bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface hardness enhancement in ion-implanted amorphous carbon
Authors: Lee D. H. , Park B. , Poker D. B. , Riester L. , Feng Z. C. , Baglin J. E. E.
Published: July 2002

The feasibility of producing carbon nitride has been studied by ion implantation into amorphous carbon. Thin films were formed with 100 keV N+ or 80 keV C+ ions at various target temperatures and ion doses. The apparent surface hardness measured by nanoindentation with load-displacement data shows an optimum value of 22.3±0.4 GPa with the ion dose of 2×1017 N+/cm2 implanted at −100 °C, while the hardness of the unimplanted amorphous carbon is 6.0±0.2 GPa. Self-implantation by carbon also produces similar hardness enhancement with a narrow temperature window. The maximum enhanced surface hardness is well correlated with the asymmetric diffuse peak at around 1500 cm−1 in Raman spectroscopy.
Vacuum ultraviolet radiative properties of formed-ferrite discharge with prepulse initiation
Authors: Tcheremiskin V. I. , Sentis M. L. , Delaporte Ph. C. , Zuev V. S. , Mikheev L. D.
Published: July 2002

A study of radiative properties of formed-ferrite discharge driven by a new electrical circuit scheme with a prepulse initiation has been carried out. With the prepulse electrical initiation, we demonstrated that the jitter time of main discharge onset can be decreased from a few tens of microseconds to ∼0.1 μs allowing us to synchronize a number of formed-ferrite discharges switched in parallel. Discharge plasma surface radiation characteristics in the spectral region of the XeF2 photodissociation band (140–180 nm) were measured by the dynamic actinometry method. The plasma surface radiation intensity is 2.3×1023 photons/cm2 s in 10 μs which corresponds to a brightness temperature of 22 000 K.
Atomic force microscopy study on topography of films produced by ion-based techniques
Authors: Wang X. , Liu X. H. , Zou S. C. , Martin P. J. , Bendavid A.
Published: July 2002

The evolution of surface morphologies of films prepared by ion-based deposition techniques has been investigated by atomic force microscopy. Two deposition processes, filtered arc deposition (FAD) and ion-beam-assisted deposition, where low-energy (<100 eV) ion irradiation and high-energy (several tens of keV) ion-beam bombardment concurrent with film growth were involved, respectively, have been employed to prepare TiN and Al films. Comparative studies on the effect of energetic ions on the development of topography have been performed between the low-ion-energy regime and high-ion-energy regime. In addition, the relationship between topography and mechanical properties of thin films has been revealed, by involving thin films prepared by thermal evaporation deposition (TED), where almost all depositing particles are neutral. In the images of the TED TiN and Al films, a large number of porous and deep boundaries between columnar grains was observed, suggesting a very rough and loose surface. In contrast, the FAD films exhibited much denser surface morphologies, although still columnar. The root-mean-square roughness of the FAD films was less than 1 Å. Hardness test and optical parameter measurement indicated that the FAD films were much harder and, in the case of optical films, much more transparent than the TED films, which was considered to arise from the denser surface morphologies rather than crystallization of the films. The high density and super smoothness of the FAD films, and the resultant mechanical and optical properties superior to those of the TED films, were attributed to the enhancement of surface migration of the deposited adatoms in the FAD process, which could provide intensive low-energy ion irradiation during film growth. As for topography modification by high-energy ion-beam bombardment concurrent with film growth, in addition to the increase of surface diffusion due to elastic collision and thermal spikes, physical sputtering must be considered while explaining the development of the film topography. Both surface migration enhancement and sputtering played important roles in the case of high-energy heavy-ion-beam bombardment, under which condition surface morphology characterized by dense columns with larger dimension and deep clean boundaries was formed. However, under high-energy light-ion-beam bombardment, the sputtering was dominant, and the variation of sputtering coefficient with position on the surface of growing film led to the formation of cones.
Relaxation behavior of undoped InxGa1−xP 0.5<x<0.7 grown on GaAs by atomic layer molecular-beam epitaxy
Authors: González L. , González Y. , Aragón G. , Castro M. J. , Dotor M. L. , Dunstan D. J.
Published: July 2002

We present a study of the relaxation behavior of compressive InxGa1−xP layers grown by atomic layer molecular-beam epitaxy at Ts=420 °C with x=56%±3% and x=67%±3%. Similar (thickness and composition) InxGa1−xP layers were grown under different growth conditions in order to assess the influence of the stoichiometry of the growth front on the structural properties and the relaxation process of this material system. All InxGa1−xP layers were characterized by double-crystal x-ray diffraction, transmission electron microscopy, and Nomarski interference. Our results show that surface stoichiometry during growth does not affect the relaxation behavior of InxGa1−xP layers but strongly determines their structural characteristics related to composition modulation features which appear in all our InxGa1−xP layers. We have established an empirical relation between residual strain and thickness. This relation makes predictable the residual strain of more complicated structures which can be introduced as buffer layers in lattice-mismatched heteroepitaxial systems.
Thermal relaxation phenomena across the metal/window interface and its significance to shock temperature measurements of metals
Authors: Tang Wenhui , Jing Fuqian , Zhang Ruoqi , Hu Jinbiao
Published: July 2002

