bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling pinchoff and reconnection in a Hele-Shaw cell. I. The models and their calibration
Authors: Lee Hyeong-Gi , Lowengrub J. S. , Goodman J.
Published: July 2002

This is the first paper in a two-part series in which we analyze two model systems to study pinchoff and reconnection in binary fluid flow in a Hele-Shaw cell with arbitrary density and viscosity contrast between the components. The systems stem from a simplification of a general system of equations governing the motion of a binary fluid (NSCH model [Lowengrub and Truskinovsky, Proc. R. Soc. London, Ser. A 454, 2617 (1998)]) to flow in a Hele-Shaw cell. The system takes into account the chemical diffusivity between different components of a fluid mixture and the reactive stresses induced by inhomogeneity. In one of the systems we consider (HSCH), the binary fluid may be compressible due to diffusion. In the other system (BHSCH), a Boussinesq approximation is used and the fluid is incompressible. In this paper, we motivate, present and calibrate the HSCH/BHSCH equations so as to yield the classical sharp interface model as a limiting case. We then analyze their equilibria, one dimensional evolution and linear stability. In the second paper [paper II, Phys. Fluids 14, 514 (2002)], we analyze the behavior of the models in the fully nonlinear regime. In the BHSCH system, the equilibrium concentration profile is obtained using the classical Maxwell construction [Rowlinson and Widom, Molecular Theory of Capillarity (Clarendon, Oxford, 1979)] and does not depend on the orientation of the gravitational field. We find that the equilibria in the HSCH model are somewhat surprising as the gravitational field actually affects the internal structure of an isolated interface by driving additional stratification of light and heavy fluids over that predicted in the Boussinesq case. A comparison of the linear growth rates indicates that the HSCH system is slightly more diffusive than the BHSCH system. In both, linear convergence to the sharp interface growth rates is observed in a parameter controlling the interface thickness. In addition, we identify the effect that each of the parameters, in the HSCH/BHSCH models, has on the linear growth rates. We then show how this analysis may be used to suggest a set of modified parameters which, when used in the HSCH/BHSCH systems, yield improved agreement with the sharp interface model at a finite interface thickness. Evidence of this improved agreement may be found in paper II.
A heuristic quantum dissipation algorithm applied to new neutron scattering data of the free ammonia rotations in the Ni–Ni–biphenyl Hofmann clathrate
Authors: Rogalsky Olaf , Vorderwisch Peter , Hüller Alfred , Hautecler Serge
Published: July 2002

This is a joint effort of an experimental and a theoretical group to understand neutron scattering linewidths of almost free quantum rotors. In the first part we introduce a new method for simulating the dynamics of small quantum systems at finite temperatures. Thermal fluctuations are implemented through a stochastic perturbation potential, which drives the system to thermal equilibrium. It is modeled in analogy to the classical Langevin equation. In the second part we present neutron scattering data for the NH3-rotor in Ni–Ni–2C12H10 Hofmann clathrates. By comparison of the experimental with the simulated data we find a classical behavior for the coupling strength of the phonons to the rotor.
Gain in a free-electron laser with planar wiggler and ion-channel guiding
Authors: Esmaeilzadeh Mahdi , Mehdian Hassan , Willett Joseph E.
Published: July 2002

A theory of ion-channel guiding in a planar wiggler is presented. For the sake of generality, a uniform static axial magnetic field is included in addition to the ion electrostatic field and wiggler magnetic field. The pendulum equation that describes the interaction between an electron and the radiation field is derived. A formula for the gain is then derived in the low-gain-per-pass limit. The results of a numerical study of gain enhancement due to the ion channel are presented and discussed.
Thermal stability and structure of ultrathin Co/Fe2O3 films on the Cu(110) surface
Authors: Pflitsch Christian , Verheij Laurens K. , David Rudolf , Franchy René
Published: July 2002

