bluuebunny/crossref_metadata_2025 huggingface dataset:
Conduction and valence band photoemission mechanisms in two-dimensional–three-dimensional structures
Authors: Lin Benjamin S. M. , Hwang J.
Published: July 2002

The active layers in heterojunction internal photoemission infrared detectors are always of thickness in the order of few hundred Å, and thus the behavior of the excited carriers should be described in a two-dimensional (2D) mode. Conduction and valence band photoemission mechanisms have been introduced in 2D–three-dimensional structures. The carrier behaviors of two carrier types, electron and hole, were discussed in the cases of PtSi/p-Si and p+-SiGe/p-Si, respectively. Parallel momentum conservation were well preserved for these two cases during carrier transportation across the heterojunction ballistically. Theoretical simulations were in good agreement with experimental data for both the electron and hole cases. The reason why p+-SiGe/p-Si photodiodes exhibit higher quantum efficiency than PtSi/p-Si diodes has also been discussed.
Epifilm thickness measurements using Fourier transform infrared spectroscopy: Effect of refractive index dispersion and refractive index measurement
Authors: Zhou Zhen-Hong , Choi Byungin , Flik M. I. , Fan S. , Reif Rafael
Published: July 2002

Procedures and results of refractive index measurements using a Fourier transform infrared spectrometer are reported. These measurements were performed on both lightly and heavily doped silicon samples over the midinfrared (2.5–25 μm) spectrum region. A strong dependence of refractive index as a function of substrate dopant concentration was observed. Moreover, it was observed that the refractive index of heavily doped silicon also varies significantly with wavelength. Furthermore, it was also observed that the refractive index of silicon decreases with increasing wafer temperature for long wavelengths. Finally, the effect of refractive index dispersion on epifilm thickness measurement was examined. The results suggest that the spectral dispersion of the refractive index cannot be neglected for epifilm thickness measurements.
Electrical transport properties and confinement potential analysis of buried AlGaAs/GaAs quantum wires
Authors: Nakata S. , Tomizawa M. , Yamamoto M. , Ikuta K. , Mizutani T.
Published: July 2002

Buried wires with lateral potential barriers provided by AlGaAs/GaAs heterointerfaces have been fabricated by wet etching and metalorganic chemical vapor deposition regrowth. Burying the wire was proven to enhance the electrical transport properties; for example, it decreased the critical width and increased the subband energy separation. Two-dimensional simulation using the Poisson equation was performed to obtain the potential profile of both as-etched and buried wires. The quantum energy levels corresponding to the lateral confinement were calculated for the obtained potential profiles. The calculated energy separations agreed well with the experimental ones and the subband energy separation of the buried wire was larger than that of the as-etched wire for the same effective width. These results show that burying wire is effective for creating strong lateral confinement.
Chemically vapor deposited diamond films grown on titanium nitride coated and uncoated iron substrates
Authors: Weiser Paul S. , Prawer Steven , Hoffman Alon , Paterson Peter J. K. , Manory Rafael R.
Published: July 2002

The nature of the interfaces of chemically vapor deposited diamond films on Fe substrates with and without a protective TiN coating is investigated. For unprotected Fe substrates a thick graphitic soot containing 6.5% Fe grows upon the Fe in the first few minutes of exposure to the plasma and, once this soot completely covers the substrate, diamond can nucleate and grow upon it into an average quality unfaceted continuous diamond film. However, adhesion is poor, the weak link being the lack of structural integrity of the soot layer itself. A TiN coating is found to prevent soot formation, C diffusion into the Fe bulk, and Fe diffusion into the diamond films. In the initial stages of growth the TiN is covered with a thin layer of amorphous carbon (a-C), and it is on this layer that diamond nucleates and grows. Here, again, adhesion is not strong, with delamination occurring at the TiN/a-C interface.
Epitaxial orientation of PtSi grown by Pt deposition on heated Si(001) substrate
Authors: Konuma K. , Utsumi H.
Published: July 2002

PtSi film epitaxial orientation and morphology depend greatly on the Si(001) substrate temperature during Pt deposition. 15 nm Pt films were evaporated on the heated substrate in room-temperature to the 1100 K range. The PtSi on the Si substrate forms a continuous film below 850 K and forms islands at above 920 K. In the entire heated range, except for around 1000 K, the dominant epitaxial orientation is PtSi(11̄0)‖‖Si(001) (A orientation). At around 1000 K, the dominant orientation is PtSi(12̄1)‖‖Si(001) (called B orientation). In the range from 920 to 950 K, B orientation grains partially exist in the predominant A orientation. Below 1000 K, the predominant factor governing the epitaxial orientation is area mismatch, which is the difference between the PtSi and Si superlattice areas at the interface. The predominantly A-oriented grains observed at above 1050 K can be explained by the superlattice area mismatch concept.
Study of classical Hall effect in an amorphous As2S3 semiconducting glass
Authors: Singh K. , Asiamah Stephen
Published: July 2002

An experimental study of the classical Hall effect was carried out on amorphous As2S3 semiconducting glass. The experiments were performed on thin samples of As2S3 which were prepared in an oxygen-free atmosphere. The Hall coefficient, conductivity, and Hall mobility were determined in the temperature range of 300–393 K. The Hall coefficient was found to decrease exponentially with temperature. The range of values for Hall coefficient was 1.8×1012–0.81×108 cm3 C−1. The Hall mobility decreases slightly with temperature and the values obtained were within the range 0.090–0.032 cm2 V−1 s−1. The conductivity of As2S3 at room temperature was of the order of 10−13 Ω−1 cm−1 and this increases exponentially with temperature.
Transport coefficients and thermoelectric figure of merit of n-Hg1−xCdxTe
Authors: Sofo J. O. , Mahan G. D. , Baars J.
Published: July 2002

