry samples; (b) measurement of the DB parameters as a function of temperature from room temperature to 200 °C; and (c) observation of the outgassing of heated samples simultaneously with the DB measurements. We interpret the measurements in terms of positron annihilation with both valence and core electrons and discuss how the DB parameters depend on the major chemical constituents (lithology) and to a lesser degree on the structure of the samples. The major result of this work is that the positron parameters such DB parameters (S, SW, and W) and mean lifetime values are shown to be reproducible and reliable bulk properties of well-characterized rock samples and can be used to categorize rocks of geophysical interest in the same way as the more familiar bulk properties such as density, porosity, resistivity, etc. These results provide the first set of data for and a physical description of some of the processes involved in the interaction of positrons with rocks. This research provides a basis for further laboratory studies and is necessary for the eventual development of a new nuclear well-logging tool based on Doppler broadening of the positron annihilation gamma ray.
Growth and optical characterization of indirect-gap AlxGa1−xAs alloys
Authors: Purón E. , Martı́nez-Criado G. , Riech I. , Almeida-Garcı́a J. , Cantarero A.
Published: July 2002

Nonintentionally doped AlxGa1−xAs layers with 0.38⩽x⩽0.84 were grown on (100) GaAs substrates by liquid phase epitaxy (LPE) under near-equilibrium conditions. The crystalline quality of the samples was studied by photoluminescence at 2 K and room temperature Raman spectroscopy. The peculiar behavior in the photoluminescence intensities of the indirect bound exciton line and the donor–acceptor pair transition is explained from the evolution of the silicon donor binding energy according to the aluminum composition. It was also possible to observe the excitonic transition corresponding to the AlxGa1−xAs/GaAs interface, despite the disorder and other factors which are normally involved when growing high-aluminum-content layers by this technique. Furthermore, Raman measurements show the quadratic variations of longitudinal optical phonon frequencies with aluminum concentration in good agreement with previous experimental results. In this work we show that high quality indirect-gap AlxGa1−xAs samples can be grown by LPE under near-equilibrium conditions.
Capacitance–voltage characteristics of selectively doped AlxGa1−xAs/GaAs heterostructures containing deep traps
Authors: Prokhorov Eugenio F. , Gorev Nikolai B. , Kodzhespirova Inna F. , Kovalenko Yury A.
Published: July 2002

The low- and high-frequency capacitance–voltage characteristics of a selectively doped AlxGa1−xAs/GaAs heterostructure containing deep traps in the AlxGa1−xAs layer are calculated. It is shown that the low-frequency capacitance–voltage characteristic features a rising portion when the deep traps are localized at the heterointerface, this rising portion being due to the onset of emptying of the deep traps. The calculated results agree with the experimental data.
Unifying the strain and temperature scaling laws for the pinning force density in superconducting niobium-tin multifilamentary wires
Authors: Cheggour Najib , Hampshire Damian P.
Published: July 2002

Systematic variable temperature measurements of the transport critical current density (Jc) tolerance to strain (ε), performed on a bronze processed niobium-tin multifilamentary wire in high magnetic fields up to 15 T, are reported. The results show that Bc2*(T,ε), the field at which the pinning force density (Fp) extrapolates to zero, can be written as Bc2*(0,ε)g[T/Tc*(ε)], where g is a function of the reduced temperature T/Tc*(ε) and Tc*(ε) is the temperature at which Bc2* extrapolates to zero. We propose a magnetic field, temperature, and strain scaling law for Fp which unifies Ekin’s strain scaling law and the Fietz–Webb variable temperature scaling law. It is of the form Fp=Jc×B=A(ε)[Bc2*(T,ε)]nbp(1-b)q, where n, p, and q are constants, A(ε) is a function of strain alone, and b is the reduced field B/Bc2*.
Jordan block structure of two-dimensional quantum neural cellular network
Authors: Qiao Bi , Ruda Harry E.
Published: July 2002

