bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of high-energy x-radiation using a plasma flow switch
Authors: Turchi P. J. , Davis J. F. , Alme M. L. , Bird G. , Boyer C. , Coffey S. K. , Conte D. , Seiler S. W. , Baker W. L. , Degnan J. H. , Hall D. J. , Holmes J. L. , McCullough W. F. , Price D. W. , Buff J. , Frese M. H. , Peterkin R. E. , Roderick N. F. , Graham J. D. , Lopez E. A.
Published: July 2002

Experiments with coaxial plasma guns at currents in excess of ten megamperes have resulted in the production of high-voltage pulses (0.5 MV) and hard x radiation (10–200 keV). The x-radiation pulse occurs substantially after the high-voltage pulse suggesting that high-energy electrons are generated by dynamic processes in a very high speed (≳106 m/s), magnetized plasma flow. Such flows, which result from acceleration of relatively low-density plasma (10−4 vs 1.0 kg/m3) by magnetic fields of 20–30 T, support high voltages by the back electromotive force-u×B during the opening switch phase of the plasma flow switch. A simple model of classical ion slowing down and subsequent heating of background electrons can explain spectral evidence of 30-keV electron temperatures in fully stripped aluminum plasma formed from plasma flows of 1–2 × 106 m/s. Similar modeling and spectral evidence indicates tungsten ion kinetic energies of 4.5 MeV and 46 keV electron temperatures of a highly stripped tungsten plasma.
Antiferromagnetism in epilayers and superlattices containing zinc-blende MnSe and MnTe
Authors: Kl/osowski P. , Giebul/towicz T. M. , Rhyne J. J. , Samarth N. , Luo H. , Furdyna J. K.
Published: July 2002

New zinc-blende Mn-based magnetic semiconductors offer unique insights into the magnetic order and critical behavior of frustrated antiferromagnets. Molecular beam epitaxy grown samples exist both in the form of strained superlattices, with strain induced anisotropies (MnSe/ZnSe and MnTe/ZnTe), and in the form of near-isotropic bulk MnTe. The tetragonally distorted strained superlattices exhibit a second-order phase transition, consistent with symmetry arguments favoring such behavior. In contrast, the bulk-like epitaxial layers of MnTe have a first-order phase transition, associated with magnetostriction, unknown to an unaccounted for by the previous studies.
Backscattering of electromagnetic missile by a perfectly conducting elliptical cylinder
Authors: Geyi Wen , Chengli Ruan , Weigan Lin
Published: July 2002

Recently the backscattering of an electromagnetic (EM) missile with an energy decreasing as r−ε by a perfectly conducting plate has been studied and the analysis shows that the energy backscattered varies as r−ε with the same exponent as the incident EM missile. Scattering of an EM missile by other shapes is left as an open question. In this paper we study the backscattering of an EM missile with energy decreasing as r−ε by a perfectly conducting elliptical cylinder. It has been shown that the rate of the energy decay for the backscattered field far from the cylinder is proportional to r−(1+ε), which is faster than r−ε for plate scattering but still slower than the ordinary energy decay r−2 for a two-dimensional backscattered field.
Pulsed laser deposition of stoichiometric LiNbO3 thin films by using O2 and Ar gas mixtures as ambients
Authors: Ogale S. B. , Nawathey-Dikshit Rashmi , Dikshit S. J. , Kanetkar S. M.
Published: July 2002

Stoichiometric thin films of LiNbO3 have been deposited on (100) silicon by a pulsed excimer laser deposition technique. By studying several cases of depositions in vacuum, oxygen, and argon, as well as oxygen-argon mixture as ambients, it is brought out that the desired stoichiometry in the film can be attained only by having an appropriate oxygen-argon mixture during deposition. The films have been characterized by x-ray diffraction, infrared transmission, and spectroscopic ellipsometry techniques.
Crystallization of sputtered lead zirconate titanate films by rapid thermal processing
Authors: Vasant Kumar C. V. R. , Pascual R. , Sayer M.
Published: July 2002

A rapid thermal annealing (RTA) technique has been employed to process lead zirconate titanate (PZT) films prepared by reactive magnetron sputtering. The films were fabricated by dc sputtering a multielement metal target in an oxygen ambient at a substrate temperature of 200 °C. A subsequent postdeposition RTA at 600 °C for 5 s crystallizes the films into a perovskite-type structure through various intermediate phases. Due to the short postdeposition processing times inherent in the RTA method, the initial nature of the as-grown films has a critical influence on the crystallization kinetics. The reaction sequence in the formation of perovskite PZT from the films deposited at low substrate temperatures by the sputtering technique has been evaluated, and various key factors influencing the crystallization of PZT have been identified. As-grown films are constituted of polycrystalline orthorhombic lead oxide in an amorphous matrix of titania and zirconia. During annealing lead oxide transforms into a cubic phase, and the lead oxide stoichiometry determines the processing route to PZT. In the case of lead-rich films, intermediate compounds of lead with titania and zirconia are observed, which react during the final stage of annealing at 600 °C to form PZT. In lead-deficient films, the formation of a pyrochlore phase has been observed, which crystallizes into perovskite at 750 °C. The Zr/Ti ratio also influences the crystallization sequence. In the case of Ti-rich PZT, the intermediate compounds initially involve a zirconium-rich rhombohedral PZT, with which residual titanium reacts to form tetragonal PZT. The films showed good ferroelectric and other electrical properties with a remanent polarization of 24 μC/cm2, coercive field of 32 kV/cm, ε′=950, tan δ=0.02, and σdc (300 K)=10−12 Ω−1 cm−1 with an activation energy between 0.9 and 1.4 eV.
Interactions of silicon point defects with SiO2 films
Authors: Dunham Scott T.
Published: July 2002

