bluuebunny/crossref_metadata_2025 huggingface dataset:
Features of the electromagnetic absorption in high-J
c melt-textured samples
Authors: Fisher L. M. , Voloshin I. F. , Makarov N. M. , Yampol’skii V. A. , López-Cruz E. , Pérez-Rodríguez F.
Published: July 2002

The surface resistance ℛ of melt-processing samples is investigated as a function of the radio wave amplitude ℋ in the low frequency range. We found an unusual dependence ℛ(ℋ) for samples with high critical current density. The rate of change of ℛ(ℋ) has a minimum at amplitudes ℋ around the lower critical magnetic field Hc. This phenomenon is explained within the proposed theoretical model which takes into account the critical intercrystallite currents as well as the intracrystallite ones. The nature of the effect is connected with the increase of the crystallite screening capability which occurs at the transition of the crystallites to the Bean critical state.
Simulation of electromigration based on resistor networks
Authors: Patrinos A. J. , Schwarz J. A.
Published: July 2002

A two-dimensional computer simulation of electromigration based on resistor networks is presented. The model utilizes a realistic grain structure generated by the Monte Carlo method and takes specific account of the local effects through which electromigration damage progresses. The dynamic evolution of the simulated thin film is governed by the local current and temperature distributions. The current distribution is calculated by superimposing a two-dimensional electrical network on the lattice whose nodes correspond to the particles in the lattice and the branches to interparticle bonds. The current distribution problem is solved by applying Kirchoff’s rules on the resulting electrical network. The calculation of the temperature distribution in the lattice proceeds by discretizing the partial differential equation for heat conduction, with appropriate material parameters chosen for the lattice and its defects. The simulation was tested by applying it to common situations arising in experiments with real films. Specifically, the model successfully reproduces the expected grain size, linewidth and bamboo effects, the log-normal failure time distribution, and the relationship between current-density exponent and current density.
Calculation of coherent effects in x-ray transition radiation produced by relativistic electrons
Authors: Neighbours John R. , Buskirk Fred R. , Hartmann H.-J.
Published: July 2002

Several effects in coherent transition radiation which may be understood analytically and which influence the character of the x-ray spectrum resulting from a stack of foils are discussed. In particular, coherence effects may produce a radiation pattern narrow in angle, but the possibility to make a narrow frequency spectrum is severely limited.
Single-frequency antiguided laser array with buried distributed feedback grating
Authors: Mawst L. J. , Tu C. , Zmudzinski C. , Botez D. , Martin R. , Mazed M.
Published: July 2002

Single-frequency, in-phase mode operation has been obtained from a ten-element antiguided phase-locked array with Talbot-type filters by incorporating a buried distributed feedback grating using a three-step metalorganic chemical vapor deposition process. Stabilized frequency with 25 dB side-mode suppression ratio is achieved to 50 mW pulsed output power. In-phase or a mixture of in-phase and out-of-phase modes are observed in nonresonant (nonoptimized) devices. Means of improving device performance (e.g., use of resonant longitudinally uniform arrays or nearly resonant Talbot-filter arrays without gratings in the filter) are discussed.
Effects of doping in Kondo insulators (invited)
Authors: Schlottmann P.
Published: July 2002

Kondo insulators like Ce3Bi4Pt3 and CeNiSn are compounds with small-gap semiconductor properties. Nonmagnetic impurities, so-called Kondo holes, break the translational invariance and hence the coherence of the ground state. Impurity states can be introduced by (i) substituting the rare earth (actinide) ion or (ii) by replacing (or adding, removing) one of the ligand atoms. Isolated impurities usually give rise to bound states in the gap. Depending on the nature of the impurity (charge neutral or a dopand) the Fermi level is pinned by the impurity level or lies in the gap. In the former case the Kondo hole has magnetic properties (Curie susceptibility and Schottky anomaly in the specific heat), while in the latter situation the properties are nonmagnetic. For a finite concentration of Kondo holes the situations (i) and (ii) are qualitatively different. In (i) it gives rise to an impurity band inside the gap of the semiconductor. The height and width of the impurity band in the f-electron density of states are proportional to c1/2 for small concentrations. If the impurities are charge neutral the Fermi level lies in the impurity band giving rise to a specific heat proportional to T and a Pauli-like susceptibility. If the impurities dope the bands the properties remain semiconducting with a strongly reduced gap. For ligand impurities [case (ii)] tails of impurity states develop close to the gap edges, suppressing in this way the gap. The system remains a semiconductor if the impurities are charge neutral, but C∝T and χ is finite if they are dopands.
Recent developments in multi-channel Kondo physics (invited) (abstract)
Authors: Ingersent Kevin , Jones Barbara A.
Published: July 2002

The two-channel Kondo model for a single spin-one-half magnetic impurity coupled to two degenerate bands of conduction electrons predicts the existence of a zero-temperature quantum critical point, leading to ‘‘marginal-Fermi-liquid’’ behavior at low temperatures. A two-channel Kondo description has been proposed1 for several U and Ce-based heavy-fermion materials whose thermodynamic and transport properties exhibit anomalous temperature and magnetic field dependence.2 This talk will review recent theoretical work on multi-channel Kondo behavior. Two factors may be particularly important in determining the relevance of the multi-channel Kondo effect for real systems: (1) Channel asymmetry: Any difference between the coupling of the two conduction bands to the impurities leads to the recovery of Fermi-liquid behavior at sufficiently low temperatures.3 (2) Inter-impurity interactions: Numerical renormalization-group calculations for a two-impurity model show that any nonzero Ruderman–Kittel–Kasuya–Yoshida (RKKY) coupling between the impurities destabilizes the single-impurity critical point.4 Depending on the strength and sign of the RKKY coupling, the system may be driven either to a Fermi-liquid or to a novel regime of non-Fermi-liquid behavior governed by nonuniversal critical exponents. The experimental implications of these findings will be discussed.
Equivalent structures of permanent magnets and electric currents designed to generate uniform fields
Authors: Abele M. G.
Published: July 2002

