bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface layers on superconducting Y-Ba-Cu-O films studied with x-ray photoelectron spectroscopy
Authors: Chang C. C. , Hegde M. S. , Wu X. D. , Dutta B. , Inam A. , Venkatesan T. , Wilkens B. J. , Wachtman J. B.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) examines only a thin surface layer (<5 nm) that may not be representative of the bulk. We separated the information from the surface and bulk by using laser-deposited superconducting films that have nearly atomically flat surfaces for which quantitative analysis formalisms exist. The chemical compositions of high Tc (90 K) and high Jc (>106 A/cm2) Y-Ba-Cu-O films on SrTiO3 (001) substrates were examined. From the relative intensities of the surface and bulk components of the Ba(3d) and Ba(4d) spectra taken at different take-off angles and different escape depths [using Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) excitations], we have determined the nonsuperconducting surface layer thickness to be 1 nm and the layer composition to be BaCuO2. The surface layer thickness for a superconducting film only 8 nm thick was also 1 nm. By detecting the substrate Ti signal through this film, and ruling out a high density pinholes, we provide evidence that the XPS data contain information from the superconducting phase. A polycrystalline pellet scraped in vacuum had a surface layer only 0.4 nm thick. Since typical photoelectron escape depths are about 2 nm, about 80% of the detected signal originates in the bulk. The surface layer contains Cu2+ and oxygen with a photoelectron binding energy of 531 eV.
Antiferromagnetic standing-spin-wave resonance in epitaxial films of MnF2
Authors: Lui M. , Ramos C. A. , King A. R. , Jaccarino V.
Published: July 2002

Standing-spin-wave resonance (SSWR) modes have been observed in an antiferromagnet for the first time. The experiments were performed on MnF2 films of thicknesses t=0.98 and 0.23 μm, grown epitaxially on carefully polished (001) Mn0.99Fe0.01F2 substrates. An intervening 1.0-μm-thick homoepitaxial, buffer layer of Mn0.99Fe0.01F2 was first deposited to improve both the epitaxial growth and thickness uniformity of the final MnF2 film. The observed SSWR modes exhibit spacings ΔHn appropriate to the boundary conditions in which the spins are pinned at both interfaces. From the spacings of the modes, a value of the spin-wave stiffness constant is obtained which is in good agreement with the calculated value from the known exchange and anisotropy interactions of bulk MnF2.
The magnetic properties of doubly oriented 3% Si-Fe and its application
Authors: Arai S. , Mizokami M. , Yabumoto M.
Published: July 2002

Doubly oriented 3% Si-Fe having high permeability was prepared by a cross-rolling method and secondary recrystallization. We examined two applications of this doubly oriented material. One application is a three-phase power transformer. Watt loss of the transformer using doubly oriented 3% Si-Fe in yokes was found to be lower than that of the transformer using singly oriented 3% Si-Fe, especially at high induction. The lower loss of the transformer using doubly oriented 3% Si-Fe was explained by magnetic domain wall movement observation and FEM calculation. The magnetic flux can rotate more smoothly in the yokes of the transformer using doubly oriented 3% Si-Fe than in those parts of the singly oriented 3% Si-Fe transformer. Doubly oriented 3% Si-Fe is also effective to decrease the loss at ‘‘hot point.’’ Another application is a punched U-I shaped core, in which the flux is rotated at the corner. Doubly oriented 3% Si-Fe core was magnetized to high induction easier than those of singly oriented 3% Si-Fe and nonoriented 3% Si-Fe. These results show that doubly oriented 3% Si-Fe is one of the best materials for the applications in which magnetic flux is rotated.
Spin-wave auto-oscillations still in need of a good model
Authors: Rezende S. M. , de Aguiar F. M. , Azevedo A.
Published: July 2002

The onset of spin-wave auto-oscillations has been studied in spheres of yttrium iron garnet pumped by a microwave field parallel or perpendicular to the dc field. Data were taken for the auto-oscillation threshold field, amplitude, and frequency as a function of sample dimension, shape, and the spin-wave relaxation rate and wave vector. The behavior near the onset confirms the theoretical prediction that the auto-oscillation results from a Hopf bifurcation. However, data for the auto-oscillation frequency and threshold field are in clear disagreement with predictions. These results indicate that although the area of nonlinear spin-wave phenomena has been under investigation for over 30 years, a good model is still needed for the auto-oscillations.
Mössbauer study of 57Fe single-crystal films grown on (110) GaAs by MBE
Authors: Volkening F. A. , Jonker B. T. , Prinz G. A. , Koon N. C.
Published: July 2002

We have performed conversion electron Mössbauer spectroscopy measurements on a series of 57Fe (28–130 Å thick) single-crystal films grown on GaAs(110) by molecular-beam epitaxy and overcoated with Al. The spectra were fit using a four-component model consisting of ‘‘bulk’’ Fe (HHF =330 kG), a component with broad lines and reduced HHF (HHF =290 kG), corresponding to Fe with As, Ga, and Al in solid solution, a paramagnetic component (most likely corresponding to the Fe/Al interface), and a component corresponding to antiferromagnetic Fe2 As. For the thinnest film measured (30 Å), the spectral areas of the components are approximately 44%, 36%, 5%, and 15%, respectively.
Internal stresses and antiferromagnetism in evaporated Cr films
Authors: Cheriet L. , Arajs S. , Helbig H. F.
Published: July 2002

We have prepared Cr films with thicknesses up to 100 nm by evaporation in ultrahigh (10−7 Torr) vacuum onto Corning glass (7059) substrates. The resistance of the films was measured in situ as a function of temperature and the shifts in Néel temperatures were measured as a function of film thickness. By using different substrate temperatures, it is possible to create compressive or tensile stresses in these films, which have a remarkable effect on the spin-density wave-paramagnetic transition. The results can be nicely correlated with the direct stress measurements of Abermann and Martinz.
High magnetic-field facilities: Opportunities in research and technology (invited)
Authors: Foner S.
Published: July 2002

