bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation of switching volume with magnetic properties, microstructure, and media noise in CoCr(Pt)Ta thin films
Authors: Gao C. , Shan Z. S. , Malmhäll R. , Liu Y. , Richter H. J. , Barney A. , Rauch G. C. , Sellmyer D. J.
Published: July 2002

The technique of “field sweep-rate dependence of coercivity” together with the delta-M curves, transmission electron microscopy, and media noise measurement have been employed to reveal the correlation among the magnetic switching volume, physical grain size, intergrain interaction, and noise properties. The switching volume for both CoCrTa and CoCrPtTa films was found to be ∼15 nm in diameter, and weakly dependent on grain size and coercivity for the systems studied. In films with physical grain size larger than the switching volume, strong intergrain interaction and high media noise is observed. In films with the physical grain size close to the switching volume, smaller interactions and lower media noise is found. It is argued that a change of the grain size results in a change in the nature of the magnetic interactions.
Elastic neutron scattering and rhombohedral antiferromagnets
Authors: Rastelli E. , Tassi A.
Published: July 2002

Infinite isoenergetic helix spin configurations minimize the energy of the classical Heisenberg rhombohedral antiferromagnet with nearest neighbor interactions. We comment about elastic neutron scattering data on ACrO2 (A=Cu, Ag, Pd) compounds which seem to support the degenerate helix scenario modified by quantum fluctuations in agreement with the theoretical expectation. We investigate by a microscopic approach the existence of static short range order in β-oxygen as suggested on the basis of recent elastic neutron scattering experiment.
Some transport properties of the two-channel Kondo impurity
Authors: Schlottmann P. , Zvyagin A. A.
Published: July 2002

We consider conduction electrons moving along a ring in two different orbital channels interacting with a spin-1/2 impurity via isotropic spin exchange. The exchange is the same for both channels, but a crystalline field breaks the orbital symmetry. The tower structure of the finite size corrections to the ground state energy is derived from the Bethe ansatz equations and used to discuss the Aharonov–Bohm–Casher interference pattern in the persistent current and the magnetoresistivity.
Magnetic moment correlations in transition metal alloys determined by neutron scattering
Authors: González A. C. , Parra R. E.
Published: July 2002

Pd and Pt have been shown to be highly polarizable when mixed with ferromagnets like Ni, Fe, and Co. In these alloys the total moment, or magnetization cloud, is produced by the magnetic moment plus the induced moment on the surrounding atoms. With diffuse magnetic scattering of neutrons, it is possible to measure these magnetization clouds, M(K), as Fourier transform of the moments; M(K) represents the site occupation magnetic moment correlation. We consider that the atoms of the matrix polarize around the atoms of the impurity, following the orientation of its moments and forming giant moments. To compare with experimental data, we obtained an expression for the magnetic moments assuming a local magnetic environment. We then developed an expression for the magnetization cloud that includes the total moment associated with a giant moment, M(0), determined by magnetization measurements; we used M(0)=dμ/dc, where μ is the average moment of the alloy and c the impurity concentration, to establish a relation between neutron and magnetization measurements.
Thermodynamic properties of YBa2Cu4O8 crystal near upper critical fields Hc2(T)
Authors: Kim Mun-Seog , Kim Wan-Seon , Lee Sung-Ik , Yu Seong-Cho , Kim Jin-Tae , Dabrowski B.
Published: July 2002

High-field magnetization with external magnetic fields parallel to the c-axis has been measured in high-quality YBa2Cu4O8 single crystal. The fluctuation-induced magnetization above Tc and a crossover of magnetization curves M(T) at T*=77.4 K were clearly observed for various magnetic fields, and analyzed by the high-field scaling law and the vortex fluctuation theory for three-dimensional systems. In the reversible temperature region, the magnetization is well described by the model of Hao et al. [Phys. Rev. B 43, 2884 (1991)]. From this analysis, the thermodynamic parameters such as upper critical field slope −dHc2/dT=1.2 T/K near Tc, penetration depth λab(0)=1943 Å, and coherence length ξab(0)=21.7 Å were extracted.
A new model for dark solitons in magnetic films
Authors: Zaspel C. E. , Slavin A. N.
Published: July 2002

In magnetic thin films, the nonlinear Schroedinger (NLS) equation predicts the formation of a pair of dark solitons with equal and opposite phase shifts from an input pulse without phase modulation. To consider the effects of higher input power, a modified NLS equation is derived including higher order dispersive and nonlinear terms. An analytic solution to the modified equation is obtained which has the form of a single dark soliton in agreement with recent experiments using higher input power.
GHz ferromagnetic resonances in laminated microscale objects (abstract)
Authors: Walser R. M. , Valanju A. P.
Published: July 2002

