bluuebunny/crossref_metadata_2025 huggingface dataset:
Erbium-doped silicon films grown by plasma-enhanced chemical-vapor deposition
Authors: Rogers J. L. , Andry P. S. , Varhue W. J. , Adams E. , Lavoie M. , Klein P. B.
Published: July 2002

Epitaxial growth of erbium-doped silicon films has been performed by plasma-enhanced chemical vapor deposition using an electron-cyclotron-resonance source. The goal was to incorporate erbium as an optically active center (ErO6) through the use of metal-organic dopant sources. The characteristic 1.5 μm emission was observed by photoluminescence. Chemical analysis of the film revealed, however, that the organic ligands were decomposing and contributing to the carbon contamination of the films. Analysis of the molecular flux to the substrate indicated that the metal-organic compound used, tris(2,2,6,6-tetramethyl-3-5-heptanedionato)erbium(III), was most likely to decompose, and supply unbonded atomic erbium and not the optical active species, ErO6. Excessive carbon contamination lowered epitaxial quality and reduced the photoluminescent intensity. Photoluminescent intensity was improved by a 600 °C anneal but was strongly quenched by a 900 °C anneal. The low-temperature anneal improved crystal quality, and the high-temperature anneal resulted in silicide formation.
Microstructural and magnetic investigations into the origins of high coercivity in die-upset praseodymium–cobalt–carbon based magnets
Authors: Lewis L. H. , Bian Wei-Min , Zhu Y. , Welch D. O.
Published: July 2002

Melt-quenched and thermomechanically deformed samples with the nominal compositions Pr18Co82, Pr18Co81C, and Pr18Co76C6 were examined with optical and electron microscopy, differential thermal analysis, and superconducting quantum interference device magnetometry that was performed in the temperature range 20 K≤T≤300 K. At room temperature, Pr18Co82 exhibits poor coercivity and remanence, 600 Oe and 2.9 kG, respectively. Pr18Co81C exhibits relatively superior remanence, 7.7 kG, but poor coercivity, 2.7 kOe, while Pr18Co76C6 exhibits the opposite trend, BR=4.5 kG and Hci=12 kOe. The main phase present in all samples, PrCo5, has basically the same character and morphology for all three samples and shows no evidence of intragranular carbon, which is demonstrated to reside in the impurity phases. The superior coercivity found in Pr18Co76C6 is attributed to a previously unknown triclinic phase, Pr3Co4Cx (x≊3–4) that appears to undergo a magnetic–nonmagnetic transition with increasing temperature around T=80 K. The variety of magnetic properties exhibited by each sample is due to the variety of minor phases present in each sample, which may be a product of the effect that carbon has on the solidification rate of the parent alloy.
Mössbauer and x-ray study of the structure and magnetic properties of amorphous and nanocrystalline Fe81Zr7B12 and Fe79Zr7B12Cu2 alloys
Authors: Kopcewicz M. , Grabias A. , Nowicki P. , Williamson D. L.
Published: July 2002

The specialized technique of radio-frequency-induced collapse of Mössbauer spectra combined with conventional Mössbauer spectroscopy, x-ray diffraction (XRD), small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) are used to investigate in detail the magnetic and structural properties of the two magnetic materials Fe81Zr7B12 and Fe79Zr7B12Cu2. Thermal treatments to convert the as-quenched, fully amorphous state into mixtures of nanocrystalline and amorphous states and the effect of the small Cu addition were of primary interest due to the improved magnetic behavior in the mixed state. DSC shows that the Cu leads to a lowering of the onset temperature for formation of the nanocrystalline phase and also to an increase in the range of temperatures over which this phase forms. XRD and Mössbauer data show the nanoscale phase to be bcc Fe and the Mössbauer spectral parameters demonstrate it to be essentially pure Fe (i.e., with little or no Zr, B, or Cu substitutional impurities). The electron density contrast between the amorphous matrix and the bcc Fe permits the detection of the Fe grains by SAXS and significant volume fractions with sizes of only 2.8–8 nm are shown to exist. Larger sizes are also present as demonstrated by the XRD and Mössbauer data and a bimodal size distribution is suggested. The Mössbauer experiments in which the radio-frequency-induced effects (rf collapse and rf sidebands) are used, allows the nanocrystalline bcc phase to be distinguished from magnetically harder microcrystalline α-Fe. The complete rf collapse of the magnetic hyperfine structure occurs only in the amorphous and nanocrystalline phases and is suppressed by the formation of larger grains. The rf sidebands disappear when the nanocrystalline phase is formed, revealing that magnetostriction vanishes. The rf-Mössbauer studies are shown to be particularly sensitive to magnetic softness of the material in that large changes in the spectra are observed for applied field changes as small as 2 Oe.
Exchange coupling between NiO and NiFe thin films
Authors: Shen J. X. , Kief M. T.
Published: July 2002

NiO/NiFe bilayer thin films were prepared by rf reactive and dc magnetron sputtering, respectively. The exchange coupling strength between NiO and NiFe as a function of NiO texture and interface roughness was investigated by using different sputtering pressures, Au, and Cu buffer layers. The experimental results show that the exchange coupling field strongly depends on the NiO/NiFe interface roughness. In addition, we found the exchange coupling is largest for the (200) texture compared to the (111)-texture films. This is surprising since the bulk spin structure of NiO predicts the (200) plane to be compensated while the (111) plane is to be uncompensated.
Impact of new magnetoresistive materials on magnetic recording heads (invited)
Authors: Brug J. A. , Tran L. , Bhattacharyya M. , Nickel J. H. , Anthony T. C. , Jander A.
Published: July 2002

