bluuebunny/crossref_metadata_2025 huggingface dataset:
Luminescence method for the study of Nd3+ ions diffusion in AgBr crystals
Authors: Nagli L. , German A. , Katzir A.
Published: July 2002

This study of diffusion of Nd in AgBr is motivated by our interest in the preparation of Nd-doped AgBr crystals that show lasing effects in the near and midinfrared. The doping can be done in several ways. We propose that monitoring the infrared luminescence along the direction of diffusion of activator ions in crystals of AgBr is a sensitive and minimally invasive method of determining the diffusion coefficient, the diffusion enthalpy, and the temperature dependence of these parameters for ions of interest. We have used this technique to study the diffusion of Nd3+ in AgBr crystals. To establish the concentration range in which the luminescence method is useful, the dependence of the luminescence kinetics on the Nd-ion concentration was investigated. The temperature dependence of the diffusion coefficient (D0=1.2×10−5 cm2/s at room temperature), and the diffusion enthalpy (H=0.51 eV) were determined. We found that high values of the diffusion coefficient and low values of the diffusion enthalpy facilitate preparation of high optical quality Nd-doped AgBr crystals. Such crystals are candidates for lasing action.
High resolution electron microscopy study of molecular beam epitaxy grown CoSi2/Si1−xGex/Si(100) heterostructures
Authors: Buschmann V. , Rodewald M. , Fuess H. , Van Tendeloo G. , Schäffer C.
Published: July 2002

Two CoSi2/Si1−xGex/Si(100) heterostructures, with different Ge content, made by molecular beam epitaxy are characterized by high resolution electron microscopy. In general, the interface between the CoSi2 thin film and the Si1−xGex layer is of a high structural quality and the strained Si1−xGex layer exhibits few defects. For both samples, different interface structures are present, although the dominant interfacial configuration is similar to the unreconstructed interface present at the CoSi2/Si(100) interface. Only occasionally (2×1) reconstructed interface regions are found which are just a few nanometers in length. Phenomena such as Ge segregation and the introduction of defects are also observed in the Si1−xGex layer. We attribute the minimal presence of the reconstructed interface to both the (2×8): Si1−xGex(100) surface reconstruction and the Ge segregation that takes place.
Raman spectroscopy and x-ray diffraction study of PbTiO3 thin films prepared by sol–gel technique
Authors: Li Ai-Dong , Wu Di , Ge Chuan-Zhen , Lü Peng , Ma Wen-Hui , Zhang Ming-Sheng , Xu Cun-Yi , Zuo Jian , Ming Nai-Ben
Published: July 2002

PbTiO 3 (PT) thin films were prepared by the sol–gel technique on Si(111) single crystal, fused quartz, and NaCl substrates using two precursor solutions: methanol and 2-methoxyethanol. Raman spectroscopy and x-ray diffraction were used to determine the effects of the precursor solution, substrate, annealing temperature, and film thickness on the film structure and crystallization behavior. The results indicate that the precursor solution plays an important role in the fabrication of pure perovskite PbTiO3 films. The films deposited from methanol solution are able to form the perovskite structure at lower crystallization temperature (∼430 °C). In contrast, the films deposited from 2-methoxyethanol solution easily form the pyrochlore phase. This is ascribed to the different mechanism of hydrolysis and condensation polymerization of the two precursor solutions. The Raman scattering results on various substrates have significant and interesting differences, especially for the films deposited from 2-methoxyethanol solution. In addition, with increasing the annealing temperature and film thickness, the full width of the Raman peaks at half-maximum decreases and the intensity increases. As to the position of the peaks, most Raman modes of PT films shift to lower frequencies due to the pressure effect in the films.
Effects of CeO2 incorporation on the performance of a Ta diffusion barrier for Al metallization
Authors: Kim Jaehwa , Seop Kwak Joon , Yoon Dong-Soo , Koo Baik Hong , Lee Sung-Man
Published: July 2002

The effects of CeO2 incorporation on the performance of a Ta diffusion barrier in the Al/Si system were investigated in the temperature range of 450–550 °C. When Ta film was deposited without CeO2 incorporation, the reaction between Ta and Al occurred at 500 °C, leading to the formation of Al3Ta. In the case of CeO2-incorporated Ta barriers, however, the reaction between Ta and Al was suppressed up to 550 °C. The suppression of the reaction of Ta with Al was attributed to the strong chemical bonding of Ta–Ce–O or Ta–O and the amorphous-like microstructure of the CeO2-incorporated Ta barrier, followed by the reduction of the chemical driving force for the initial stage of Al3Ta formation.
Resonant tunneling through a diode accumulation layer
Authors: Morris Daniel P. , Price Peter J.
Published: July 2002

The phenomenon equivalent to resonant tunneling, in a double-barrier diode with a spacer layer and consequent accumulation layer on the cathode (source) side, is elucidated. The accumulation layer and the inter-barrier well each support an electron quasi-level (or several) which may be expected to combine coherently into a doublet of states that are filled by in-scattering of the source electrons and emptied by tunneling to the “anode” side. The resulting current depends on the distribution of electron density (norm) between these two component orbitals, with a peak (as a function of bias) at the “level crossing” point where the norms are each 1/2. The analysis draws on a previous computational study of Gamow-like tunneling out of a double-well system, as a model of the electron states. The in-scattering rate due to acoustic-mode phonons in particular is calculated. Corresponding diode current characteristics are obtained.
Elastic activity of the chiral medium
Authors: Ro Ruyen
Published: July 2002

