bluuebunny/crossref_metadata_2025 huggingface dataset:
Model for the effect of tensile and compressive stress on ferromagnetic hysteresis
Authors: Sablik M. J. , Kwun H. , Burkhardt G. L. , Jiles D. C.
Published: July 2002

A model is presented for the stress-dependent effective field, which when used in conjunction with the Jiles–Atherton theory, qualitatively accounts for (1) the change in slope and shape of the hysteresis curves with uniaxial stress and (2) the convexity of the curves depicting remanent and peak magnetization as a function of stress. Also, the model can produce the Villari reversal if parameters are selected appropriately.
Magnetic viscosity in different Nd-Fe-B magnets
Authors: Givord D. , Tenaud P. , Viadieu T. , Hadjipanayis G.
Published: July 2002

Measurements of magnetic viscosity in Nd-Fe-B magnets of different kinds (sintered magnets, MQII magnets prepared from ribbons, and LDC magnets prepared by liquid dynamic compaction) are reported. In all magnets, the viscosity S and the irreversible susceptibility χirr are found to be proportional. The temperature dependence of the coefficient Sv =S/χirr exhibits a sharp maximum at about 100 K. The temperature dependence of the activation energy E0 which governs magnetic aftereffects is deduced from these data. In sintered and LDC magnets, but not in MQII ones, E0 is found to be approximately proportional to the domain wall energy. It is concluded that the mechanism of coercivity is different in sintered and LDC magnets than in the MQII ones.
Characteristics of electron-beam-excited Kr*2 at low pressures as a vacuum ultraviolet source
Authors: Eckstrom D. J. , Nakano H. H. , Lorents D. C. , Rothem T. , Betts J. A. , Lainhart M. E. , Triebes K. J. , Dakin D. A.
Published: July 2002

The performance of Kr*2 as a 146 nm fluorescence or laser source when pumped by a low-current, long-pulse electron beam has been determined. The fluorescence efficiency of Kr*2 is near the theoretical limit of ∼46% at modest pressures over a range of pump rates up to 3×105 W/cm3. Lasing could not be achieved under the conditions studied here. An improved kinetics and extraction code has been developed to model the performance of the Kr*2 system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Estimated rates for these processes are presented. With this model, good absolute agreement of model predictions with experimental results has been obtained at pressures as low as 0.5 atm.
Water-induced relaxation of the vitreous silica surface
Authors: Feuston B. P. , Garofalini S. H.
Published: July 2002

The formation of a vitreous silica surface in the presence of water vapor is investigated through the molecular dynamics simulation technique. Three-body potentials are employed to describe the interatomic interactions. The structure of the reconstructed surface is analyzed with respect to the concentration and type of defects. Comparison between surfaces created in the presence of water and those created in a vacuum indicate that H2O-surface reactions substantially reduce the number of topological (two- , three- , and four-membered rings) and bonding defects (under- and overcoordinated species) incurred during the relaxation process. Due to the dissociation of water molecules, the wet surface contains excess oxygen with a concentration of 3.1±0.6 silanols per 100 Å2, involving approximately 13% geminal sites.
Pt layer thickness dependence of magnetic properties in Co/Pt multilayers
Authors: Zhang Z. , Wigen P. E. , Parkin S. S. P.
Published: July 2002

The magnetic properties of Co/Pt multilayer films prepared by dc sputtering have been investigated using ferromagnetic resonance and a vibrating sample magnetometer. The thickness of the Co layer is constant at 25 Å while the thickness of the Pt layer varies from 0.9 to 18 Å. The saturation magnetization per unit volume of Co is nearly constant for all the films and close to that of bulk Co. In the ferromagnetic resonance spectra, the presence of two resonance modes having an amplitude ratio that varies substantially with the Pt thickness is strong evidence that a significant coupling between cobalt layers exists even at 18 Å of Pt. For Pt thicknesses greater than 3.6 Å, a uniaxial perpendicular magnetic anisotropy field component increases with Pt thickness and saturates at about 9.0 Å. If the induced anisotropy energy is attributed to a surface anisotropy energy at the Co/Pt interface, it has a value Ks of approximately 0.57 erg/cm2. The temperature dependence of the magnetic hysteresis loop and the ferromagnetic resonance spectrum indicate an increase in the uniaxial anisotropy field with decreasing temperature.
A thin-film Schottky diode fabricated from flame-grown diamond
Authors: Glesener J. W. , Morrish A. A. , Snail K. A.
Published: July 2002

Schottky diodes have been fabricated out of polycrystalline diamond thin films grown in an atmospheric pressure turbulent flame. Doping was accomplished by bubbling the source gas through a solution of diboric trioxide in acetone. The diodes were grown on metallic, noncrystalline substrates. A device fabrication technique utilizing carbide-forming refractory metals as a buffer layer was used to bond the diamond film to the substrate and act as an ohmic contact. The resulting structure avoided the difficulties in flame growth of working with thin detached films. Diode rectification ratios in excess of 300 were observed.
On the modification of the Kondorsky function
Authors: Schumacher F.
Published: July 2002

The validity of Kondorsky’s switching field distribution for pinning magnets with rectangular hysteresis loops is extended to cases where the hysteresis loops are influenced by reversible rotation of the magnetization vectors in the domains. Caused by this influence, the monotonic Kondorsky function changes to modified Kondorsky functions HP(ϑ0) with minimal switching fields for orientation angles ϑ0≳0° when HP(ϑ0 = 0°) reaches the anisotropy field strength Ha. It is shown that the modified Kondorsky function is a more suitable description of experimental results in the literature than the Kondorsky function itself.
Strain relaxation kinetics in Si1−xGex/Si heterostructures
Authors: Houghton D. C.
Published: July 2002

