bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarity determination of a GaN thin film on sapphire (0001) with x-ray standing waves
Authors: Kazimirov A. , Scherb G. , Zegenhagen J. , Lee T.-L. , Bedzyk M. J. , Kelly M. K. , Angerer H. , Ambacher O.
Published: July 2002

The x-ray standing wave technique was used to determine the polarity of a 1 μm thick GaN film grown by molecular beam epitaxy on an α-Al2O3(0001) single crystal. The standing wave was generated by x-ray diffraction from the GaN film. The Ga Kα fluorescence yield was recorded as a function of incidence angle within the range of the GaN(0002) reflection. Analysis of the data reveals that the film has grown with N polarity, i.e., the nitrogen atoms occupy the top half of the wurtzite (0001) bilayers.
Doping effects in organic electroluminescent devices
Authors: Yang Jie , Shen Jun
Published: July 2002

Several possible doping effects in organic electroluminescent devices are studied within the framework of the trap-charge limited conduction models. A significant increase in the trapped charges in the doped layer can cause an overall trap charge redistribution and a shift of the location of the recombination zone. Some experimental data are analyzed and their possible relevance to our predictions is discussed. An experimental verification scheme is also proposed.
Intrinsic and extrinsic surface states of single crystalline SrTiO3
Authors: Adachi Yuji , Kohiki Shigemi , Wagatsuma Kazuaki , Oku Masaoki
Published: July 2002

We have no evidence of intrinsic surface states in the band gap for in situ fractured SrTiO3:La, SrTiO3:Nb and SrTiO3. The valence band maximum of the samples lie at 3.3 eV below the Fermi level, indicating that the band bending is negligibly small. No emission from the region of bulk band gap indicated that the intrinsic surface states are largely empty. Ar+ ion bombardment of in situ fractured samples gave rise to a peak at around 11.6 eV, indicating the Ti+3 state.
Relaxations of nonpolar zinc blende (110) surface of GaN, AlN, and BN
Authors: Li Zhi-Qiang , Chen Hao , Kong Fan-Quan , Sun Qiang , Kawazoe Yoshiyuki
Published: July 2002

The relaxations of zinc blende GaN, AlN, and BN (110) surfaces have been studied systematically by ab initio total energy calculations. We find that the surface rotation angle of GaN(110) is larger than that of AlN(110) and the surface bond contraction of GaN(110) is smaller than that of AlN(110). The surface relaxations of GaN(110) and AlN(110) could be understood by electrostatic attraction between anion and cation. The BN(110) surface shows the largest surface rotation angle and surface bond contraction and this behavior could be explained by the covalent and double bonding characters between boron and nitrogen atoms.
Spectral filtering for time isolation of intensive picosecond optical pulses from a Q-switched laser diode
Authors: Vainshtein S. N. , Kostamovaara J. T.
Published: July 2002

A significant wavelength separation is found between the shot-pulsed mode and stationary lasing in the emission band of a Q-switched single-heterostructure laser. This feature allowed a 35 ps/100 W single optical pulse to be achieved by means of band-pass optical filtering. An attendant long-duration optical response, attributed to optical transitions with at least one localized tail state involved, is suppressed by a factor of 104 as compared with the peak power of the single picosecond pulse. A complicated time-resolved spectrum is interpreted in terms of a qualitative model involving saturable absorption and carrier heating.
Low-temperature epitaxial growth of Ge-rich Ge–Si–C alloys: Microstructure, Raman studies, and optical properties
Authors: Yang B.-K. , Krishnamurthy M. , Weber W. H.
Published: July 2002

Low-temperature (∼200 °C) molecular beam epitaxy of Ge-rich Ge1−x−ySiyCx alloys grown on Si(100) have been investigated by in situ reflection high-energy electron diffraction, ex situ x-ray diffraction, transmission electron microscopy, Raman scattering, and ellipsometry. The Si contents were either ∼20 or ∼40 at % and the C concentrations were nominally varied from zero up to ∼8 at %. Selected samples were annealed in an Ar ambient at 750 °C to evaluate the stability of the thin films. With increasing C concentration, the epitaxial growth mode changes from two-dimensional (2D) layer growth to 3D island growth. Under the growth conditions studied, the GeSiC films have a tendency to form planar defects, whose density increases with increasing C and Si concentrations. The x-ray diffraction data show that the lattice parameter decreases with increasing C concentration. It is estimated that a maximum of ∼2–3 at % C is substitutionally incorporated into these films. Raman spectra of the alloy films show that the effects of C on the strong Ge–Ge and Ge–Si local modes are far less than the effects due to Si. We are unable to observe any systematic change in the Ge–Ge mode, whereas the Ge–Si mode appears to shift to lower frequency with the small addition of C. Ge1−x−ySiyCx films formed by annealing Ge1−xCx films on Si are also discussed. Spectroscopic ellipsometry determinations of the film’s optical constants show that the primary effect of C is to reduce the strength of the E1 critical point feature.
Irreversibility line and granularity in Chevrel phase superconducting wires
Authors: Cheggour N. , Decroux M. , Fischer O/. , Hampshire D. P.
Published: July 2002

