bluuebunny/crossref_metadata_2025 huggingface dataset:
Light-emitting devices based on a poly(p-phenylene vinylene) derivative with ion-coordinating side groups
Authors: Morgado J. , Friend R. H. , Cacialli F. , Chuah B. S. , Moratti S. C. , Holmes A. B.
Published: July 2002

We report the phase-separation and the optoelectronic properties of an alternating poly(p-phenylene vinylene)-based copolymer, to which poly(ethylene oxide), PEO, and or lithium triflate are added. The alternating units of this copolymer (DB-BTEM-PPV) are 2,3-dibutoxy-1,4-phenylene vinylene, and 2,5-bis(triethoxymethoxy)-1,4-phenylene vinylene, a moiety containing side groups allowing ion solvation and transport. Upon addition of the ion-transporting polymer PEO to DB-BTEM-PPV blended with lithium triflate, we have found a sizeable efficiency increase (from 0.9 to 1.5 cd/A), with a concomitant increase of the response time. We propose that this is due to solvation and complexation of lithium triflate by the PEO, which simultaneously reduces the quenching of photoluminescence (and electroluminescence) efficiency by the ionic charge, and the effectiveness of formation of highly doped, low-barrier, polymer/electrode interfaces. We discuss charge transport and injection in the copolymer and in the blend with reference to the diodes characteristics, and to the phase separation of PEO, which we investigated with atomic force microscopy.
Electrostatic capacitance of two unequal overlapping spheres and the rate of diffusion-controlled absorption
Authors: Felderhof B. U. , Palaniappan D.
Published: July 2002

An exact expression is derived for the electrostatic capacitance of two overlapping conducting spheres of arbitrary radii and with arbitrary angle of intersection. The capacitance is expressed as a single integral, which can be performed if the angle of intersection is a rational fraction of π, i.e., the angle can be expressed as mπ/n with m and n integers. In earlier work by the authors (J. Appl. Phys. 86, 3418 (1999)) the electrostatic potential and the capacitance were found only for discrete angles of intersection π/n with n integer. The single integral derived here can be evaluated numerically for any angle, and expressed as a finite sum if the angle is a rational fraction of π. The capacitance can be used to find the steady-state flux to the body in diffusion-controlled absorption of particles, or the steady-state heat flux from the hot body placed in a thermally conducting environment of lower temperature. An approximation is proposed for the time-dependent rate of absorption of particles or the time-dependent flow of heat, starting from a uniform initial state of the environment.
Lorentzian noise in the two-dimensional electron gas of AlxGa1−xAs/GaAs quantum wells
Authors: Chen Yuping , Van Vliet Carolyne M. , Koenraad Paul M. , Larkins Grover L.
Published: July 2002

Current noise spectra SI(ω) are reported on samples grown by the molecular beam epitaxy technique, with current-carrying contacts, acting as source and drain, and two probes extending into the two-dimensional electron gas (2DEG) of the AlGaAs/GaAs quantum well, in the range 77–295 K for frequencies of 10 Hz to 1 MHz. The time constants are almost independent of temperature and the current dependence is close to linear. The noise is interpreted as Lorentzian-modulated shot noise of the 2DEG current.
Excimer laser ablation of microstructures: A numerical model
Authors: Paterson C. , Holmes A. S. , Smith R. W.
Published: July 2002

A new model for the ablation of microstructures with excimer laser radiation is presented. The model is based on an interactive two-step approach. The local distribution of the light over the developing structure is evaluated for each pulse. This distribution is then used to calculate the local etch rate, and hence the change to the structure. Despite the assumptions inherent in the model, in particular assumptions made about the propagation of the light through a developing structure and about the etching behavior of the materials, results from the model agree with actual aspects of ablated structures. The model has been used with some success to predict the wall angles of trench structures ablated using partially coherent illumination from a fly’s eye homogenizer. Predictions of the model show good agreement with the experiment results. In particular, the model correctly predicts the variation of the wall angles with incident fluence and also predicts structures with well defined wall angles over depths much greater than the depth of focus of the image.
Monte Carlo method for the investigation of electron diffusion in degenerate semiconductors
Authors: Borowik P. , Thobel J. L.
Published: July 2002

We propose an efficient Monte Carlo method for calculating diffusion coefficients in degenerate semiconductors by simulating two populations of particles: one obeying the nonlinear Boltzmann equation and the other obeying the linearized Boltzmann equation. The required numbers of particles and observation times are very different for the two populations. With the aim of improving computing efficiency, we have developed a rejection technique in order to account for the coupling between the two populations. We apply this method to the study of highly degenerate GaAs. We compare diffusivity and noise spectral density in order to investigate the noise reduction induced by degeneracy. We find that the magnitude of this effect is strongly sensitive to the applied field. We also suggest a possible application of our method to the accurate determination of low-field mobility.
Characterization of an AlGaN/GaN two-dimensional electron gas structure
Authors: Saxler A. , Debray P. , Perrin R. , Elhamri S. , Mitchel W. C. , Elsass C. R. , Smorchkova I. P. , Heying B. , Haus E. , Fini P. , Ibbetson J. P. , Keller S. , Petroff P. M. , DenBaars S. P. , Mishra U. K. , Speck J. S.
Published: July 2002

