bluuebunny/crossref_metadata_2025 huggingface dataset:
Thickness dependence of interfacial magneto-optic effects in Pt/Co multilayers
Authors: Gao Xiang , DeVries Michael J. , Thompson Daniel W. , Woollam John A.
Published: July 2002

Spectroscopic ellipsometry and magneto-optic Kerr effects are measured on Pt/Co multilayers with a series of Co layer thicknesses from 0.08 to 1 nm. An electromagnetic theory of multilayered structures allows regression analysis fits between acquired data and parameter dependent model analysis. Recently, we determined the single layer Co magneto-optic Voigt parameter and found that it depends on Co layer thickness. In the present work, we report an in-depth study of interfacial magneto-optic effects for a large number of Pt/Co multilayer samples. Kerr rotation and ellipticity were measured over the spectral range from 200 to 1700 nm. Voigt parameters of the magnetic layers for these Pt/Co multilayer samples with different thicknesses were compared, and the Pt–Co interface thicknesses were determined in terms of the material dielectric tensor.
Collisional-radiative model for an argon glow discharge
Authors: Bogaerts Annemie , Gijbels Renaat , Vlcek Jaroslav
Published: July 2002

An extensive collisional-radiative model for the argon atoms in a glow discharge has been developed. Sixty-five effective argon atomic levels are considered. The processes taken into account are radiative decay, electron, fast argon ion and argon atom and thermal argon atom impact ionization, excitation and deexcitation between all the levels, electron-ion radiative recombination, and electron-ion three-body recombination where the third body is an electron, fast argon ion or atom, or a thermal argon atom. Some additional processes are incorporated for the two 4s metastable levels, i.e., Penning ionization of sputtered atoms, two- and three-body collisions with argon ground state atoms, collisions between two atoms in a metastable level, and diffusion and subsequent deexcitation at the walls. Typical results of the model are the populations of the various excited levels as a function of distance, and the relative contributions of different populating and depopulating processes for all levels.
Effect of plasma treatment on the density of defects at an amorphous Si:H-insulator interface
Authors: Umezu Ikurou , Kuwamura Takahiro , Kitamura Kazuaki , Tsuchida Takatsugu , Maeda Keiji
Published: July 2002

Interface state density between a-Si:H and an insulating film (a-SiN1.7:H or a-SiO2.0) was measured by photothermal deflection (PD) spectroscopy and electron spin resonance. While the interface state density in a-SiN1.7:H on a-Si:H structure was smaller than the free surface state density on a-Si:H, that in a-SiO2.0:H on a-Si:H structure was larger than the free surface state density of a-Si:H. The difference in the surface state density between these specimens was discussed in terms of plasma surface reaction. The effect of plasma reaction was examined by treating the surface of the a-Si:H layer by the plasma of NH3 or N2O gas which were dominant constituents of the source gases used to deposit the insulating layers. The PD spectral shape of a-SiO2.0 on a-Si:H was similar to N2O plasma-treated a-Si:H and that of the a-SiN1.7:H on a-Si:H structure was similar to NH3 plasma-treated a-Si:H. These results indicate that the interface defects in the a-SiO2.0 on a-Si:H structure were induced by the plasma reaction of the source gas with the surface of a-Si:H at the initial stage of deposition. The interface state densities in a-Si:H on a-SiN1.7:H and a-Si:H on a-SiO2.0 interfaces were smaller than those of a-SiN1.7:H on a-Si:H and a-SiO2.0 on a-Si:H, respectively. These differences in the interface defect density were due to the difference in the precursor used to deposit the upper layer. Chemical reaction on the surface of a-Si:H with a source gas induces interface defects at the initial stage of the deposition of the insulating layer. This surface chemical reaction was investigated by varying the condition of plasma species at the surface of a-Si:H.
Electrical and structural properties of poly-Si films grown by furnace and rapid thermal annealing of amorphous Si
Authors: Girginoudi S. , Girginoudi D. , Thanailakis A. , Georgoulas N. , Papaioannou V.
Published: July 2002

In the present work, the effect of rapid thermal annealing (RTA) on the electrical and structural properties of poly-Si thin films, grown by the crystallization of a-Si films deposited by rapid thermal low-pressure chemical vapor deposition, has been studied. Structural and electrical results were obtained using atomic force microscopy, transmission electron microscopy, electron spin resonance, electrical resistivity, and Hall mobility techniques. The effects of the grain size, grain boundaries, and surface roughness on the electrical characteristics of poly-Si films have been investigated. Amorphous Si (a-Si) films crystallized by RTA at 850 °C for 45 s result in the formation of poly-Si with small grains, an electron spin density Ns=5.2×1016 cm−3, and a Hall mobility μH=30 cm2 V−1 s−1. A two-stage annealing, involving low-temperature annealing at 600 °C for 6 h, followed by RTA at 850 °C in five steps of 30 s each, results in the formation of poly-Si films with large grains free of in-grain defects, low surface roughness, and higher Hall mobility μH=43 cm2 V−1 s−1, characteristics rendering such poly-Si films suitable for the fabrication of good performance thin film transistors.
Dynamical response in an array of quantum-dot cells
Authors: Wang C.-K. , Yakimenko I. I. , Zozoulenko I. V. , Berggren K.-F.
Published: July 2002