We present a Jordan type spectral decomposition for the Hamiltonian operator of a two-dimensional (2D) Ising model driven by a local external field. The results reveal that the external field induces a dynamic contribution to the Jordan block structure, while the interaction between lattice spins contributes to the energy spectrum, tending to attract the system to a minimum energy state. Both interactions contributing to the principle vectors of the Hamiltonian operator reflect their correlation. Since we adopt a first-principle approach the results are useful to general 2D Ising spin systems subject to time-independent external fields irrespective of whether the system is in equilibrium.
Electrostatics of a thick disk
Authors: Cade R.
Published: July 2002

An integral equation method is developed and applied to solve approximately the electrostatic problem of a drum-shaped conductor, one whose plane faces have a separation small compared with their diameter and what is called a thick disk. The approximate method is an asymptotic process by which capacitance formulas are obtained corresponding to the first two orders of the process. Comparison indicates that convergence is rapid, and enables rules to be proposed for estimating the accuracy of the formulas.
Impedance spectroscopy study of the electrical conductivity and dielectric constant of polycrystalline LiNbO3
Authors: Lanfredi S. , Rodrigues A. C. M.
Published: July 2002

The electrical properties of LiNbO3 ceramic pellets were investigated by impedance spectroscopy in the temperature range of 450–800 °C. Impedance data are presented in the Nyquist plot which, in addition to the separation of grain and grain boundary contributions, allows the determination of the relaxation frequency of samples. The results of bulk electrical conductivity and its activation energy, confirmed by measuring samples with different geometrical parameters, are presented. Relaxation frequencies follow an Arrhenius behavior with the same activation energy found for the electrical conductivity. Relaxation frequencies were also used to calculate bulk dielectric constants and the results are compared to the behavior of the real part of the sample dielectric constant.
Effect of sputtering with hydrogen dilution on fluorine concentration of low hydrogen content fluorinated amorphous carbon thin films with low dielectric constant
Authors: Yokomichi Haruo , Masuda Atsushi
Published: July 2002

The fluorine concentration of fluorinated amorphous carbon (a-C:F) thin films with a low dielectric constant was controlled using magnetron sputtering with hydrogen gas dilution into CF4 gas. The structural, optical and defect properties of these films were investigated by infrared (IR) absorption, x-ray photoelectron spectroscopy, ultraviolet visual spectroscopy and electron spin resonance as well as dielectric constant measurements. IR absorption spectra of the films with various fluorine concentrations prepared by hydrogen gas dilution showed no vibration modes between hydrogen and carbon atoms in CH, CH2, and CH3 configurations, suggesting that the hydrogen concentration of these films is less than a few at. %. Furthermore, the structural properties change at a fluorine concentration of approximately 50 at. %. However, the dielectric constant of 2.5 that is required for the interlayer materials for the next generation of ultralarge scale integrated devices remains unchanged at this fluorine concentration. Based on these results, the thermal stability of these a-C:F films is discussed briefly.
Spectroscopic ellipsometry study of Pb1−xEuxSe(0⩽x⩽0.45)
Authors: Kanazawa Hideyuki , Adachi Sadao , Yamaguchi Tsutomu , Murashige Shinichi , Murakami Kensuke
Published: July 2002

We have measured the complex dielectric function, ε(E)=ε1(E)+iε2(E), of Pb1−xEuxSe in the 1.2–5.0 eV photon-energy range by spectroscopic ellipsometry at room temperature. The Pb1−xEuxSe crystals were grown by the conventional Bridgman method (x=0) and by hot-wall epitaxy on (111) BaF2 substrates at 300 °C (0.09⩽x⩽0.45). The measured ε(E) data revealed distinct structures at two critical-point (CP) energies, E1 and E2. The second-derivative spectra of the complex dielectric function, d2ε(E)/dE2, were analyzed using a theoretical model called the model dielectric function. The CP parameters, such as the CP energy, amplitude, and broadening, as a function of alloy composition x were fit determined from these derivative analyses. This parameterization of the CP parameters enabled us to calculate ε(E) and its related optical constants for optional alloy composition and photon energy.
Microstructural evolution and giant magnetoresistance in melt spun and annealed Cu85(FeCo)15 alloy
Authors: Ravishankar N. , Park J. , Aoki K. , Masumoto T. , Chattopadhyay K.
Published: July 2002

