bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructure and homogeneity in (In,Mn)As III-V-based diluted magnetic semiconductor epitaxial films
Authors: Guha S. , Munekata H.
Published: July 2002

Microstructural evaluation of thick (In,Mn)As epitaxial films grown by molecular-beam epitaxy on InAs/GaAs(100) substrates is carried out by transmission electron microscopy (TEM). Films grown at the substrate temperature of Ts=300 °C show the inclusion of MnAs crystallites in the zinc-blende host matrix, in which two types of crystallite morphologies, rod and approximately round shapes, are identified. In contrast, the MnAs crystallites are not observed in films grown at Ts=200 °C, and TEM studies confirm that the films are primarily of zinc-blende structure. Microstructural defects in the films are also discussed to assess the quality of epitaxy.
A Monte Carlo simulation of resonance radiation transport in the rare-gas–mercury positive column
Authors: Sommerer Timothy J.
Published: July 2002

A Monte Carlo simulation is presented for the transport of mercury 254 nm radiation in the cylindrical positive column of an argon-mercury fluorescent lamp. Of particular interest are factors influencing the fraction f of photons which, once initially created by electron impact excitation in the volume of the lamp, actually escape to the outer wall of the lamp rather than being lost to nonresonant quenching. Increases in f of up to ≊6%, reported earlier from a similar model [Anderson et al., Phys. Rev. A 31, 2968 (1985)], are confirmed for artificial increases in the abundance of the 196Hg isotope to ≊10%. Other manipulations of the isotopic mercury content are investigated here, but none increase f by more than a few percent. Externally applied axial magnetic fields up to 0.15 T are found to increase f by up to 15%.
Evaluation of ohmic contacts formed by B+ implantation and Ti-Au metallization on diamond
Authors: Venkatesan V. , Malta D. M. , Das K. , Belu A. M.
Published: July 2002

Low-resistance ohmic contacts have been fabricated on a naturally occurring B-doped diamond crystal and on polycrystalline diamond films by B ion implantation and subsequent Ti/Au bilayer metallization. A high B concentration was obtained at the surface by ion implantation, a post-implant anneal, and a subsequent chemical removal of the graphite layer. A bilayer metallization of Ti followed by Au, annealed at 850 °C, yielded specific contact resistance (ρc) values of the order of 10−5 Ω cm2 for chemical vapor deposition grown polycrystalline films and the natural IIb crystal. The ρc values from transmission line model measurements on three different contact configurations, namely, standard rectangular pads, rectangular pads on diamond mesas, and three-ring circular structures have been compared. These contacts were stable to a measurement temperature of ∼400 °C and no degradation due to temperature cycling was observed. Chemical analysis by x-ray photoelectron spectroscopy (XPS) in conjunction with Ar+ sputter depth profiling of the annealed samples indicated that the Au overlayer was not effective in preventing oxidation of the underlying Ti. The XPS study also indicated the formation of TiC at the Ti/diamond interface.
Effects of oxygen on the melting and freezing of Bi2Sr2Ca1Cu2O8+δ
Authors: Polonka J. , Xu Ming , Goldman A. I. , Finnemore D. K.
Published: July 2002

Crystallization of Bi2Sr2Ca1Cu2O8+δ has been studied as a function of the partial pressure of oxygen, using in situ high temperature x-ray measurements, in order to determine the phases that form and the kinetics of growth. As the partial pressure of oxygen is systematically reduced, the dominant second phase changes from (Sr,Ca)2Cu1O3 at 150 Torr of oxygen, to (Sr,Ca)1Cu1O2 at 15 Torr of oxygen, to CaO in a pure nitrogen atmosphere. Normally, in 150 Torr of oxygen, Bi2Sr2Cu1O6+δ is the first Bi cuprate to form from the melt but if the oxygen partial pressure is reduced to 75 Torr, then Bi2Sr2Ca1Cu2O8+δ is favored due to the decreased amount of 2:1 phase in the melt.
Density changes and viscous flow during structural relaxation of amorphous silicon
Authors: Volkert C. A.
Published: July 2002

Structural relaxation of amorphous silicon (a-Si) surface layers made by ion irradiation has been studied during heating using wafer curvature measurements. These measurements, which determine the stress in the amorphous layer, are sensitive to both plastic deformation and density changes. The amorphous layer first expands (0.1%) on heating from room temperature to 250 °C and then densifies (0.1%) on heating further to 500 °C. A larger expansion (≥0.3%) is observed on heating liquid-nitrogen-temperature irradiated a-Si to room temperature. This behavior reveals the existence of two distinct relaxation regimes, and is explained in terms of the annihilation of complementary features of the amorphous covalent network. In addition to density changes, shear deformation was observed during heating the a-Si layers. This deformation was characterized by a Newtonian shear viscosity of roughly 3×1012 N s/m2. The thermal-expansion coefficient of the a-Si was determined to be roughly 6.5% smaller than that of crystalline Si. Stress changes due to crystallization by epitaxial regrowth were observed between 600 and 700 °C and revealed evidence for the existence of large compressive stresses at the amorphous-crystalline interface.
Electron-cyclotron-resonance plasma-assisted radio-frequency-sputtered strontium titanate thin films
Authors: Belsick J. R. , Krupanidhi S. B.
Published: July 2002