Recent research1 has indicated that ferromagnetic laminates are potentially useful in the design of integrated monolithic microwave integrated circuit (MMIC) devices operating at GHz frequencies. Laminates with n=10–50 insulated, uniaxial magnetic films can have large permeabilities and device fill factors, and are process compatible with monolithic integration. The effect of lamination disappears, however, for frequencies greater than the frequency of a dimensional resonance (DR) associated with the easy axis dimension of the laminated object.23 Thus, when the ferromagnetic resonance (FMR) occurs at a frequency greater than the DR, the FMR spectra of the magnetic films may be distorted or, possibly, not observed. The bandwidths of both the DR and the FMR will be significantly broadened, if either lies above the eddy current relaxation (ECR) frequency of the individual magnetic films. Prior models23 of the permeability of laminated objects were developed for magnetic recording head laminates operating at frequencies below 10 MHz. At these frequencies, the effects of FMR are negligible, and easy axis dimensions e>1 cm are required to observe DR. In this work we modified our equivalent circuit model (ECM)3 to include the effects of DR, FMR, and the ECR in each layer. FMR spectra in the GHz frequency range, were computed for laminates with different dimensions, a variable number of different magnetic and insulating films, and a range of film thicknesses. A single domain, coherent rotational model was used to determine the layer permeability. The effect of the dc and rf shape demagnetizing fields of the laminated object on the FMR of the individual layers was included. The results show that the FMR spectra are shifted and distorted when the DR is near the FMR, and significantly broadened for insulator resistivities <1000 Ω cm and, as expected, when the FMR>ECR. To eliminate DR in the GHz range, the laminate must have e<100 μm.
Magnetohistory effects and spin reorientations of Nd3Fe29−xTx and Nd3Fe29−xTxN4 (T=V and Cr) compounds
Authors: Han X. F. , Wang J. L. , Pan H. G. , Wang Y. G. , Liu P. , Han B. S. , Yang F. M.
Published: July 2002

The novel compounds Nd3Fe29−xTx (T=V and Cr) have been successfully synthesized. Their x-ray patterns can be indexed with a monoclinic symmetry and the A2/m space group. The Curie temperatures of Nd3Fe29−xTx are 455 and 413 K for T=V and Cr, the saturation magnetization at 4.2 K and room temperature are 54.7 and 46.8 μB/f.u. for T=V and 49.9 and 32.2 μB/f.u. for T=Cr, and the anisotropy fields at 4.2 K and room temperature are 12.8 and 2.4 T for T=V and 9.1 and 1.2 T for T=Cr, respectively. The spin reorientations of the easy magnetization direction of Nd3Fe29−xTx are observed at around 230 K for T=V and 135 K for T=Cr, respectively. After nitrogenation the relative volume expansion was about 6%, Curie temperature of each nitride increased about 50%, and the transition temperature Tsr of the spin reorientations of the nitride also increased 145 K for T=V and 250 K for T=Cr, respectively, compared with its parent compound. Nitrogenation resulted in remarkable improvements in the saturation magnetization and anisotropy fields at 4.2 K and room temperature. Magnetohistory effects for Nd3Fe29−xTx and Nd3Fe29−xTxN4 (T=V and Cr) in a low field have been observed.
Magnesium outdiffusion through magnetite films grown on magnesium oxide (001) (abstract)
Authors: Shaw K. A. , Lochner E. , Lind D. M. , Anderson J. F. , Kuhn M. , Diebold U.
Published: July 2002

Scanning tunneling microscopy (STM) studies of 1 μm thick films of single crystalline Fe3O4 grown on MgO(001) indicate that repeated annealing of the sample in UHV causes Mg diffusion through the Fe3O4 film. The onset of this effect was clearly seen by STM at room temperature for samples raised above 400–430 °C. It appears that the annealing process causes the migration of Mg from the substrate entirely through the Fe3O4 lattice, and that the migration tends to fill the surface layer first, with lower layers filling as anneal time is increased. Upon detection of this effect, several complementary sample analysis techniques were employed to determine the extent of the changes observed. X-ray diffraction studies indicate shifts in the lattice constant from the cubic constant of magnetite, Fe3O4, (8.396 Å), which is already strained in thin-film growth on a substrate, further toward the cubic lattice constant of magnesioferrite, MgFe2O4, (8.375 Å) in order to accommodate the Mg that has migrated to the surface. Superconducting quantum interference device magnetometry studies reveal a significant change in the magnetic behavior of the film and large decreases in the saturation moment, remanence, and coercive field. The Verwey transition is greatly altered in these films after the annealing sequence. X-ray photoelectron spectroscopy studies of the films confirm the presence of magnesium in the uppermost layers of the film, and indicate a concentration gradient, with the highest concentrations of magnesium in the surface layer. X-ray fluorescence in scanning electron microscopy qualitatively indicate the presence of magnesium throughout the film, consistent with migration of the magnesium from the substrate. These results are compared with those on an unannealed Fe3O4 film of the same thickness and growth parameters, which shows no magnesium migration into the film during growth up to substrate temperatures of 300 °C.
Performance of a fractional dc electric motor equipped with plastic bonded Nd2Fe14B stator poles
Authors: Nicolaides G. K. , Atanassova Y. K. , Ioannides M. G. , Tsamakis D. M. , Gamari-Seale H.
Published: July 2002

Injection molding Nd2Fe14B plastic bonded magnetic material is pressed into the form of cylindrical ring segments in order to investigate its performance when used in the manufacturing of stator poles of fractional power dc motors. Measurements of speed and armature current versus different load torques were performed. The experimental results obtained for stator poles made by three plastic bonded Nd2Fe14B magnetic materials of different densities, are compared to those results obtained by using a pair of typical barium ferrite stator poles. The torque versus speed curves, the obtained mechanical power versus speed and the efficiency of the motor as a function of the speed are presented. The torque speed data in high speeds follow a linear law, as is expected by theory, while at low speeds, below a crossover point, a deviation from this linearity appears. This is attributed to temperature effects. In this work it is shown that in the region of light loads and high speeds, at a certain speed, the injection molded Nd2Fe14B permanent magnet stators produce a higher electromagnetic torque, higher mechanical power, and higher efficiency than the barium ferrite ones.
Spectroscopic properties of anisotropic absorption in a neodymium-doped YAlO3 laser crystal
Authors: Zhang Xinghong , Jiang Bingxi , Yang Yufen , Wang Zhanguo
Published: July 2002