The time-dependent behavior of quantum cellular automata (QCA), chains of quantum-dot cells, is examined. Each cell consists of four quantum dots on the four corners of a square and contains two electrons. Cells interact with nearby cells via Coulomb potentials. We study the time evolution of the polarization of the cells due to the switching of a driver cell for the particular cases of linear chains and L-shaped configurations. Two different approaches are discussed. In the first one dissipation is not considered. For this case numerical results demonstrate that adiabatic switching can be used to implement QCA only for chains of two cells. In the second approach we explicitly include energy dissipation processes in a phenomenological way and obtain the time evolution of the polarization of the cells due to the switching of the driver cell. Dissipation processes are found to give rise to satisfactory response characteristics. For the cases studied no metastable states are found.
Optical probe of microwave current distributions in high temperature superconducting transmission lines
Authors: Culbertson James C. , Newman Harvey S. , Wilker Charles
Published: July 2002

This work develops two techniques for optically probing the spatial profile of microwave frequency superconducting currents. As an application, we measured the effects of high microwave powers on the spatial distribution of current on coplanar superconducting transmission lines fabricated using YBa2Cu3O7−δ and Tl2Ba2CaCu2O8. For both techniques, a focused light-spot served as the spatial probe whose effect was measured through the change in transmitted microwave power. For resonant geometries, the change was due to the kinetic-inductance bolometric effect; for nonresonant geometries, the change was due to the resistive-transition bolometric effect. Kinetic-inductance photoresponse measurements were acquired by setting the microwave frequency on the shoulder of a resonance and measuring the change in the microwave power transmitted through the device that occurred when the absorption of light shifted the frequency of the resonance. These kinetic-inductance measurements were performed as a function of microwave power on a YBa2Cu3O7−δ coplanar device at 74 K and on a Tl2Ba2CaCu2O8 coplanar device at 80 K. Because the photoresponse in this technique is proportional to the square of the local current density underneath the light spot (as opposed to directly proportional), this technique is sensitive to current redistribution on length scales much smaller than the ∼6 μm spatial resolution of our measurements. Extrinsic (defect and grain boundary associated) and intrinsic photoresponses were measured. Both coplanar samples showed no change in the intrinsic spatial distribution of the current as the microwave power was varied. At all temperatures the Tl2Ba2CaCu2O8 sample exhibited substantial extrinsic spatial variations on a distance scale equal to the film’s ∼5 μm grain size; these spatial variations became more dramatic as the temperature T approached the critical temperature Tc. The spatial variations for the finer-grained YBa2Cu3O7 sample were much less pronounced. Behavior consistent with Tc being several degrees lower at the film edges was observed as T approached Tc; the edge-current photoresponse started to narrow, increase faster than linearly with light power, and move away from the film edges toward the center of the strip; lowering the light power lessened these effects. Resistive-bolometric photoresponse measurements performed at temperatures within the resistive transition exhibited the same light power effects near Tc. The kinetic-inductance photoresponse can also be used as a probe of the local quality of unpatterned superconducting films; for this usage an unpatterned film serves as one wall of a resonant cavity and a focused light beam is scanned through the transparent substrate onto the underside of the superconducting film while monitoring the shift in the cavity resonance.
Modification on the electron field emission properties of diamond films: The effect of bias voltage applied in situ
Authors: Chen Yung-Hsin , Hu Chen-Ti , Lin I-Nan
Published: July 2002

In this work, we have systematically examined the effect of bias voltage applied in situ on the characteristics of diamond films. Raman spectroscopic and scanning electron microscopic examinations indicate that the applied positive bias voltage changes the thin films’ morphology and the Raman spectroscopy insignificantly, but markedly lowers their effective work function (Φe) and turn-on field (E0). The enhancement of the field emission properties of these films is assumed to result from the introduction of impurity and surface states. By contrast, the negative bias voltage applied during a chemical vapor deposition process leads to pronounced modification of the morphology of diamond films due to an etching effect. Such a process results in a fine granular structure for the diamond films, significantly improving their field emission behavior via the enhancement of field concentration effect.
Interactions of SiH radicals with silicon surfaces: An atomic-scale simulation study
Authors: Ramalingam Shyam , Maroudas Dimitrios , Aydil Eray S.
Published: July 2002

A comprehensive study is presented of the interactions of SiH radicals originating in silane containing plasmas with crystalline and amorphous silicon surfaces based on a detailed atomic-scale analysis. The hydrogen concentration on the surface is established to be the main factor that controls both the surface reaction mechanism and the reaction probability; other important factors include the location of impingement of the radical on the surface, as well as the molecular orientation of the radical with respect to the surface. On the ordered crystalline surfaces, the radical reacts in such a way as to maximize the number of Si–Si bonds it can form even if such bond formation requires dissociation of the radical and introduction of defects in the crystal structure. The radical is established to be fully reactive with the pristine Si(001)-(2×1) surface. This chemical reactivity is reduced significantly for the corresponding H-terminated surface with a hydrogen coverage of one monolayer. SiH is found to be highly reactive with surfaces of hydrogenated amorphous silicon films, independent of radical orientation and the location of impingement. Our simulations predict an average reaction probability of 95% for SiH with a-Si:H film surfaces, which is in excellent agreement with experimental data.
Model for the current–voltage characteristics of ultrathin gate oxides after soft breakdown
Authors: Houssa M. , Nigam T. , Mertens P. W. , Heyns M. M.
Published: July 2002