Difficulties in connection with the radiometry measurement of shock temperatures for metals, due to our insufficient knowledge of the high-pressure thermophysical properties of materials, are discussed in this paper. The thermal radiation histories from shocked iron film/sapphire interfaces are observed. It is found that after the passage of a shock wave, the thermal relaxation process at the metal/window interface is not always so rapid that the available pyrometric system is unable to respond. Shock temperature measurement experiments show that the spectral radiance history observed at this interface consists of an initial spike followed by a plateau, demonstrating distinctly the temperature relaxation behavior. This fact is contrary to the previous arguments in which the interface temperatures instantly approach their time-independent equilibrium value. Release temperatures are obtained directly from the observed amplitude of the irradiance spike, these values are comparable to those calculated theoretically from the plateau via the instantaneous equilibrium temperature model based on the well-known thermophysical properties of iron and sapphire at high pressure and high temperature. The observation of the thermal relaxation process shows that it is possible to obtain directly the release temperature of the metal samples even though their thermophysical properties at high pressures are not known. Therefore all the difficulties mentioned above can be avoided.
A transmission electron microscopy study of microstructural defects in proton implanted silicon
Authors: Gao Min , Duan X. F. , Li Jianming , Wang Fenglian
Published: July 2002

The microstructure of silicon on defect layer, a new type of silicon-on-insulator material using proton implantation and two-step annealing to obtain a high resistivity buried layer beneath the silicon surface, has been investigated by transmission electron microscopy. Implantation induced a heavily damaged region containing two types of extended defects involving hydrogen: (001) platelets and {111} platelets. During the first step annealing, gas bubbles and {111} precipitates formed. After the second step annealing, {111} precipitates disappeared, while the bubble microstructure still remained and a buried layer consisting of bubbles and dislocations between the bubbles was left. This study shows that the dislocations pinning the bubbles plays an important role in stabilizing the bubbles and in the formation of the defect insulating layer.
Study of positron yield of W(100) single crystalline foil with in situ surface characterization
Authors: Amarendra G. , Canter K. F. , Schoepf D. C.
Published: July 2002

The slow positron yield of a 1 μm thick W(100) single crystalline foil moderator used in transmission mode geometry is measured as a function of successive heat treatments with and without annealing in partial pressure of oxygen. In situ surface characterization of the moderator foil is carried out using Auger spectroscopy. The yield gradually improves after each heat treatment leveling off beyond five treatments. The improvement in the yield does not seem to be correlated with low surface carbon content but rather due to removal of bulk carbon due to heat treatments. The reemitted positron flux reproducibly goes through a maximum within an hour after surface treatment and approaches an equilibrium level after about 12 h which is about 20% less than the maximum. The initial increase in the positron flux is correlated with an oxygen adsorption rate which is higher than for carbon immediately after surface treatment while the subsequent decrease in flux seems to correlate with a more gradual increase in contamination due to carbon adsorption with the oxygen contamination remaining fairly constant. Energy spectra of the reemitted positrons using a retarding field analyzer show a beam of FWHM∼0.25 eV with a low energy tail whose intensity remains at a minimum after surface treatment and during the approach to maximum flux, but whose intensity increases after the flux passes through a maximum. Preliminary surface preparation studies with annealing in partial pressure of hydrogen show evidence of an improved FWHM and a slight shift in positron work function.
Electron-paramagnetic-resonance investigation of SrS:Ce powders and thin films
Authors: Kreissl J. , Hüttl B. , Troppenz U. , Velthaus K.-O. , Mauch R. H. , Fouassier C.
Published: July 2002

The electron paramagnetic resonance (EPR) of cerium and manganese impurities has been studied in SrS powders and thin-film electroluminescent devices (TFELD) containing an SrS luminescent layer. In both types of material the EPR signal at g=1.31 was attributed to the Ce3+ charge state. A variation of the g value, larger linewidth, and variation of the line shape in the case of TFELD samples are due to larger strains compared to the powders. The spectrum of Mn2+ has been observed in the SrS:Ce powders and in the intentionally doped SrS:Ce,Mn layers. Contrary to the undisturbed octahedral behavior of the Mn2+ centre in the powders, an additional axial distortion oriented in the growing direction of the layer was found in the TFELD structures.
Molecular-beam epitaxial growth and characterization of Bi–Sr–Co–O thin films
Authors: Tsukada I. , Nose M. , Uchinokura K.
Published: July 2002

The thin-film growth of Co oxides and layered Bi–Sr–Co–O compounds was investigated in detail under an ozone ambient. All samples were prepared using a molecular-beam-epitaxy machine modified for the growth of Bi-based high-temperature superconductors. The temperature versus pressure phase diagram of Co oxidation was determined for growth on Nd:YAlO3 substrates. According to the phase diagram, Bi2Sr2CoO6+α and Bi2Sr3Co2O9+β were grown on Nd:YAlO3 (001). The boundary between the stable regions of them is basically the same as that between CoO and Co3O4. Bi2Sr2CoO6+α was an insulator with an activation energy of approximately 0.20 ± 0.01 eV. Bi2Sr3Co2O9+β shows metallic behavior down to 80 K, and the carriers are revealed to be holes by Hall-effect measurement.
Characteristics of γ-ray-induced absorption bands in oxygen deficient silica
Authors: Kawaguchi Yoshizo , Kuzuu Nobu
Published: July 2002