A Chevrel phase monofilamentary wire was hot isostatically pressed (HIPed) under 2 kbar of argon pressure at 940 °C for 1.5 h. Transport critical current densities (Jc) were measured from 2.7 to 11 K, in magnetic fields up to 15.5 T. Compared to previous results for a sample HIPed at 900 °C for 0.5 h, the irreversibility field [Birr(T)] has been improved by 3 T to 34.5 T at 4.2 K. The Jc has also been enhanced at 14 T and 4.2 K to the highest reported value of 6.7×108 A/m2. The reduced pinning force data show excellent temperature scaling. Resistivity measurements in high magnetic fields confirm an improvement in Birr(T) and provide evidence that the improved properties are due to better superconducting properties at the grain boundaries.
Reliability of metal semiconductor field-effect transistor using GaN at high temperature
Authors: Yoshida Seikoh , Suzuki Joe
Published: July 2002

The reliability of a GaN metal semiconductor field-effect transistor (MESFET) was investigated. We used Au/Pt as a Schottky gate and Ti/Al as a source drain of a GaN MESFET. The thermal stress test of the GaN MESFET at high temperature was investigated. It was found that no change of FET characteristics was observed even after the device was heated at 400 °C for 1000 h. Furthermore, using a GaN MESFET which was heated at 400 °C for 1000 h, a life of FET at 350 °C was examined by a continuously current injection at 350 °C. We confirmed that the FET performance did not change at 350 °C for 150 h. No degradation of metal–semiconductor interface was observed by secondary ion mass spectrometry and a transmission electron microscope. The reliability of the GaN MESFET as a high-temperature operation transistor was fully demonstrated.
Temperature effect on the roughness of the formation interface of p-type porous silicon
Authors: Setzu S. , Lérondel G. , Romestain R.
Published: July 2002

We have studied the influence of the anodization temperature on the formation of porous Si for different current intensities. We have monitored the porosity, growth rate, luminescence, refractive index, and porous Si/bulk Si interface roughness. A strong decrease of the roughness was obtained for low temperature anodization. These results were used to fabricate distributed Bragg reflectors with a remarkable optical quality (Rmax=99.5%) for low doped p-type silicon.
Finite-element analysis of quantum wires with arbitrary cross sections
Authors: Ogawa M. , Kunimasa T. , Ito T. , Miyoshi T.
Published: July 2002

A finite-element method is developed for the analysis of eigenstates in the valence band of quantum wires which have arbitrary potential profiles. Our method is basically based on the Galerkin procedure and triangle linear elements are used as finite elements. In our formulation the effect of the band mixing in the valence band is duly taken into account. Boundary conditions at heterointerfaces are also taken into account in the multiband envelope function space. Numerical examples are presented for circular, square, rectangular, and triangular quantum wire structures. The relation is clarified between the degeneracy in the E-ky dispersion curve and the symmetricity of the confinement potential.
The approximate analytic expressions for field dependence of surface impedance of high-Tc superconducting thin film in low- and intermediate-field region
Authors: Jin B. B. , Wu R. X.
Published: July 2002

Assuming that a high-Tc superconducting thin film is composed of a network of superconducting grains connected by grain-boundary weak links, the equation describing its response under microwave current irf is solved using a perturbation method to the third order of irf. Based on the obtained results, analytic expressions for film’s microwave surface impedance are derived, revealing its dependence on a rf magnetic field in low and intermediate field limit. Good qualitative agreement with experiments is observed.
Effect of silicon substitution on the magnetic properties of Tb2Fe17 single crystals
Authors: Du J. , Wu G. H. , Tang C. C. , Li Y. X. , Zhan W. S.
Published: July 2002

The growth of single crystals Tb2Fe17−xSix (x=0, 1, 2, 3, and 3.3) by the Czochralski method is performed. The crystal structure and magnetic properties are investigated. The substitution of silicon for iron leads to the reduction of the lattice parameters, and the significant increase of the Curie temperature from 413 to 526 K. The mean field analysis shows that the Fe–Fe exchange interaction is much strengthened while the Tb–Fe one is slightly weakened by the introduction of silicon. The anisotropy constants up to the sixth order are calculated by fitting the magnetization process of the samples at 1.5 K under the field up to 65 kOe. In addition, a narrow domain wall pinning-dominated process caused by the substitution of Si for Fe is confirmed and analyzed.
Enhancement in electron emission from polycrystalline silicon field emitter arrays coated with diamondlike carbon
Authors: Mimura Hidenori , Hashiguchi Gen , Okada Morihiro , Matsumoto Takahiro , Tanaka Masaki , Yokoo Kuniyoshi
Published: July 2002

The gated polysilicon field emitter arrays coated with diamondlike carbon (DLC) were fabricated by a transfer mold method using anisotropic etching of Si and the deposition of DLC using methane ion beam. This paper describes significant enhancement in electron emission and considerable reduction in turn-on voltage for the emitter arrays by the DLC coating. In addition, the paper discusses the emission mechanism from the energy band diagram of a field emitter coated with a sufficiently thin dielectric material having a low electron affinity, and shows that an effective work function of the DLC coated polysilicon field emitter is about 2.9 eV.
Response to “Comment on ‘Dispersion of birefringence in AgGaS2 and CuGaS2’ ” [J. Appl. Phys. 84, 2364 (1998)]
Authors: Yu Peter Y. , Choi In-Hwan , Eom Sung-Hwan
Published: July 2002

