bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetization of amorphous Fe0.82B0.18 and Fe0.90Zr0.10 compounds with additions of Tb
Authors: Clegg S. J. , Purdy J. H. , Greenough R. D. , Jerems F.
Published: July 2002

The addition of Tb to melt spun iron-boron or iron-zirconium compounds raises the magneto- striction to practically useful levels (≥450 ppm) at room temperature. The present work examines the corresponding compositional and temperature dependencies of the magnetization in Tbx (Fe0.82B0.18)1−x for 0≤x≤0.5 and Tb y (Fe0.90Zr0.10)1−y for 0≤y≤0.1. Increasing the Tb content in Fe-B compounds decreases the room-temperature magnetization towards a compensation point and lowers the Curie temperature Tc. In Fe-Zr compounds, Tb increases the room-temperature magnetization and raises Tc. Magnetic phase diagrams are presented for each group of compounds and exchange frustration effects identified by comparing the observed temperature dependencies of the reduced magnetization with predictions from the Handrich mean field model. Only 3 at. % Tb in Fe-Zr is sufficient to annul exchange fluctuations, but in Fe-B compounds exchange frustration persists throughout the compositional range.
Impurity hyperfine fields in metastable body centered cubic Co
Authors: Dekoster J. , Swinnen B. , Rots M. , Langouche G. , Jedryka E.
Published: July 2002

A new spectrum component is observed in Mössbauer spectra of thin body centered cubic Co layers doped with 57Co which have previously been shown to be in this metastable state with nuclear magnetic resonance. It is characterized with a large magnetic hyperfine field (31.2 T) and an isomer shift nearly equal to that of α-Fe. The decrease of the isomer shift in bcc Co with respect to hcp Co is consistent with a smaller s–d charge transfer in bcc Co as compared to hcp Co. The influence of structure on the hyperfine field in a Co crystal is reflected in the decrease of the magnetic hyperfine field of Fe in bcc Co as compared to hcp Co and is similar to the volume dependence of the magnetic hyperfine fields in a Co crystal.
Electrical resistivity and thermoelectric power of heavy fermions and mixed-valence systems
Authors: Panwar Sunil , Singh Ishwar
Published: July 2002

Representing the heavy fermions and mixed-valence systems by the periodic Anderson model, we have used the variational method to study the temperature dependence of electronic transport properties of these systems. The electrical resistivity ρ(T) and thermoelectric power Q(T) calculated show the features experimentally observed in these materials. In the low-temperature region ρ(T) and Q(T) increase rapidly. Toward high-temperature region, Q(T) changes sign.
Study of desorbed hydrogen-decrepitated anisotropic Nd-Fe-B powder using x-ray diffraction
Authors: Meisner G. P. , Panchanathan V.
Published: July 2002

The intrinsic magnetic coercivity (Hci) of Nd-Fe-B-based permanent magnet material is profoundly affected by hydrogen absorbed during the hydrogen decrepitation (HD) process for producing anisotropic powders from bulk anisotropic hot-deformed MAGNEQUENCH (MQ) magnets. Hydrogen (H) content and x-ray diffraction measurements clarify the effects of H and desorption temperature (Td) on the intrinsic magnetic anisotropy (IMA) of the Nd2Fe14B-type phase and the nature of the intergranular phases, both of which are crucial for high Hci. The Nd-rich intergranular phase disproportionates during HD, initially forming a microcrystalline Nd-hydride phase, possibly Nd2H5. For Td≤220 °C, H remains in the Nd2Fe14B-type phase, severely degrading the IMA, which causes a low Hci. For 220 °C≤Td≤250 °C, enough H desorbs from the Nd2Fe14B-type phase and the IMA recovers its large prehydrided value, and the microstructure supports a high Hci≤10 kOe in spite of the H disproportionated intergranular phase. Only for Td≳250 °C is Hci degraded by the microstructure, corresponding to further H desorption and the microcrystalline Nd-hydride phase becoming well-crystallized NdH2. The NdH2 phase decomposes with continued H desorption and at Td≳580 °C recombines to re-form the Nd-rich intergranular phase of prehydrided MQ material. H is completely desorbed above 580 °C and Hci≳11 kOe, nearly that of the prehydrided MQ magnets.
Magnetic structures and interactions in Ho/Y, Ho/Lu, and Ho/Er superlattices (invited)
Authors: Cowley R. A. , McMorrow D. F. , Simpson A. , Jehan D. , Swaddling P. , Ward R. C. C. , Wells M. R.
Published: July 2002

Ho/Y and Ho/Er superlattices have been grown by molecular-beam epitaxy using a Balzers UMS 630 instrument. The superlattices were grown on a sapphire substrate with an Nb buffer and Y seed layer. X-ray-diffraction techniques were used to characterize the crystallographic structure and neutron-diffraction techniques to determine the magnetic structures. The results for the Ho/Y systems were consistent with long-range order being formed coherently through the whole superlattice. The moments in the Ho layers were aligned in the basal plane and most of the structures could be described by helical structures with a turn angle between holmium planes of ΨH and between nonmagnetic Y planes ΨY. ΨY is found to be largely independent of temperature or superlattice, while ΨH decreases with decreasing temperatures and at low temperatures takes a commensurate value, so as to take advantage of the basal plane anisotropy. The results for the Ho/Er superlattices differ because the Er has a magnetic moment and the anisotropy favors alignment along the c axis. Between the ordering temperature of bulk Ho and bulk Er, the results are similar to those of the Ho/Y superlattices. The ordering propagates through the Er layers but the Er moments are not ordered. At lower temperatures the Er moments order in a cycloidal (a/c) structure with the basal plane components having fairly long-range coherence with the Ho moments, but the c-axis components having no coherence from one Er layer to the next.
Interlayer coupling in antiferromagnetic EuTe/PbTe superlattices (abstract)
Authors: Giebultowicz T. M. , Nunez V. , Springholz G. , Bauer G. , Chen J. , Dresselhaus M. S. , Furdyna J. K.
Published: July 2002