Chiral materials are noncentrosymmetric due to handedness in their microstructures. The elastic field behavior in a chiral medium is readily described herein using the governing equations and constitutive relations for noncentrosymmetric, isotropic micropolar materials. Accordingly, linearly polarized longitudinal waves and left and right circularly polarized transverse waves are eigenstates for elastic waves in the chiral medium. Consequently, by using appropriate field representations along with prescribed boundary conditions, scattering characteristics at chiral interfaces can be readily determined. In this paper, transmission characteristics, i.e., magnitudes and rotation angles, of the chiral slab bounded by achiral media for normally incident transverse waves are the focus of this investigation. Simulation results show the Cotton effect in the magnitude and rotation angle curves, verifying theoretically the elastic activity of a chiral medium.
Donor-acceptor pair recombination in AgIn5S8 single crystals
Authors: Gasanly N. M. , Serpengüzel A. , Aydinli A. , Gürlü O. , Yilmaz I.
Published: July 2002

Photoluminescence (PL) spectra of AgIn5S8 single crystals were investigated in the 1.44–1.91 eV energy region and in the 10–170 K temperature range. The PL band was observed to be centered at 1.65 eV at 10 K and an excitation intensity of 0.97 W cm−2. The redshift of this band with increasing temperature and with decreasing excitation intensity was observed. To explain the observed PL behavior, we propose that the emission is due to radiative recombination of a donor-acceptor pair, with an electron occupying a donor level located at 0.06 eV below the conduction band, and a hole occupying an acceptor level located at 0.32 eV above the valence band.
Hot free-electron absorption in nonparabolic III–V semiconductors at mid-infrared wavelengths
Authors: Shkerdin G. , Stiens J. , Vounckx R.
Published: July 2002

A quantum mechanical model based on the second order perturbation theory was constructed for the calculation of the free-electron absorption coefficient in nonparabolic III–V semiconductors. The implemented model allows for the calculation of the absorption changes when the free-electron gas temperature differs from the lattice temperature. Several mechanisms, which assist in the photon absorption process, were taken into account. At the considered lattice and electron temperature range and doping concentrations, the most important scattering mechanisms are impurity scattering, thermal and hot longitudinal optical phonon scattering, and finally acoustic phonon scattering. For all the interaction potentials we included the effect of screening by the conduction electrons. The model was developed in a fully consistent nonparabolic way. The electron dispersion relation as well as the interaction probabilities feature nonparabolic effects. Computations are performed for GaAs, InAs, and InSb at different mid-IR wavelengths, doping concentrations, and lattice and electron temperatures. Our nonparabolic hot-electron model is validated with experimental results available in the literature. It turns out that our model is much more accurate and consistent than other more relaxed models. The competition between different hot free-electron absorption mechanisms is discussed.
X-ray scattering from magnetic, rough surfaces
Authors: Osgood R. M. , Sinha S. K. , Freeland J. W. , Idzerda Y. U. , Bader S. D.
Published: July 2002

We present expressions for the resonant magnetic x-ray scattering from rough surfaces; using our formulas we analyze the structural and magnetic roughnesses, as well as the correlation between them. We demonstrate that the leading contribution to the difference in the diffuse scattering between left- and right-circularly polarized light for a rough surface with in-plane magnetization vanishes unless the structural and magnetic roughnesses are correlated. The effects of magnetic domain structure and magnetic dead layers on the surface scattering are also discussed.
A unifying view on some experimental effects in tapping-mode atomic force microscopy
Authors: Marth M. , Maier D. , Honerkamp J. , Brandsch R. , Bar G.
Published: July 2002

Several experimental effects that occur in tapping-mode atomic force microscopy are examined: apparent hysteresis effects in force probes and frequency sweeps and distortions in imaging. It is found through examinations of the phase space that they can all be reduced to one common cause: the existence of more than one stable state of the tip vibration for certain parameters. It is shown that the experimental effects can be explained considering only the phase space under the assumption that measurement noise exists. Numerical simulations confirm the theoretical and experimental findings.
Simulation of ZnSe-based self-electro-optic effect devices
Authors: Merbach D. , Schöll E. , Gutowski J.
Published: July 2002

We present computer simulations of self-electro-optic effect devices in different circuit environments, based on the quantum confined Stark effect in wide-gap materials with strong excitonic features. Our work is founded on a microscopic model of the electric field dependent absorption in ZnCdSe/ZnSSe quantum well structures, taking full account of Coulomb induced intersubband coupling and strong excitonic effects, which is essential for all wide-gap materials and distinguishes our theory from standard models of III–V compounds. Optical bistability and even multistability are predicted from the electro-optical and optical input–output characteristics for a wide range of operating conditions. The dependence upon the optical frequency, bias voltage, length of the waveguide and quantum well width, and possible optimization of the performance of the electro-optic modulator are discussed.
Pd
2
Si
-assisted crystallization of amorphous silicon thin films at low temperature
Authors: Lee Seok-Woon , Lee Byung-Il , Kim Tae-Kyung , Joo Seung-Ki
Published: July 2002

An ultrathin palladium layer was deposited on top of amorphous silicon films and the crystallization enhancement of the underlying amorphous silicon films was observed. The crystallization temperature was lowered down to 350 °C while that of intrinsic silicon films is around 600 °C. The degree of enhancement was found to be dependent on the thickness of the palladium layer as well as the annealing temperature. From the microstructure analysis, the formation of Pd2Si precipitates in the amorphous silicon was observed at the initial stage of crystallization and after further annealing, the crystallization of amorphous silicon was observed with the simultaneous split of the preformed Pd2Si precipitates into many small pieces. The mechanism of this abnormal phenomenon is discussed with the theory of epitaxial growth.
Thermal stability of Mo–V epitaxial multilayers
Authors: Dudás A. , Langer G. A. , Beke D. L. , Kis-Varga M. , Daróczi L. , Erdélyi Z.
Published: July 2002