Advances in magnetoresistive materials have recently enabled magnetic recording heads to achieve higher levels of performance. This article describes why higher signal outputs are necessary for improvements to be made in areal density. The requirements for recording at an areal density of 16 Mb/mm2 (10 Gb/in.2) are discussed with regards to both the channel and the head design. Increased output from new multilayer magnetoresistive materials is required to counteract the decrease in output due to the reduction in the size of the head geometry. An areal density of 16 Mb/mm2 is shown to be feasible with spin valve recording heads using materials with magnetoresistance ratios of 10%. Fabrication issues relating to the manufacturing of these materials are shown to be more stringent than previously required.
Two-phase coexistence in Fe–Ni alloys synthesized by ball milling
Authors: Hong L. B. , Fultz B.
Published: July 2002

We used mechanical alloying with a Spex 8000 mixer/mill to synthesize a series of Fe100−xNix alloys from x=0 to x=49. The Spex mill was modified so that it could also operate at a reduced milling intensity, and we compared the alloys synthesized after long times with the normal and reduced milling intensities. X-ray diffractometry and Mössbauer spectrometry were used to measure the volume fractions of the bcc and fcc phases in the alloys, and to determine the chemical compositions of the individual phases. We found that the composition ranges of the bcc and fcc single phase regions were extended well beyond their equilibrium ranges. At the higher milling intensity, we found that the bcc phase was destabilized with respect to the fcc phase, and the two-phase region shifted to lower Ni concentrations. For those alloys with coexisting bcc and fcc phases, we present evidence that the chemical compositions of the two phases are nearly the same. We explain the destabilization of the bcc with milling intensity as originating with a higher defect density in the bcc alloys than in the fcc alloys. We argue that this defect density is not homogeneous throughout the alloy, however, and the distribution of defect enthalpies can explain the two- phase coexistence in the as-milled alloys.
A position sensor based on magnetoimpedance
Authors: Valenzuela R. , Vazquez M. , Hernando A.
Published: July 2002

A magnetic-field sensor based on the giant magnetoimpedance phenomenon is presented. It is shown that a low, negative magnetostriction CoFeBSi amorphous wire can be used to detect the presence or passage of moving pieces or vehicles, simply by pasting a small permanent magnet on the vehicles/pieces. The detection is observed as a decrease in the ac voltage on the wire’s ends. A system of such devices can be used to monitor and control a number of industrial processes.
CoFe2O4 thin films grown on (100) MgO substrates using pulsed laser deposition
Authors: Dorsey P. C. , Lubitz P. , Chrisey D. B. , Horwitz J. S.
Published: July 2002

Thin films (≊0.4 μm) of cobalt ferrite (CoFe2O4) have been grown on single-crystal (100) MgO substrates using pulsed laser deposition (PLD). The phase, orientation, and microstructure of the as-deposited films were investigated as a function of substrate temperature (i.e., 200–800 °C) at a constant oxygen deposition pressure of 30 mTorr. The as-deposited films were found to be single phase, well oriented, and approximately matching the stoichiometry of the target, but the cubic lattice constant of the films depended on the substrate temperature indicating that the films were strained. The greatest effect of the substrate temperature was on the magnetic properties of the as-deposited films. At 800 °C, 4πMs was measured to be 5370 G which is approximately the accepted bulk value for cobalt ferrite. In addition, PLD cobalt ferrite films grown at substrate temperatures of 600 and 800 °C exhibited a uniaxial magnetic anisotropy with an easy direction normal to the film plane. Films grown at 200 and 400 °C also exhibited a uniaxial magnetic anisotropy but possessed a planar easy direction.
Factors affecting performance of NiO biased giant magnetoresistance structures
Authors: Cheng S. F. , Teter J. P. , Lubitz P. , Miller M. M. , Hoines L. , Krebs J. J. , Schaefer D. M. , Prinz G. A.
Published: July 2002

We have made spin-valve structures of Permalloy/Cu/Co by sputtering or electron-beam deposition onto the antiferromagnetic oxide NiO. The oxides were made either by deposition of the metals and subsequent oxidation or by growing them in situ using reactive sputtering. The magnetic properties of the giant magnetoresistance structures were studied by magnetoresistance, vibrating sample magnetometry, and ferromagnetic resonance methods. The oxides were characterized by x-ray diffraction and atomic force microscopy. We studied surface roughness and structure as functions of thickness and oxidation temperature and correlated the oxide properties with the magnetic performance. We found that the metal layer roughened during the postdeposition oxidation process and that the resulting oxide layers were very effective in pinning the direction of the magnetic moment of adjacent metal films. Coercive fields over 500 Oe were obtained for Co overlayers on NiO films but the exchange bias field was generally less than 100 Oe and was not strongly dependent on the roughness. The beneficial effects of this strong pinning were offset to some degree by higher switching fields required in spin-valve structures deposited over the Co. We also made reactively sputtered oxide antiferromagnetic films which had smoother surfaces than those made by postdeposition oxidation.
Finite-size scaling in thin antiferromagnetic CoO layers
Authors: Ambrose T. , Chien C. L.
Published: July 2002