An AlxGa1−xN/GaN two-dimensional electron gas structure with x=0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. X-ray diffraction maps of asymmetric reciprocal lattice points confirmed that the thin AlGaN layer was coherently strained to the thick GaN layer. Methods for computing the aluminum mole fraction in the AlGaN layer by x-ray diffraction are discussed. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm−2 and a mobility of 1.9×104 cm2/V s at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov–de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov–de Haas oscillations. The quantum lifetime was about one-fifth of the transport lifetime of 2.3×10−12 s.
Images of cleaved GaAs(110) surfaces observed with a reflection optical second harmonic microscope
Authors: Sano H. , Shimizu T. , Mizutani G. , Ushioda S.
Published: July 2002

We have developed a reflection optical second harmonic (SH) microscope as a new surface probe. Using the combination of the developed SH microscope, a near infrared microscope, a confocal laser microscope, a Raman microprobe spectrometer, and an electron probe microanalyzer for x-ray fluorescence, we have observed microstructures on cleaved GaAs(110) surfaces. We have demonstrated that slab structures on these surfaces are unambiguously identified by the analysis using a combination of these microscopes. We have found that the reflection SH microscope is especially sensitive to the slab structures. The enhancement of second harmonic generation by the slab structure could be well accounted for by an electromagnetic calculation of the SH intensity.
Annealing properties of defects during Si-on-insulator fabrication by low-dose oxygen implantation studied by monoenergetic positron beams
Authors: Uedono Akira , Tanigawa Shoichiro , Ogura Atsushi , Ono Haruhiko , Suzuki Ryoichi , Ohdaira Toshiyuki , Mikado Tomohisa
Published: July 2002

The depth distributions and species of defects in Si on insulator (SOI) fabricated by low-dose oxygen implantation were determined from measurements of Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons. The temperature range for the annealing of the defects in the subsurface region (<100 nm) was divided into three stages. Annealing behaviors of the defects in stages I (600–800 °C) and II (800–1100 °C) were identified as the introduction of vacancy clusters and their recovery process, respectively. The major species of the defects in stage III (1100–1300 °C) was identified as oxygen-related defects, and the mean size of the open volume of such defects was close to that of a hexavacancy. The oxygen-related defects in the SOI layer were found to be present even after annealing at 1350 °C. The effect of the presence of vacancy-type defects on the depth distribution of oxygen atoms is also discussed.
Formation and propagation of failure in shocked glasses
Authors: Feng R.
Published: July 2002

Under plane shock wave compression, a glass may undergo elastic deformation at the shock wave front, and fail catastrophically at a later time. Since this time delay increases with the distance into the material, the phenomenon has been interpreted as a failure wave. In this article, a new theory of failure formation and propagation in shocked glasses is presented. Stress concentration due to the defects and transient loading conditions on the impact surface is assumed as the origin for initiating heterogeneous microdamage. The progressive percolation of microfissures into the material bulk gives rise to the failure wave phenomenon. Through the failure process, the deviatoric strain energy in the intact material is converted to the volumetric potential energy in the comminuted and dilated material. The state of material damage is measured in terms of the dilated volume of comminuted material at full release. The failure propagation is governed by coupled nonlinear diffusion and time-dependent evolution of the dilated volume. Numerical results are presented and compared to the lateral stress gauge measurements in two shocked glasses. It is shown that the proposed theory and simple modeling can capture the essence of the failure wave phenomenon. The theory also eliminates the ambiguity in the previous modeling work on the failure wave phenomenon in shocked glasses.
Optical and photoelectrical properties of oriented ZnO films
Authors: Tomm J. W. , Ullrich B. , Qiu X. G. , Segawa Y. , Ohtomo A. , Kawasaki M. , Koinuma H.
Published: July 2002

ZnO films that have been used as active layers of optically pumped lasers are investigated by a number of optical and photoelectric techniques, such as transmission, reflectivity, photoluminescence and direct and alternating photocurrents. Homogeneity with respect to “bulk” and surface properties is evaluated. The spectral position of the lowest intrinsic exciton peak Ex(A,B) is determined to be at (3.323±0.002) eV at 295 K. In spite of the rather small thickness of about 0.5 μm, the samples do not exhibit strain-induced modifications of the optical properties at the band edge. A deep level is found to be responsible for the room temperature photocurrent, and excitonic features appear as absorption lines. On the other hand, excitonic photocurrent peaks are observed at low temperatures; however, the conduction mechanism still remains defect related.
Visualization of 180° domain structures in uniaxial ferroelectrics using confocal scanning optical microscopy
Authors: Tikhomirov Oleg , Red’kin Boris , Trivelli Angelo , Levy Jeremy
Published: July 2002

Confocal scanning optical microscopy (CSOM) is used to visualize ferroelectric domain structure in materials with no natural optical contrast between domains. By applying an in-plane ac electric field, the orientation of the in-plane ferroelectric polarization may be determined. Experiments on LiNbO3 single crystals show high-quality images of domain structures. The domain contrast is found to depend on the ac field orientation and the incident light polarization. This investigation demonstrates the utility of CSOM for studying static and dynamic processes of domain structure and the possibility for studying domain kinetics on the nanoscale level.
Magnetic force microscopy study on the effect of Cu in melt-spun Sm–Fe–Si–C ribbons
Authors: Sun Zhi-gang , Zhang Hong-wei , Zhang Shao-ying , Han Bao-shan , Shen Bao-gen
Published: July 2002