In a comment preceding this response [J. Appl. Phys. 84, 2364 (1998)], Bhar noted that the expression we proposed to explain the dispersion of birefringence in AgGaS2 and CuGaS2 near their band gaps did not fit the long wavelength dispersion as well as the Sellmeier equation approach. We point out that our model can be improved to fit the long wavelength dispersion provided we can include contributions to the birefringence from the higher energy transitions.
Auger electron spectroscopy studies of nitridation of the GaAs(001) surface
Authors: Aksenov Igor , Nakada Yoshinobu , Okumura Hajime
Published: July 2002

Auger electron spectroscopy has been used to investigate the processes taking place during the initial stages of nitridation of the As-stabilized GaAs(001)-2×4 surface by active nitrogen species generated by a radio-frequency plasma source. The results of analysis of the spectral shape of core-level Auger electron signals from Ga, As, and N, as well as dependencies of the intensities of those signals on the duration of nitridation combined with reflection high-energy electron diffraction results show that nitridation occurs in two distinct steps: the first step (with duration of only a few minutes) being the formation of 1 ML of nitrogen (partially mixed with arsenic) on the surface, and the second stage being the formation of the disordered GaAsN phase, which may be the GaAsxN1−x surface phase. The subsequent thermal annealing for several minutes at 600 °C leads to the desorption of arsenic and the resulting crystallization of the GaAsN phase into a cubic GaN layer of about 20 Å thickness.
The nucleation process and magnetic interaction in nanocrystalline alloy Nd4Fe77Co3B16
Authors: Youhui Gao , Jinghan Zhu , Yuqing Weng , Park Eon Byung , Yang Choong Jin
Published: July 2002

The temperature dependence of coercivity of nanocrystalline Nd4Fe77Co3B16 is analyzed in this article. The best linear fit of HC(T)/Ms(T) vs 2K1(T)/μ0Ms2(T) indicates that HC is controlled by a nucleation process in the nanocrystalline two-phase magnet. The effects of additives (Hf, Ga) and magnetic field heat treatment on the microstructure and magnetic interaction are also studied. It is found that the condition of the grain shapes is improved after adding Hf and Ga, but the surfaces are deteriorated by some precipitates present on the boundary. Taking into account the values of α, Neff, the results of transmission electron microscopy (TEM) and Mossbauer spectroscopy, the magnetic field heat treatment not only induces grain refinement but also causes a uniform distribution of the soft and hard phases, and therefore it is a recommendable method for improving the condition of microstructure. From the analysis of high resolution TEM, amorphous phase with scale of <10 Å exists between the different phase’s grains due to the mismatch of their lattices. The existence of amorphous phase reduces the exchange-coupled interaction between the two magnetic phases and results in a relatively low coercivity in the nanocrystalline magnet. The magnetic interaction in Nd2Fe14B/Fe3B+αFe based nanocrystalline magnet was studied using δM plots. The results show that the exchange-coupled interaction is greatly enhanced in the sample annealed with magnetic heat treatment, achieving the highest energy product (BH)max=15.8 MGOe.
Proximity gettering of platinum in proton irradiated silicon
Authors: Schmidt D. C. , Svensson B. G. , Keskitalo N. , Godey S. , Ntsoenzok E. , Barbot J. F. , Blanchard C.
Published: July 2002

Epitaxial silicon samples of n type have been implanted with 850 keV protons at doses of 5.8×1011 to 5×1013 H+ cm−2. Subsequent indiffusion of platinum at 700 °C for 30 min resulted in the presence of a single deep level, which is attributed to the platinum acceptor level, at 0.23 eV below the conduction band edge. Depth profiling of this level shows that the substitutional platinum is following the vacancy profile in the peak region around the projected range for the protons. In addition, at more shallow depths, a strong increase of the platinum concentration is also observed. Without ion implantation, no deep levels are detected after in-diffusion at 700 °C, while at 800 °C, the Pt deep level concentration is inferior to the one reached after preimplantation of hydrogen with a dose of at least 5×1012 H+ cm−2. In-diffusion at 600 °C into 5×1013 H+ cm−2 implanted samples did not lead to an enhanced platinum accumulation. A tentative explanation of this proximity gettering of Pt is proposed, which is for the first time observed after light-ion irradiation.
Soft-solution process of Fe–Tb–O films for use in high frequency micromagnetic devices
Authors: Lim Pang Boey , Yamada Koji , Inoue Mitsuteru , Arai Ken’ichi , Izaki Masanobu , Fujii Toshitaka
Published: July 2002

An attempt for fabricating Fe–Tb–O films with heterogranular structures was made using a soft-solution process with an alternative current for deposition. The resultant film composition was found to be sensitive to the concentration of complex agent, KNaC4H4O6, the intensity i, and the frequency f of deposition current. These dependencies are well explained from the model for film formation. When KNaC4H4O6=283.47 mM, i=6.7 mA/cm2, and f=6 mHz, the film with very high Tb content exceeding 29 at. % were obtained, which exhibited a relatively soft magnetic magnetization of Hc=29 Oe and Mr/Ms=0.93.
Effect of magnetization ripple at high frequencies in amorphous ferromagnetic thin films
Authors: Spenato D. , Fessant A. , Gieraltowski J. , Le Gall H. , Tannous C.
Published: July 2002