Absorption spectra of YAlO3:Nd for the three crystallographic axes are investigated at room temperature. The spectral strengths indicate that the absorption coefficient of YAlO3:Nd is anisotropic. The anisotropy of the local electric field acting on the rare-earth ion in a laser crystal is considered. An extended Judd–Ofelt theory is applied to calculate the absorption cross sections and oscillator strengths of the electric-dipole transitions in the different principal directions. Three groups of the phenomenological parameters are derived from a least-squares-fitting procedure. We also analyze theoretically the anisotropy of the optical absorption of YAlO3:Nd crystal in detail.
Anisotropy and field dependence of critical current density in YBa2Cu3O7−δ epitaxial thin film
Authors: Cao X. W. , Wang Z. H. , Fang J. , Xu X. J. , Li K. B.
Published: July 2002

The critical current density Jc of the epitaxial YBa2Cu3O7−δ thin film was measured as a function of applied magnetic field H up to 6 T for three special configurations: (1) H∥c axis and H⊥J; (2) H⊥c axis and H⊥J; (3) H⊥c axis and H∥J. We observed a large anisotropy of Jc(H) between H∥c axis and H⊥c axis with H⊥J, and a very small anisotropy of Jc(H) between H∥J and H⊥J with H⊥c axis. The field dependence of the critical current density can be well described by the modified Kim–Anderson model taking into account the Kramer scaling law. The weak dependence of the transport critical current density on its orientation relative to the applied magnetic field is discussed.
Exchange coupling coefficient and domain wall mobility of (Dy,Tb)FeCo magneto-optical films
Authors: Hwang Wein-Kuen , Wu Te-Ho , Shieh Han-Ping D.
Published: July 2002

The magnetic properties and recording characteristics of (Dy,Tb)FeCo films are dependent upon the anisotropy dispersion of rare earth constituents and the exchange integral between rare earth and transition metal sub networks. The exchange coupling coefficient λ and the domain wall mobility μeff are found to be interrelated by both measurements and calculations. The higher the λ or the lower the μeff, the higher bias field that is required for magneto-optical recording. The temperature independent λ is demonstrated to be an important parameter in the assessment of magnetic properties and recording characteristics of sperimagnetic films.
The operational principle of a new amorphous silicon based p-i-i-n color detector
Authors: Brüggemann R. , Neidlinger T. , Schubert M. B.
Published: July 2002

The operational principle of a new type p-i-i-n color sensor is described with the aid of numerical modeling. The modeling results account for the color detection mechanism recently presented that this kind of structure exhibits [T. Neidlinger, M. B. Schubert, G. Schmid, and H. Brummack, in Amorphous Silicon Technology—1996, edited by E. A. Schiff et al. (Materials Research Society, Pittsburgh, 1996), p. 147]. By band gap engineering the experimental red response is maximized at larger reverse bias voltage whereas the green response has its maximum at low reverse bias voltage. The numerical modeling qualitatively reproduces the characteristic shape of the steady-state current-voltage curves at different illumination wavelengths. At low and at high reverse bias voltages the influence of the internal variables and parameters is identified and leads to the experimentally observed response. The potential profile of the p-i-i-n structure is of crucial importance to the color detection mechanism. At larger wavelengths the large potential drop across the two highly defective front layers assists recombination in the back part of the device, which thus leads to the drop in the red response at low reverse voltage. For the voltage-dependent shift in spectral sensitivity it is important that photogenerated carriers under green bias illumination are lost by recombination in the front part of the device.
Giant dipole effect and second-harmonic generation in quantum wires biased with a magnetic field
Authors: Svizhenko A. , Balandin A. , Bandyopadhyay S.
Published: July 2002

We have theoretically studied giant dipoles associated with transitions between magneto-electric subbands in a quantum wire subjected to a transverse magnetic field. The strengths of these dipoles and their resonant frequencies can be varied with the magnetic field which then allows one to tune the emission wavelength of these transitions. The large magnitude of the dipole moments also leads to a strong second-harmonic component of the dielectric susceptibility that can be utilized for nonlinear optical applications such as second-harmonic generation, limiting, mixing, optical switching, etc.
Nitriding of coarse powders of Sm2Fe17 based carbides
Authors: Chen X. , Altounian Z. , Ström-Olsen J. O.
Published: July 2002

Sm2Fe17 based carbides can be aligned, sintered, and then nitrided to form anisotropic magnets. In this approach, it is critical to fully nitride the coarse powders of Sm2Fe17 based carbides, without introducing soft magnetic phases such as α-Fe. We have successfully nitrided, with very small amounts of precipitated α-Fe, the coarse powders in less than three days. The physical integrity of these powders is retained after nitrogenation. We have found that when nitriding is carried out in a gas mixture of N2 and H2, the latter plays a catalytic role in accelerating the nitrogen diffusion. In order to maximize the catalytic role of H2, a nitriding temperature range between 723 and 773 K is used to obtain good quality specimens in a reasonably short time.
Importance of spin-independent scattering potentials in giant magnetoresistance
Authors: Tsymbal E. Yu. , Pettifor D. G.
Published: July 2002