Characteristics of γ-ray-induced absorption spectra in oxygen deficient vitreous silica were investigated. The spectra were well fitted with five Gaussian absorption bands, the same bands as those reported before in x-ray- and excimer-laser-induced absorption spectra. Absorption intensity in oxygen deficient silica was much larger than those in silicas containing Si–OH or Si–Cl units. Creation mechanisms for radiation induced defects and role of terminal structures such as Si–OH and Si–Cl on resisting defect formation have been discussed.
Scandium correlated deep levels in silicon
Authors: Achtziger Norbert
Published: July 2002

Deep band-gap states in silicon were studied by deep level transient spectroscopy during the elemental transmutation of the radioactive isotope 45Ti to Sc. The parent isotope 45Ti was implanted into n- and p-type silicon by recoil implantation using the nuclear reaction 45Sc(p,n) 45Ti. Repeated measurements reveal a concentration decrease of the three well-known levels of interstitial titanium in Si and an increase of three levels at 0.21, 0.47, and 0.50 eV below the conduction-band edge. Since the concentration versus time curves exactly reflect the nuclear transmutation (half-life 3.08 h), these new levels are identified to be scandium correlated. Since an involvement of other defects is unlikely, these levels are most probably associated with different charge states of interstitial Sc in Si.
Electrical conduction in polyetherimide
Authors: Suh Kwang S. , Nam Jin Ho , Lim Kee Joe
Published: July 2002

Electrical conduction characteristics in polyetherimide (PEI) have been investigated at electric fields of 0.9–27 MV/m over temperatures of 50–100 °C. It is found that the current density of PEI increases with the increase of electric field and temperature and that its conduction is governed by the thermally activated ion hopping process. The hopping distance of ions in the PEI decreases from 5.69 nm at 50 °C to 3.34 nm at 100 °C due to the increase of chain motion with the increase of temperature. The activation energy for the electrical conduction is 0.74 eV.
Intrinsic noise temperatures of YBa2Cu3O7−δ Josephson devices on bicrystal substrates and the upper frequency limit for their operation
Authors: Chen J. , Myoren H. , Nakajima K. , Yamashita T. , Wu P. H.
Published: July 2002

Following a method proposed by Divin and Modovets [Sov. Tech. Phys. Lett. 9, 108 (1983)], we have measured at millimeter waveband the intrinsic noise temperatures TN of YBa2Cu3O7−δ Josephson junctions or dc superconducting quantum interference devices (SQUIDs) fabricated on SrTiO3, yttria-stabilized ZrO2, or Si bicrystal substrates. Over wide ranges of physical temperatures TP and the junction’s normal resistance RN, it was found that TN follows TP pretty well. This indicates that the intrinsic noise in the devices is dominated by Johnson noise. TN was also measured in cases where there is external magnetic field applied, or where there is another microwave radiation like the local oscillator in a mixer. The magnetic field or microwave radiation does not seem to affect TN in any appreciable way. To estimate the high frequency performance of the junctions on Si bicrystal substrates, direct irradiation by a far infrared laser at 1.81 THz is carried out and the clear first Shapiro step is observed.
Submicrosecond switching with a Fabry–Perot interferometer containing a nematic liquid crystal
Authors: Baier-Saip J. A. , Eichler H. J. , Iryanto I. , Macdonald R.
Published: July 2002

A study of the temporal response of a biased Fabry–Perot etalon with a nematic liquid crystal exposed to short (∼10 μs) voltage pulses is presented. It is found that the finesse is the crucial factor determining the achievable switching time. For a given maximum transmission the reflectivities of the Fabry–Perot mirrors are optimized to achieve minimum switching times. In particular, switching times of 0.5 μs with an initial transmission of 3% and a maximum value of 70% have been obtained. The experimental results are in good agreement with theory.
Low-temperature transport properties of the mixed-valence semiconductor Ru0.5Pd0.5Sb3
Authors: Nolas G. S. , Harris V. G. , Tritt T. M. , Slack G. A.
Published: July 2002

We have measured the transport properties of Ru0.5Pd0.5Sb3 from 300 down to 4 K and compared them to those of the binary-skutterudite antimonides. In particular, the lattice thermal conductivity of this compound is substantially lower than that of CoSb3 and IrSb3. This is attributed to the mixed-valency of ruthenium in this compound. Using near-edge extended absorption fine structure analysis, it is observed that ruthenium in this compound is in the Ru4+- and Ru2+-valence states in approximately equal proportions. The potential for thermoelectric applications of this material is also discussed.
Resonant electron tunneling in ZnSe/BeTe double-barrier, single-quantum-well heterostructures
Authors: Lunz U. , Keim M. , Reuscher G. , Fischer F. , Schüll K. , Waag A. , Landwehr G.
Published: July 2002

We report on resonant tunneling through ZnSe/BeTe double-barrier, single-quantum-well structures. Negative differential resistance has been observed in the current–voltage characteristics up to room temperature. Due to a conduction-band offset of more than 2 eV, four resonances with negative differential resistance could be detected for this semiconductor material combination at liquid-helium temperature. The structures exhibit a peak-to-valley ratio up to 6:1 at 4.2 K. Current–voltage characteristics as a function of temperature have been studied and analyzed.
Compensation of shallow impurities in oxygen-doped metalorganic vapor phase epitaxy grown GaAs
Authors: Huang J. W. , Bray K. L. , Kuech T. F.
Published: July 2002