We report neutron diffraction studies of magnetic correlations in MBE-grown (111) EuTe/PbTe multilayers. Bulk EuTe is a type-II fcc antiferromagnet. This structure consists of ferromagnetic (FM) sheets of spins on (111)-type planes which are coupled antiferromagnetically to one another. In EuTe/PbTe the lattice mismatch strain selects an arrangement of the FM sheets parallel to the multilayer plane. Thus, in the case of an odd number of EuTe monlayers, there should be a nonzero magnetic moment—in other words, such superlattices are expected to behave as ferrimagnets, not anitferromagnets. This has been confirmed by recent SQUID studies. As suggested by the above model, the dipolar fields arising from the lack of total moment compensation may introduce interlayer magnetic coupling, even though in this system there are no carriers that can transfer magnetic interactions across the nonmagnetic spacers through the RKKY mechanism, which is usually the case in coupled magnetic multilayered systems. Our experiments have indeed revealed clear magnetic superlattice peak patterns in several samples. The results of measurements in high external fields (up to 6 T) bring further support for the dipolar nature of the observed interlayer coupling. However, in some contrast to the expected scenario, the most pronounced coupling effects were seen in the case of an even number of monolayers. Possible explanations of this fact (e.g., higher-order terms in dipolar fields, or structural imperfections) are discussed.
Quadrupolar effects in PrCu2Si2
Authors: Osborn R. , Goremychkin E. A.
Published: July 2002

As part of a systematic study of the crystal-field (CF) potential in RCu2Si2 compounds (R=Ce, Pr, Nd, Tb, Ho, Er) using inelastic neutron scattering, the CF level scheme and potential in the antiferromagnet PrCu2Si2 both above and below TN has been determined. There have been recent speculations that the quadrupole moment of PrCu2Si2 has been quenched by the quadrupolar Kondo effect. Using the CF potential derived from the results, the quadrupole moment Q2 of PrCu2Si2 has been calculated and it has been compared to the other members of the RCu2Si2 series. The temperature dependence of the quadrupole moment of PrCu2Si2 is significantly weaker than all the other compounds, e.g., it is approximately five times smaller than in HoCu2Si2. Therefore, it is the CF potential which is responsible for quenching Q2 at low temperature rather than a quadrupolar Kondo effect. Furthermore, the CF Schottky contribution to C/T vs T2 is approximately linear above TN and explains the anomalously high linear term in the specific heat. However, the evolution of the CF potential across the rare-earth series provides evidence of an enhanced hybridization contribution to the CF potential of PrCu2Si2, intermediate between the heavy fermion CeCu2Si2 and the other rare-earth compounds.
Consequences of competing hybridization for magnetic ordering in correlated-electron lattices
Authors: Sanchez-Castro Carlos , Cooper Bernard R. , Bedell Kevin S.
Published: July 2002

A method is presented for calculating the f-f ion interaction in systems where a d-electron species also hybridizes with the same Fermi sea. For the physical systems of interest, typically the f species is a light rare earth (e.g., Ce or Pr) or a light actinide (U) involving a partially filled f shell and the d species is a transition-metal ion; and there may be experimental evidence of competition between magnetic ordering of the f and d electron systems (e.g., when f is U and d is Mn). The method treats first the strong hybridization between the d and the conduction electrons to obtain a new ground state with delocalized d electrons. Then it calculates the f-f ion interaction by perturbation theory. The mechanisms by which the d electrons modify the f-f ion interaction are identified.
Field-dependent susceptibility aging in CuMn spin glasses
Authors: Fenimore P. W. , Weissman M. B.
Published: July 2002

A ‘‘hole’’ is found to develop in the dc field-dependent ac susceptibility of CuMn spin glasses when the material is cooled or aged in a field. Comparisons are made with analogous experiments on structural glasses at low temperature. The hole is not compatible with independent two-level systems. The significance of the width (in H) of this hole is discussed.
Remanent magnetization of AgMn spin glasses (abstract)
Authors: Engle Emily , Dahlberg E. Dan
Published: July 2002

Plots of the dc demagnetization remanence, Id, versus the isothermal remanent magnetization, Ir, have been used to determine the presence of interactions in concentrated magnetic systems. In terms of interacting entities within the system, the standard interpretation of these plots is that positive (negative) curvature indicates the presence of demagnetizing (magnetizing) interactions between the magnetic entities. We will report the first such analysis made on a spin glass (the archetypal spin glass system, AgMn, with three Mn concentrations). As prescribed, Id and Ir were normalized and plotted against each other. All of the plots of zero-field quenched specimens have positive curvature, indicating the presence of demagnetizing interactions. In these measurements the sample shape dependent demagnetizing field was not a factor because the magnetizing field was applied parallel to the long axis of the foils (0.50 in. × 0.15 in. ×∼0.005 in.). These results are consistent with a recent model for slow relaxation wherein interactions, not disorder, determine the quasilogarithmic time dependence of relaxation processes in these systems.
Transverse susceptibility and magnetic anisotropy of CoTi-doped barium hexaferrite single crystals
Authors: Zimmermann G. , Hempel K. A.
Published: July 2002