Generation and confinement of a magnetic field by means of permanent magnets as well as time-independent electric currents are analyzed, and the conditions of existence of uniform field solutions are established in the general case of structures that enclose a region of arbitrary geometry. Basic properties of distributions of the electric currents are compared with the properties of yokeless structures of permanent magnets intended to generate the same uniform field within the same region.
Susceptibility of current-carrying iron whiskers
Authors: Lee J. -G. , Arrott A. S.
Published: July 2002

The magnetization pattern of an iron whisker can be altered by passing a dc current along its axis. Changes in the resistance and the ac susceptibility are used to deduce the domain structures and how they change with dc currents and/or dc magnetic fields, applied along the axis. The magnetization curls around a central core that is magnetized along the whisker axis.
The effects of demagnetizing and stray fields on magnetoacoustic emission
Authors: Ng D. H. L. , Lo C. C. H. , Jakubovics J. P.
Published: July 2002

Magnetoacoustic emission (MAE) has been measured from a Ni bar and a mild steel bar. The magnetizing field was produced by a c-core electromagnet, which was separated from the specimen surface by a nonmagnetic spacer layer (thickness la). In Ni, the MAE profile contains a single peak when la is small. As la increases, a second peak appears and then a third peak, indicating that MAE is not only caused by domain wall creation and annihilation, but also by domain wall motion. In the case of the mild steel bar, only two peaks are detected, which are increasingly separated as la increases. The different results between the two materials can be explained by the different mobility of non-180° domain walls. The present results also indicate that the MAE profile obtained from Ni is less sensitive to la than that obtained from mild steel. This difference can be explained by considering the reluctance of the magnetic circuit.
IR spectroscopy on FeNi and FePt Invar alloys
Authors: Buchholz B. , Wassermann E. F. , Pepperhoff W. , Acet M.
Published: July 2002

UHV IR emission spectroscopy has been carried out at energies 9–33 mRy (wavelength 2.8–9.8 μm) in the temperature range 430–800 K on single crystals of FeNi and FePt Invar, with Curie temperatures in the respective range. The results show a drastic (up to 30%) decrease of the IR absorptivity around Tc in the energy range 23–33 mRy (2.8–4 μm), while at small energies around 9 mRy (10 μm) the absorptivity is temperature independent, as expected for 3d metals, and found on pure Ni. The decrease in absorptivity on the Invar alloys cannot be understood by the common temperature dependence of interband transitions; rather, it calls for the existence of moment-volume instabilities, characterized by transitions between a high-spin ground state and a temperature-induced low-spin state. Qualitative agreement of the absorptivity with respective total density-of-states calculations on Fe3Ni is demonstrated.
Phase transitions in the one-dimensional frustrated quantum XY model and Josephson-junction ladders
Authors: Granato Enzo
Published: July 2002

A one-dimensional quantum version of the frustrated XY (planar rotor) model is considered which can be physically realized as a ladder of Josephson junctions at half a flux quantum per plaquette. This system undergoes a superconductor to insulator transition at zero temperature as a function of charging energy. The critical behavior is studied using a Monte Carlo transfer matrix applied to the path-integral representation of the model and a finite-size-scaling analysis of data on small system sizes. Depending on the ratio between the interchain and intrachain couplings the system can have single or double transitions which is consistent with the prediction that its critical behavior should be described by the two-dimensional classical XY-Ising model.
Spectroscopic studies of magnetic transitions in TbPO4
Authors: Liu G. K. , Loong C.-K. , Trouw F. , Abraham M. M. , Boatner L. A.
Published: July 2002

In TbPO4, an antiferromagnetic-phase transition is accompanied by a cooperative Jahn–Teller effect involving a Tb ion-lattice coupling that induces a tetragonal-to-monoclinic distortion of the crystal lattice and an associated readjustment of the Tb electronic states as required by the lowering of the rare-earth-site symmetry. The laser-excitation and emission spectra of TbPO4 and 0.1% Tb-doped YPO4 single crystals were studied at temperatures below and above the phase transitions. The line shapes of transitions in the stoichiometric compound are unusually broad and asymmetric while those observed for the dilute compound are relatively sharp. In addition, the TbPO4 spectra show an anomalous temperature dependence near the phase transition, indicating strong interactions of the rare-earth ions with their local environments. In the present work, a comparison of the optical results with the results of new neutron-scattering measurements is made.
An analysis of magnetization patterns measured using a magnetic force scanning tunneling microscope (abstract)
Authors: Burke E. R. , Gomez R. D. , Mayergoyz I. D.
Published: July 2002