The microstructure and magnetic domain structure of nanocrystalline melt-spun Sm2Fe15−xCuxSi2C (x=0 and 1) ribbons are investigated by magnetic force microscopy (MFM). The MFM signal, such as Δφmax and root-mean-square, which is sensitive to the sample magnetic stray field, is much stronger in Cu-doped MFM image than that in Cu-free MFM image. This is consistent with the result of the effective demagnetization factor Neff derived from the temperature dependence of coercivity. The exchange–decoupling effect of Cu which precipitates the grain boundary and forms nonmagnetic boundary phase is discussed. Magnetic domains ranging over 300–500 nm are found in both samples.
Half-point fields for microwave magnetoabsorption in colossal magnetoresistance manganite powders
Authors: Patil S. I. , Lofland S. E. , Shu Q. Q. , Bhagat S. M.
Published: July 2002

We reexamine the recently discovered giant microwave magnetoimpedance in manganite powders. The absorption consists of two components, a resistive part which is only weakly field dependent, and a low-temperature spin-dependent part, which gives rise to the giant effects. We propose that the half-point field H0, required to reduce the spin-dependent absorption to one half of its value at zero field, arises from the magnetic anisotropy consequent on the strains in the grains.
Investigation of the influence of edge structure on the micromagnetic behavior of small magnetic elements
Authors: Herrmann M. , McVitie S. , Chapman J. N.
Published: July 2002

Thin film elements of a soft magnetic nickel–iron alloy have been fabricated with structured edges in order to determine their effect on the magnetization reversal processes. Lorentz microscopy was used to study acicular elements with different edge structures and these were compared with standard elements with straight edges. The presence of the structured edges results in deviations of the magnetization along the length of the elements in the remanent state. Switching processes are described for a number of different elements and the effect of structuring the edges is discussed.
Magnetostriction study of structural and magnetic transitions in La1−xSrxMnO3 (0.1<x<0.2)
Authors: Dabrowski B. , Gladczuk L. , Wisniewski A. , Bukowski Z. , Dybzinski R. , Szewczyk A. , Gutowska M. , Kolesnik S. , Kimball C. W. , Szymczak H.
Published: July 2002

Magnetostriction measurements were performed for stoichiometric La1−xSrxMnO3 samples with x=0.11, 0.13, and 0.165 in magnetic fields to 12 T. By combining temperature and magnetic field dependent magnetostriction data with magnetization and resistivity results, the complex relationship between structural and physical properties was clarified. In the paramagnetic phase, large magnetic fields suppress a transition near x=0.13 from a structure with disordered Jahn–Teller (JT) orbital ordering (O*) to a structure with large coherent JT orbital ordering (O′), and near x=0.165 remove a transition from a structure with no coherent JT orbital ordering (R) to O*. The reentrant structural transition from O′ to O* appears between ferromagnetic and spin–canting transitions for x=0.11 and 0.13; temperatures of all transitions increase with magnetic field. The low temperature structural O* to O′ transition in the ferromagnetic phase for x=0.165 remains unchanged by application of an external magnetic field.
Calculation of ion energy distributions from radio frequency plasmas using a simplified kinetic approach
Authors: Misakian Martin , Wang Yicheng
Published: July 2002

Using an elementary kinetic approach, a procedure is described for calculating ion energy distributions (IEDs) from radio frequency (rf) plasmas. The calculated distributions, which are in the form of histograms, are used to fit experimental argon and CF3+ IEDs measured in a Gaseous Electronics Conference rf reactor modified to operate in a pulsed inductively coupled mode. Given the average plasma potential profile and its time dependence, the calculation incorporates a number of parameters used in more comprehensive treatments of the problem to determine the shape of the IED. The reverse calculation that determines the average potential profile, given an experimental IED, cannot be uniquely done, but some insights may be gained in some cases if a sufficient number of plasma related parameters are known, e.g., the shape and amplitude of the rf modulation. The results of the calculation indicate that argon ions forming the IEDs during the bright (H) mode come nearly exclusively from a presheath region that extends far into the interior of the plasma. The calculations also suggest that the CF3+ ions forming the IEDs observed during the dim (E) mode may preferentially come from near the “edge” of the bulk plasma. Possible significances of this difference are noted.
A study of effect of cyanine dyes on ionic conductivity of AgBr grains
Authors: Furuuchi Hideo , Tani Tadaaki
Published: July 2002

(1) On adsorption to cubic AgBr grains, both a 9-methyl-thiacarbocyanine (dye 1) and a 5,5′-dichloro-9-ethyl-thiacarbocyanine (dye 2), increased the conductivity of the grains with interstitial silver ions as carriers, repelling silver ions at the surface kink sites into interstitial positions. (2) On adsorption to octahedral AgBr grains, both dyes 1 and 2 increased the conductivity of the grains with mobile silver ions on the surface as carriers, whereas neither dyes 1 nor 2 increased the conductivity of the grains with interstitial silver ions as carriers. It is considered that the dye molecules repelled silver ions at surface sites, not into interstitial positions, but into surface sites where silver ions are mobile. (3) Dye 1 was preferably adsorbed to (100) faces on the surface of tetradecahedral AgBr grains, increasing the conductivity of the grains with interstitial silver ions as carriers, and was then adsorbed to the (111) faces after the saturation of the (100) faces by the dye, giving little influence on the ionic conductivity. On the contrary, dye 2 was preferably adsorbed to the (111) faces, giving little influence on the conductivity, and was then adsorbed to the (100) faces after the saturation of the (111) faces by the dye, increasing the conductivity. This result provided a method to examine the kind of faces on the surface of silver halide grains. (4) The observation of preferential adsorption on different crystal faces was successfully used to prove the presence of (100) faces on the side surfaces of a twinned tabular grain.
Photoinduced optical second-harmonic generation of SiNO films deposited on Si〈111〉 substrate
Authors: Pluciński K. , Kityk I. V. , Makowska-Janusik M. , Mefleh A. , Kaddouri H. , Benet S.
Published: July 2002