The conductivity, Hall coefficient, and thermopower of Hg1−xCdxTe (MCT) are calculated for 0<x<0.3 at temperatures below 400 K and donor doping lower than 1019 cm−3. The results are obtained by solving the Boltzmann equation with the iteration method, including scattering of the carriers by ionized impurities, optical and acoustic phonons, and alloy potential. The agreement between the calculation and a large amount of compiled experimental data is excellent for bulk samples and thick films grown by molecular-beam epitaxy. Using tabulated data for the thermal conductivity the thermoelectric figure of merit Z is calculated, and in the most favorable conditions of doping and composition ZT is less than 0.33 at T=300 K. The potential for increasing the ZT of MCT is discussed.
Field-assisted self-propagating synthesis of β-SiC
Authors: Feng A. , Munir Z. A.
Published: July 2002

We report on the effect of an electric field on the self-propagating reaction between silicon and carbon to synthesize β-SiC. In the absence of a field at a threshold level, no self-sustaining reaction is observed, and as the applied voltage increases, a linear increase in the velocity of the reaction wave is observed. This first demonstration of the effect of fields on such reactions is explained in terms of a model in which the role of the field is confined to the reaction zone.
Out-of-plane magnetoresistivity for fields parallel to the c axis in single-crystalline (La1−xSrx)2 CuO4
Authors: Yuan S. L. , Kadowaki K. , Yang Z. J. , Li J. Q. , Chen J. L. , Kimura T. , Takeya H. , Kishio K.
Published: July 2002

The c-axis magnetoresistivity as a function of temperature T and field H for fields parallel to the c axis is experimentally investigated for single-crystalline (La1−xSrx)2CuO4 (x=0.068). It is argued that the observed Lorentz force free magnetoresistive phenomena cannot be accounted for by previously considered mechanisms. By contrast, they can be explained by the extended Josephson coupling model, which takes into account both effective thermal energy and anisotropy. Based on this extended model, it is shown that all the magnetoresistivity curves obtained in H//I//c at various constant temperatures could be nicely scaled onto a single curve without any adjustable parameter in a wide transition region.
Correlation of transport and magnetic critical currents in melt-processed YBa2Cu3O7−δ thick films
Authors: Jones A. R. , Cardwell D. A. , Ingle N. J. C. , Ashworth S. P. , Campbell A. M. , Alford N. McN. , Button T. W. , Wellhofer F. , Abell J. S.
Published: July 2002

Transport critical current measurements have been carried out on melt-processed thick films of YBa2Cu3O7−δ on yttria-stabilized zirconia in fields of up to 8 T both within grains and across grain boundaries. These measurements yield Jc values of ∼3000 A cm−2 at 4.2 K and zero magnetic field and 400 A cm−2 at 77 K and zero magnetic field, taking the entire sample width as the definitive dimension. Optical and scanning electron microscopy reveals that the thick-film grains consist typically of a central ‘‘hub’’ region ∼50 μm in diameter, which is well connected to radial subgrains or ‘‘spokes’’ which extend ∼1 mm to define the complete grain structure. Attempts have been made to correlate the transport measurements of inter- and intra-hub-and-spoke (H-S) critical current with values of this parameter derived previously from magnetization measurements. Analysis of the transport measurements indicates that current flow through H-S grains is constrained to paths along the spokes via the grain hub. Taking the size of the hub as the definitive dimension yields an intra-H-S grain Jc of ∼60 000 A cm−2 at 4.2 K and 0 T, which is in reasonable agreement with the magnetization data. Experiments in which the hub is removed from individual grains confirm that this feature determines critically the Jc of the film.
Oxidation of silicon by low energy oxygen bombardment
Authors: Williams J. S. , Petravić M. , Svensson B. G. , Conway M.
Published: July 2002

High resolution Rutherford backscattering and channeling has been used to study the formation of surface oxides during room temperature bombardment of silicon with oxygen in a secondary ion mass spectrometry system. Stoichiometric SiO2 is formed at angles of incidence (to the surface normal)≤25° and the angular dependence is adequately modeled using the profile code. A linear dependence of oxide thickness on energy is obtained in the energy range 3–40 keV (per oxygen ion) and this is consistent with trim code calculations. The suboxide damage has also been measured and studied during annealing. Our data are consistent with a simple model of oxygen build up and formation of strong Si–O bonds during room temperature bombardment. Once a buried SiO2 layer is reached and Si bonds are saturated, oxygen can migrate in SiO2 to extend the oxide towards the surface.
Influence of neutron irradiation on the thermal conductivity of vapor-deposited diamond
Authors: Uher Ctirad , Morelli Donald T.
Published: July 2002

The influence of neutron irradiation on the thermal conductivity κ of diamond films fabricated by hot filament (HF) and microwave plasma assisted (MPA) deposition has been studied. The additional thermal resistivity induced by irradiation is similar to that found in single crystal diamond and is due mainly to the formation of clusters of disordered carbon material. Despite a significant difference in κ prior to irradiation, the thermal conductivity of the HF and MPA films is almost the same after a cumulative dose of 2.7×1017 neutrons cm−2.
Determination of bending stress of Si wafer using concentrated load
Authors: Chen L. D. , Zhang M. J. , Zhang S.
Published: July 2002