Strontium titanate thin films were deposited by electron-cyclotron-resonance plasma-assisted radio-frequency magnetron sputtering. Electron-cyclotron-resonance plasma assistance was employed because of its ability to be used as a source of low-energy bombardment by a high density of species that are highly activated. It was found that both the structure and composition improve with the application of microwave plasma during the deposition. Analysis of the capacitance-voltage characteristics of metal-insulator-semiconductor devices revealed that the quality of the film/substrate interface is dependent on the pressure, atmosphere, and temperature of the deposition. Interfacial traps which give rise to charged surface states and silicon oxide formation have detrimental effects on films deposited on bare silicon substrates. Films on platinum-coated silicon substrates show good dielectric properties. The small-signal dielectric constant and dissipation factor at a frequency of 100 kHz were 170 and 0.033, respectively. For a 0.37-μm-thick film a charge storage density of 28 fC/μm2 and a unit area capacitance of 3.7 fF/μm2 were obtained at an applied electric field of 200 kV/cm.
Nanoscale study of the as-grown hydrogenated amorphous silicon surface
Authors: Stutzin G. C. , Ostrom R. M. , Gallagher Alan , Tanenbaum D. M.
Published: July 2002

A scanning tunneling microscope has been used to study the topography of the as-grown surface of device-quality, intrinsic, hydrogenated amorphous silicon deposited by rf discharge from silane. The substrates were atomically flat, oxide-free, single-crystal silicon or gallium arsenide. No evidence for island formation or nanoscale irregularities was seen in studies of 100-Å-thick films on either silicon or gallium arsenide. The topography of 1000- and 4000-Å-thick films has much variation; many regions can be characterized as ‘‘rolling hills,’’ but atomically flat areas have also been observed nearby. Generally, it appears that surface diffusion plays a role in smoothing the film topography. In most regions, the observed slopes were 10% or less from horizontal, but some steep-sided valleys, indicating incipient voids, were observed. The effect of the finite size of the scanning tunneling microscope probe tip is considered; this has an effect on the observed images in some cases.
The interfacial layer formation of the Al2O3/Si structures grown by low-pressure metalorganic chemical vapor deposition
Authors: Kim T. W. , Kang W. N. , Yoon Y. S. , Yom S. S. , Lee J. Y. , Kim Chayeon , Lim H. , Park H. L.
Published: July 2002

Metalorganic chemical vapor deposition of Al2O3 using Al(O-C3H7)3 and N2O via pyrolysis was investigated with the goal of producing Al2O3 epitaxial films on p-Si (100) substrates. Room-temperature capacitance-voltage measurements clearly showed metal-insulator-semiconductor behaviors for the samples with the Al2O3 insulator gate, and the interface state densities at the Al2O3/p-Si interface were approximately 1011 eV−1 cm−2 at the middle of the Si energy gap. Auger depth profiles demonstrated that the Al2O3/Si interface was not abrupt, and transmission electron microscopy verified the formation of an interfacial layer in the Al2O3/Si interface and the formation of a polycrystalline Al2O3 thin film. These results indicated that the failure to form Al2O3 epitaxial films was due to the formation of an interfacial layer prior to the growth of the Al2O3 layer.
Study of the Rayleigh acoustic mode in a-Si:H/a-SiNx:H superlattices
Authors: Xia Hua , Carlotti G. , Socino G. , Chen K. J. , Zhang Wei , Li Z. F. , Zhang X. K.
Published: July 2002

The Brillouin light-scattering technique has been exploited for studying the Rayleigh acoustic mode in superlattice films consisting of alternate layers of a-Si:H and a-SiNx:H. The phase velocity of the Rayleigh mode has been measured on superlattices with different thicknesses of the a-Si:H layer. The results obtained have been interpreted in terms of the effective medium model within the elastic continuum approximation; no anomalous behavior is observed, in contrast with previous ultrasonic measurements in superlattice with a minor nitrogen content.
A confocal photoluminescence study of metalorganic chemical vapor deposition growth on patterned GaAs substrates
Authors: Fong Yu-Chue , Armour E. A. , Hersee S. D. , Brueck S. R. J.
Published: July 2002

New insight is provided into the properties of GaAs/AlGaAs quantum wells and associated heterostructure layers grown by metalorganic chemical vapor deposition over grooves etched into GaAs substrates, an important class of geometries for multidimensional quantum confinement. A new, noncontact, simple characterization technique, confocal photoluminescence (CPL), provides composition, thickness, and electronic quality information with ≲1 μm spatial resolution, significantly improved over the traditional diffusion length limit of photoluminescence. Room temperature CPL spectra show: AlGaAs composition variations along the groove sidewall, including large compositional fluctuations on a 1 μm scale; variations in AlGaAs composition and material quality extending several μm’s from the groove edges; and rapid variations in quantum well thickness and optical properties, particularly along high-index growth planes.
Configuration transition of a closed-cylinder nematic liquid crystal in an external electric field
Authors: Chen Shu-Hsia , Wei Jong-Guang
Published: July 2002

This study reports on the director-field configurations of a closed-cylinder nematic liquid crystal with negative dielectric anisotropy in an external electric field. Raising the applied voltage causes the director orientation with a volume point defect to be transformed from a normal to a twisted configuration. The voltage at which the configuration is transformed depends on the aspect ratio of the cylinder.
A combined x-ray diffraction and Raman analysis of Ni/Au/Te-ohmic contacts to n-GaAs
Authors: Watté J. , Wuyts K. , Silverans R. E. , Van Hove M. , Van Rossum M.
Published: July 2002