Finite-size scaling effects in CoO, an antiferromagnetic insulator with localized moments, have been observed in CoO/SiO2 multilayers. The Néel temperatures of the CoO layers, as determined by susceptibility measurements, shows a finite-size scaling relation with a shift exponent of λ=1.6±0.1.
Piezoelectric effect in (001)- and (111)-oriented double-barrier resonant tunneling devices
Authors: Cong L. , Albrecht J. D. , Nathan M. I. , Ruden P. P.
Published: July 2002

We show experimentally that current–voltage characteristics of double-barrier resonant tunneling devices (DBRTDs) can be modified by internal polarization fields due to the piezoelectric effect induced by external uniaxial stresses. Electric polarization fields, perpendicular to the interfaces, arise in DBRTDs grown on (001)-oriented substrates under uniaxial, compressive stresses parallel to the (110) or (11̄0) crystal orientations, and in DBRTDs grown on (111)B-oriented substrates under stress parallel to (111) crystal orientation. The voltages at which the resonant tunneling current peaks occur (peak voltages) are sensitive to the polarization fields induced by external stresses. The peak voltages can shift to more positive voltages or more negative voltages depending on the directions of applied stresses. We measured current–voltage characteristics of AlAs/GaAs/AlAs double-barrier resonant tunneling structures as a function of external stresses at 77 K. Uniaxial stress was applied parallel to the (110) and the (11̄0) crystal orientations for (001)-oriented DBRTDs, as well as to the (111) orientation for (111)B-oriented DBRTDs. With the substrates grounded in all the measurements, we found that the peak voltages shift to more positive voltages for (001)-oriented DBRTDs under stress along the (110) orientation and for (111)-oriented DBRTDs under the stress along (111) orientation, and to more negative voltages for (001)-oriented devices under stress along the (11̄0) orientation. The results are in agreement with our calculations published in the preceding article, which take into account the piezoelectric effect and band alignment under stress.
Electromigration in isolated aluminum vias probed by resistance changes and 1/f noise
Authors: Alers G. B. , Beverly N. L. , Oates A. S.
Published: July 2002

1/f noise and high-resolution resistance measurements have been performed in isolated aluminum via interconnects under electromigration stress. The test structures had a volume of roughly 1 μm3 and a resistance of 0.1 Ω with an internal TiN diffusion barrier. The 1/f resistance noise of the vias was found to be larger than the 1/f noise in the connecting aluminum runners due to the high intrinsic noise, high resistivity TiN layer within the structure. The direct currents induced reversible increases and decreases in the resistance of the via due to electromigration damage but had no effect on the 1/f noise. The magnitude and polarity dependence of the dc current induced resistance changes are most consistent with the initial electromigration of copper. Above 200 °C the resistance changes became dominated by abrupt jumps in the resistance. Possible mechanisms for the abrupt resistance changes are reviewed.
Anisotropic strain relaxation of GaInP epitaxial layers in compression and tension
Authors: Matragrano M. J. , Ast D. G. , Shealy J. R. , Krishnamoorthy V.
Published: July 2002

We have investigated the strain relaxation of intentionally lattice mismatched (±0.5%) GaInP layers grown on GaAs substrates by organometallic vapor phase epitaxy. Double axis x-ray diffraction was used to measure the relaxation in these epitaxial layers in perpendicular 〈110〉 directions as a function of thickness. For samples in tension, the difference in relaxation between [11̄0] and [110] increases from 10% to 48% as the layer thickness increases from 7 to 28 times the critical thickness, hc. For samples in compression this difference is 28% at 24hc while no relaxation is measured for a sample at 6hc. These results indicate that strain relaxes anisotropically and that the anisotropy is more pronounced for samples in tension than in compression. Furthermore, the major relaxation axis was found to be [11̄0] regardless of the sign of the strain. Reciprocal space maps, generated using triple axis x-ray diffraction, showed that the amount of microtilt of the epitaxial layers was also anisotropic. This anisotropy and the direction of the maximum dislocation density which was measured by cathodoluminescence and transmission electron microscopy, changed from [110] in tension to [11̄0] in compression. The fact that the major relaxation axis remained stationary while the high misfit dislocation density direction rotated indicates that a substantial number of dislocations with Burgers vectors of the ‘‘wrong sense’’ for strain relief are formed in compressed epilayers. A model in which α type dislocations are more mobile than the β type misfit dislocations regardless of the sign of the strain is consistent with all of the experimental observations.
Thermal treatment dependence of the dynamic magnetic behavior of Gd2CuO4
Authors: Mira J. , Rivas J. , Fiorani D. , Caciuffo R. , Rinaldi D. , Vázquez Vázquez C. , Mahía J. , López Quintela M. A. , Oseroff S. B.
Published: July 2002

dc and ac magnetization measurements in Gd2CuO4 ceramic samples synthesized at different annealing temperatures show that their magnetic properties are influenced by the thermal treatment. The changes consist of variations of the onset point for differences between zero field cooled and field cooled dc magnetizations, in the coercive fields, and in the position and frequency dependence of a maximum in the real part of ac susceptibility. An explanation suggesting a temperature-dependent homogeneity of the oxygen distortions causing weak ferromagnetism in the CuO2 planes is given.
Transparent ion intercalation films of Zr–Ce oxide
Authors: Veszelei M. , Kullman L. , Azens A. , Granqvist C. G. , Hjörvarsson B.
Published: July 2002