A semiempirical kinetic model is presented which maps out the thermal budget for processing of strained layer devices through epitaxial growth and postgrowth anneals. Misfit strain relaxation in Si1−xGex/Si heterostructures by the injection and propagation of a/2 〈110〉 60°-type misfit dislocations has been studied for a range of geometries and dimensions. Strained layer superlattices, Si1−xGex alloy layers, 0<x<0.3, and alloy layers with unstrained Si capping layers of thickness 0 to 400 nm were grown by molecular-beam epitaxy on (100) Si substrates and subjected to post-growth thermal cycles. Velocity and nucleation rate data from Nomarski interference microscopy of defect-etched surfaces were correlated with electron beam induced current microscopy transmission electron microscopy and x-ray diffraction results to define Arrhenius relationships for misfit dislocation injection rates and propagation velocities. A unified kinetic model for misfit strain relaxation that incorporates both nucleation and propagation is then developed, which is applicable for all heterostructures and thermal cycles in the low dislocation density regime <103 mm−1.

Nonuniform strain distribution in graded device heterostructures is considered by defining the effective stress acting on misfit dislocations for an arbitrary geometry. The effective stress was varied from 0 to 750 MPa in Si1−xGex/Si heterostructures by varying both layer dimensions and Ge concentration. Misfit dislocation nucleation rates varied from 10−3 to 103 mm−2 s−1 and misfit extension velocities of 25 nm s−1 to 2 mm s−1 were obtained over the temperature range 450–1000 °C for anneals of duration 5–2000 s. Activation energies, stress exponents, and prefactors in the Arrhenius relations were found to be independent of Ge concentration, effective stress, and heterostructure geometry allowing a comprehensive model to be developed. The onset of strain relaxation during epitaxial growth cycles (the ‘‘apparent critical thickness’’ or metastability limit) characteristic of molecular-beam epitaxy and chemical vapor deposition was measured and correlated with the simulation of misfit dislocation injection and propagation in typical growth sequences. The kinetic model is also used to define the maximum time-temperature envelope, or thermal budget (t, T), for the misfit dislocation-free processing of Si1−xGex/Si heterostructures subjected to post-growth thermal treatme
Response times of electrorheological fluids
Authors: Hill John C. , Van Steenkiste Thomas H.
Published: July 2002

Potential automotive applications of electrorheological (ER) fluids have stimulated renewed interest in their development. The inherent simplicity of a suspension of particles in a fluid that responds to an applied electric field with a two- or three-order-of-magnitude increase in viscosity is clearly attractive. However, there are some important parameters that are not well delineated. Among them is the ER-fluid response time, which needs to approach the millisecond range for effective device operation. We report results from recent measurements of response times for glass spheres suspended in silicone oil.
Magnetic properties of fine crystalline Fe–P–C–Cu–X alloys
Authors: Fujii Y. , Fujita H. , Seki A. , Tomida T.
Published: July 2002

The crystallization process and magnetic properties of Fe–P–C–Cu–Ge–Si amorphous alloys were investigated. After annealing above the crystallization temperature, the bcc Fe particles, with nanoscale grain size, precipitated in the amorphous alloy for a wide P concentration range. The volume ratio of the bcc Fe phase to amorphous was about 30%. The mixed phase of these alloys shows a drastic decrease of coercive force (Hc) and particle diameter (d) as P concentration increases. The lowest Hc was obtained for Fe78P16C2Cu0.5Ge3Si0.5, and Hc and d were 1.8 A/m and 16 nm, respectively. Observation by in situ Lorentz scanning electron microscopy (SEM) revealed that the width of magnetic domains were 0.01 or 0.1 mm and the domain walls with smaller bcc Fe particles moved in a weaker magnetic field than those with larger ones. So the low Hc is attributed to the decrease of magnetocrystalline anisotropy caused by fine structures. The core loss of fine crystalline Fe–P–C–Cu–Si–Mo alloy, W14/50, was 0.22 W/kg after annealing under a magnetic field of 2.4 kA/m.
Charging of pattern features during plasma etching
Authors: Arnold J. C. , Sawin H. H.
Published: July 2002

The localized charging of a rectangular trench during the plasma etching of a perfectly insulating surface was modeled assuming an isotropic electron flux and monodirectional ion bombardment. The field set up by the localized charging acts to deflect arriving ions, modifying the ion flux densities within the feature, and thus, etching rates. Preliminary simulations indicate that this may be important in the shaping of etching profiles.
Determination of the elastic constants of anisotropic materials using laser-generated ultrasonic signals
Authors: Castagnede B. , Kim Kwang Yul , Sachse Wolfgang , Thompson Michael O.
Published: July 2002

This paper presents the solution of the materials characterization problem in which the elastic constants of an anisotropic material are determined from ultrasonic wavespeed measurements made in nonprincipal directions of a specimen. The ultrasonic waves were generated via the point-source/point-receiver technique using a pulsed laser as a source and a miniature, point-like transducer as a receiver. Data were acquired during a scan of the source along one of the principal acoustic axes of symmetry of the material. In each waveform the arrivals of the quasi-longitudinal and the two quasi-shear bulk modes were measured and the elastic constants of the material were then recovered using an optimization algorithm. Experimental results are presented for a transversely isotropic, unidirectional fiberglass/polyester and a single crystal specimen of silicon. It was found that the nonlinear fit between the measured and the recovered longitudinal slowness values is excellent. Some discrepancies are observed in the data for the two shear modes. These are shown to be related to the complexity of the detected signals.
Electronic structure of double-layer graphene tubules
Authors: Saito Riichiro , Dresselhaus G. , Dresselhaus M. S.
Published: July 2002