An analytical calculation of the permeability frequency spectra using Landau–Lifshitz theory is presented and compared to experimental results obtained on soft amorphous ferromagnetic thin films with an in-plane uniaxial magnetic anisotropy. The disagreement between theory and experiment is attributed to a small magnetic anisotropy dispersion in the film (magnetization ripple). We have measured the real part of the transverse biased permeability μt′ at different frequencies of the exciting field in order to estimate the ripple effects in our films and to improve our model. We have plotted the value of the bias field Hm, at which μt′ versus exciting field frequency reaches a maximum. Hm is found to be frequency independent at low frequencies (up to 280 MHz). Above a critical frequency it increases rapidly, suggesting that ripple effects are strongly frequency dependent.
Investigation of secondary recrystallization kinetics in grain-oriented electrical steel using the ultrasonic interferometry
Authors: Sadahiro Kenichi , Hayakawa Yasuyuki , Kurosawa Mitsumasa , Komatsubara Michiro
Published: July 2002

Measurements of the growth rate for the individual secondary recrystallized Goss grain in Fe–3%Si containing AlN and MnSe as grain growth inhibitors were made by the ultrasonic interferometry. It was found that the growth rates in the rolling direction for secondary recrystallized grains varied from 0.3 to 0.6 mm/min, and that the growth rate in the rolling direction is 1.2–1.9 times higher than that of the cross-rolling direction. The set of results were explained in terms of the concept of the “growth advantage” which is defined as the fraction of high-energy grain boundaries having the misorientation angles between 20° and 45° with respect to the specific secondary recrystallized grain in the primary recrystallized grains.
The effect of mechanical working on the in-plane magnetic properties of Hiperco 50
Authors: Storch M. L. , Rollett A. D. , McHenry M. E.
Published: July 2002

Due to its high permeability and low coercivity Hiperco 50 is a premier soft magnetic material. It is often utilized, in laminate form, in rotors for aircraft power generators. These laminates are usually linearly cold rolled from larger billets. The rolling procedure results in the development of specific texture components within the material. This work shows the effect of rolling schedule on the texture of resulting material. Several rolling schedules were considered including straight rolling, cross rolling, clock rolling, and intermediate schedules between these. Varying the rolling schedule has been shown to have a dramatic effect on the resulting texture. In addition, the connection between texture and magnetic anisotropy is explored through a consideration of the angular distribution of easy axes and magnetostrictive strains within the material. This is done through theoretical calculations based on published values of the magnetostrictive and magnetocrystalline anisotropy constants for FeCo. We have observed a strong relationship between the mechanical history of the material and its texture. This implies a strong dependence of the in-plane magnetic properties on the mechanical history.
Effect of La substitution on the CuO2 chain spin gap in Sr14Cu24O41
Authors: Wang Qiheng , Lin Chin , Ni Yongming , Zhang Yingzi
Published: July 2002

The compound Sr14Cu24O41 has a unique structure. It consists of an alternating stack of the plane containing edge-sharing CuO2 chains, and the plane containing two-leg Cu2O3 ladders sandwiched by Sr layers. The CuO2 chain also has a singlet ground state originating from a dimerization. The dimerized state in the chain depends critically on the number of holes. The effect of La substitution for Sr on the CuO2 chain spin gap is investigated by measuring the magnetic susceptibility of Sr14−xLaxCu24O41 with x⩽1. Due to the large spin gap, the Cu2O3 plane is magnetically inert below room temperature. For x=0, the susceptibility shows a characteristic decrease below 80 K, followed by an upturn below 20 K due to a Curie contribution. After subtracting the Curie term, the susceptibility is fitted by a dimer description. The dimer number ND=1.45 and the dimer coupling constant JD=139 K are obtained. With increasing La content, both ND and JD decrease gradually, indicating the weakening of the antiferromagnetic interaction between Cu2+ spins in the CuO2 chain. The Curie contribution enhances at the same time, and the gap feature becomes gradually smeared. It is difficult to fit the data when x=1.0.
The quantum Heisenberg antiferromagnet on the square lattice
Authors: Cuccoli Alessandro , Tognetti Valerio , Verrucchi Paola , Vaia Ruggero
Published: July 2002

The thermodynamic behavior of the quantum Heisenberg antiferromagnet on the square lattice (QHAF) is tackled by the pure-quantum self-consistent harmonic approximation, a semiclassical method based on the path-integral formulation of quantum statistical mechanics. Results for various properties are obtained for different values of the spin and the effects of the dipolar anisotropy are considered. Furthermore, the validity of the quantum nonlinear σ model approach is discussed and shown to be inadequate to describe the behavior of the QHAF with spin value S⩾1 in the experimentally accessible temperature region.
Study of optimum design on magnetizing fixtures for the ring magnet in spindle motors
Authors: Yao Y. D. , Huang D. R. , Huang S. Z. , Wang S. J. , Ying T. F.
Published: July 2002

The magnetization profiles of the ring magnet in spindle motors have been investigated theoretically by a computer-aided simulation with two dimensional modeling processes of finite element analysis. Under the three topologies of magnetizing fixture, the current was varied below 15 kA. The double-sided fixture shows a minimum cogging torque near 8 kA. However, the single-sided fixture internal to the magnet shows a maximum torque constant near 8 kA. Experimental data are in good agreement with theoretical calculation. A high efficiency and low cogging torque spindle motor has been achieved by optimum design of the magnetizing fixture to magnetize the ring magnet in a spindle motor.
Imaging of refractive index change by the reflection-mode scattering-type scanning near-field optical microscope: Simulation and observations
Authors: Sasaki Hiroko , Sasaki Yasuo
Published: July 2002