The current–voltage characteristics of metal-oxide-semiconductor capacitors with a 4.2 nm SiO2 gate oxide are investigated. After the occurrence of soft breakdown, which is observed during constant current stress of the devices, the gate current is shown to behave like a power law of the applied gate voltage. We propose that this power law behavior is due to the formation of a percolation path between the electrons traps generated in the SiO2 layer during current stress of the capacitor. We describe a simple model which accounts for the current–voltage characteristics between two neighbor trapping sites, as well as a distribution of percolation thresholds in these (finite size) ultrathin SiO2 layers. The prediction of the model is in fair agreement with the experimental results in a large voltage range, and leads to a better description of the data than previously reported models. Furthermore, it is shown that this percolation model can also explain the temperature dependence of the gate current after the occurrence of soft breakdown.
Density functional theory for calculation of elastic properties of orthorhombic crystals: Application to TiSi2
Authors: Ravindran P. , Fast Lars , Korzhavyi P. A. , Johansson B. , Wills J. , Eriksson O.
Published: July 2002

A theoretical formalism to calculate the single crystal elastic constants for orthorhombic crystals from first principle calculations is described. This is applied for TiSi2 and we calculate the elastic constants using a full potential linear muffin-tin orbital method using the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. From this the calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline TiSi2 are also calculated and compared with corresponding experimental values. The directional bulk modulus and the Young’s modulus for single crystal TiSi2 are estimated from the elastic constants obtained from LDA as well as GGA calculations and are compared with the experimental results. The shear anisotropic factors and anisotropy in the linear bulk modulus are obtained from the single crystal elastic constants. From the site and angular momentum decomposed density of states combined with a charge density analysis and the elastic anisotropies, the chemical bonding nature between the constituents in TiSi2 is analyzed. The Debye temperature is calculated from the average elastic wave velocity obtained from shear and bulk modulus as well as the integration of elastic wave velocities in different directions of the single crystal. The calculated elastic properties are found to be in good agreement with experimental values when the generalized gradient approximation is used for the exchange and correlation potential.
Thermal nonequilibrium of the shock-compressed state of polymers realized by 1 GPa shock waves
Authors: Nagayama Kunihito , Mori Yasuhito
Published: July 2002

A thermodynamic theory has been developed to calculate the shock temperature of polymers in the 1 GPa pressure region. For this purpose, the Grüneisen parameter has been calculated based on the available nonlinear shock velocity–particle velocity Hugoniot function for polymers. The calculated large values of the parameter correspond to that of phonon frequencies in the collective motion of polymer molecules, and not to that of the intramolecular vibration frequencies. Then, the equilibrium shock temperature as well as the cold potential energy function is calculated using the calculated Grüneisen parameter and the available nonlinear shock velocity–particle velocity Hugoniot function. Shock temperature and cold potential function have also been calculated using the equilibrium thermodynamic values of the Grüneisen parameter and the specific heat. From these calculations, the following unphysical result has been derived; that is, the cold potential function for these polymers has no minimum value at zero-temperature volume, but monotonically decreases with compression. It is demonstrated that this kind of anomaly can be resolved only when large values of the Grüneisen parameter and the corresponding small values of the specific heat are assumed. These results strongly suggest that the state of the shocked polymers observed in the experiment of μs duration may be the thermodynamic state, which is very different from the thermal equilibrium state in that phonon temperature is sufficiently high but not in equilibrium with molecular vibrational temperature.
Atomic structure and magnetism of ordered and disordered Al0.5Fe0.5−xMnx alloys
Authors: Harris V. G. , Fatemi D. J. , Hathaway K. B. , Huang Q. , Mohan Amitabh , Long Gary J.
Published: July 2002

The equiatomic FeAl alloy has been modified by partial substitution of Mn for Fe, and its magnetic and structural properties investigated by neutron diffraction (ND), x-ray absorption fine structure (XAFS) spectroscopy, Mössbauer spectroscopy (MS), and SQUID magnetometry both for the ordered (B2) and disordered states. The unit cell volume is measured to increase linearly with Mn concentration. XAFS measurements indicate local structural displacements occur at the Mn sites in both ordered and disordered states that may act to frustrate long-range magnetic order (LRMO). Although MS and ND show no evidence of LRMO, SQUID magnetometry indicates an induced movement in the ordered state that increases with disorder but does not saturate at fields up to 5 T.
Sizing particles on substrates. A general method for oblique incidence
Authors: de la Peña J. L. , González F. , Saiz J. M. , Moreno F. , Valle P. J.
Published: July 2002