Granular alloys of Cu with FeCo were prepared by the melt-spinning technique. The alloy was characterized by x-ray, transmission electron microscopy, vibrating sample magnetometer, and magnetoresistance measurements. The alloys were heat treated for different temperatures to optimize the magnetoresistance properties. Structural characterization reveals that the FeCo phase initially precipitates out as fcc and later transforms to the bcc structure by martensitic transformation. It is seen that the trend in the magnetoresistance properties is different for the measurements carried out at room temperature and 4.2 K. This has been attributed to the transformation of fine fcc precipitates to the bcc structure during the low temperature measurements. It is seen that the presence of fine particles causes an increase in the field for saturation and is not suitable for applications where moderate field giant magnetoresistance is required.
Modulation speed of an efficient porous silicon light emitting device
Authors: Cox T. I. , Simons A. J. , Loni A. , Calcott P. D. J. , Canham L. T. , Uren M. J. , Nash K. J.
Published: July 2002

Trends in the efficiency and small signal modulation behavior of porous silicon light emitting diodes (LEDs) are reported for devices formed by the anodization of bulk silicon p–n junctions. As the average size of the silicon skeleton is decreased, the external electroluminescence (EL) efficiency increases from 0.001% to 0.18% and there is a corresponding blue shift in the EL peak from 776 to 633 nm. An associated tenfold increase is observed in the photoluminescence efficiency while the diode resistance, at 2 V, increases from 3×103 to 1×106 Ω. Under small signal pulsed operation, the voltage dependence of the rising edge of the EL is well described by a carrier mobility of 3×10−4 cm2 s−1 V−1 which is independent of the average size of the luminescent regions of the silicon nanostructure. The falling edge of the EL transient is dominated by radiative recombination of quantum confined excitons. The modulation speed is found to be limited by a combination of carrier mobility in the silicon wires and radiative recombination processes. Evidence of charge trapping and discharge is found in an EL overshoot phenomenon. The major application of this type of porous silicon LED, with modulation speeds below 1 MHz, appears to be for displays integrated with circuitry rather than for optical interconnection.
Optical anisotropy of (11l)-oriented strained quantum-wells calculated with the effect of the spin-orbit split-off band
Authors: Kajikawa Yasutomo
Published: July 2002

We theoretically investigate the polarization anisotropies of the interband transitions in heteroepitaxial layers of zinc-blende type semiconductors pseudomorphically grown on (11l) substrates with arbitrary l. As the grown layers, quantum well (QW) layers, as well as layers which are thick enough to be regarded as the bulk, are investigated on the assumption that the strain due to the lattice mismatch is not released at all and is fully involved. Calculation of polarization-dependent optical matrix elements at the Brillouin-zone center is carried out based on the multiband effective-mass theory with and without the spin-orbit split-off (SO) band. For the QW, an infinite-high-barrier model is adopted. The calculated results are presented for GaxIn1−xP heteroepitaxial layers having various Ga contents grown on (11l) GaAs substrates. The dependence of polarization anisotropies upon lattice mismatch, quantum confinement, and substrate orientation is discussed in connection with the effect of mixing between heavy- and light-hole band as well as the coupling effect to the SO band. The calculation including the SO band predicts that the interband transitions in layers grown on (11l) substrates except for (001) and (111) are strongly polarized along the [ll2̄], [1̄10], or [11l] direction when the quantum confinement effect or the strain is large. The predicted large anisotropy in narrow QWs or in highly strained layers, which is due to the effect of the SO band, is expected to improve the suppression ratio of the mode competition between orthogonally polarized lasing modes in vertical-cavity surface-emitting lasers.
Magnetic force microscopy and micromagnetic study of cross-tie wall structures in Co91Nb6Zr3 amorphous thin films
Authors: Huo S. , Pan G. , Mapps D. J. , Clegg W. W. , Heydon G. , Rainforth W. M. , Davies H. A. , Bishop J. E. L. , Tucker J. W. , Gibbs M. R. J.
Published: July 2002