Measurements of the reversible transverse susceptibility (RTS) were performed on BaFe12−x−yCoxTiyO19 single crystals with doping concentrations x≊y≤0.77 under different angles ϑ0 between the easy axis and applied field. For intermediate doping concentrations the occurrence of a peak in the RTS curve for ϑ0=90° indicates that the domain walls are not free to move. This assumption is confirmed further by the large component of the RTS being out of phase with respect to the excitation field HE. The phase angle of the initial susceptibility measured parallel to the easy axis becomes maximum at a characteristic excitation field value HE,C and a relaxation of the susceptibility is observed if HE steps from zero to a value larger than HE,C. Using the RTS curve for ϑ0=0 and assuming that the peak of the RTS is located at the same field strength as if the particle were single domain, the first-order anisotropy field HA=2K1/(μ0Ms) and the second-order anisotropy constant K2 can be determined. The values found this way are compared to results from magnetization-curve analysis and from other authors.
Incommensuration and the distorted oxygen octahedron of antiferroelectric tin-modified lead zirconate titanate
Authors: Forst Donald , Viehland Dwight
Published: July 2002

A study of the thermal-mechanical properties of incommensurate antiferroelectric tin-modified lead zirconate titanate has been performed as functions of Sn and Ti contents. Deviation from a linear temperature dependence in the thermal-expansion strain (ε) was observed below ∼400 °C. The deviation resulted from a contraction of the lattice relative to an extrapolation of a high-temperature linear behavior. Dielectric property measurements were also performed. Strong deviations from Curie–Weiss behavior were found to high temperatures, however with increasing Ti content, the degree of deviation from linearity in ε decreased, whereas that in the reciprocal dielectric constant increased. This indicates that the nonlinearity in ε does not arise due to an electrostrictive coupling of polarization fluctuations to the lattice. Rather, an oxygen octahedra tilt mechanism is proposed. The existence of 1/2 [111] superlattice reflections (F spots) to temperatures close to that of the onset of nonlinearity in ε gives support to this possibility.
Interdiffusion between Y-Ba-Cu-O thin films and Al2O3 substrates studied by applying Rutherford backscattering spectrometry combined with atomic force microscopy
Authors: Igarashi Y. , Fujino Y. , Suzuki N. , Iimura K.
Published: July 2002

Interdiffusion between sputter-deposited thin Y-Ba-Cu-O films and Al2O3 substrates has been studied by applying 3.2 MeV 4He+ Rutherford backscattering spectrometry. The effects of morphology variation of the films during annealing have been successfully excluded using the data of thickness distribution measured by applying atomic force microscopy. The formalism used for the analysis is presented. The obtained values of the diffusivities are more than 20 times smaller than those reported by other workers.
Structure and magnetic properties of Nd2Fe14B fine particles produced by spark erosion (abstract)
Authors: Wan H. , Berkowitz A. E.
Published: July 2002

At present Nd2Fe14B is the best permanent magnet because of its extremely high coercivity and energy product. Optimum properties of Nd2Fe14B magnets can be attained by producing single domain particles, and then aligning and compacting them. Due to the high reactivity of the Nd constituent, it is challenging to produce and handle a large amount of fine particles of this material. We have prepared fine particles of Nd2Fe14B by spark erosion with various dielectric media. Yield, size, size distribution, structure, and magnetic properties are discussed. The Nd2Fe14B particles were made by the shaker pot spark erosion method. Relaxation oscillators or a pulse generator were used to power the spark erosion. Commercial Neomax 35 was employed as the primary material. The dielectric media were liquid Ar, Ar gas, and hydrocarbons, which provided an oxygen free environment. Structure and size were studied by TEM, SEM, and x-ray diffraction. Magnetic properties were measured by VSM with temperatures in the range of 4.2–1200 K. The particles produced in these three different dielectric media had different microstructures and crystal structures. The particles made in Ar gas were pure Nd2Fe14B phase. The particles made in liquid Ar were a mixture of amorphous and crystalline Nd2Fe14B, because the liquid Ar provided a much higher quench rate than Ar gas, which produced some amorphous Nd2Fe14B. Upon annealing, the amorphous particles became crystalline. The fine particles produced in hydrocarbons, such as pentane and dodecane, had more complex mixed phases, since the rare earth reacted with the hydrocarbons during the sparking process. The phases were NdC2, α-Fe, and amorphous and crystalline Nd2Fe14B. The effects of power parameters, such as voltage and capacitance, on particle size were investigated. Particle sizes from 20 nm to 50 μm were obtained. We concentrated on Nd2Fe14B fine particles made in liquid Ar and Ar gas. Particles were classified by centrifuging and sizes were confirmed by TEM and x-ray diffraction. The size dependence of coercivity, anisotropy, and magnetization of these Nd2Fe14B particles will be discussed.
Magnetic response of ultrathin Fe on MgO: A polarized neutron reflectometry study
Authors: Adenwalla S. , Park Yongsup , Felcher G. P. , Teitelman M.
Published: July 2002

The magnetization of ultrathin bcc Fe films (two and three monolayers) on MgO was measured and compared with the behavior predicted for a two-dimensional ferromagnet. The experiment indicated that no hysteresis was present in the magnetization. Instead, the magnetization at low temperature was affected by a marked field cooling effect. These observations lead to the conclusion that films of Fe on MgO of such thickness exhibit superparamagnetic behavior as if they were not entirely continuous. In contrast, films thicker than five monolayers exhibit a magnetic response close to that of bulk iron.
Calculation of magnetic moments in Ho2C3 nanocrystals
Authors: Majetich S. A. , Artman J. O. , Tanaka C. , McHenry M. E.
Published: July 2002