Interactions of point defects with SiO2 films play a central role in integrated circuit fabrication processes. In this work, a model is developed for the Si-SiO2 system that considers the segregation of excess silicon between the oxide and the silicon substrate and diffusion and reaction of that excess silicon in SiO2. The model is able to explain a broad range of experimental observations under both oxidizing and nonoxidizing conditions in a consistent manner including: the variation of interstitial supersaturation with oxidation rate in steam and dry O2 ambients, oxidation enhanced and retarded diffusion results in 〈100〉 and 〈111〉 silicon, the large interstitial supersaturation resulting from the nitridation of SiO2, the reduction of SiO2 in argon and the corresponding decrease in interstitial concentration, contradictory calculations of effective interstitial diffusivity, and the greatly reduced effective recombination velocities for nitrided oxides relative to capped oxides.
Diamond nucleation on cleaved Si(111)
Authors: Polini Riccardo
Published: July 2002

Diamond crystallites have been nucleated and grown by hot filament chemical vapor deposition at 600 °C on the untreated fracture surface of a cleaved Si(111) sample. The flat surface of the cleaved crystal was inactive towards diamond nucleation while, on the terraced surface formed by the propagating crack, a high density of nuclei was found. The crystallites were nucleated in correspondence of edges between (111) terrace planes and step planes. The occurrence of edges, as determined by scanning electron microscopy (SEM) observation, is a necessary but not sufficient condition for the nucleation and this fact suggests that particular atomic arrangements are required for the diamond nucleus formation.
Frequency dependence of ultrasonic velocity and attenuation in two-phase composite systems with spherical scatterers
Authors: Alig Ingo , Lellinger Dirk
Published: July 2002

The frequency dependence of the scattering contribution of ultrasonic attenuation and sound velocity of longitudinal waves was measured in a frequency range from 2 to 50 MHz. For model systems we used gelatine samples with 20 wt % glass beads of 315–400 μm and 180–250 μm diameter and with 10 wt % glass beads of 42–174 μm diameter. The application of a recently developed Fourier transform technique allowed us to observe both ultrasonic attenuation and velocity with sufficiently high accuracy for quantitative comparison with sound scattering theory. The frequency dependence of sound attenuation shows multiple maxima, whereas in the frequency dependence of the velocity difference a sharp minimum on the order of 1% of the absolute velocity occurs. For the simulation of ultrasonic attenuation and sound velocity we combined a multiple scattering approach from Waterman and Truell [J. Math. Phys. 2, 512 (1961)] with a acoustic resonance scattering theory for a viscoelastic sphere developed by Ayres and Gaunaurd [J. Acoust. Soc. Am. 81, 301 (1986)]. Supplementary in the computations, the attenuation of the matrix and the particle size distribution were taken into account. Comparison of the simulated data to the experimental results shows good agreement.
Minority-carrier recombination kinetics and transport in ‘‘surface-free’’ GaAs/AlxGa1−xAs double heterostructures
Authors: Gilliland G. D. , Wolford D. J. , Kuech T. F. , Bradley J. A. , Hjalmarson H. P.
Published: July 2002

We have measured room-temperature band-to-band recombination decay kinetics in superior quality GaAs heterostructures, and have observed the longest lifetime (2.5 μs) observed for any GaAs/AlxGa1−xAs structure to date. Additionally, using a novel time-resolved optical photoluminescence imagining technique, analogous to the Haynes–Shockley experiment, we have also measured room-temperature minority-carrier transport in this series of ‘‘surface-free’’ GaAs/Al0.3Ga0.7As double heterostructures, measurements only possible in high-quality samples with long lifetimes and intense photoluminescence. We find the transport to be diffusive with diffusion lengths of ≳100 μm. Further, we find, for thick structures, minority-carrier transport is hole-dominated ambipolar diffusion, as expected for high-purity n-type material. However, for thinner structures, we find that the minority-carrier transport is time dependent, changing from ambipolar diffusion at early times, as in thick structures, to electron-dominated diffusion at later times. We show that these structures become effectively p-type modulation doped due to the relative ‘‘impurity’’ and thickness of the AlxGa1−xAs compared to the GaAs. As a result, the minority-carrier species changes from holes to electrons for decreasing GaAs layer thicknesses. Cumulatively, we show the band-to-band recombination decay kinetics and carrier transport results to be in excellent qualitative and quantitative agreement. Moreover, our results are in excellent agreement with electrical transport measurements of electron and hole mobilities. Finally, with our measured room-temperature lifetimes and minority-carrier transport measurements versus GaAs layer thickness, we accurately calculate the interface recombination velocity for these structures, with the result S∼40 cm/s, among the lowest ever reported for any GaAs/AlxGa1−xAs structure.
Supercritical bifurcation at a phase transition
Authors: Gumbs Godfrey , Kolár̆ Miroslav
Published: July 2002