Recently the NSF Division of Materials Research Panel on Large Magnetic Fields concluded that the opportunities for research and technology in high magnetic fields are compelling for support of a new national magnet laboratory with the following components as goals: (i) dc hybrid magnet facilities (water-cooled magnets surrounded by superconducting magnets) of at least 45 T; (ii) state-of-the-art superconducting magnets and NMR magnet facilities greater than 25 T; (iii) intense pulsed magnet facilities for ms to 75 T and above; (iv) short pulsed magnets for μs intervals to 200 T; (v) a variety of water-cooled magnets reaching 25 T; and (vi) quasistatic magnets for transient fields of 0.5 s or more up to 65 T. A 20-MW highly regulated solid-state power supply is central for the dc magnets and separate pulsed power sources are required for the pulsed facilities. This paper outlines the main conclusions of the panel report and discusses some of the opportunities for high-field research and technology. Active overseas high magnetic facilities also are outlined.
Magnetic ordering in strain-controlled epitaxial rare-earth films (abstract)
Authors: Farrow R. F. C. , Parkin S. S. P. , Bennett W. , Segmuller A. , Chien J. , Bravman J. C.
Published: July 2002

The magnetic properties of epitaxial rare-earth films and multilayers are known to be sensitive to lattice strain arising from the misfit between the substrate and the epitaxial rare earth overlayer. We find that rare-earth films grow epitaxially, with no coherency strain, onto rare-earth fluoride films on GaAs(111) substrates. This decoupling of the rare-earth film from substrate-imposed coherency strain permits controlled amounts of strain to be introduced into the rare earths by suitably designed sandwich structures. We report quantitative x-ray measurements of strain in films of Dy, Ho, and Er sandwiched by rare-earth fluorides and in Er/Dy/Er sandwich structures grown onto rare-earth fluoride buffer films on GaAs(111). SQUID magnetometry of these structures shows that the unstrained Dy, Ho, and Er films exhibit similar Curie temperatures to bulk crystals. On the other hand, in the case of Er/Dy/Er structures the Curie temperature of Dy is increased as a result of a designed, coherency-induced c-axis expansion of the Dy. The results of a study of strain in Y/Dy/Y structures will also be reported.
New approaches to the quantum Heisenberg models: Schwinger boson representations (invited)
Authors: Auerbach Assa , Arovas Daniel P.
Published: July 2002

Schwinger boson representations allow us to study ferromagnetic and antiferromagnetic Heisenberg models in a rotationally invariant formulation. The large-N SU (N) Heisenberg models are approximated by the Schwinger boson mean-field theory (SBMFT). In most cases, even for N=2 (the physical model), the SBMFT is surprisingly successful: We review recent comparisons with numerical results, spin-wave theory, and renormalization group analysis of the nonlinear sigma model. The mean-field theory, like the nonlinear sigma model, does not include the effects of topological Berry phases, which can appear in the antiferromagnetic spin liquid phases.
Magnetic properties of stage-1 CoCl2-graphite intercalation compounds under pressure
Authors: Nicholls J. T. , Murayama C. , Takahashi H. , Mōri N. , Tamegai T. , Iye Y. , Dresselhaus G.
Published: July 2002

We report ac magnetic susceptibility (χ=χ′−iχ″) measurements of stage-1 CoCl2-graphite intercalation compounds (GICs) carried out under hydrostatic pressures of 0–15 kbar. For stage-1 CoCl2 -GICs, it is found that the three-dimensional ordering temperature Tc2=TN=9.7 K, where the ferromagnetic planes of Co2+ spins become ordered into an antiferromagnetic arrangement along the c axis, shifts to a higher temperature with increasing pressure. Additionally, the transition field Ht, measured when a magnetic field is applied in the x-y plane, shifts to a higher field value with increasing pressure. From the increase of the transition field, we can estimate the pressure dependence of the antiferromagnetic interlayer coupling J′.
Rotational and transverse dynamics of some random anisotropy magnets (invited)
Authors: Barbara B. , Dieny B. , Filippi J.
Published: July 2002

After rapid survey of the behavior of random anisotropy magnets, the time and frequency evolution of the magnetization vector of a rotating disk of amorphous DyNi is focused on, as measured by a two-SQUID detectors setup. The results show that the dynamics of this system is completely determined by two macroscopic variables: the magnetizations M1 and M2, respectively associated with short and long relaxation times. The first one is fixed with respect to the applied field and therefore responsible for dissipation (magnetic friction) whereas the second one is fixed with respect to the sample (rotating anisotropy). Furthermore, transverse ac susceptibility experiments are used to confirm some of the results obtained. In particular low-temperature dynamical scaling (T→0) permits the existence of a new type of frequency crossover, when the volume samples by the ac field becomes of the order of Imry and Ma domains, to be shown.
Analytical magnetostatic field calculation for a conductor with uniform current in the longitudinal direction
Authors: Babic S. , Andjelic Z. , Krstajic B. , Salon S.
Published: July 2002

New closed-form expressions are derived and presented to compute the three-dimensional magnetostatic field of a hollow cylinder of finite thickness when the current has a longitudinal component. The results are valid for problems with no magnetic materials. It is shown that the solution can be written in terms of complete and incomplete Jacobian elliptic functions of the first and second kind. Results of a specific configuration are compared with a numerical solution.
NMR evidence of metal-support interaction in syngas conversion catalyst Co-TiO2
Authors: Murty A. N. , Seamster M. , Thorpe A. N. , Obermyer R. T. , Rao V. U. S.
Published: July 2002