Intentional oxygen incorporation, using diethyl aluminum ethoxide [(C2H5)2AlOC2H5] during metalorganic vapor phase epitaxy GaAs was found to compensate C and Zn shallow acceptors as well as Si and Se shallow donors, due to the oxygen-related multiple deep levels within the GaAs band gap. Deep level transient spectroscopy (DLTS) was used to characterize the energy levels associated with these oxygen deep centers. The total measured trap concentration from DLTS can account for the observed compensation in n-type GaAs:O. The total trap concentration in p-type GaAs:O, however, was found to be lower than the observed compensation by a factor of ∼100. These oxygen deep centers exhibit multiple electronic states which have been associated with the local number of Al nearest neighbors and the microscopic structure of the defect. The concentration and nature of these deep levels were not influenced by the chemical identity of the shallow dopants.
The enhancement effect in the domain and domain wall in Fe57 nuclear magnetic resonance
Authors: Dho Joonghoe , Kim Mincheol , Lee Soonchil , Lee Won-Jong
Published: July 2002

We observed both the nuclear magnetic resonance (NMR) signals from the domain and the domain wall in pure iron particles. In the multi-domain state, the total signal was divided into the signal from the domain and that from the domain wall and the enhancement factors were evaluated separately. The average enhancement factors in the domain walls and the domains were estimated to be about 136 and 34, respectively, in zero field, and remained constant in external dc field in the multi-domain state. In the single domain state, it was observed that the enhancement factor is inversely proportional to external field, and the resonance frequency decreases linearly with external field. From the comparative study between the magnetization curve and the variation curves of the domain wall signal and the resonance frequency with external field, we showed that NMR could give useful information on the magnetization process.
Dependence of wavelength control on dielectric structure for vertical-cavity surface-emitting lasers
Authors: Huffaker D. L.
Published: July 2002

Calculated data are presented that compare the control of the lasing wavelength for different dielectric vertical-cavity surface-emitting lasers that use a GaAs tuning layer of variable thickness to control the longitudinal cavity resonance. The advantages of high-contrast Bragg reflectors and a half-wave cavity spacer thickness become obvious in achieving both a wide tuning range and minimizing reflectivity changes.
Resonant absorption due to electron–electron interaction in coupled quantum dots under an electromagnetic field
Authors: Ugajin Ryuichi
Published: July 2002

In a pair of coupled quantum dots under an external electro-magnetic field, new types of resonant absorption are caused by electron–electron interaction. Quasi-resonance between states of multiple electrons caused by electron–electron interaction can be controlled by an external electric field in the direction of coupling of quantum dots. This kind of resonance produces two types of resonant absorption of a photon, so that far-infrared absorption spectra have additional peaks. These quasi-resonant states of multiple electrons are investigated by optical transitions modulated by an external magnetic field.
Artificially layered films of CuBa2 (Ca1−xSrx)n−1CunOy grown using pulsed laser deposition
Authors: Aruta C. , Balestrino G. , Martellucci S. , Paoletti A. , Petrocelli G.
Published: July 2002

We have shown that the pulsed laser deposition technique (PLD) can be successfully used to grow artificially layered films of the CuBa2(Ca1−xSrx)n−1CunOy compound using only two targets having nominal composition BaCuOy and (Ca1−xSrx)CuOy, respectively. n was varied between 2 and 5. We have demonstrated, by a kinematic analysis of the x-ray diffraction spectra that the average random discrete thickness fluctuations which affect both the BaCuOy and (Ca1−xSrx)CuOy layers are much smaller than one atomic layer. Such features are confirmed by the appearance of sharp peaks even for the n=2 artificially layered structure where only one (Ca1−xSrx)CuOy cell is deposited in the stacking sequence. These results show that truly new structures can be obtained by a layer by layer deposition technique with a low interfacial disorder and give strong support to the idea of synthesizing new artificial high Tc structures by the PLD technique.
Theoretical calculation of electron mobility in HgCdTe
Authors: Yoo Sang Dong , Kwack Kae Dal
Published: July 2002

The low field electron mobility in n-type HgCdTe is calculated by using the relaxation time approximation method. Scattering mechanisms considered in the analysis are ionized impurity, electron–hole, alloy, and polar optical-phonon (two types) scatterings. The calculation also retains band-structure effects such as nonparabolic conduction band, electron wave function admixture, and velocity degradation as the electron energy increases. Furthermore, degeneracy is incorporated without approximation. For polar optical-phonon momentum relaxation time, we employ a model that can be applicable at low temperature, when the thermal energy is lower than the optical-phonon energy. The calculation results for drift mobility are in good agreement with the Monte Carlo results. The effects of donor level, compensation, and ionicity of impurity on Hall mobility are also presented.
Single-electron traps: A quantitative comparison of theory and experiment
Authors: Matsuoka K. A. , Likharev K. K. , Dresselhaus P. , Ji L. , Han S. , Lukens J.
Published: July 2002

We have carried out a coordinated experimental and theoretical study of single-electron traps based on submicron metallic (aluminum) islands and Al/AlOx/Al tunnel junctions. The results of geometrical modeling using a modified version of MIT’s FASTCAP were used as input data for the general-purpose single-electron circuit simulator MOSES. The analysis indicates reasonable quantitative agreement between theory and experiment for those trap characteristics which are not affected by random offset charges. The observed differences (ranging from a few to fifty percent) can be readily explained by the uncertainty in the exact geometry of the experimental nanostructures.
X-ray diffraction analysis of InGaP/GaAs heterointerfaces grown by metalorganic chemical vapor deposition
Authors: Nittono Takumi , Hyuga Fumiaki
Published: July 2002