A general approach to the computation of effective magnetic moments in rare-earth carbides is described, and details of this calculation for Ho3+ ions in Ho2C3 are presented. This calculation is designed to explain the reduced magnetic moments, relative to free ion values, measured by dc SQUID magnetometry for Ho2C3 nanocrystals. Crystal-field splittings of the rare-earth ion in a particular symmetry site are determined by the operator equivalent method. Using the eigenvalues and eigenfunctions of the crystal-field Hamiltonian, the effective magnetic moment is then determined. For Ho3+ ions in Ho2C3, this method predicts a reduced magnetic moment, but the degree of reduction depends on the energy-level splittings and, therefore, the temperature. This magnetic moment is compared with previous experimental results, and the implications of the formal carbon charge, screening, and temperature are discussed.
Native oxides on Si surfaces of deep-submicron contact-hole bottoms
Authors: Aoto Nahomi , Nakamori Masaharu , Yamasaki Shinya , Hada Hiromitsu , Ikarashi Nobuyuki , Ishida Koichi , Teraoka Yuden , Nishiyama Iwao
Published: July 2002

The effects of cleaning and treatment on the characteristics of contact-hole-bottom Si surfaces are investigated in order to reveal the origin of the increased contact resistance and to find treatment processes that can be used to obtain low contact resistance. Contact-hole-bottom Si surfaces were analyzed by using thermal desorption spectroscopy, transmission electron microscope, and energy-dispersive x-ray spectroscopy. Nonpatterned Si surfaces, which roughly simulate the properties of the contact-hole-bottom Si surfaces, were also analyzed by using x-ray photoelectron spectroscopy. It is revealed that suboxide-rich native oxide layers are formed on dry-etch-damaged Si surfaces. The oxide layer persists after the samples are cleaned with a mixture of NH4OH, H2O2, and H2O, and with a mixture of HCl, H2O2, and H2O, and even after dipping in diluted HF. The roughly 1.3-nm-thick oxide layer remains at the plugging-poly-Si/Si–substrate interface, increasing the contact resistance. The carbon contamination in the dry-etch-damaged layer contributes less to the increase in contact resistance. The dry-etch-damaged Si layer is removed by chemical dry etching. On the resultant damage-free surfaces, native oxides with low suboxide density appear after NH4OH/H2O2/H2O and HCl/H2O2/H2O cleaning. Such oxides are easily removed by treatment with diluted HF, resulting in low contact resistance. An integrated contact-hole treatment sequence is thus achieved to control the Si surface condition. The resultant low-contact-resistance deep-submicron contact holes, plugged with P-doped poly-Si, can be applied for deep-submicron contacts of 256 Mbit and larger dynamic random access memories.
Effects of irradiation temperature on radiation damage in InP solar cells
Authors: Yamaguchi Masafumi
Published: July 2002

1-MeV electron irradiation damage in n+-p junction InP solar cells has been studied as a function of irradiation temperature, in comparison with those in GaAs cells. Solar cell property degradation of InP solar cells due to electron irradiation has been found to strongly decrease with an increase in irradiation temperature while that in GaAs cells is independent of irradiation temperature up to 150 °C. These results are explained by thermal annealing phenomena of radiation-induced defects in InP solar cells. The irradiation temperature dependence of radiation damage in InP cells has been analyzed based on annealing characteristics of radiation-induced defects, determined by deep-level transient spectroscopy measurement, in InP.
Electrical and optical properties of ruthenium-related defects in silicon
Authors: Pettersson H. , Grimmeiss H. G. , Schmalz K. , Knecht A. , Pässler R.
Published: July 2002

The electrical and optical properties of defects due to ion implantation of ruthenium in silicon have been studied by means of junction space-charge techniques. Two energy levels were observed with energy positions at Ec−0.184 eV (A-level) and EV+0.265 eV (B-level), respectively, at 77 K. The changes in enthalpy due to the capture of electrons and holes were −8 meV (A-level) and 1 meV (B-level). Gibb’s free energies at different temperatures were calculated for both levels. Good agreement with the corresponding optical threshold energy was found for the B-level suggesting a small Frank–Condon shift. A tentative model for the origin of the observed defects is discussed.
Current-biased superconducting strips as position-sensitive particle detectors
Authors: Scherschel M. A. , Hagen C. W. , Jaggi A. , Maier Th. , Zehnder A. , Zhao S. P. , Ott H. R.
Published: July 2002

The feasibility of superconducting strips as position-sensitive detectors for α particles was investigated. For this purpose films have been prepared of different materials and dimensions using standard evaporation and laser ablation in an UHV system. Films were patterned into strips and current-biased strips were irradiated with 5.5 MeV α particles. The energy deposited by the particle in the strip creates a normal-conducting region, which in turn causes a voltage drop. The time evolution of the voltage drop across the strip as a function of bath temperature and bias current has been investigated. Information about the site of the strip hit by the α particle was obtained from monitoring the propagating normal-conducting zones. A lateral position sensitivity of ±30 μm for a tantalum strip detector has been achieved. The propagation of normal-state zones in the strip cannot provide information on the energy of the particle. A model describing the time evolution of a normal-conducting zone in a current-biased superconducting strip was used to test and interpret the experimental results and allows one to estimate the thermal conductivity of the strip material and the heat transfer from the strip into the substrate.
Influence of the DX center on capacitance-voltage profiling for Si δ -doped AlxGa1−xAs
Authors: Yang G. M. , Seo K. S. , Choe Byung-Doo
Published: July 2002