By use of transmission electron microscopy and x-ray diffraction the degradation of the structure of epitaxial crystalline Mo–V multilayers has been studied during annealing between 1000 and 1250 K. In low vacuum (10−4 mbar), first, a polygonization (driven by the elastic mismatch due to the oxidation of V) took place, which led to a grain size in the order of the thickness of the individual layers (i.e., 1–2 nm). At longer annealing times, a partial recrystallization of Mo—parallel with a grain-boundary assisted discontinuous structural transformation—was observed. The (200) texture was preserved during the degradation process. At low temperatures and in low vacuum, this process was fast, while with improving the vacuum (up to 10−7 mbar) the polygonization was slower and above 1200 K the bulk diffusion controlled intermixing was observed with a final state of completely homogeneous solid solution.
Electron-induced gasification reactions in the fabrication process on graphite surface using scanning tunneling microscopy
Authors: Wu Ji , Wang Chen , Qiu Xiaohui , Shang Guangyi , Wang Naixin , Bai Chunli
Published: July 2002

We have studied the dependence of nanofabrication on the pulse duration explicitly with scanning tunneling microscopy, using graphite as an example. It is demonstrated that, under comparable conditions, the depth of the as-generated craters has monotonic correlation with the pulse duration, while the apparent surface diameters do not show significant changes. This is believed to be direct evidence that the electron-induced gasification reaction of the carbon atoms did occur during fabrication, rather than field evaporation mechanism. The removal rate of the carbon atoms is estimated to be on the order of 105/s. In addition, the characteristics of the fabrication is shown to be related to the specific reaction type.
Thermally induced conduction type conversion in n-type InP
Authors: Fung S. , Zhao Y. W. , Beling C. D. , Xu X. L. , Sun N. F. , Sun T. N. , Chen X. D.
Published: July 2002

n-type semiconducting InP is changed into p-type semiconducting by short time annealing at 700 °C. Further annealing for a longer time leads to a second conduction-type conversion changing the material back to n type again but with a much higher resistivity. These conduction conversions indicate the formation of both acceptor and donor defects and the progressive variation of their relative concentrations during annealing.
Enhancement of mercuric iodide detector performance through increases in wafer uniformity by purification and crystal growth in microgravity
Authors: Steiner Bruce , van den Berg Lodewijk , Laor Uri
Published: July 2002

Wafers from mercuric iodide crystals grown in microgravity on two occasions have previously been found to be characterized by a higher hole mobility-lifetime product, which enables energy dispersive radiation detectors with superior resolution. In the present work, we have identified the specific structural modifications that are responsible for this enhanced performance. As a result of this study, the performance of terrestrial wafers also has been improved but not yet to the level of wafers grown in microgravity. High resolution synchrotron x-ray diffraction images of a series of wafers, including those grown both in microgravity and on the ground, reveal two principal types of structural changes that are interrelated. One of these, arrays of inclusions, affects performance far more strongly than the other, variation in lattice orientation. Inclusions can be formed either from residual impurities or in response to deviations from ideal stoichiometry. The formation of both types is facilitated by gravity-driven convection during growth. As the level of inclusions is reduced, through growth from material of higher purity, through the achievement of balanced stoichiometry, or by suppression of convection mixing during crystal growth, the hole mobility-lifetime product is enhanced in spite of an accompanying decreased uniformity in lattice orientation. Sixfold enhancement in the performance of x- and γ-ray detectors has been accomplished to date. Further augmentation in performance appears likely.
Three-dimensional tree constructs of “constant” thermal resistance
Authors: Neagu M. , Bejan A.
Published: July 2002

This article extends to three-dimensional heat flow the constructal method of minimizing geometrically the thermal resistance between a heat-generating volume and one point. Optimized is the geometry of each volume element, and the shape and distribution of high-conductivity inserts. The new feature is the maximization of the amount of heat-generating material that operates at temperatures close to the hot-spot level (Tmax). Volume elements and subsequent constructs acquire optimal shapes where all the external surfaces are isothermal at Tmax. The same, constant thermal resistance separates each surface point (Tmax) and the common heat-sink point (Tmin). The optimized architecture is pine-cone-like, with high-conductivity nerves and low-conductivity filling (and heat-generating) material. The similarities between the constant-resistance structures and the three-dimensional tree networks found in nature are discussed. The analogy between evolutionary flow systems and evolutionary mechanical support systems is reasoned based on the same (constructal) principle of pursuing objective (purpose) subject to global and local constraints.
Photoinduced alignment of nematic liquid crystal on the polymer surface microrelief
Authors: Parfenov A. , Tamaoki N. , Ohnishi S.
Published: July 2002

A polymer film surface modified by laser illumination was used as a command surface for nematic liquid crystal (LC). Azobenzene-containing polyurethanes were taken for the material of the film. A relief grating inscribed by laser illumination onto the surface of these polymers induced a reorientation of the LC director from the initial homeotropic alignment. Possible applications of the described processes include command-surface creation for liquid-crystal displays, reversible and adaptable liquid-crystal structures for laser-beam targeting and aiming, etc.
Nanoscale compositional fluctuations in multiple InGaAs/GaAs quantum wires
Authors: Catalano M. , Taurino A. , Lomascolo M. , Vasanelli L. , De Giorgi M. , Passaseo A. , Rinaldi R. , Cingolani R. , Mauritz O. , Goldoni G. , Rossi F. , Molinari E. , Crozier P.
Published: July 2002