We demonstrate that the giant magnetoresistance (GMR) effect in magnetic multilayers can be explained quantitatively in terms of the scattering of electrons from a spin-independent random potential that arises from the grown-in defects within the multilayer. We have calculated the GMR ratio for Co4/Cu4 and Fe4/Cr4 (001) multilayered systems within the Kubo–Greenwood formalism assuming that the on-site atomic energies are disordered randomly within a realistic spd tight-binding model. Our predictions are in good agreement with experiment and demonstrate that (i) increasing disorder causes a drop of GMR in multilayers, (ii) GMR for the current perpendicular to the plane is typically by a factor of two higher than GMR for the current in the plane, and (iii) the semiclassical treatment of conductivity applied to magnetic multilayers results in overestimated values of GMR due to the neglect of interband transitions.
T⋅ln(t/τ) scaling approach and fluctuation field analysis in interacting particulate systems
Authors: García del Muro M. , Batlle X. , Labarta A. , González J. M. , Montero M. I.
Published: July 2002

The time dependence of the magnetization in samples having a different degree of interparticle interactions is analyzed. The selected compound was a nanocrystalline powder of the composition BaFe10.4Co0.8Ti0.8O19 with and without an admixture of nanosized SiO2 particles. Two different approaches were considered for the analysis of the thermally activated demagnetization: the T⋅ln(t/τ0) scaling of the thermomagnetic relaxation in zero applied field, and the classical fluctuation field and activation volume analysis. The energy barrier distribution obtained from the former approach shows that the occurrence of larger demagnetizing interactions leads to a relative enhancement of the lowest energy barriers with respect to the largest barriers. Activation volumes are found to increase with demagnetizing interactions and the leading demagnetizing mechanism appears to shift from an individual particle mode (coherent or nucleation) to a collective one. Both analyses are suggested to be taken into account in order to ascertain the influence of interactions on the magnetic properties of nanocrystalline particles.
Enhanced temperature effect on magnetoresistance in Fe/Mo multilayers
Authors: Yan M. L. , Lai W. Y. , Yin L. , Liou S. H.
Published: July 2002

We have observed an enhancement of the temperature effect on magnetoresistance (MR) in Fe/Mo multilayers with the increase of the Mo layer thickness. MR at the first antiferromagnetic maximum is large and insensitive to the temperature whereas at the third antiferromagnetic maximum it is small and relatively sensitive to the temperature. We found that the mean free path of the conduction electrons increased with the increase of the Mo layer thickness at a fixed temperature and it decreased very slightly with the increase of the temperature at a fixed Mo layer thickness. The interlayer coupling, similar to MR, also has an enhanced temperature effect with the increase of the Mo layer thickness. There is a scaling relationship between the temperature effect on saturation field Hs and MR, which suggests that the decrease of the interlayer coupling strength is responsible for the enhanced temperature effect on MR.
Study of magnetization reversal in ultrathin magnetic multilayers using in situ MOKE
Authors: Li Ning , Mei Lin , Ho Kuoksan , Schouterden Kris , Lairson Bruce M.
Published: July 2002

The magnetization reversal mechanism in Co/Pt and CoCrTa/Pt thin film multilayers was investigated in situ using a magneto-optical Kerr effect (MOKE) apparatus. Multilayers Nx(Co/Pt) and Nx(CoCrTa/Pt) with different amounts of doped Cr17Ta5 were sputtered in high vacuum. The growth was interrupted at intervals of two bilayers to perform perpendicular MOKE loops in the range from N=2 to N=20. The Draaisma and de Jonge model was adapted to analyze the hysteresis loops with the domain wall energy density as a fitting parameter. The broad consistence of the experimental data and theory shows that the incorporation of Cr and Ta in the magnetic layer lowers the domain wall energy density. The model successfully predicts the thickness dependence of the loop squareness, which decreases with increasing N. According to the model, the domain repetition length d is in the range of 900–2100 Å, and the domain wall energy density σw is in the range of 1.68–11.1 erg/cm2.
The relationship between the crystallization process and the soft magnetic properties of nanocrystalline Fe–M–B–Cu (M=Zr, Nb) alloy
Authors: Bitoh Teruo , Makino Akihiro , Hatanai Takashi , Inoue Akihisa , Masumoto Tuyoshi
Published: July 2002

The relationship between the crystallization process and the soft magnetic properties of nanocrystalline Fe84Nb3.5Zr3.5B8Cu1 alloy has been studied by comparison with that of Fe73.5Si13.5B9Nb3Cu1 alloy. When the annealing temperature Ta is slightly above the crystallization temperature, high permeability can only be obtained for Fe–Nb–Zr–B–Cu after annealing for very short times. The Ta dependence of the coercive force of Fe–Nb–Zr–B–Cu cannot be explained by the change of the grain size of the bcc phase. The soft magnetic properties of Fe–Nb–Zr–B–Cu is dominated by not only the grain size but also the Curie temperature of the intergranular amorphous phase. It is concluded that the magnetic softness of Fe–Nb–Zr–B–Cu is related directly to the degree of the reduction in the apparent anisotropy, while that of Fe–Si–B–Nb–Cu is strongly affected by the Si content of the bcc phase.
Effect of the Cu and Nb additives on the effective magnetic anisotropy in FeSiB alloys
Authors: Garcı́a Tello P. , Blanco J. M. , González J. , González J. M.
Published: July 2002