The response of the reflection-mode scattering-type scanning near-field optical microscope (RS-SNOM) to the refractive index change was investigated numerically and experimentally. Numerical results indicated that the signal of the RS-SNOM reflected the optical property of the sample and the image resolution was drastically improved in an ac mode SNOM image. The experimental results using alternating layers of GaAs and Al0.55Ga0.45As sample were in quite good coincidence with the numerical results and the spatial resolution of the RS-SNOM reached 10 nm in the case of ac mode SNOM operation.
Electric current in dc surface flashover in vacuum
Authors: Neuber A. , Butcher M. , Hatfield L. L. , Krompholz H.
Published: July 2002

Dielectric surface flashover in vacuum is characterized by a three-phase development, as shown by current measurements covering the range from 10−4 to 100 A, assisted by x-ray emission measurements, high speed photography, and time-resolved spectroscopy. Further information is gained from a comparison of the flashover dynamics at 77 and 300 K. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification, with a duration on the order of 100 ns, reaching currents in the ampere level. The final phase three is characterized again by a fast (nanoseconds) current rise up to the impedance-limited current on the order of 100 A in this specific apparatus. The development during phase two and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the surface. The feedback mechanism towards a self-sustained discharge is due to space charges leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as field enhancement factors, outgassing rate, and the buildup of the gas density above the surface, are determined by fitting calculated results to experimental data.
Vacancies and interstitial atoms in e−-irradiated germanium
Authors: Ehrhart P. , Zillgen H.
Published: July 2002

High purity Ge wafers have been irradiated at low temperatures with MeV electrons and have subsequently been investigated by x-ray diffraction methods. The intensity of the Huang diffuse scattering shows that a high concentration of defects (>1019 cm−3) can be frozen in at 4 K. A large fraction of the defects is stabilized in the form of close Frenkel pairs which are characterized by the nearly perfect cancellation of the long range displacement fields of the interstitial atom and the vacancy. We discuss the absolute size of these displacements as well as the introduction rate of the defects, which is of the order of ∑=3 cm−1. The high defect introduction rates are at variance to the results of electrical and optical investigations and indicate that these methods detect only a few percent of the total defect concentration which is produced and frozen in at 4 K. The consequences for the understanding of the defect production in Ge and for the assumption of an athermal migration of interstitial atoms are discussed in close relation to similar results for Si. In addition, we discuss the differences between the defect patterns observed after 4 K irradiation to those observed after room-temperature irradiations and the thermally activated defect reactions up to the final annealing at 600 K.
Effective diffusion lengths for minority carriers in solar cells as determined from internal quantum efficiency analysis
Authors: Brendel Rolf , Rau Uwe
Published: July 2002

We introduce a general relationship between the effective diffusion length LQ of solar cells derived from spectral quantum efficiency Q and the effective diffusion length LJ that determines the saturation current j0=qn0D/LJ of the diode in the dark. The general relation LQ⩾LJ holds in the presence of grain boundaries and of spatially nonhomogeneous doping profiles. We find the relation LQ=LJ for solar cells without potential fluctuations at the collecting junction.
Intrinsic Josephson junctions in oxygen-deficient YBa2Cu3O7−δ film deposited on a substrate step
Authors: Wang H. B. , Chen J. , Tachiki T. , Mizugaki Y. , Nakajima K. , Yamashita T.
Published: July 2002

Using c-axis-oriented oxygen-deficient YBa2Cu3O7−δ film deposited across a low-angle step on a SrTiO3 substrate, we successfully demonstrated intrinsic Josephson effects. In addition to several voltage jumps of large amplitudes (a few millivolts) and remarkable hysteresis on the dc current–voltage curves, we observed upturns on the current–voltage curves under microwave irradiation which appeared at increasingly high voltages with increasing microwave power. We proposed to explain this observation in terms of high-order microwave-induced Shapiro steps.
Microwave study of Nd0.7Sr0.3MnO3 thin film resistivity
Authors: Claycomb J. R. , Tralshawala N. , Xie L.-M. , Wosik J. , Miller J. H.
Published: July 2002

We report on microwave studies of Nd0.7Sr0.3MnO3 thin film losses in a shielded TE011 dielectric cavity. The cavity quality (Q) factor and resonant frequency are measured as a function of temperature with the dielectric cavity loaded with a thin film on a LaAlO3 substrate. A reference Q measurement is then made without the film enabling the extraction of the film-Q factor Qfilm. Here the temperature dependence of the Q factor is discussed in terms of resistive losses in the thin film. A numerical finite difference time domain code is then used to extract the microwave resistivity as a function of temperature from the measured Q factors. The numerical method involves the discretization of Maxwell’s equations on an axisymmetric space-time grid coupled to a discrete Fourier transform to determine the resonant frequency.
Raman spectra of the chalcopyrite compound CuInTe2
Authors: Rincón C. , Wasim S. M. , Marı́n G. , Huntzinger J. R. , Zwick A. , Galibert J.
Published: July 2002

The lattice vibrational modes of the chalcopyrite compound CuInTe2 have been obtained at several temperatures by the unpolarized Raman spectra. Lines observed at 48, 74, 129, 138, 155, and 185 cm−1, identified with E modes, and at 60, 106, and 170 cm−1, assigned to B2 modes, agree quite well with the peaks reported from infrared reflectivity data. On the other hand, using the available theoretical models, the peaks that appear at 123 cm−1 and those observed around 65, 116, and 223 cm−1, which are only Raman active, are assigned to the A1 and B1 modes, respectively.
1/f
γ
noise in polycrystalline silicon thin-film transistors
Authors: Dimitriadis C. A. , Brini J. , Lee J. I. , Farmakis F. V. , Kamarinos G.
Published: July 2002