Zr–Ce oxide films were made by reactive dc magnetron cosputtering. The elemental composition was determined by Rutherford backscattering spectrometery and the crystalline structure by x-ray diffraction. Li intercalation/deintercalation was accomplished potentiodynamically in a liquid electrolyte. The films remained fully transparent irrespective of their degree of lithiation, which may be reconciled with a population/depopulation of Ce 4f levels.
Competing magnetic interactions in manganese perovskites
Authors: Fontcuberta J. , Garcı́a-Muñoz J. L. , Suaaidi M. , Martı́nez B. , Piñol S. , Obradors X.
Published: July 2002

Structural and magnetotransport properties of L2/3A1/3MnO3 (A=Ca, Sr) oxides reveal the gradual increase of the magnetic frustration when bending the Mn–O–Mn bonds. The relative strength of competing magnetic interactions and thus the ferromagnetic ordering temperature TC are controlled not only by R0 (R0 is the mean radius of the lanthanides) but also by the electronegativity of the divalent cation. Consequently, the curve TC(R0) is not universal but it is sensitive to the alkaline ions.
Magnetic studies in (Co/V) multilayers
Authors: Driouch L. , Krishnan R. , Porte M. , Rao K. V. , Tessier M.
Published: July 2002

The magnetization and anistoropy of (Co/V)n multilayers prepared by e-beam evaporation under ultrahigh vacuum are presented. The magnetization at 5 K suggests one atomic layer of Co to be nonmagnetic. The overall magnetization is found to have an unusual linear temperature dependence, which follows the relation M(T)/M(5 K)=1−AT and the constant A depends on t(Co). The linear temperature dependence of magnetization is attributed to the lowered dimensionality of the system and rather weak interlayer coupling between two Co layers. The surface (Ks) and bulk anisotropy (Kv) constants at room temperature are determined to be 0.5 ergcm−2 and 9.4 106 ergcm−3, respectively. Even though the major contribution to Kv, in the (Co/V)n system, arises from the demagnetization energy, some arise from the magnetoelastic interactions as well.
Bonded permanent magnets: Current status and future opportunities (invited)
Authors: Ormerod John , Constantinides Steve
Published: July 2002

Permanent magnets play a vital role in modern society as a component in a wide range of devices utilized by many industries and consumers. In 1995, the world production of permanent magnets was estimated to be valued at $3.6 billion and growing at an annual rate of 12%. Bonded permanent magnets are the fastest growing segment of this market. Bonded magnet technology enables a wide variety of magnetic powders to be combined with several polymer and binder systems to produce magnetic components utilizing several processing options. In this article, we review the development of bonded magnet technology. The major classes of magnetic powders, binder systems, and processing technologies are described. Recent developments in magnetic material grades, e.g., anisotropic NdFeB, rare earth lean NdFeB, SmFe(N,C) are outlined. The current status of processing and binder options aimed at increasing the upper application temperature limit of these materials is highlighted. Finally, the improvements and future opportunities for bonded magnets are discussed.
Determination of the crystallographic orientation of CuInSe2 thin films by Raman and infrared spectroscopy
Authors: Vorlı́ček V. , Železný V. , Tiwari A. N. , Krejci M. , Zogg H.
Published: July 2002

Polarized Raman spectra were measured at room temperature from the (001)- and (112)-oriented CuInSe2 heteroepitaxial layers grown on (001)- and (111)-oriented Si substrates. The analysis of the results proved the expected orientation and good crystalline quality of the (001) layers. No special crystallographic direction was identified unambiguously on the (112) plane. Similar behavior was confirmed by infrared transmission spectroscopy. This can be explained by the existence of rotational twins, which have been observed by x-ray diffraction.
In situ transmission electron microscopy study of plastic deformation in passivated Al–Cu thin films
Authors: Jawarani D. , Kawasaki H. , Yeo I.-S. , Rabenberg L. , Stark J. P. , Ho P. S.
Published: July 2002

Plastic deformation in passivated Al-1 wt %Cu thin films was studied in situ using a straining device in the transmission electron microscope. Both edge and screw dislocations were found to have caused slip on inclined {111} planes. Multiple slip was frequently observed as two or more sets of intersecting slip traces. Microstructural investigations of both unpassivated and passivated Al-1 wt %Cu films indicate that grain size and encapsulation by passivating layers are major contributors to strength of a thin film with a particular thickness. Additional strengthening is also provided by interactions between dislocations on multiple slip systems. The roles of grain orientation and precipitates in plastic deformation are also discussed.
Comparison of magnetocaloric properties from magnetic and thermal measurements
Authors: Foldeaki M. , Schnelle W. , Gmelin E. , Benard P. , Koszegi B. , Giguere A. , Chahine R. , Bose T. K.
Published: July 2002