Results are presented on the evolution according to the thermal treatment parameters of the effective anisotropy and the dispersion field of two amorphous melt-spun samples having nominal compositions Fe74.5Nb3Si13.5B9 and Fe76.5Cu1Si13.5B9, respectively. From these data we conclude that the softening of the samples prepared by nanocrystallization annealings is closely linked to the particularities of the devitrification processes of the two considered alloys.
Mössbauer analysis of the phase distribution of Fe64.5Co18B16SiC0.5 soft magnetic samples in powder form
Authors: Partiti C. S. M. , Rechenberg H. R. , González J. , Garcia Tello P. , González J. M.
Published: July 2002

We report on data, obtained from the analysis of Mössbauer spectra, on the phase distribution present in FeCoBSiC samples which were prepared by milling a melt spun amorphous precursor in a high energy ball mill. The milling process reduced the material to a powdered form, induced the precipitation of FeCo and Fe3C crystallites and resulted in relatively high coercive force values. In contrast with this, short time thermal treatments, carried out in the as-milled samples at temperatures in the range from 575 up to 650 K, produced a very remarkable softening which, according to our phase distribution results, should be associated to the partial dissolution of a certain percentage of the atoms involved in the grain boundary structures. This resulted in improved coupling between the crystalline and amorphous phases forming the sample and consequently, in an easier displacement of the domain walls.
Perpendicular media signal and noise analysis by magnetic force microscopy on written transitions (abstract)
Authors: Mei L. , Schouterden K. , Liu W. H. , Ho K. , Lairson B.
Published: July 2002

We have quantitatively analyzed the signal and noise properties of perpendicular media from magnetic force microscopy images of written transitions. Four sets of Pt/CoCrTa multilayer thin films were prepared under different sputtering conditions to yield virgin media domain sizes ranging from 50 to 5000 nm. The magnetic recording transitions on each disk were written in the frequency range from 2 to 30 MHz, which corresponds to recording densities from 20 to 300 kfci. We performed Fourier analysis on the written and unwritten areas of the media to obtain signal and noise spectra. The media noise and signal-to-noise ratios from these spectra are comparable with those from recording head read-back spectra. These spectra are indicative of the exchange coupling existing in the films and correlate with the synthesis conditions for the various media. By analyzing the unwritten areas in the sample disks, we also obtained the average virgin domain sizes for these four kinds of disks.
Magnetism of Co thin film with perpendicular anisotropy grown on Au layer
Authors: Yamazaki H. , Tanaka Y. , Katsumata K.
Published: July 2002

Single Co film with a thickness of d (d=0.3–1.8 nm) was prepared on a Au buffer layer in an ultrahigh vacuum (10−10 Torr) system using electron beam evaporation. Temperature dependence of the uniaxial anisotropy constants was evaluated up to the fourth order from the result of electron spin resonance study. The critical exponent γ′ for the sample of d=0.3 nm was evaluated to be γ′=1.71±0.01, which is close to the theoretical value of 7/4 for the two-dimensional Ising system.
Superparamagnetic ultrathin films
Authors: Skomski R. , Sander D. , Shen J. , Kirschner J.
Published: July 2002

The finite-temperature magnetism of ultrathin films such as sesquilayer Fe/W(110) and submonolayer Fe/Cu(111) is investigated. Based on renormalizations of Onsager’s exact solution of the two-dimensional Ising model it is shown that superparamagnetism is a common phenomenon in imperfect ultrathin films. The ultimate reason for this behavior is the existence of two structural length scales: the lattice constant, usually considered in renormalization-group theory, and the characteristic size of the film inhomogeneities.
Time-resolved photoluminescence studies of cross-well transport in a biased GaAs/AlGaAs multiple quantum well p-i-n structure
Authors: Massa J. S. , Buller G. S. , Walker A. C.
Published: July 2002

Time-resolved photoluminescence has been used to study the cross-well carrier dynamics in a biased multiple quantum well p-i-n structure at temperatures in the range 5–350 K and for electric fields <200 kV cm−1. The photoluminescence decays have been parameterized using reconvolution analysis with a coupled rate equation model and this has provided strong evidence for the successive recapturing of carriers in adjacent wells. For temperatures <100 K, and for electric fields <60 kV cm−1, the carrier escape appears to be strongly affected by resonant tunneling between hole subbands in adjacent wells. At higher temperatures an increase in the carrier escape rate is observed which corresponds to a field dependent thermal activation energy.
Passive artificial molecule realizations of dielectric materials
Authors: Ziolkowski Richard W. , Auzanneau Fabrice
Published: July 2002

The design of dielectric materials with artificial molecules formed by electrically small dipole antennas loaded with passive electrical circuit elements is considered. Variations in the antenna loads lead to known and generalizations of known dielectric material models. These artificial dielectrics are characterized in terms of their equivalent susceptibilities and polarization vectors both in the frequency and time domains. With suitable choices in the antenna loads one can design the loss and dispersion properties of the resultant materials.
Formation of titanium nitride by annealing Ag/Ti structures in ammonia ambient
Authors: Zou Y. L. , Alford T. L. , Zeng Yuxiao , Deng F. , Lau S. S. , Laursen T. , Amali A. I. , Ullrich B. M.
Published: July 2002

