bluuebunny/crossref_metadata_2025 huggingface dataset:
Low resistance silver contacts to indium phosphide: Electrical and metallurgical considerations
Authors: Weizer Victor G. , Fatemi Navid S.
Published: July 2002

The electrical and metallurgical behavior of the Ag-InP contact system has been investigated. Specific contact resistivity (Rc) values in the low 10−6 Ω cm2 range are readily achieved on n-InP (Si: 1.7×1018 cm−3) after sintering at 400 °C for several minutes. The low Rc values, however, are shown to be accompanied by dissolution of InP into the metallization, resulting in device degradation. An analysis of the sinter-induced metallurgical interactions indicates that this system is quite similar to the well-characterized Au-InP system, although there are some fundamental differences. The similarities include the dissociative diffusion of In, the reaction-suppressing effect of SiO2 capping, and, most importantly, the formation of a phosphide layer at the metal-InP interface. It is suggested that the low post-sinter Rc values in the Ag-InP system are due to the presence of a AgP2 layer at the metal-InP interface. We show that it is possible to achieve low values of Rc without incurring device degrading metallurgical interactions by introducing a thin AgP2 layer between the InP and the current carrying metallization.
High-performance diffusion disordered AlxGa1−xAs lasers via a self-aligned process and conventional open-tube annealing
Authors: Burton R. S. , Schlesinger T. E. , Holmgren D. J. , Smith S. C. , Burnham R. D.
Published: July 2002

Process conditions for fabricating Si-O impurity-induced layer disorder defined AlxGa1−xAs-GaAs buried heterostructure quantum well lasers utilizing a fully self-aligned planar process and conventional As free open-tube-furnace annealing are presented. An SiO2 layer, deposited by sputtering, was used as a diffusion source for Si and O impurities as well as a source for Ga vacancies that enhance impurity diffusion and allow for a reduction in the required annealing temperature and time. A self-aligned native oxide of the AlxGa1−xAs cladding layer was used to form a Zn diffusion mask and dielectric layer. Lasers fabricated using this process exhibited threshold currents as low as 2.72 mA and external differential quantum efficiencies of 79% at room temperature in continuous operation.
Structural and magnetic properties of rare earth iron nitride series R2(Fe1−xCox)17N3−δ
Authors: Xu Xie , Shaheen S. A.
Published: July 2002

The structural and magnetic properties of ternary nitride compounds R2(Fe1−xCox)17N3−δ with R=Ce, Pr, and Nd have been studied. X-ray diffraction results show that nitrides are formed in a limited range of x in each series and the rhombohedral Th2Zn17 structure of their parent compounds is retained with an expansion in the unit cell volume. The room temperature saturation magnetization of nitrides is greater than that of the parent compounds for low Co concentrations. The 3d sublattice anisotropy is modified upon nitrogenation. All of the terminal compounds R2Fe17N3−δ have their easy direction of magnetization in the basal plane. However, easy direction of magnetization shifts from planar to axial for x≊0.5 in the R2(Fe1−xCox)17 series with R=Ce and Pr. This planar to axial transition of easy direction of magnetization shifts towards lower x (x≊0.15) in the nitride series R2(Fe1−xCox)17N3−δ for R=Ce, and is not observed for R=Pr and Nd. The results are interpreted in terms of enhancements in competing 3d sublattice (axial) and R sublattice (planar) anisotropies on nitrogenation, with Ce being essentially in the nonmagnetic intermediate valence state.
Desorption and other effects of pulsed-laser annealing of hydrogen-implanted silicon
Authors: Boivin R. , Terreault B.
Published: July 2002

The behavior of H-ion-implanted c-Si was investigated at temperatures up to the melting point, on the 10−8 s time scale, using pulsed-laser annealing in conjunction with quantitative analysis of the released gas. Laser reflectivity, scanning electron microscopy, and surface profilometry were also used to characterize implantation and annealing effects. Computational kinetic modeling of H release as a function of laser energy is applied to the interpretation of the data. The desorption of H implanted at 1 or 2 keV takes place at ≥1000 K, without extensive surface deformation, and can be fitted by detrapping with an activation energy (≊2 eV) that slowly decreases with the H/Si ratio in the range 4%–27%; contrary to expectation, no limitation by diffusion is observed. Implanted at 5 or 10 keV and H/Si≥20%, H is released at ≥550 K by blister rupture. In spite of the differences between the results for low and high implantation energies, a unified picture emerges, involving a layer with a high-temperature H mobility greater than that of ordinary atomic diffusion.
Nonequilibrium phase transformation and magnetic properties of Ni3Sn2 during mechanical milling
Authors: Zhou G. F. , Di L. M. , Bakker H.
Published: July 2002

A nonequilibrium phase transformation in stoichiometric Ni3Sn2 during mechanical impact in a high-energy ball mill was monitored by the measurements of high-field magnetization, ac susceptibility, and x-ray diffraction. X-ray diffraction showed that the material transformed from the low-temperature Ni3Sn2 phase with Ni3Sn2-type orthorhombic structure to the high-temperature Ni3Sn2 phase with partially filled-up B8-type structure after 40 h of milling. Quenching was used to confirm the phase transformation during ball milling. The magnetization of the ball-milled high-temperature phase (metastable) is much higher than that of the quenched high-temperature phase, where both ball-milled and quenched high-temperature phases have higher magnetization than the low-temperature phase. The Curie temperature of our metastable high-temperature phase is quite close to that of the equilibrium phases (high- and low-temperature phases). The result is discussed in terms of atomic disordering induced by mechanical milling.
High responsivity YBa2Cu3O7−x microbolometers with fast response times
Authors: Li Kin , Johnson Joseph E. , Aker Bruce W.
Published: July 2002