InGaP/GaAs heterointerfaces grown by metalorganic chemical vapor deposition have been characterized by a high-resolution x-ray diffraction analysis of multiple quantum well structures. The flow of AsH3 to InGaP surface produces an InGaAs-like interfacial layer at the GaAs-on-InGaP interface, indicating P atoms of the InGaP surface are easily replaced by As atoms. The flow of PH3 to GaAs surface, on the other hand, does not make any detectable interfacial layer, indicating that almost no As atoms of the GaAs surface are replaced by P atoms. It is also found that the flow of trimethylgallium (TMG) to the InGaP surface produces a GaP-like interfacial layer. This interfacial layer is probably formed by the reaction between TMG and excessive P atoms on the InGaP surface or residual PH3 in the growth chamber.
The effects of particle size and surface recombination rate on the brightness of low-voltage phosphor
Authors: Yoo Jae Soo , Lee Jong Duk
Published: July 2002

A theoretical expression for the brightness of powder phosphor is derived in order to investigate the effects of phosphor’s properties on low-energy cathodoluminescence. It contains the particle size, surface recombination rate, lifetime of holes, carrier diffusion coefficients, and anode voltage. As such, it can be applied to systematically optimize the design parameters of phosphors for maximum efficiency at low-voltage excitation. Use of the given expression is illustrated and a typical case is analyzed in this study.
Influence of ultraviolet illumination on microdischarge behavior in dry and humid N2, O2, air, and Ar/O2: The Joshi effect
Authors: Falkenstein Zoran
Published: July 2002

The influence of UV illumination on the behavior of silent electrical discharge streamers (microdischarges) in dry and humid gases (nitrogen, oxygen, air, and Ar/O2 mixtures) is presented. A commercial low-pressure mercury lamp, which was spectrally calibrated, provides UV light. The UV irradiance power coupled into the discharge cell is also calculated. The transferred charge per mean microdischarge (in a voltage half-cycle) was measured for a pin electrode assembly, configured to deliver either M−G+ transitions (electrons going from metal towards dielectric) or M+G− transitions (electrons going from dielectric to metal). The total transferred charge is recorded by monitoring a charge versus voltage plot (Lissajous figure). When illumination is restricted to the gas alone, both the total transferred charge and the individual-microdischarge transferred charge remain constant for either transition (M−G+ or M+G−). However, when the dielectric is illuminated, a marked decrease in the individual-microdischarge transferred charge is observed for M+G− transitions in some gases. For this case, a greater number of microdischarges must occur in a half-cycle to keep the total transferred charge constant, thereby creating more homogeneous discharge conditions.
Reorientational magnetic transition in nanostructured epitaxial cobalt patches (abstract)
Authors: Ounadjela K. , Hehn M. , Ferré R. , Rousseaux F.
Published: July 2002

Tiny magnets are currently studied for their fundamental and technological properties in the form of particles or patterned submicron dots in close interaction with a substrate. While very small monodomain Co particles (a few tens of atoms) do not show any hysteretic behavior at room temperature, bigger particles, relevant to high-density data storage materials, on the contrary, build up anisotropies that lock the magnetic moment in a given direction. We will discuss here how the geometrical constraints affect the magnetic domain structure in the case of particles dimensions comparable to the characteristic nanoscopic magnetic length scales. We present a study of magnetic dot array 0.5 μm lateral dimension and 1 μm periodicity fabricated out of an epitaxial (0001) hcp cobalt film. While the magnetic properties of the overall particles were determined by conventional magnetometry, the domain structure of each patch was studied by local magnetic force microscopy. This study shows the effects induced by the nanostructuration of the cobalt films in which the patch height determines the domain size and the patch shape controls the pattern of the magnetic domain structure.1 How domain confinement proceeds is particularly interesting in the 25 nm thick cobalt dot array in which the magnetization curves are characteristic of mostly in-plane magnetization with small alternately up and down perpendicular components. The in-plane magnetostatic energy reduced for in-plane magnetic moments parallel to the edges of the dots results in concentric magnetic domain pattern. This implies that a singularity occurs at the center of the dot where the in-plane magnetization reorients fully perpendicular to form a so-called vortex structure. When the temperature is decreased from room temperature, interestingly we show that a reorientational transition occurs at 200 K from the quasi-in-plane magnetization to perpendicularly oriented domain structure. This reorientational transition is attributed to an increase of the perpendicular anisotropy and gives rise to an elongated stripe domain structure.
Magnetism of Gd and Tb nanoscale particles (abstract)
Authors: O’Shea M. J.
Published: July 2002