Titanium nitride thin films have been formed in the temperature range of 400–600 °C by annealing Ag/Ti bilayer films on oxidized Si substrates in an ammonia ambient. Rutherford backscattering spectrometry and Auger depth profiling have shown the segregation of Ti at the surface and at interface. Ti diffused out through the silver layer and reacted with ammonia to form a TiN layer that self-encapsulated the silver film. A near-bamboo structure in the encapsulated Ag films was observed using cross-sectional transmission electron microscopy. Such a structure is expected to improve the electromigration resistance of the silver metallization. The kinetics of the Ti-nitride growth was studied by investigating its dependence on time, temperature, and Ag/Ti bilayer thicknesses. We also found that two processes govern the nitridation reaction. A dominant nitridation process takes place initially at fast growth rates. After 15 min anneals the nitride growth can be described by x2=B t, where B is a parabolic rate constant for the growing nitride phase. The parabolic rate constants follow an Arrhenius behavior with an apparent activation energy of ∼0.4 eV. These observations led to further discussion regarding the diffusion mechanism as well as the rate-limiting step.
Extrinsic photoresponse and photoluminescence of CuInSe2 crystals grown with a deviation from valence stoichiometry
Authors: Medvedkin G. A. , Magomedov M. A.
Published: July 2002

Extrinsic photoresponse spectra (T=300 K) and photoluminescence spectra (T=77 K) of CuInSe2 single crystals grown with the deviation from valence stoichiometry are studied for point defect identification. The main peaks of photoresponse and photoluminescence correlate each other. The characteristic spectral singularities at 1.00, 0.98–0.97, 0.94, 0.92–0.89, and 0.72 eV have been attributed to photoactive and radiative transitions through the donor (VSe,InCu) and acceptor (VCu,CuIn,Sei) levels. The shift of the extrinsic photoresponse peak to long wavelengths (0.92→0.89 eV) by the VSe point defect has been found at increasing deviation from valence stoichiometry ΔZ<0. The impurity subband formation has been suggested as very probable in the crystals with elevated ΔZ.
Detection of the metastable state of the EL2 defect in GaAs
Authors: Bourgoin J. C. , Neffati T.
Published: July 2002

Using a classical photocapacitance technique, we have transformed the well-known EL2 defects, related to the As antisite in GaAs, into their metastable states. Using the capacitance, we have monitored the temperature dependence of the electron occupancy of these metastable states at thermal equilibrium. From this study, we deduce that a level located at 40 meV below the conduction band is associated with electron ionization from the metastable EL2 states.
Stefan problem for a finite liquid phase and its application to laser or electron beam welding
Authors: Kasuya Tadashi , Shimoda Nobuyuki
Published: July 2002

An exact solution of a heat conduction problem with the effect of latent heat of solidification (Stefan problem) is derived. The solution of the one dimensional Stefan problem for a finite liquid phase initially existing in a semi-infinite body is applied to evaluate temperature fields produced by laser or electron beam welding. The solution of the model has not been available before, as Carslaw and Jaeger [Conduction of Heat in Solids, 2nd ed. (Oxford University Press, New York, 1959)] pointed out. The heat conduction calculations are performed using thermal properties of carbon steel, and the comparison of the Stefan problem with a simplified linear heat conduction model reveals that the solidification rate and cooling curve over 1273 K significantly depend on which model (Stefan or linear heat conduction problem) is applied, and that the type of the thermal model applied has little meaning for cooling curve below 1273 K. Since the heat conduction problems with a phase change arise in many important industrial fields, the solution derived in this study is ready to be used not only for welding but also for other industrial applications.
Influence of a finite energy width on the hot electron double-slit interference experiment: A design of the emitter structure
Authors: Hongo Hiroo , Miyamoto Yasuyuki , Gault Michael , Furuya Kazuhito
Published: July 2002

The influence of electron energy width in the hot electron double-slit experiment is investigated quantitatively. The required condition on the Fermi level in the emitter and the slit-spacing is derived for the experiment. In order to achieve a coherent electron source, a single-barrier graded emitter structure is discussed and its characteristics are considered. For application to the hot electron double-slit experiment, the graded emitter diode is fabricated and the current–voltage relation is measured in a supplementary experiment.
Optical properties of Fe1−xSix alloys (x⩽0.1) studied by spectroscopic ellipsometry
Authors: Kim Kwang Joo , Kang Ji Hoon , Bahng Jae Ho , Lee Myoung Hee , Lynch D. W.
Published: July 2002

Optical properties of dilute Fe1−xSix (x⩽0.1) alloys were studied by spectroscopic ellipsometry in the 1.5–6 eV photon energy region. The absorption strength of the alloys is found to decrease in the entire energy region as the Si composition increases. The interband-transition edges of Fe at about 2.5 eV in the minority-spin bands and at about 5 eV in the majority-spin bands shift to lower energies as the Si composition increases. The decrease in the absorption strength is interpreted as due to a decrease in the density of states of the Fe host and also due to a reduction of conduction-electron concentration in the alloys. The shifts of the 2.5 and 5 eV interband-transition edges can be explained in terms of a perturbative variation in the band structure of Fe induced by its alloying with Si.
Spectral diffusion within the porous silicon emission wavelength range on the nanosecond to millisecond time scale
Authors: Song Li , El-Sayed M. A. , Chen Peter C.
Published: July 2002