Optical photoinduced second-harmonic generation (PISHG) of SiON films deposited on Si〈111〉 substrates has been studied. Nitrogen laser (λ=337 nm) was used as a source of pumping light. We have found that with an increase of photoinducing power and nitrogen-to-oxygen (N/O) ratio, the PISHG for probe YAG-Nd laser (for the doubled frequency λ=0.53 μm) signal increases and achieves its maximum value at photoinducing power about 0.96 GW/cm2. The maximal value of the PISHG was equal about 1 pm/V. With decreasing temperature, the PISHG signal strongly increases below 28 K. Femtosecond probe-pump measurements indicate the existence of the PISHG maximum at pump-probe time delay about 0.4 ps. We explain these dependencies within a framework of the quantum chemical approach and molecular dynamics interface structure optimization. Role of photoinduced anharmonic electron-phonon interaction is demonstrated. We have revealed that Si–N chemical bonds play a key role in the observed photoinduced nonlinear optics effect.
Cathodoluminescence analysis of cleaved facets of a ZnSe p–n junction
Authors: Wang C. , As D. J. , Buda B. , Lübbers M. , Schikora D. , Mimkes J. , Lischka K.
Published: July 2002

ZnSe diodes grown on (100) GaAs substrates by molecular beam epitaxy were investigated using cathodoluminesence (CL) measurements at sample temperatures between 50 and 300 K. The CL line scans at different photon energies were performed on cleaved p–n junctions at 50 and 300 K, respectively. Taking into account surface recombination, carrier generation volume, carrier diffusion and internal built-in electric field and related carrier drift, the CL measurements from cleaved p–n junctions could be qualitatively explained. The charge depletion layer has a strong influence on the CL measurements. The calculated charge depletion width is in good agreement with CL measurements. The experimental data from the spatially resolved CL on the cleaved ZnSe diodes revealed important information of the carrier dynamics and recombination processes in these devices.
Combining x-ray and optical spectroscopies in the study of dilute semiconductor nanoparticle composites
Authors: Persans P. D. , Lurio L. B. , Pant J. , Yükselici H. , Lian G. , Hayes T. M.
Published: July 2002

We discuss a methodology for the cooperative analysis of optical and x-ray spectroscopies to deduce the thermophysical properties of dilute suspensions of semiconductor nanoparticles in a wide band gap host. X-ray spectroscopy is used to determine concentration and bonding of selected elements. Resonant Raman spectroscopy establishes limits on composition and strain in particles. Analysis of optical absorption, with constraints provided by x-ray and Raman measurements, yields the particle size distribution and concentration. As an example of this approach, we study borosilicate glasses doped with ∼0.1 wt% CdS and heat treated to produce nanoparticles.
Gold and silver Schottky barriers on ZnS(110)
Authors: Wolfframm D. , Evans D. A. , Neuhold G. , Horn K.
Published: July 2002

The evolution of the Schottky barrier between Au and Ag metal films and ZnS(110) has been studied using photoemission. Clean and well-ordered ZnS(110) surfaces were prepared by molecular beam epitaxy on cleaved GaP(110) surfaces. Chemical reaction and/or intermixing between the metal and substrate were not observed upon room temperature deposition. Substrate Zn 3d attenuation plots indicate that an initial layer-by-layer growth is followed by island growth at higher depositions. The Schottky barrier heights were found to be φBAu=2.19 and φBAg=1.81 eV, indicating a considerable dependence on metal work function. This observation agrees well with predictions of Schottky barrier heights based on the concept of metal-induced gap states and the influence of charge transfer based on electronegativities, and discussed in the light of current concepts of Schottky barrier characteristics.
Determination of magnetic properties of ultrathin iron films using microwave stripline technique
Authors: Su Jun , Tsai Chen S. , Lee Chin C.
Published: July 2002

Microwave microstrip line technique is utilized to determine the magnetic properties of ultrathin iron (Fe) films. Wideband tunable microwave filters are fabricated, either in integrated or flip-chip type, on ultrathin Fe films grown on GaAs substrates using molecular beam epitaxy. Taking advantage of the ferromagnetic resonance effects of the Fe films, four-fold variation of the resonance frequency is found to relate directly to the tetragonal symmetry of in-plane anisotropy. As a result, effective magnetization 4πMeff, four-fold in-plane anisotropy K1, second order term of perpendicular uniaxial anisotropy Ku⊥, and the Gilbert damping factor of the ultrathin films are derived from Landau–Lifshitz equation and transmission characteristics of the microwave filters involved.
Atom probe analysis of planar multilayer structures
Authors: Larson D. J. , Martens R. L. , Kelly T. F. , Miller M. K. , Tabat N.
Published: July 2002