To examine the relation between catalytic and magnetic properties, the zero-field NMR spectra and hysteresis loops of cobalt supported on silica, alumina, magnesia, titania, and ZSM-5 with and without the promoter thoria were investigated. Cobalt was incorporated on the support by simple physical admixture of precipitated cobalt and support, and by aqueous impregnation technique. Our studies indicate that the particle sizes are consistently lower in the presence of thoria. Of all the catalysts examined, the Co/Th/TiO2 catalyst exhibits a high saturation magnetization value—about 20% higher than pure cobalt. In addition, the NMR spectrum of the aqueous impregnation Co/TiO2 catalyst is distinctly different from the rest. All the NMR lines are shifted to a higher frequency by about 4 MHz. These two features—enhancement of the magnetic moment of cobalt atoms and increases in the hyperfine field at the Co nucleus—clearly indicate that there occurs strong metal-support interaction between cobalt and titania support. The higher hydrocarbon yields observed by the earlier investigators with Co/TiO2 catalysts might be related to this phenomenon.
Magnetic properties of a sulfur-bridged iron dimer of potassium bis(dithiooxalato)nitrosyliron (abstract)
Authors: Qian Lingqian , Ro HyeKyeong , Hatfield William E. , Singh Phirtu
Published: July 2002

A sulfur-bridged iron dimeric compound of potassium bis(dithiooxalato)nitrosyliron monohydrate, K2Fe(C2O2S2)2NO⋅H2O, crystallized in the monoclinic space group C2/c with unit-cell dimensions of a=18.416(6) Å, b=15.800(6) Å, c=9.834(4) Å, and β=111.26(3)° with z=8. The coordination around the iron atom is distorted tetragonal pyramidal with an average basal Fe-S distance of 2.27 Å. The sixth coordination position of iron is filled by a sulfur atom from an adjacent complex ion with an Fe-S distance of 3.823 Å. The Fe-N-O fragment has an angle of 161.8(3)°, and the N-O distance is 1.148(3) Å. The loosely bound dimer exhibits surprisingly strong intramolecular anitferromagnetic interactions with the coupling constant J=−23.8 cm−1. This antiferromagnetic exchange occurs through an unsymmetrical unit with two short Fe-S distances and two rather long Fe-S distances. From the extended Hückel calculation of a monomer fragment, the relatively large superexchange interaction results from the effective overlap of iron 3dz2 and 4pz orbitals with the orbital of the bridging sulfur atom.
Magnetochemistry of the tetrahaloferrate (III) ions: Crystal structure and magnetic ordering in [(4-chloropyridinium)FeCl4]2⋅4-chloropyridinium chloride and related salts (abstract)
Authors: Lowe C. B. , Shum D. P. , Carlin Richard L.
Published: July 2002

The yellow compound of stoichiometry (4-chloropyridinium)3Fe2Cl9 belongs to the space group P21/n with four formula units in the unit cell. Structural analysis shows that the iron is present as the FeX4− ion. Magnetic measurements on single crystals show that the material orders as a canted antiferromagnet at 2.725 K. Replacement of the 4-chloro cation by the 4-bromo analog yields an isomorphous material and shifts the transition temperature to 2.34 K. The phase diagrams of both salts have also been determined. For [(4-chloropyridinium)FeCl4]2⋅4-chloropyridinium chloride, the bicritical point is at 2.40 K and 7 kOe, and HSF(0)=5.5 kOe. With Hc(0)=56 kOe, α=HA/HE=1.9×10−2. In the case of [(4-bromopyridinium)FeCl4]2⋅4-bromopyridinium chloride, HSF(0)=3 kOe, Hc(0)=42 kOe, α=1.0×10−2, and the bicritical point is at 2.24 K and 4.2 kOe. Crystallographic investigations at 25 K are reported on the isomorphous (4-chloropyridinium)3Fe2Br9 and on (4-bromopyridinium)3Fe2Cl1.3Br7.7. Magnetic measurements on single crystals show that (4-bromopyridinium)3Fe2Cl1.3Br7.7 orders as a canted antiferromagnet at 5.67 K, while (4-chloropyridinium)3Fe2Br9 orders similarly at 7.96 K. These compounds behave approximately as S=5/2, three-dimensional Heisenberg antiferromagnets.
NMR study of rapidly quenched Fe95M5 crystalline alloys (M=C, B, P, Si, and Al)
Authors: Zhang Y. D. , Budnick J. I. , Sanchez F. H. , Hasegawa R.
Published: July 2002

Rapidly quenched crystalline Fe95M5 alloys were studied by spin-echo NMR experiments for frequencies ranging from 10 to 300 MHz at 4.2 K. While x-ray diffraction for all these samples show the same bcc Fe pattern, NMR measurements reveal the details of their local structural character. The C atoms form an interstitial bcc solid solution with Fe as well as Fe3C-like short-range order (SRO). Similarly, an orthorhombic Fe3B-like SRO exists in the B-containing alloy. The Si- and P-containing alloys are substitutional bcc solid solutions in which the Si and P atoms replace Fe randomly. A SRO is also found in Fe95Al5 the Al atoms avoid being close to each other. The results demonstrate that atomic size plays a key role in forming the various kinds of atomic configurations in these alloys.
Magnetic behavior of layered films of submicronic Fe3O4 particles
Authors: Arajs Sigurds , Amin Nurul , Anderson Elmer E.
Published: July 2002