The isothermal change of the magnetic entropy of a magnetically ordered material upon application of external magnetic field can be calculated from the temperature and field dependence of the magnetization or of the specific heat. The adiabatic temperature change, i.e., the magnetocaloric effect (MCE) can be measured directly or can be calculated via different methods using the field-dependent specific heat values, or a combination of data obtained via magnetization and thermal measurements. In the present study, magnetic and thermal measurements were carried out on Gd75Y25(TC=232 K) and Gd48Y52(TC=161 K) samples, for applied fields ranging between 0 and 7 T. From both datasets, the magnetic entropy change and MCE values were calculated and compared, in order to assess the mutual reliability of the methods applied. The magnetically or thermally deduced specific heat discontinuities show a reasonable agreement within experimental error. Similar comparison of the calculated magnetic entropy changes reveals that the measured transition temperature and the shape of the curve do not depend on the method selected. It is demonstrated that the choice of an integration constant during entropy calculation has a significant impact on the adiabatic temperature change deduced from the field and temperature dependence of the entropies. For the MCE, a better approximation can be obtained using the magnetically acquired magnetic entropy change and the field-dependent specific heat. The results prove that magnetic measurements carried out in high enough magnetic fields provide reliable information on the isothermal magnetic entropy change and, when combined with field-dependent specific heat measurements, on the magnetocaloric effect as well.
Li deficiencies in LiNbO3 films prepared by pulsed laser deposition in a buffer gas
Authors: Gonzalo J. , Afonso C. N. , Ballesteros J. M. , Grosman A. , Ortega C.
Published: July 2002

The origin of Li deficiency in films grown by laser ablation of single-crystal LiNbO3 targets in a buffer gas has been investigated by analyzing the stoichiometry of the deposited films as a function of the following parameters: the distance target-substrate, the nature of the buffer gas (He, O2, and Ar) and the deposition configuration. The results show that significant Li losses are related to scattering processes during the expansion regime which are higher the higher the mass of the gas species. The results show that the Li content of the films can be enhanced by setting the substrate either at distances larger than the plume length or in a configuration in which the substrate is not facing the target.
A study of GaAs/AlGaAs p-type quantum well infrared photodetectors with different barrier heights
Authors: Liu H. C. , Li L. , Buchanan M. , Wasilewski Z. R. , Brown G. J. , Szmulowicz F. , Hegde S. M.
Published: July 2002

We report on a systematic study of a set of GaAs/AlGaAs p-type quantum well infrared photodetectors with varying barrier heights. The detector cutoff wavelength decreases with increasing barrier height. Experimental photoresponse spectra are in good agreement with calculated ground-state to continuum absorption spectra, based on a multiband envelope-function model. The measured dark current decreases with increasing barrier height. An estimate of the low-field dark current gives good agreement with the measured dark current.
A time-resolved reflectivity study of the amorphous-to-crystalline transformation kinetics in dc-magnetron sputtered indium tin oxide
Authors: Ow-Yang Cleva W. , Spinner Doren , Shigesato Yuzo , Paine David C.
Published: July 2002

Amorphous tin-doped indium oxide (ITO) was deposited to a thickness of 110 nm on 〈100〉-oriented Si substrates at ∼40–60 °C by dc-magnetron sputtering under a total Ar pressure of 2 Pa. The kinetics of crystallization of the a-ITO films in flowing N2 were investigated by in situ time-resolved reflectivity. The microstructure of the films in the as-deposited, partially recrystallized and fully regrown conditions was established using a combination of plan view and cross-section transmission electron microscopy and atomic force microscopy. The experimental reflectivity vs time curves were analyzed using classical nucleation and growth kinetic analysis. Various transformation models are proposed and are combined with Fresnel reflectivity calculations for direct comparison to the experimentally obtained data. The activation energy for the crystallization in flowing N2 of these amorphous ITO films in N2 gas was found to be 0.67±0.18 eV.
Intrinsic strain in SiO2 thin films
Authors: Delph T. J.
Published: July 2002

The oxidation of silicon to form SiO2 is accompanied by large changes in volume. Existing models of silicon oxidation indicate that almost all of this volume change takes place normal to the oxidation front. However, available experimental results clearly indicate that there exists a small component of expansion in the plane of oxidation, leading to what may be termed as intrinsic, in-plane strain in SiO2 films on silicon substrates. We reanalyze the experimental data of Kobeda and Irene (J. Vac. Sci. Technol. B 6, 720, 1986) to estimate this quantity, arriving at a value of approximately 2×10−3. The effect of this intrinsic strain upon the oxide film stresses is then examined for planar and nonplanar oxidation. Implications of these results for standard oxidation prediction schemes are discussed.
Effect of annealing on the structure and magnetic properties of graphite encapsulated nickel and cobalt nanocrystals
Authors: Host J. J. , Block J. A. , Parvin K. , Dravid V. P. , Alpers J. L. , Sezen T. , LaDuca R.
Published: July 2002

We report the structure and magnetic studies of carbon coated nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio arc chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fcc nickel or cobalt and major peaks of graphite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a function of temperature in the range 20–900 °C give a Curie temperature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M–H hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20–650 °C for Ni, and 20–900 °C for Co. The dependence of room temperature saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature is reported. These data are described by transition of particles form single domain to multidomain as a result of particle growth due to annealing. We also present the particle size distribution measurements that show log-normal behavior, and indicate substantial particle size growth due to annealing.
Carbon encapsulated nanoparticles of Ni, Co, Cu, and Ti
Authors: Jiao Jun , Seraphin Supapan
Published: July 2002