Polycrystalline silicon thin-film transistors show a 1/fγ (with γ<1) low frequency noise behavior. The 1/fγ noise is explained by emission and trapping processes of carriers between trapping states located within an energy range of kT around the Fermi level and the exponential tail states. We have derived a simple relationship between the frequency index γ of the noise spectrum and the energy distribution parameter of the tail states. From the experimental noise data, the theoretical model allows us to determine the tail states energy distribution and the grain boundary trap density.
Band-engineered semiconductor optical waveguides for integral transform implementation
Authors: Dragoman D. , Dragoman M.
Published: July 2002

The article proposes the implementation of integral transforms using band-engineered semiconductor waveguides. This application could open a new perspective regarding optical computing.
Theory of localized spin wave modes at surface steps on ferromagnetic films
Authors: Jiang L. J. , Cottam M. G.
Published: July 2002

A Green function matrix method has been developed to investigate the spin dynamics in low-dimensional Heisenberg ferromagnetic nanostructures. The theory is based on a generalization of earlier work on the spin wave spectra of magnetic films and their interfaces, and in the present case applications are made to the surface step produced at a perpendicular interface between two films of the same material but different thickness. Analytic expressions are obtained for the dispersion relation and spectral intensities of the localized surface and edge spin waves and the quantized bulk spin waves near a step. The effects of varying the step height and modifying the exchange interactions near the step are studied. These factors influence the number of localized modes, as well as their frequency, intensity, and attenuation.
Effect of damping constant on the switching limit of a thin-film recording head
Authors: Mao Chris Y. , Zhu Jian-Gang , White Robert M. , Min Tai
Published: July 2002

This article presents a micromagnetic study on the dynamics of magnetic flux transfer in the pole tip region of a stitched-pole magnetoresistance/thin-film head. Three-dimensional vortices are found inside the pole tips. The flux at the ABS surface is conducted by the motion of the vortex moving in the cross-track direction. The resulting head field well follows the driving flux with little phase delay and little dependence on the damping constant, α. The vortex motion under the driving flux maintains similar speed even when the damping constant is decreased to zero. In this case, it is found that in addition to the motion of the vortex, spins precess locally with small cone angles, resulting in small head field fluctuations. It is concluded that the speed of magnetization flux reversal in the pole tip region is not limited by small damping constant.
Magnetic anisotropy in Gd/(FeCo) and Gd/Fe multilayers for high density magneto-optical recording
Authors: Stavrou E. , Röll K.
Published: July 2002

Very recently, new techniques in the magneto-optical recording were proposed which promise the readout of domains smaller than the optical diffraction limit. For this purpose Gd–(Fe,Co) alloy films are used, which change their magnetization direction with temperature. In this experimental study for the first time spin reorientation effects in multilayers of Gd/(FeCo) and Gd/Fe were investigated with varying composition and periodicity. Both Gd/(FeCo) and Gd/Fe films show a compositional dependence of the anisotropy similar to a phase transition. An in-plane to perpendicular spin reorientation near the compensation temperature Tcomp can be obtained in rare earth rich films for small periodicities λ∼1.1 nm, but for increasing λ the anisotropy change occurs at temperatures different from Tcomp. The opposite effect, i.e., spin reorientation from perpendicular to in-plane, can be observed in transition metal rich Gd/(FeCo) multilayers but not in Gd/Fe multilayers. The anisotropy behavior and the reorientation effects are explained qualitatively within a new model.
Effects of oxidation and abnormal grain growth on the magnetic properties of thin Fe–SiO2 granular films
Authors: Zhou J. N. , Butera A. , Jiang H. , Yang D. H. , Barnard J. A.
Published: July 2002

In the Fe–SiO2 granular system the effects of oxidation and abnormal grain growth on the magnetic behavior are significant. As film thickness is reduced a larger fraction of Fe grains intersect the free surface and are prone to oxidation even in good vacuum conditions. Films coated with protective SiO2 layers were found to be unaffected by oxidation. The presence of a surface during annealing also affects the microstructural evolution. Transmission electron microscopy observations revealed a bimodal distribution of Fe grain sizes in uncoated films due to grain merger assisted by surface diffusion. This bimodal distribution of Fe grain sizes is associated with a coercivity (Hc) vs vol. % Fe curve containing two peaks. By contrast, SiO2 coated films exhibited a uniform microstructure without unusually large grains and a smoother Hc vs vol. % Fe curve with a well defined maximum. Room temperature coercivities of ∼1000 Oe can be routinely achieved in Fe–SiO2 granular films as thin as 10 nm. X-ray photoelectron spectroscopy studies showed evidence for the chemical interaction between Fe and SiO2 in SiO2/Fe/SiO2 trilayer model film samples.
Magnetic and recording properties of CoCrPtTa thin film media with CrW underlayer
Authors: Malhotra Sudhir S. , Stafford Donald C. , Lal Brij B. , Gao Chuan , Russak Michael A.
Published: July 2002