We have developed an experimental light-scattering method to size metallic spherical and cylindrical particles on flat substrates using obliquely incident light. We modify an earlier model to include effects such as the shadowing of the incident and scattered beams. We provide empirical expressions for sizing spheres and cylinders on substrates based on the positions of their minima. The results have been experimentally verified for both types of particle. The extension of the method to oblique incidence angles allows an alternative dynamic procedure.
Occupancy level of the DX center in Te-doped AlxGa1−xSb
Authors: Baraldi A. , Ghezzi C. , Parisini A. , Magnanini R. , Tarricone L. , Franchi S.
Published: July 2002

Hall effect measurements were performed in Te-doped AlxGa1−xSb layers grown by molecular beam epitaxy to investigate the composition dependence of the DX center occupancy level. The investigated samples have AlSb molar fractions in the 0.25⩽x⩽0.50 range and n-type doping of about 1018 cm−3. A family of x≈0.40 samples of different doping (5×1015–1018 cm−3) were also studied. The Hall electron density data versus temperature were analyzed at high temperatures (T⩾150 K) where the DX center is at equilibrium, by assuming the negative-U model for the DX level and by taking into account the multivalley conduction effects. The DX level, degenerate in energy with the conduction band at low x values, enters the forbidden gap at x≈0.25 and then it becomes deeper with increasing x. In lightly doped samples, the introduction of a second level of the same Te impurity is required to fit the data; such level can be identified with the nonmetastable level which controls the low temperature electrical properties of the material. A critical discussion on the choice of the conduction band parameters for the fitting is reported.
Effect of transverse current injection during air annealing on the formation of oxides in thin Ti films
Authors: Parkansky N. , Alterkop B. , Goldsmith S. , Boxman R. L. , Rosenberg Yu.
Published: July 2002

Thin titanium films, deposited by a filtered vacuum arc on glass substrates, were oxidized by annealing in air at 400 °C for up to 4 h. The film thickness was 180 nm, and lateral dimensions were 3×8 mm. During the annealing a dc voltage was applied between the ends of the sample. The oxidation of the samples was studied by x-ray diffraction and x-ray photoelectron spectroscopy. The generated oxides were nonstoichiometric forms of TiO and TiO2 depending on the applied voltage. The application of a voltage, or the injection of current, also affected the distribution in the sample of the oxygen to titanium density ratio. In the case of samples run with Us=10 or 100 V, the oxygen to titanium density ratio had a minimum at a 120 nm depth, but increased inside the sample up to the substrate. The density of oxygen at a 120 nm depth was 43%, 34%, and 20% of the surface density for Us=0, 10, and 100 V, respectively.
On the axial structure of a nitrogen surface wave sustained discharge: Theory and experiment
Authors: Tatarova E. , Dias F. M. , Ferreira C. M. , Ricard A.
Published: July 2002

A model for a surface wave sustained nitrogen discharge accounting in a self-consistent way for electron and heavy particles kinetics and discharge electrodynamics has been developed. The system under analysis is a plasma column produced by a traveling, azimuthally symmetric (m=0 mode) surface wave. The model is based on a set of coupled equations consisting of the electron Boltzmann equation and the rate balance equations for the most important excited species—vibrationally, N2(X 1Σg+, ν), and electronically excited states, N2(A 3Σu+, a′ Σu−, B 3Πg, C 3Πu, a 1Πg)—and charged particles (e, N2+, N4−) in the discharge. Electron collisions with nitrogen molecules of the first and the second kind and electron–electron collisions are accounted for in the Boltzmann equation. The field strength necessary for steady-state operation of the discharge is obtained from the balance between the total rates of ionization (including direct, stepwise, and associative ionization) and of electronic losses (due to diffusion to the wall and bulk recombination). The transfer of wave power to the discharge occurs through collisional processes, thus the set of equations is closed by an ordinary differential equation (stemming from basic electrodynamical relations) which associates the axial gradient of the electron density to the wave attenuation. As a result, a self-consistent interdependence between wave propagation and discharge characteristics is obtained over the whole plasma column. The axial profile of the gas temperature and the initial value of the electron density at the position of the wave launcher are used as input parameters. The model determines the axial structure of the discharge—axial variations of the electron energy distribution function and its moments, the vibrational distribution function of the electronic ground state, and the densities of the most important electronically excited states and positive ions—consistently with the electric field and the surface wave dispersion characteristics. A spatially resolved experimental investigation of the electron energy distribution function, the gas and the vibrational temperatures, and the population densities of some electronically excited states along with wave propagation characteristics measurements provides a verification of the model. Strong correlation between different plasma balances, governing the discharge production, and discharge electrodynamics—the basis of surface-wave discharge physics—has been demonstrated both theoretically and experimentally.
Chain/column evolution and corresponding electrorheological effect
Authors: Wen Weijia , Zheng D. W. , Tu K. N.
Published: July 2002

We present an investigation about chain/column evolution and the corresponding electrorheological (ER) effect performed with glass/oil ER fluid. Our results demonstrate that once the field applied to the ER fluids surpasses a certain time period, the particles begin aggregating to form chains. These chains then coarsen and eventually form columns in the direction of the external field. We found that different column structures can be obtained depending on how the electric field is applied to the ER fluid. Only a loose column structure can be achieved if a square pulse field is applied to the ER fluid, yet a compact column is formed when the field strength is increased slowly. We have measured the ER effect with a sensitive yield stress testing device as the structure varies. The results indicate that there exist three increasing tendencies of interaction among particles corresponding to three processes of sequential transition between states; they are (1) random spatial configuration to chain, (2) chain to metastable column, and (3) metastable column to stable column.
Improved stability of deuterated amorphous silicon thin film transistors
Authors: Wei Jeng-Hua , Lee Si-Chen
Published: July 2002