The ohmic contact formation mechanism in furnace alloyed Ni/Au/Te/Au/GaAs structures is investigated by the combined application of x-ray diffraction and Raman spectroscopy. It was found that the Ni top layer promotes a more uniform dissociation of the GaAs substrate by the formation of a ternary Nix GaAs phase in the initial stages of contact formation (T=350 °C). At an alloy temperature T=550 °C, which induces low resistive electrical behavior, the formation of epitaxial Ga2Te3 was observed, indicating the formation of a Ga2Te3/GaAs heterojunction. After alloying at 600 °C, Ga2Te3 was still found to be present in the contact zone. In addition, evidence for a regrowth of GaAs crystallites could be derived from an increase of the intensity ratio ITO/ILO of the GaAs Raman signals. No indication for the presence of a high density of shallow donors (≳1019 cm−3) in the GaAs top layers could be adduced.
Effects of dissolved oxygen in a de-ionized water treatment on GaAs surface
Authors: Hirota Y.
Published: July 2002

This article presents a new passivation method with an As and/or hydrogenated As layer on a GaAs surface. X-ray photoelectron spectroscopy was employed to investigate the influences of dissolved oxygen on a GaAs surface during the de-ionized water treatment. It is shown that reduction of dissolved oxygen in de-ionized water accelerates the removal of Ga and As oxides from the surface, and that the de-ionized water treatment in an extremely low concentration of dissolved oxygen produces arsenic and/or hydrogenated arsenic layer due to the liberation of Ga atoms near the surface. These phenomena are discussed based on the chemical reaction between the GaAs surface and dissolved oxygen in de-ionized water. The arsenic and hydrogenated arsenic layer is available for preserving the oxide-free and damage-free GaAs surface.
The geometry of misfit dislocations with respect to the strained interface in [001] In0.1Ga0.9As/GaAs single heterostructures
Authors: Zou J. , Cockayne D. J. H. , Jiang S. S.
Published: July 2002

The position of misfit dislocations with respect to the strained interface in [001] In0.1Ga0.9As/GaAs single heterostructures is studied by transmission electron microscopy. The observation that, for h≳hc, some misfit dislocations glide away from the strained interface towards the substrate is explained by an analysis based on the elasticity theory of interacting dislocations.
Preanneal effect on the ring-shaped distribution of oxygen precipitates in Czochralski-grown silicon
Authors: Ono Haruhiko , Ikarashi Taeko , Kimura Shigeru , Tanikawa Akio
Published: July 2002

An anomalous ring-shaped distribution of oxygen precipitates in Czochralski-grown Si is investigated, using x-ray-diffraction topography and Fourier transform infrared spectroscopy. The appearance of the ring area, corresponding to a region of oxidation-induced stacking faults, strongly depends on the preanneal before the precipitation anneal at 1000 °C. The single precipitation anneal at an elevated temperature without any preanneal maintains the distribution of precipitation nuclei formed at 450 °C during cooling from crystal growth; however, the ring-shaped distribution is not affected by the existence of thermal donors. Therefore, it is concluded that the precipitation nuclei formed at 450 °C do not have any relationship to thermal donors.
X-ray photoelectron diffraction study of YbAs/GaAs(001) and ScAs/GaAs(001) heterostructures
Authors: Lépine B. , Quémerais A. , Sébilleau D. , Jézéquel G. , Agliz D. , Ballini Y. , Guivarc’h A.
Published: July 2002

Polar angle distributions of core level photoemission intensities recorded on YbAs/GaAs(001) and ScAs/GaAs(001) heterostructures are presented. They allow first the surface roughness of thin YbAs overlayers to be seen, second to estimate the tetragonal distortion of a strained ScAs film and, third, the most interesting point, to demonstrate in a direct fashion that the mixed (Yb-As) (010) planes of YbAs grow in the prolongation of the As planes of GaAs. The results are compared to those obtained by other authors with various techniques. The main advantage of the photoelectron diffraction method over the other techniques is that it can be performed on very thin epitaxial films (some monolayers) directly in situ under ultrahigh vacuum.
Study of point defects in CdTe and CdTe:V by cathodoluminescence
Authors: Pal U. , Piqueras J. , Fernández P. , Serrano M. D. , Dieguez E.
Published: July 2002

Cathodoluminescence in the scanning electron microscope has been used to investigate the relationship of point defects in CdTe and CdTe:V with luminescence bands at 1.40 and 1.13 eV. V has been found to inhibit the 1.40 eV luminescence. Annealing experiments indicate that Cd and Te vacancies are involved in the mentioned emission bands.
Donor-doping characteristics of gas-source molecular beam epitaxial Si and Si1−xGex using phosphine
Authors: Li S. H. , Bhattacharya P. K.
Published: July 2002

Well-behaved and reproducible n-type doping of Si and Si1−xGex by phosphine during gas-source molecular beam epitaxy is demonstrated. No significant reduction of growth rate of these materials in the presence of phosphine is recorded in the doping range of 1017–1019 cm−3 and perfect surface morphologies are observed. The incorporated P atoms are fully activated without ex situ annealing. The doping profiles are well defined in both Si and Si1−xGex layers. A p-Si0.9Ge0.1/n-Si heterojunction diode made with boron and phosphine doping has demonstrated excellent rectifying characteristics.
X-ray analysis of strain relaxation in strained-layer superlattices
Authors: Li J. H. , Mai Z. H. , Cui S. F.
Published: July 2002

Theoretical simulations of x-ray double-crystal-diffraction rocking curves for strain-relaxed superlattices have been successfully carried out based on x-ray dynamical diffraction theory. The strain relaxation, the misorientation between the superlattice layers and the substrate, and the effect of peak broadening due to the formation of misfit dislocations have been taken into account. The influence of possible strain relaxation mechanisms and relaxation ratios on the rocking curves have been investigated. It was found that both the mechanism and degree of the strain relaxation of the superlattice can be determined by fitting the angular positions and the relative intensities of the experimental superlattice satellites. By using this method, an InxGa1−xAs/GaAs superlattice sample and a GexSi1−x/Si superlattice sample were analyzed. The different strain-relaxation mechanisms were found in these two samples.
Proportional regime of a double superconducting tunnel junction detector
Authors: Esposito E. , Ivlev B. , Pepe G. , Scotti di Uccio U.
Published: July 2002