High resolution images of cross-tie domain wall structures have been obtained by magnetic force microscopy (MFM) for a 37.5 nm Co91Nb6Zr3 film using a NiFe thin film coated tip. Between successive cross ties, the main or spinal wall was found to be consistently subdivided unequally into pairs of oppositely oriented Néel wall sections separated by circular Bloch lines. Main and wing walls intersect at cross Bloch lines. A reversed-contrast MFM image of the same uneven cross-tie wall structure was obtained after reversing the tip magnetization. MFM images reflect only the field from the divergence of the underlying magnetization M and contain no direct information on curl M. Accordingly they are best interpreted by comparison with the magnetization pattern of a similar cross-tie structure obtained by micromagnetic computation. This enables the cross and circular Bloch line singularities to be distinguished in the MFM images of the cross-tie structure. By combining repeated observations made with opposite tip magnetizations, disturbance of the main and wing wall structures by the tip was extracted from the MFM signal which was then compared with the signal computed for a two-dimensional model wall. The main wall was found to be an asymmetric Néel wall with a weak S shaped magnetic structure. The wing walls were found to be Néel walls of acute angle, decreasing with distance from the spine.
Production and structural characterization of BN/TiN multilayers
Authors: Méndez J. M. , Qu B. D. , Evstigneev M. , Prince R. H.
Published: July 2002

A pulsed excimer laser was used to evaporate targets of boron nitride and titanium nitride in an attempt to produce hard thin films on crystalline silicon substrates. The films were either pure TiN or BN layers, as well as alternating multilayers and mixed layers. Deposition could be assisted by ion bombardment. The films were characterized by Auger electron spectroscopy, Fourier transform infrared spectroscopy (FTIR), and x-ray diffraction. A selection of films was also studied by profilometry in order to determine deposition rate and the type of stress present. The level of stress in TiN films was also a function of the deposition temperature and could be varied with the use of ion bombardment. Amorphous, cubic, and hexagonal BN films were produced and the effect of the stress of the substrate on these layers was investigated. Multilayers were stressed, having alternating layers of nanocrystalline TiN and amorphous BN. Mixtures consisted of nanometer-sized regions of crystalline TiN and sp2 coordinated boron nitride. FTIR spectra and high-resolution transmission electron microscope pictures suggested that in the mixtures, boron nitride planes tended to parallel the surface of the TiN grains. No sign of stress-driven formation of cubic BN was observed in the multilayers nor in the nanosized mixtures, regardless of the stress level present in them; neither was there any sign of titanium borides or other structures that might increase the hardness of the films.
Nanoscale compositional analysis of Fe/Tb multilayers: A field ion microscopy and tomographic atom probe study
Authors: Veiller L. , Danoix F. , Teillet J.
Published: July 2002

Sputtered Fe/Tb multilayered systems have been successfully deposited on needle-shaped substrates. These specimens have been observed by field ion microscopy and the layered sequence is evidenced together with the preferential field evaporation of terbium versus iron. The first direct atomic scale concentration data of Fe/Tb multilayers by means of the tomographic atom probe are given. The three-dimensional reconstructions of the layers were obtained and the compositional modulation was observed across the specimens.
Short-wavelength laser diodes based on AlInAs/AlGaAs self-assembled quantum dots
Authors: Hinzer K. , Lapointe J. , Feng Y. , Delâge A. , Fafard S. , SpringThorpe A. J. , Griswold E. M.
Published: July 2002

Self-assembled quantum dots (QDs) of highly strained InAlAs have been grown by molecular beam epitaxy in separate-confinement p–i–n heterostructures on (001) GaAs substrates. Results from a systematic study of samples with varying amounts of deposited material relates the observed emission peaks with QD levels, wetting layer states, or barrier materials. For samples with high-QD concentration, lasing is observed in the upper-QD shells. A sample with contact layers improving carrier and optical confinement operates up to room temperature and displays lowered threshold current densities. A threshold current density of ∼4 A/cm2 is measured for this structure at T=5 K and continuous-wave operation is obtained up to T∼77 K. A material gain larger than 1.7×104 cm−1 is measured for this single-layer structure. Lasing is observed in the upper-QD shells for small gain media, and progresses towards the QD lower states for longer cavity lengths representing an emission shift of 45 meV. A minor dependence of the threshold on QD density is found for samples having densities between 20 and hundreds of QDs per micron squared. For samples with multiple QD layers displaying vertical self-assembling, an increase in the emission linewidth is observed compared with single-layer samples and multilayer samples with uncorrelated growth.
Photonic band gap airbridge microcavity resonances in GaAs/AlxOy waveguides
Authors: Ripin D. J. , Lim Kuo-Yi , Petrich G. S. , Villeneuve Pierre R. , Fan Shanhui , Thoen E. R. , Joannopoulos J. D. , Ippen E. P. , Kolodziejski L. A.
Published: July 2002