Atom probe field ion microscopy has been used to analyze a planar-deposited layered structure in plan view. The specimens were prepared with a newly developed method that involves a combination of photolithography and focused ion-beam milling. A multilayer structure consisting of {Ta/CoFe/(Cu/CoFe)15/Ru/(CoFe/Ru)5/Ru/NiFe} was sputter deposited for use as a test stack. The corresponding thicknesses of these layers were 7/13(3/3)/50/(3/1)/50/150 nm. The nanometer-scale periodicity of the Cu/CoFe stack is readily apparent in transmission electron microscopy images of a field ion specimen fabricated from this material, suggesting that the specimen preparation procedure does not lead to destruction of the multilayer structure. Atom probe analysis of the bulk NiFe layer and the Ru/NiFe interface revealed the distribution of impurity atoms in the film, and these may affect the magnetic properties of the multilayers. Whereas a uniform distribution of C, N and Ar was observed, segregation of O was observed in the NiFe layer within ∼0.25 nm of the interphase interface, with a concentration greater than 20 times that found in the bulk of the NiFe layer.
Absolute magnetic moment measurements of nickel spheres
Authors: Shull R. D. , McMichael R. D. , Swartzendruber L. J. , Leigh S. D.
Published: July 2002

The preparation and measurement of nickel spheres for use in the calibration of magnetometers are described. The absolute value of the magnetic moment of a set of these spheres near room temperature was measured using the Faraday method. The variations of moment for temperatures near room temperature and for applied fields between 0.3 and 5 T were also determined.
Magnetism and incommensurate waves in Zr3(Rh1−xPdx)4
Authors: Bennett L. H. , Waterstrat R. M. , Swartzendruber L. J. , Bendersky L. A. , Brown H. J. , Watson R. E.
Published: July 2002

Unique properties of the Zr3(Rh(1−x)Pdx)4 alloy system are reported here. Contrary to most metallically bonded systems which are not quick to change magnetic behavior with alloy concentration, this system does. Pure Zr3Rh4 and pure Zr3Pd4 are each nonmagnetic, while in the limited range 0.02<x<0.08, there is a substantial increase in paramagnetism, attended by a similarly varying incommensurate density wave. At x=0.04 the system exhibits ferromagnetism and displays some superconductivity at x=0.06. The crystal structure involves disclination lines of Rh atoms, or channels, which are equivalent to those of the superconducting A15 structures and bear resemblances to those of the hard magnets.
Weak ferromagnetism in low-dimensional cuprates Ba2Cu3O4Cl2 and Ba3Cu2O4Cl2
Authors: Müller K.-H. , Wolf M.
Published: July 2002

Ba 2 Cu 3 O 4 Cl 2 is a two-dimensional quantum antiferromagnet with two Cu sublattices showing a small spontaneous magnetization M0. A group theoretical analysis shows that M0 may be due to (i) induced magnetization in the Cu II sublattice or (ii) spin canting of the Cu I moments, i.e., weak ferromagnetism (WFM). In case (i) the interaction is of pseudodipolar type. In case (ii) the Cu I moments are subject to some kind of anisotropy. The main reason for the occurrence of M0, compared to similar high-Tc cuprates related structures, is that the center between two Cu I moments is not an inversion center. Ba3Cu2O4Cl2 behaves like a classical antiferromagnet with a spin-flop transition. For this compound our analysis shows that, in the case where the magnetic and the crystallographic unit cells are identical, the Cu moments of equivalent sites should be parallel and the antiferromagnetic order is related to opposite directions of the moments at different sites. Weak antiferromagnetism but no WFM is permitted, in agreement with the experimental results.
NiCl
2
⋅
H
2
O
:
A quasi-one-dimensional Heisenberg antiferromagnet
Authors: DeFotis G. C. , McMahon J. G. , Berlin J. M. , Duling J. A. , Jeffers R. B.
Published: July 2002

The magnetic behavior of NiCl2⋅H2O has been studied for the first time. A Curie–Weiss fit, χM=C/(T−θ), to the susceptibility between 90 and 300 K yields g=2.23±0.01(S=1) and θ=30.4±0.05 K. Systematic curvature in χ−1(T) is evident below 90 K. Despite the very positive θ, NiCl2⋅H2O appears to order antiferromagnetically at Tc=5.65±0.1 K, somewhat below a maximum in χ(T) at T(χmax)=8.4±0.1 K, with χmax=0.1297±0.0005 emu/mol. The ratios Tc/T(χmax)=0.67±0.01 and Tc/θ=0.186±0.005 suggest lower magnetic dimensionality, most likely one-dimensional character. Plausible looking fits to the low temperature susceptibility based on a one-dimensional antiferromagnetic Heisenberg model can be obtained. However, these presuppose antiferromagnetic intrachain exchange, and in NiCl2⋅H2O such exchange is almost certainly ferromagnetic, with weaker antiferromagnetic interchain interactions. Well above Tc the susceptibility can be accounted for assuming axial and rhombic crystal-field distortions, i.e., D[Ŝz2−S(S+1)/3] and E[Ŝx2−Ŝy2] spin Hamiltonian terms, with exchange included in a mean field approximation. In the absence of single crystal data the parameters are only provisional, but clearly |D| and |E| are very large; the associated mean exchange interaction zJ/k=25.1 K is consistent with the observed θ value. Magnetization versus field isotherms exhibit an unusual evolution in shape with varying temperature; significant hysteresis is present even for temperatures somewhat above Tc.
High thermal stability tunnel junctions
Authors: Cardoso S. , Freitas P. P. , de Jesus C. , Soares J. C.
Published: July 2002