In a previous paper, we made a complete analysis of the interaction between the probe tip of a magnetic force scanning tunneling microscope (MFSTM) and the magnetic fields emanating from a typical recorded pattern. In this paper we show how the magnetization distribution in the recorded media can be determined from the measurements by obtaining expressions for the magnetic fields from a Fourier series expansion for the recorded magnetizations. We have used these techniques to find the magnetic fields from many different distributions, including all those we could find in the literature. The probe tip displacement, which is the quantity measured using the MFSTM, can be calculated using these magnetic fields. The results can then be compared to the experimental data. For one set of experiments on high density recording we have found that the best fit is with a magnetization that has a modified arctan transition. The modification eliminates the discontinuity in the slope of the transitions as they are joined together, giving a more realistic representation of the magnetic distribution. The transition width can then be used as an adjustable parameter to find the best fit to the data. The MFSTM can, therefore, be used as a quantitative tool to find the magnetic recording transition widths. These theoretical techniques are not necessarily restricted to the use of a MFSTM, but can be applied to other problems in magnetic recording. For instance, we show how the probe tip displacement corresponds to the flux picked up by a conventional read head. The response of the head as a function of different magnetization patterns can then be studied and compared to experimental results. The measurable quantities are expressed in Fourier series but we show how these series can be easily evaluated with a PC and the appropriate software.
First-principles calculation of orbital moment distribution in amorphous Fe
Authors: Zhong Xue-Fu , Ching W. Y.
Published: July 2002

A model calculation on a large periodic unit cell of amorphous Fe containing 200 atoms is carried out to determine the distribution of its local orbital moments. A spin-polarized orthogonalized linear combination of atomic orbitals method including spin-orbit coupling is used for the calculation of the electronic structure. It is shown that spin-orbit coupling brings about a small change in the density of states for amorphous Fe. The average orbital moment in amorphous Fe is found to be 0.01 μB, which is much smaller than the value of 0.09 μB for crystalline fcc Fe. The distribution of orbital moments over various sites is rather broad, indicating a strong quenching effect as a result of variation in the local structure in the amorphous case. It is speculated that such ground-state properties are favorable to the formation of the spin glass phase.
Depth distribution of birefringence in magneto-optical recording disk substrates
Authors: Kono Raymond-Noel , Jhon Myung S. , Karis Thomas E.
Published: July 2002

Polycarbonate substrates are the industry standard for optical storage. Optical anisotropy is frozen into the substrate as polymer orientation during mold flow and thermal stress during cooling. The substrate optical anisotropy degrades the readback of the data [carrier-to-noise ratio (CNR)] by increasing the noise level on Kerr rotation measurement. There are also synergistic effects of the disk optical properties with focus and tracking optics. For optimum CNR, the beam is focused on the magneto-optical (MO) film. For optimum tracking, the beam is focused to maximize track error signal (TES). For the disks considered the difference between the offset for optimum CNR and that for optimum TES was between 300 and 600 μm. This results in a compromise between optimum CNR and TES, which could limit MO storage density and performance. Conventional methods of bulk optical anisotropy measurement are unsuccessful in explaining this variance. These give an average or effective optical property. The optical properties are also known to vary with the depth in the substrate and especially near the mold walls as determined by optical polarizing microscope. A scanning micropolarimeter instrument is presented to rapidly measure the depth distribution of the optical properties. The interaction between the obliquely incident polarized beam with the microscopic variation in the index of refraction throughout the depth of the substrate is proposed as the source of the difference between the optimum focus offsets.
Macroscopic ferrimagnets as magneto-optic media (abstract)
Authors: Gambino Richard J. , Ruf Ralph R. , Bojarczuk Nestor
Published: July 2002

We have discovered a class of phase separated materials in which the two phases couple antiferromagnetically across the phase boundary. A prototypical example is cobalt containing ≂100-Å-diam precipitate particles of EuS with the NaCl structure. These materials show many of the properties of ferrimagnets such as compensation points so we have adopted the term ‘‘macroscopic ferrimagnet’’ to distinguish them from ferrimagnets in which the antiferromagnetic exchange couples individual atoms. As a result of the ferrimagnetic exchange between the Co and the EuS the net magnetization of the composite is low in the vicinity of compensation so the anisotropy field is high. Electron beam evaporated thin films have a perpendicular easy axis as a result of growth induced anisotropy. Another effect of the exchange between the Co and EuS is to order the EuS well above its normal Curie point of 16 K. A film with 80 mole % Co and 20 mole % EuS has a compensation point of 55 K showing that the Eu2+ has essentially its full moment of 7μB at this temperature in order to compensate the 1.65μB of Co. The EuS even shows evidence of magnetic order at room temperature. The wavelength dependence of the sign of the magneto-optic Kerr effect can only be explained if the EuS has sufficient rotation at high photon energies to be larger than the rotation of the Co. We have prepared more complex alloys that show square loop properties at room temperature, for example, Tb-Co-EuS and Tb-FeCo-EuO. X-ray diffraction analysis shows that these films consist of an amorphous matrix (Tb-Co or Tb-FeCo) with precipitates of a NaCl structure phase (e.g., defect solid solutions like Eu1−xTbxS1−x⧠x). Both atomic and macroscopic ferrimagnetism are operating in these compositions. Films with high remanence and coercivities over 4 kOe at room temperature have been obtained. Some of these compositions may be useful as magneto-optic storage media.
New complex permeability measurement device for thin magnetic films
Authors: Hayano S. , Marinova I. , Saito Y.
Published: July 2002