Photonic band gap waveguide microcavities were designed, fabricated, and measured in a high-dielectric-contrast GaAs/AlxOy III–V compound semiconductor structure. The photonic crystal is defined by a regularly spaced one-dimensional array of holes in the waveguide. By controlling the spacing between the two central holes, the microcavity is formed. The waveguide microcavity is suspended in the airbridge geometry to further increase optical confinement. Resonance states with cavity quality factors as high as 360 were measured at wavelengths near 1.55 μm, with modal volumes as small as 0.026 μm3.
Boundary condition applicable to free-standing quantum films
Authors: Kim J. U. , Lee Hong H.
Published: July 2002

An alternative to the usual infinite boundary condition is obtained for free-standing quantum films in the framework of effective-mass approximation. This alternative is applied to silicon quantum film to arrive at the confinement conditions in terms of envelope functions. The confinement conditions contain surface effects as well. The present boundary condition yields the energy gap of the film that is comparable to that by ab initio calculation. The approach is also applicable to quantum films embedded by high-energy barriers.
Microstructure and optical properties of submicron porous silicon thin films grown at low current densities
Authors: Riley David W. , Gerhardt Rosario A.
Published: July 2002

The mechanism of porous silicon (PS) film growth remains a debate among researchers. This study investigates the effect of current density (J) and anodization time (t) on the film thickness, optical properties, and microstructure of films grown at low current densities. The samples reported herein were generated from polished and unpolished 〈100〉 single crystal wafers, with resistivities of 0.1 and 0.6 Ω cm, respectively. Current densities of 1–10 mA/cm2 and anodization times ranging from 3 to 900 s were used. This work shows that film growth is at first linear but beyond a given anodization time (dependent on J), the film produced is not continuous, but is best described as a finite number of stacked sublayers with each sublayer having a unique microstructure, porosity, and optical response. This article also shows that as the PS film grows, second and third order peaks appear in the reflectance spectra. This data, together with modulations in the index of refraction spectra and visual evidence from cross-sectional scanning electron microscopy (SEM) images demonstrate that submicron PS thin films grown at low current densities have a multilayer structure. Above a certain thickness, the modulation in the properties due to the sublayers phases out and the films appear as if they were single layered films. SEM is used to show the progression, from cross-sectional imaging, from a layered or stacked PS film structure to an apparently homogeneous film with depth. When using low current densities, single layer films are only obtained at very short times.
Effects of plasma emission on optical properties of phosphor layers in surface-type alternate current plasma display panel
Authors: Jang Sang-Hun , Tae Heung-Sik , Lee Jeong-Hae , Choi Kyung Cheol
Published: July 2002

This study uses helium and xenon gas mixture discharges to determine the effects of helium plasma emission on the characteristics of the visible emission from the stimulation of the red, green, and blue (RGB) phosphor layers in a surface-type alternate current plasma display panel. With a mixture of less than 2% xenon to helium, it was found that the luminance of the RGB phosphor layers decreases with a decrease in the helium plasma emission intensity. However, with a mixture of above 2% xenon to helium, the luminance of the RGB phosphor layers increases regardless of a decrease in the helium plasma emission intensity. Furthermore, the color purity of the RGB phosphor layers improves as the helium plasma emission intensity decreases. Accordingly, it can be concluded that the optical properties of the phosphor layers, including color purity and luminance, depend on the helium plasma discharge emission as well as the visible emission from the stimulation of the phosphor layers.
Asymmetrical spin-polarized tunneling and magnetoresistance in ferromagnet/insulator/insulator/ferromagnet junctions
Authors: Xu Hui-Ying , Yu Wen-Xue , Kang Jian , Mai Zhen-Hong
Published: July 2002