The proportional regime of a double superconducting tunnel junction detector is described in terms of the diffusion equation of the nonequilibrium quasiparticles excited in the absorber by the impinging radiation. The expression of the output current signals through the junctions is obtained. Some numerical results are shown and discussed also in connection with the operation of the detector in the charge collection mode.
Low-temperature carrier distributions in wide quantum wells of different shapes from capacitance-voltage measurements
Authors: Sundaram M. , Allen S. J. , Gossard A. C.
Published: July 2002

The carrier distributions in modulation-doped wide graded quantum wells that a measurement of the capacitance C between a surface gate and an ohmic contact to the carriers as a function of the applied bias V would yield are calculated. These capacitance-voltage (C-V) distributions are found to agree inexactly, but closely, with the calculated true carrier distributions. Density modulation features, induced by superlattices or by abrupt changes in the curvature of band-gap grading, are strikingly reproduced. Electron distributions extracted from actual measurements on a wide parabolic well and on a parabolic well with superimposed superlattice are in good agreement with theory. For the case of the parabolic well, the occupancy of a finite number of subbands is manifested as structure in the C-V distributions. This technique is relevant to the measurement of carrier distributions in any wide carrier system with more than one electric subband occupied.
Dependence of ion-matrix dose on density for planar, cylindrical, and spherical electrodes
Authors: Kissick M. W. , Hong M. P. , Shamim M. M. , Callen J. D. , Conrad J. R. , Emmert G. A.
Published: July 2002

Ion-matrix dose dependence calculations for plasma source ion implantation in planar, cylindrical, and spherical geometries are presented. It is demonstrated that in the high plasma density limit (in relation to the applied electrode potential and electrode size), the spherical and cylindrical cases approach the planar case. However, in the low relative density limit, the density dependencies diverge with the dependence vanishing for the spherical case, remaining unchanged for the planar case, and with the cylindrical case lying between the previous two.
Guided-mode interactions in thin films with surface corrugation
Authors: Seshadri S. R.
Published: July 2002

The guided modes in a thin-film planar dielectric waveguide sandwiched between a cover and a substrate (two different dielectrics) are considered. The interface between the cover and the film has a smooth corrugation in the longitudinal direction. For weak corrugations, the guided-mode interactions are investigated using the expansion in terms of ideal normal modes. A corresponding treatment is given for the not-so-weak corrugations using the expansion in terms of local normal modes. The coupling coefficients are evaluated and reduced to simple forms. The theories are specialized for the treatment of contradirectional coupling between two guided modes taking place selectively in the neighborhood of the Bragg frequency. The coupled-mode equations governing the contradirectional interaction obtained from the local normal mode expansion procedure, in the limit of weak periodic corrugations, are identical to those deduced directly using the ideal normal mode expansion technique. The treatments for both the transverse electric and the transverse magnetic modes are included.
Surface modification during scanning tunneling microscope measurements on YBa2Cu3O7 thin films
Authors: Geyer Ulrich , von Minnigerode Gunther , Krebs Hans-Ulrich
Published: July 2002

During scanning tunneling microscopy measurements on YBa2Cu3O7 thin films with positive sample bias voltages, different kinds of surface modifications appear. While in the topography condition no surface modification occurs, the cleaning and etching conditions irreversibly change the surface topography. In the deformation condition a strong elastic deformation of the tip and/or the surface is observed. It is shown that field evaporation of material from the surface to the tip is responsible for the observed surface modification. In consequence, the tip is contaminated with different oxides. Localized states in the oxides make resonant tunneling of electrons through the oxide layer possible. The configuration of these localized states determines the kind of surface modification by the scanning tunneling microscope.
Characterization of Pt-Si interface by spectroscopic ellipsometry
Authors: Liu Y. C. , Chen T. P. , Fung S. , Beling C. D.
Published: July 2002

Spectroscopic ellipsometric measurements for Pt/n-Si samples with different thickness of Pt films have been performed. The thickness of the Pt films determined with the three-phase model (air/Pt/Si) changes with the wavelength λ while that with the four-phase model (air/Pt/interface layer/Si) remains unchanged, showing the existence of an interface layer. At the same time, the apparent optical dielectric constants of the interface layer as a function of λ are also obtained. A calculation based on the effective medium theory is carried out to simulate the optical dielectric data of the interface layer. Some structural information of the interface layer is obtained from the calculation.
The magnetic properties of R2(Fe1−xSix)17 compounds (R=Dy, Y)
Authors: Zhang D. M. , Gao Y. H. , Yu B. M. , Tang C. Q. , Tang N. , Zhong X. P. , Lin W. G. , Yang F. M. , de Boer F. R.
Published: July 2002

The structures and magnetic properties of R2(Fe1−xSix)17 compounds (R=Dy, Y) were investigated by x-ray phase analysis and magnetic measurements. It is shown that Si substitution for Fe leads to a phase transition at x=0.2, and that the mean iron magnetic moment μFe of R2(Fe1−xSix)17 compounds (R=Dy, Y) decreases as Si concentration x increases; moreover, there is a maximum of dependence Tc(x) at about x=0.16. It is found that the exchange interaction constant AFeFe in R2(Fe1−xSix)17 compounds (R=Dy, Y) increases with x increasing by means of mean-molecular-field analysis of Tc.
Device oriented magnetoelastic properties of TbxDy1−xFe1.95 (x=0.27, 0.3) at elevated temperatures
Authors: Prajapati K. , Greenough R. D. , Jenner A. G.
Published: July 2002