The thermal stability of CoFe/Al2O3/CoFe/MnIr spin-tunnel junctions fabricated by ion beam, using two oxidation methods is studied for annealing temperatures up to 450 °C. Tunnel magnetoresistance (TMR) is 40% after annealing at 300 °C, and a TMR of 15% is still achieved after annealing at 380 °C. The TMR decay with anneal depends on the oxidation process (O2 beam or remote plasma) and on the thickness of the MnIr exchange layer. Overoxidation of the Al, or higher kinetic energy of the O2 ions, during oxidation, lead to faster thermal degradation of the TMR. Thinner MnIr retards thermal degradation. Rutherford backscattering shows a strong interdiffusion of Mn into the CoFe electrode above 300 °C, but no significant changes are detected at the CoFe/Al2O3/CoFe interfaces, even after annealing at 435 °C (the junction resistance remains high confirming barrier stability). The pinned layer moment decreases upon annealing as expected from Mn incorporation. Also, the strong TMR decrease may indicate polarization loss when Mn reaches the Al2O3/CoFe interface. At 410 °C, the TMR is still 5%, with the fast switching characteristics of the free layer unchanged.
Surface anisotropy and the Kondo effect in restricted geometry (invited)
Authors: Újsághy O. , Borda L. , Zawadowski A.
Published: July 2002

In recent years, several groups have demonstrated experimentally that in thin films the Kondo effect has a smaller amplitude but roughly the same Kondo temperature compared to the bulk values. We have developed a theory of surface anisotropy proportional to KdSz2, where Sz is the component of the impurity spin perpendicular to the surface and Kd is an amplitude which is proportional to the inverse of the distance measured from the surface. The anisotropy is due to the fact that conduction electrons interact not only with the magnetic impurity by the exchange interaction, but also with the host atoms by the spin-orbit interaction, which allows the impurity spin to get information about the shape of the sample. Near the surface of the sample and at low temperature, the spin degrees of freedom freeze in the singlet ground state for spin S=2 so that the impurities at the surface cannot contribute to the Kondo anomaly, in contrast to the case S=5/2 where the ground state is degenerate. The theory of the surface anisotropy is in good agreement with experiments examining Kondo resistivity and magnetoresistance in the ballistic regime, while in the dirty limit, a different theory is required, which has recently been developed by I. Martin, Y. Wan, and P. Phillips.
Effect of magnetic field on the electrodeposition of Fe, Co into the pores of anodic film on aluminum
Authors: Kwon H. W. , Kim S. K. , Jeong Y.
Published: July 2002

Ferromagnetic Fe or Co was electrodeposited into the pores in aluminum anodic oxide film to produce a magnetic anodic film, and the effects of magnetic field during electrodeposition on the deposition characteristics of the metals and coercivity of the produced anodic film were investigated. Fe was electrodeposited into the pores in anodic film using a mixed solution (pH=3.8) of FeSO4(NH4)SO4.6H2O (0.20 mol/l) and H3BO4 (0.48 mol/l) at 20 °C with constant applied voltage of 10 V, and Co using a mixed solution (pH=4.5) of CoSO4,7H2O (0.10 mol/l) and H3BO4 (0.48 mol/l). It has been found that for the Fe-deposited anodic film the coercivity increased with increasing length of deposited Fe particles and decreased with increasing diameter (or porosity). The application of magnetic field during electrodeposition of Fe had little influence on the coercivity of the anodic film. For the Co-deposited anodic film the coercivity increased with increasing length of deposited Fe particles and increased unexpectedly with increasing diameter. The perpendicular coercivity of the Co-deposited anodic film was significantly increased by the application of magnetic field during the electrodeposition. The unexpected coercivity increase with diameter and the increase of the perpendicular coercivity of Co-deposited film by the application of magnetic field were closely related to the preferred orientation of Co crystals in the anodic film.
An approach to the magnetic disaccommodation in Ti-substituted Ba–W hexaferrites
Authors: Hernández-Gómez P. , de Francisco C. , Muñoz J. M. , Alejos O. , Torres C. , Bercoff P. G. , Bertorello H. R.
Published: July 2002

The relaxation of the initial permeability has been measured in polycrystalline Ti-doped Ba–W hexaferrites with nominal composition BaTixFe18−xO27, with x ranging among x=0 and 0.3. The samples have been sintered at different temperatures in the 1250 °C<T<1350 °C range in a reducing CO2 atmosphere and rapidly quenched to promote the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as main phase. In the temperature range between 80 and 500 K, the time decay of the initial permeability after sample demagnetization has been plotted by means of isochronal curves. The isochronal disaccommodation spectra show an increase in the amplitude of the different disaccommodation peaks in substituted samples regarding pure Ba–W hexaferrites. These peaks are centered at 180, 300, and 380 K and they are related to the presence of both ferrous cations and lattice vacancies. The increase of the disaccommodation maxima in Ti-substituted hexaferrites is ascribed to the additional amount of ferrous cations induced by the presence of Ti cations, in order to maintain electrical neutrality. On the other hand, the isochronal spectrum in pure Ba–W shows another relaxation peak at 240 K which almost disappears in substituted hexaferrites, even with the lowest Ti doping analyzed. This behavior is explained in terms of the increasing hindrance to the mechanisms responsible for this ionic relaxation process, owing to the site occupation by the nonmagnetic Ti cations within the hexagonal structure.
Power generating device using compound magnetic wire
Authors: Matsushita A. , Takemura Y.
Published: July 2002