We consider a magnetic tunneling junction (MTJ) composed of two ferromagnetic electrodes separated by two adjacent insulating barriers, which have different dielectric constants and barrier heights. Based on the two-band model and nearly-free-electron approximation, the tunnel current, tunnel conductance, and tunnel magnetoresistance effect of the MTJ under the forward and reverse biases are discussed, respectively. The numerical results are compared with the experimental results of the single-barrier MTJs. We find that there exists a directional and spin-polarized tunneling in this structure. It suggests that this structure will provide additional functions to the traditional MTJs.
Properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells
Authors: Zheng Ruisheng , Taguchi Tsunemasa , Matsuura Mitsuru
Published: July 2002

The fundamental properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are investigated theoretically. The mode number and the energies of optical phonons, the high frequency and static dielectric constants of Ga1−xInxN crystals, are obtained by using an improved mixed crystal theory. The carrier-longitudinal optical (LO)-phonon interaction in Ga1−xInxN mixed crystals is studied by a perturbational polaron theory. The feature of optical phonons in Ga1−xInxN/GaN quantum wells is studied based on the dielectric continuum approach. The frequencies and the dispersion relation of the interface phonons are obtained. The 1s and 2s exciton binding energies and the exciton–LO-phonon interaction energies in Ga1−xInxN/GaN quantum wells are calculated numerically. Based on the present theoretical results, the general properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are discussed.
Numerical simulation of the tunneling current and ballistic electron effects in field emission devices
Authors: Fonseca L. R. C. , von Allmen Paul , Ramprasad R.
Published: July 2002

Using a two-dimensional model, we have considered the effects of spatially changing fields and potentials, stochastic electron emission, and ballistic electron motion on the anode current and on the width of the electron beam in field emission displays. We have solved the electrostatic problem using the boundary element method. Our electron emission model evaluates the current density at the cathode surface from the tunneling transmission coefficient, which is calculated from the solution of the one-dimensional Schrödinger equation using a potential barrier which includes the effect of image charges and nonuniform electric field. The current density is used to calculate the rate of electron emission for each segment of the emitter’s surface. The emission time is assumed to follow a Poisson distribution. The electron’s velocity magnitude and angle with the normal to the surface are also stochastically generated following the probability distribution of field emitted electrons. Ballistic transport is used to propagate electrons through the device. For very sharp tips the electric field changes from its surface value over a very short distance away from the surface, which can be comparable to the tunneling distance. We found that the resulting current density is considerably lower for the calculated barrier profile than for the triangular one, especially at low values of the electric field. We have also shown that the effect of the lateral kinetic energy and emission angle distribution on the electron beam width at the anode is negligible for sharp emitters, where the angular spread is dominated by the curvature of the emitting surface.
Effect of crystal orientation and doping on the activation energy for GaAs oxide growth by liquid phase method
Authors: Wang Hwei-Heng , Chou Dei-Wei , Wu Jau-Yi , Wang Yeong-Her , Houng Mau-Phon
Published: July 2002

We have investigated the oxide growth kinetics of near-room-temperature liquid phase chemical enhanced oxidation on differently oriented and doped GaAs substrates. Oxidation reactions have been studied by analyzing their activation energies and have been found to depend on the bond configuration of crystal planes. Experimental results indicate that the activation energies are independent of the doping of GaAs. The oxidation rates are dopant selective (n−:p+-GaAs∼4:1 at 30 °C under illumination) and sensitive to illumination (without:with illumination∼1:25 at 30 °C for a n+-doped GaAs). In the oxidation reactions, photogenerated holes are found to play an important role. Finally, we have proposed a mechanism based on the band bending and the carrier transport near the oxide-GaAs interface to interpret the experimental observations.
AP-pinned spin valve GMR and magnetization
Authors: Beach Robert S. , Pinarbasi Mustafa , Carey Matthew J.
Published: July 2002