The time evolution of the spin distribution for the XY model in mean-field theory is described by the Fokker–Planck transport equation. This equation is Fourier transformed to generate an infinite-dimensional nonlinear dynamical system. The otherwise well-known behavior of the XY model is then explained using a fully dynamical-systems approach. The evolution of the phase-space trajectories of this dynamical system from initial nonequilibrium states is obtained. Regardless of the initial spin distribution, its attractors (all are point attractors) determine the equilibrium magnetic state of the model. The phase transition at a critical temperature is accompanied by a supercritical bifurcation. Numerical results for the time dependence of the phase-space trajectories and the order parameter are presented.
Displacement current and multiple pulse effects in plasma source ion implantation
Authors: Wood Blake P.
Published: July 2002

In plasma source ion implantation (PSII), a target to be implanted is immersed in a weakly ionized plasma and pulsed to a high negative voltage. Plasma ions are accelerated toward the target and implanted in its surface. In this article, two factors in the analysis of these discharges are examined for the first time: (1) displacement current across the expanding sheath results in increased implant current and decreased implanted ion energy, with respect to existing models; and (2) ion depletion around the target due to high pulse repetition rates results in decreased implant current and dose. These effects are studied with analytic models and particle-in-cell simulations. Simulation results are compared to previously published PSII models.
Multistaging in free-space laser particle accelerators
Authors: Steinhauer L. C. , Kimura W. D.
Published: July 2002

One important issue with regard to acceleration of electrons in free space using intense laser beams is the phase slippage of the electrons relative to the electromagnetic field. This arises from a phase velocity mismatch between the electron and light wave. Left uncontrolled this slippage can result in degradation of the e-beam characteristics (e.g., emittance) and dispersion of the electron bunches. By a method similar to microwave accelerators, multistaging offers a means to control the e-beam evolution by tuning each successive stage. The phase of the electron bunches relative to the laser field at the entrance of each section determines the acceleration and/or focusing that ensues in that section; therefore, the entrance phase is a natural tuning parameter. It is shown that by controlling the entrance phase it is possible to preserve the e-beam quality, both transverse (emittance) and longitudinal (bunching, energy spread). Calculations of the longitudinal and transverse beam dynamics are performed to determine the evolution of a finite-emittance e beam from stage to stage. By this method the conditions on entrance phase that allow successful e-beam trapping are found. It is also shown that conditions that assure e-beam trapping automatically preserve the overall beam quality.
Control of powder formation in silane discharge by cathode heating and hydrogen dilution for high-rate deposition of hydrogenated amorphous silicon thin films
Authors: Banerjee Ratnabali , Sharma S. N. , Chattopadhyay S. , Batabyal A. K. , Barua A. K.
Published: July 2002

Hydrogenated amorphous silicon (a-Si:H) films have been deposited at high growth rates by increasing the rf power density while the optoelectronic quality of the films has been concurrently taken care of by controlling powder formation due to gas-phase polymerization in the plasma. This has been achieved by heating the cathode together with the anode in the capacitive coupling arrangement and keeping the cathode temperature close to that of the anode. This, together with hydrogen dilution of the source gas, has been used to control powder formation in the silane discharge. The films have been evaluated by optical and infrared vibrational spectroscopy, dark conductivity, secondary photoconductivity, and internal quantum efficiency measurements.
Synthesis of x-ray intensity profiles for x-ray optical systems with curved diffractors
Authors: Chang W. Z. , Wittry D. B.
Published: July 2002

A procedure for synthesizing the x-ray intensity profile for x-ray optical systems employing curved diffractors is presented. The synthesis takes account of the effects of misalignment, the solid angle, and the convergence or divergence of the diffracted x rays. Broadening effects due to finite source size, wavelength spread, and presence of polychromatic radiation have also been included. The broadening effects on the intensity profile are demonstrated analytically. By comparing experimental results with results from the synthesis method, curved diffractor’s parameters (rocking curve width and peak reflectivity) were quantitatively evaluated for mica, silicon, graphite, and LiF crystals at three x-ray wavelengths.
Time dependence of radiation-induced generation currents in irradiated InGaAs photodiodes
Authors: Shaw G. J. , Walters R. J. , Messenger S. R. , Summers G. P.
Published: July 2002

The annealing behavior of the reverse bias current-voltage curves of 1 MeV electron irradiated In0.53Ga0.47As photodiodes has been measured at 300 K. The observed decay is shown to be correlated with the reduction of the E2 peak height with time, as measured by deep level transient spectroscopy. The reverse current is found to decay with a logarithmic time dependence, which can be explained by a model in which the annealing of the E2 defects is controlled by a distribution of thermal energy barriers.
Laser-stimulable transparent KCl:Eu crystals for erasable and rewritable optical memory utilizing photostimulated luminescence
Authors: Nanto Hidehito , Murayama Kazuhiko , Usuda Takayuki , Endo Fumitaka , Hirai Yoshiaki , Taniguchi Shin-ichi , Takeuchi Nozomu
Published: July 2002

Intense photostimulated luminescence (PSL) with a peak at 420 nm is observed in ultraviolet (UV)-light-irradiated europium-doped potassium chloride (KCl:Eu) single crystals. The PSL characteristics of UV-irradiated KCl:Eu for optical memory application are studied. The excitation and emission mechanisms of the 420 nm PSL, which are consistent with the results obtained, are discussed.
Investigation of the oxidation kinetics of NiSi2 on (111)Si by transmission electron microscopy
Authors: Huang G. J. , Chen L. J.
Published: July 2002