The technique of concentrated load with a simple O-ring supporter is used to measure the deflection of Si wafers. The load varies so that the ratio of the deflection to the wafer thickness changes from 0 to 1. For some samples, this ratio goes up to 1.4 at which the samples are fractured. It is observed in the experiment that the stress of the wafer can be described by the linear deflection theory for small deflection. However, when the deflection is larger than 1/5 of the wafer thickness, nonlinear deflection theory should be used and the stress in the middle plane cannot be ignored anymore. The total stress for large deflection is approximately equal to the sum of the stress of a linear elastic plate and the stress in the middle plane. The experiment also shows that the bending strength of the Si wafer strongly depends on the surface conditions. Mirror finishing surface gives high mechanical strength.
Strain buildup in InxGa1−xAs partially relaxed quantum well
Authors: Buchinsky O. , Brafman O. , Brener R. , Fekete D. , Blumin M.
Published: July 2002

Partially relaxed InxGa1−xAs strained-well layers of a fixed thickness above critical, grown on GaAs and capped by GaAs of different thicknesses, were studied by Raman spectroscopy in conjunction with Auger electron spectroscopy and secondary-ion-mass spectroscopy. It is shown that further strain relaxation of the well takes place with increasing GaAs capping thickness initially, but for cap thickness above 100 Å this trend is reversed. This puzzling result is explained by the 3D growth mode. The presence of a high concentration of dislocations intensifies the diffusion of indium to the extent that the two layers become indistinguishable by all three methods. This diffusion of indium is severely reduced with increasing strain.
Structural properties of ZnSe on GaAs grown by atomic layer epitaxy
Authors: Lee C. D. , Kim B. K. , Kim J. W. , Chang S. K. , Suh S. H.
Published: July 2002

ZnSe epilayers were grown on GaAs (100) substrates by atomic layer epitaxy modified from chemical-vapor deposition with thicknesses ranging from 600 to 6000 Å. X-ray-diffraction and micro-Raman scattering measurements were carried out to study the effects of strain in the ZnSe epilayers with different thicknesses. The increase in full width at half-maximum of double-crystal x-ray rocking curves was observed for layers thicker than the critical thickness, which indicates that the crystallinity gets strongly degraded when the layers are grown over the critical thickness. The critical thickness estimated by x-ray rocking curves is 1500 Å, while that obtained by micro-Raman scattering is 1000 Å. This difference suggests that the elastic strain depends on the layer depth for ZnSe epilayers around the critical thickness.
Dependence of charging on crystallographic orientations of an as-grown α-quartz
Authors: Gong H. , Taijing Lu , Ong C. K.
Published: July 2002

The scanning electron microscope copper-detector technique introduced most recently by H. Gong and C. K. Ong [J. Appl. Phys. 75, 449 (1994)] is employed for the investigation of charging on faces m {101̄0}, R {101̄1}, and χ {516̄1} of single-crystalline α-quartz. It is found that the charging ability decreases in the order of m, R, and χ, revealing the dependence of charging on crystallographic orientations, and these results are confirmed by the well-established mirror-image method. In addition, the experimental results also suggest that not only electron-irradiation-induced defects but also intrinsic defects are responsible for charge trapping.
Thermal strain and viscous flow of glass films
Authors: Murray Paul E.
Published: July 2002

The effect of viscous flow on thermal strain in fluids is especially significant for materials such as glass. Thermally strained glass flows to accommodate the volume change caused by expansion or contraction. Viscous flow is responsible for the observed phenomenon of stress relaxation which occurs during annealing of glass. In this paper we present a mathematical solution to the problem of viscous flow in thermally strained compressible fluids. This result is used to study thermal stress in glass films undergoing heating and cooling. The effect of viscous flow on thermal stress is demonstrated numerically and is used to explain experimental observations of stress relaxation in glass films which were reported elsewhere. This paper includes a general formulation of the mechanics of slow viscous flow of compressible Newtonian fluids in the presence of a thermal strain.
Optical transitions of infinite and finite strained Si/Ge superlattices
Authors: Tserbak C. , Theodorou G.
Published: July 2002

A systematic study of the optical transitions of pseudomorphically strained (Si)n/(Ge)n superlattices grown on Si1−xGex(001) buffers is presented. The influence of period (n+m), synthesis (n/m), and strain on the transition energies and transition probabilities at the Γ point is studied. This is performed with the use of a realistic tight-binding model in the three-center representation. The transition energies and probabilities for the finite superlattices (Si)4/(Ge)4 inside Si and (Si)5/(Ge)5 superlattice inside Ge are also studied. It is proposed that the most promising material for optoelectronic applications is the strain-symmetrized (Si)4/(Ge)6 strained layer superlattice.
Symmetry of optically active Yb-related centers in InP and In1−xGaxP (x≤0.13)
Authors: Buyanova I. A. , Neuhalfen A. J. , Wessels B. W. , Sheinkman M. K.
Published: July 2002

The symmetry of Yb3+-related luminescent centers in InP and In1−xGaxP layers with alloy composition less than x=0.13 is studied using polarized excitation spectroscopy. An induced polarized luminescence under above gap excitation is detected and is explained in terms of an excitation mechanism involving 4f→5d shell electron transitions. It is shown that the Yb3+-related centers responsible for 1.220–1.237 eV luminescence are optically anisotropic with C3v symmetry or its subgroup. The effect of alloy composition on the symmetry of Yb centers is analyzed in terms of the Yb3+-GaIn defect model.
Investigation of the photoresponse of lipid monolayers containing azobenzene derivatives by a Maxwell-displacement-current-measuring technique
Authors: Iwamoto Mitsumasa , Ohnishi Koji
Published: July 2002