The emission spectrum from porous silicon (PS) at room temperature was recorded after different delay times ranging from 30 ns to 2.0 ms after pulsed laser excitation by using a gated charge-coupled device camera. In agreement with previous studies, the photoluminescence of porous silicon was found to redshift with delay time in the ns to 100 μs time scale. However, a study of the normalized band shape of the redshifted emission reveals that the emission spectrum retains its band shape rather than giving a distorted band shape that increases in intensity on the longer wavelength side. This behavior suggests that the redshift in the emission spectrum of porous silicon is a result of spectral diffusion resulting from energy transfer among emitters within the inhomogeneously broadened absorption spectrum. Furthermore, on the longer time scale (0.8–2 ms), the much weaker, long wavelength emission spectrum is found to blueshift as the delay time is increased. Two peaks were resolved in the photoluminescence spectrum. One is centered around 650 nm and the other is centered around 750 nm. The photoluminescence at 600 and 800 nm have lifetimes of 0.35 and 0.19 ms, respectively. This observation suggests the existence of two distinct molecular species responsible for the observed photoluminescence in PS.
Measurement of doping concentration in boron-doped diamond film from capacitance spectroscopy
Authors: Liu Kun , Johnston C. , Chu J. H. , Roth S. , Zhang Bo , Wan Mingfang
Published: July 2002

Schottky barrier structure and metal-insulator-semiconductor structure have been fabricated on boron-doped diamond film samples, which were grown using the microwave enhanced chemical vapor deposition method. By measuring the capacitance-voltage (CV) spectroscopies of both structures, boron acceptor concentration of ∼1017 cm−3 in the diamond samples has been measured. In the CV measurement, the influences from interface states, non-boron deep centers, emission/capture time constant distribution of boron acceptors and diamond resistance have been considered. It shows that proper ac modulation frequency should be selected in the measurement. It also shows that, in the measurement of dopant concentration in wide gap semiconductors, the CV measurement of the semiconductor Schottky barrier structure is a better choice, because Ohmic contact problem can be avoided.
Photoluminescence studies of erbium-doped GaAs under hydrostatic pressure
Authors: Culp T. D. , Hömmerich U. , Redwing J. M. , Kuech T. F. , Bray K. L.
Published: July 2002

The photoluminescence properties of metal-organic chemical vapor deposition GaAs:Er were investigated as a function of temperature and applied hydrostatic pressure. The I13/24→4I15/2Er3+emission energy was largely independent of pressures up to 56 kbar and temperatures between 12 and 300 K. Furthermore, no significant change in the low temperature emission intensity was observed at pressures up to and beyond the Γ-X crossover at ∼41 kbar. In contrast, AlxGa1−xAs:Er alloying studies have shown a strong increase in intensity near the Γ-X crossover at x∼0.4. These results suggest that the enhancement is most likely due to a chemical effect related to the presence of Al, such as residual oxygen incorporation, rather than a band structure effect related to the indirect band gap or larger band gap energy. Modeling the temperature dependence of the 1.54 μm Er3+ emission intensity and lifetime at ambient pressure suggested two dominant quenching mechanisms. At temperatures below approximately 150 K, thermal quenching is dominated by a ∼13 meV activation energy process which prevents Er3+ excitation, reducing the intensity, but does not affect the Er3+ ion once it is excited, leaving the lifetime unchanged. At higher temperatures, thermal quenching is governed by a ∼115 meV activation energy process which deactivates the excited Er3+ ion, quenching both the intensity and lifetime. At 42 kbar, the low activation energy process was largely unaffected, whereas the higher activation energy process was significantly reduced. These processes are proposed to be thermal dissociation of the Er-bound exciton, and energy back transfer, respectively. A model is presented in which the Er-related electron trap shifts up in energy at higher pressure, increasing the activation energy to back transfer, but not affecting thermal dissociation of the bound exciton through hole emission.
Optical characterization of AlInGaAs/InGaAs quantum well structures on InGaAs substrates
Authors: Jedral L. , Edirisinghe C. , Ruda H. , Moore A. , Lent B.
Published: July 2002

We report on photoluminescence and photoreflectance studies of metalorganic chemical vapor deposition grown InGaAs/InGaAlAs quantum well structures on lattice matched InGaAs substrates. The optical characteristics of the substrates are also discussed. The quantum wells exhibit a high degree of relaxation, despite their thickness being below the critical values, which may be related to the growth conditions. A correlation was found between the optical characteristics and the quality of the layer structures grown on the InGaAs substrates.
Carrier heating and the power independent linewidth in semiconductor lasers
Authors: Ongstad Andrew P. , Dente Gregory C. , Tilton Michael L.
Published: July 2002