To make high-responsivity YBa2Cu3O7−x microbolometers with response times in the tens of microseconds, heat capacity and hence physical size must be small. We measured the responsivities and response times of 2.5×2.5 μm and 10×10 μm YBa2Cu3O7−x microbolometers fabricated on ZrO2 substrates and illuminated by an argon-ion laser focused to a 2 μm spot. Responsivities were found to be 975 and 700 V/W, respectively, with response times of 10 and 40 μs.
High-amplitude photoemission oscillation of highly As-rich GaAs (001) 2×4-γ phase during molecular beam epitaxy
Authors: Tsuda Hiroshi
Published: July 2002

Photoemission oscillation from GaAs(001) surfaces illuminated by a deuterium lamp is studied for a highly As-rich 2×4-γ phase during molecular beam epitaxy. The 2×4-γ phase shows higher oscillation amplitude than the As-poorer 2×4-α phase. Oscillation of 2×4-γ occurs as a result of two-dimensional nucleation, which is confirmed by the suppression of oscillation on highly misoriented GaAs(001) surfaces. Furthermore, the oscillation has its maxima at nearly half-monolayer growth, which correspond to maximum step-edge density. For this particular 2×4-γ phase, this implies that the surface with a higher step-edge density emits more photoelectron and has a lower ionization energy threshold.
Integral equation solution of low-pressure transport of gases in capillary tubes
Authors: Flory C. A. , Cutler L. S.
Published: July 2002

The molecular flow, or effusion limit, of transport through a capillary is extended to include the regime where the effects of intermolecular scattering become important. This is accomplished through a generalization of the Clausing equation, which gives a fundamental kinetic theory description of molecular flow. The generalized integral equation obtained is solved numerically to obtain both throughput and angular distribution properties of the gas flow. The results show good agreement with experimental curves which clearly demonstrate the effects of intermolecular scattering.
Aberration calculation for combined magnetic focusing deflection systems with curved axes
Authors: Ximen Jiye , Liu Zhixiong
Published: July 2002

The general aberration theory for combined electromagnetic focusing-deflection systems with curved axes will be applied to a scanning electron microscopic system and an electron lithographic system. All third order geometric and first order chromatic aberrations have been calculated along rectilinear/curvilinear axes. The important identities between some aberration coefficients will be verified numerically. The geometric and chromatic aberrations calculated along the matching curvilinear axis will be compared with those along the rectilinear axis. The computational trials may be of help for explaining the mechanism of moving/swinging objective lens and variable axis lens systems.
Barrier height lowering of Schottky contacts on AlInAs layers grown by metal-organic chemical-vapor deposition
Authors: Fujita Shinobu , Naritsuka Shigeya , Noda Takao , Wagai Aki , Ashizawa Yasuo
Published: July 2002

Schottky characteristics of undoped AlInAs grown by metal-organic chemical-vapor deposition have been investigated. I-V characteristics and their dependence on donor concentrations in AlInAs layers were explained well by assuming the existence of an interfacial layer. The presence of oxygen atoms at the metal-AlInAs interface was revealed by Auger electron spectroscopy measurements. The oxide formation on AlInAs surface may be an inevitable result because of the high composition of aluminum atoms in AlInAs. Reduction in donor concentration in AlInAs layers is effective in minimizing the influence of the interfacial layer on barrier height lowering. Reduction in gate leakage current was successfully demonstrated for fabricated GaInAs/AlInAs high-electron-mobility transistors by reducing donor concentration in AlInAs Schottky layers.
Deceleration of magnetic dipoles interacting with YBa2Cu3Ox superconductors
Authors: Kuze Hiroaki , Tachikawa Maki , Onae Atsushi
Published: July 2002

When a magnetic dipole passes through a hollow YBa2Cu3Ox cylinder, the dipole is decelerated as a result of the energy dissipation in the superconductor. This energy loss is evaluated on the basis of a macroscopic theory that incorporates both the hysteresis and flux-flow losses. The results are in reasonably good agreement with the experiments conducted using Nd-Fe-B magnets.
Analyses of photodischarges induced by bulk generation
Authors: Chen Inan
Published: July 2002

Photoinduced discharge is a common experimental technique for studying photogeneration and carrier transport in photoconductors. Although the technique was first devised for discharge with strongly absorbed light and trap-free transport, its application to systems with bulk-generated carriers and/or trap-controlled transport is becoming more important. It is noted that in the latter case, a finite error can be introduced by using the existing formula for data analyses. This paper describes a generalized method of deducing electronic transport and photogeneration properties from photoinduced discharge characteristics.
Trapped-hole centers in irradiated Li3VO4
Authors: Murata Takuya , Miki Toshikatsu
Published: July 2002

Defects in lithium vanadate (Li3VO4) exposed to x rays were studied by electron spin resonance (ESR). 77 K irradiation produces two types of trapped-hole centers. One is supposed to be carbonate radical CO3−, which is thermally stable even at room temperature. Another, having a hyperfine interaction with a vanadium nucleus (I=7/2, 99.76% natural abundant) is attributable to an intrinsic O− type center with one neighboring vanadium ion. The center is thermally unstable and anneals out around 200 K. In spite of the prominent detection of trapped-hole centers, ESR signals due to trapped-electron centers have not been clearly observed.
Structural control of nonequilibrium WSi2.6 thin films by external stress
Authors: Fujimura Norifumi , Tachibana Shoji , Ito Taichiro , Hosokawa Norio
Published: July 2002