Mixed monolayers composed of azobenzene and phospholipids on a water surface were investigated in the context of the phase diagram by means of the Maxwell-displacement current measurement. Transient displacement-current pulses were found to be produced in the entire range of molecular area when successive isomerizations of azobenzene were induced in the mixed monolayers by irradiation with ultraviolet and visible light. It was suggested that the condensation of pure azobenzene monolayers is loosened as a result of the introduction of phospholipids into the monolayers, and molecular aggregation in pure azobenzene monolayers is also weakened by the introduction. The displacement current generated during monolayer compression was also investigated in connection with phase transitions. Finally, we concluded that Maxwell-displacement current measurement is useful for the investigation of artificial lipid membranes containing azobenzene derivatives as a model of visual light processing, from both scientific and technological points of view.
Effect of tunneling electrons in Fowler–Nordheim regime on the current-voltage characteristics and model of degradation of metal-oxide-semiconductor capacitors
Authors: Elrharbi Simon , Jourdain Marc , Meinertzhagen Anne
Published: July 2002

The evolution of the shift of current-voltage characteristics after Fowler–Nordheim stress has been investigated. In some cases, a diminution of the slope of current-voltage characteristics was observed. It has been studied and discussed with respect to the mode of the injection of carriers in the oxide, the type of carrier trapping, and the generation of defects. The variation of the slopes of current-voltage characteristics after high-electric-field stress is due to a nonuniformity of the field in the oxide and mainly depends on the cathode field. The variation of the cathode field is basically related to the location of the oxide charge (positive or negative) near the substrate-Si/SiO2 and gate/SiO2 interfaces which modify the shape of the tunneling barrier. A model of degradation of the metal-oxide-semiconductor structures has been deduced from a qualitative analysis of voltage shifts of current-voltage characteristics depending on the magnitude of electric field stress, on the stress mode with a positively or negatively biased gate, and on the injected charge density. The degradation of the oxide is consistent with both mechanisms of trap creation and/or impact ionization.
Iterative spectral solution of the Poisson-Boltzmann equation in semiconductor devices
Authors: Floyd Brian H. , Le Coz Yannick L.
Published: July 2002

We present an iterative spectral method for solving the coupled Poisson–Boltzmann equation in semiconductor devices. Both theory and preliminary numerical results, in one spatial dimension (three phase-space dimensions), are reviewed. The method relies on a multidimensional Hermite–Gaussian-product expansion of the carrier distribution function’s velocity dependence. Pseudotemporal and spatial variables are discretized using finite differences. Stability of the resulting numerical spectral equations is achieved using an added pseudoviscous term. The present formulation allows full inclusion of realistic phonon-scattering probability rates by means of a ‘‘collision matrix.’’ Numerical solutions have been obtained for a variety of one-dimensional semiconductor diode problems. These include ballistic, collisional, equilibrium, and nonequilibrium cases. Comparison, where possible, with analytical solutions has confirmed the validity and accuracy of this spectral approach.
Electrical characterization of rapid thermal nitrided and reoxidized plasma-enhanced chemical-vapor-deposited silicon dioxide metal-oxide-silicon structures
Authors: Ang S. S. , Shi Y. J. , Brown W. D.
Published: July 2002

The electrical characteristics of rapid thermal nitrided and reoxidized plasma-enhanced chemical-vapor-deposited (PECVD) silicon dioxide metal-oxide-silicon (MOS) structures were investigated. Both nitridation temperature and time affect the properties of the MOS structures as revealed by capacitance-voltage (C-V) characteristics. Nitridation at 1000 °C for 60 s followed by reoxidation for 60 s at 1000 °C in an oxygen/nitrogen ambient was found to be superior to the same nitridation followed by reoxidation in pure oxygen. Typical values of fixed charge and interface state densities for devices subjected to nitridation and reoxidation in a mixture of oxygen and nitrogen were 4×1010 cm−2 and 7×1010 eV−1 cm−2, respectively. Avalanche electron injection using electric fields of 3–5 MV/cm produced negative shifts in flatband voltage for low fluence levels and positive flatband voltage shifts for larger fluence levels. Furthermore, the magnitudes of both positive and negative shifts and the electron fluence level at which turnaround occurs increase with electric field. However, independent of the electric field, the flatband voltage saturates very close to its preinjection value. These results strongly suggest that device quality MOS dielectrics can be realized by nitridation/reoxidation of PECVD oxide.
Characteristics of hydrogenated amorphous silicon thin-film transistor
Authors: Kim Jeong Hyun , Oh Eui Yeol , Hong Chan Hee
Published: July 2002

The effects of N2 plasma interface treatment and atmospheric pressure chemical vapor deposition (APCVD) silicon dioxide (SiO2) gate insulator were investigated for the performance and the stability of hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). To improve an a-Si:H/SiO2 interface, the N2 plasma treatment was carried out on the SiO2 surface. As a result, both the threshold voltage and subthreshold voltage swing were decreased remarkably. The high performance of a-Si:H TFTs was achieved with a field-effect mobility of 1.27 cm2/V s, threshold voltage of 3.5 V, and subthreshold voltage swing of 0.4 V/decade. For both positive and negative gate bias stresses, the threshold voltage was shifted in the positive direction and was dominated by the defect creation near the a-Si:H/SiO2 interface. There was little charge trapping into the APCVD SiO2 gate insulator. For the positive gate bias and time stress, the threshold voltage shifts were smaller than those of the conventional a-Si:H TFTs with the a-Si:H/SiNX interface.
Plasma-filled applied B ion diode experiments using a plasma opening switch
Authors: Renk T. J.
Published: July 2002