An accurate analysis of nanoscale compositional fluctuations in InGaAs/GaAs quantum wires grown by metalorganic chemical vapor deposition on V-grooved substrates was performed by means of high-spatial-resolution transmission electron microscopy techniques. Small In fluctuations (2%–3% excess indium), spatially localized over approximately 5 nm, were detected and related to changes in the photoluminescence and photoluminescence excitation spectra.
Effect of Mg ion implantation on electrical properties of CuInSe2 thin films
Authors: Tanaka Tooru , Wakahara Akihiro , Yoshida Akira , Ohshima Takeshi , Itoh Hisayoshi , Okada Sohei
Published: July 2002

The effects of Mg ion implantation on the electrical properties of CuInSe2 epitaxial thin films have been investigated. The implantation was carried out using the multienergy implantation technique to obtain a constant profile of the Mg concentration along the depth direction. After implantation, the layer was annealed at 400 °C in N2 atmosphere for 60 min. From the results of reflection high-energy electron diffraction, it was confirmed that the damages due to ion implantation were removed by the thermal annealing. The conductivity type in all implanted films was n type, and the carrier concentration was increased with increasing Mg concentration in the films. Consequently, it is concluded that the Mg atom acts as a donor in CuInSe2.
Valence state of CeAl2 nanoparticles studied by Ce L3-edge x-ray absorption spectroscopy
Authors: Chang C. L. , Dong C. L. , Huang C. L. , Chen Y. Y.
Published: July 2002

We report measurements of the Ce L3-edge x-ray absorption near-edge structure on CeAl2 nanoparticles with average diameters of 80 Å. The Ce exhibits a mixed valence with a small amount of tetravalent Ce, which is in contrast to the purely trivalent Ce observed in bulk CeAl2. A shift in the absorption edge to higher energy and a narrower linewidth are also observed in the nanoparticle samples. These spectral differences are attributed to surface effects caused by the small particle size, including a lower coordination number and higher surface pressure. The observation of nonmagnetic tetravalent Ce with the 4f0 configuration is in good agreement with the small values of magnetic entropy seen in low temperature specific heat measurements.
Influence of substrate dopant concentration on electrical properties and residual defects in pn junction formed by low-temperature post-implantation annealing
Authors: Tamai Yukio , Oka Mauricio Massazumi , Nakada Akira , Ohmi Tadahiro
Published: July 2002

Dependence of the leakage currents in BF2+ implanted junctions on the dopant concentration of the n-type substrate was investigated. It was clarified that the leakage currents of low-temperature-annealed junctions increase as the substrate dopant concentration increases, while high-temperature-annealed junctions have the opposite dependence. We demonstrate that low-leakage currents can be achieved in low-temperature-annealed junctions by using lightly doped silicon substrate. It was confirmed that the higher leakage currents in low-temperature-annealed junctions originate from the implantation-induced defects existing deeply in the substrate. Considering the results of both BF2+ implanted p+n junction and As+ implanted n+p junction, we discuss an implantation-induced defect generation mechanism.
Metal diffusion barriers for porous SiO2
Authors: Kumar A. , Bakhru H. , Jin C. , Lee W. W. , Lu T.-M.
Published: July 2002

The diffusion characteristics of metal barrier materials in porous films having low dielectric constant are studied using the Rutherford backscattering spectrometry technique. It is shown that barrier materials such as TaN and TiN do not diffuse into the xerogel film with porosity of 70%–75% after 450 °C annealing in N2 ambient for 2 h. This result points to an interesting possibility that one may not require a dielectric liner for a porous film (to prevent the diffusion of metal barrier materials into the porous material) in the Cu porous film interconnect technology.
Temperature dependence of particle–particle interactions in electrorheological fluids
Authors: Gonon P. , Foulc J.-N.
Published: July 2002

We report on the temperature dependence of particle–particle interactions in electrorheological (ER) fluids for the temperature range 20–100 °C. The attraction force between polyamide spheres immersed in silicone oil is measured as a function of temperature. The force–temperature characteristic shows a broad maximum around 40 °C, corresponding to an increase of about 30% compared to the force measured at room temperature. In view of these results we proposed that the temperature dependence of the shear stress in ER fluids is directly related to the variation of the local particle–particle attraction forces. Data are discussed in light of models which were proposed in the literature to describe particle–particle interactions. At high electric fields “conduction models” could explain the observed temperature dependence through the variations of the oil breakdown field with temperature. However, limitations of such models are also clearly evidenced by data obtained at low electric fields.
Ferromagnetic properties and structures of the Mn-implanted GaAs semiconductor
Authors: Chen Chenjia , Cai Ming , Wang Xuezhong , Xu Shifa , Zhang Ming , Ding Xiaomin , Sun Yunxi
Published: July 2002

Submicron ferromagnets have been successfully incorporated into semi-insulating (001) GaAs crystals by Mn+ ion implantation and subsequent rapid annealing. Magnetization measurements reveal room-temperature ferromagnetism. The structural and compositional properties of crystallites have been analyzed by transmission electron microscopy, energy dispersion x-ray spectrum, and electron microdiffraction. The results show that crystallites of MnGa and MnAs with a small amount of Ga are formed. Atomic force microscopy and magnetic force microscopy images indicate that the single-domain magnetic state is dominant in submicron ferromagnets under our annealing conditions (750 °C–900 °C).
First-principles calculations for the structural and magnetic properties of ordered NiFe(001) thin films with and without a Ta overlayer
Authors: Kim Miyoung , Geng W. T. , Freeman A. J. , Zhong Lieping , Fernandez-de-Castro Juan
Published: July 2002