Si δ-doped AlxGa1−xAs structures grown by metalorganic chemical vapor deposition have been studied by capacitance-voltage (C-V) measurements against temperature and frequency. The potential lowering due to the ionized DX centers is responsible for hysteresis and abnormal phenomena in C-V curves at low temperatures. Two electron-accumulation peaks are observed in C-V profiles measured in the temperature range of 200–300 K. One is related to the occupancy of localized DX centers, and the other is related to the two-dimensional electron gas at the planar layer of dopants. The apparent C-V profile, which is widely used to determine the dopant distribution, also depends on the measuring frequency. This behavior is interpreted as an artifact caused by the emission kinetics of the DX centers localized in the dopant plane.
Electric field effects in superconducting YBa2Cu3O7−x thin films using field-effect transistor structures
Authors: Taheri E. H. , Cochrane J. W. , Russell G. J.
Published: July 2002

All-thin-film SrTiO3/YBa2Cu3O7−x/SrTiO3/Cu structures fabricated on sapphire substrates were used to study large electric field effects in YBCO superconducting films. The YBCO films (∼100 nm) were deposited by off-axis radio-frequency sputtering while the top SrTiO3 films (∼200 nm) were reactively vacuum deposited at low temperatures to avoid significant deoxygenation of the superconducting YBCO underlayer films. Cu was used in the field-effect transistor-type YBCO/SrTiO3/Cu multilayered structures as the gate electrode and as Ohmic contacts for the source and drain electrodes. This avoided probable electroforming processes taking place in the SrTiO3 insulating matrices under the large electric fields arising from the applied gate voltages. The YBCO films were polycrystalline with Tc onsets ∼72–82 K. The gate voltages were found to change both the Tc and Tco values and hence the resistivity of the relatively thick superconducting YBCO layers in the normal and superconducting states.
Influence of the interface composition of InAs/AlSb superlattices on their optical and structural properties
Authors: Spitzer J. , Höpner A. , Kuball M. , Cardona M. , Jenichen B. , Neuroth H. , Brar B. , Kroemer H.
Published: July 2002

A series of five short-period (InAs)6/(AlSb)6 superlattices, grown either with AlAs-like, InSb-like, or alternating interfaces, were studied by means of x-ray diffraction, high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, photoluminescence and ellipsometry. The combination of these techniques allows us to explain the pronounced differences in the optical and structural properties of both types of interfaces. In samples with an AlAs-like bottom interface x-ray, HRTEM and Raman results demonstrate the differing structural quality to be related to inhomogeneous strain relaxation and As intermixing. The energies of the critical points E0, E1 and E1+Δ1 of the samples with pure AlAs-like interfaces are shifted by more than 100 meV to higher energies with respect to those of the samples with InSb-like interfaces. These differences can be understood on the basis of the different interfacial atomic structure and strain in the samples.
Systematic study of PbTe (111) molecular-beam epitaxy using reflection high-energy electron-diffraction intensity oscillations
Authors: Springholz G. , Bauer G.
Published: July 2002

Molecular beam epitaxy of PbTe on BaF2 (111) is studied using reflection high-energy electron diffraction (RHEED). The influence of growth parameters (substrate temperature and growth rate) on surface kinetics and the steady-state growth surface morphology is investigated employing dynamical RHEED measurements (RHEED oscillations). For a well adjusted stoichiometric PbTe beam flux composition, two-dimensional layer-by-layer growth can be achieved from substrate temperatures as high as 410 °C down to temperatures below 95 °C, with a maximum number of 230 RHEED oscillations observed at substrate temperatures in the 160 °C range. At temperatures above 400 °C, the growth kinetics start to be modified by PbTe reevaporation from the layer surface. The dependence of the RHEED oscillations on substrate temperature and growth rate indicates the importance of adatom surface diffusion for the surface morphology developed under steady-state growth conditions, and for all growth conditions, a close correlation between steady-state growth surface step density and damping of RHEED oscillations is observed. Furthermore, it is shown that even very small changes in the beam flux composition have a dramatic influence on the RHEED intensity oscillations as well as the surface processes involved in the growth. With only a small additional Te2 flux used for the growth, an abrupt growth mode transition from layer-by-layer to step flow growth is induced. This is the first evidence that PbTe molecules impinging on the layer surface do not dissociate upon adsorption, but remain in a molecular state until incorporated in the crystal lattice.
Luminescent Sr1−xErxF2+x thin films grown on InP(100) substrates. II. Optical properties
Authors: Barrière A. S. , Césaire T. , Hirsch L. , Lambert B. , Raoux S.
Published: July 2002

Photoluminescence and cathodoluminescence studies, performed at room temperature, on Sr1−xErxF2+x/InP(100) layers exhibit strong light emissions in both the visible range and in the infrared domain. It is shown that in the visible range the highest intensities were found for x=0.02, while for the 4I13/2→4I15/2 infrared emission (1.53 μm) the maximum of luminescence was found for x close to 0.2. For a 4I15/2→4I11/2 (980 nm) excitation, up conversion leads to visible light emissions (red and green). A photoluminescence study at low temperature (2 K) shows that, due to the low growth temperature of the layers, the Er3+ environment is more simple than in bulk crystals. For x=0.03 a single charge compensated Er3+ ion site was evidenced. Up to x=0,15 this site predominates but a clustering process is evidenced.
Control of composition and structure for molybdenum nitride films synthesized using ion beam assisted deposition
Authors: Mudholkar M. S. , Thompson L. T.
Published: July 2002