We have prepared Gd and Tb particles in a matrix of Ti by sputtering to understand the effects of finite size and large interface area on the magnetic properties of magnetic systems with large anisotropy. Ti is chosen as the matrix since Gd and Tb have low miscibility in it. Electron microscopy shows that the particle diameters for a given sample have a distribution characterized by a standard deviation that ranges from 15% to 25% of the mean value. We have already shown that the coercivity in nanoscale Tb particles at 4.5 K is enhanced over that in thick films by a factor of 2.4 reaching a value of 22 kOe for particles of average diameter of 7 nm.1 Here, we find that in the Gd particles the coercivity at 4.5 K and transition temperature are reduced for particles with average diameter below about 10 nm. The coercivity in the Gd particles is more than a factor of 10 smaller than for Tb particles due to the lack of any significant microscopic anisotropy in the Gd system (Gd is an s-state ion in its metallic state). A strong time dependence of the magnetization is observed in the Tb particle systems for particle diameters above about 5 nm while only a weak time dependence is seen in the Gd systems for all particle diameters studied. We find that in the Tb system the relaxation rate as characterized by the magnetic viscosity S shows a maximum as a function of temperature in the range 6–20 K. We shall discuss these results in terms of models involving finite size and strong magnetic anisotropy.
Designing and testing of a sensor based on a magnetoresistive manganese perovskite thick film
Authors: Balcells LL. , Enrich R. , Calleja A. , Fontcuberta J. , Obradors X.
Published: July 2002

In this paper we report on the growth of thick films of magnetoresistive La2/3Sr1/3MnO3 by using spray and screen printing techniques on various substrates (Al2O3 and ZrO2). The growth conditions are explored in order to optimize the microstructure of the films. The films display a room-temperature magnetoresistance of 0.0012%/Oe in the 1 kOe field region. A magnetic sensor is described and tested.
Magnetic films for GHz applications (abstract)
Authors: Korenivski V. , van Dover R. B.
Published: July 2002

Tremendous growth of the communications industry and the increasingly high demand for low-cost light-weight/small-size products drive technology to designs with a high degree of integration. In particular, planar inductors used in integrated circuits with significantly improved inductance per unit area characteristics are needed for further miniaturization of cellular phones operating at 0.95 and 1.9 GHz. Little has been done, however, to use magnetic films to improve the performance and/or reduce size of planar magnetic flux devices. The successful thin-film material would have a high ferromagnetic resonance (FMR) frequency (well above the operating frequency of the device), large permaeability, and low magnetic loss, and very importantly be technologically attractive, i.e., be process compatible with IC technology and have as few preparation steps as possible. Here, we report on fabrication of metallic ferromagnetic films of CoNbZr, CoNbZr/AlN mulitilayered laminates, and exchange-biased structures suitable for GHz applications. Lamination of CoNbZr with thin insulating layers of AlN is shown to significantly improve the microstructure and dc magnetic properties of the films having thicknesses >0.2 μm, as well as to be effective in suppressing eddy current losses at frequencies up to 1–2 GHz. We use exchange biasing to increase the FMR frequency of soft CoNbZr. In-plane unidirectional anisotropy fields of ∼50 Oe are achieved, which result in FMR frequencies >2 GHz. Permeability values of ∼200 with quality factors of ∼10 at 1 GHz are demonstrated. The films are deposited at room temperature and require no postdeposition processing. Application of these films in planar inductors is discussed.
Anisotropic properties of molecular beam epitaxy-grown colossal magnetoresistance manganite thin films
Authors: O’Donnell J. , Onellion M. , Rzchowski M. S. , Eckstein J. N. , Bozovic I.
Published: July 2002

We show that both the magnetoresistance and magnetism in tetragonal MBE-grown films of La1−xCaxMnO3 show anisotropic effects that depend on both temperature and magnetic field. We show that the “colossal” magnetoresistance depends on the angle between the magnetization and the transport current and that the size of this effect is temperature-dependent. Below the Curie temperature this results in an unusual upturn in the magnetoresistance for small magnetic fields normal to the plane of the film as the magnetization rotates out of the plane. Low-field hysteresis of the in-plane magnetoresistance is also observed, and also shows an anisotropy with respect to the current and magnetization directions. We also find an in-plane biaxial magnetocrystalline anisotropy with easy axes along the {100} (Mn–O) crystal directions, and evidence for c-axis magnetocrystalline anisotropy.
Induced magnetism of 4d transition metals: Rh and Ru/Fe(001) overlayers and sandwiches
Authors: Zhong Lieping , Freeman A. J.
Published: July 2002

The structural, electronic, and magnetic properties of mono and bilayer Rh and Ru transition metals deposited on a Fe(001) substrate or sandwiched by an additional Fe layer have been determined by using the highly precise local density full-potential linearized augmented plane-wave method. Significantly, the atomic force determinations demonstrate that a large interfacial relaxation plays an important role in their electronic and magnetic properties. Both mono and bilayer Ru and Rh in the systems considered are found to be able to retain large spin magnetic moments. The ground state of bilayer Rh is found to have its moments coupled antiferromagnetically. In good agreement with x-ray magnetic circular dichroism (MCD) measurements, the deposition of Rh layers on Fe(001) is found to enhance the Fe spin magnetic moments in the interface. Calculated values of the spin and orbital parts of the magnetic moments and the MCD spectra are presented, and the role of overlayer-substrate hybridization is discussed.
Superconductivity and magnetism in the system Y1−xPrxBa1.5Sr0.5Cu3O7−δ (0⩽x⩽1.0)
Authors: Jhans H. , Malik S. K. , Awana V. P. S. , Yelon W. B.
Published: July 2002