In order to assess the potential of the highly magnetostrictive compound Terfenol-D (composition TbxDy1−xFe1.95, where x=0.3 or 0.27) for device applications in the temperature range 20–300 °C, measurement of magnetostrictive strain (λ), differential strain coefficient (d33), and magnetomechanical coupling coefficient (k33) have been made against applied dc field (≤120 kA/m), uniaxial pressure (≤20 MPa), and temperature. Comparing the two compositions at elevated temperatures has shown that large grain oriented material of the composition Tb0.3Dy0.7Fe1.95 has higher strains and d coefficients at elevated temperatures than small grain oriented material of composition Tb0.27Dy0.73Fe1.95, but for optimum coupling efficiency Tb0.27Dy0.73Fe1.95 is preferable.
The effect of Mn on the magnetic properties of YFe2
Authors: Cai Jian-Wang , Feng Yuan-Bing , Luo He-Lie , Zeng Zhi , Zheng Qing-Qi
Published: July 2002

The fully self-consistent discrete variational method within the local-spin-density framework has been employed to obtain the electronic structure and magnetic moments of the Mn-containing cubic Laves phase pseudobinary compound of YFe2. The calculated results show that the substitution of Mn atoms on Fe sites weakens the ferromagnetism of this compound. The magnetic moments on distinct atomic sites were derived from the calculation. We found that when the Fe atom has one Mn atom in its nearest neighbor sites, its magnetic moment decreases rapidly, while the Mn moment is 0.71μB, parallel to the Fe moment. Based upon our results, the various experimental data have been satisfactorily explained.
Properties of polycrystalline samples of Nd2−xCexCuO4−y obtained from a sol-gel precursor
Authors: Barbeta V. B. , Jardim R. F. , Ben-Dor L. , Maple M. B.
Published: July 2002

Polycrystalline samples of Nd2−xCexCuO4−y(0.0≤x≤0.17) obtained from a sol-gel precursor were sintered below the eutectic temperature and reduced under different conditions. From the results of x-ray powder diffraction, the tetragonal T′ structure is preserved for Ce content up to x=0.17. A decrease in the lattice parameter c and a small increase in the lattice parameter a are observed with increasing Ce concentration. Electrical resistance R(T) measurements performed on reduced samples with x≥0.12 reveal superconducting behavior below T≊25 K. We also found that all reduced samples display a striking double resistive superconducting transition and that the zero resistance state is achieved through two distinct drops in R(T) at upper and lower transition temperatures Tci and Tcj. The upper transition temperature Tci has a maximum for Ce concentration x=0.14, which is independent of the reduction process. The lower transition temperature Tcj is observed in reduced samples with Ce content x≥0.14 and decreases monotonically with increasing Ce concentration. Magnetic susceptibility measurements confirm bulk superconductivity in several reduced samples, and a significant diamagnetic contribution is only observed for temperatures below Tcj. The results are discussed within the framework of a granular superconductor model.
Spin-polarized photoemission from quantum well and interface states (invited)
Authors: Carbone C. , Vescovo E. , Kläsges R. , Eberhardt W. , Rader O. , Gudat W.
Published: July 2002

We examine the role of quantum well and interface states in mediating the coupling between magnetic films. We have studied with spin- and angle-resolved photoemission the electronic structure of Cu on Co(100) and Ag on Fe(100). Noble metal states of sp-derived character are found to be spin-polarized upon contact with the magnetic materials. In Cu films up to 10 monolayer thickness polarized states have been observed. These observations are well described within the framework of one-dimensional quantum-well states, for film thicknesses above a few monolayers. In the low coverage regime, the hybridization with the magnetic states of the substrate strongly influences the character and the dispersion of the conduction bands.
Effect of coupling on magnetic properties of uniaxial anisotropy NiFeCo/TaN/NiFeCo sandwich thin films
Authors: Yeh T. , Berg L. , Witcraft B. , Falenschek J. , Yue J.
Published: July 2002

The coercivity of the coupled NiFeCo thin films is a function of nickel-iron-cobalt (NiFeCo) film thickness and tantalum nitride (TaN) intermediate layer thickness. This can be attributed to the variation of energy and magnetization gradients of a domain wall. The magnetization gradient can be affected by a thickness-induced coupling effect associated with altering the exchange and magnetostatic interactions between the two NiFeCo films. Decreasing the exchange coupling and increasing the magnetostatic coupling between the coupled NiFeCo films tends to decrease the energy and magnetization gradients of domain walls and results in decreased coercivity.
Structure, transport and thermal properties of UCoGa
Authors: Purwanto A. , Robinson R. A. , Prokeš K. , Nakotte H. , de Boer F. R. , Havela L. , Sechovský V. , Tuan N. C. , Kergadallan Y. , Spirlet J. C. , Rebizant J.
Published: July 2002

By means of neutron powder diffraction, we find that UCoGa crystallizes in the hexagonal ZrNiAl structure and orders ferromagnetically at low temperatures with magnetic moments stacked along the c axis. The magnetic-ordering temperature is reflected in anomalies in the temperature dependencies of the electrical resistivity and the specific heat at TC =47 K. Furthermore, the strong anisotropy in the electrical resistivity for i∥c and i⊥c indicates a significant contribution of the magnetic anisotropy to the electrical resistivity.
Surface precession solitons (surface ‘‘Magnetic drops’’) in uniaxial magnetics (abstract)
Authors: Bespyatykh Yurij , Dikshtein Igor , Nikitov Sergey
Published: July 2002