We present a general analytical coherent rotation model for AP-pinned spin valve magnetoresistance (MR) and magnetization (M). The magnetic structure is determined by minimizing the sum of the Zeeman and the two interfacial exchange energies, σAF and σAP. We compare the model to M(H) and MR(H) taken from an experimental series of AP-pinned spin valves (NiO/AP1/Ru/AP2/Cu/free layer) measured in magnetic fields −5⩽H⩽5 kOe, and at temperatures 20⩽T⩽240 °C. The Ru thicknesses were 4.5 Å⩽tRu⩽10.4 Å. We fit the model to data by varying σAF and σAP. The fits reproduce the data quantitatively, and the resulting exchange strengths agree well with published studies of Co/Ru/Co multilayers and ferromagnet/NiO exchange couples.
Atomic migration in MgFe2−xCrxO4
Authors: Lee Seung Wha , An Sung Yong , Ahn Geun Young , Kim Chul Sung
Published: July 2002

Mg–Cr ferrite has been studied with Mössbauer spectroscopy, x-ray diffraction, and vibrating sample magnetometer. The crystal structure for this system is spinel structure, and the lattice constant is in accord with Vegard’s law. The Mössbauer spectra consist of two six-line patterns corresponding to Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Curie temperature decreases linearly with Cr concentration, suggesting the superexchange interaction Fe(A)–O–Fe(B) link is stronger than that for the Fe(A)–O–Cr(B) link. Debye temperatures for the A and B sites of MgFe1.9Cr0.1O4 are found to be θA=515±5 K and θB=265±5 K, respectively. Atomic migration of MgFe1.9Cr0.1O4 starts near 350 K and increases rapidly with increasing temperature to such a degree that 50% of the ferric ions at the A sites have moved over to the B sites by 600 K. The temperature dependence of both the magnetic hyperfine field and magnetization of MgFe1.9Cr0.1O4 is explained by the Néel theory of ferrimagnetism using three superexchange integrals: JA–B=−17.9kB, JA–A=1.9kB, and JB–B=7.1kB.
Switching behavior of Stoner-like magnetic thin film elements
Authors: Bauer M. , Fassbender J. , Hillebrands B.
Published: July 2002

We report results of the switching properties of thin magnetic films described by Stoner-like magnetic blocks subject to ultrashort, laterally localized magnetic field pulses, obtained by numerical investigations. Perpendicularly magnetized films exhibit a magnetization reversal due to a 4 ps magnetic field pulse, which can be explained with a coherent rotation model. Although the magnetic field pulse is short compared to the ferromagnetic resonance precession time, the time needed for the relaxation of the magnetization to the equilibrium state is rather large. In addition, we show numerical results for the magnetization reversal of an in-plane magnetized sample with uniaxial in-plane anisotropy.
Charge ordering and Mössbauer studies of single crystal R1/3Sr2/3FeO3 (R=Pr, Sm, and Nd)
Authors: Uhm Young Rang , Wha Lee Seung , Park Key-Taeck , Tomioka Y. , Tokura Y. , Kim Chul Sung
Published: July 2002

Single crystals of R1/3Sr2/3FeO3 (R=Pr, Nd, and Sm) were synthesized by the floating zone method and their magnetic properties and charge ordering (CO) transition related to lattice dynamics were systematically investigated. Mössbauer spectra of R1/3Sr2/3FeO3 were taken at various temperatures ranging from 12 K to room temperature. The charge disproportionation in Pr1/3Sr2/3FeO3 was detected below 190 K, in which two kinds of iron with valence states Fe3+ and Fe5+ were found with ratio of 2:1. The iron with valence state Fe4+ in Pr1/3Sr2/3FeO3 coexists at and above 150 K, and its ratio increased from 13% to 66% as the temperature rose. The (Nd1−ySmy)1/3Sr2/3FeO3 (y=0.0, 0.2, 0.4, 0.6, and 0.8) with least lattice distortion underwent a CO phase transition at and below TCO=163 K and accompanying the charge disproportionation into nominally Fe3+ and Fe5+ sites as well as a canted antiferromagnetic spin ordering. In this charge ordering state, a sequence of Fe+3Fe+3Fe+5Fe+3Fe+3Fe+5 exists aligned along the [111] direction of the pseudocubic perovskite structure. In this study, the CO at x=2/3 disappeared in the case of R=Sm. Indeed, disappearance of the CO transition was detected by systematic decrease of a spontaneous magnetization with increase of y in the system of (Nd1−ySmy)1/3Sr2/3FeO3. This result shows that the charge ordering state was realized with strong hybridization between the Fe and O atoms.
Effect of pinning field on the magnetization in patterned synthetic antiferromagnetic spin valves
Authors: Cho Hae Seok , Chen Jian , Cool Stephen , Michel Richard , Tabat Ned
Published: July 2002