The results of superconducting and magnetic studies on the Y1−xPrxBa1.5Sr0.5Cu3O7−δ system with 0⩽x⩽1.0 are reported. Superconducting transition temperature (Tc) decreases with increasing x. The behavior of Tc(x)/Tc(0) versus Pr concentration x in this system is similar to that observed in the Y1−xPrxBa2Cu3O7−δ system but the critical Pr concentration (xcr) required for complete suppression of superconductivity appears to be higher in the present system. Surprisingly, the Néel temperature TN of PrBa1.5Sr0.5Cu3O7−δ is found to be higher (∼21 K) than that for PrBa2Cu3O7−δ (∼17 K) with nearly the same oxygen content. This may imply stronger interaction of Pr-4f moments with the carriers in the Cu–O2 planar network in the present system and yet the Tc suppression is relatively weaker.
Temperature dependence of coercivity of the two-phase Co/Co2TiSn magnet
Authors: Kim T. W. , Gambino R. J.
Published: July 2002

We report a new composite magnet consisting of two metallic phases, Co/Co2TiSn. This Co/Co2TiSn system shows an unusual change of coercivity with temperature. The coercivity shows the normal decrease with temperature in the low-temperature region then suddenly increases at the temperature at which the magnetization of the low-Tc phase disappears. We suggest that this phenomenon can be explained by exchange coupling at the phase boundary. The change in coercivity was modeled assuming the formation of a wall at the phase boundary. On the basis of this model, the wall energy at the phase boundary was extracted. Initial magnetization curves show that magnetization takes place by a different process at high and low temperatures.
Thermodynamic properties and stability of shock waves in metals
Authors: Rutkevich I. , Zaretsky E. , Mond M.
Published: July 2002

A thermodynamic description of metals in the range of high pressures and temperatures is obtained and applied to the problem of the stability of strong shock waves under spontaneous emission of sound. A three-term form of the equation of state is employed to describe the contributions of the cold elastic pressure, the thermal atomic pressure, and the thermal pressure of the free electrons to the total pressure. The full determination of the equation of state is performed from the experimental Hugoniot data and from a model (such as the Slater–Landau model) connecting the atomic Grüneisen parameter and the atomic cold pressure. From the equation of state, the behavior of the Mach number, the temperature, and the entropy along the Hugoniot adiabatic is obtained and analyzed. The calculation of the sound velocity behind the shock enables the application of the Kontorovich criterion for the spontaneous emission of sound from the shock’s front. It is shown that metals with a relatively low slope of the shock velocity versus the particle-velocity straight line exhibit such an unstable behavior for high enough shock intensities. The effects of the electrons’ thermal pressure on both the thermodynamic properties of the metals and the stability of shocks are analyzed and discussed. In particular, it is shown that the electronic contribution to the total pressure has a stabilizing effect.
Passivation, structural modification, and etching of amorphous silicon in hydrogen plasmas
Authors: McQuaid S. A. , Holgado S. , Garrido J. , Martı́nez J. , Piqueras J. , Newman R. C. , Tucker J. H.
Published: July 2002

Atomic hydrogen from plasma discharges dissolves in silicon previously amorphized by ion implantation (aSi) in the form of Si–H bonds, giving rise to infrared (IR) absorption at ∼1990 cm−1 and causing partial activation of implanted dopants. Passivation of aSi does not affect the rate at which the material subsequently undergoes solid phase epitaxy. Exposure giving rise to [H]>6 at. % causes the appearance of an additional IR absorption band at ∼2080 cm−1 and coloration of the layer. Despite annealing, the Si–H defects, normal solid phase epitaxy does not occur during subsequent heat treatment. The structural modification by H-plasma exposure coincides with etching of the layer. The observations can be understood in terms of void formation in aSi resulting from the clustering of Si–H.
Effect of SiOH and SiCl on ArF-excimer laser-induced absorption in soot-remelted silica
Authors: Kuzuu Nobu , Taga Toshiyuki , Kamisugi Naoyoshi
Published: July 2002

Characteristics of ArF-excimer laser-induced absorption in various types of fused silica synthesized by the vapor-phase axial deposition-soot remelting method were investigated. Fused silica containing 2−6×1018 cm−3 of OH and no Cl shows an absorption band at 5.8 eV ascribed to the E′ center (≡ Si ⋅) and an absorption band at 4.8 eV ascribed to oxygen related centers. A sample containing about 3×1019 cm−3 of Cl and no OH shows only the 5.8 eV band. An OH- and Cl-free silica had a preexisting absorption band at 5.0 eV ascribed to an oxygen deficient center (ODC; ≡ Si ⋅⋅⋅ Si ≡) and a weak band at 5.8 eV; the 5.0 eV band changed into a 5.8 eV band by irradiating it with the ArF laser. These phenomena can be interpreted as follows: typical silica glasses are considerably deformed from the ideal network and terminal groups such as Si–OH and Si–Cl reduce some amount of highly strained bonds in the glass network. In the sample containing a considerable amount of Cl and no OH, Si–Cl could be the major precursor of the E′ center because only a weak 5.8 eV band is observed. In silica containing a relatively small amount of OH and no Cl, bond breakage is the major process by which laser induced defects are produced.
Structural ordering and chemical configuration of Ba-based infinite-layer superlattices
Authors: Del Vecchio A. , Mirenghi L. , De Caro L. , Tapfer L. , Aruta C. , Petrocelli G. , Balestrino G.
Published: July 2002