In order for a plasma opening switch (POS) to open quickly and transfer power efficiently from an inductively charged vacuum transmission line to an applied B ion diode, the load impedance of the ion diode may be required to have an initial low impedance phase. A plasma-filled diode has such an impedance history. To test the effect of a plasma-filled diode on POS-diode coupling, a drifting plasma was introduced from the cathode side of an applied B ion diode operated on the LION accelerator (1.5 MV, 4 Ω, 40 ns) at Cornell University. This plasma readily crossed the 2.1 T magnetic insulation field of the diode, and resulted in both increased diode electrical power, and an increased ability of the ion beam to remove material from a target. The plasma did not appear to have a noticeable effect on local beam steering angle.
Model for calculating T
c of pluralistic magnetic component compounds
Authors: Gao Y. H. , Zhang D. M. , Tang C. Q. , Yu B. M. , Qin W. D. , Lu Z. H. , Yang F. M.
Published: July 2002

A coupling model of the pluralistic magnetic component R-T intermetallic compounds (R denotes rare earth, T denotes transition metal) has been proposed by means of the mean-molecular-field analysis. Using the model, the Curie temperature Tc of (Sm1−xPrx)2Fe17 compounds has been calculated and the calculated values have been compared with the experimental values. It is found that the former are consistent with the later.
Ga substitution effect on magnetic and magnetostrictive properties of TbFe2 compounds
Authors: Tang Y. J. , Feng Y. B. , Luo H. L. , Pan S. M.
Published: July 2002

In the present paper magnetic and magnetostrictive properties of Tb(Fe1−xGax)2 (x=0–0.2) compounds were investigated. It was found that the iron moment of the compounds does not seem to vary much for x≤0.12. The Curie temperatures of the compounds decrease continuously by substituting Ga for Fe, which was attributed to the decrease of the R-T coupling strength due to Ga substitution. The intersublattice coupling constant JRFe was evaluated by molecular field model. The decrease of JRFe with increasing Ga content was found and related to the decrease of the number of the Tb-Fe interaction pairs when replacing Fe with Ga. By using an x-ray diffractometer the samples were step scanned with Cu radiation at a higher Bragg angle 2θ ranging from 71° to 74° to study the cubic (440) reflection. The splitting of (440) reflection for X≤1.2 was clearly seen and the easy direction magnetostriction λ111 of the compounds was calculated. It was found that λ111 decreases with increasing Ga content. This was attributed to the decrease of magnetic properties of the compounds. The polycrystal magnetostriction λs of the compounds has also been studied.
Effects of InGaAs/GaAs strained-layer superlattices in optimized molecular-beam-epitaxy GaAs on Si with Si buffer layers
Authors: Georgakilas A. , Christou A.
Published: July 2002

The effectiveness of In0.10Ga0.90As/GaAs strained-layer superlattices (SLSs) as barriers for the threading dislocation propagation, in molecular-beam-epitaxy GaAs-on-Si structures with Si buffer layers, has been investigated. It is shown that the interaction of threading dislocations with the strain field of SLSs is effective in limiting their propagation. The interaction is stronger as the total thickness of In0.10Ga0.90As (i.e., SLS periods) is increased. SLSs with thinner individual layers resulted in a lower dislocation density and a better structural quality at the GaAs/Si interface.
Magnetic layer thickness dependence of the interlayer exchange coupling in (001) Co/Cu/Co
Authors: Bloemen P. J. H. , van de Vorst M. T. H. , Johnson M. T. , Coehoorn R. , de Jonge W. J. M.
Published: July 2002

A dependence of the strength of the antiferromagnetic coupling across Cu on the Co layer thickness has been observed. The Co thickness dependence displays two clear peaks consistent with the recently predicted oscillation period of 6.2 Å Co. Apart from the two peaks also several small peaks are visible on a scale of about 1 monolayer Co. Free-electron calculations indicate that these rapid variations in strength may result from slight differences between the slopes and starting points of the two Co wedges that were involved in the experiment.
Observation of large biquadratic coupling of FeCo through Mn (invited) (abstract)
Authors: Filipkowski M. E. , Gutierrez C. J. , Krebs J. J. , Prinz G. A.
Published: July 2002

A new system, exhibiting unusually large interlayer biquadratic exchange coupling, has been discovered, consisting of single crystal FeCo/Mn/FeCo sandwiches epitaxially grown on GaAs substrates. Normalized m(H) curves yield a remanent moment of approximately 0.5 for all cases studied, with a number of examples having saturation fields as large as several Tesla. This implies a biquadratic coupling constant as large or larger than many known bilinear coupling constants. None of the normalized m(H) data exhibits a remanence of less than 0.5, indicating the absence of comparable contributions from bilinear coupling. Angular dependent FMR at 9 GHz implies a fourfold anisotropy of opposite sign to that measured for single FeCo layers, whereas FMR at 35 GHz agrees in sign with the single layer anisotropy. Detailed analysis of this contradiction shows that this is an apparent anisotropy reversal which emerges within the theory of FMR in the presence of large biquadratic coupling, requiring values of the coupling constant in excess of −j2=2 ergs/cm2, where the coupling term in the energy is written J2[(m1⋅m2)/m1m2]2. This large biquadratic coupling, together with the absence of bilinear coupling, appears to contradict existing theories of interlayer exchange.
Magnetic dipole mechanism for biquadratic interlayer coupling (abstract)
Authors: Demokritov S. , Tsymbal E. , Grünberg P. , Zinn W. , Schuller Ivan K.
Published: July 2002