The influence of external stresses on a crystallization and a precipitation process for amorphous WSi2.6 films has been studied. An as-deposited amorphous WSi2.6 has crystallized as a hexagonal WSi2 by annealing at 400 °C for 1 h, and excess Si has precipitated to the WSi2/substrate interface. To verify whether the driving force of the precipitation to the interface is a stress relaxation, calculations of the volume change by a crystallization, precipitations (in the film and to the interface), and the stress measurement were performed. It was revealed that as-deposited film had a tensile stress, and it increased by precipitation of excess Si to the interface. That is, the driving force of the precipitation to the interface is not the stress relaxation. Then we tried to apply larger stress (compression and tension) to make clear an effect of stress on the structural change. By annealing at 350 °C under external tensile stress,the Si precipitated at the interface was not observed. On the other hand, by annealing under external compressive stress, a greater amount of Si has precipitated than that without external stress. These phenomena seem to be related to the stress-assisted diffusion similar to Nabarro–Herring creep.
A new technique to decompose closely spaced interface and bulk trap states using temperature dependent pulse-width deep level transient spectroscopy method: An application to PT/CdS photodetector
Authors: Wang C. W. , Wu C. H. , Boone J. L.
Published: July 2002

A novel method is presented here to experimentally decompose nonexponential capacitive transients into the appropriate components from the closely spaced deep trap states. Using temperature dependent pulse-width deep level transient spectroscopy (TDP-DLTS) technique, we show for the first time that two bulk trap states and one continuously distributed interface states in (Pt/CdS) photodiodes can be successfully separated. The basic principle is to set the applied pulse width to follow an averaged temperature-dependent capture time constant and divide the DLTS spectrum. In the example of Pt/CdS photodiodes, we show that all physical parameters including thermal activation energies, capture cross sections, and trap densities are more accurately calculated after each component is separated from others. The origins for those bulk traps and interface states are also discussed. Without any complicated mathematics or program, TDP-DLTS can be applied to both large and small voltage pulse DLTS measurements.
Charge density excitations of two-dimensional magnetoplasma in semiconductor superlattices
Authors: Kushwaha Manvir S.
Published: July 2002

This paper presents the theoretical investigation of collective charge density excitations in two-dimensional semiconductor superlattices subjected to a perpendicular magnetic field. We use a simple electromagnetic theory with a two-dimensional Dirac-δ function for the charge density profile. The magnetoplasmon dispersion relations are derived for an infinite and semi-infinite superlattices, both type II (InAs-GaSb systems) and type I (GaAs-AlxGa1−xAs systems). We emphasize on the magnetoplasma polaritons of semi-infinite superlattices. A detailed analytical diagnosis has been made of the dispersion relations to study the various limiting situations. The surface charge density excitations in the type II superlattices display a broken degeneracy in the retarded limit due to the presence of an applied magnetic field. This interestingly happens with the surface plasmon branch lying within the gap between two bulk bands. Moreover, the Q (Bloch wave vector)=0 edge of the bulk bands exhibits the splitting of the dispersion curve at a frequency (ω) greater than the cyclotron frequency (ωc). Application of a magnetic field results, in general, in pushing the whole spectrum lying above ω=ωc. The numerical results, including the retardation effects, both for zero and nonzero magnetic fields, have been presented for several illustrative cases.
Electromigration-induced compressive stresses in encapsulated thin-film conductors with and without the presence of drift velocity
Authors: Hemmert R. S. , Shatzkes M.
Published: July 2002

Electromigration-induced compressive stresses in encapsulated thin-film conductors were studied by using NPN transistors as pressure sensors. If the current exits at the midpoint of a conductor through an ideal blocking boundary, an emitter of an NPN transistor, then the compressive-stress gradient with and without the presence of the drift velocity can be studied. Based on the observed compressive-stress gradients, estimates for the diffusion constant, D=8.0×10−13 cm2/s, and the drift velocity, V=1.4×10−11 cm/s, were made for a temperature-bias condition of 1470 kA/cm2 and 180 °C. The time-dependent buildup of the concentration for different boundary conditions was consistent among the experiments and the models presented.
A macroscopic time dependent electrical conduction model for thin phosphosilicate glass films
Authors: Popa O. , Cobianu C.
Published: July 2002

Starting from the general laws for the macroscopic electric field and charge conservation in a solid material and taking into account the permanent polarization and heat transfer phenomena in chemically vapor deposited phosphosilicate glass thin films, we propose a macroscopic model of the current-time dependence through such layers. This model predicts (i) exponential variations of the current decay and film temperature as a function of time and (ii) the actual time when the steady state current density is reached after the application of a certain dc electric field. Based on the specific chemical states of phosphorus incorporated in the host SiO2 network and the structural changes thus obtained in phosphosilicate glass (permanent polarization, viscous flow, P-related trapping centers), we propose a physical model of transient-time current in terms of permanent dipoles (one dipole associated with one P4O10 incorporation complex) that rotate more easily in the network as the P content or the applied electric field increases. This mathematical model agrees with our experimental results. The model allows the computation of a dipole concentration related constant and of a thermal dissipation coefficient.
Effects of thin, semi-rigid coatings on the adhesion-induced deformations between rigid particles and soft substrates
Authors: Rimai D. S. , DeMejo L. P. , Vreeland W. B. , Bowen R. C. , Gaboury S. R. , Urban M. W.
Published: July 2002