To improve the bias-induced degradation in hydrogenated amorphous silicon thin film transistor, the hydrogen in the amorphous silicon film should be replaced by deuterium. The stability of deuterated amorphous silicon thin film transistors, i.e., the shifts of threshold voltage and subthreshold swing, is indeed improved compared to that of the hydrogenated ones. This result is consistent with the improvement of the light-induced degradation in deuterated amorphous silicon films and this improvement can be explained by the efficient coupling between the Si–D wagging mode and the amorphous silicon phonon mode.
Thermal stresses in hermetically double-coated optical fibers
Authors: Shiue Sham-Tsong
Published: July 2002

The thermal stresses in hermetically double-coated optical fibers are analyzed. Several thermal stresses and forces are important in a hermetically double-coated optical fiber. First, the shear stress at the interface of the glass fiber and primary coating will make hermetical coatings be delaminated from the glass fiber. Second, the compressive radial stress at the interface of the glass fiber and primary coating would produce microbending loss. Third, the compressive axial force in the glass fiber would induce the buckling of the fiber and also results in an increase of bending loss. Finally, the normal stresses in the hermetical coatings will make the hermetical coatings be broken. To minimize these thermal stresses and forces, the thermal expansion coefficient of the primary coating and the Poisson’s ratios of the primary coating and secondary coating should be decreased. The thermal stresses in the hermetically double-coated optical fibers are compared to those in the hermetically single-coated optical fibers. It was found that the hermetical coatings of double-coated optical fibers are more easy to be delaminated from the glass fiber than that of single-coated optical fibers, and double-coated optical fibers will induce larger microbending loss than single-coated optical fibers. Based on the strength consideration, a best selection of hermetical coatings to minimize the thermal stresses is also discussed.
Nanolithography with an atomic force microscope by means of vector-scan controlled dynamic plowing
Authors: Klehn B. , Kunze U.
Published: July 2002

We present a nanolithography technique based on an atomic force microscope. A thin resist layer on the sample surface is plastically indented by a vibrating tip. Controlling of the vibration amplitude and tip movement enables one to plow a narrow furrow along line segments of arbitrary length and direction. Different line segments which form a complex pattern can be plowed at a scan speed up to 5 μm/s. The geometric distortion of the resist pattern is less than 50 nm, where at scan speed in excess of 1 μm/s an interrupt of at least 10 ms is necessary between the line segments. The minimum offset error in positioning a pattern with respect to existing features is less than 4% of the scanning field. The patterns are transferred into SiO2, Si, GaAs, Ti, and Au by wet-chemical etching. Minimum linewidth is 25 nm in 1.5 nm oxide layers, 75 nm in 10 nm Ti film and 40 nm in 10 nm Au. On semiconductor surfaces smooth and perfectly shaped V grooves of 55 nm width are obtained.
>Study of NiFe/Al/Al2O3 magnetic tunnel junction interfaces using second-harmonic magneto-optic Kerr effect
Authors: Russek S. E. , Crawford T. M. , Silva T. J.
Published: July 2002

Using the second-harmonic magneto-optic Kerr effect (SHMOKE), we have measured the interfacial magnetic properties of NiFe/Al/Al2O3 heterostructures as a function of Al2O3 thickness and processing conditions. The samples were prepared like magnetic tunnel junctions except that the top electrodes were not deposited. A large change in SHMOKE contrast was observed for different oxidation processes and Al2O3 thicknesses. The magnetic SHMOKE contrast from a plasma oxidized sample in which the oxidation front is thought to be inside the original NiFe film is 70%, whereas the SHMOKE contrast from a thermally oxidized NiFe/Al sample, in which the oxidation front is ∼1 nm from the Al/NiFe interface is only 15%. Further, the phase of the signal is reversed between the two structures. The SHMOKE data have been correlated with tunneling measurements on similarly fabricated structures. For the two samples listed above, the junction resistivity varied from 10−1 Ω cm2 (plasma oxidized) to 10−6 Ω cm2 (thermally oxidized). These results indicate that SHMOKE may be useful for assessing tunnel junction quality during processing.
Artificial antiferromagnetic tunnel junction sensors based on Co/Ru/Co sandwiches
Authors: Tiusan C. , Hehn M. , Ounadjela K. , Henry Y. , Hommet J. , Meny C. , van den Berg H. , Baer L. , Kinder R.
Published: July 2002

A novel method is used for pinning the magnetization of the magnetically hard subsystem in micron-size magnetic tunnel junctions: the so-called artificial antiferromagnetic structure. The latter uses the strong antiparallel exchange coupling between two Co layers through a Ru spacer layer to ensure a high rigidity of the hard subsystem magnetization. The tunnel barriers were formed by sputter etching previously deposited Al layers in a rf Ar/O2 plasma. Wafers, 3 in. in diameter, were patterned into arrays of square junctions with lateral sizes of 20 and 50 μm. All junctions of a given size show resistances reproducible within several percents. The tunnel magnetoresistance (TMR) is found to be independent of the junction size and TMR ratios of 14%–16% are achieved at room temperature.
One monolayer-periodic oscillations in the magnetization induced second harmonic generation signals during the growth of Co films on Cu(001)
Authors: Jin Q. Y. , Vollmer R. , Regensburger H. , Kirschner J.
Published: July 2002