Surface spin waves, localizing near the surface of magnets as a consequence of nonlinear properties of material, have been studied in Ref. . According to the Lighthill criterion this wave is modulationally unstable. This can lead to the creation of nonunidimensional surface states. In this work, a new class of nonlinear exitations in a pure exchange-coupled uniaxial ferro- and antiferromagnets is considered. These exitations are 3D surface precession solitons or ‘‘magnetic drops’’ localized near the surface of a crystal. Such solitons are the space localized solutions of the Landau–Lifshits equation of motion for magnetization field with appropriate boundary conditions for the spins on the surface of magnetics. They keep their dynamic structure during the motion in space. The conditions of their stability are found. The problem of soliton motion is considered. It is shown that energy and precession frequency for surface 3D solitons is less than for volume 3D solitons, which was studied in Ref. . The possibility of experimental registration of such solitons is discussed.
Correlation of magneto-volume effects and local properties of the Fe2Ti-laves phase (abstract)
Authors: Pelloth J. , Brand R. A. , Keune W.
Published: July 2002

Magneto-volume effects have been calculated and shown to exist in Fe2Ti alloys. Since Mössbauer spectroscopy provides insight into local magnetic properties we have performed such measurements in the temperature range 4.2 K<T<800 in Bext=0 and ≠0 on Fe2+xTi1−x powder samples with x=−0.05 to +0.1. The spectra of Fe2Ti at 4.2 K show two subspectra, one belonging to the antiferromagnetically coupled 6h sites, and the other one to the paramagnetic 2a sites, in agreement with Ref. . A detailed analysis shows that the electric field gradient of the 6h sites changes in sign and value when going from the antiferromagnetic to the paramagnetic state at 310 K. Moreover, the isomer shift values of the two sites are found to be different at 4.2 K, while they are equal above 310 K. Both changes suggest a change in the local electronic band structure between the paramagnetic and antiferromagnetic states. An increase of the linewidth is observable in the temperature region of 200 K<T<310 K that could be caused by a distribution of hyperfine fields on the 6h sites or by spin fluctuations during the lifetime of the excited state of the Mössbauer nucleus. For x≳0 and 4.2 K, an additional magnetically split subspectrum appears that is associated with 6h sites and neighboring Fe antisite atoms. For x<0 the spectra are similar to those of Fe2Ti.
A systematic study on stability of flux in Nd-Fe-B magnets consolidated by direct joule heating
Authors: Fukunaga H. , Tomita H. , Wada M. , Yamashita F. , Toshimura T.
Published: July 2002

Magnetic aftereffect and irreversible flux loss at elevated temperatures were studied for fully dense isotropic and anisotropic Nd-Fe-Co−B magnets consolidated from rapidly quenched powder by direct joule heating. The consolidation process and the subsequent alignment of c axis do not affect the aftereffect constant Sv and the prepared magnets have roughly the same Sv as a bonded magnet, independent of coercivity at room temperature (650–1200 kA/m), exposure temperature (60–120 °C), and anisotropy of the magnets. Thus the magnitude of the aftereffect is determined mainly by the magnitude of the irreversible susceptibility χirr. It was also clarified empirically that the irreversible flux loss FL is proportional in magnitude to χirr. However, the slopes of the FL vs χirr line for isotropic magnets are much larger than those for the anisotropic ones. Thus FL can be reduced by decreasing χirr and/or making magnets anisotropic.
Metastable Nd2(Fe1−xCox)23B3 (0≤x≤1.0) compounds with the 2:23:3-type structure
Authors: Shen Bao-gen , Zhang Bo , Wang Fang-wei , Zhang Jun-xian , Liang Bing , Zhan Wen-shan , Guo Hui-qun , Zhao Jian-gao
Published: July 2002

Amorphous Nd2(Fe1−xCox)23B3 (0≤x≤1.0) alloys were prepared by melt spinning at a speed of 47 m/s. When the amorphous samples were annealed at 900 K for 20 min, they crystallized to the metastable 2:23:3 single phase for all x. A detailed study of structure and magnetic properties of metastable Nd2(Fe1−xCox)23B3 compounds has been made by x-ray diffraction and magnetization measurements. They are body-centered cubic, and the lattice constant decreases linearly from a=14.16 Å for x=0 to 13.86 Å for x=1.0. The Fe atom moment μ̄Fe for Nd2(Fe1−xCox)23B3 was found to increase with x from 2.02μB for x=0 to 2.17μB for x=0.8, when the Co atom moment μ̄Co is assumed to be constant, as made in previous papers for crystalline Fe–Co alloys. The Curie temperature Tc is found to increase monotonically with increasing x from 659 K for x=0 to about 1218 K for x=1.0. The thermomagnetic measurements showed that when the Nd2(Fe1−xCox)23B3 compounds were heated to certain temperatures, they decomposed to Nd2(Fe,Co)14B and α-(Fe,Co) phases. The decomposition temperature of metastable compounds was about 990±15 K, and was almost independent of the Co concentration x.
Ferromagnetic filaments fabrication in porous Si matrix (invited)
Authors: Gusev Sergey A. , Korotkova Natalia A. , Rozenstein Dmitry B. , Fraerman Andrey A.
Published: July 2002

We have fabricated Ni filaments by nickel precipitation into a porous silicon matrix, making Ni filaments of about 200 Å in diameter and 30 μm in length separated by ∼500 Å silicon walls. This composite material demonstrates strong magnetic anisotropy perpendicular to the surface due to shape anisotropy of the Ni filaments.
Wannier states in magnetite
Authors: Her Trao , Boekema Carel
Published: July 2002