A special aggregate of six glass slides, glued together, has been prepared. Each side of each slide contained a thin layer (∼0.2 μm) of dispersed Fe3O4 particles of mean diameter ∼0.17 μm prepared by reduction of α-Fe2O3 particles. Magnetization measurements on the above aggregate have been carried out at 300 K as a function of the angle between the direction of the magnetic field and the plane of the slide aggregate as it is rotated along the long axis of symmetry. The angular dependence of the coercivity and the ratio of the remanent to saturation magnetization imply that magnetic-moment reversals in this system are likely to be incoherent fanning rotations.
Quasicrystalline Al37Mn30Si33: Ferromagnet or concentrated spin glass? (abstract)
Authors: Chatterjee Ratnamala , Dunlap R. A. , Srinivas V. , O’Handley R. C.
Published: July 2002

Recently, some Al-Mn-Si (Ref. 1) and Al-Mn-Ge (Ref. 2) quasicrystals (with high-Mn and metalloid content) have been reported to show ferromagnetic behavior. The ferromagnetic Al-Mn-Si quasicrystals show low Curie temperature, Tc∼107 K and very weak, strongly field-dependent magnetization (Ms∼0.1 emu/g). The Al-Mn-Ge ferromagnetic quasicrystals are also reported to have low magnetization (∼0.2 emu/g in a field of 20 kOe), fairly high Curie temperature (maximum Tc was quoted for Al52.5Ge22.5Mn25 to be ∼533 K), and also a coercivity in the range of 0.7–1.1 kOe. A crystalline compound close in composition to these quasicrystals, equiatomic AlMnGe,3 is strongly ferromagnetic with a moment at 77 K∼1.7 μB/Mn atom and Tc∼518 K. Many questions are unanswered about these new quasicrystals: Why do they have such low magnetization yet relatively high Tc? Why does Tc increase so strongly when Ge replaces Si? What kind of magnetic order exists in these alloys; is it really ferromagnetic? How many of the transition-metal species are participating in moment formation and what kind of exchange interactions are coupling them? Detailed magnetic measurements on the Al37Mn30Si33 quasicrystals are presented. It has been established that this high-Mn content quasicrystal is not a real ferromagnet and instead shows a concentrated spin-glass behavior with a reentrant ferromagnetism. For the first time a coherent picture of the magnetic phase-diagram of different Mn content Al-Mn-Si, Ge quasicrystals is presented and some of the crucial questions concerning the magnetism in quasicrystal structure have been answered.
Berry’s phase and topological structures in micromagnetics
Authors: Zhang X. Y. , Fredkin D. R.
Published: July 2002

We calculate Berry’s phase for a particle passing through a magnetic slab with nontrivial topological structure. In cases where the magnetic structure is known, it provides a way to detect Berry’s phase; in cases where the structure is unknown, it provides a novel imaging method for the study of micromagnetics. Possible noise sources are discussed.
The ferrimagnetic compounds CoM[M′(EDTA)]2⋅4H2O(M,M′=Co,Ni): Magnetic characterization of CoCo[Ni(EDTA)2]⋅4H2O
Authors: Sapiña F. , Coronado E. , Gomez-Romero P. , Beltran D. , Burriel R. , Carlin R. L.
Published: July 2002

We report on the magnetic properties of the ordered bimetallic compound CoCo(NiEDTA)2⋅4H2O (abbreviated as [CoCoNi]). The structure consists of ordered bimetallic layers formed by alternating octahedral sites of Co and Ni(II); tetrahedral Co sites connect different Co-Ni layers. We discuss the low-dimensional ferrimagnetic behavior of this compound in terms of a model that assumes three spin sublattices exchange coupled by an Ising interaction.
Pulsed-laser crystallization of amorphous silicon layers buried in a crystalline matrix
Authors: Polman A. , Stolk P. A. , Mous D. J. W. , Sinke W. C. , Bulle-Lieuwma C. W. T. , Vandenhoudt D. E. W.
Published: July 2002

Ion implantation, employing Si, Ar, and Cu ions in the energy range from 275 to 600 keV, was used to form amorphous silicon layers buried in a crystalline matrix. Different layer geometries were produced, with 150–620-nm-thick amorphous layers, separated from the surface by 120–350-nm-thick crystalline layers. Crystallization of the amorphous layers was induced by 32-ns pulsed ruby laser irradiation. Real-time reflectivity and conductivity measurements indicate that internal melting can be initiated at the amorphous-crystalline interface, immediately followed by explosive crystallization of the buried layer. Channeling and cross-section transmission electron microscopy reveal that in both Si(100) and Si(111) samples explosive crystallization proceeds epitaxially with twin formation, the twin density being higher in Si(111) than in Si(100). The measured crystal growth velocities range from 15 to 16 m/s, close to the fundamental limit for crystalline ordering at a Si liquid-crystalline interface. Computer modeling of heat flow and phase transformations supports the experimental data.
High-T
c oxide/metal composite superconductors produced by oxidation of unidirectionally solidified Ag-Yb-Ba-Cu alloys
Authors: Togano K. , Kumakura H. , Dietderich D. R.
Published: July 2002

This communication presents a new method for producing a superconducting high-Tc oxide/metal matrix composite by unidirectional solidification of metallic precursors and subsequent oxidation. A lamellar eutecticlike structure which is aligned parallel to the solidification direction has been produced in Ag-Yb-Ba-Cu alloys. During oxidation the superconducting perovskite phase nucleates preferentially within some of the lamellae and at the lamellae boundaries. It is hoped that such preferential nucleation will ultimately improve the continuity of the superconducting phase, and thus the transport current. The highest Tc, zero resistance, obtained so far is 85 K.
Anisotropy and loss effects on MSWs in layered ferrite films (abstract)
Authors: Buris N. E.
Published: July 2002