A transmission electron microscopy study of oxidation kinetics of NiSi2 for both dry and wet oxidation has been carried out. Care was taken to determine the activation energies of oxidation in the temperature and time regime where the islanding of NiSi2 did not occur. For dry oxidation, activation energies for parabolic and linear growth were found to be 1.87 and 1.94 eV (with an error bar of ±0.1 eV), respectively. On the other hand, activation energies for parabolic and linear growth were found to be 1.72 and 1.59 eV (with an error bar of ±0.1 eV), respectively, for wet oxidation. The activation energy of parabolic rate constant is seen to be substantially different from those obtained previously. The difference is attributed to the occurrence and absence of islanding during oxidation in the previous and present study, correspondingly. Compared to the oxidation of TiSi2 and pure silicon, a model based on the dominant diffusing species through silicide, i.e., metal and Si for NiSi2 and TiSi2, respectively, is proposed to explain the substantial difference and closeness in linear activation energies of wet oxidation between NiSi2, TiSi2, and pure silicon, respectively.
Model of the microwave response in granular Bi-Sr-Ca-Cu-O thin films
Authors: Grabow B. E. , Boone B. G. , Sova R. M.
Published: July 2002

A model used to approximate the measured response of a Bi-Sr-Ca-Cu-O detector to both microwave frequency and HeNe laser light over a temperature range from zero up to the critical transition temperature of the superconductor Tc is reported. The nonbolometric (microwave) response model is based on microwave enhancement of a thermal fluctuation voltage occurring in the networks of inherent Josephson junctions. Modeling the film as a distribution of granular connections with varying critical currents and temperatures yields results similar to those observed in recent microwave response measurements. These results may be useful when optimizing the nonbolometric response for detector applications.
Carbon incorporation in GaAs and AlxGa1−xAs layers grown by molecular-beam epitaxy
Authors: Giannini C. , Gerardi C. , Tapfer L. , Fischer A. , Ploog K. H.
Published: July 2002

GaAs:C and AlxGa1−xAs:C films, grown by solid-source molecular-beam epitaxy with doping levels beyond 1019 cm−3, have been studied by high-resolution double-crystal x-ray diffraction, Hall-effect measurements, and secondary-ion-mass spectroscopy (SIMS). Comparison between x-ray diffraction and Hall-effect data indicate that carbon is preferentially incorporated as acceptor on As lattice sites both in the GaAs:C and in the AlxGa1−xAs:C films. It was found that the higher the AlAs mole fraction the higher is the concentration of carbon incorporated on As sites (CAs). Moreover, SIMS results showed that the total amount of carbon in the host lattices largely exceeds CAs. Our findings are explained by supposing that carbon atoms are incorporated on As sites and on interstitial sites. Furthermore, it is shown that the carbon interstitial concentration can be reduced growing at higher arsenic flux and higher substrate temperature in GaAs:C as well as in AlxGa1−xAs:C layers.
Theoretical calculations of energy transfer from noble gases to surfaces
Authors: Luo Z. , Hou Q.
Published: July 2002

Using the bipartition model of ion transport in a solid, we have calculated the energy reflection coefficients R(E) or equivalently the energy deposition coefficients F(E) for the ion-target combinations of He-Si, He-Cu, He-Ag, He-Au, Ar-Ag, Ar-Au, and Xe-Au. To test the validity of the theoretical method, we compared our results with corresponding experimental data and Monte Carlo data. The comparison shows that the results of the bipartition model reach a reasonable agreement with experimental data and Monte Carlo data.
A steady-state model for a low-pressure Cs-Ba diode
Authors: Murray Christopher S. , El-Genk Mohamed S. , Wernsman Bernard , Kaibyshev Vladimir Z.
Published: July 2002

A steady-state model of a low-pressure Cs-Ba discharge is developed. It calculates the plasma density, electron temperature, Cs atom densities, and output voltage, assuming uniform conditions across the discharge gap and neglecting radiation losses. The model predictions are in good agreement with experimental data for a low-pressure Cs discharge. Comparing the model to experimental data from a Cs-Ba diode reinforce the validity of the system of equations and also confirms the assumption that barium has practically no effect on the current transport characteristics. The calculated I-V curves for the Cs-Ba diode match well with the experimental data below the emission current; however, above the emission current the calculated currents are much lower. This difference occurs when the model incorporates normal Schottky emission instead of anomalous Schottky. The effect of the latter is determined from estimates of the effective changes in the emitter work function for agreement between the model and experiments at high discharge currents. For such currents, results suggest that changing the Cs pressure strongly affects the anomalous Schottky emission, while changes in the Ba pressure seem to have a negligible effect.
Effects of nitridation temperature on the electron trap characteristics of nitrided-oxide metal-oxide-semiconductor capacitors
Authors: Fleischer S. , Lai P. T. , Cheng Y. C.
Published: July 2002