We present linewidth measurements on AlGaAs double heterostructure lasers as a function of temperature and power. The data, which plot observed linewidth as a function of reciprocal power, reveal decreasing slopes and increasing vertical axis intercepts, or power independent linewidths, with falling temperature, consistent with previous experimental observation [D. Welford and A. Mooradian, Appl. Phys. Lett. 40, 560 (1982)]. To explain the power independent linewidth broadening observed in these continuous wave single frequency lasers, we propose that carrier heating processes create a thermal nonequilibrium between electrons and the semiconductor lattice, and that this leads to a power dependent linewidth enhancement factor. From a carrier thermal model we find that the power independent linewidth is mediated by the longitudinal optical (LO)-phonon lifetime (τLO); as τLO is increased (for example, by decreasing the lattice temperature), the noneqilibrium condition is enhanced, and, consequently, the power independent linewidth increases.
Atomic force and scanning tunneling microscopy study of current-voltage properties of TiB2 microcontacts
Authors: Heuberger M. , Dietler G. , Strümpler R. , Rhyner J. , Isberg J.
Published: July 2002

Scanning probe microscopy was used to investigate electrical microcontacts in the nanometer range. With the atomic force microscope current flowing through the contact as well as current-voltage characteristics of the contact as a function of the force acting on the contact itself were recorded. With the scanning tunneling microscope current-voltage characteristics and voltage at constant current characteristics were measured as a function of the contact position. From these experiments we conclude that a TiB2 microcontact can sustain a maximum voltage of about 0.1–1 V before melting. These results were confirmed by a theoretical model. The implications for the functioning of so-called positive temperature coefficient current limiting devices based on filled polymers is discussed.
Cathodoluminescence and electron beam induced current study of partially relaxed AlGaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions
Authors: Sjölund O. , Lin H. T. , Rich D. H. , Ghisoni M. , Larsson A. , Wang S. , Thordsson J. , Andersson T. G.
Published: July 2002

We have studied time-resolved cathodoluminescence (CL) and electron beam induced current (EBIC) on AlGaAs/GaAs/InGaAs heterojunction phototransistors under operating conditions, i.e., at room temperature and under bias. Devices from four wafers, with a different amount of lattice relaxation, were tested. It is shown that the CL intensity increases more than one order of magnitude as the voltage is increased and the current gain of the device turns on. The voltage dependence of the CL signal is analogous to the current–voltage curve of the transistor. The buildup in CL intensity was found to be much less in devices with low current gain showing that the CL intensity correlates to the electrical gain of the device. Time resolved CL showed two distinct CL decay times, one very short, a few nanoseconds, and one long, of the order of microseconds. This indicates that two fundamental recombination processes are present, which we attribute to a spatially direct recombination between carriers in the base and a spatially indirect recombination. This spatially indirect recombination is believed to come from recombination of electrons trapped in the notch formed at the conduction band discontinuity and holes in the base. By studying EBIC as a function of beam current for devices from the different wafers we found that relaxed devices have a complex current-gain relationship. They require higher current densities than nonrelaxed devices to reach high gain. At low current densities the gain is very low and the ideality factor is high indicating a high degree of trap related recombination. At high current densities, on the other hand, these traps become filled and the associated recombination quenched. This results in a gain and an ideality factor comparable to those of nonrelaxed devices.
Calculation of levitation forces in permanent magnet-superconductor systems using finite element analysis
Authors: Camacho D. , Mora J. , Fontcuberta J. , Obradors X.
Published: July 2002

In this paper we present calculations of levitation forces between a cylindrical permanent magnet and a cylindrical superconductor using a commercial finite element program. Force limits for zero field cooled and field cooled processes have been obtained using the Meissner effect and the perfect pinning hypothesis, respectively. Comparison of the experimentally determined forces with respect to these limits provides a simple estimation of the sample quality. The hysteretical behavior of the forces has been reproduced assuming a critical state model for the superconductor. Results are compared with experimental data. Excellent agreement has been found for forces measured after zero field cooled process thus allowing us to estimate the critical current of the samples. As a further exploitation of the software capabilities we have investigated the effects of the superconducting sample geometry and the effects of different strategies of flux conditioning to optimize the levitation forces.
Paley–Wiener criterion for relaxation functions
Authors: Macdonald J. Ross
Published: July 2002

The conclusion [K. L. Ngai, A. K. Rajagopal, R. W. Rendell, and S. Teitler, Phys. Rev. B 28, 6073 (1983)] that simple exponential decay is a nonviable model for electrical relaxation, because it fails to satisfy the fundamental Paley–Wiener Fourier transform criterion, is shown by direct analysis to be inapplicable to small-signal electrical relaxation situations. Thus, not only is exponential decay and its associated single-relaxation-time Debye frequency response a valid model for relaxation, but, by extension, all distributions of relaxations times and energies which use a superposition of simple exponentials or Debye functions are also acceptable descriptions of relaxation phenomena. Reasons why the earlier conclusion is nonviable in the present context are discussed.
Effect of inner-shell excitation of disilane on the reaction yield of synchrotron-radiation excited high-vacuum chemical vapor deposition
Authors: Utsumi Yuichi
Published: July 2002

The reaction yields of synchrotron-radiation excited high-vacuum chemical vapor deposition at the core-excitation and valence-excitation regions of disilane are obtained by measuring both the incident photon numbers of the synchrotron-radiation beams which pass through C and Al filters and the rates of Si deposition induced by these synchroton-radiation beams. It is confirmed that the reaction yield for the core-excitation energy region is about three times larger than the value for the valence-excitation region. It is also demonstrated that synchrotron-radiation excited high-vacuum chemical vapor deposition proceeds by the pure-photochemical reaction, not by the reaction induced by secondary electrons due to synchrotron-radiation irradiation on the substrate.