Previously, we have proposed a strategic dual image (SDI) method to obtain the finite element solution of open boundary magnetic field problems. Also, we have derived a complex permeability from a Chua-type magnetization model. Combination of this complex permeability with the SDI method is now applied to exploit a new high frequency characteristic measurement device for thin magnetic films. Numerical simulation suggests that our new measurement method yields far superior results compared with those of the conventional one.
Effect of recorded transition shape on spatial noise distributions and correlations
Authors: Arnoldussen T. C. , Zhu J. G.
Published: July 2002

Improved error rate models require better understanding of noise distributions and correlations in magnetization reversals. An analytical model is described for micromagnetic transition noise. Magnetic charge conservation is the basis for the model. The noise distribution is shown to be dependent primarily on the average transition shape. While magnetic charge variance is always peaked at the center of the transition, readback voltage noise may peak at the center for arctangent transitions or may show leading and trailing edge peaks for hyperbolic tangent or error-function transitions. Moreover, noise in the voltage derivative shows yet different distributions. Principal noise eigenfunction modes are determined for the contrasting transition shapes studied. Analytical and full micromagnetic modeling results are compared.
Magnetic Davydov splitting in 2D AFM (CH2)2(NH3)2MnCl4 (abstract)
Authors: Eremenko V. , Kachur I. , Piryatinskaya V. , Shapiro V.
Published: July 2002

Recently it was shown that the light absorption spectrum of (CH2)2(NH3)2MnCl4 (EDAMnCl4) differs strongly from those of other compounds of this family (CH2)n(NH3)2MnCl4. Additional peculiarities in the spectral distributions of two-magnon Raman scattering and the coefficient of exciton-magnon absorption were found. It was also determined that the pure exciton band in the absorption spectrum is of the electric dipole type of polarization. All these peculiarities are out of the framework of the usual tetragonal two-sublattice antiferromagnet model and indicate the inverse center loss and magnetic cell doubling. Results of studying the absorption spectrum of EDAMnCl4 presented in the report show that not less than three Davydov components of the exciton band σA1g(σS)→4Eg(4G) optical transition may be detected with confidence in magnetic fields oriented along the c axis. It confirms the four-sublattice magnetic structure of EDAMnCl4. The presence of exciton line splitting at H=0 and its nonlinear character at H∥α (that is practically parallel to l) allows us to make a conclusion about the resonance interaction between ions with antiparallel spins and, probably, about the initial noncollinearity of magnetic structure. Parameters characterizing the magnetic Davydov splitting are determined using comparison of the experimental data and theoretical analysis.
Spin diffusion in classical Heisenberg magnets with uniform, alternating, and random exchange
Authors: Srivastava Niraj , Liu Jian-Min , Viswanath V. S. , Müller Gerhard
Published: July 2002

We have carried out an extensive simulation study for the spin autocorrelation function at T=∞ of the one-dimensional classical Heisenberg model with four different types of isotropic bilinear nearest-neighbor coupling: uniform exchange, alternating exchange, and two kinds of random exchange. For the long-time tails of all but one case, the simulation data seem incompatible with the simple ∼t−1/2 leading term predicted by spin diffusion phenomenology.
Kinematical pairing and magnetism in layered systems
Authors: Ivanov Valery A. , Zhuravlev Michail Ye. , Z’ubin Pavel B.
Published: July 2002

Hubbard model of two layers of strongly correlated electrons with interlayer hopping is investigated. The temperature Tc of superconducting transition and magnetic suspectibility are calculated. The external pressure dependence of Tc is discussed.
Ground-state properties and excitations of an integrable one-dimensional model with δ-function interaction involving several bands
Authors: Schlottmann P.
Published: July 2002

We consider an integrable model consisting of n parabolic bands of electrons of equal mass with a δ-function exchange interaction. The interaction is attractive or repulsive depending on whether the interacting particles are in a spin-singlet or triplet state. The structure of the Bethe ansatz and the classification of states is similar to that of the n-channel Kondo problem. The attractive interaction leads to Cooper-pair-like boundstates. Some ground-state properties and the excitation spectrum are discussed for the special situation of degenerate bands.
Peculiarities of inelastic neutron scattering on magnons in high-T
c materials of stoichiometrical composition Nd2CuO4, La2CuO4, and YBa2Cu3O6 (abstract)
Authors: Pashkevich Yu. , Larionov M.
Published: July 2002

This paper focuses on the phenomenon of the ‘‘exchange amplification’’ of inelastic neutron scattering as compared to the elastic one around some Bragg peaks on small transferred momenta and energies. The effect is closely connected with increasing of the amplitude of uniform vibrations of some spin linear combinations. Since all the above-mentioned magnetics are multisublattice quasi-two-dimensional, those with an anomalously large intralayer exchange as compared to the interlayer one has the exchange amplification effect for the exchange and acoustic modes for all wave vectors directed perpendicularly to layers right up to the Brillouin zone edge. The same effect takes place in the region of the dispersion law being linear for small (compared with the intralayer exchange) transferred energies. The greater intensity of inelastic magnetic scattering as compared with the elastic one was observed in some experiments. The selection laws defining Bragg peaks around which the inelastic scattering is exchange amplified have been obtained. The peculiarities of the inelastic neutron scattering in Nd2CuO4 for the different magnetic structures—collinear and noncollinear—are considered.
Spin magnetic moment of conduction quasiparticle (abstract)
Authors: Safonov Vladimir L.
Published: July 2002