In this work the effects of nitridation temperature on trap parameters have been studied. Using a high-field technique, no significant change in capture cross section (σ) could be seen with all devices having σ∼10−15 cm2, indicating the traps are neutral. Trap energies, extracted using a novel technique based on a simple trap-assisted tunneling model, were found to be ∼2.7 eV for nitridation temperatures below 1100 °C, falling to ∼2.4 eV above this temperature. Trap densities, also extracted using the same method, followed a similar trend.
Carrier mobility degradation in metal-oxide-semiconductor field-effect transistors due to oxide charge
Authors: Phanse A. , Sharma D. , Mallik A. , Vasi J.
Published: July 2002

Radiation causes oxide charge buildup which can degrade carrier mobility in the inversion layer of a metal-oxide-semiconductor field-effect transistor (MOSFET). An expression for the carrier mobility in MOSFETs due to oxide charge scattering has been derived. The model predicts the mobility degradation given any specified charge density profile in the oxide. It accounts for screening of oxide charges by channel carriers. To validate the proposed model we performed an experiment to place a measured quantity of charge at a definite position in the gate oxide and then measured the mobility degradation. The experimental results are in good agreement with the predictions of the model.
Barrier height temperature dependence of Au/InP from photoreflectance spectroscopy
Authors: Wang D. P. , Shen T. L.
Published: July 2002

The barrier height of Au/InP was measured at various temperatures from Franz–Kelydsh oscillations (FKO) of photoreflectance spectroscopy. It appears that the photoinduced voltage can not be neglected especially at low temperature. The presently observed barrier height temperature dependence confirms the theory of Hecht [M. Hecht, Phys. Rev. B 41, 7918 (1990)]. Also, the mixture between FKO and E0+Δ0 oscillations was observed. The degree of mixture varies with temperature.
Analysis of the electronic properties of CoCrPt thin films using parallel electron energy loss spectroscopy (PEELS)
Authors: Glijer Pawel , Sivertsen John M. , Judy Jack H.
Published: July 2002

The effects of the Pt content in CoCrPt thin films on the appearance of the near-edge structure of the Cr and Co L2,3 absorption edges were studied using parallel electron energy loss spectroscopy (PEELS). These reflect local changes in the 3d band of the material. It was found that addition of Pt leads to local fluctuations in the 3d band occupancy in CoCrPt films which causes a decrease of the number of 3d electrons per Co atom. An increase of the L3 to L2 white lines intensities ratio on the L2,3 Co edge with Pt content probably indicates an increase of the magnitude of exchange interactions in the CoCrPt thin films.
Time-resolved scanning probe microscopy: Investigations and applications of dynamic magnetostriction (invited)
Authors: Nunes G. , Freeman M. R.
Published: July 2002

We have measured the magnetostrictive response of small nickel and Tb0.3Dy0.7Fe1.95 cylinders to sudden step changes in applied magnetic field with a time-resolved scanning fiber interferometer. Spatially resolved measurements reveal the nonuniform distortion of the face of the rod as the induced magnetization propagates in toward the center. A crossover is observed, as a function of decreasing cylinder diameter, from eddy current- to longitudinal phonon-limited response times. The ability to characterize dynamic magnetostriction in small samples has also enabled us to perform time-resolved measurements of fast transient signals with a scanning tunneling microscope. Modulation of the tip-sample separation on short time scales is provided by the use of a nickel magnetostrictive tip driven by a local magnetic field coil. The technique offers a broadly applicable means of obtaining fast dynamical information in tunneling microscopy, and is demonstrated here on nanosecond time scales. These experiments illustrate, in the context of magnetics, the general promise of time-resolved scanning probe methods.
Ground state and spin dynamics in hexagonal antiferromagnet CsCuCl3 (abstract)
Authors: Stefanovskii E. P. , Sukstanskii A. L.
Published: July 2002

Modulated magnetic structure in CsCuCl3 was obtained by the neutron diffraction technique and theoretically discussed in Refs. by means of the model including Dzyaloshinskii–Moriya and exchange interactions as well as sixth order anisotropy in the basal plane. The present abstract aims at presenting some new aspects of a magnetization distribution in the ground state of the system and more adequate theory of its dynamic properties. We described the so-called ‘‘space mutation’’ effect caused by an external magnetic field parallel to the hexagonal axis. Sufficiently large hexagonal anisotropy results in appearing of the soliton lattice structure and enables us to explain nonmagnetic resonance (NMR) experiments. Dynamic properties of the magnet were investigated by means of the effective Lagrange function method using Cartan differential forms. The spin wave spectrum and the magnetic susceptibility tensor were obtained. If, the hexagonal anisotropy being neglected, the spectrum was shown to consist of three branches in contrast to Ref. where there was a single branch only, antiferromagnetic (AFMR) frequencies dependence on an external magnetic field occurred to be in a good agreement with experimental data. Taking into account the hexagonal anisotropy, the usual zone structure of the spectrum was obtained.
Enhanced coercivity due to a local anisotropy increase
Authors: Jatau J. A. , Pardavi-Horváth M. , Della Torre E.
Published: July 2002

In most of the cases, a local wall energy decrease, usually associated with defects, leads to domain wall pinning. Another mechanism for coercivity is to create local maxima in the anisotropy energy. It is observed that in epitaxial garnet crystals, locally enhanced anisotropy due to stress arising from iridium inclusions embedded in the substrate is the source of coercivity. By modifying our previous energy barrier model, the coercivity has been calculated as a function of the size and position of the iridium inclusions in the substrate. It is shown that the coercivity varies nonlinearly with inclusion size and depth in the substrate material, in agreement with experimental observation.
A detailed study of nitride precipitates in Nd2Fe17
Authors: Colucci C. C. , Gama S. , Ribeiro C. A. , Cardoso L. P.
Published: July 2002