A mechanism resulting in biquadratic interlayer coupling is proposed and analyzed theoretically. This mechanism is connected with the magnetic dipole field, created by magnetic layers with roughness. This field decays exponentially with the distance from the layer, but it shows oscillating behavior in the lateral direction. The scale of both exponential and oscillating dependencies corresponds to the scale of the interface roughness and can reach 20–30 nm. The oscillating variation of the field makes 90° alignment of the magnetization energetically favorable in analogy to the Slonczewski’s mechanism. Computer simulations and estimates show that this mechanism can provide a coupling strength of the order of 0.01 erg/cm2 for Fe films with 1 nm interlayer thickness. The part of the work done in Moscow and Jüelich was supported by Collaborative Research Grant CRG 921170 of the NATO Scientific Exchange Programmes. Work at UCSD was supported by the U. S. National Science Foundation. One of us (E.Ts.) is pleased to thank the Alexander von Humboldt Foundation for support.
Polarized neutron reflectivity studies of biquadratic coupling in [Fe/Cr] (100) and [Fe/Al] (100) superlattices and films (invited) (abstract)
Authors: Ankner J. F. , Schreyer A. , Zabel H. , Borchers J. A. , Majkrzak C. F. , Schäfer M. , Wolf J. A. , Grünberg P. , Filipkowski M. E. , Gutierrez C. J. , Krebs J. J. , Prinz G. A.
Published: July 2002

Sensitivity to magnetic atoms and low intrinsic absorption characterize the interaction of neutrons with matter. Consequently, polarized neutron reflectivity provides a unique means of performing depth-resolved vector magnetometry. We have used this technique to determine the magnetization depth profiles of Fe/Cr superlattices. Superlattices of bilayer composition [55 Å Fe/17 Å Cr], grown at 523 K, exhibit biquadratic coupling with large saturation fields (∼3 kOe), while those grown at 293 K are ferromagnetically ordered. We have directly measured the evolution of the coupling angle between adjacent Fe layers as a function of applied field and will discuss how bilinear, biquadratic, and external field terms produce the observed order. The weaker coupling found in the Fe/Al system makes possible the investigation of a range of spin configurations at temperatures that do not endanger the sample. We have mapped the phase diagram of a [42 Å Fe/12 Å Al/39 Å Fe] (100) trilayer and find evidence of biquadratic coupling at low temperatures and fields (e.g., when H=180 Oe, the Fe layer spins relax away from ferromagnetic alignment below T≊170 K). Our measurements agree qualitatively with energy minimization calculations and the results of bulk magnetometry.
Two-magnon absorption in Nd2CuO4
Authors: Sobolev V. L. , Pashkevich Yu. G. , Huang H. L. , Vitebskii I. M. , Blinkin V. A.
Published: July 2002

The two-magnon absorption of the electromagnetic waves in the exchange noncollinear magnetic phase of four-sublattice antiferromagnet Nd2CuO4 has been considered. The spin-wave density of states and the frequency dependencies of the absorption coefficient have been calculated. It is shown that the intensity of the two-magnon absorption by exchange magnons in exchange-noncollinear magnets is substantially larger than in magnets collinear in the exchange approximation.
Ferromagnetic resonance and Brillouin light scattering from epitaxial FexSi1−x films on Si(111) (abstract)
Authors: Mendik M. , Frait Z. , von Känel H. , Onda N.
Published: July 2002

The magnetic properties of epitaxial FexSi1−x films on Si(111) have been determined by means of ferromagnetic resonance (FMR) and Brillouin light scattering (BLS). The investigated films are (111) oriented, with thicknesses h=150 Å, 250 Å, 710 Å, and Fe concentrations x=0.75, 0.79, and 0.75, respectively. All experiments have been carried out at room temperature. For BLS, the frequencies of both surface and bulk magnons have been measured as a function of the external in plane field H and the in plane direction of magnon propagation versus the main crystallographic axis. Moreover, the wave vector dependence has been used to identify the surface and bulk magnons present in the thicker films. FMR has been used in the parallel configuration (PC) and normal configuration (NC), where the external applied field lies in the sample plane and normal to the sample, respectively. Several waveguide setups were used to cover the frequency range from 18 to 92 GHz. For the numerical analysis we used the resonance conditions for a thin single crystalline film grown in the (111) plane. From our fits we obtained for the Landé g value 2.1, for the saturation induction 4πMs=8.8 kG, 10.7 kG, 13.7 kG for h=150 Å, 710 Å, and 250 Å, respectively. The magnetic parameters have been found to depend strongly on the Fe concentrations and FexSi1−x-Si substrate interface interdiffusion. The magnetic parameters of epitaxial FexSi1−x films are in agreement with the data obtained from single crystals by Hines et al. The Landau–Lifshitz FMR relaxation constant is very small, ranging from 5 to 8×107 rad/s.
Theory of spin dynamics in the metallic cuprates (invited)
Authors: Si Qimiao , Zha Yuyao , Levin K.
Published: July 2002

The spin dynamics in the metallic cuprates are studied theoretically. In the normal state, it is demonstrated that the interplay of correlation effects and fermiology lead to a natural understanding of the contrasting wave vector dependences in the neutron scattering cross sections of the two best characterized cuprates: LaSrCuO and YBaCuO, as well as the anomalous frequency and temperature dependences in the neutron scattering cross sections and in the NMR relaxation rates 1/T1 and 1/T2G. In the superconducting state, the compatibility of the d-wave pairing state with magnetic data is explored.
Hydrostatic pressure on HgBa2CaCu2O6+δ and HgBa2Ca2Cu3O8+δ
Authors: Chen F. , Gao L. , Meng R. L. , Xue Y. Y. , Chu C. W.
Published: July 2002