Magnetization induced optical second harmonic generation is used to investigate the surface magnetic properties during the layer-by-layer growth of Co films on Cu(001). The second harmonic (SH) signal changes as a function of Co thickness and an oscillatory behavior of the SH asymmetry with one monolayer period is observed for the transverse Kerr-geometry and s-polarized incident light, while the average SH signal has no such oscillations. The magnetization induced SH signal at half filled layers is enhanced with respect to that of filled layers, which agrees with the enhancement of magnetic moment of edge atoms and the higher step density at half filled layers. Its dependences on the growth rate and temperature are discussed.
Magnetization of Fe3O4/MgO multilayers studied with Mössbauer spectroscopy
Authors: Strijkers G. J. , Kohlhepp J. T. , van der Heijden P. A. A. , Swagten H. J. M. , de Jonge W. J. M. , Gaines J. M.
Published: July 2002

The magnetic behavior and structure of Fe3O4/MgO multilayers have been investigated with Mössbauer spectroscopy in field, magnetization measurements, and scanning tunneling microscopy. It is shown that detailed structural knowledge of the size of the structural domains and the width of the antiphase boundaries are indispensable in understanding the magnetization behavior of the films and the contradictory results reported in the literature.
Magnetic and structural properties of face-centered-cubic FexCo1−x alloys on diamond
Authors: Ambrose T. , Krebs J. J. , Bussmann K. , Prinz G. A.
Published: July 2002

Molecular beam epitaxy has been used to stabilize FexCo1−x alloy films over the entire composition range on diamond (100) substrates. Below a critical thickness, the films are single crystal face- centered-cubic (fcc) as determined from reflection high energy electron diffraction. Superconducting quantum interference device magnetometry studies reveal a nonmonotonic increase in the magnetization of the fcc alloys with Fe concentration, while ferromagnetic resonance indicates a small magnetic anisotropy for x>20%.
Faraday rotation and magnetic properties of neodynium trifluoride under high magnetic field
Authors: Guillot M. , Schmiedel T. , Xu You
Published: July 2002

Faraday rotation (Fr) and magnetization (M) measurements were performed in the trifluoride of neodymium NdF3 in high continuous magnetic field (17 T below 77 K and 30 T above this temperature). The temperature range extends from 15 to 300 K (Fr) and from 1.6 to 300 K (M), respectively; the external field was applied along the (c) direction of the hexagonal structure at 633 nm wavelength. M was found to be linear versus H above 40 K; the magnetic susceptibility (χ) follows a Curie–Weiss law. At low temperature, only the lowest doublet is populated and the saturation of M is observed. Below 30 K, Fr presents a nonlinear field dependence in H higher than 7 T but remains linear in the 30–300 K range. The Verdet constant (V) was then deduced. All the data confirm a complex and unusual temperature variation of the ratio V/χ which cannot be attributed to a strong superexchange coupling between the spins of the Nd3+ ions as previously proposed; they underline a paramagnetic behavior which is influenced by crystal field effects.
On the extraction of linear and nonlinear physical parameters in nonideal diodes
Authors: Mikhelashvili V. , Eisenstein G. , Garber V. , Fainleib S. , Bahir G. , Ritter D. , Orenstein M. , Peer A.
Published: July 2002

We describe a parameter extraction technique for the simultaneous determination of physical parameters in nonideal Schottky barrier, p-n and p-i-n diodes. These include the ideality factor, saturation current, barrier height, and linear or nonlinear series, and parallel leakage resistances. The suggested technique which deals with the extraction of bias independent parameters makes use of the forward biased current–voltage (I–V) characteristics and the voltage-dependent differential slope curve α(V)=[d(ln I)]/[d(ln V)]. The method allows (a) establishment of the current flow mechanisms at low and high bias levels, (b) extensive of the permissible ranges of determined parameters beyond what is possible in other published methods, and (c) to automation and computerization of the measurement processes. The method is verified experimentally using metal–semiconductor structures based on Si, InGaP, and HgCdTe as well as an InGaAs/InGaAsP multiple quantum well laser diode exemplifying a p-n junction.
Engineering issues study of ring-shaped head recording on single layered perpendicular media
Authors: Liu Bo , Zhuang Pingwei , Yuan Zhimin , Futamoto Masaaki , Hirayama Yoshiyuki
Published: July 2002