We report electron microprobe measurements of the diffusion profile in partially nitrided samples of the Fe17Nd2 phase. The results are fully consistent with a step-like profile. This confirms that the nitrogenation process is one of precipitating a fully nitrided phase from a nitrogen-free phase. We report also x-ray diffraction patterns for the partially nitrided samples as a superposition of the pure and nitrided patterns, with displacements and broadenings of lines due to mutual stresses between the phases.
Three-dimensional eddy current solution of a polyphase machine test model (abstract)
Authors: Pahner Uwe , Belmans Ronnie , Ostovic Vlado
Published: July 2002

This abstract describes a three-dimensional (3D) finite element solution of a test model that has been reported in the literature. The model is a basis for calculating the current redistribution effects in the end windings of turbogenerators. The aim of the study is to see whether the analytical results of the test model can be found using a general purpose finite element package, thus indicating that the finite element model is accurate enough to treat real end winding problems. The real end winding problems cannot be solved analytically, as the geometry is far too complicated. The model consists of a polyphase coil set, containing 44 individual coils. This set generates a two pole mmf distribution on a cylindrical surface. The rotating field causes eddy currents to flow in the inner massive and conducting rotor. In the analytical solution a perfect sinusoidal mmf distribution is put forward. The finite element model contains 85824 tetrahedra and 16451 nodes. A complex single scalar potential representation is used in the nonconducting parts. The computation time required was 3 h and 42 min. The flux plots show that the field distribution is acceptable. Furthermore, the induced currents are calculated and compared with the values found from the analytical solution. The distribution of the eddy currents is very close to the distribution of the analytical solution. The most important results are the losses, both local and global. The value of the overall losses is less than 2% away from those of the analytical solution. Also the local distribution of the losses is at any given point less than 7% away from the analytical solution. The deviations of the results are acceptable and are partially due to the fact that the sinusoidal mmf distribution was not modeled perfectly in the finite element method.
Magnetoresistive characterization of thin-film structures by a gradient-field method (abstract)
Authors: Trouilloud P. L. , Suits F. , Jahnes C. V. , Russak M. A. , Spada E. J. , Chang J. W.
Published: July 2002

Magnetoresistance measurements can be used during manufacturing to characterize basic magnetic parameters at the device level. A fruitful approach is to take advantage of the gradient field generated by the current flowing through the sample. This method was employed by Smith et al. in an elegant series of experiments that led to precise measurements of exchange constant in soft magnetic thin films. In the present paper, local coupling in bilayers, anisotropy, and interface quality are investigated using magnetoresistive response to gradient fields. Permalloy samples consisting of two sputtered layers (0.1 μm thick) separated by thin SiO2 spacers were patterned in the form of long stripes parallel to the easy axis. Variation in resistance was measured as a function of external field applied in the easy-axis direction and of current density. For similar geometries, the samples have similar resistance. The curves display a sharp decrease in the resistance near zero field that is associated with the reversal of the magnetization in the stripe. Once the sample has switched, coupled layers have a flat response versus field: exchange coupling between the layers constrains the fanning of the magnetization, as previously described. For uncoupled or weakly coupled samples, the curves show curvature at higher fields, and the curvature increases markedly with current density. The fanning within each layer is constrained by exchange, but the average magnetization in one layer rotates under the influence of the field generated by the current in the other layer.
Spin-1/2 Heisenberg antiferromagnet on the square and triangular lattices: A comparison of finite temperature properties
Authors: Elstner N. , Singh R. R. P. , Young A. P.
Published: July 2002

We compare and contrast the behavior of the spin-half Heisenberg antiferromagnet on the square and triangular lattices. For both models, high temperature expansions are obtained for the structure factor at the ordering vector, the correlation length, the uniform susceptibility, and the entropy. We show that the antiferromagnetic correlations are significantly larger for the square lattice than for the triangular lattice. Analysis of the structure factor and the correlation length shows that the spin stiffness is an order of magnitude smaller in the triangular lattice than in the square lattice. These results indicate that the triangular lattice, although weakly ordered at T=0, is close to a quantum critical point. Thus, in contrast to the square lattice, a semiclassical spin-wave picture may not be sufficient to describe the ground state.
ac susceptibility on the dilute antiferromagnet MnxZn1−xF2 close to the percolation threshold (abstract)
Authors: Machado F. L. A. , Montenegro F. C. , Montarroyos E. , de Jesus J. C. O. , Rosales-Rivera A. , Rezende S. M.
Published: July 2002

ac susceptibility measurements have been performed on the dilute antiferromagnet Mn0.35Zn0.65F2 in the absence of an external uniform field. For a driving field of amplitude Hac∼1.0 Oe applied parallel to the easy [001] direction, a frequency-dependent susceptibility has been observed at low temperatures. When the driving field is applied perpendicular to this direction, a departure from the Curie–Weiss behavior indicates that some spin clusters are spatially oriented at low temperatures. The presence of these randomly oriented magnetic moments may cause the random-exchange Ising model to be inappropriate1 in explaining the zero-field behavior near to the percolation concentration xp. A random-anisotropy model may be useful to explain the magnetic features of MnxZn1−xF2 close to xp.
Elongated superparamagnetic particles
Authors: Aharoni Amikam
Published: July 2002