The first-principles calculations within the local density approximation using the full potential linearized augmented plane wave (FLAPW) method were performed to investigate the structural and magnetic properties of the Ta/NiFe interface for both clean NixFe1−x (001) thin films and with a Ta overlayer. To study the composition dependence, we adopted x=0.5 (L10 structure with either Fe or Ni layers on the surface) and 0.75 (L12 structure with mixed Fe-Ni layers or with Ni on the surface). The equilibrium overlayer/substrate distance and the preferred site position of Ta were obtained by structural optimization employing atomic-force calculations and total energy comparisons for several possible adsorption sites of Ta. By comparing with results for the clean surface of five-layer NixFe1−x (001) films, we found that Ta has a significant detrimental effect on the magnetic properties of NiFe with its induced magnetic moment coupled ferro- or antiferro-magnetically with the substrate depending sensitively on the surface layer.
Quantitative x-ray magnetic circular dichroism microspectroscopy of Fe/Co/Cu(001) using a photoemission microscope
Authors: Kuch W. , Gilles J. , Kang S. S. , Offi F. , Kirschner J. , Imada S. , Suga S.
Published: July 2002

Photoelectron emission microscopy is combined with soft x-ray magnetic circular dichroism (XMCD) absorption spectroscopy to obtain local element-resolved quantitative magnetic properties with microscopic resolution. This is applied to study 0–14 ML Fe wedges with a slope of 0.055 ML/μm on 6 ML Co/Cu(001). Local XMCD spectra at the Fe L2,3 edge confirm the presence of three magnetically different thickness regions of Fe with effective spin moments of 2.5μB (0–4.5 ML), 0.7μB (4.5–11 ML), and 1.8μB (>11 ML). The value of 0.7μB in the second phase is consistent with an fcc Fe phase containing nonferromagnetic layers underneath a ferromagnetic surface.
Temperature dependence of the magnetomechanical effect in metal-bonded cobalt ferrite composites under torsional strain
Authors: Chen Y. , Snyder J. E. , Dennis K. W. , McCallum R. W. , Jiles D. C.
Published: July 2002

Metal-bonded cobalt ferrite composites are promising candidates for torque sensors and other magnetostrictive sensing and actuating applications. In the present study, the temperature dependence of the magnetomechanical effect in a ring-shape cobalt ferrite composite under torsional strain has been investigated in the temperature range of −37 to 90 °C. The changes of external axial magnetic field were measured as a function of applied torque. Magnetomechanical sensitivity of ΔHext/Δτ=65 A N−1 m−2 was observed with a magnetomechanical hysteresis of Δτ=±0.62 N m at room temperature (22 °C). These were then measured as a function of temperature. Both decreased as the temperature increased throughout the entire range. The magnetomechanical hysteresis became negligible at temperatures higher than 60 °C, above which there was a linear change in external magnetic field with applied torque. These temperature dependences are explained by the changes of magnetostriction, anisotropy, spontaneous magnetization, and pinning of domain walls caused by the availability of increased thermal energy.
Simulations of a classical spin system with competing superexchange and double-exchange interactions
Authors: Tsai Shan-Ho , Landau D. P.
Published: July 2002

Monte Carlo simulations and ground-state calculations have been used to map out the phase diagram of a system of classical spins, on a simple cubic lattice, where nearest-neighbor pairs of spins are coupled via competing antiferromagnetic superexchange and ferromagnetic double-exchange interactions. For a certain range of parameters, this model is relevant for some magnetic materials, such as doped manganites, which exhibit the remarkable colossal magnetoresistance effect. The phase diagram includes two regions in which the two sublattice magnetizations differ in magnitude. Spin-dynamics simulations have been used to compute the time- and space-displaced spin–spin correlation functions, and their Fourier transforms, which yield the dynamic structure factor S(q,ω) for this system. Effects of the double-exchange interaction on the dispersion curves are shown.
Colossal magnetoresistance and magnetic phase transitions in bulk Nd0.7Ba0.3MnO3
Authors: Wang Kai-Ying , Sangregorio Claudio , Wiggins Jason , Wiemann Joan , Tang Jinke
Published: July 2002

Perovskite-type Nd0.7Bao0.3MnO3 has been synthesized and its magnetic and magnetotransport properties studied in the temperature range from 5 K to 300 K. X-ray diffraction indicates that the crystal-symmetry is orthorhombic with a=0.5508(1) nm, b=0.5496(2) nm, and c=0.7764(3) nm. There exist two magnetic phase transitions. It first undergoes a paramaganetic-to-ferromagnetic phase transition at Curie temperature, TC=115 K, and then partial ordering of Nd magnetic moments develops at TNd=40 K as temperature is further decreased. Resistivity measurements indicate corresponding insulator-to-metal transition near TC and a reentrant metal-to-insulator transition near 40 K. Colossal magnetoresistance exists over a wide temperature range, MR=[R(H)-R(0)]/R(0) is over 90% below 120 K. The maximum MR (98%) appears at 95 K, close to the paramagnetic-to-ferromagnetic phase transition.
Magnetic properties of frustrated spin ladder
Authors: Sakai Tôru , Okazaki Nobuhisa
Published: July 2002