The superconducting transition temperature Tc and its pressure dependence dTc/dP of HgBa2CaCu2O6+δ (Hg-1212) and HgBa2Ca2Cu3O8+δ (Hg-1223) were measured up to 17 kbar. Tc increases with pressure approximately linearly for both compounds before oxidation. However, the nonlinearity in the Tc-P correlation shows up after oxidation in both compounds. For Hg-1212, the average dTc/dP decreases as oxygen doping increases, while it increases in Hg-1223. These observations are in conflict with the modified pressure-induced charge-transfer model, but might be attributed to the possible existence of fine electronic structure. These observations suggest that a Tc much higher than 140 K might be achievable in Hg-1223 by means of higher physical or chemical pressure.
Evidence of domain-wall pinning in W-doped (NdDy)(FeCo)B sintered magnets
Authors: Chu T. Y. , Chin T. S. , Lin C. H. , Yao J. M.
Published: July 2002

Addition of tungsten (1–2 at. %) is very effective in improving coercivity of (Nd,Dy)-(Fe,Co)-B sintered magnets both at room temperature and elevated temperatures in addition to improving thermal stability. Transmission electron microscopy shows that there exists a WFeB precipitate phase inside the Nd2Fe14B (2-14-1) matrix grain. The WFeB phase is plateletlike, and occasionally appears to have its largest dimension running parallel to the c axis of the 2-14-1 matrix phase. The orientation relationship between the WFeB precipitate and the matrix is found to be (102)WFeB∥(220)2-14-1, and [010]WFeB∥[002]2-14-1. Domain walls jump from a position alongside the WFeB particles to a new position alongside others upon turning on magnetic field of the objective lens, as evidenced by Lorentz microscopy, denoting the ability of domain-wall pinning by the strain field around the WFeB precipitate. As a result, this provides extra hindrance to domain-wall motion and leads to enhanced coercivity and thermal stability, particularly at elevated temperatures.
Magnetic alignment in powder magnet processing
Authors: Liu S.
Published: July 2002

A strong magnetic field is used to align single-crystal powder particles in the process of producing sintered powder permanent magnets, including hard ferrites and rare-earth permanent magnets. The applied magnetic field aligns the easy direction of magnetization of each particle, owing to strong crystalline anisotropy. Shape anisotropy, existence of particles containing multigrains, and physical interlock between particles reduce the degree of alignment. This study provides a quantitative analysis of magnetic alignment in powder magnet processing. We assume (1) the powder particle is a single crystal; (2) it has the shape of an oblate spheroid and its short axis is the easy direction of magnetization; and (3) the applied magnetic field is strong enough to overcome the resistance of alignment. By applying the minimum-energy principle, it was concluded that the necessary and sufficient condition for a complete magnetic alignment is that the magnetocrystalline anisotropy constant K1 of the particles is greater than its shape anisotropy constant Ks, provided the applied magnetic field is strong enough. When Ks≳K1+2K2, the angle between the short axis of the oblate particle and the direction of applied magnetic field is 90°, and when K1≤Ks≤K1+2K2, the angle is arcsin√(Ks−K1)/2K2.
Temperature-dependent electronic structure and ferromagnetism of bcc iron
Authors: Nolting W. , Vega A.
Published: July 2002

The influence of electron correlations on the temperature dependence of the electronic structure of ferromagnetic bcc iron is investigated by the use of a many-body evaluation of a generalized model of magnetism. The one-particle part of the model Hamiltonian is taken from an LDA band-structure calculation. The model contains only two parameters, an intraband Coulomb interaction U, and an interband exchange J. With U=1.8 eV and J=0.2 eV the self-consistent model solution yields a T=0 moment of about 2.04 μB and an exact Curie temperature of 1044 K. Details of the magnetic behavior of Fe can be traced back to a striking temperature variation of the quasiparticle density of states. Typical differences in the magnetic behavior of Fe and Ni are worked out.
Verwey transition in magnetite: Mean-field solution of the three-band model
Authors: Mishra S. K. , Zhang Z. , Satpathy S.
Published: July 2002

The nature of the Verwey transition in magnetite (Fe3O4) within a three-band spinless model Hamiltonian is examined. These bands, which arise from the minority-spin t2g orbitals on the Fe(B) sublattice, are occupied by half an electron per Fe(B) atom. The Verwey order–disorder transition is studied as a function of the ratio of the intersite Coulomb repulsion U1 and the bandwidth W. It is found that the electrons are ordered beyond the critical value of U1/W≊0.25 in essential agreement with the results of the one-band Cullen–Callen model. For larger values of U1/W, a Verwey-like order is exhibited where the electrons occupy alternate (001) planes. The model predicts a transition from the metallic to the semiconducting state with the band gap increasing linearly with U1 beyond the transition point.
Magnetic properties of Tm2Fe17Cx (0≤x≤2.8) compounds prepared by melt spinning
Authors: Shen Bao-Gen , Kong Lin-Shu , Cao Lei , Gong Hua-Yang , Wang Fang-Wei , Cheng Zhao-Hua , Zhao Jian-Gao
Published: July 2002