The electronic structure of coaxial, graphene double-layer tubules is predicted for various combinations of metallic and insulating constituent inner and outer monolayers, depending on the diameter and chirality of the tubule. For the examples chosen, some of the energy bands of the inner and outer tubules are coupled to each other by commensurate interlayer interactions. Nevertheless, because of symmetry, the energy bands of metallic monolayer tubules remain metallic even after interlayer interactions are considered. The possible implications of these results on molecular metal-insulator devices are discussed.
Structural chemistry, magnetism and 119Sn Mössbauer spectroscopy of ternary compounds REMn6Sn6 (RE=Pr,Nd,Sm)
Authors: Weitzer F. , Leithe-Jasper A. , Hiebl K. , Rogl P. , Qi Qinian , Coey J. M. D.
Published: July 2002

Ternary compounds (Pr,Nd,Sm)Mn6Sn6 have been synthesized by argon arc melting starting from the pure elements. All compounds were found to be isotypic with HoFe6Sn6 (space group Immm-D2h25, no. 71; Z=6). The HoFe6Sn6 type is a stacking variant composed of slabs of the HfFe6Ge6 type and of the ScFe6Ga6 type. The compounds order ferrimagnetically slightly above room temperature exhibiting a strong magnetocrystalline anisotropy. The 119Sn Mössbauer spectra at 15 and 293 K were fitted with five magnetic sextets, with hyperfine fields ranging from 12 to 32 T at 15 K.
X-ray photoelectron spectroscopy and Auger electron spectroscopy studies of ferroelectric (Pb,La)TiO3 thin films prepared by a multi-ion-beam reactive cosputtering technique
Authors: Qian Zhenghong , Xiao Dingquan , Zhu Jianguo , Li Zisheng , Zuo Changming
Published: July 2002

X-ray photoelectron spectroscopy and Auger electron spectroscopy studies were performed for determining the chemical composition, compositional depth profile, and the chemical bondings of ferroelectric (Pb,La) TiO3 thin films deposited by a multi-ion-beam reactive cosputtering technique. It was found that the chemical composition of the films and the chemical bondings of lead in the near surface region are different from those in the ‘‘bulk’’ region of the films. The films are Pb enriched and the lead atoms exist mainly in the forms of PbOads (i.e., oxygen-chemisorbed lead) and PbTiO3 in the near surface region, whereas in the bulk region, the chemical composition is in good agreement with stoichiometry and lead atoms exist in the form of PbTiO3. It is believed that the effect of Pb enrichment in the near surface regions is an intrinsic phenomenon of lead-contained thin films, which may be attributable to the oxygen-chemisorbed lead.
Cathode sheath formation in a discharge-sustained XeCl laser
Authors: Belasri A. , Boeuf J. P. , Pitchford L. C.
Published: July 2002

A one-dimensional longitudinal model of a XeCl discharge including the cathode region, the plasma, and the external circuit has been used for conditions close to experiments for 50–100 ns laser pulse durations and electron power deposition in the MW/cm3 range in a 300 cm3 chamber. This model provides the space and time variations of the electric field, electron and positive-ion densities in the cathode region, as well as the time variations of the charged-particle densities and excited species concentrations in the plasma obtained with a simplified kinetic model. Results show that under normal conditions of operation the cathode electric field can reach values as high as several 106 V/cm. The influence of photoemission and secondary emission due to ion impact on the cathode is discussed. A transition from capacitive to resistive behavior of the sheath is seen to occur rapidly due to the multiplication of cathode-emitted electrons in the large sheath electric field, leading to a fast increase in the ion conduction current in this region. The possible mechanisms of streamer formation in the sheath are also discussed. Results are presented for different values of the secondary electron emission coefficient due to ion impact from the cathode and it is shown that under conditions of low secondary emission (on the order of 10−2 or less), the electrical and chemical behavior of the plasma can be strongly affected by the presence of the sheath. Finally, the possibility of discharge instabilities related to the sheath evolution is briefly discussed on the basis of the numerical results.
Cobalt particles wrapped in graphitic carbon prepared by an arc discharge method
Authors: Saito Yahachi , Yoshikawa Tadanobu , Okuda Mitsumasa , Fujimoto Naoya , Yamamuro Saeki , Wakoh Kimio , Sumiyama Kenji , Suzuki Kenji , Kasuya Atsuo , Nishina Yuichiro
Published: July 2002

Fine particles of cobalt and cobalt carbide nesting in multilayered graphitic sheets, which were synthesized by an electric arc discharge of carbon rods containing cobalt oxide (CoO), were studied by transmission electron microscopy, including microdiffraction and energy dispersive x-ray analysis. The size of the wrapped particles was typically in a range from 50 to 200 nm. Three phases of nested materials, hcp(α)-Co, fcc(β)-Co, and Co3C, were identified.
Characterization of electron beams generated in a high-voltage pulse-line-driven pseudospark discharge
Authors: Ramaswamy K. , Destler W. W. , Segalov Z. , Rodgers J.
Published: July 2002