Magnetite exhibits the well-known metal–insulator transition at the Verwey temperature (TV) near 123 K. Neither the physical origin nor the conduction process above TV is well understood. Using the Cullen and Callen tight-binding Hamiltonian, the ‘‘extra’’ fully spin-polarized 3d conduction electrons in the B sublattice are described. By introducing a covalency parameter Γ, the Wannier states for these conduction electrons can be characterized. Only for the lower singlet subband, does the Γ value point toward delocalization. The results reveal that the Wannier states in magnetite are a mixture of localized and delocalized electron states.
Effects of domains on magnetoresistance (abstract)
Authors: Zhang Shufeng , Levy Peter M.
Published: July 2002

Our present understanding of the giant magnetoresistance observed in magnetic multilayered structures is based on the premise that the layers are either monodomains or that the sizes of the domains are much larger than all other length scales (mean free path, spin-diffusion length, and layer thickness) relevant to the magnetoresistance. We extend our theory of magnetoresistance to include multilayers with small in-plane domains where there is the possibility that their size may be limited by columnar growth. We study magnetoresistance in the presence of in-plane domains both for the current parallel and perpendicular to the plane of layers. The domain boundaries introduce additional scattering and the direction of the internal current will be different from that of the driven current. In the presence of spin-dependent scattering, a transverse spin current develops so that the current in each of the spin channels is ‘‘mixed’’ even without spin-flip processes. Analytical expressions will be given for limiting cases where the mean free path is either much larger or much smaller than the layer thickness and the domain size. We find that the domain size is a relevant length scale and contributes to the spin diffusion length λsdl in the equations that govern the spin diffusion attendant to charge transport in magnetic multilayers. If one does not know the domain structure, nor the amount of spin-flip scattering, both mechanisms are equally plausible for producing spin diffusion which suppresses the magnetoresistance for the current perpendicular to the plane of the layers.
Perfluorocyclobutane containing aromatic ether polymers as planarization materials for alternative magnetic media substrates
Authors: Perettie Donald J. , Judy Jack , Chen Qixu , Keirstead Rick
Published: July 2002

Perfluorocyclobutane aromatic ether polymers (PFCB) are being researched as planarization materials for alternative magnetic media substrates allowing smoother surfaces for lower head flying recording. The results of current work reported herein have shown that PFCB can be used to affect surfaces on canasite with RA’s less than 2 nm. In addition, magnetic media can be produced of a quality comparative to that obtained on standard NiP-coated Al as well as that produced on regular canasite with equivalent coercivities at about 1500–1600 Oe and squarenesses of 0.8 or better. In addition to the above magnetic properties the recording performance was excellent with signal-to-noise ratios of planarized media 3.5 dB higher than that on regular canasite.
Magnetic properties of nanometer-sized Fe4N compound (abstract)
Authors: Feng Y. B.
Published: July 2002

Nanometer-sized magnetic materials have attracted a lot of attention because of their potential applications in the fields of ferrofluids, high density magnetic recording, and magnetic refrigeration. The sol-gel method has been considered to be effective for creating dispersions of small metal particles in nonmetallic materials. In this work, nanometer-sized Fe-N compound particles were prepared by nitrogenating the iron-boron oxide glass powders, which were synthesized by the sol-gel method from ethylene glycol gel. The magnetic properties of nanometer-sized iron nitride were measured by a vibrating sample magnetometer (VSM). The specific magnetic moment σ is equal to 132 emu/g and the coercivity Hc is 150 Oe. The phase composition was determined by using an x-ray diffractometer (XRD). The diffraction pattern shows that the sample consisted of a main phase of Fe4N and a small α-Fe phase. The crystallite size d of the Fe4N particles was estimated by Scherrer’s formula, and is about 12 nm. The nanometer-sized Fe-N compound was also studied by using Mossbauer spectroscopy. The Mossbauer absorption pattern consisted of a ferromagnetic component superimposed on a superparamagnetic doublet. The ratio of superparamagnetic fraction to ferromagnetic is about 13%. It is shown that the sol-gel technique could be used to prepare ultrafine particles of Fe-N compound.
Magnetoresistance and magnetization oscillations in Fe/Cr/Fe trilayers
Authors: Schad R. , Potter C. D. , Beliën P. , Verbanck G. , Moshchalkov V. V. , Bruynseraede Y. , Schäfer M. , Schäfer R. , Grünberg P.
Published: July 2002

The 2-ML (monolayer) oscillation period has been observed in the magnetization as well as in the magnetoresistance of Fe/Cr/Fe trilayers. Kerr effect measurements were performed in order to verify the periodicity and determine the kind of the coupling between the Fe layers. The magnetoresistance loops show characteristic steps at magnetic field values at which the size of the magnetization changes.
Fluctuation effects in ultrathin films
Authors: Chui S. T.
Published: July 2002

We discuss finite temperature fluctuation of the domain walls and the magnetization M in ultrathin magnetic films where spins interact with short range exchange (J), anisotropy (K), and long range dipolar (g) couplings. Phase boundaries for a triangular lattice are obtained from Monte Carlo simulations as a function of J, g, and K. The transition temperature for the disappearance of the z magnetization as a function of the effective anisotropy field K−g/0.1208 is essentially unchanged as g becomes zero. This suggests a new physical picture that the change in the direction of M is closely connected with the Ising transition for g=0. Mean field calculation suggests three transitions where only Mz is finite at low temperature. As the temperature is increased (1) Mx becomes nonzero, (2) Mz becomes zero and eventually, (3) Mx becomes zero. To investigate fluctuation of domain walls their elastic energy are calculated. For Bloch walls perpendicular to the x axis separating spins along the z axis this energy is negative for a small enough wave vector. The competition of the stabilizing long range dipolar interaction and low dimensional fluctuation suggests the possibility of a finite temperature roughening of an array of one-dimensional Néel walls in the film.
Magnetic studies of fcc Co films grown on diamond (abstract)
Authors: Wolf J. A. , Krebs J. J. , Idzerda Y. U. , Prinz G. A.
Published: July 2002