A new approach for the description of charged quasiparticles (conduction electrons and holes) with the spin of 1/2 in crystals is proposed. The crystalline-field anisotropy is considered as a space-time geometry which is introduced by means of the metric tensor into the dirac equation. In the framework of this approach both the kinetic energy and the Zeeman energy depend on the tensor of effective masses m*j=Gjm0, where Gj is the diagonal metric tensor. Components of the g factor are gj=2m0/m*cj, m*cj being the cyclotron mass. This relation is in good agreement with the experiment for three-dimensional (3D) anisotropic metals such as Bi. For 2D motion we have only one nonzero spin magnetic component, perpendicular to the plane. This fact can be used for an explanation of two dimensional spin confinement observed experimentally in strained quantum wells. For 1D motion the spin of the quasiparticle falls out of the Hamiltonian. The fact that the spin is a hidden coordinate in this case can originate in para-Fermi statistics in which the maximal number of quasiparticles at the same energy level is equal to two (quasiparticles with up and down spins are identical). According to the theory of superconductivity, the system of quasiparticles with para-Fermi statistics has much greater critical temperature than the system with Fermi statistics.
Particle interactions in granular Co films (abstract)
Authors: Tsoukatos A. , Wan H. , Hadjipanayis G. C.
Published: July 2002

The interparticle interactions of hcp-Co particles embedded in silica (SiO2) and boron nitride (BN) matrices have been studied via isothermal remanence curve measurements. The films were prepared by a combination of dc/rf magnetron sputtering. The strength of the interaction fields was varied as a function of metal content (xv) and temperature. The average particle size was kept below 10 nm for all the samples investigated. The volumetric fraction of Co was varied between 0.035 and 0.66. Henkel plots (normalized Mr vs Md plots) showed that in hcp granular Co the interactions are negative, i.e., they have the tendency to destabilize the saturation state. Switching field distribution (SFD) plots showed a decreased width with increasing xv and temperature. In the low metal volume fraction specimens, magnetizing and demagnetizing SFD curves indicate that the dominant interactions are dipolar in origin due to the fact that they nearly overlap. However, as xv increases, the presence of exchange interactions is also evident. A detailed discussion of these studies will be presented in an effort to determine the interaction mechanisms controlling the magnetic behavior of these films as compared to existing theoretical models.
Magnetic properties of Dy-Lu alloys
Authors: Everitt B. A. , Salamon M. B. , Flynn C. P. , Park B. J. , Borchers J. A. , Erwin R. W. , Tsui F.
Published: July 2002

Although many binary heavy rare-earth alloy systems have been studied extensively, there has been little work reported on the Dy-Lu system. The properties of single-crystal DyxLu1−x films grown by molecular beam epitaxy are reported. SQUID magnetometer and neutron diffraction measurements on samples with x=0.4, 0.5, and 0.6 show that the samples order helimagnetically with Néel temperatures of TN=90, 105, and 120 K, respectively. The helical turn angle was mapped as a function of temperature for each of the three alloys. Magnetic x-ray scattering, the first in an alloy, was observed at the (002)± positions at 15 and 60 K in the x=0.4 sample using resonant exchange scattering of synchrotron radiation at the Dy LIII edge.
Exchange interactions among ferromagnetic clusters in Cu-Co heterogeneous alloy films (abstract)
Authors: Berkowitz A. E. , Parker F. T. , Rao D.
Published: July 2002

Sputtered thin films of Cu-Co contain ferromagnetic clusters, even in the as-deposited condition, due to the immiscibility of Co and Cu. This structural heterogeneity is indicated by diffraction data, transport properties, and strongly temperature dependent magnetization and hysteresis behavior. In addition, these films exhibit some unusual properties that suggest the existence of both strong intercluster ferromagnetic exchange interactions and features usually associated with an exchange anisotropy coupling of ferromagnetic and antiferromagnetic materials. Some of these properties are (a) field-cooled samples exhibit strongly shifted hysteresis loops in measuring fields of 7 T, (b) coercive force depends on the magnitude of the cooling field, (c) initial susceptibility measurements given an interparticle ordering temperature ≳100 K, (d) Curie–Weiss analysis above the interparticle ordering temperatures indicates cluster sizes of a few nm and interparticle molecular fields ≊10 T, (e) fitting the magnetization curves with a Langevin analysis above the interparticle ordering temperatures yields similar cluster sizes and molecular fields. Thus, the films exhibit behavior associated with both ‘‘micromagnetism’’ and ‘‘superferromagnetism’’. We shall discuss our attempts to model this behavior in a consistent manner.
Photothermally modulated ferromagnetic resonance investigations of epitaxially grown thin films (abstract)
Authors: Meckenstock R. , Schreiber F. , von Geisau O. , Pelzl J.
Published: July 2002

Photothermally modulated (PM) ferromagnetic resonance (FMR) is applied, for the first time, to locally investigate the magnetic anisotropy of thin epitaxially grown Fe and Co films. The PM-FMR measurements were performed at 9.2 GHz in an optical cavity. The microwave resonance absorption was thermally modulated by an intensity modulated laser beam and the change of the microwave power reflected at the cavity was synchronously detected by a lock-in amplifier. By focusing the laser beam a lateral resolution of 20 μm could be achieved. The PM-FMR signal was studied as a function of the position of the laser spot on the film, of the modulation frequency (10 Hz–10 MHz) and of the temperature. The PM-FMR measurements were complemented by temperature (5–700 K) and angle dependent conventional FMR studies. The temperature variation of the PM-FMR and its comparison with the behavior of the conventional signal yields evidence for a signal generation process of the PM-FMR that is governed by anisotropy effects. Therefore the local variation of the PM-FMR signal was used to deduce the lateral inhomogeneity of the magnetic anisotropy in the plane of the films.
Effect of interface on the properties of Ti/NiFe thin films
Authors: Li Shuxiang , Yan Minglang , Yu Chengtao , Lai Wuyan
Published: July 2002