Emittance and energy measurements have been performed on a high-brightness electron beam (≳1010 A/m2 rad2) with diameter in the range 1–3 mm and energy in the range 150–170 keV. This electron beam is generated by the mating of a hollow-cathode discharge device operating in the pseudospark regime to the output of a high-power pulse line accelerator. The measured effective emittance lies in the range between 30 and 90 mm mrad and increases with axial distance. Electron energy measurements indicate that the high-energy electrons are generated during the first 20–30 ns of the discharge. Both the emittance and energy experiments were performed at two different ambient argon gas pressures (92 and 152 mtorr). Beam expansion as a function of axial position has also been studied and a lower bound on the beam brightness has been obtained.
Single-domain magnetic pillar array of 35 nm diameter and 65 Gbits/in.2 density for ultrahigh density quantum magnetic storage
Authors: Chou Stephen Y. , Wei Mark S. , Krauss Peter R. , Fischer Paul B.
Published: July 2002

Using electron beam nanolithography and electroplating, arrays of Ni pillars on silicon that have a uniform diameter of 35 nm, a height of 120 nm, and a period of 100 nm were fabricated. The density of the pillar arrays is 65 Gbits/in.2—over two orders of magnitude greater than the state-of-the-art magnetic storage density. Because of their nanoscale size, shape anisotropy, and separation from each other, each Ni pillar is single domain with only two quantized perpendicular magnetization states: up and down. Each pillar can be used to store one bit of information, therefore such nanomagnetic pillar array storage offers a rather different paradigm than the conventional storage method. A quantum magnetic disk scheme that is based on uniformly embedding single-domain magnetic structures in a nonmagnetic disk is proposed.
Models of slow relaxation in particulate and thin film materials (invited)
Authors: Chantrell R. W. , Lyberatos A. , El-Hilo M. , O’Grady K.
Published: July 2002

The development of a number of models of slow relaxation in magnetic materials is reviewed. A phenomenological theory based on the intrinsic energy barrier is shown to explain the form of time dependence of the magnetization, including the logarithmic time dependence observed for systems with a relatively wide distribution of energy barriers. This formulation gives rise to useful analytical results which give generally good qualitative agreement with experiment. The slow relaxation is related to the irreversible magnetic behavior via a fictitious fluctuation field Hf which itself determines a quantity called the activation volume Vact. Both Hf and Vact are related to the magnetization reversal process. For granular materials Vact is generally smaller than the grain size. Computer simulations based on the Monte Carlo method are applied to the investigation of the behavior of thin films with perpendicular anisotropy. Detailed comparisons of the simulation with experimental data demonstrate the relationship between Vact and the micromagnetic magnetization reversal mode. Some recent models introducing thermal agitation into the micromagnetic formalism are discussed.
High-speed photography of compressive failure waves in glasses
Authors: Bourne N. K. , Rosenberg Z. , Field J. E.
Published: July 2002

A mechanism for the compressive failure of soda-lime and borosilicate glasses is proposed based upon high-speed photography of impact on glasses. Shock loading was produced by the impact of a 50 mm diameter projectile so inducing shock states of one-dimensional strain in glass targets. The shock waves and failure fronts were visualised using the shadowgraph technique. The failure appeared to occur at discrete nucleation sites and propagated out to form a continuous front. The velocity of this front increased with higher impact stresses and varied with the glass composition.
Interactions between the interface of titanium and fullerene
Authors: Wang W. H. , Wang W. K.
Published: July 2002

X-ray diffraction, photoemission, and Auger electron spectroscopy studies are reported of the interactions between the interface of titanium and C60 solid film during low-temperature annealing. The structure of C60 at the Ti/C60 interface is disrupted by the Ti atoms when Ti is deposited onto the surface of C60 film. Titanium atoms react with carbon atoms to form amorphous Ti carbide during low-temperature annealing. This interaction is related to the solid state amorphization reaction of the deposited Ti layer with C60 film, which occurs with a driving force of a negative heat of formation in the Ti–C system and a dominant diffusion of carbon into the Ti overlayer.
Ohmic contact formation mechanism of the Au/Ge/Pd/n-GaAs system formed below 200 °C
Authors: Wang L. C. , Hao P. H. , Cheng J. Y. , Deng F. , Lau S. S.
Published: July 2002

We report the ohmic contact formation mechanism of the low resistance (∼1×10−6 Ω cm2) Au/Ge/Pd contact to n-GaAs annealed at 175 °C. Cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry were utilized in this study. It is found that the solid phase regrowth process, interdiffusion between Au and Ge, and the enhancement of the conductivity of the excess Ge layer are responsible for the observed low contact resistivity.
A novel integrated electric motor/pump for underwater applications
Authors: Cho C. Peter , Fussell Barry K. , Hung John Y.
Published: July 2002

This article presents a novel electric motor/pump system that combines an axial field, permanent magnet motor with a centrifugal pump. This system, unique because the motor permanent magnet rotor and pump impeller vanes are a single unit, provides a compact, reliable, low-noise, and high-power density electrically driven centrifugal pump suitable for underwater applications in which minimizing noise, vibration, and volume are major design objectives. Performance tradeoffs for the electromagnetic analysis were made by three-dimensional finite element analysis models in conjunction with a lumped parameter magnetic circuit model.
Structural and optical properties of laser deposited ferroelectric (Sr0.2Ba0.8)TiO3 thin films
Authors: Cheng Hsiu-Fung
Published: July 2002