The magnetic properties of the antiferromagnetic spin ladder with the next-nearest neighbor interaction, particularly under external field, are investigated by the exact diagonalization of the finite clusters and size scaling techniques. It is found that there exist two phases, the rung-dimer and rung-triplet phases, not only in the nonmagnetic ground state but also magnetized one, where the phase boundary has a small magnetization dependence. Only in the former phase, the magnetization curve is revealed to have a possible plateau at half the saturation moment, with a sufficient frustration.
Electron spin resonance in the S=1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C5H14N2)2N3(PF6)
Authors: Honda Z. , Katsumata K. , Hagiwara M. , Tokunaga M.
Published: July 2002

We have performed electron spin resonance (ESR) measurements on the S=1 quasi-one-dimensional Heisenberg antiferromagnet, Ni(C5H14N2)2N3(PF6) in the field induced long range ordered (LRO) phase. From the ESR experiments, two branches are observed, one of which is assigned to an antiferromagnetic resonance mode and the other to a one-dimensional excitation that persists in the LRO phase.
Magnetic systems containing quenched, long-range, spherically symmetric impurities
Authors: Nicolaides Demetris
Published: July 2002

Magnetic systems containing randomly distributed fields with long-range correlations, decaying as a power law x−b exhibit new critical behavior at phase transitions in comparison to short-range random systems. When b<d+2, where d is the dimension of space, the critical behavior of the random system is the same as that of the pure one in an effective spatial dimension (b−2).
Tetragonal equilibrium states of Mn and Fe
Authors: Qiu S. L. , Marcus P. M. , Ma Hong
Published: July 2002

Tetragonal equilibrium states of Mn and Fe have been found by total-energy calculations at constant volume as a function of c/a with the full-potential linearized-augmented-plane-wave method using two different potentials: (1) the local-spin-density approximation without relativistic corrections and (2) the Perdew–Burke–Ernzerhof exchange-correlation potential in a generalized-gradient approximation with relativistic corrections. Comparison of potential (1) with potential (2) shows that the energy curves relative to the lowest minimum of each are very similar and have minima at the same c/a values. However, potential (2) makes the magnetic phases more magnetic. Both Mn and Fe are shown to have stable and metastable tetragonal equilibrium states in each of several magnetic phases. The antiferromagnetic (AF) energy versus c/a curve of Mn shows a stable tetragonal state at c/a=0.96, close to the experimental value for γ-Mn at c/a=0.95, and a metastable body-centered-tetragonal state at c/a=0.60. However the bcc state at c/a=0.707 is inherently unstable. The calculation on Fe in tetragonal structure shows that AF Fe has a tetragonal equilibrium state at c/a=1.08, and ferromagnetic Fe has a tetragonal equilibrium state at c/a=1.17.
Dependence of exchange coupling on the surface roughness and structure in α-Fe2O3/NiFe and NiFe/α-Fe2O3 bilayers
Authors: Bae Seongtae , Judy Jack H. , Egelhoff W. F. , Chen P. J.
Published: July 2002

The exchange coupling characteristics for both Si/α-Fe2O3/NiFe and Si/NiFe/α-Fe2O3 bilayers have been investigated. These two bilayers showed completely different exchange coupling characteristics. The Si/α-Fe2O3 (50 nm)/NiFe(5 nm) bilayer had Hex=62.5 Oe, Hc=137.5 Oe, while the Si/NiFe(5 nm)/α-Fe2O3(50 nm) bilayer had Hex=4.5 Oe, Hc=9.5 Oe. The larger exchange bias field of α-Fe2O3/NiFe bilayer was mainly attributed to good crystallinity of α-Fe2O3 and the smooth interface between NiFe and α-Fe2O3. The interfacial exchange energy, Jex was also calculated for these two bilayers. The case of the rougher surface of bilayer (Si/NiFe/α-Fe2O3) exhibited smaller interfacial exchange energy. In order to verify the effect of α-Fe2O3 crystal structure on the exchange bias coupling characteristics, Ti (hcp), Zr (hcp), Ta (bcc), and Cu (fcc) were used as buffer layers for the Si/buffer layer/α-Fe2O3/NiFe structures. Ti and Zr buffer layers showed good exchange coupling performance, which was strongly related to good crystal matching with α-Fe2O3, but there was no exchange coupling improvement in the case of Ta and Cu buffer layers.
Magnetization and susceptibility of coupled ferromagnetic trilayers calculated with a Green’s function type theory
Authors: Jensen P. J. , Bennemann K. H. , Baberschke K. , Poulopoulos P. , Farle M.
Published: July 2002

The effect of the interlayer exchange interaction Jinter in coupled Co/Cu/Ni trilayers is studied theoretically, as motivated by recent experiments. In these systems, Jinter induces a considerable Ni magnetization for temperatures above the Curie temperature TC,Ni of the Ni film. We will show that this strong effect can only be understood if magnetic fluctuations in these 2D-like systems are taken into account. A mean field estimate, neglecting these fluctuations, yields a ten times smaller increase of the Ni magnetization. The induced magnetization increases for both a ferromagnetic (Jinter>0) and an antiferromagnetic (Jinter<0) interlayer coupling. The Ni susceptibility, exhibiting a resonance-like peak at T≳TC,Ni, looks different for these two cases. The calculations are performed with the help of a Heisenberg Hamiltonian and a Green’s function approach.
Nonlinear magneto-optical effect in Fe/Au superlattices modulated by noninteger atomic layers
Authors: Sato Katsuaki , Kodama Akihiro , Miyamoto Masaaki , Takanashi Koki , Fujimori Hiroyasu , Rasing Theo
Published: July 2002