The microstructure, anisotropic magnetoresistance, and magnetic properties of Ti/Ni81Fe19 thin films have been investigated with respect to annealing temperatures (TA) up to 450 °C as well as to anneal times (tA). It has been shown that 120 °C is the optimum annealing temperature and that tA has no significant influence on the properties with annealing at 270 °C. Auger depth profiling was used to study interdiffusion kinetics. The diffusivity D and activation energy ε have been estimated at about 2.0×10−17 cm2/s and 1.69 eV, respectively.
Spin-wave study of the magnetic excitations in a layered structure with bilinear and biquadratic interlayer exchange
Authors: Macciò M. , Pini M. G. , Politi P. , Rettori A.
Published: July 2002

Using a microscopic and quantum approach, we study the ground state configuration and the spin-wave excitations of a model of two ferromagnetic monolayers, interacting via both bilinear and biquadratic exchange, and subject to an easy-plane anisotropy, a quartic in-plane anisotropy, and an external field. Peculiar features are found in the field dependence of the acoustic and optical frequency gaps in correspondence to the critical fields for which the canting angle between the magnetizations of the two coupled layers presents discontinuities. In particular, we suggest that a minimum in the optical gap at the saturation field could be experimentally observed by Brillouin light scattering in real systems, like Fe/Cr/Fe trilayers.
Self-stabilization of domain walls in antiferromagnetically coupled multilayered magnetic films
Authors: Fujiwara Hideo , Ishikawa Tomohiro , Doyle W. D.
Published: July 2002

In our previous work, it was shown that in antiferromagnetically coupled multilayered films there is the possibility of the appearance of both Bloch-type walls and Néel-type walls at the center of which the magnetizations point parallel (PB,PN) or antiparallel (AB,AN) to each other between the adjacent layers when a high field is applied in one direction and then removed. Here it is shown that the shift of the walls in each layer from their vertical alignment will give substantially lower energy for the AB walls and for the PN walls, giving rise to self-stabilization, and that the effect is greater for the latter than the former for [Co/Cu]N systems with the thickness ratio tCo/tCu=2. This will result in a greater possibility of the PN walls formation than was expected in the previous work. In the region where the number of layers and the coupling field are of practical interest, Néel walls are favored, and AN walls have lower energy than PN walls. In this case, it is likely that the transition of the wall types from PN to AN and vice versa will occur during the magnetization process, which will cause noise in sensor applications.
Relationship between weak ferromagnetism, superconductivity, and lattice parameter in the A2−x−y>BxCeyCuO4 (A,B≡La, Pr, Nd, Sm, Eu, Gd, Y) compounds
Authors: Schultz T. , Smith R. , Fondado A. , Maley C. , Beacom T. , Tinklenberg P. , Gross J. , Saylor C. , Oseroff S. , Fisk Z. , Cheong S.-W. , Jones T. E.
Published: July 2002

We report the results obtained by x-ray, dc magnetization, ac magnetic susceptibility, microwave magnetic absorption, and electron paramagnetic resonance of the magnetic and superconductive properties of a large series of compounds of the form A2−x−yBxCeyCuO4 with A,B≡La, Pr, Nd, Sm, Eu, Gd, and Y. For y=0 antiferromagnetic ordering of the copper moments is present at TN≊280 K for all the compounds formed in the tetragonal T′ phase. Weak ferromagnetism (WF) associated with the canting of the copper moments is observed in all the compounds with a lattice parameter, a<a0, a0≊3.91 Å, regardless of the rare-earth (R) ions present in the system. Superconductivity is observed only in the Ce-doped compounds with a≳a1, a1≊3.92 Å. The maximum onset temperature Tc is found for a≊3.95 Å. A phase diagram was obtained by varying the lattice parameter size as a function of A, B, and x. The correlation between the appearance of WF in the undoped compounds and the disappearance of superconductivity in the Ce-doped systems is discussed.
Influence of quantum wire electron confinement on intrasubband polar-optical scattering rates
Authors: Shadrin V. D. , Kistenev F. E. , Serzhenko F. L.
Published: July 2002

Analytical calculations of the polar-optical intrasubband scattering rates in a cylindrical quantum wire using modeling wave functions, which fit very closely to the exact wave functions, are presented. The actual scattering rates are shown to be almost twice as large as those of earlier calculations when the spatial dependency of the wave functions of the confined electrons was neglected.
Nonequilibrium characteristics of the gate-controlled diode in 6H-SiC
Authors: Sheppard S. T. , Cooper J. A. , Melloch M. R.
Published: July 2002

N+-P gate-controlled diodes are fabricated in the wide band gap semiconductor 6H-SiC by thermal oxidation and ion implantation of nitrogen. Room temperature capacitance-voltage characteristics display a ‘‘hook and ledge’’ hysteresis, which has been observed in Si gate-controlled diodes at 77 K. In these samples of p-type doping 2.8×1016 cm−3, the surface state density is about 4×1012 cm−2.
Some analyses of potential profiles of forward-biased high-low junctions
Authors: De S. S. , Ghosh A. K.
Published: July 2002