The relaxation time for superparamagnetic behavior is calculated for prolate spheroidal particles with a uniaxial, crystalline anisotropy superimposed on their shape anisotropy. Possible applications to the analysis of some experiments, in particular for the case of quantum tunneling, are discussed.
Core-level magnetic circular dichroism in 3d and 4f magnetic systems (invited) (abstract)
Authors: Koide T.
Published: July 2002

With the recent availability of circulary polarized synchrotron radiation over a wide photon energy range from VUV to hard X rays, the magnetic circular dichroism (MCD) in core-level photoabsorption has rapidly attracted growing interest, both experimentally and theoretically. This novel technique can provide element-specific and site-selective information about the magnetic and the electronic states in various magnetic substances because the core-level MCD process involves optical transitions in which the one-electron initial states are well localized and have well-defined angular momenta. In order to get insight into the local magnetic states in 3d and 4f magnetic systems, we have studied MCD of ferrites, Fe1−xPtx alloys, and mixed-valence CeRh3B2 at the core-absorption edges in the VUV∼soft x-ray region. The experiments were performed by utilizing directly characterized, circularly polarized undulator radiation and off-plane synchrotron radiation1 in conjunction with an ultrahigh vacuum compatible superconducting magnet of special design.2 Clear MCD signals were observed for CeRh3B2 in the prethreshold region of the Ce 4d→4f (N4,5) edges. A comparison of the experimental MCD spectrum with theoretical ones3 for uniaxial crystal fields of Δc=0 and 0.2 eV shows that the experimental spectrum qualitatively agrees with the theoretical one for Δc=0 eV. Theory predicts that the MCD pattern for Δc<0.14 eV will be essentially the same as that for Δc=0 eV. Considering a Δc value of 0.1–0.2 eV, which was determined from the linear dichroism measurement,4 the present result leads to the conclusion that Δc∼0.1 eV. With a model based on the strong hybridization between the neighboring Ce 4f0(lz=0) and Ce 5d0(lz=0) orbitals together with the value of Δc∼0.1 eV, we can successfully explain many of the extraordinary magnetic properties of CeRh3B2. We will also present the MCD data in the M2,3 core-absorption region for ferrites (Fe3O4 and CoFe2O4) and Fe1−xPtx alloys, discussing the results.
Soft x-ray magneto-optical Kerr effect (invited) (abstract)
Authors: Kao C. C. , Chen C. T. , Johnson E. D. , Idzerda Y. U. , Hastings J. B.
Published: July 2002

Recently, there has been a great deal of interest in x-ray magnetic circular dichroism (MCD) and its applications to the study of magnetic thin films and multilayers due to the dramatically enhanced dichroic effect and its element specific nature. In this work, the soft x-ray magneto-optical Kerr effect, an effect closely related to MCD, is investigated. Detailed comparison between MCD and the Kerr effect is discussed. As an example, recent results from a thin bcc Co film will be presented. In the Kerr effect measurement, specular reflectivity as a function of incident angle and photon energy was measured using both linearly polarized and circularly polarized x rays. To give the largest effect, in the case of linearly polarized light, specular reflectivity was measured in the transverse configuration, while in the case of circularly polarized light, the measurement was carried out in the longitudinal configuration. In both cases very large changes in reflectivity, up to 40% in the linearly polarized case and 75% in the circularly polarized case, were observed near the Co LII and LIII absorption edges upon reversal of the direction of the magnetic field. These results agree very well with a macroscopic model calculation.
Application of the spreading of damage technique to the S=1/2 Ising thin film
Authors: Rojdestvenski I. V. , Costa U. M. S.
Published: July 2002

We apply the spreading of the damage technique to a S=1/2 Ising ferromagnetic thin film. We use Metropolis dynamics to calculate the temperature dependence of the Hamming distance as well as the Hamming distance distributions along the Markov chain. We suggest a modification of the algorithm for calculating the Hamming distance in the case of Metropolis dynamics and we discuss the results for the two-dimensional model obtained by the modified approach. For a film with three layers we present the thermal dependences of the above quantities. We also suggest a new way to introduce damage to the system as the difference in the boundary conditions. We discuss the results of this new approach for the two-dimensional model.
Thermodynamical properties of a Heisenberg model with Dzyaloshinski–Moriya interactions
Authors: Lacerda F. , Ricardo de Sousa J. , Fittipaldi I. P.
Published: July 2002