Magnetic second harmonic generation (MSHG) signals from the Fe/Au superlattices grown on MgO (100) substrate have been investigated. The measurements were carried out in the longitudinal Kerr geometry with a 45° incidence. The magnetic field of ±0.2 T was applied to provide a magnetic saturation. The polar plot of azimuth dependence of the MSHG intensity showed a four-fold symmetry pattern similar to that observed in Fe/Au superlattices modulated by integer atomic layers. Azimuthal patterns show 45° rotation by reversing the magnetic field. Nonlinear Kerr rotation angle of 23.2° was observed in the x=3.75 superlattice.
Giant magneto-optical Kerr effect enhancement in layered CoMn/Si structures
Authors: Shen Z. C. , Liu Z. , Wang S. Y. , Zhang R. J. , Yang Y. M. , Zhou S. M. , Chen L. Y. , Song J. T.
Published: July 2002

For sputtered Co57Mn43/Si film samples with different ferromagnetic thickness, the spectra of the complex magneto-optical polar Kerr rotation and optical constants were ex-situ measured. For 5 nm-thick Co57Mn43 film, the Kerr rotation around 4.3 eV was enhanced by a factor of about 10 times with lower loss of the signal intensity as compared to the thick film. Numerical calculations show that the enhancement effect is attributed to an interplay between optical properties of Co57Mn43 and substrate Si that has a strong interband transition (E2) near 4.3 eV.
Magneto-optical spectra of epitaxial ferromagnetic MnAs films grown on Si and GaAs substrates
Authors: Nazmul Ahsan M. , Shimizu H. , Tanaka M.
Published: July 2002

We have studied magnetic circular dichroism (MCD) spectra of ferromagnetic MnAs epitaxial thin films of (1̄101), (1̄100), and (0001) crystallographic growth orientations. The films were grown on Si(001), Si(111), GaAs(001), and GaAs(111)B substrates by molecular-beam epitaxy. We have found strong crystal orientation dependence of MCD spectra of MnAs films in terms of characteristic features and absolute intensity of MCD signals. We have also measured temperature dependence of MCD spectra on a (0001) MnAs film. Features at low temperatures are found to be different from those at room temperature. Our experimental results are compared with the ionic model for the electronic structure of NiAs-type MnAs and with magneto-optical Kerr ellipticity spectra recently calculated using an all electron full potential linear muffin-tin orbital method.
Magnetic and thermal properties of ErMn4Al8 and ErCr4Al8 in high magnetic fields
Authors: Hagmusa I. H. , Klaasse J. C. P. , Brück E. , de Boer F. R. , Buschow K. H. J.
Published: July 2002

The magnetic properties and specific heat of ErMn4Al8 and ErCr4Al8 compounds have been studied between 0.5 and 30 K in magnetic fields up to 12 T. We find clear indications that the specific heat is dominated by CEF effects rather than by magnetic ordering. The very high low-temperature specific heat of ErMn4Al8 may be attributed to an isolated |Jz=±3/2〉 doublet, which is strongly affected by the applied magnetic field. For ErCr4Al8, the zero-field data very much resemble the ErMn4Al8 data but the influence of the magnetic field is quite different, which excludes a possible explanation in terms of a doublet ground state.
Direct current plasma implantation using a grounded conducting grid
Authors: Kwok Dixon T. K. , Zeng Xuchu , Chan Chung , Chu Paul K.
Published: July 2002

A novel plasma implantation technique performed in a low pressure steady state dc mode utilizing a grounded conducting grid on top of the wafer stage is presented. By numerically simulating the ion paths by the particle-in-cell method, it is observed that the ion paths are optimized for certain implant geometry. In the optimal configuration, the directional angle of the acceleration vector does not depend on the mass and charge state of the ions, and the ratio of the partial differential of the scalar potential φ along the radial and longitudinal directions remains constant for varying applied voltages. The retained dose and impact energy uniformity are totally determined by the ratio of the radius of the wafer stage r, radius of the vacuum chamber R, distance between the wafer stage and the grid H, and thickness of the wafer stage D. The optimal ratio is r:R:H:D=1:4:2.5:2, that is, suggesting a disk shape vacuum chamber, which is quite different from that of a conventional plasma immersion ion implanter. In addition to retaining the large area and parallel processing advantages of plasma immersion ion implantation (PIII), the implantation energy can be extended far beyond the limit of PIII as the technique obviates the use of the power modulator, which not only limits the implantation energy but also is the most expensive and technologically complex hardware component in a PIII system.
Microstructural evolution in a multiple composite layer of GaN on sapphire by organometallic vapor phase epitaxy
Authors: Yang Chien-Cheng , Wu Meng-Chyi , Lee Chih-Hao , Chi Gou-Chung
Published: July 2002

Using cross-section transmission electron microscopy and grazing incidence x-ray diffraction measurements, this work investigates the defect reduction in a wurtize GaN thin film with a multiple composite layer grown by atmospheric-pressure organometallic vapor phase epitaxy on sapphire substrate. According to those results, the sequential composite layer can terminate most threading dislocations and only a few dislocations can be deeply propagated into the next epitaxial layer. Moreover, the multiple composite layer structure significantly reduces the density of threading dislocation generated from the GaN/Al2O3 interface.
Effect of SiO2 cladding on properties of annealed proton-exchanged LiNbO3 waveguides
Authors: Korkishko Y. N. , Fedorov V. A. , Feoktistova O. Y. , Morozova T. V.
Published: July 2002