We report the first growth and characterization of fcc Co epitaxial films (t=10–130 nm) on diamond. Growth was carried out under UHV conditions in a commercial MBE machine and monitored during growth using RHEED which showed single crystal growth oriented with 〈100〉Co∥〈100〉C. X-ray diffraction studies of the 130 nm sample demonstrated the single crystal fcc growth throughout the entire sample. Separate studies to determine growth mode and structure were carried out using angle-resolved Auger forward scattering. Vibrating sample and SQUID magnetometry yields a magnetic moment of (1.05±0.1) 103 emu/cm3. Ferromagnetic resonance measurements carried out at 35 GHz yield a large cubic anisotropy K1/MS=(480±30)Oe and linewidth of only ΔH=100 Oe. Spin waves were observed in the thicker samples and the exchange constant determined to be ACo=1.09×10−6 erg/cm. This work has been supported by the Office of Naval Research.
Circular magnetic x-ray dichroism for rare earths
Authors: König H. , Wang Xindong , Harmon B. N. , Carra P.
Published: July 2002

Circular magnetic x-ray dichroism is a promising new tool for the study of spin and orbital moments in ferromagnetically ordered materials. For rare-earth systems the L2 and L3 edge dichroic spectra give information about the magnetic properties of the 5d conduction electrons. This article addresses a few of the key issues concerning the interpretation of the spectra. In particular, the quadrupole nature of the dichroic spectra below the L absorption edges and its angular dependence is discussed. The important role of the 4f-5d exchange in causing a large and important spin dependence in the 2p–5d matrix elements is likewise examined.
Elemental determination of the magnetic moment vector (abstract)
Authors: Lin H.-J. , Meigs G. , Chen C. T. , Idzerda Y. U. , Prinz G. A. , Ho G. H.
Published: July 2002

Recently, we have shown that magnetic circular dichroism (MCD) can be used to obtain element specific magnetic hysteresis (ESMH) loops of heteromagnetic systems. By using magnetic circular dichroism (MCD) we map the three orthogonal components of the magnetic moment vector of each element of a strongly (90°) coupled Fe30Co70/Mn/Fe30Co70 single crystal trilayer heterostructure as a function of applied magnetic field. The intensity of the MCD of the Mn L2,3 absorption edges clearly shows that the Mn possesses a ferromagnetically aligned net magnetic moment. Furthermore, the individual element specific hystersis curves, when followed through the magnetic reversal process, reveals a detailed description of the magnetization reversal in the presence of both cubic anisotropy and strong exchange. This vividly demonstrates the capability of this technique to analyze the magnetic moment reversal process in the presence of strongly competing interactions.
Giant magnetoresistance in NiFe-Ag granular alloys
Authors: Badía F. , Labarta A. , Batlle X. , Watson M. L.
Published: July 2002

Some FeNi-Ag granular films of composition Fe11.43Ni6.35Ag82.22 (sample A) and Fe7.62Ni16.4Ag75.98 (B) were prepared by using rf magnetron sputtering, and once deposited were rapidly annealed at 600, 650, and 750 °C. All samples displayed giant magnetoresistance. The zero-field-cooled and field-cooled processes evidence the segregation of ferromagnetic particles with a broad size distribution. The temperature and magnetic field dependence of the resistance is analyzed. The magnetoresistance follows a Hn law at high fields and it decays from its maximum value with a Tm behavior, with m approaching 1 at high fields.
Critical magnetic susceptibility of gadolinium
Authors: Dunlap R. A. , Fujiki N. M. , Hargraves P. , Geldart D. J. W.
Published: July 2002

An extensive analysis of ac magnetic susceptibility measurements of single-crystal gadolinium is presented. The demagnetization-corrected c-axis data are analyzed on the basis of a power law of the form χc=At−γ where t is the reduced temperature above Tc. Our results yield effective values of Tc=293.57 K and γ=1.327. The basal plane susceptibility is expressed as a parametric equation in terms of the c-axis susceptibility χ−1ab=B′+C′χ−y/γc. The exponent y, which is related to the critical exponent associated with the specific heat α as y=1−α, is determined by our analysis to be y=1.01(2). This gives a temperature scale associated with the anisotropy as tanis=2.0×10−3. These results are interpreted in the context of the effects of dipolar interactions in the critical region.
Study of critical properties of the Potts model by the modified variational cumulant expansion method
Authors: Fazleev N. G. , Che Hao , Fry J. L. , Lin D. L.
Published: July 2002

The modified variational cumulant expansion (VCE) method valid for the whole temperature range and for arbitrary fields is developed to study the critical properties of the general q-state Potts model used to stimulate various magnetic systems. The series expansion is generated directly in terms of the Potts Hamiltonian. The free energy and the internal energy are calculated up to the third-order cumulant terms for the d-dimensional hypercubic q-state Potts model. The expression for the critical temperature Tc is derived in a closed form which allows to determine Tc up to arbitrary order cumulant terms. The modified VCE method eliminates the unphysical first-order phase transition at the mean-field critical point in the internal energy of the Potts model which plagues the conventional VCE method.