The thermal stability of very thin Co(0.9 nm≈7 ML)/Fe2O3(1 nm) double layers, deposited on Cu(110), was studied by Auger electron spectroscopy, thermal energy helium atom scattering, and low energy electron diffraction. Ordering of the double layer requires annealing to 400–450 K. After annealing, the film consists mainly of Co(110). Part of the Co(110) surface shows a (3×1) reconstruction which could be related to the presence of oxygen floating on the Co film. In addition, evidence was found for the presence of c(2×2) reconstructed domains of fcc Co(100). The Co/Fe2O3 double layer is essentially stable up to 400 K. Above this temperature, the Co and Fe2O3 start to mix, resulting in segregation of Fe2O3 to the surface at around 700 K. Similar as in the case of the single layer system Fe2O3/Cu(110), copper is found to diffuse towards the surface at temperatures >600 K, but the surface itself remains covered by iron oxide up to 1100 K. Already below 1000 K, Co is completely dissolved in the bulk of the copper crystal.
Ferromagnetic resonance spectra in Co/Nb multilayers with large Co thickness
Authors: Ajan Antony , Prasad Shiva , Krishnan R. , Venkataramani N. , Tessier M.
Published: July 2002

The perpendicular ferromagnetic resonance spectra for four Co/ Nb multilayered films have been studied. Three of these multilayers had the same thickness of Co (210 Å) with Nb layer thicknesses varying between 18 and 135 Å. The fourth film was a trilayer sandwich film. A multimode spectrum was observed for all the films. The results indicated that the Co films are magnetically coupled even when the Nb film thickness was 135 Å. A new approach to understand the multimode ferromagnetic resonance in magnetic/nonmagnetic multilayer thin film structures was worked out. In this approach, the equation of motion is solved in the magnetic layers, which are assumed to be coupled through a boundary condition. The theoretical calculations gave a good agreement with the experimentally observed field positions when it was assumed that the magnetic properties of the top and the bottom Co layers are slightly different from the intermediate ones.
Classical Hamiltonian systems with sl(2) coalgebra symmetry and their integrable deformations
Authors: Ballesteros Ángel , Ragnisco Orlando
Published: July 2002

Several families of classical integrable systems with two degrees of freedom are derived from phase-space realizations of sl(2) Poisson coalgebras. As a remarkable fact, the existence of the N-dimensional integrable generalization of all these systems is always ensured (by construction) due to their underlying dynamical coalgebra symmetry. By following the same approach, different integrable deformations for such systems are obtained from the q-deformed analogues of sl(2). The well-known Jordan-Schwinger realization is also proven to be related to a (non-coassociative) coalgebra structure on sl(2) and the 2 N dimensional integrable Hamiltonian generated by such Jordan-Schwinger representation is obtained. Finally, the relation between complete integrability and the properties of the initial phase-space realization is elucidated through two more examples based on the Heisenberg-Weyl and so(3,2) Poisson coalgebras.
X-ray characterization of annealed iridium films
Authors: Kohli Sandeep , Rithner Christopher D. , Dorhout Peter K.
Published: July 2002

The changes in the mechanical and structural properties of sputter-deposited iridium films are described as a function of annealing temperature from 673 to 1073 K. Glancing angle x-ray diffraction and x-ray reflectivity measurements indicated the growth of an IrO2 overlayer by annealing at 873 K. An increased annealing temperature of 1073 K led to the oxidation of the underlying iridium layer yielding a film comprising IrO2 (major) and Ir (minor) phases. Annealing the films at 873 and 1073 K also led to an increased surface roughness of the films. As-deposited as well as films annealed at 673 and 873 K exhibited tensile stresses along the normal to the plane in consideration. Annealing at 673 K and cooling within a tube furnace relaxed normal and shear stress present in as-deposited 20 nm Ir metal films. Slow cooling, formation of oxygen–iridium bonding, and increased roughness caused the preferential relaxation of the shear stresses for the film annealed at 873 K. The oscillations observed in the strain-sin2 ψ curve for the films annealed at 1073 K could be due to texture in the film and/or stress gradient across the thickness of the film. The residual stress has, therefore, not been evaluated for these films. X-ray photoelectron spectroscopic measurements on the iridium film annealed at 1073 K show that the film is oxygen rich at the surface and oxygen deficient near the substrate.
Monte Carlo study of hole mobility in Al-doped 4H–SiC
Authors: Martinez A. , Lindefelt U. , Hjelm M. , Nilsson H.-E.
Published: July 2002