A model calculation for the variation of the normalized potential against the normalized position for a forward-biased, below 2 thermal voltage, high-low junction has been made through numerical computations of Poisson’s equation. The model is oriented to a junction with a sufficiently large doping difference between the two sides.
Surface plasmon field-enhanced Raman spectroscopy with double gratings
Authors: Nemetz Axel , Fernandez Ulrich , Knoll Wolfgang
Published: July 2002

Plasmon surface polaritons (PSPs) are excited via a grating structure to enhance Raman signals from an alkyl-thiol monolayer HS(CH2)21OH on a silver-coated grating. This is usually done with an experimental setup consisting of a turnable periscope in order to excite the PSPs under variable angles and to observe the inelastically Raman scattered PSPs under a different angle. A technique that surpasses the use of such a turnable device is presented, and consists of a double-grating structure—in contrast to the conventionally used single grating—which helps to carry out PSP-Raman spectroscopy with fixed incoupling and outcoupling angles.
Relationship between strain effect and martensitic transformation in multifilamentary Nb3Sn wires
Authors: Watanabe K. , Katagiri K. , Noto K. , Awaji S. , Goto K. , Sadakata N. , Saito T. , Kohno O.
Published: July 2002

For bronze-processed multifilamentary pure Nb3Sn and 2.3 at. % Ti added (Nb,Ti)3Sn wires, which were prepared under the condition of the same wire parameters such as the filament size, filament number, and bronze ratio, the strain effects on the critical current density were measured. The large difference of the residual strain between Nb3Sn and (Nb,Ti)3Sn wires was obtained. It was found that the martensitic transformation for Nb3Sn compound largely contributes to the residual strain through the variation of the elastic modulus in bronze-processed multifilamentary Nb3Sn wires. It was verified that the ternary element addition of Ti surely suppresses the martensitic transformation in the bronze processed multifilamentary Nb3Sn wire.
Carbon-doped impurity induced layer disorder 0.98 μm lasers
Authors: Bylsma R. B. , Hobson W. S. , Lopata J. , Zydzik G. J. , Geva M. , Asom M. T. , Pearton S. J. , Thomas P. M. , Bridenbaugh P. M. , Washington M. A. , Roccasecca D. D. , Wilt D. P.
Published: July 2002

We have fabricated high power carbon-doped InGaAs/AlGaAs lasers using an impurity-induced layer disordering process to define the active region. The advantage of carbon doping is that it exhibits significantly lower diffusivity compared to other p-type dopants, thereby avoiding displacement of the p-n junction, even at the high temperatures and long diffusion times required by the disordering process. Secondary ion mass spectrometry (SIMS) measurements before and after Si diffusion show the p-n junction position to be unchanged during processing. The carbon was introduced using CCl4 as an extrinsic precursor, giving improved control over doping levels and ternary growth conditions that is not available with intrinsic carbon doping. Thresholds of 20 mA and slope efficiencies of 0.44 mW/mA at 25 °C were obtained for lasers with cavity lengths of 500 μm and coated facets.
Electron tunneling lifetime of a quasibound state in a double-barrier resonant tunneling structure
Authors: Fisher D. J. , Zhang C.
Published: July 2002

The level width of a quasibound state due to electron tunneling in a double-barrier resonant tunneling structure in an external electric field is investigated. The result for the width rests on the S-matrix resonant tunneling formalism. The inverse lifetime obtained in this manner is free of small parameter expansion and is exactly the width at the half-maximum of the resonant tunneling rate. It is found that for a system exhibiting current bistability, the level broadening of a quasibound state is not a monotonic function of the applied bias and has a minimum at some intermediate field strength.
Growth optimization of n-type GaAs on GaAs(201) substrates
Authors: Williams J. P. , Westwood D. I. , Sobiesierski Z. , Aubrey J. E.
Published: July 2002

A study of the growth by molecular-beam epitaxy of Si-doped n -type GaAs on the GaAs(201) surface is presented. The motivation for attempting growth on this particular plane, apart from fundamental considerations, is in connection with an investigation of off-axis transport in GaAs. The effects of growth temperature and doping on GaAs(201) and GaAs(100) samples have been compared using the Hall effect, low-temperature photoluminescence (PL), and Nomarski interference contrast microscopy. These studies showed that the PL, onset of conduction, and mobility behavior were very similar for both orientations. It was possible to dope n-GaAs/GaAs(201) reliably from NSi∼4×1014 to 6×1018 cm−3, the highest mobility of 96 000 cm2 V−1 s−1 measured at 77 K, being obtained for a sample doped at NSi∼4×1014 cm−3.
As precipitate redistribution in Si δ-doped low-temperature GaAs
Authors: Cheng T. M. , Chang C. Y. , Huang J. H.
Published: July 2002