Glass particles, having nominal radii of approximately 20 μm, were deposited onto polyurethane substrates, which had been overcoated with a 5-μm-thick thermoplastic layer, and the surface-force-induced contact radii were measured using scanning electron microscopy. It was found that the size of the deformations depends only on the modulus of the coating and not on the modulus of the underlying substrate. The measured contact radii were found to scale with the Young’s modulus of the substrate for the uncoated polyurethane substrates according to the predictions of the JKR model [K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Soc. London Ser. A 324, 301 (1971)]. However, the contact radii on the overcoated substrates appear to be more accurately predicted by the DMT theory [B. V. Derjaguin, V. M. Muller, and Yu P. Toporov, J. Colloid Interface Sci. 53, 314 (1975)].
Nonrandom gold-YBa2Cu3O7−x composites
Authors: Veretnik D. , Reich S.
Published: July 2002

In this article we present a new method for the preparation of nonrandom Au-YBa2Cu3O7−x (YBCO) composites. We describe the structural and the morphological properties of this novel system. Gold introduction into this system causes an increase in the c axis value of the unit cell upon sintering at 950 °C. A systematic increase of the zero resistance transition temperature, up to 96 K as measured by a dc transport method, is observed upon the introduction of gold into the YBCO lattice. Introduction of gold above ≊3% by weight causes weakening of the intergranular links of the composite material.
Pulsed laser deposition of SiC films on fused silica and sapphire substrates
Authors: Rimai L. , Ager R. , Hangas J. , Logothetis E. M. , Abu-Ageel Nayef , Aslam M.
Published: July 2002

350-nm-wavelength laser ablation of ceramic SiC targets has been used to deposit SiC films on fused silica and R-cut sapphire at substrate temperatures from 300 to 1150 °C. The films deposited above 800 °C show (111) and (222) x-ray-diffraction bands from crystal planes parallel to the substrate. The bandwidths decrease and the integrated intensities increase with deposition temperature. The crystallite dimension for the highest-temperature films is in the order of 50 nm. The diffraction peaks are absent for the films deposited at the lower temperatures. Analysis of optical transmission spectra of the high-temperature films shows a lowest-energy gap near 2.2 eV which is the value for cubic SiC. The low-temperature films show smaller and variable gaps. The room-temperature resistivities of the former are low, from 0.02 to 0.1 Ω cm whereas the latter are insulating. Film thicknesses and deposition rates ranging from 0.2 to over 0.6 Å/pulse are obtained from the spectra and by monitoring of the interference oscillations in the infrared emission through the film during deposition.
Magnetic structure of the solid solution UNiCuSi2: Neutron diffraction versus ac-susceptibility measurements
Authors: Kuznietz Moshe , André Gilles , Bourée Françoise , Pinto Haim , Ettedgui Hanania , Melamud Mordechai
Published: July 2002

Polycrystalline samples of the solid solution UNiCuSi2 have ThCr2Si2-type crystallographic structure. ac susceptibility indicates an antiferromagnetic (AF) transition at TN=150±5 K, followed by four transitions with ferri/ferromagnetic (F) character at 110±5, 90±5, 70±5, and 45±5 K. Neutron diffraction shows that apart from the AF transition at TN=150±1 K, in accordance with the ac susceptibility, none of the other F transitions is observed. The magnetic structure, down to 1.7 K, is AF-I, with uranium moments (2.25±0.10μB at 1.7 K) along the tetragonal axis in ferromagnetic planes, stacked antiferromagnetically (+−+−) along the tetragonal axis, with a wave vector k=(0,0,1). A possible explanation for the difference between the ac susceptibility and neutron-diffraction results is discussed.
Biquadratic exchange coupling in Fe/Cr/Fe(100) and Fe/Ag/Fe(100) thin film sandwiches (abstract)
Authors: Unguris J. , Celotta R. J. , Pierce D. T. , Stroscio J. A.
Published: July 2002

We have investigated the magnetic exchange coupling between an Fe film and an Fe(100) single crystal whisker substrate separated by varying thickness Cr or Ag spacer layers. The magnetization of the Fe was measured using scanning electron microscopy with polarization analysis. Reflection high-energy electron diffraction and scanning tunneling microscopy (STM) were used to analyze the film structure. In addition to the oscillatory thickness-dependent bilinear exchange coupling, biquadratic exchange coupling was observed in both films. At spacer thicknesses that correspond to the transitions between ferromagnetic and antiferromagnetic coupling the Fe film magnetization was in-plane and orthogonal to the ferromagnetic alignment. Our observations generally agree with the model proposed by Slonczewski1 in which the biquadratic coupling originates from the bilinear coupling through spatial fluctuations of the spacer thickness. If these fluctuations occur over small enough length scales and the bilinear coupling varies between ferromagnetic and antiferromagnetic with monolayer thickness changes, then the Fe film intralayer exchange stiffness forces biquadratic coupling of the film to the substrate. We have observed the necessary short-period coupling in both Fe/Cr/Fe and Fe/Ag/Fe and we have measured the length scale of the thickness variations on a similar sample using a STM. One unexpected result in Fe/Ag/Fe was that the amount of biquadratic coupling increased with increasing top Fe layer thickness. No similar dependence was observed for Fe/Cr/Fe.
Oscillatory exchange coupling in MBE-grown Fe(110)/Ag(111) multilayers (abstract)
Authors: Storm D. F. , Keavney D. J. , Wieczorek M. D. , Freeland J. W. , Walker J. C.
Published: July 2002