The effects of cubic anisotropy on the resonance and magnetostatic wave (MSW) propagation properties of thin and lossy YIG films are discussed. It is known that cubic anisotropy fields result in a misalignment between the magnetization vector and the applied magnetic field.1,2 We show that when the applied field is in a plane perpendicular to the [111] crystallographic axis of the YIG cubic lattice the misalignment can be avoided in some special cases. We give expressions for the permeability tensor in these special cases and we show that the cubic anisotropy renders surface waves with piecewise planar-phase fronts. The phase of the surface mode is different at the two sides of the YIG film. This phase shift can be used to characterize the cubic anisotropy of YIG films. In addition, loss effects are discussed and the insertion loss of composite modes in layered structures is compared with the traditional dB/μs loss figures of single-film modes.
Magnetic exchange interactions in the heteropoly complexes [M4(H2O)2(PW9O34)2]10− [M=Co(II) and Cu(II)]
Authors: Gomez-Garcia C. J. , Casañ-Pastor N. , Coronado E. , Baker L. C. W. , Pourroy G.
Published: July 2002

The magnetic properties of the heteropolyanions [M4(H2O)2(PW9O34)2]10− [M=Co(II) and Cu(II)] down to 4 K are reported. Their individual heteropoly molecules contain a rhomblike arrangement formed by four coplanar MO6 octahedra sharing edges. The magnetic properties support an intramolecular ferromagnetic exchange in the Co(II) tetramer, and antiferromagnetic exchange in the Cu(II) tetramer. These behaviors are discussed assuming anisotropic exchange in the Co(II) compound, and Heisenberg exchange in the Cu(II) compound.
Preparation and characterization of ultrathin Bi2Sr2CaCu2O8 single crystals
Authors: Forro L. , Mandrus D. , Keszei B. , Mihaly L. , Reeder R.
Published: July 2002

Optically transparent, free-standing single crystals of the high-temperature superconductor Bi2Sr2CaCu2O8 have been prepared with a typical size of a×b×c=1 mm×1 mm×1500 Å. The thickness of the samples has been determined by x-ray transmission measurements and was found to be uniform to within a lattice spacing. Transmission electron microscopy and diffraction revealed a pseudotetragonal lattice with a twinning free, incommensurate superstructure. X-ray microanalysis shows that the composition of the sample is homogeneous. Electrical resistivity and magnetic susceptibility measurements indicate that the crystals are good quality superconductors.
Photoluminescence study of Si+- and Si++P+-implanted InP
Authors: Shen Honglie , Yang Genqing , Zhou Zuyao , Huang Wei , Zou Shichang
Published: July 2002

Low-temperature photoluminescence was used to study the Si+- and Si++P+-implanted InP. A broadband at ∼ 1.26 eV appears in photoluminescence spectra for Si+-implanted InP after annealing. The intensity of this broadband decreases with increasing the coimplanted P+ dose and increases with increasing the implanted Si+ dose. The temperature dependence of the photoluminescence data shows that the change in the half-width of the broadband can fit the configuration coordinate model. This band is believed to be due to VP-SiP complex. The results indicate that silicon is an amphoteric species in InP.
Direct measurement of dispersive optical nonlinearities in GaAs using the photoreflectance method
Authors: Wang Chun , Zhang Chunping , Zhang Guangyin
Published: July 2002

Nonlinear refractive index changes for the wavelengths near the fundamental edge in an ultrapure epitaxial layer of GaAs were obtained directly by the photoreflectance method. The nonlinearities are due to the electric field effect for pump intensities lower than 0.06 W/cm2 and the plasma screening and the band filling effect for pump intensities higher than 0.06 W/cm2.
Structures and magnetic properties of a Nd15Fe77B8 alloy produced by twin-roller quenching
Authors: Harada Tetsuji , Ando Teiichi , O’Handley Robert C. , Grant Nicholas J.
Published: July 2002

The relationship between the magnetic properties and microstructures of flakes of a Nd15Fe77B8 alloy produced by twin-roller quenching was studied. Flakes produced at low to intermediate roller speeds (1.4–8 m/s) showed a dendritic structure of Nd2Fe14B oriented normal to the chilled surface, while flakes produced at higher roller speeds showed fine, equiaxed Nd2Fe14B grains. The flakes with a dendritic structure were found to be magnetically anisotropic and showed a high-energy product (BH)max (15.2 MGOe) in the direction perpendicular to the chilled surface. The flakes that had fine equiaxed grains were magnetically isotropic. The maximum coercivity of the flakes exceeded 20 kOe. The microstructural transition from columnar dendrites to equiaxed grains is explained in terms of morphological changes in the solidification of undercooled melts. Optimization of twin-roller quenching parameters for the production of desirable microstructures in Nd-Fe-B magnets is discussed.
Optimal electric bias for the diffraction efficiency of degenerate four-wave mixing process in nematics
Authors: Kuo Chen-Lung , Chen Shu-Hsia
Published: July 2002

The optimal quasi-static-electric bias for the diffraction efficiency of degenerate four-wave mixing process in a homeotropically aligned nematic liquid-crystal film is studied. The nonlinearity of this process is characterized by the behavior of field-induced molecular reorientation. In our theoretical calculation, it is pointed out that the determination of optimal field is crucially influenced by the twist deformation in the reorientational grating and is also subjected to the competing influences of splay and of bend elastic torques on the molecular reorientation. In experiments, the careful measurement of diffraction efficiency is carried out automatically by using a microcomputer to investigate the optimal bias for this nonlinear effect in nematic N-(4-methoxybenzylidene)-4-butylaniline films. It is unambiguously shown both by theoretical and experimental results that the optimal bias is decreasing with the increasing of grating period and optical intensity.
Effects of Xe post-bombardment on carbonitrides produced in a low-carbon nitrogen-implanted steel
Authors: Behar M. , Amaral L. , Ramos S. M. M. , Vasquez A. , Zawislak F. C.
Published: July 2002