CoCrPtTa thin film media using Cr as an underlayer has been developed as a candidate for future high density longitudinal recording. In an effort to reduce the lattice mismatch between the Cr underlayer and the magnetic layer, the Cr underlayer has been doped with various elements. In this work the effect of adding Wx (x=10 and 15 at. %) to the Cr underlayer on the magnetic, crystallographic, and recording properties are reported. CrW10(Cr90W10) underlayer produced about 400 Oe higher coercivity and higher S* at Mrt≈0.60 memu/cm2 compared to the Cr underlayer. Also at 240 kfci recording density for CoCrPtTa/CrW10 media the normalized media noise was similar and signal-to-noise ratio was about 2 dB higher compared to the CoCrPtTa/Cr media. For the CrW15(Cr85W15) underlayer at Mrt≈0.60 memu/cm2 the coercivity was about 300 Oe higher with S*=0.88, but the normalized media noise was also higher than the CoCrPtTa/Cr media. The x-ray diffraction data suggest better lattice match between the CoCrPtTa/CrW10 media due to the addition of W to the Cr underlayer compared to the CoCrPtTa/Cr media. The magnetic and recording properties are correlated to the crystallographic properties of the CoCrPtTa/CrW thin film media.
Ultrathin Si buffer layers to improve the exchange bias between Ni–Fe and FeMn layers
Authors: Nakagawa S. , Nishimura K. , Shimizu Y. , Naoe M.
Published: July 2002

Ni–Fe/FeMn bilayers were deposited by using dual ion beam sputtering apparatus. In order to promote (111) orientation in γ-FeMn layers and to exhibit exchange bias field Hex, Si and/or silicon nitride (Si3N4) layers were deposited as buffer layers. Although silicon nitride (Si3N4) buffer layers did not improve (111) orientation in FeMn layers, as well as in Ni–Fe layers, an amorphous Si buffer layer only 1 nm thick improved (111) orientation in FeMn layers and increased Hex to above 100 Oe. FeMn/Ni–Fe bilayers deposited on Si(0.5 nm)/Si3N4(1 nm) bilayered buffer layer also exhibited high Hex of about 130 Oe. This result implies that the ultrathin a-Si buffer layer was effective in improving (111) orientation in Ni–Fe layers. Very thin Si/Si3N4 bilayers may be applicable as dielectric layers in tunneling magnetoresistive spin valve devices.
Modification of the magnetic properties of Fe/Cr(001) by controlling the compensation of a vicinal Cr(001) surface
Authors: Escorcia-Aparicio Ernesto J. , Choi Hyuk J. , Wolfe J. H. , Ling W. L. , Kawakami R. K. , Qiu Z. Q.
Published: July 2002

The degree of compensation of a normally uncompensated Cr(001) surface is controlled by using a curved substrate with steps parallel to the [100] direction. In this way, the degree of frustration caused by steps at the interface between an Fe overlayer and the Cr substrate can be systematically varied. Previous work on flat Cr(001) at temperatures below the Cr ordering temperature (311 K) has identified a critical Fe thickness of ∼35–38 Å, below which the Fe films display a reduced remanence. For our curved Cr substrate, below this critical Fe thickness three phases are observed for low (<∼2.5°), intermediate and high (>∼5°) miscut angle respectively: (i) multidomain; (ii) single domain with magnetization perpendicular to the step edges; and (iii) single domain with magnetization parallel to the step edges. At the same temperature, for Fe films above the critical thickness, region (i) disappears and only regions (ii) and (iii) remain. In a second experiment, the adsorption of submonolayer Au on the Fe is observed to increase the strength of the step-induced anisotropy and accordingly vary the position of the transition from regions (ii) to (iii).
Model of media noise generation based on modulation theory
Authors: Minuhin Vadim B.
Published: July 2002

A random profile of transitions in magnetization on thin-film media causes fluctuations of transition-region widths and locations. The readback transducer reacts to an averaged derivative of magnetization, and this results in modulation of magnetization derivative pulses, in shape and amplitude as well as in position (jitter). Due to bandwidth limitations in the readback transducer the high-frequency sideband of the modulation is almost totally suppressed. Thus, readback filtration converts a double-sideband write-cycle modulation into a single-sideband readback modulation.
Experimental determination of the distribution of relaxation time for thermally activated transition in thin film magnetic recording media
Authors: Yang Cheng , Sivertsen John M. , Li Tan , Judy Jack H.
Published: July 2002

Although grain size distribution has significant effects on the time decay behavior of thin film magnetic recording media, the grains size itself is not a truly independent parameter due to intergranular exchange coupling. Relaxation time incorporates the interaction. Its distribution will determine the time decay performance of thin film magnetic recording media. An algorithm is proposed to determine the unique distribution of relaxation time in magnetic recording media directly from the experimental data without any presumption. It was found for the first time that the distribution of relaxation time is very well described by Weibull distribution.
Thermal stability of Co/Pt multilayers
Authors: Chen Xufeng , Kryder Mark H.
Published: July 2002

Thermally assisted domain wall motion of perpendicular Co/Pt multilayers was characterized by observing domain expansion using a Kerr microscope and measuring magnetization decay curves. Unlike conventional longitudinal recording media, the viscosity coefficient was found to be highly time dependent over a large range. Activation energies of the domain wall motion were found to be in the range from 60 to 140 kT for the samples examined. A simple model was developed to determine the thermal stability of a circular domain in Co/Pt multilayers and used to project the stability of circular domains in these materials. Based upon the data collected, circular domains with diameter down to 20 nm are predicted to be thermally stable with a relaxation time constant of greater than 10 years.
Dynamic coercivity measurements in thin film recording media using a contact write/read tester
Authors: Moser A. , Weller D. , Best M. E. , Doerner M. F.
Published: July 2002