The purpose of the research described in this article was to synthesize molybdenum nitride films with well-defined structures and stoichiometries using ion beam assisted deposition (IBAD). Approximately 400 nm thick films were prepared by the evaporative deposition of molybdenum while simultaneously bombarding the growing film with low energy (250–1000 eV) nitrogen ions. The effects of ion-to-atom arrival rate ratio, ion angle of incidence, and ion energy on the film composition and phase constituents were examined. The film nitrogen to molybdenum stoichiometry increased linearly with increasing arrival rate ratio irrespective of the ion energy and varied significantly with changes in the ion angle of incidence. The latter was interpreted based on sputtering and reflection effects. The phase constituents were functions of all of the deposition parameters investigated. We propose that a single parameter, the effective energy density per deposited atom, can account for the effects of ion energy, mass, and angle of incidence. The effective energy density is approximately the ion energy divided by the ion range. The range incorporates the effects of ion mass and angle of incidence, as well as the energy. For low energy ions the energy density per depositing atom is proportional to E1/2, a dependence that it shares with other models that have been developed to account for phase formation during IBAD. The advantage of the energy density treatment is that it has a more obvious influence on the temperature in the growth zone, a factor controlling phase formation.
The adhesion of metallic films to laser-irradiated alumina
Authors: Pedraza A. J. , DeSilva M. J. , Kumar R. A. , Lowndes Douglas H.
Published: July 2002

The adhesion strength of copper and nickel films deposited onto polycrystalline alumina (Al2O3) substrates is greatly increased by pulsed ultraviolet excimer (XeCl, 308 nm) laser irradiation of the alumina prior to metal-film deposition. Adhesion enhancement occurs for pulsed-laser energy densities, Ed, both below and above alumina’s melting threshold (Edth∼0.7 J/cm2). Cross-section transmission electron micrographs of alumina irradiated at Ed≳ Edth reveal an outer amorphous layer 40–60 nm thick that is formed during the rapid solidification process that follows pulsed-laser melting. Our results for gold, copper, and nickel films demonstrate that several factors contribute to metal-alumina bonding: (1) the chemical nature of the metal, as indicated by quite different adhesion strengths of sputter-deposited metals on unirradiated alumina substrates: 0.1 MPa for Au, 13 MPa for Cu, and 32 MPa for Ni; (2) the type and extent of laser-generated disorder/damage, since for gold significant bonding enhancement is obtained only if pulsed-laser melting of the alumina occurs; and (3) the irradiation atmosphere, as the adhesion strength of these metals to alumina is greatest if pulsed-laser irradiation is performed in an oxidizing atmosphere.
Transport mechanism of spray pyrolytic-grown indium tin oxide/indium phosphide junctions
Authors: Vasu V. , Manivannan P. , Subrahmanyam A.
Published: July 2002

Indium tin oxide (ITO)/p-indium phosphide (InP) junctions have been prepared by the spray pyrolysis technique and the photovoltaic properties have been reported earlier. Continuing our efforts to understand the transport mechanism across these junctions, XPS, current-voltage (dark), and capacitance-voltage measurements have been carried out on the samples having 10.2% photovoltaic efficiency under 100 mW/cm2 illumination. The XPS studies have confirmed an interfacial layer consisting of In2O3 and InPO4. The transport mechanism above 300 K is found to be dominated by recombination at the depletion region. The MIS model proposed in our earlier paper to account for the photovoltaic and transport properties is confirmed for sprayed ITO/p-InP junctions in the present study.
Studies of binding energies of various components in bismuth-based cuprate superconductors through secondary ion mass spectrometry and x-ray photoelectron spectroscopy
Authors: Rajasekar P. , Ray N. , Dey S. D. , Bandyopadhyay S. K. , Barat P. , Sen Pintu , Chakraborty P. , Caccavale F. , Bertoncello R.
Published: July 2002

Surface analysis of bismuth-based high-Tc superconductors, like Bi2Sr2CaCu2O8+δ (Bi-2212) and Bi2Sr2Ca2Cu3O10+δ (Bi-2223) has been carried out by secondary ion mass spectrometry and x-ray photoelectron spectroscopy techniques. The results have been compared with that of similar measurements on Bi2O3, CuO, and CaCO3 samples so as to have detailed information about the surface binding energy as well as the chemical nature of the various individual components that exist inside these high-Tc compounds.
Thin-film composite mixtures of YBa2Cu3O7−δ and Y2O3
Authors: Broussard P. R. , Cestone V. C. , Allen L. H.
Published: July 2002

Thin-film, composite mixtures of YBa2Cu3O7−δ (YBCO) and Y2O3 (yttria) have been grown by off-axis sputtering onto (100) SrTiO3 and (100) and (110) MgO. X-ray-diffraction measurements for films grown on (100) substrates show the presence of c-axis-oriented YBCO and (h00)-ordered yttria. The composites deposited on (110) MgO are insulating, and x-ray analysis shows the presence of Y2Ba1Cu1O5 along with c-axis YBCO. Scanning electron microscopy analysis shows the films to be very granular. The superconducting transition temperature and resistivity ratios between 300 and 100 K of the composites grown on the (100) substrates have been measured. A dramatic change is seen in these transport properties around 70% (volume) of YBa2Cu3O7−δ, which may be explained by the phase diagram for Y-Ba-Cu-O.
Etching characteristics of thin films of tungsten, amorphous silicon carbide, and SAL-603 resist submitted to a surface-wave driven SF6 magnetoplasma near electron cyclotron resonance conditions
Authors: Bounasri F. , Moisan M. , St-Onge L. , Margot J. , Chaker M. , Pelletier J. , El Khakani M. A. , Gat E.
Published: July 2002