Laser deposition technique has been applied to synthesize (Sr0.2Ba0.8)TiO3 thin films. The crystal structure of the films and the effect of deposition parameters on them have been systematically examined. Among the important deposition parameters, the substrate temperature and the chamber oxygen pressure affect the characteristics of the films most prominently. The crystalline phase of (Sr0.2Ba0.8)TiO3 films can only be obtained when the substrate temperature is higher than 550 °C. Furthermore, both the low deposition oxygen pressure and the high substrate temperature during deposition facilitate the formation of (100) grains. The metal-insulator-metal structure of the films deposited on Pt-coated Si substrates, possesses good ferroelectric properties, which are indicated by butterfly-shaped capacitance-voltage characteristics and demonstrated by hysteresis loop of remanence Pr=1.56 μC/cm2 and coercivity Ec=13.5 kV/cm. The charge-discharge test reveals that the effective dielectric constant can reach a value as high as εr=788. The energy gap (Eg) and index of refraction (n) are estimated to be Eg=2.8 eV and n=1.68–1.73, respectively.
Nanolithographically defined magnetic structures and quantum magnetic disk (invited)
Authors: Chou Stephen Y. , Krauss Peter R. , Kong Linshu
Published: July 2002

Isolated and interactive arrays of magnetic nanostructures as small as 15 nm are fabricated using nanolithography and related technologies, and are characterized using magnetic force microscopy. It has been demonstrated that manipulating the size, aspect ratio, and spacing of these nanostructures can lead to unique control of their magnetic properties. A quantum magnetic disk based on discrete single-domain nanomagnetic structures with storage density of 65 Gbits/in.2 is demonstrated along with a low-cost method for mass producing such disks. Other impacts that nanofabrication can bring to the development of future magnetic storage are discussed.
Determination of molecular orientation of very thin rubbed and unrubbed polyimide films
Authors: Sakamoto K. , Arafune R. , Ito N. , Ushioda S. , Suzuki Y. , Morokawa S.
Published: July 2002

To determine the molecular orientation of rubbed and unrubbed poly[4,4′-oxydiphenylene- pyromellitimide] (PMDA-ODA) films, we have measured the infrared (IR) absorption spectra of PMDA-ODA films on Si substrates as a function of the incident angle. The molecular orientation was determined by fitting the incident angle dependence and the IR dichroic ratio with the theoretical curves. The IR absorption was calculated by a transfer matrix method to take account of the multiple reflection and refraction at the interfaces. The vibrational response of the polyimide film in the IR region was represented by a Lorentz oscillator model. We found that the molecules in a spin-coated polyimide film are oriented parallel to the substrate surface with a standard deviation of 6.5°. For a rubbed film, the polymer chains are oriented along the rubbing direction and are tilted up on average by 8.5° from the surface plane. We observed a blueshift of the C=O asymmetric stretching band at oblique incidence. The blueshift is explained by anomalous dispersion of the dielectric constant of the polyimide film, and is reproduced by our calculation.
Influence of postdeposition annealing on the structural and optical properties of sputtered zinc oxide film
Authors: Gupta Vinay , Mansingh Abhai
Published: July 2002

The influence of postdeposition annealing on the structural and optical properties of rf sputtered insulating zinc oxide films has been investigated. The as-grown films deposited on quartz substrates were highly c-axis oriented and in a state of stress. These films become almost stress free after a postdeposition annealing treatment at 673 K for 1 h in air. Above 673 K, a process of coalescence was observed which causes major grain growth resulting in microcrack formation and surface roughness. The refractive index shows a strong frequency dispersion below the interband absorption edge. The optical dispersion data have been fitted to (1) a single oscillator model and (2) the Pikhtin–Yas’kov model. The origin of optical dispersion at different annealing temperatures has been discussed in the light of these models. A packing density of more than 99% is estimated in the film annealed at 673 K, indicating that these films are almost void free.
Electron spin resonance study of NiFe2O4 nanosolids compacted under high pressure
Authors: Sui Y. , Xu D. P. , Zheng F. L. , Su W. H.
Published: July 2002

In this article, NiFe2O4 nanosolids are prepared by compacting nanoparticles, synthesized by the coprecipitation method, under different pressures. The variations of structure and interface states of the NiFe2O4 nanosolids with compacting pressures are studied by x-ray diffraction and electronic spin resonance (ESR). It is found that the crystal structure of the nanosolids has not changed under pressure up to 6.0 GPa, but the linewidth and g-factor values of their ESR spectra increased significantly with increasing pressure up to 4.5 GPa and then decreased slightly with further increase of pressure. These variations are discussed in terms of the changes of interparticle magnetic dipole interaction and superexchange interaction in NiFe2O4 nanosolids under different pressures. The experimental results suggest that 4.5 GPa is the optimum forming pressure, under which the NiFe2O4 nanoparticles can be compacted into a dense solid with their structure and nanoproperties still preserved.
Comparative study of phonon-limited mobility of two-dimensional electrons in strained and unstrained Si metal–oxide–semiconductor field-effect transistors
Authors: Takagi Shin-ichi , Hoyt Judy L. , Welser Jeffrey J. , Gibbons James F.
Published: July 2002