The nature of the interlayer coupling in epitaxial Fe(110)/Ag(111) multilayer structures was investigated using a highly surface-sensitive Mössbauer spectroscopy technique. The films were grown by molecular beam epitaxy and analyzed with x-ray diffraction and in situ RHEED to verify their crystallinity and orientation. All the films took the general form [56Fe3057Fe2Agx]15, where x=3 to 22 monolayers (ML). The entire film is invisible to the Mössbauer effect except for the 2 ML 57Fe layers, which therefore act as a probe of the magnetic environment of the Fe surfaces. Information regarding the surface spin-wave spectrum can then be obtained by measuring the temperature dependence of the hyperfine field at the surface. In the limit of no Ag interlayer, the 57Fe probe layers would be in direct contact with 56Fe on both sides and therefore should display a (1−BT3/2) hyperfine field temperature dependence, with B=5.2×10−6 K−3/2. As the Ag interlayer thickness is increased, we expect the hyperfine field to follow a (1−kBT3/2) temperature dependence, with 1<k<3.5 depending on the strength of the interlayer exchange. In addition, we expect an enhancement in H(0), the hyperfine field at 0 K, as the surface exchange decreases. The RHEED and x-ray studies showed all the samples to be single-crystalline and well-oriented, with no Bragg reflections other than Fe(110) and Ag(111) present. As the Ag interlayer thickness increased, the films showed the expected softening of the spin-wave spectrum, with k increasing from 1 at x=0 to 2.2 at x=22. However, a clear oscillation in k was observed at x=6 ML, and a possible second oscillation visible at x=12 ML. This implies an oscillation superimposed upon the monotonically decreasing interlayer exchange. The value of H(0) also shows a similar oscillation at x=6 ML.
Frequency dependence of magnetization in thick film heads for VCR
Authors: Fujita J. , Tanabe S.
Published: July 2002

The magnetization in a VCR thick film magnetic head with 10-μm-thick poles was measured by a scanning Kerr effect microscope (SKEM) and calculated by a three-dimensional finite element method (FEM) including a time dependent term. The SKEM measurements were achieved at the rubbing surface of the heads and it was observed that the vertical magnetization component to the surface has peaks at both side edges of the poles and the peak height ratio to height at the pole center becomes larger as the driving frequency increases. These measurement results were in good agreement with the FEM calculations. The comparison with the measurements and the calculations for a ferrite head shows that the magnetization peaks are attributed to both the eddy currents and the geometrical effects.
Influence of ion beam mixing on the structure and magnetic properties of multilayered Co–B thin films
Authors: Choe G. , Walser R. M.
Published: July 2002

Multilayered Co–B films with various bilayer periods (BP=TCo+TB, TCo=4×TB) and ratios of bilayer thickness were ion beam mixed with 150 keV 1016 Ar ions/cm2. The morphologies of both the as-deposited and the ion beam mixed films changed with BP. The ion beam mixed films exhibited amorphous structures, and the morphologies differed as a function of layer thickness in the as-deposited state. The morphological differences appeared to influence magnetic properties such as in-plane anisotropy and coercivity. Magnetization measurements revealed that the compositional inhomogeneities resulted in the anomalous extension of ferromagnetic composition range compared to melt-quenched films.
Optical and magnetic studies of NdFeB films
Authors: Yao Y. D. , Wu K. T. , Chen Y. Y. , Hsu D. N.
Published: July 2002

The optical, electrical, and magnetic properties of magnetic Nd15Fe77B8 and nonmagnetic Nd5Fe62B33 films have been investigated. It has been found that within the visible (wavelengths 4400–6400 Å) the films are transparent with thickness less than 800 Å for magnetic films and 1500 Å for nonmagnetic films. This is due to the difference of the magnetic permeability μ between films. The oscillatory behavior with thickness of the films is observed in the film samples. The anomalous behaviors near the Curie temperature for both bulk- and film-type samples are similar to that of the type-III ferromagnets. The temperature at which the electrical resistance of the film-type samples increases abruptly varies from 620 K for 1000 Å to 650 K for 2000 Å.
Measurement of the switching properties of a regular 2D array of Preisach-type particles (abstract)
Authors: Pardavi-Horvath M. , Vertesy G.
Published: July 2002