The etching of thin films of tungsten, amorphous silicon carbide (a-SiC:H), and SAL-603 resist has been investigated in a magnetoplasma sustained by microwaves under conditions close to the electron cyclotron resonance. This was done in a newly developed large diameter (280 mm i.d.) reactor operating in a uniform static magnetic field and supplied with the plasma from a surface-wave-sustained discharge. A pure SF6 discharge was used. The etch rate of W, a-SiC:H, and SAL-603 resist materials was systematically determined as functions of operating parameters such as the SF6 pressure in the 0.1–5 mTorr range, the absorbed microwave power, and the distance of the substrate to the surface-wave launching gap. It is shown, in particular, that the etching selectivity of W with respect to a-SiC:H and SAL-603 resist is strongly influenced by the SF6 pressure. Optimal etching selectivities as high as 10 and 25 were obtained for W relative to a-SiC:H and SAL-603 resist, respectively, when the SF6 gas pressure is close to 4 mTorr. At this pressure, a high etch rate of about 1700 Å/min for W was recorded. This SF6 magnetoplasma was characterized using two classical diagnostics: electrostatic probe and optical emission spectroscopy (actinometry method). The electron temperature, the sheath potential, the ion current density, and the fluorine atom concentration in the reactor were determined in this way as functions of the above-mentioned operating parameters. Analyzing the variations of both the plasma characteristics and the etch rates of these three materials as functions of the operating parameters, it was found that (i) the etch rate of W is linearly correlated to the concentration of neutral fluorine atoms whereas (ii) the etch rates of a-SiC:H and SAL-603 resist are linearly correlated to the ion current density of the SF6 plasma. It was also shown that, in contrast to W, the etching of a-SiC:H and SAL-603 resist is strongly influenced by the biasing voltage applied to the substrate.
Capacitance and hysteresis study of AlAs/GaAs resonant tunneling diode with asymmetric spacer layers
Authors: Wei T. , Stapleton S. , Berolo O.
Published: July 2002

By measuring scattering parameters, the impedance of an AlAs/GaAs double barrier resonant tunneling diode with asymmetric spacer layers has been obtained up to 40 GHz. It is found that the impedance at various biases can be described well by the lumped equivalent circuit representation. A hysteresis is observed in the impedance measurement in the negative differential resistance region. This hysteresis, unlike the one observed in the dc current-voltage measurement, is not distorted by oscillations and it may be attributed to the load-line effect due to the internal series resistance. Self-consistent calculations have been used to predict the capacitance-voltage curve derived from the impedance measurements. Peaks in capacitance found in the negative differential resistance region are accompanied by a smaller peak. Calculation indicates that the smaller peak is due to electrons discharging from the accumulation region in the undoped region adjacent to the emitter barrier.
Lateral potential modulation in periodically line-doped structures
Authors: Takagaki Y. , Ploog K.
Published: July 2002

The electrostatic potential and the carrier distribution in periodically line-doped structures are calculated within the semiclassical approximation. When the distance between the doped lines is 8 nm, which is a typical periodicity experimentally realized on the (100) GaAs surface, the system is regarded as a lateral superlattice instead of parallel wires because of strong overlap of the electron charge. Moreover, the modulation is anticipated not to be sufficient to probe the electronic state unambiguously. Alternative possible structures to enhance the modulation, which may provide more suitable systems for experiments, are investigated. It is shown that the confinement is stronger for heavier carriers by virtue of their smaller screening length. The coupling among the wires is suppressed by inserting weak p-type dopant lines in between the channels. An application of the line doping to a heterojunction is also examined.
Dynamics of selective reflections of cholesteric liquid crystals subject to electric fields
Authors: Scaramuzza N. , Ferrero C. , Carbone V. , Versace C.
Published: July 2002

In this paper we report on the time behavior of the apparent pitch of a cholesteric mixture undergoing a step-like dc electric field applied parallel to the helical axis. We show that the relative variation of the apparent pitch Δλ(t)/λ0 decreases from the initial value to negative ones (λ<λ0) by two different characteristic times and tends finally to a saturation value with Δλ/λ0<0 after a long time. An attempt to explain this phenomenon in a (semi-)quantitative way has been made by considering relaxation mechanisms on different spatial scales.
Amplitude and frequency dependence of hysteresis loss in a magnet-superconductor levitation system
Authors: Yang Z. J. , Hull J. R. , Mulcahy T. M. , Rossing T. D.
Published: July 2002

Using an electromagnetically controlled mechanical pendulum, we measured the energy loss for different amplitudes in a magnetic levitation system that contained high temperature superconductors (HTSs). Two procedures were followed to measure losses at 77 K for frequencies of 93.8 mHz to 80 Hz. In the first procedure, the distance between the permanent magnet and the HTS levitator was the same as that during (field) cooling. In the second procedure, the magnet was lowered (after cooling) closer to the HTS levitator before the measurements were performed. The experimental data show that these two procedures give essentially the same results at the same distance despite different cooling (and magnetization) histories for melt-textured YBaCuO levitators, and the frequency-independent energy loss is a power-law function of amplitude. We attribute the energy loss to magnetic hysteresis in the superconductor.
Ordinary and extraordinary continuous wave lasing at 1.092 and 1.082 μm in bulk Nd:LiTaO3 crystal
Authors: Abedin Kazi Sarwar , Sato Manabu , Ito Hiromasa , Maeda Toshiki , Shimamura Kiyoshi , Fukuda Tsuguo
Published: July 2002