The phonon-limited mobility of strained Si metal–oxide–semiconductor field-effect transistors (MOSFETs) fabricated on a SiGe substrate is investigated through theoretical calculations including two-dimensional quantization, and compared with the mobility of conventional (unstrained) Si MOSFETs. In order to match both the mobility of unstrained Si MOSFETs and the mobility enhancement in strained Si MOSFETs, it is necessary to increase the coupling of electrons in the two-dimensional gas with intervalley phonons, compared to the values used in conventional models. The mobility enhancement associated with strain in Si is attributed to the following two factors: the suppression of intervalley phonon scattering due to the strain-induced band splitting, and the decrease in the occupancy of the fourfold valleys which exhibit a lower mobility due to the stronger interaction with intervalley phonons. While the decrease in the averaged conductivity mass, caused by the decrease in the occupancy of the fourfold valleys, contributes to the mobility enhancement in bulk strained Si, it is not necessarily adequate to explain the mobility enhancement for two-dimensional electrons in strained Si. This is suggested by the fact that the mobility limited by intravalley acoustic phonon scattering, which is the dominant scattering mechanism, has almost the same value in the two- and the fourfold valleys, because the difference in the conductivity mass is compensated by differences in the inversion-layer thickness and the valley degeneracy.
Band structure, deformation potentials, and carrier mobility in strained Si, Ge, and SiGe alloys
Authors: Fischetti M. V. , Laux S. E.
Published: July 2002

Using nonlocal empirical pseudopotentials, we compute the band structure and shear deformation potentials of strained Si, Ge, and SiGe alloys. Fitting the theoretical results to experimental data on the phonon-limited carrier mobilities in bulk Si and Ge, the dilatation deformation potential Ξd is found to be 1.1 eV for the Si Δ minima, −4.4 eV for the Ge L minima, corresponding to a value for the valence band dilatation deformation potential a of approximately 2 eV for both Si and Ge. The optical deformation potential d0 is found to be 41.45 and 41.75 eV for Si and Ge, respectively. Carrier mobilities in strained Si and Ge are then evaluated. The results show a large enhancement of the hole mobility for both tensile and compressive strain along the [001] direction, but only a modest enhancement (approximately 60%) of the electron mobility for tensile biaxial strain in Si. Finally, from a fit to carrier mobilities in relaxed SiGe alloys, the effective alloy scattering potential is determined to be about 0.7 eV for electrons, 0.9±0.1 eV for holes, and the low-field mobilities in strained alloys can be evaluated. The results show that alloy scattering completely cancels any gain expected from the lifting of the valleys/bands degeneracy caused by the strain.
The effect of external field symmetry on the antiferroelectric-ferroelectric phase transformation
Authors: Yang P. , Payne D. A.
Published: July 2002

The antiferroelectric (AFE)-ferroelectric (FE) phase transformation under dc bias and hydrostatic pressure conditions in tin-modified lead zirconate titanate ceramics [Pb(Zr,Sn,Ti)O3, i.e., PZST] was investigated. The shifting of transformation temperature under these conditions and an electric field induced lattice softening are reported. Depending on the symmetry of external applied fields, the thermal stability region of one phase can be expanded at the expense of another. Experimental results indicate that a symmetric external field (such as hydrostatic pressure) tends to stabilize the AFE phase region, whereas an asymmetric external field (such as electric field) tends to extend the FE phase region. These observations are found to be generally consistent with many ferroelectric and antiferroelectric materials exhibiting a displacive structural phase transformation. Results are compared with the Clausius–Clapeyron relationship, and fundamental issues underlying the thermodynamic relationship and field-induced lattice softening behavior are discussed from the perspective of lattice dynamics theory.
Effect of vapor composition on polytype homogeneity of epitaxial silicon carbide
Authors: Konstantinov A. O. , Hallin C. , Kordina O. , Janzén E.
Published: July 2002

The silicon to carbon precursor ratio is demonstrated as strongly affecting the spontaneous nucleation of cubic SiC upon the growth of epitaxial layers of 4H and 6H silicon carbide using the chemical vapor deposition (CVD) technique. High C/Si ratios appear to promote the nucleation of cubic SiC. A model of CVD process chemistry that relates the effect to a decrease of SiC surface mobility with an increase of the C/Si ratio is proposed. The resulting increase of supersaturation at the surface terraces promotes the spontaneous nucleation of cubic SiC.
Magnetic and magnetoelastic behavior of mechanically alloyed FeRh compound
Authors: Marquina C. , Ibarra M. R. , Algarabel P. A. , Hernando A. , Crespo P. , Agudo P. , Yavari A. R. , Navarro E.
Published: July 2002

Mechanical milling and subsequent annealing have been applied in order to obtain FeRh compound, as an alternative to bulk sample preparation methods (plasma jet- and arc-melting, induction, etc). Structural and magnetic characterization of the as-milled sample show that heat treatment at the proper temperature is necessary in order to obtain the equilibrium ordered α′-bcc phase, in which a first order ferro–antiferromagnetic transition accompanied by a large magnetovolume effect takes place. In order to study the magnetoelastic behavior of the mechanically alloyed FeRh compound, magnetostriction measurements were performed using high pulsed magnetic fields up to 14.2 T within the temperature range 4–450 K. The large magnetostriction observed in the room temperature range has been related to the field instability of the rhodium magnetic moment within the bcc ordered phase.
A theory of magnetostriction oriented towards applications
Authors: DeSimone Antonio , James Richard D.
Published: July 2002