Despite intensive research on the encapsulation of metal nanoparticles into carbon clusters deposited by arc discharge, the detailed pathways of the formation of these novel forms of materials remain unclear. The growth of a rich variety of morphologies is not well understood. Studies are reported here on the growth phenomena of different metals encapsulated into carbon cages that emphasize the effect of carbon and metal supply on the size of particles. Post-deposition annealing was introduced as a process that induces structural rearrangements, and thus enables changes in morphologies. A set of carbon encapsulated Ni, Co, Cu, and Ti particles were prepared by an arc discharge process modified in the geometry of the anode and flow pattern of helium or methane gas. The samples were then annealed under flowing argon gas. Three annealing temperatures were used (600, 900, and 1100 °C). Samples were characterized by transmission and scanning electron microscopy. Particles made under the same experimental conditions are of roughly the same size. When the supply of metal in the reactor space was increased by using a larger diameter of the metal pool, the average diameter of the particles is bigger than those of produced from the smaller metal pool. The thickness of the carbon cages of Ni and Co particles increased during the annealing. The carbon cages of Cu particles, however, did not change their thickness, while some carbon coatings of Ti particles disappeared under annealing. This suggests that the addition of layers for the Ni and Co cages results from a precipitation of carbon previously dissolved in the metal, while the much lower solubility of C in Cu prevents this possibility. The Ti of high reactivity, on the other hand, may further react with the available carbon under annealing to form TiC. It is suggested that annealing provides additional thermal energy that makes structural re-arrangement possible long after the initial deposition process was terminated. This may explain the rich variety of morphologies of deposit obtained at different locations of the reaction chamber.
Magnetic properties of Nd/Fe double layers grown on Si(111) by electron beam evaporation
Authors: Fermin José R. , Azevedo Antonio , Rezende Sergio M. , Pereira Luiz G. , Teixeira Sergio
Published: July 2002

In-plane ferromagnetic resonance (FMR) has been used to study the room-temperature magnetic and crystalline properties of Nd/Fe bilayers. Several samples were grown by electron beam evaporation onto Si(111) substrates, for various Fe and Nd thicknesses. The symmetry of the resonance field as a function of the azimuthal angle departs from the sixfold one expected for a (111)Fe film. The FMR data were analyzed in the framework of a phenomenological model, which takes into account first- and second-order magnetocrystalline, uniaxial and perpendicular anisotropies. Our analysis reveals that the deviation from the sixfold symmetry is due to a combined effect of an induced uniaxial in-plane anisotropy with a small misorientation of the substrate surface from the (111) plane. We suggest that the in-plane uniaxial anisotropy may originate from an arrangement of (111) terraces separated by well oriented atomic steps formed during the small miscut. The effect of the Nd overlayer is to induce a perpendicular anisotropy which is not sufficiently large to overcome the demagnetizing field and push the magnetization entirely out-of-plane.
New permanent magnet couplings for screwing devices
Authors: Quellec L. , Lemarquand V. , Lemarquand G.
Published: July 2002

The use of permanent magnet coaxial synchronous couplings in screwing devices allows one to adjust the screwing torque very precisely, but the symmetrical behavior of classical systems always leads to a partial unscrewing. This article shows that the dissymmetry of this behavior can be enhanced only by the modification of the shape of the magnets stuck on the rotors. The consideration of some basic geometrical parameters (number and length of poles) and the application of rules to reach the dissymmetry lead to improved couplings for screwing devices.
Conversion electron Mössbauer studies on strontium ferrite films with in-plane and perpendicular anisotropies
Authors: Ajan Antony , Acharya B. Ramamurthy , Prasad Shiva , Shringi S. N. , Venkataramani N.
Published: July 2002

Hexagonal strontium ferrite thin films with in-plane and perpendicular magnetic anisotropies have been studied using conversion electron Mössbauer spectroscopy. The hyperfine parameters of the films were found to be similar to the one obtained for bulk strontium ferrite. However, the intensity ratios of the first and second lines of the Mössbauer spectra were found to be different for films deposited at different rf powers. This indicated a change in the anisotropy of the deposited film. The resultant change in the magnetization direction and crystallization texture was compared to other measurements.
Phenonemological model for magnetoimpedance in soft ferromagnets
Authors: Atkinson D. , Squire P. T.
Published: July 2002

The dependence of magnetoimpedance (MI) on transverse susceptibility in soft ferromagnets is used in conjunction with an existing phenomenological domain structure model to study the relative importance of moment rotation and domain-wall motion in MI. It is shown that moment rotation is predominantly responsible for the observed behavior in amorphous wires at frequencies above 1 MHz. It is also necessary to include anisotropy distributions in both magnitude and direction.
Line width of manganese–zinc ferrite polycrystals with oxygen partial pressure
Authors: Byeon Soon Cheon , Hong Kug Sun , Kim In-Tae
Published: July 2002