The original Preisach model of magnetic hysteresis assumes an assembly of single domain particles, each having a rectangular hysteresis loop. The loops are characterized by up and down switching fields. The halfwidth of the loop is the coercivity Hc of the given particle. The loops might be shifted depending on the magnetic state of the environment, described by an effective interaction field, Hi. In the present work the switching properties of an artificially structured, highly uniaxial, magneto-optic garnet film (courtesy of R. Belt, AIRTRON), corresponding to the assumptions of the original Preisach model, are investigated. In this case the parameter identification is straightforward. The garnet film is etched into 42 μm rectangular pixels, separated by 15 μm grooves. Each pixel has a rectangular hysteresis loop. Up and down magnetized pixels give a black and white contrast due to Faraday effect. Magnetization can be measured by counting the number of pixels switched in a given field. The distribution of the coercive field has been obtained by measuring individual loops on each of several hundred pixels in an optical magnetometer. The average Hc=265±99 Oe, Hi=0±26 Oe. The interaction field has been measured on the same pixel, depending on the magnetization state of the nearest neighbor pixels. The contribution to Hi of oppositely magnetized pixels is dHi/dn=26 Oe/pixel. These data are compared to macroscopic hysteresis loops measured in VSM.
Flux-flop in high temperature superconducting crystals with columnar defects
Authors: Klein L. , Yacoby E. R. , Wolfus Y. , Yeshurun Y. , Burlachkov L. , Ya Shapiro B. , Konczykowski M. , Holtzberg F.
Published: July 2002

Measurements of magnetization curves of Y-Ba-Cu-O crystals with columnar defects reveal pronounced peaks around H=0 for applied fields not parallel to the defect direction. The magnitude and direction of the remanent magnetization are independent of the direction of the applied field for a large range of angles. We show that the unidirectional character of the defects induces, in the low field limit, a flux-flop from a direction determined by the field to the defect direction. Implications to planar pinning are discussed.
Remanent magnetization in single crystals of YBa2Cu3O7
Authors: Moloni Katerina , Dahlberg E. Dan
Published: July 2002

Measurements of the isothermal remanent magnetization Mr and the dc demagnetization remanence Md have been made on single crystals of YBa2Cu3O7 with the applied magnetic field parallel to the c axis. Plots of Md vs Mr indicate the presence of interactions when the usual interpretation developed for ferromagnetic systems is applied. However, this interpretation neglects the intrinsic response of a superconductor to expel the flux with supercurrents. It is found that the plots, although suggesting repulsive interactions, are qualitatively explained by the Bean critical state model. In addition the use of the Md vs Mr plots in connection with the Bean model provides an alternative technique to determine critical currents.
Probe the vortex state of YBa2Cu3O7−δ under small magnetic field using short time magnetic moment relaxation method (abstract)
Authors: Gao L. , Xue Y. Y. , Hor P. , Chu C. W.
Published: July 2002

At the present time, there is a large amount of convergent experimental evidence pointing toward the existence of a vortex-glass state in the high temperature superconductor YBa2Cu3O7−δ in the presence of a high magnetic field, as predicted by Fisher1 and Fisher et al.2 However, the situation at low magnetic field is not so clear. The nature of the vortex state separating the Meissener state at low field and the vortex-glass state at very high field remains unsolved both theoretically and experimentally. To shed some light to this problem, we have measured the remnant magnetic moment relaxation of melt-textured bulk YBa2Cu3O7−δ samples in various magnetic fields from ∼10−4 to ∼300 s. Even at zero field, the time dependence of the relaxation follows the vortex-glass model prediction m(t)∝1/[ln(t)]1/μ but with a μ=1.4±0.2, contradicting the vortex-glass model. As the field is increased, the μ value decreases until it reaches a value of ∼0.2–0.3 above 1 T. The larger than one value of μ seems to point to a collective pinning behavior at low magnetic field.
Evidence for a magnetic hard gap in the density of states of a magnetic semiconductor near the metal insulator transition (abstract)
Authors: Penney T. , Terry I. , von Molnár S. , Becla P.
Published: July 2002

A family of resistivity curves with carrier concentrations approaching the metal insulator transition is obtained in a single sample of the dilute magnetic persistent photoconductor Cd0.91Mn0.09Te:In. In zero applied magnetic field, these curves exhibit a cross-over from an exp[(T0/T)1/2] for variable range hopping with interactions to an exp(EH/T) form upon reducing temperature. The energy EH is associated with a hard gap in the density of states which is magnetic in origin. All of the data for different carrier concentrations are shown to scale on to a single curve.1 The localization length is found to have the same critical dependence as the electronic part of the dielectric constant which has been previously measured.2 In a magnetic field, the hard gap is suppressed, and only variable range hopping is observed. The orientation of the Mn spins by the carriers contributes both to the localization (bound magnetic polaron) and is the relaxation responsible for the hard gap. A large peak in the magnetoresistance is also understood within this model. The localization length ξ and hence T0=2.8e2/κξk and the resistivity are field dependent. The magnetic field first aids the BMP formation and then destroys it. The result is first a peak and then in some cases negative magnetoresistance.
Magnetic properties of (GdxY1−x)2Co7B3 compounds
Authors: Ballou R. , Burzo E. , Pop V. , Pentek A.
Published: July 2002

The (GdxY1−x)2Co7B3 compounds crystallize in a hexagonal structure having the P6/mmm space group. Magnetic measurements performed in the temperature range 4–800 K show that Y2Co7B3 is a ferromagnet, while the compounds with x≥0.2 are ferrimagnetically ordered. The gadolinium and cobalt magnetization compensate at a composition x≂0.26. The mean cobalt moment is strongly dependent on the composition, increasing from 0.29μB/atom (x=0) up to 0.82μB/atom (x=1.0). Above the Curie points the magnetic susceptibilities follow a Curie–Weiss law for Y2Co7B3 and hyperbolic-type dependencies for the ferrimagnetic compounds. The effective cobalt moments are only a little composition dependent. The composition dependence of the mean cobalt moments is analyzed in correlation with the exchange interactions.
Spin dynamics in the diluted three-dimensional classical Heisenberg antiferromagnet
Authors: Chen Kun , Landau D. P.
Published: July 2002