This article presents results of experimental study on the engineering issues of using ring-shaped thin-film head and single layered perpendicularly orientated media for high density magnetic recording. Comparing with the experiment conducted with longitudinal media, this combination is of smaller sensitivity of its nonlinear transition shift to the variation of the head–disk spacing, smaller writing current for saturation recording even if the media coercivity is 25% higher than that of the longitudinal counterpart. Furthermore, the width of its erasure band is quite comparable to the case using longitudinal media, if the writing current is selected properly. On the other hand, the combination is of slightly larger asymmetry of its track profile and slightly more sensitive to the skew angle increment, if the anisotropic magnetoresistive head is used for reading.
Change of universality class of metal–insulator transition due to magnetic ordering
Authors: de Oliveira N. A. , Tovar Costa M. V. , Troper A. , Japiassú Gloria M. , Continentino M. A.
Published: July 2002

Using a two-band model we report a theory to describe the metal–insulator (MI) transition as a function of an external applied magnetic field in Kondo insulators. To deal with electronic correlations we use a functional integral approach in the static approximation. We show the existence of a critical value of the Coulomb correlation Uc, such that for UUc, the transition is to a ferromagnetic metal and it is described by different critical exponents.
Dispersion of Nd spin correlations in heavy-fermion like Nd1.85Ce0.15CuO4
Authors: Pyka N. M. , Loewenhaupt M. , Metz A.
Published: July 2002

The T′-phase compound Nd2−xCexCuO4 exhibits at T<2 K resemblance with heavy-fermion behavior above a critical Ce content which has been established by specific heat, magnetic susceptibility, magnetoresistance, and μSR experiments and described qualitatively by the Zeeman route to heavy-fermion behavior. We present inelastic neutron scattering measurements at the onset of this doping regime at x=0.15 performed on a single crystal at T=0.05 K. The magnetic response from correlated Nd spins is reminiscent of the spin waves in Nd2CuO4. The dispersion of the Nd–Nd correlations has been determined in several Brillouin zones along the line ΓMAZΓ. Its medium energy is shifted to lower energies by more than 40% compared to the undoped compound. An energy gap of the Nd–Nd correlations is still observed but also an incipient soft mode has been found between the points A and M, which might indicate a magnetic instability. In addition, single-site scattering from slowly relaxing Nd spins contributes to the total magnetic scattering, indicating a hybridization of the Nd-4f electrons with the strongly correlated Cu-3d conduction electrons.
Spin-dependent tunneling and properties of ferromagnetic (Ga,Mn)As (invited)
Authors: Ohno H. , Matsukura F. , Omiya T. , Akiba N.
Published: July 2002

Low-temperature molecular beam epitaxy allows one to dope GaAs with Mn over its solubility limit, making it possible to realize a III–V-based diluted magnetic semiconductor (Ga,Mn)As. Magnetization measurements revealed that (Ga,Mn)As is ferromagnetic at low temperatures. The ferromagnetic transition temperature Tc can be as high as 110 K for 5% Mn. Magnetotransport properties of (Ga,Mn)As were found closely related to its magnetic properties. The temperature and magnetic-field B dependence of resistivity ρ can be understood in terms of spin-dependent scattering. The p–d exchange determined from the B dependence of ρ is shown to be consistent with the exchange expected from Tc based on the Ruderman–Kittel–Kasuya–Yosida interaction. The anomalous Hall effect dominates the Hall resistance up to room temperature, allowing one to determine the magnetic properties from the magnetotransport measurements alone. Since (Ga,Mn)As can be grown pseudomorphically on GaAs, one can introduce ferromagnetism in GaAs-based heterostructures such as resonant tunneling diodes and observe spin-dependent phenomena in current–voltage characteristics. Magneto-optical properties of (Ga,Mn)As films are also presented.
Phase formation in Zr–Fe multilayers: Effect of irradiation
Authors: Motta A. T. , Paesano A. , Birtcher R. C. , Brückmann M. E. , Teixeira S. R. , Amaral L.
Published: July 2002

We have conducted a detailed in situ study of phase formation in Zr–Fe metallic multilayers using irradiation and thermal annealing. Metallic multilayers with near equiatomic and Fe-rich overall compositions and with repetition thicknesses ranging from 7.4 to 33 nm were either irradiated with 300 keV Kr ions at various temperatures (from 17 to 623 K) or thermally annealed at 773 K while being observed in situ. The kinetics of multilayer reaction were monitored by following the diffraction patterns. For near equiatomic samples, irradiation causes complete amorphization. The dose to amorphization increases in proportion to the square of the wavelength, indicating a process controlled by atomic transport. Amorphization was also achieved by 900 keV electron irradiation at 25 K showing that displacement cascades are not required. The critical dose to amorphization was independent of temperature below room temperature and decreased above room temperature. The activation energy for this second process is 0.17 eV. For the temperature range studied, diffraction from Zr disappears first, indicating that amorphization takes place in the Zr layer by atomic transport of Fe from the Fe layers. These results are consistent with a combination of simple ballistic mixing at low temperature and either simple diffusion or radiation-enhanced diffusion at higher temperatures. Thermal annealing of the equiatomic samples at 773 K produced the same reaction products with slower kinetics. Ion irradiation of Fe-rich samples did not cause complete amorphization and intermetallic compounds Zr3Fe and ZrFe2 were observed in longer wavelength samples. Amorphization of Fe-rich samples was more sluggish, likely because there was competition with formation of other phases. The reaction kinetics were not proportional to square of wavelength for Fe-rich samples, indicating a process that depends on more than atomic transport. Thermal annealing at 773 K of a long wavelength, 57% Fe sample resulted in intermetallic compounds Zr3Fe and ZrFe2 which amorphized during subsequent irradiation. The ease of amorphization of equiatomic samples relative to Fe-rich samples can be explained by a narrower, single minimum free energy curve for the amorphous phase.
Response of boron carbide subjected to large strains, high strain rates, and high pressures
Authors: Johnson Gordon R. , Holmquist Tim J.
Published: July 2002