The effects of Xe bombardment on carbonitrides produced by N implantation in a low-carbon steel are studied via conversion electron Mössbauer spectroscopy and nuclear reaction analysis. The results show two main features: dissolution and reprecipitation of the produced carbonitrides and modification of the thermal behavior of the precipitates. Recently we have performed similar experiments bombarding samples of the same steel with He and Ar. Comparison of the experiments shows that irradiation with Ar ions provides the best retention of carbonitrides at 450 °C. On the other hand, irradiation with Xe ions to the same dpa level is most effective in producing the dissolution and reprecipitation process.
Quantization effect on capacitance-voltage and current-voltage characteristics of an InAs/AlSb/GaSb interband tunneling diode
Authors: Yang L. , Wu M. C. , Chen J. F. , Chen Y. K. , Snider G. L. , Cho A. Y.
Published: July 2002

Capacitance-voltage measurements were performed on the InAs/AlSb/GaSb interband tunneling diode at various frequencies. Theoretical analyses using a self-consistent Schrödinger–Poisson solver were found in agreement with the experimental results under the forward-bias condition. The quantization energy of each subband of the electron in the InAs accumulation region is used to predict the tunneling current cutoff voltage in agreement with that of the current-voltage measurements. Therefore, the cutoff of the interband tunneling process is mainly caused by the crossover of the electron subband energy in the InAs conduction band with respect to the valence band of the GaSb electrode due to the increased external bias voltage.
Harmonic mixing in a superconducting tunnel junction
Authors: Tong C. E. , Chernin L. M. , Blundell R.
Published: July 2002

A theoretical analysis of harmonic mixing in a quasiparticle tunnel junction at Vbias=0 shows that harmonic mixing for submillimeter frequencies is a viable alternative to fundamental mixing. It is also shown that the signal gain from the first harmonic above the pump frequency to the intermediate frequency is independent of the embedding admittance at the pump frequency. In fact, we show that the five-port model reduces to the three-port model as a result of the inversion symmetry of the I-V curve and, moreover, no large-signal analysis is necessary. Full simulations of harmonic mixing using a five-port model confirm these results. The simulations have been done for the case of a superconductor-insulator-normal junction in which there is no pair current interference. For this type of harmonic mixer we show that the local oscillator (LO) power requirement can be less than a fundamental mixer operating at the same signal frequency, which is particularly useful in circumventing the problems of high-frequency LO power generation.
Spontaneous current oscillations in optically pumped semi-insulating InP
Authors: Nolte D. D. , Olson D. H. , Glass A. M.
Published: July 2002

Current oscillations arising from voltage-controlled negative differential resistivity are observed in optically pumped semi-insulating InP at low temperature. The presence of high-field domains sweeping through the sample are detected using photorefractive four-wave mixing. Direct evidence of the role of defects in these current oscillations is presented using deep-level photodiffractive spectroscopy. A deep hole trap with an energy Ev +85 meV is observed. The current oscillations occur only when this defect level is partially occupied.
Evidence for surface recombination at mesa sidewalls of self-electro-optic effect devices
Authors: Swaminathan V. , Freund J. M. , Chirovsky L. M. F. , Harris T. D. , Kuebler N. A. , D’Asaro L. A.
Published: July 2002

We measured the forward current-voltage characteristics of self-electro-optic effect devices (SEED). These devices consist of p-i-n diodes where the i region is a GaAs/AlxGa1−xAs (x∼0.3) multiple-quantum-well structure. It is found that the diode current varies as exp(qV/2 kT) and that it also scales with the junction perimeter for diodes of different mesa sizes, indicating nonradiative surface recombination at the mesa sidewalls. We also measured minority-carrier lifetimes from photoluminescence decay experiments. They revealed that the recombination rate increases with decreasing mesa size, once again indicating that surface recombination at the mesa sidewalls limits carrier lifetime. A value of 6×105 cm s−1 for the surface recombination velocity for the sidewalls is determined. The implication of the nonradiative surface recombination at the mesa sidewalls is that it undermines the performance of the SEED as the mesa size decreases by reducing the photocurrent, thereby leading to higher bistability voltage threshold and hence higher switching energy.
Relationship between photoluminescence spectra and low-field electrical properties of modulation-doped AlGaAs/GaAs quantum wells
Authors: Dodabalapur Ananth , Sadra K. , Streetman B. G.
Published: July 2002

We describe the influence of sheet charge density and crystalline quality on the photoluminescence (PL) spectra of AlGaAs/GaAs n-type modulation doped quantum wells (MDQWs). We discuss the various contributions to the PL linewidth at 4.2 K. The linewidth at 77 K is approximately equal to the Fermi energy, and is independent of crystalline quality, making it a good measure of sheet carrier density. At 4.2 K, the crystalline quality also influences the PL linewidths; however, the carrier density can be deduced from the high-energy cutoff point of the PL spectra. The ratio of 77 K to 4.2 K linewidths correlates fairly well with the crystalline quality, as measured by the 77 K Hall mobility. Our calculations of the band diagram, wavefunctions, and carrier densities provide a deeper understanding of these structures. Results of this work have applications in nondestructive testing of large area wafers for uniformity in sheet carrier density and mobility as well as in the design of novel optoelectronic devices.
Rapid thermal annealing of Si-implanted GaAs with CF4/O2 plasma treatment
Authors: Tamura Akiyoshi , Watanabe Atsushi , Nambu Syutaro
Published: July 2002