Within the framework of a new correlated effective-field theory (CEF) the effects of the Dzyaloshinski–Moriya (DM) interactions on magnetic properties of the spin-1/2 anisotropic Heisenberg model are discussed. The CEF theory is based on a generalized but approximate Callen–Suzuki spin relation for cluster with two spins, and makes use of the Honmura–Kaneyoshi exponential operator technique. The phase diagram and the thermal behavior of magnetization are analyzed for the simple cubic lattice, and compared with the corresponding two-spin cluster mean-field (MFA) predictions. It is shown that for the easy direction (D=Dz; where D is the DM vector coupling), the model exhibit a tricritical point (TCP), at which the phase transition changes from second to first order. The TCP is explicitly obtained, and the tricritical temperature, Tt, is independent of the exchange anisotropy parameter Δ (Δ=0 and Δ=1, correspond the isotropic Heisenberg and Ising models, respectively), while the tricritical parameter, Dt, has dependence on Δ. In spite of its simplicity, the present CEF formalism yields results, which represent a remarkable improvement on the usual MFA treatment.
Coexistence of Brillouin and Van Vleck spin exchange in Zn1−xMnxSe/Zn1−yFeySe spin superlattice structures
Authors: Jonker B. T. , Abad H. , Fu L. P. , Yu W. Y. , Petrou A. , Warnock J.
Published: July 2002

We have studied a new type of semiconductor quantum well system in which the carrier-ion exchange interactions are determined by the spin of the carrier. The samples consist of alternating layers of Zn1−xMnxSe and Zn1−yFeySe grown by molecular-beam epitaxy (MBE). At zero field, the carriers initially interact randomly with both transition metal species. When a magnetic field is applied, the excitonic wave functions are increasingly localized in one or the other of the magnetic layers according to their spin state as the competing spin exchange interactions define the confining potential. The spin components of the heavy hole exciton are subsequently dominated by different exchange interactions as revealed by their temperature and field dependence: the behavior of the spin-down component (−3/2,−1/2) is described by exchange interactions of the carriers with the Mn2+ ions and exhibits Brillouin paramagnetic behavior, while the spin-up component (+3/2,+1/2) is dominated by interactions with Fe2+ ions and exhibits Van Vleck paramagnetism. These structures are thus characterized by an initial competition and eventual coexistence of Brillouin- and Van Vleck-like paramagnetic behavior for the exciton.
Direct imaging of track edge fringing fields on recorded media
Authors: Gomez R. D. , Mayergoyz I. D. , Burke E. R.
Published: July 2002

The capability to image individual vector components of surface magnetic fields locally has been developed recently, and this component-selective technique has been applied to understand the field distribution of recorded patterns. Apart from the commonly measured in-plane component in the direction of the track Hx, a cross-track component Hy has been observed along the edges. While the Hx field images show side written patterns extending laterally from the edges, the Hy field images reveal localized bright and dark spots on both sides of the track. This suggests that pairs of mutually opposite magnetic dipole layers persist on both edges which are oriented transverse to the track direction. A systematic investigation was made of these side fringing fields to understand their behavior as a function of recording wavelength.
Experimental Preisach analysis of the Wohlfarth relation (abstract)
Authors: Vajda Ferenc , Della Torre Edward
Published: July 2002

The Wohlfarth relation1 states that, for noninteracting particles, the ascending normalized remanent major loop, mD(H), is twice the shifted normalized virgin remanent magnetization. Researchers2 claim to measure the interaction field and characterize it as ‘‘positive’’ and ‘‘negative’’ depending on whether it is ‘‘above’’ or ‘‘below’’ the axis on a Henkel plot or a ΔM plot. It has been shown3 that the moving Preisach model can compute all observed types of ΔM plots by varying only the moving parameter, α, and over only positive values. This type of behavior is characterized by the ratio of α to the standard deviation of the interaction field. In this paper the measured ΔM plot for two commercial recording media, namely longitudinal BaFe and γ-Fe2O3, is presented and the positive moving parameter of each sample is identified. Let us define the operative field of the system by Hop=H+αM, where M is the total magnetization. It is then shown that, when plotted as a function of Hop, all samples deviate from the Wohlfarth relation in the same way, even though they exhibit ‘‘positive’’ and ‘‘negative’’ ΔM when plotted as a function of the applied field, H, as seen in Fig. 1. It will also be shown that deviations from the Wohlfarth relation are due to a combination of parameters, not only the interaction field. The model and the identification algorithm has been used to simulate all the observed ΔM plots.
Brillouin light scattering study of spin wave instability magnon distributions in yttrium iron garnet thin films (abstract)
Authors: Kabos P. , Wiese G. , Patton C. E.
Published: July 2002

There has been increased interest recently in nonlinear microwave phenomena in bulk and thin film single crystal ferrite materials. These nonlinear phenomena show a variety of novel effects such as spin wave instability, auto-oscillation, multistability, period multiplication, intermittency, and chaos. Emerging theories to explain these phenomena have generally been based on simple two mode models. It is clear that an experimental determination of the number and distribution of the modes excited at and above the spin wave instability threshold could play a key role in the refinement of these theories. This work is concerned with a Brillouin light scattering (BLS) experimental study of the wave vector distribution of the parametric spin wave modes excited at and above the subsidiary absorption spin wave instability threshold in yttrium iron garnet (YIG) films. The in-plane magnetized 4.15-μm-thick films were transverse pumped at 8.47 GHz, with the microwave field also in-plane. The BLS data were obtained with a tandem, multipassed, high contrast Sandercock Fabry–Perot interferometer. The data show a complicated wave vector distribution for the excited modes. The spin wave propagation direction is sharply peaked at the value expected from standard instability theory. However, the wave number distribution is quite broad, even at threshold. For power levels above threshold, this distribution evolves in a complicated manner that depends on field and propagation direction. These results show that simple two mode models are inadequate to deal with high frequency nonlinear processes in ferrite thin films.