A simplified theory of magnetostriction, derived from micromagnetics, is presented. The theory is designed to provide a systematic, yet manageable, quantitative tool in the design of magnetostrictive actuators, by making the “calculation” of the overall response of magnetostrictive specimens subjected to applied magnetic fields and loads a feasible task. Indeed, for simple geometries, the computation of energy minimizing domain patterns is reduced to a finite dimensional problem. As an application of the theory, a mechanism explaining the anisotropic magnetostriction observed in Terfenol-D rods, based on computed energetically optimal domain arrangements, is proposed.
NiFe/NiO bilayers with high exchange coupling and low coercive fields
Authors: Han De-Hua , Zhu Jian-Gang , Judy Jack H.
Published: July 2002

NiO/Ni 81 Fe 19 bilayers with high exchange coupling, Hex, and low coercivity, Hc, were fabricated. These bilayers with NiO crystalline textures of (111), (200), and (220), and different surface/interface roughness were deposited onto a Si(100) wafer by changing the rf reactive sputtering conditions. A new parameter of exchange coupling to coercivity ratio, Hex/Hc, was used to quantify the effectiveness of the exchange bias for the bilayer film. An Hex=39 Oe and Hc=4.1 Oe were obtained from a NiO (45 nm)/NiFe (10 nm) bilayer, yielding a ratio of Hex/Hc=9.51, which was significantly higher than any previously reported results. An atomic force microscopy study showed that the key to the low coercivity was the reduction of surface roughness of the NiO/NiFe bilayer. It is proposed that the roughness-induced magnetostatic coupling in the NiFe film could be one of the main mechanisms for the relating high coercivity in NiO exchange-biased NiFe bilayer. NiO/NiFe/Co/Cu/Co/NiFe spin-valve films were deposited and a giant magnetoresistance of 6.62% and a field sensitivity of 0.83%/Oe were obtained at room temperature.
Phosphorus and boron implantation in 6H–SiC
Authors: Rao Mulpuri V. , Gardner Jason A. , Chi P. H. , Holland O. W. , Kelner G. , Kretchmer J. , Ghezzo M.
Published: July 2002

Phosphorus and boron ion implantations were performed at various energies in the 50 keV–4 MeV range. Range statistics of P+ and B+ were established by analyzing the as-implanted secondary ion mass spectrometry depth profiles. Anneals were conducted in the temperature range of 1400–1700 °C using either a conventional resistive heating ceramic processing furnace or a microwave annealing station. The P implant was found to be stable at any annealing temperature investigated, but the B redistributed during the annealing process. The implant damage is effectively annealed as indicated by Rutherford backscattering measurements. For the 250 keV/1.2×1015 cm−2 P implant, annealed at 1600 °C for 15 min, the measured donor activation at room temperature is 34% with a sheet resistance of 4.8×102 Ω/□. The p-type conduction could not be measured for the B implants.
Local structure of amorphous GeTe and PdGeSbTe alloy for phase change optical recording
Authors: Hirota K. , Nagino K. , Ohbayashi G.
Published: July 2002

The local structures of amorphous Ge0.52Te0.48 and Pd0.01Ge0.17Sb0.26Te0.56 were examined by extended x-ray absorption fine structure. In amorphous GeTe film, only Ge atoms were observed in the nearest-neighbor of Te atoms. The average coordination number around Te atoms in amorphous GeTe is 1.5 which is close to twofold coordination. These results support a chemically ordered structure model. In amorphous PdGeSbTe film, Ge and Sb (and/or Te) atoms were observed in the nearest-neighbor of Te atoms at distances of 2.61 and 2.84 Å, respectively. Debye–Waller factors of the Te–Ge bonds in TeGe and PdGeSbTe are 0.076 and 0.081 Å, respectively, and are larger than that of Te-(Sb and/or Te); 0.063 Å. From these results, we argue that the softened Te–Ge bond plays an important role in the elementary process of crystallization of GeTe and PdGeSbTe alloys. The total average coordination number of Pd atoms in amorphous PdGeSbTe alloy is 4.0. This result suggests Pd atoms play the role of cross-linking element.
Evidence of domain wall contribution to the dielectric permittivity in PZT thin films at sub-switching fields
Authors: Taylor D. V. , Damjanovic D.
Published: July 2002

Through the use of relations analogous to that of the Rayleigh law, it is demonstrated that the ac electric field dependence of the permittivity of ferroelectric thin films can be described. It is further shown that both reversible and irreversible components of the permittivity decrease linearly with the logarithm of the frequency of the ac field. The results demonstrate that the models describing the interaction of domain walls and randomly distributed pinning centers in magnetic materials can be extended to the displacement of domain walls in ferroelectric thin films.
Growth of Si-doped GaAs(110) thin films by molecular beam epitaxy; Si site occupation and the role of arsenic
Authors: Tok E. S. , Neave J. H. , Ashwin M. J. , Joyce B. A. , Jones T. S.
Published: July 2002

We have studied the relationship between the surface morphology, Si doping behavior, and arsenic incorporation kinetics for GaAs(110) thin films grown on singular substrates by molecular beam epitaxy. To obtain films with good surface morphology, homoepitaxial growth requires low substrate temperatures and high As:Ga flux ratios. Under these conditions, the Si-doped layers exhibit n-type behavior. Growth at higher temperatures and lower As:Ga flux ratios produces films with a poorer morphology, the n-type layers become increasingly compensated, and p-type layers are eventually formed. This growth-related site switching behavior and corresponding variation in surface morphology can be attributed to a low arsenic surface population, a consequence of the small and temperature-dependent arsenic incorporation coefficient for growth on GaAs(110).
Electric field effects on the states of a donor impurity in rectangular cross-section vacuum/GaAs/vacuum quantum-well wires
Authors: Montes A. , Duque C. A. , Porras-Montenegro N.
Published: July 2002