Using spin dynamics simulations we have studied magnetic excitations in the randomly site-diluted, body-centered cubic, classical Heisenberg antiferromagnet at a low temperature. With site configurations and initial spin configurations generated by Monte Carlo simulations, equations of motion for spins in 40×40×40 systems with periodic boundary conditions and only nearest-neighbor interactions were integrated using a fourth-order predictor-corrector method. Neutron-scattering functions were then determined from the space-time Fourier transform of averaged spin-spin correlation functions convoluted with a resolution function. We found that at occupation fraction of magnetic ion p=1 only spin-wave excitations exist, while at p=0.245 (percolation threshold) only fracton excitations are present. In the intermediate region spin-wave excitations at long length scales cross over to fracton excitations at short length scales.
Calorimetric studies of the multicritical behavior of the spin-density wave induced by the magnetic field in (TMTSF)2ClO4
Authors: Tsobnang F. , Pesty F. , Garoche P. , Héritier M.
Published: July 2002

Specific heat and magnetocaloric investigations have been performed at low temperature in the molecular conductor (TMTSF)2ClO4. We have studied the effect of anion disorder on the spin-density wave (SDW) induced by the magnetic field. This communication is focused on the tetracritical behavior that we observe experimentally along the phase transition line separating the normal metal from the quantized SDWs. It is found that this behavior evolves from a bicritical to a tetracritical behavior (following Griffiths’ terminology), as the amount of disorder is decreased. This change corresponds to the appearance of a treelike transition line in the diagram. We suggest that the mean free path plays a key role in the electronic phase transitions, contrary to what is expected for a conventional density wave.
Spin dynamics in a diluted Heisenberg NN antiferromagnet on a HCP lattice: Zn1−xMnxSe (abstract)
Authors: Giebultowicz T. M. , Fernandez-Baca J. A. , Nicklow R. M. , Furdyna J. K. , Debska U.
Published: July 2002

The nature of magnetic excitations in disordered and short-range ordered antiferromagnetic systems continues to be an interesting problem. The family of AII1−xMnxBVI alloys (AII=Zn, Cd, Hg, BVI=S, Se, Te) offers highly interesting prototypes for such studies. In these systems, which crystallize in the zinc blende (ZB) or wurtzite structure, the Mn ions form, respectively, diluted FCC or HCP Heisenberg spin lattices with dominating nearest-neighbor AF coupling. Due to the strong frustration inherent in such lattices these systems do not exhibit long-range spin order even for Mn concentrations as high as x=0.70. Neutron scattering studies of spin dynamics in the zinc-blende forms (Zn1−xMnxTe and Cd1−xMnxTe) have been reported in a recent article.1 In the present paper we report the results of experiments on a wurtzite-type system, Zn1−xMnxSe, with x=0.55 and 0.40. As in the previous case of ZB alloys, the data reveal the existence of magnon-like modes that are extremely strongly damped, but still show a pronounced dispersive behavior. The range of static AF spin–spin correlations in Zn1−xMnxSe is extremely short (ξ=κ−1=5–15 Å), and exhibits a significant anisotropy. The energy of the observed excitation is clearly correlated with ξ, i.e., it is higher in the Q-space directions for which the correlation range is longer. The results are discussed in the context of the existing theoretical models.
Quantum size effect and the giant magnetoresistance in magnetic multilayers
Authors: Ehrlich A. C. , Gillespie D. J.
Published: July 2002

A theoretical model which accounts for the variations of the giant magnetoresistance, Δρ/ρ, with electron mean free path, L, interface roughness, r, and magnetic, tM, and normal layer, tN, thickness has been previously presented for sandwich films. It employs the quantum-size-effect theory of resistivity in thin films and relies on spin dependent transmission or reflection at individual layer boundaries to establish the metallic quantum-well states. This model has now been extended (i) to films where L can be different in the magnetic, LM, and nonmagnetic, LN, layers, (ii) to an electron/atom number, n, of 0.131 as well as 1.047, and (iii) to films from sandwiches to those with as many as 64 (63) magnetic (nonmagnetic) layers. The focus is on films with relatively thin tM and tN where quantum, as distinguished from semiclassical, effects should dominate. Typical results can be summarized as follows: for tM∼tN∼10 monolayers, ML, and L’s∼70 ML, Δρ/ρ increases more rapidly as LM than LN, but this effect is reduced as one goes from sandwiches to superlattices; Δρ/ρ is approximately 50% in a superlattice with r=5 ML, which is an order of magnitude larger than in a superlattice with r=1 ML. In a sandwich the difference between r=5 and r=1 is closer to a factor of 5. In the sandwiches Δρ/ρ is smaller for n=0.131 than for n=1.05.
Structure and recording characteristics for CoPtB(O) rigid disk media
Authors: Okabe A. , Yamamoto T. , Kagawa K. , Hayashi K. , Aso K. , Onodera S.
Published: July 2002