The effect of the pinning field (Hp) on the distribution of the average magnetization direction in the pinned layer (PL) and the reference layer (RL) of spin valve (SV) sensors with synthetic antiferromagnetic layers was investigated with respect to the magnetic fields that can cause canting in the PL and RL magnetization. Hp was controlled by annealing temperatures and seed layers. It was found that the direction of the average magnetization in the PL and RL is largely governed by shape anisotropy in the patterned SV sensors with a low Hp. Both experimental and modeling results support the idea that the canting of the PL and RL can occur in both SV sheet films and patterned SV sensors, when Hp is not large enough to overcome the interfacial magnetostatic coupling (H1) between the free and reference layer.
History dependent crystallization of Zr41Ti14Cu12Ni10Be23 melts
Authors: Schroers Jan , Johnson William L.
Published: July 2002

The crystallization of Zr41Ti14Cu12Ni10Be23 (Vit 1) melts during constant heating is investigated. (Vit 1) melts are cooled with different rates into the amorphous state and the crystallization temperature upon subsequent heating is studied. In addition, Vit 1 melts are cooled using a constant rate to different temperatures and subsequently heated from this temperature with a constant rate. We investigate the influence of the temperature to which the melt was cooled on the crystallization temperature measured upon heating. In both cases the onset temperature of crystallization shows strong history dependence. This can be explained by an accumulating process during cooling and heating. An attempt is made to consider this process in a simple model by steady state nucleation and subsequent growth of the nuclei which results in different crystallization kinetics during cooling or heating. Calculations show qualitative agreement with the experimental results. However, calculated and experimental results differ quantitatively. This difference can be explained by a decomposition process leading to a nonsteady nucleation rate which continuously increases with decreasing temperature.
Graphitization of ultrathin amorphous carbon films on Si(001) by Ar+ ion irradiation at ambient temperature
Authors: Kim Sang Sub , Hishita Shunichi , Cho Tae Sik , Je Jung Ho
Published: July 2002

We report the graphitization of ultrathin (8 nm) amorphous carbon films on Si(001) by 2 MeV Ar+ ion irradiation at ambient temperature. The resulting graphite films show the 〈001〉 preferred orientation in the film normal direction, but random distribution in the film plane direction. The smooth surface and interface suggest that the activation energy for the graphitization is supplied mostly by the electronic stopping process while Ar+ ions travel through the amorphous carbon film.
Electron transport across one-dimensional modulated superlattices in a quantum waveguide in magnetic fields
Authors: Gu Ben-Yuan , Zhang Yan , Wang Xue-Hua
Published: July 2002

Electron transport in a quantum waveguide containing a finite length superlattice (SL) with periodically modulated potentials or Gaussian envelope modulation in magnetic fields is investigated with use of the scattering matrix method. When the modulated potential of the SL takes a Gaussian function, we found that the resonant peaks in conductance at zero field are prominently smoothed out and the conductance spectrum exhibits a monotonically increasing line with a finite rise at the beginning. The corresponding one-dimensional structure presents flat minibands and minigaps. When electron energy is aligned with the miniband, electrons are allowed to pass through the SL to be nearly unscattered. When applying magnetic fields, the conductance plateaus are observed again. From the analyses of the individual modal transmission probabilities, the conductance spectra can be understood well. It is expected that implanting a finite length SL with different modulation potentials may provide a way to tailor conductance spectrum to match the practical demands in devices.