We have investigated the effect of CF4/O2 plasma treatment before ion implantation on the activation efficiency for Si-implanted GaAs by rapid thermal annealing. WSiN-capped, SiO2-capped, and capless rapid thermal annealings at 900 °C for 10 s in an Ar atmosphere were compared. The annealed samples were characterized by Hall-effect, capacitance-voltage, secondary-ion mass spectrometry, and 77-K photoluminescence measurements. The large decrease in activation efficiency by CF4/O2 plasma treatment was observed in SiO2-capped rapid thermal annealing. On the other hand, the decrease in activation efficiency by this plasma treatment was small in both WSiN-capped and capless rapid thermal annealings.
Theoretical considerations relating to the characteristic curves of the silver chalcogenide glass inorganic photoresists
Authors: Das Amitabha , Bello Abner , Jishi Radi A.
Published: July 2002

We present a calculation of the characteristic curves of the silver/chalcogenide glass inorganic photoresists which is based on a theory of photoinduced diffusion of silver into the chalcogenide glasses. The silver concentration profiles in the glasses are calculated for the cases of semi-infinite as well as finite slabs. We also consider the effect on the silver concentration profiles of the intrinsic absorption of photons, which results when illumination by photons whose energies are above the band-gap energies of the glass is employed. It is found that very high values of the contrast parameter are attainable for prolonged development.
Migration of Si in molecular-beam epitaxial growth of δ-doped GaAs and Al0.25Ga0.75As
Authors: Jansen Ph. , Meuris M. , Van Rossum M. , Borghs G.
Published: July 2002

We have analyzed the incorporation of Si δ doping during the molecular-beam epitaxial (MBE) growth of GaAs and Al0.25Ga0.75As using secondary-ion mass spectroscopy. At high substrate temperatures (≥580 °C) a significant and asymmetric broadening is observed in both GaAs and Al0.25Ga0.75As. This is due to the combined effect of thermal diffusion and migration towards the surface during MBE growth. This study points out that a low substrate temperature (≤540 °C) and a short time are required during MBE crystal growth to achieve confined δ doping.
Submicron structuring of YBa2Cu3O7 thin films with electron beam lithography
Authors: Albrecht W. , Langheinrich W. , Kurz H. , Poppe U. , Soltner H. , Schubert J.
Published: July 2002

Thin films of high-temperature superconducting YBaCuO deposited by dc-sputtering on SrTiO3 substrates are structured by electron beam lithography on a submicron scale. Details of the technology processes involved are presented. The sudden transition of structures below 1 μm into the semiconducting state is discussed.
Depth profiles of Xe ions at energies from 50 to 500 keV in ladderlike polyphenylsilsesquioxane
Authors: Wang Ke-Ming , Liu Ji-Tian , Shi Bo-Rong , Liu Xiang-Dong , Zhao Qing-Tai , Huang Mu-Bing , Deng Shu-Mei
Published: July 2002

Xe ions at energies from 50 to 500 keV have been implanted into the polymer ladderlike polyphenylsilsesquioxane ([(C6H5)8Si8O12]n). The corresponding depth profiles have been measured by 2.1-MeV 4He++ Rutherford backscattering. The experimental data were compared with our calculation based on Biersack’s angular diffusion model and Monte Carlo simulation. The results show that a good agreement is found between the experimental data and calculated values for the mean projected range, but significant deviations from theoretical values are observed for range straggling.
Photoluminescence study of Li- and Na-implanted ZnSe epitaxial layers grown by atmospheric pressure metalorganic vapor-phase epitaxy
Authors: Yodo Tokuo , Ueda Kazuhiro , Morio Kenji , Yamashita Ken , Tanaka Shuhei
Published: July 2002

We have investigated p-type doping of Li and Na impurities by ion implantation into ZnSe heteroepitaxial layers grown at very low temperatures (250 °C) by atmospheric pressure metalorganic vapor-phase epitaxy. The activation rates of shallow acceptor levels, the density of self-activated centers in the implanted-ZnSe epitaxial layers (epilayers), and the degradation of crystalline quality due to radiation damage are dependent on the ion implantation and annealing conditions. As the dose density changes, so do the optimum annealing conditions for the degrees of activation and the recovery of radiation damage. Also the depth profile of the implanted ions and damage is greatly influenced by the acceleration energy. It is difficult to decide on the optimum annealing conditions at the different acceleration energies and to assess the crystalline quality of the implanted epilayers with a nonuniform depth profile of the implanted ions, because the region analyzed is restricted near to the surface and does not always coincide with the implanted region in the epilayer. We have confirmed that the high-temperature annealing does not cause the damage to propagate deeply into the implanted epilayers, although it has been a cause of much concern in II-VI compounds.
Electromigration of Ga and Sb during anodization of Al/GaSb structures
Authors: Teare S. W. , Fischer C. W.
Published: July 2002

This paper presents a study of the electric field induced migration of Ga and Sb ions during anodic oxidation of Al films on GaSb. Rutherford backscattering spectrometry was used to examine the composition and thickness of the anodic oxides. Oxide thickening was found to occur at the electrolyte oxide and also at the GaSb oxide substrate interface. It was found that Ga and Sb were mobile under the growth conditions and mobile ions resulted in the formation of a surface GaSb oxide. From measurments of the thickness we are able to report on the transport number for the anion and cations in this system. The growth of the surface GaSb oxide is discussed with reference to the intrinsic oxide growth rates in the Al and GaSb.
Ion milling damage in InP and GaAs
Authors: Pearton S. J. , Chakrabarti U. K. , Perley A. P. , Jones K. S.
Published: July 2002