We present a study of the role of the electric field on the binding energy of the ground and first few excited states of a shallow impurity in a rectangular cross-sectional area vacuum/GaAs/vacuum quantum-well wire (QWW), where the electric field is applied perpendicular to the symmetry axis of the wire. Using the effective-mass approximation within a variational scheme we calculate the binding energy of the 1s-like ground state as well as that of some excited states (2s-, 2px-, and 2pz-like) as a function of the geometry, applied electric field, and donor-impurity position. We found that the presence of the electric field breaks down the degeneracy of states for impurities symmetrically positioned within the structure, and that the geometric confinement and the electric field are determinant for the existence of bound excited states in these structures. Future interpretations of optical phenomena related with shallow donor impurities in vacuum/GaAs/vacuum QWWs, in which the effects of an applied electric field competes with the quantum confinement, must take into consideration these results.
Properties of large-area nanomagnet arrays with 100 nm period made by interferometric lithography
Authors: Savas T. A. , Farhoud M. , Smith Henry I. , Hwang M. , Ross C. A.
Published: July 2002

A method is presented for the fabrication of large-area arrays of magnetic particles with 100 nm period, using achromatic interferometric lithography combined with etching, electrodeposition, or an evaporation and liftoff processes. These “nanomagnet” arrays have applications in patterned magnetic media, in magnetic memories, and for studies of particle interactions.
Magnetism and compressive modulus of magnetic fluid containing gels
Authors: Mitsumata Tetsu , Ikeda Kazuo , Gong Jian Ping , Osada Yoshihito , Szabó Dénes , Zrı́nyi Miklós
Published: July 2002

We have investigated the magnetism and have measured the compressive modulus of magnetic fluid containing gels, called ferrogels, in the presence of magnetic field. No hysteresis was shown in the magnetization curve of the ferrogel suggesting a super paramagnetic response. The equilibrium compressive modulus of the ferrogel was measured up to 4 kOe and the modulus with field was much higher than that without field. The mean change in modulus increased with increasing magnetic field, and it saturates above 2 kOe. The mean and maximum change in modulus at 4 kOe was 31 and 71 Pa which corresponds to 19% and 46% increases of that without field, respectively. The change in modulus has been analyzed theoretically and is well explained in terms of the magnetism of ferrogels.
Optical and structural properties of laser annealed Er-doped amorphous silicon thin films
Authors: Bell M. J. V. , Nunes L. A. O. , Zanatta A. R.
Published: July 2002

Er-doped hydrogenated amorphous silicon (a-SiEr:H) thin films were deposited by cosputtering. After deposition, the samples were submitted to annealing treatments employing Ar+ and Nd-YAG lasers. Thermal anneals in a temperature-controlled furnace were also performed for comparison purposes. Photoluminescence, optical absorption in the infrared energy region, and Raman spectroscopies were carried out after each annealing treatment. Based on the experimental data, some mechanisms associated with the different annealing procedures and Er3+ ion excitation are proposed and discussed.
Electrical response of piezoelectric materials under mechanical excitation
Authors: Ghosh A. K. , Munshi T. K. , Chakravorty H. S.
Published: July 2002

The piezoelectric responses of different piezocrystals, rigidly backed, have been studied under constant and impulsive stress. Equations connecting electrical and mechanical interactions have been coupled to find the response. The rates of growth and decay of piezovoltage have been found to vary widely from material to material.
Magnetic ordering and granularity effects in La1−xBaxMnO3
Authors: Budhani R. C. , Roy Chaitali , Lewis Laura H. , Li Qiang , Moodenbaugh A. R.
Published: July 2002

We report detailed measurements of the magnetic behavior of La1−xBaxMnO3 bulk samples over a wide range of composition (0⩽x⩽1), temperature (4.2–775 K), and magnetic field strength (1 G–50 kG). The end members of this series namely LaMnO3+δ (with δ⩾0) and BaMnO3 crystallize into pseudocubic perovskite and hexagonal lattices, respectively. The perovskite structure is stable for x⩽0.35 while a phase separation into La0.65Ba0.35MnO3 and BaMnO3 occurs for x>0.35. The ordered moment on LaMnO3+δ depends sensitively on the processing conditions and the lowest value of 0.006 μB per Mn ion at 4.2 K is realized in the antiferromagnetic compound of δ=0. The complex magnetic behavior of BaMnO3 suggests antiferromagnetic ordering below ∼150 K. The Ba-substituted compounds order ferromagnetically with a monotonic increase of TC and saturation magnetization Ms with x<0.35. In the two-phase region, Ms scales with the fraction of the ferromagnetic phase present in the sample. Magnetic granularity, superparamagnetism, and an effective medium approach for susceptibility of the two-phase mixture are discussed.
Electrical resistivity of molten indium–antimony alloys
Authors: Wang Qiang , Chen Xiumei , Li ChenXi , Lu Kunquan
Published: July 2002

The resistivity of liquid indium–antimony (In1−xSbx) alloys has been measured as a function of temperature and concentration. On the In-rich side, the resistivity increases linearly with raising temperature and rapidly with addition of Sb concentration. For Sb-rich alloys, clear turning points are found in the temperature coefficient of resistivity; the resistivity behaves smoothly with concentration and shows a peak around x=0.8.