A new method of generating power using compound magnetic wires is proposed. The coercive force of twisted Vicalloy (40Fe–50Co–10 V) wire is about 20 Oe in the outer shell near the surface and 60 Oe in the inner core. By applying an external magnetic field to the wire, a pulse voltage, owing to a large Barkhausen jump, is induced in a search coil wound around the wire. The voltage of 4 V for 1.2 ms was obtained with a bundle of 150 compound magnetic wires and a 2000 turn search coil.
Micromagnetic calculation for superlattice magnetic recording media
Authors: Xue Jianhua , Victora R. H.
Published: July 2002

A micromagnetic model with realistic grain configuration (planar Voronoi cells) is developed to simulate perpendicular magnetic recording media. The model is applied to an experimental Co/Pd superlattice film. The calculated perpendicular hysteresis loop exhibits a coercivity of 6560 Oe, in approximate agreement with the experimental value. It is found that the coercivity is largely invariant to grain irregularity, but the initial switching field depends greatly on grain size uniformity for ideally oriented material. In particular, an assumption of uniform grains produces a knee in the hysteresis loop that is not found with the more realistic Voronoi grains or in experiment.
Magnetization reversal processes in exchange-biased systems
Authors: Portier X. , Petford-Long A. K. , de Morais A. , Owen N. W. , Laidler H. , O’Grady K.
Published: July 2002

The magnetization reversal mechanism in sputtered bilayer films of NiFe coupled to a range of antiferromagnets has been studied using Lorentz microscopy and magnetic measurements. The reversal mechanism on the forward and recoil loops appears different and the results have been interpreted in terms of a recently published seven-point model. Reversal is controlled by the magnetic domain structure in the antiferromagnet. Time-dependent studies show that the reversal field for the NiFe layer decreases for both the forward and recoil loops, as the time for which the film is held above the saturation field of the NiFe layer increases. This can be explained by viscous rotation of the magnetization in thermally activated domains in the antiferromagnetic layer.
Temperature dependence of magnetic anisotropy of La0.8Ca0.2MnO3 epitaxial thin films
Authors: Smoak M. C. , Ryan P. A. , Tsui F. , Nath T. K. , Rao R. A. , Lavric D. , Eom C. B.
Published: July 2002

Magnetic anisotropy of epitaxial La0.8Ca0.2MnO3(LCMO) thin films grown by pulsed laser deposition has been studied as a function of temperature and lattice strain. Pseudomorphic growth on (001) SrTiO3 substrates imposes an in-plane tensile stress on the LCMO films, which produces an easy-plane magnetic anisotropy in the growth plane, accompanied by a weak biaxial anisotropy in the easy plane. In contrast growth on (001) LaAlO3 substrates produces an in-plane compressive stress, which leads to an out-of-plane uniaxial easy axis. The observed behavior indicates the presence of a positive magnetostriction, and that the anisotropy energy is dominated by the epitaxial strain induced effects. Uniaxial magnetic anisotropy constants of 250 and 500 Å LCMO films have been determined by fitting the magnetization curves for temperatures between 5 and 200 K, and magnetostriction constants have been estimated subsequently.
Study of the barrier height in exchange coupled Fe/Fe1−xSix (x>0.70) multilayers
Authors: Endo Yasushi , Kitakami Osamu , Shimada Yutaka
Published: July 2002

Fe/Fe 1−x Si x multilayers show distinct antiferromagnetic (AF) coupling for a wide spacer composition range 0.50<x⩽1.00. As the Si content x increases, the spacer changes from metallic to insulating and the AF coupling strength (J) is significantly enhanced from 0.05 to 1.20 erg/cm2. We have explained the temperature dependence of the coupling constants J1 and J2 in terms of the quantum interference model by taking an unknown energy difference Δ(=U−εF) as a fitting parameter, where εF is the Fermi level of Fe and U is the potential of the Fe1−xSix. The aim of the present work is to determine the quantity Δ experimentally for the insulating composition range of x>0.70. The quantity Δ was evaluated both from I–V characteristics and the temperature dependence of the resistivity with the current perpendicular to the sample plane using a crossed electrode geometry junction. It is found that the barrier height increases from 0.15 to 0.70 eV with increasing the Si content x. These values almost agree with the parameter Δ deduced from the temperature dependence of J1 and J2. This agreement supports the validity of our previous calculations based on the quantum interference model.
Observation of dia- and paramagnetic domains in beryllium and white tin by muon spin rotation spectroscopy
Authors: Solt G. , Baines C. , Egorov V. S. , Herlach D. , Zimmermann U.
Published: July 2002

Dia- and paramagnetic (Condon) domains in simple metals, produced at low temperatures and in high magnetic fields via the cooperative effect of electrons in Landau orbitals, are observable by the muon spin rotation (μSR) technique. For beryllium, domains were seen for T<3.5 K in fields 1<H<3 T through a well resolved splitting of the muon precession frequency, the split lines reappearing in each de Haas–van Alphen (dHvA) period as the applied field H varies. The beat in the dHvA oscillations allowed varying of the amplitude χ0 of the differential susceptibility, the key parameter determining domain properties. The data points in the (B,T) plane show that the standard formula for χ0(B,T) fails to describe the phase diagram, a consequence of the nearly cylindrical Fermi surface of Be. For white tin, preliminary data indicate the presence of domains by the oscillatory behavior of the μSR linewidth at T=0.1 K in fields of H≈1 T, arising from electrons in the pocket of the Fermi surface in the sixth zone.
Thermal behavior of quantum cellular automaton wires
Authors: Ungarelli C. , Francaviglia S. , Macucci M. , Iannaccone G.
Published: July 2002