Structure, magnetic properties and recording characteristics of the CoPtB(O) media with various underlayers were investigated. Recording performance of the CoPtB(O) perpendicular media is closely related to the crystallinity. Magnetic properties and recording characteristics are much improved by the Au seed layer, and higher reproduced output can be obtained at 200 kFRPI. CoPtB(O) perpendicular media show higher resolution than the CoPtB(O)/Cr longitudinal one. The reproduced output of perpendicular media at 200 kFRPI is more than 10 dB higher than that of the longitudinal one. Noise characteristics of both media are quite different in the recording density response. With increasing recording density, the media noise of longitudinal media slightly increases, while that of the perpendicular one gradually decreases. This means for the perpendicular media that the media noise might originate not from the magnetic transition but from the irregular magnetic reversals within recorded bits.
S/N ratio of thin-film media estimated by computer simulation (abstract)
Authors: Yoshida Kazuetsu , Hara Miki , Uesaka Yasutaro , Nakatani Yoshinobu , Hayashi Nobuo
Published: July 2002

This paper derives a formula that shows a direct relationship between the noise spectrum and the magnetization configuration of a thin-film medium. This formula was obtained by Fourier transforming the down-track autocorrelation function of jMx-Mz, where Mx and Mz are the longitudinal and vertical components of magnetization. The formula takes into account not only the variance of the magnetization deviation, which is generally used to estimate media noise, but also the correlation between deviations in magnetization. This lets us calculate the noise power spectrum as well as the signal power, once the magnetization configuration in a medium is obtained, which means that the magnetic microstructures such as vortex, zig-zag domain, etc., are automatically included in noise calculation. To investigate the relationships between the noise properties and the magnetic properties of media with various anisotropy fields (Hk), axis orientation of easy magnetization, and stiffness constants (A), recorded magnetization configurations were simulated using the Landau–Lifshitz–Gilbert equation.1,2 The easy axis was oriented from uniaxial direction to random direction in plane. The simulated noise power spectra were maximized at a relatively long wavelength, which was varied by changing Hk, or the easy axis orientation. The noise power was strongly influenced by Hk and the axis orientation of magnetization. This confirms that preferential orientation along the longitudinal direction and a large Hk are effective for improving the S/N ratio. On the other hand, the stiffness constant less improved media noise than Hk and axis orientation, suggesting that the preferential orientation of the easy axis is more effective to reduce media noise than isolating magnetic grains.1
Observation of giant magnetoresistance in Ag-Ni-Fe alloy films
Authors: Watson M. L. , Barnard J. A. , Hossain S. , Parker M. R.
Published: July 2002

We report the observation of a large magnetoresistance in as-deposited co-sputtered Ag-Ni-Fe films. The size of the magnetoresistance has been measured as a function of the film composition. Measurement of the samples’ hysteresis indicates that smaller Ag concentrations result in regions of the films becoming ferromagnetically coupled while greater concentrations result in the formation of a superparamagnetic state. Crystallographic evidence obtained using x-ray diffraction shows the samples to have the crystal structure of a contracted Ag lattice possibly caused by the inclusion of Ni and Fe.
Magnetothermopower of Co/Cu multilayers
Authors: Shi Jing , Yu R. C. , Parkin S. S. P. , Salamon M. B.
Published: July 2002

We have studied the magnetothermopower S(H,T) for a series of Co/Cu multilayers with a constant Co layer thickness of 10 Å but with Cu layer thickness varying between 9 and 21 Å. The thermopower is negative, and its magnitude increases with magnetic field. We also found the magnetothermopower to be linear in the conductance for all samples. In two samples studied, the linear relationship holds from room temperature down to 50 K, with the proportionality constant linear in temperature. The zero-field thermopower is roughly the same for all samples despite different Cu layer thickness. This behavior can be understood from the spin-dependent interface scattering model.
Magnetic ordering in the A3Fe2X9 salts (abstract)
Authors: Carlin Richard L. , Shaviv Roey
Published: July 2002

The specific heat of several salts in this series of canted antiferromagnets, where A is 4-halopyridinium and X is chloride, bromide or a mixture of the two, is reported over the temperature interval 1.6–40 K.1 All contain the monomeric tetrahedral [FeX4]− ions. The substances include 2[4-Br(py)H] [FeCl4]⋅[4-Br(py)H]Cl (Tc=2.28 K), [4-Cl(py)H]3Fe2Cl9 (2.685 K), [4-Cl(py)H]1.1[4-Br(py)H]1.9Fe2Br9 (7.52 and 7.80 K) and (pyH)3Fe2Cl9 (2.12 K). Odd behavior is observed with several compounds such as [4-Br(py)H]3Fe2Br9, which exhibits two λ features in the specific heat (at 5.23 and 7.635 K) but only one ordering temperature in the ac susceptibility (at 7.70 K). The ordering phenomena are largely three dimensional in nature. The magnetic properties will be correlated with the structures and the likely superexchange paths.
Determination of the interlayer superexchange in the two-dimensional ferromagnet Rb2CrCl4−xBrx by means of quasielastic neutron scattering (abstract)
Authors: Visser D. , Bramwell S. T.
Published: July 2002

The 2D ferromagnets Rb2CrCl4−xBrx; x=0, 1, 2 were studied by means of quasielastic neutron scattering. The compounds order between Tc=52.2 and 57.0 K. Rods of quasielastic diffuse scattering can be observed along the [001] direction. The wave vector dependent modulation of the width and the intensity of the diffuse scattering can be explained as resulting from the effect of the weak interplanar exchange coupling J′. By means of a mean field model, we describe the wave vector dependent susceptibility as function of the scattering vector components with respect to the reciprocal lattice vector. From the data around Tc, we derive that the ratio of the inter/intralayer superexchange changes from J′/J=2.3×10−4 to 10.2×10−4 upon increasing value of x.