In this work, we present a detailed structural and microanalytical study of BaCuO2/CaCuO2 superlattices grown on (001) SrTiO3 substrates by pulsed laser deposition technique. Our analyses were performed by using high-resolution x-ray diffraction, reciprocal space mapping, x-ray specular reflectivity, and x-ray photoelectron spectroscopy. We found that the BaCuO2/CaCuO2 superlattices are grown pseudomorphically on the SrTiO3 substrate. X-ray photoelectron spectroscopy measurements show that the Ca is partially oxidized having both Ca–O and Ca–Ca bonds, while Ba is completely oxidized (Ba–O bonds). The results of our analyses demonstrate that the pulsed laser deposition technique allows one to fabricate high quality infinite-layer superlattices of high structural and chemical ordering.
High field transport in semi-insulating GaAs: A promising material for solid-state detectors
Authors: Mareš J. J. , Krištofik J. , Hubı́k P. , Jurek K. , Pospı́šil S. , Kubašta J.
Published: July 2002

Semi-insulating GaAs detector structures have been studied by transport [current–voltage (I–V) curves] and scanning electron microscope voltage (potential) contrast methods. The saturated part of I–V curves, suitable for radiation detection, is due to the velocity controlled high field transport mechanism while the sharp increase of current with voltage is ascribed to the impact ionization. It is shown that a modified voltage contrast method is able to reveal the actual potential distribution and to contribute to the optimization of the radiation detector structures.
Investigation of crystallographic and bulk strain in doped lead magnesium niobate
Authors: Blue C. T. , Hicks J. C. , Winzer S. R.
Published: July 2002

Using an in situ x-ray diffraction (XRD) technique, the electric-field induced change in the d-spacings Δd/d of the individual microcrystals or “crystallographic” strain is measured on lead magnesium niobate—lead titanate—barium titanate electrostrictive ceramics of composition 0.970(0.900PMN−0.100PT)−0.030BT from zero field to saturation. This crystallographic strain is then compared to the bulk longitudinal strain as measured by Δl/l using a photonic sensor. The crystallographic electric-field induced strain behavior for virgin samples could be fit to a constitutive model, where the induced strain is proportional to the square of the polarization with an electrostrictive coefficient Q determined to be 1.4 (×106 cm4/C2). Attempts to fit the bulk strain behavior on the same sample required lower values of the electrostrictive coefficient Q∼1, and generally yielded worse fits where the model predicted greater than the observed strain values at low electric fields. Subsequent in situ XRD measurements on previously electric-field cycled samples yielded much lower crystallographic strain values with a significantly different electric field dependence.
Resonant tunneling emitter quantum mechanically coupled to a vacuum gap
Authors: Vatannia S. , Gildenblat G. , Schiano J.
Published: July 2002

A resonant tunneling emitter quantum mechanically coupled to a vacuum gap significantly increases the current level as compared to a Fowler–Nordheim field emitter. In addition, an almost monoenergetic distribution of the emitted electrons produces well resolved oscillations of the tunneling current as a function of the vacuum gap thickness and applied voltage. These effects, which may have potential device applications, are investigated using a simple model.
Self-organization during Si1−yCy alloy layer growth on Si(001) using homogeneous coevaporation
Authors: Osten H. J. , Bugiel E. , Zaumseil P.
Published: July 2002

We show that the formation of self-organized pseudosuperlattices during homogeneous epitaxial growth of Si1−yCy on Si(001) is a fundamental phenomenon not limited to special growth techniques. For samples grown at higher temperatures, and therefore, with higher concentration of nonsubstitutional carbon, we find contrast variations in cross-sectional transmission electron microscopy, roughly periodic in the growth direction in epitaxial Si1−yCy alloy layers. The periodicity is a monotonic function of growth temperature and growth rate. Although the final explanation of this phenomenon remains an open question, we are able to rule out several possible effects. These structures do not reflect a modulation in the substitutional C content, the formation of the C-rich SinC phases, or the segregation of nonsubstitutional C-containing defect complexes up to a certain saturation level. Rather, we show that the layers formed during the different growth processes contain different amounts of carbon. There is no critical size of the C-containing defect complexes independent of growth conditions that could lead to the formation of the observed structures. The variation of the periodicity with growth rate and temperature is similar to surface diffusion processes with an activation energy of 0.94±0.04 eV, close to the value for Si adatom diffusion on a Si(001) surface.
Realization of AlGaAs antidot arrays by pulsed laser interference gratings
Authors: Nebel C. E. , Rogg J. , Kelly M. K. , Dahlheimer B. , Rother M. , Bichler M. , Wegscheider W. , Stutzmann M.
Published: July 2002

AlGaAs antidot arrays with about 107 antidots are produced by single-shot interference processing with a pulsed high-power Nd:YAG laser system. We apply magnetotransport experiments and atomic force microscopy (AFM) to explore the electronic and geometric properties of the arrays. The size of the antidot arrays are 3 mm×3 mm and the period varies from 400 to 1000 nm. The dots are elliptic or circular and have diameters ranging from 255 to 690 nm. The magnetotransport experiments are performed at 1.5 K in van der Pauw contact configuration. The laser structuring leaves the two dimensional electron density nearly unchanged but decreases the mobility by a factor of about 30. Several maxima are detected in the low magnetic field magnetoresistivity which are discussed based on the geometric data determined by AFM.