The formation, structure, and magnetic properties of Tm2Fe17Cx compounds with x=0, 0.5, 1.0, 1.5, 2.0, 2.5, and 2.8 were studied. The samples with x≤1.0 were arc melted and heat treated at 1370 K for 14 h. The carbides were prepared by melt spinning at appropriate quenching rates of vs=10–20 m/s for 1.5≤x≤2.5 and by the crystallization from corresponding amorphous for x=2.8. X-ray diffraction and thermomagnetic measurements show that all samples studied are single phase with the hexagonal Th2Ni17-type structure except for Tm2Fe17C2.8, which contains a few percent of α-Fe. The lattice parameters a,c and the unit-cell volumes v increase as the carbon concentration x increases. The Curie temperature is found to rise with x from 260 K for x=0 to 669 K for x=2.8. The saturation magnetization Ms at 1.5 K is found to be 92.2–97.1 emu/g as x varies from 0 to 2.8, and the carbon concentration dependence of the Fe moment is approximately constant (2.12±0.04μB). The spin-reorientation transitions are observed. The spin-reorientation temperature is found to increase with the carbon concentration for x≤1.5, and then has a slight decrease with x for x≳1.5.
The micromagnetics of periodic arrays of defects in trilayers with interlayer exchange coupling (abstract)
Authors: van den Berg H. A. M.
Published: July 2002

A significant loss in the giant magnetoresistive signal of magnetic stacks with antiferromagnetic coupling across nonmagnetic intermediate layers is caused by regions with a ferro- rather than an antiferromagnetic coupling. The impact of these ferromagnetic coupling regions extends itself into the lateral direction due to the bulk exchange coupling. The present micromagnetic model provides a tool by which a detailed quantitative evaluation of the impact of periodic arrays of parallel line defects is possible. These defects have deviating exchange-coupling constants, and/or anisotropy constants or directions, bulk exchange constants, saturation magnetization, etc., in specific regions. Previously, we developed a phenomenological model of trilayers with two magnetic films separated by a nonmagnetic interlayer that contains one such defect. This model, with a relatively small number of free parameters, allows one to trace complete hysteresis curves. A large number of mode branches reveal themselves and jumpwise transitions between these modes frequently occur along the hysteresis loops. The present micromagnetic model requires a sufficiently accurate assessment of the starting magnetization configuration in order to get a convergence of the code. In general, the micromagnetic code is not capable of overcoming the above irreversible mode conversions. The mode branches evaluated by the phenomenological model are applied to provide the micromagnetic model with appropriate starting configurations after meeting a situation of nonstability.

The micromagnetic theory of Brown constitutes the basis of the present approach. The micromagnetic effective field is calculated at grid points and the torque exerted by it on the magnetic dipole is made zero at each grid side by an iteration scheme. The long ranging magnetostatic fields are given by convolution integrals and are evaluated in the Fourier space by using two-dimensional fast Fourier transforms. The single defect is micromagnetically studied by zero padding techniques. Depending on the course of the external field, two different wall regions reveal themselves, to wit, the wall core and the so-called Néel tails. These tails were not incorporated into the phenomenological model. Provided that the defects are sufficiently wide spaced, the agreement between both models is rather good in the core regions. The impact on the GMR signal, in particular of the Néel tails, will be discussed with emphasis on systems with weak interlayer coupling, e.g., the decoupled systems.
Thermal decay of N coupled particles
Authors: Klik Ivo , Chang Ching-Ray , Yang Jyh-Shinn
Published: July 2002

A periodic chain of N magnetostatically coupled uniaxial particles is considered within the nearest-neighbor approximation of weak coupling. The time-dependent magnetization of the ensemble, which relaxes thermally in constant reversing field, is calculated using a master equation approach. It is found that at very small applied fields the net magnetization relaxes via simple exponential decay while at large reversing fields its relaxation becomes multiexponential. It is conjectured that at large N this effect gives rise to field induced logarithmic decay.
Enhanced magnetoresistance in chromium doped Fe/Cr multilayers (abstract)
Authors: Rensing Noa M. , Clemens Bruce M.
Published: July 2002

We report enhanced room temperature magnetoresistance of up to 11.4% in sputter deposited Fe/Cr multilayers when the iron layers are doped with additional chromium. To our knowledge, this is the largest room temperature magnetoresistance in polycrystalline Fe/Cr multilayers reported to date. In comparison, the magnetoresistance was 7.5% in a similar, undoped sample made at the same time. The magnetoresistance of antiferromagnetically coupled Fe/Cr multilayers has been the subject of considerable study in the past five years. We have investigated the possibility of doping the Fe layers with additional chromium, in order to increase the magnetoresistance by enhancing the spin dependent scattering. In one series of samples, the chromium dopant was concentrated into thin layers within the Fe layer. A 1 Å layer of chromium (1/2 monolayer) was deposited at a depth of 4, 8, or 15 Å into the 30-Å-thick iron layers of the multilayers. The magnetoresistance was enhanced in all of the samples, to 8% when the doping layer was at 4 Å and to 10.5% in the others. In the second series of samples the iron layers were alloyed with between 1.5% and 50% chromium. The maximum magnetoresistance was 11.4%, observed in the sample with 20% chromium in the Fe layers. This is larger than the magnetoresistance in any of the samples where the Cr was concentrated in a thin layer within the Fe layer, but a given amount of chromium appears to be more effective in increasing magnetoresistance when it is concentrated in a layer rather than distributed as an alloy. A 1 Å layer of Cr is roughly equivalent to a 3% alloy, but the alloyed sample only showed 8% magnetoresistance, in comparison to up to 10.5% in the samples with layered doping.