Thermally activated magnetization reversal processes become manifest in the dependence of the remanent coercivity on the time during which a magnetic field is applied opposite to the initial magnetization direction. They have important consequences for the long term stability and short time writeability of future high density recording media. In this paper, we report on a new experiment using a contact write/read tester to study the time dependence of the remanent coercivity over more than 10 orders of magnitude (from 6 ns to >60 s). Remanence coercivity and signal decay measurements of a CoPtCr recording medium with 5.5 nm thickness are presented.
Angular dependence of giant magnetoresistance properties of exchange biased spin valves
Authors: Mao Sining , Plumer Martin , Mack Anthony , Yang Zhijun , Murdock Ed
Published: July 2002

Giant magnetoresistance (GMR) responses were studied as a function of applied measurement field angle in FeMn and NiMn pinned spin valves. As the measurement field varies away from the pinning field direction, the peak GMR ratio and the free layer coercivity decrease. The GMR curve changes from unsymmetrical to symmetrical with respect to the applied field polarity. The GMR responses from the component parallel to the applied field are proportional to the cosine of the angle between the pinning and the applied field. The results can be described well by a simple vector model, and an empirical method to measure the pinning field rotation is established.
Constant height scanning tunneling spectroscopy using an alternating voltage signal
Authors: Ryan George W. , Tornallyay J.
Published: July 2002

A method of performing scanning tunneling spectroscopy (STS) at a constant height while maintaining feedback control is described. By using sinusoidal voltage oscillations the ac tunneling current can be separated from the displacement current present in current–voltage (i–V) measurements. The method allows statistically well averaged spectra spanning both occupied and unoccupied states of the sample to be obtained from a continuous set of i–V curves. The unoccupied π* state on graphite is observed using the technique described, and the utility of the technique is further demonstrated by the detection of several bulk states on NiTe2 and PdTe2, two transition metal dichalcogenides. A weighting towards k perpendicular states is seen in the STS spectra. These results show that the fast alternating current form of STS is suitable for probing low amplitude electron states on metal surfaces within 2 eV of the Fermi level.
Spontaneous formation of multiple land-and-groove structures of silica thin films
Authors: Takeda Yasuhiko , Matsuoka Yoriko , Motohiro Tomoyoshi
Published: July 2002

We found spontaneous formation of microscopic multiple land-and-groove structures of silica thin films. Silica and nickel were simultaneously deposited onto glass substrates from two opposite oblique directions to form columnar structures of silica among which nickel nanoparticles were embedded. Then nickel was dissolved in hydrochloric acid solution. After the dissolution of the nickel particles the columns of silica became very unstable and coalesced to form the multiple land-and-groove structures. The grooves are oriented to the direction perpendicular to the two deposition directions. The distances between the neighboring grooves are fairly uniform, and can be controlled between several hundred nanometers and several microns by changing the film thickness and the ratio of the nickel deposition rate to the silica deposition rate. The process found here may propose a new class of micro fabrication techniques in contrast to the artificial photolithography.
Characterization of local dielectric breakdown in ultrathin SiO2 films using scanning tunneling microscopy and spectroscopy
Authors: Watanabe Heiji , Baba Toshio , Ichikawa Masakazu
Published: July 2002

Local dielectric breakdown of ultrathin SiO2 films grown on silicon substrates has been investigated by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that STM observation can reveal individual quasibreakdown spots created by hot-electron injection into the oxide, as well as features of the topography such as atomic steps on the oxide surface. STS was used to study the local electrical properties of the oxide films before and after electrical stressing. We observed a leakage current at the quasibreakdown spots that passed through defect levels in the ultrathin oxide films. We also found that several tunneling spectra obtained from near leakage sites showed clear negative differential resistance. This phenomenon was attributed to the conductance change in the leakage path due to electron charging effects. Moreover, we confirmed the stressing polarity dependence of the leakage-site creation, and that atomic steps on the oxide and at the SiO2/Si interface did not cause any serous problem in the quasibreakdown process.
The effect of transition from a crystalline to amorphous structure on the magneto-optical Kerr rotation in Fe0.1Co0.9−xSix films
Authors: Cai J. H. , Gu G. , Yang W. , Du Y. W.
Published: July 2002

Films of Fe0.1Co0.9−xSix (x=0.13, 0.28, 0.34) alloy were fabricated on a glass substrate by direct current magnetron sputtering at a substrate temperature of 400 °C and at room temperature (RT). The samples grown at 400 °C are mainly composed of crystalline Co2Si and amorphous Co(Fe) or Si, while the samples grown at RT are mainly amorphous. The polar Kerr spectra were measured in the spectral range 1.6–3.5 eV at room temperature with an applied magnetic field 10 kOe. A 0.01°–0.10° enhancement of the Kerr rotation was found at the transition from the crystalline to amorphous structure, and can be well explained by the changes of microscopic parameters, such as the change of transition energy center, the increase of magnitude of the transition matrix elements, which were obtained from fitting to experimental results. By analyzing the microscopic parameters, we concluded that the enhancement of spin-orbit coupling strength should mainly be responsible for the enhancement of the Kerr rotation.