A systematic variation in line width at X band (9.78 GHz) with oxygen partial pressure was observed in Mn0.47Zn0.47Fe2.06O4 polycrystalline samples. The linewidth of the samples increased from 105 to 188 Oe with decreasing atmospheric parameters from 8.4 to 6.4. It was found that contribution of anisotropy and porosity to the linewidth was small compared to the variation in linewidth with oxygen partial pressure. Estimation of the Fe2+ concentration of samples by measuring their thermoelectric power revealed that an increase in the concentration from 1.88 to 2.44 wt % was accompanied by decreasing oxygen partial pressure. As the resistivity of grain does not vary with oxygen partial pressure, the contribution of eddy current will be the same irrespective of the oxygen partial pressure. Therefore, the systematic increase in linewidth observed in our present study was attributed to the increase in Fe2+ concentration with decreasing oxygen partial pressure.
Annealing behaviors of photoluminescence from SiOx:H
Authors: Ma Zhi-xun , Liao Xian-bo , He Jie , Cheng Wen-chao , Yue Guo-zhen , Wang Yong-qian , Kong Guang-lin
Published: July 2002

The strong photoluminescence (PL) of SiOx:H prepared by plasma enhanced chemical vapor deposition has been systematically studied in conjunction with infrared and micro-Raman spectra. We have found that each PL spectrum is comprised of two Gaussian components, a main band and a shoulder. The main band might originate from amorphous silicon clusters embedded in the SiOx network, and its redshift with annealing temperature is due to expansion of the silicon clusters. The shoulder remains at about 835 nm in spite of the annealing temperature and possibly comes from luminescent defect centers. The enhanced PL spectra after 1170 °C annealing are attributed to the quantum confinement effects of nanocrystalline silicon embedded in the SiO2 matrix.
Effects of recombination current on the current–voltage characteristics in metal–InGaAs Schottky diodes
Authors: Wang X. J. , He L.
Published: July 2002

Current transport mechanisms were investigated on metal–InGaAs Schottky diodes in which the metal electrodes were deposited at both room temperature and low temperature. Based on fitting of the current–voltage (I–V) characteristics at different temperatures, it was found that the recombination current in the depletion region caused by the deep defect centers was responsible mainly for the deviations of I–V characteristics from the pure thermionic emission mechanism. This effect became more significant for diodes with lower barrier height, under smaller forward bias and at lower measuring temperature. However, the generation current only had subtle influence on the reverse bias characteristics. A deep defect level located at 0.321 eV below the bottom of the conduction band in the InGaAs material, which was identified by the deep level transient spectra measurements, may act as the recombination–generation deep center. The generation lifetime of this deep center may be much longer than its recombination lifetime, therefore it may more severely affect the forward biased I–V characteristics.
Reduction of the extended defect density in molecular beam epitaxy grown ZnSe based II-VI heterostructures by the use of a BeTe buffer layer
Authors: Fischer F. , Keller M. , Gerhard T. , Behr T. , Litz T. , Lugauer H. J. , Keim M. , Reuscher G. , Baron T. , Waag A. , Landwehr G.
Published: July 2002

The reduction of extended defects in ZnSe based II-VI heterostructures grown by molecular beam epitaxy on (001) GaAs is reported, using BeTe buffer layers as a novel approach. After defect selective chemical etching three different types of etch pits could be observed by optical microscopy. By the application of a thin BeTe buffer layer the density of paired Frank type stacking faults could be strongly reduced to values below 103 cm−2. The role of Se in the background pressure for the defect nucleation at the II-VI/GaAs interface is significant. It has been found that BeTe can form a smooth interface to GaAs and ZnSe which is reflected in high resolution x-ray diffraction data.
Surface effects on current mechanisms in 6H-SiC n+pp+ structures passivated with a deposited oxide
Authors: Ortolland S. , Raynaud C. , Locatelli M. L. , Chante J. P. , Senes A.
Published: July 2002

Bipolar n+pp+ diodes fabricated by nitrogen implantation and passivated with a deposited oxide have been characterized. Current–voltage measurements in a large temperature range have been analyzed. We also used the optical beam induced current method to represent the depleted zone at the surface around a reverse-biased device. We show that phenomena as the diameter-dependent current for low reverse and forward biases, the specific value for the energy activation of current under low bias equal to 0.65 eV, the reverse current–voltage characteristics evolution with time, or the anomalous spread of the depleted layer around a reverse-biased diode can be correlated with the presence of the deposited oxide as a passivation layer. A study of the current–time characteristic, obtained for a set reverse bias, is used to prove the presence of charges in the oxide and interface states responsible for an eventual inversion channel along the mesa.
X-ray study of lateral strain and composition modulation in an AlGaAs overlayer induced by a GaAs lateral surface grating
Authors: Darowski N. , Pietsch U. , Zeimer U. , Smirnitzki V. , Bugge F.
Published: July 2002

A lateral surface grating has been prepared by holographic photolithography followed by wet chemical etching on a slightly misaligned GaAs [001] substrate. The structural parameters were investigated before and after thermal annealing by triple-axis high resolution x-ray diffraction (HRXRD) and scanning electron microscopy (SEM). In particular HRXRD was used to collect reciprocal space maps providing periodicity and shape of the grating. After overgrowth of the free standing nanostructure with AlxGa1−xAs the HRXRD technique fails. Only first order grating truncation rods remain in the (004) HRXRD map. They disappear completely running asymmetric reflections. On the other hand SEM at the cleavage plane reveals the perfection of the overgrowth process and the smoothness of the sample surface. Thus nondestructive analysis of the buried lateral nanostructure was performed by triple-axis x-ray grazing incidence diffraction using synchrotron radiation. This method is exclusively sensitive to the lateral strain profile and provides the possibility of depth resolution. Using two complementary in-plane reflections we found lateral strain modulation within the GaAlAs overlayer accompanied by a compositional modulation with the same period as the grating itself. This periodicity still appears close below the sample surface.
Low frequency oscillations in a stationary plasma thruster
Authors: Boeuf J. P. , Garrigues L.
Published: July 2002