We show SiO2 cladding effects on the optical properties of annealed proton-exchanged LiNbO3 waveguides as well as on the kinetics of annealing process. Annealing with previously deposited SiO2 film reduces the optical losses in annealed proton-exchanged LiNbO3 waveguides. The structural phase diagram of HxLi1−xNbO3 is also modified if annealing has been performed with a previously deposited SiO2 layer. The physical mechanisms, including evaporation of H2O and/or Li2O during annealing without deposited film and, as a result, formation of defects and a lithium deficit phase, are discussed.
Temperature effects in the hot wire chemical vapor deposition of amorphous hydrogenated silicon carbon alloy
Authors: Lee Moon-Sook , Bent Stacey F.
Published: July 2002

The microstructure, composition, and bonding in hydrogenated amorphous silicon carbon alloy (a-SiC:H) films grown at different substrate temperatures were investigated by a combination of multiple internal reflection-Fourier transform infrared spectroscopy and near edge x-ray absorption fine structure measurements. Hot wire chemical vapor deposition (HW–CVD) was used to grow the thin films at substrate temperatures ranging from 200 to 600 K using mono- and trimethylsilane as precursors. It is found that raising the substrate temperature during HW–CVD leads to films depleted in the higher hydrides (namely SiH3, SiH2, and CH3) in favor of the lower hydrides (SiH and CH). This change marks a transition of the film structure from a highly methylated-polysilane backbone to a polycarbosilane backbone. In addition, some crystalline characteristics appear with increasing substrate temperature, demonstrating that the change of substrate temperature affects both the hydrogen configuration and the microstructure of the film. Temperature-dependent growth of thin a-SiC:H films by HW–CVD is compared with the method of electron cyclotron resonance plasma-enhanced (PECVD).
Compositional clustering in Nd2Fe14B melt-spun ribbons
Authors: Lewis L. H. , Kramer M. J. , Dennis K. W. , McCallum R. W.
Published: July 2002

Elevated-temperature superconducting quantum interference device magnetometry was employed to investigate the detailed phase constitution of nominally glassy stoichiometric Nd2Fe14B melt-spun ribbons modified with 6 wt % Ti/C and subjected to various degrees of quenching by circumferential wheel speed variations of 40, 35, and 20 m/s. While previous microstructural characterization indicated that these materials were completely amorphous, ac susceptibility and magnetization measurements analyzed within the framework of superparamagnetism indicates that the materials actually contain a concentration of crystalline phase clusters of Nd2Fe14B and α-Fe that remained undetected by previous microstructural characterization due to their small size (diameter <5 nm) and inhomogeneous spatial distribution. The Curie temperatures of the glassy component increase with decreasing wheel speed, while the amount of glass varies in a systematic manner from 78 to 91 wt %. The remaining phases in the quenched product are Nd2Fe14B and α-Fe. Analysis of the distribution and size of the α-Fe clusters provides insight into the nucleation and growth process that ultimately produces the crystallized microstructure associated with high energy-product melt-spun Nd2Fe14B-based magnets.
Real time quantification of Monte Carlo steps for different time scales
Authors: Smirnov-Rueda R. , Chubykalo O. , Nowak U. , Chantrell R. W. , Gonzalez J. M.
Published: July 2002

Time quantification of Monte Carlo steps is studied by the implementation of a new technique which takes into account the realistic size of thermal fluctuations of magnetization along with Landau–Lifshitz–Gilbert dynamic correlations. The computational model has been specifically developed for an ensemble of isolated single-domain particles. The numerical results have been compared with Langevin dynamics calculations and theoretically predicted Brown’s asymptotes for relaxation time of single spin system. In addition we demonstrated that real time quantification of Monte Carlo steps is also possible for different time scales. Implementation of real time scales into Monte Carlo calculations for different sizes of time steps is shown to be convergent to the expected value if the Monte Carlo acceptance rate is taken into account.
Contactless measurement of giant magnetoresistance in thin films by infrared reflection
Authors: Camplin John P. , Thompson Sarah M. , Loraine Duncan R. , Pugh David I. , Collingwood Joanna F. , McCash Elaine M. , Horn Andrew B.
Published: July 2002

The magnetorefractive effect (MRE) has been demonstrated in reflection for giant magnetoresistive Co30Ag70/Si(100) granular films, 400 and 1000 Å thick. Measurements of infrared radiation in the 2–14 μm range indicate a sensitivity to polarization of the incident light and an enhanced response at oblique angles of incidence. Direct correlation with dc magnetoresistance measurements has been demonstrated but computer simulations show that current theory is unable to satisfactorily model experimental observation.
Magnetism of RhN clusters
Authors: Guirado-López R. , Villaseñor-González P. , Dorantes-Dávila J. , Pastor G. M.
Published: July 2002

The size and structural dependence of the magnetic properties of RhN clusters (3⩽N⩽27) are studied by using a d-electron tight-binding Hamiltonian which includes hybridizations, Coulomb interactions, and spin–orbit coupling on the same electronic level. We show that the spin (S) and orbital (L) magnetic moments depend strongly on the local environment of the atoms and that their behavior as a function of the cluster size present oscillations which are in good qualitative agreement with experiment. The magnetic anisotropy energy and the anisotropy in L are quantified by considering different directions of the spin magnetization S within the cluster. We conclude that orbital contributions to the average magnetization in RhN clusters could be important for precise comparison with experiment. Bond-length relaxation effects are also briefly discussed.