Near-surface damage created by Ar+ ion milling in InP and GaAs was characterized by capacitance-voltage, current-voltage, photoluminescence, ion channeling, and transmission electron microscopy. We find no evidence of amorphous layer formation in either material even for Ar+ ion energies of 800 eV. Low ion energies (200 eV) create thin (≤100 Å) damaged regions which can be removed by annealing at 500 °C. Higher ion energies (≥500 eV) create more thermally stable damaged layers which actually show higher backscattering yields after 500 °C annealing. Heating to 800 °C is required to restore the near-surface crystallinity, although a layer of extended defects forms in GaAs after such a treatment. No dislocations are observed in InP after this type of annealing. The electrical characteristics of both InP and GaAs after ion milling at ≥500 eV cannot be restored by annealing, and it is necessary to remove the damaged surface by wet chemical etching. For the same Ar+ ion energies the damaged layers are deeper for InP than for GaAs after 500 eV ion milling at 45° incidence angle. Removal of ∼485 and ∼650 Å from GaAs and InP, respectively, restores the initial current-voltage characteristics of simple Schottky diodes.
Selective suppression of photochemical dry etching using elevated surface impurity concentrations: A new technique for self-aligned etching
Authors: Ashby C. I. H. , Myers D. R. , Vawter G. A. , Biefeld R. M. , Datye A. K.
Published: July 2002

High concentrations of recombination-promoting impurities in the near-surface region of a semiconductor can produce virtually complete suppression of carrier-driven photochemical etching processes. In-diffusion of Zn to levels appropriate for ohmic contact formation on GaAs (mid-1020/cm3) has been employed to reduce etching to an undetectable level in n-GaAs with n=1.0×1017 and 1.3×1018/cm3 and to produce a greater than ten-fold reduction in etching of semi-insulating GaAs. Raman spectroscopy of the altered near-surface region shows enhanced electronic scattering, which indicates the presence of a sufficient impurity concentration to suppress etching. Transmission electron microscopy shows the near-surface region to be crystalline without significant numbers of defects following Zn diffusion. Possible applications of this process include self-aligned etching of transistor and laser structures.
Formation and characterization of a PtSi contacted n+p shallow junction
Authors: Tsui Bing-Yue , Chen Mao-Chieh
Published: July 2002

Ion implantation through metal (ITM) or metal silicide (ITS) appears to be an attractive technique for self-aligned silicided shallow junction formation. Since most of the implanted ions are confined in the barrier film, the damage generated in the silicon substrate is reduced and so is the required post-implant annealing temperature. The purpose of this work is to study the capability of using Pt and PtSi in the ITM/ITS technique. As+ ions were implanted through 30-nm Pt or 60-nm PtSi at 80 keV with doses ranging from 1×1015 to 1×1016 cm−2. The implanted samples were annealed from 650 to 800 °C. Junction depths measured by spreading resistance and secondary-ion mass spectroscopy on 750 °C annealed samples are about 0.11 and 0.13 μm for the ITS and ITM schemes, respectively. The reverse area and peripheral leakage current density (JRA and JRP) are separated by measuring diodes of different size. All samples with a dose of 1×1015 cm−2 failed due to an insufficient amount of dopants which were incorporated into Si.

For a dosage greater than 5×1015 cm−2, the JRA at −5 V is less than 0.2 nA/cm2 and the forward ideality factor is lower than 1.02 for the ITS samples annealed at 750 °C. Activation energy measurement shows that the JRA is diffusion-component dominated while the JRP is generation-component dominated at room temperature. Surface generation current and lateral implantation damage-induced current are examined and discussed in detail. A possible reason is that the lateral junction depth is less than the vertical junction depth and the depletion region may be extended to near the PtSi/Si interface. Annealing at 800 °C degrades the junction characteristics because the Pt from PtSi diffuses into the junction region. Based on the experimental results and analysis we conclude that either Pt or PtSi film can be employed as an efficient barrier film in the ITM/ITS technique. A high-quality and ultrashallow junction can be fabricated using furnace annealing at temperature as low as 750 °C. The peripheral current generated at the silicide/silicon interface dominates the reverse junction current and cannot be avoided for the silicided junction with a junction depth less than 50 nm from the interface. This factor acts as a physical limitation as the junction depth scales down.
Formation of (Ca1−xSrx)2PbO4 in Bi(Pb)-Sr-Ca-Cu-O system: Correlation with the formation of the 2223 high-T
c phase
Authors: Shiloh M. , Wood I. , Brown M. , Beech F. , Boyd I. W.
Published: July 2002

X-ray diffraction data of Bi1.4Pb0.6Sr2Ca2Cu3.6O10+x samples show a sharp change near the annealing temperature of 845 °C. Below this temperature a solid solution of (Ca0.867Sr0.133)2PbO4 is formed and prolonged annealing leads to transformation of the 2212 phase to the 2223 phase. Above this temperature, Ca2PbO4 is formed, and prolonged annealing leads to an irreversible transformation of both the 2212 phase and the 2223 phase to the 2201 phase.
Preparation of YBa2Cu3O7−δ thin films by magnetron sputtering with i
n s
i
t
u plasma oxidation
Authors: Yang Senzu , Wu Peiheng , Ji Zhengming , Sun Zhijian , Zhang Ruojian , Li Yuan , Zhang Shiyuan , Zhang Hongchai , Liu Hanmo
Published: July 2002

Superconducting YBa2Cu3O7−δ thin films with surface roughness Ra∼20 Å have been successfully prepared by either rf or dc magnetron sputtering. The substrate temperature was kept at 600–670 °C during deposition and a subsequent in situ plasma oxidation treatment was performed at 480–520 °C. The films deposited on single-crystal SrTiO3 (100) exhibited zero resistance at 91 K and had the critical current density of 3×106 A/cm2. In this paper we describe the influences of fabrication conditions on film superconductivity.