Stationary plasma thrusters are ion thrusters whose properties make them especially suitable for satellite station keeping or orbit transfer. In these thrusters, a magnetic field transverse to the electron flow towards the anode increases the electron collision frequency and makes possible the generation of a plasma at relatively low gas flow and gas density. The decrease of the plasma conductivity due to the magnetic field induces a large electric field in the plasma which accelerates the quasicollisionless ions whose trajectories are not significantly affected by the magnetic field. The purpose of this article is to clarify, using results from a simple model, the electrical properties of these thrusters and the low frequency oscillation regime which has been observed experimentally. The model is based on the assumption of quasineutrality of the plasma column and on a 1D transient hybrid treatment of electron and ion transport in the device. Electrons are considered as a fluid and ions are described with a collisionless kinetic equation. This model provides reasonable estimates of the plasma properties and is able to give a clear picture of the low frequency oscillations, qualitatively close to the experimental observations.
The growth of pinhole-free epitaxial DySi2−x films on atomically clean Si(111)
Authors: Shen G. H. , Chen J. C. , Lou C. H. , Cheng S. L. , Chen L. J.
Published: July 2002

The growth of pinhole-free epitaxial DySi2−x films on atomically clean Si(111) has been achieved by depositing a 2-nm-thick Dy layer onto Si(111) with a 1.5-nm-thick capping amorphous Si (a-Si) layer at room temperature followed by annealing at 700 °C in ultrahigh vacuum. The thickness of the a-Si was selected to be such that the consumption of Si atoms from the substrate is minimized by taking into account the formation of an amorphous interlayer at the Dy/Si(111) interface. Based on our experimental findings, a new mechanism for the formation of pinhole is proposed. The Stranski–Krastanov growth behavior of epitaxial DySi2−x on Si(111) by solid phase epitaxy leads to the apparently random formation of a high density of recessed regions at the initial stage of silicidation. Polycrystalline DySi2−x was found to be present at the areas inside and epitaxial DySi2−x outside the recessed regions. Large numbers of Si atoms from the substrate can therefore diffuse through the recessed regions. As a result, the depth and size of the recessed regions increase with annealing time. Finally, the DySi2−x thin layer inside the recessed regions with higher interface energy is thermally unstable and breaks apart to form pinholes.
Narrow band 390 nm emitting phosphors for photoluminescent liquid crystal displays
Authors: Vecht A. , Newport A. C. , Bayley P. A. , Crossland W. A.
Published: July 2002

Photoluminescent liquid crystal displays use a liquid crystal to modulate the intensity of ultraviolet light incident on visible emissive phosphors. In order to gain optimum performance from the liquid crystal in such a device, the activating light must cover a narrow spectral bandwidth very near to the visible (violet) region of the spectrum. Calcium sulphate europium phosphor offers an emission spectrum closely matching these requirements and has been incorporated inside low pressure mercury cold cathode fluorescent lamps. These have been used to backlight a photoluminescent liquid crystal display demonstrator.
Ti concentration effect on adhesive energy at Cu/TiW interface
Authors: Furuya Akira , Ohshita Yoshio
Published: July 2002

Changes in adhesive energy at Cu/TiW interfaces caused by varying the Ti concentration were evaluated by means of contact angle measurement. The adhesive energy was evaluated by applying the Young–Dupré equation to the contact angle. Copper particles were fabricated by annealing thin Cu film deposited on the TiW film. The adhesive energies at the Cu/TiW interface were evaluated as 1.5, 2.1, and 2.6 N/m for a Ti concentration of 0, 10, and 20 wt. %, respectively. The adhesive energies were found to increase almost linearly as the Ti concentration was increased. These results were applied to prevent TiW/Cu/TiW interconnects fabricated by using infrared-assisted reactive ion etching from peeling at the Cu/TiW interface. In annealing Cu films on TiW substrates at 600 °C in a vacuum, it was found that the Cu peeled from the TiW when the Ti concentration was 10 wt. %, but it stuck to that at 20 wt. %. The effect of Ti on the adhesion strength was also studied from the results of molecular calculation by using the method.
Band-gap widening of CdO thin films
Authors: Ueda Naoyuki , Maeda Hiroo , Hosono Hideo , Kawazoe Hiroshi
Published: July 2002

CdO thin films were deposited on silica glass substrates by rf sputtering. The orientation of the crystal axis in the films was changed by varying the sputtering conditions. The (100)-oriented films were annealed under various conditions to change the carrier concentration. A blueshift of the optical absorption edge was observed as the carrier concentration increased, and band-gap widening was analyzed using the Burstein–Moss formula. We found that the intrinsic direct-band-gap Eg0 (Γ15–Γ1 gap) and the reduced effective mass mvc* are 2.22±0.01 eV and (0.274±0.013)m0, respectively, at room temperature. The results are discussed in relation to the band structure of CdO.