Optical and lasing characteristics of a rare-earth-doped nonlinear crystal, neodymium-doped lithium tantalate (Nd:LiTaO3), have been studied. Continuous wave lasing lines at 1.092 and 1.082 μm with ordinary (σ) and extraordinary (π) polarizations, respectively, have been observed by pumping near the 800 nm absorption region. The effects of annealing and poling of the crystal on the fluorescence spectrum, lifetime, and polarization are investigated. A comparative study of the slope efficiency and threshold for ordinary and extraordinary polarized lasing is also made.
Physical study of a gliding arc discharge
Authors: Richard F. , Cormier J. M. , Pellerin S. , Chapelle J.
Published: July 2002

Gliding arc discharges are the subject of renewed interest in applications to a variety of chemical reactions. A gliding arc is created by a weakly ionized gas flowing between two electrodes. The reacting gas introduced at the base blows the arc column upwards. These devices are used in industry mainly for decontamination and general gas treatment. Either ac or dc can be used as the power supply. In order to analyze gliding arc motion, a multi-shot camera was designed with a very short exposure time. A high-voltage dc arc is studied and a simple theoretical model is given which enables determination of the conduction radius and axial temperature.
Photoresponse of a YBa2Cu3Ox grain-boundary junction
Authors: Park J. H. , Kim D. H. , Kim Y. H. , Kang W. N. , Choi S. S. , Hahn T. S. , Khim Z. G.
Published: July 2002

The photoresponse of a YBa2Cu3Ox grain-boundary junction has been measured as a function of chopping frequency and bias current in a temperature range of 2 K≤T≤70 K. The response was found to be mostly bolometric, but a nonbolometriclike component was also identified, which appeared as a chopping-frequency independent signal while immersing the sample in the superfluid helium. The bias-current dependence of the response showed a peak at a current corresponding to the critical current of the junction, and the peak values remained constant for 30 K≤T≤70 K. However, below 15 K the peak of the photoresponse increased very sharply with decreasing temperature following a 1/T3 dependence, consistent with the temperature dependence of the thermal boundary resistance between the film and the substrate.
Domains and giant magneto-impedance in amorphous ribbons by magneto-optical Kerr effect
Authors: Santos A. D. , Melo L. G. C. , Martins C. S. , Missell F. P. , Souche Y. , Machado F. L. A. , Rezende S. M.
Published: July 2002

By longitudinal magneto-optical Kerr effect microscopy we analyzed the development of the domain structure in Co70.4Fe4.6Si15B10 amorphous ribbons for which giant magneto-impedance had been observed. Using transverse and longitudinal magneto-optical Kerr effect magnetometry we measured the magnetization direction around the hysteresis loop. These results allowed us to model the domain structure of the sample during the giant magneto-impedance measurements.
Electrical properties of (Zr,Sn)TiO4 dielectric thin film prepared by pulsed laser deposition
Authors: Nakagawara Osamu , Toyota Yuji , Kobayashi Masato , Yoshino Yukio , Katayama Yuzo , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

We have been successful in obtaining temperature-stable crystallized thin film of (Zr,Sn)TiO4. Preferential (111)-oriented (Zr,Sn)TiO4 thin film was prepared by pulsed laser deposition. Effects of crystallization were elucidated based on a comparison of electric properties of crystallized and amorphous (Zr,Sn)TiO4 film. For crystallized film, the temperature coefficient of capacitance (TCC) was 20 ppm/°C at 3 MHz and the dielectric constant εr=38 in the microwave range of 1–10 GHz. These values are superior to those for amorphous film (TCC=220 ppm/°C, εr=27). The crystallization of this material was found quite effective for improving dielectrical properties. Atomic force microscope images showed the surface morphologies of crystallized and amorphous film of (Zr,Sn)TiO4 to differ.
Optical constants of tetragonal and cubic zirconias in the infrared
Authors: Pecharromán C. , Ocaña M. , Serna C. J.
Published: July 2002

The infrared optical constants of cubic and tetragonal zirconias were determined from the analysis of the reflectance spectra of yttria doped zirconia single crystals and undoped tetragonal zirconia powders, respectively. In the latter case, the reflectance spectrum was analyzed by an effective medium Theory, which has been proven to successfully determine the dielectric properties of small powder particles. The symmetry of the resulting modes of tetragonal zirconia was elucidated by performing theoretical calculations of the vibrational properties.
Brillouin light scattering investigations of structured permalloy films
Authors: Hillebrands B. , Mathieu C. , Bauer M. , Demokritov S. O. , Bartenlian B. , Chappert C. , Decanini D. , Rousseaux F. , Carcenac F.
Published: July 2002

The static and spin wave properties of regular square lattices of magnetic dots of 0.5–2 μm dot diameter and 1–4 μm periodicity patterned in permalloy films have been investigated by Brillouin light scattering. The samples have been structured using x-ray lithography and ion beam etching. The Brillouin light scattering spectra reveal both surface and bulk spin wave modes. The spin wave frequencies can be well described taking into account the demagnetization factor of each single dot. For the samples with smallest dot separation of 0.1 μm a fourfold in-plane magnetic anisotropy with the easy axis directed along the pattern diagonal is observed, indicating anisotropic coupling between the dots.