A high-resolution x-ray diffractometer and transmission electron microscope (TEM) are used to characterize the redistribution of As precipitates in Si δ-doped GaAs grown by molecular-beam epitaxy at low substrate temperature (230 °C). The analysis results indicate that superlattice satellite peaks, as observed for samples annealed at 700–900 °C for 10 min, are attributed to the formation of the GaAs/As superlattice. Also, the intensity of satellite peaks in x-ray rocking curves and TEM observations reveals the varying degree of As precipitates confined on the Si δ-doped planes. Furthermore, the asymmetry of the satellite peaks clearly indicates the lattice expansion and contraction of the annealed low-temperature epitaxial layers.
Excitonic structures in II–VI superlattices observed by Brewster-angle reflectance spectroscopy
Authors: Kuroda Takashi , Hayashi Masaki , Inoue Kuon , Yoshida Kouji , Minami Fujio , Fujiyasu Hiroshi
Published: July 2002

Reflectance spectra at Brewster-angle of incidence in ZnSe/ZnS superlattices are investigated together with a ZnSe thin film for comparison. The spectra for p polarization show a distinct peak profile at the energy of the fundamental excitonic transition, with a great reduction of the nonresonant background reflectance. The dependence of the reflectance spectra on the resonance width, as well as the incident angle, is numerically investigated and found to be consistent with the experimental results.
Remote hydrogen plasma chemical vapor deposition using an organopentasilane cluster as a novel film-forming precursor: Mechanism of the activation step
Authors: Wrobel A. M. , Wickramanayaka S. , Hatanaka Y.
Published: July 2002

The remote hydrogen plasma chemical vapor deposition (CVD) using tetrakis(trimethylsilyl)silane (TMSS) as a source compound has been examined in terms of the mechanism of the activation step. The deposition experiments performed for different configurations of the afterglow tube (straight, with a light trap, and with a hydrogen-radical annihilator) prove that the TMSS molecules are exclusively activated by the reactions with the hydrogen radicals. The determined temperature dependence of the film deposition rate suggests that the examined remote hydrogen plasma CVD is a nonthermally activated process. Susceptibility of particular bonds in TMSS molecule to the activation step has been characterized using suitable model source compounds. Mechanisms of the most important elementary reactions contributing to the activation step have been proposed.
High-resistivity regions in n-type InGaAsP produced by ion bombardment at different temperatures
Authors: Comedi D. , Zhao J. , Jankowska K. , Thompson D. A. , Simmons J. G.
Published: July 2002

The formation of high-resistivity regions in Si-doped (n=1×1018 cm−3) lattice-matched In0.75Ga0.25As0.54P0.46 on InP by nitrogen and boron ion irradiations at 300 K, and by helium ion bombardment at 80, 300, and 523 K has been investigated as function of ion dose (1×1012–1×1016 cm−2) and subsequent anneal temperature (70–650 °C) by sheet resistance and Hall effect measurements. The dose dependence of the sheet resistance shows two regions for all cases considered: (I) for lower doses in which the sheet resistance (resistivity) increases up to a maximum of about 6×106 Ω/⧠ (180 Ω cm), and (II) for higher doses in which the sheet resistance decreases with dose. Temperature dependent Hall measurements for materials in region (I) show thermally activated carrier densities with activation energies between 0.21 and 0.29 eV. The temperature dependence of the sheet resistance in region (II), on the other hand, is consistent with the assumption of a hopping conductivity. Varying the substrate temperature during the irradiations yields no measurable effects for samples implanted in region (I). For the case of He+ bombardments at 523 K, higher sheet resistances are obtained in region (II) as compared to samples irradiated at lower temperatures. For the case of He+ at 80 K and N+ at 300 K a third region (III) is observed for doses higher than 7 and 2×1014 cm−2, respectively, in which a renewed increase in the sheet resistance with increasing dose is detected. Rutherford backscattering-channeling results suggest that this behavior is related to the creation of an amorphouslike region in the InGaAsP layer. Annealing of samples amorphized by He+ at 80 K yields higher resistivities (up to a factor of 6×105 relative to that of the unimplanted material), and improved stability of the high resistivity as compared to the other implantation schedules investigated.
Transient phenomena on the temperature elevation of semi-insulating GaP wafers heated locally by a Nd:YAG laser beam
Authors: Kawamura Tsutomu , Shimizu Toshihiro
Published: July 2002

Transient phenomena on the temperature elevation of semi-insulating GaP wafers locally heated by a Nd:Y3Al5O12 laser beam (the wavelength:1.06 μm) were found. After the temperature was elevated once with the increase of the incident laser power, the temperature spontaneously decreased to the stationary state in spite of the continuing duration of the incident laser power. In the case of low-resistive GaP wafers, such a phenomenon could not be observed in the measured range. The physical model to explain these transient phenomena well was proposed. The different thermal behaviors of semi-insulating GaP from the low-resistive GaP should be ascribed to how the function of the free carrier densities differ and the existence of a midgap energy level in GaP wafers. The reason that such a phenomenon is not observed in GaAs in either the semi-insulating or the low-resistive wafers could also be made clear by this physical model.
Carrier energy relaxation in multisubband quantum well lasers with hot phonon effects
Authors: Tsai Chin-Yi , Eastman Lester F. , Lo Yu-Hwa , Tsai Chin-Yao
Published: July 2002

Carrier energy relaxation rates via carrier-polar optical-phonon interactions with hot phonon effects are theoretically studied in multisubband quantum well structures. The effects of changing the width and depth of quantum wells on the carrier energy relaxation rate are discussed. Compared to the result of the bulk, we find that the difference in the energy relaxation rate between quantum wells and bulk is rather small. Reducing the lifetime of longitudinal-optical phonons will effectively enhance the carrier energy relaxation rate. The implications in designing high-speed quantum well lasers are also suggested.