We investigate the effect of finite temperature on the behavior of logic circuits based on the principle of quantum cellular automata (QCA) and of ground state computation. In particular, we focus on the error probability for a wire of QCA cells that propagates a logic state. A numerical model and an analytical, more approximate model are presented for evaluation of the partition function of such a system and, consequently, of the desired probabilities. We compare the results of the two models, assess the limits of validity of the analytical approach, and show that error probabilities depend on the ratio of the energy splitting between the ground state and first excited state to the thermal energy kT. We then provide estimates of the maximum operating temperature for a few relevant cases, and discuss possible approaches for increasing it.
Voltage dependence of the magnetic state in magnetic tunnel junctions
Authors: Heide C. , Krikunov A. I. , Ogrin Yu. F. , Zilberman P. E. , Elliott R. J.
Published: July 2002

In magnetic tunnel junctions (MTJ) we study how the magnetic state depends on the tunnel current. We present data on the I–V dependence of the MTJ in strong biasing fields up to 4 V, and propose that the observed hysteresis may have its origin in spin-polarized electrons influencing locally the magnetic state of the system.
Magnetic flux penetration depth study in Nb/Co system
Authors: Lee S. F. , Liou Y. , Yao Y. D. , Shih W. T. , Yu C.
Published: July 2002

A series of Co/Nb bilayers, Nb/Co/Nb and Co/Nb/Co trilayer thin films are made by dc sputtering. Magnetic hysteresis loops are measured to study the interplay between superconductor and ferromagnet. Our results show the influence of large exchange field and small spin-orbit interaction on superconductivity. We inferred a magnetic penetration depth of ∼35–40 nm in Nb. Suppression of superconducting signal by 0.6-nm-thick Co between two 30 nm Nb films suggests confinement of the superconducting wave function within individual layers.
Investigation of the interaction between silicon interstitials and dislocation loops using the wafer bonding technique
Authors: Tsoukalas D. , Skarlatos D. , Stoemenos J.
Published: July 2002

In this work, we investigate (i) the interaction of silicon interstitial atoms during thermal oxidation of silicon with a dislocation loop layer positioned at different distances from the surface as well as (ii) the interaction between two loop layers positioned at different depth distances. In both experiments, interstitials are injected by surface oxidation. The results show a linear dependence of the injection flux of interstitials with the inverse of the distance of the loop layer from the surface and a small leakage (16%) of the injected interstitials escaping from the upper and becoming bounded to the deeper loop layer. The experiments are performed using the wafer bonding technique that allows versatility on their design.
Study of defects and strain relaxation in GaAs/InxGa1−xAs/GaAs heterostructures using photoluminescence, positron annihilation, and x-ray diffraction
Authors: Arora B. M. , Chandrasekaran K. S. , Gokhale M. R. , Nair Geeta , Rao G. Venugopal , Amarendra G. , Viswanathan B.
Published: July 2002

Strain relaxation and consequent generation of defects in GaAs/InxGa1−xAs/GaAs heterostructures with x=0.05–0.3 prepared by metalorganic chemical vapor deposition have been studied using photoluminescence, positron beam, and x-ray diffraction techniques. Photoluminescence studies have indicated peak shifts and broadening in the spectra as the In concentration is increased. Broadening is attributed to defect generation, caused by In substitution beyond the critical limit. Depth resolved defect-sensitive S-parameter measurements, using a low energy positron beam, exhibit an increase in the S parameter over a depth range corresponding to the In substituted layers as the In concentration is increased. The results are suggestive of the production of open volume defects like misfit dislocations in the In substituted layer. A simplified analysis of positron beam data shows that a 5% In sample is defect-free, indicating that it is pseudomorphic to the substrate. A 10% In sample is in the transition region, while higher In concentration samples indicate a large concentration of defects. X-ray diffraction studies have revealed the in-plane and out-of-plane strains in the samples and it is found that increased In substitution leads to larger strains and an increased degree of strain relaxation. Using the experimentally determined strain parameters, a dynamical theory based simulation of x-ray profiles has been made for comparison with the measured profiles. The combined study of the three techniques clearly establishes the interplay between the In substitution and strain relaxation leading to defect generation in the InGaAs system.
Enhanced evaporation from a highly strained Si crystal surface
Authors: Sun Yong , Miyasato Tatsuro , Wigmore J. Keith
Published: July 2002

We observed enhanced evaporation from regions of a single crystal silicon surface subject to very large local strain. The strain was created across narrow breaks in silicon carbide films produced by change of phase due to annealing. The films were grown in an amorphous form on a (111) Si substrate at 600 °C by hydrogen plasma sputtering. Annealing the SiC/Si structure at temperatures above 850 °C, resulted in contraction due to recrystallization, giving breaks in the film which approximately followed the 〈112〉 crystal axes of the Si substrate. We found that the silicon immediately beneath a break in a film was preferentially removed leaving a deep ditch. An enhancement of evaporation of at least 2 orders of magnitude and a corresponding decrease in activation energy were found from the dependence of the effect on film thickness and annealing temperature.