This article presents an analysis of the response of boron carbide (B4C) to severe loading conditions that produce large strains, high strain rates, and high pressures. Experimental data from the literature are used to determine and/or estimate constants for the JH-2 constitutive model for brittle materials. Because B4C is a very strong material, it is not always possible to determine the constants explicitly. Instead they must sometimes be inferred from the limited experimental data that are available. The process of determining constants provides insight into the constitutive behavior for some loading conditions, but it also raises questions regarding the response under other loading conditions. Several Lagrangian finite element and Eulerian finite difference computations are provided to illustrate responses for a variety of impact and penetration problems.
Full band Monte Carlo simulation of electron transport in 6H-SiC
Authors: Nilsson Hans-Erik , Hjelm Mats , Fröjdh Christer , Persson Clas , Sannemo Ulf , Petersson C. Sture
Published: July 2002

A study of electron transport in 6H-SiC is presented using a full band Monte Carlo simulation model. The Monte Carlo model uses four conduction bands obtained from a full potential band structure calculation based on the local density approximation to the density functional theory. Electron–phonon coupling constants are deduced by fitting the Monte Carlo simulation results to available experimental data for the mobility as a function of temperature. The saturation velocity perpendicular to the c axis is found to be near 2.0×107 cm/s, which is in good agreement with the experimental data available. In the c-axis direction the saturation velocity is much lower (4.5×106 cm/s). There are no direct experimental results available for the saturation velocity in the c-axis direction. A comparison between two-dimensional simulations of a 6H-SiC permeable base transistor, using transport parameters obtained from the Monte Carlo simulations, and experimental I–V characteristics confirms the low value. The physical mechanism behind this result can be explained in terms of the small group velocity in the c-axis direction for reasonable energy levels in combination with band structure effects that limits the energy range that an electron can reach by drift. This effect reduces the mean energy of the carriers for an electric field applied along the c axis and at 1.0 MV/cm the difference in mean energy compared with perpendicular directions is almost one order of magnitude. The mean energy increases with increasing temperature for electric fields in the c-axis direction, while the situation is reversed in perpendicular directions. In general the impact ionization coefficient has the same temperature dependence as the mean energy and this indicates that the impact ionization coefficient for electrons has a positive temperature derivative along the c axis. This may be a serious drawback in the design of high power vertical metal–semiconductor field effect transistors.
Electroplastic effect under the simultaneous superposition of electric and magnetic fields
Authors: Pinchook A. I. , Savenko V. S.
Published: July 2002

In recent years a number of models were proposed in which the electroplastic effect in the process of thermal fluctuation motion and generation of dislocations was considered to be a result of lowering potential barriers by the electromagnetic field. The experiments carried out so far reveal the surface character of the electroplastic deformation. To test the effect, the free surface has been perturbed by an electromagnetic field. The deformation of a bismuth crystal in response to application of mechanical stress and concurrent constant magnetic field and an impulse electric current has been studied. To illustrate the deformation process in single crystals of bismuth under the simultaneous effect of the electric and magnetic fields, a special device was designed as a supplement to the standard microhardness measuring equipment. There are two factors that increase plasticity in the case of the concurrent application of magnetic and electric fields. The first one is the damped elastic sonic vibrations caused by the Ampere force. In the case of deformation by twinning, these vibrations result in an increase of wedge-shaped twin around the imprint of the diamond indentor. The second factor is the electric charge on the free surface. The electric charge on the cleavage face increases the run of the twinning dislocations.
Aggregation and out diffusion of iron atoms for Fe ion implanted silica films
Authors: Ding Xing-zhao , Chiah M. F. , Cheung W. Y. , Wong S. P. , Xu J. B. , Wilson I. H. , Wang Hui-min , Chen Li-zhi , Liu Xiang-huai
Published: July 2002

Iron ions were implanted into a thermally grown silica film with a dose of about 6.5×1016 Fe/cm2 by using a metal vapor vacuum arc ion source at an accelerating voltage of 62 kV. The structure evolution of the film during the postimplantation annealing processes was studied by x-ray diffraction, Rutherford backscattering spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy experiments. It was found that the implanted iron atoms diffused out of the film and aggregated with the formation of particle islands on the film surface gradually after annealing. The out diffusion and island formation phenomena are particularly evident when the annealing temperature was elevated up to 800 °C. It was also demonstrated that in the early stage of annealing the growth of iron islands closely depended on the out diffusion of the implanted iron atoms, and both the size and areal density of the islands increased progressively with increasing annealing time. After a certain annealing period, the size of the islands continued to increase, but the areal density of the islands decreased significantly, which implied that most of the implanted iron atoms had diffused out of the film, and the islands could grow further by consuming the initially formed smaller ones through a surface diffusion process.