The ohmic transport of holes in p-type Al-doped 4H–SiC samples is investigated using a Monte Carlo tool based on a full-potential band structure. The temperature and doping dependence of the hole mobility and its anisotropy are calculated and discussed from a physical point of view, where we stress the importance of considering two-band conduction. Acoustic and optical phonon scattering, as well as ionized and neutral impurity scattering, have been considered. The Monte Carlo program also considers incomplete ionization of impurity atoms compatible with an Al ionization energy of 0.2 eV.
L
p
spectral independence of the streaming operator
Authors: Zhang Xianwen
Published: July 2002

The Lp spectrum of the streaming operator in spherical geometry with detailed balancing wall is discussed. First, the Lp-independence of the spectrum is established. Second, the Browder essential spectrum is proved to be empty except for ∞ under the assumption that the wall is nonmultiplying. Finally, we give a simple example to show that the Lp spectral independence of the operator is not valid if the wall is not detailed balancing.
Optical properties of GaN grown on Si (111) by gas source molecular beam epitaxy with ammonia
Authors: Zubrilov A. S. , Nikishin S. A. , Kipshidze G. D. , Kuryatkov V. V. , Temkin H. , Prokofyeva T. I. , Holtz M.
Published: July 2002

We report a study of the optical properties of GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia. Temperature dependence of edge luminescence was studied in the range of 77–495 K for samples with low background carrier concentrations, as determined by capacitance voltage profiling and Raman spectroscopy, and the results were fitted using Passler’s and Varshni’s models. We also demonstrate strong correlation between electron concentration in GaN and relative Raman intensity of A1 (longitudinal optical) and E22 modes. The binding energy of free excitons is estimated to be 29±2 meV. The contributions of different mechanisms to free exciton line broadening are discussed.
Nonlinear evolution of electromagnetic waves driven by the relativistic ring distribution
Authors: Matsukiyo S. , Hada T.
Published: July 2002

Relativistic ring distribution of plasma particles generates electromagnetic waves via the relativistic cyclotron resonance. The long time evolution of this so-called cyclotron maser instability at null wave number (k=0) is studied in detail, by performing particle simulations using a plasma which consists of relativistic ring electrons, background positrons, and background electrons. The linear and nonlinear stages of the system evolution are discussed for both gyrotropic and nongyrotropic ring distributions. The linear theory predicts that, when the initial ring energy is strongly relativistic, there appears a critical initial ring momentum at which the system is marginally stable. Numerical simulations show, however, that the system is nonlinearly unstable even when the initial ring momentum exceeds the critical momentum. The final saturation level of the wave energy is obtained analytically.
Influence of Penning effects on high-pressure discharge in the plasma display panel
Authors: Uhm Han S. , Whang Ki W. , Choi Eun H. , Kim Sung S.
Published: July 2002

The influence of the Penning effects on the high-pressure discharge in a neon–xenon mixed gas was investigated in connection with applications to the plasma display panel. A theoretical model of the breakdown voltage in a mixed gas is developed, based on the Townsend criteria. The breakdown temperature Tb and voltage Vb are obtained in terms of the gas mixture ratio. As an example, electrical breakdown properties in neon gas mixed with xenon are investigated. It is shown that the electron breakdown temperature Tb decreases monotonically as the xenon mole fraction χ increases. The Penning effects modify the electron temperature significantly, particularly in the range of a small mole fraction. A preliminary experiment using the plasma display panel is carried out to verify some of the theoretical models. The Paschen curves of the breakdown voltage are experimentally obtained in terms of the pressure parameter (pd) and the xenon mole fraction. It is shown that the breakdown voltage is reduced significantly at the xenon mole fraction of χ=0.015, which agrees remarkably well with experimental data.
Multilayer optics for a wiggler beamline (invited)
Authors: Headrick Randall L. , Smolenski Karl W. , Kazimirov Alexander , Liu Chian , Macrander Albert T.
Published: July 2002

A double crystal, multilayer monochromator was designed and fabricated for a wiggler beamline at the Cornell High Energy Synchrotron Source. The monochromator consists of an internally water-cooled first substrate and a fixed-radius sagittally focusing second substrate, each coated with a multilayer consisting of 100 bilayers of tungsten/carbon with a 27 Å d spacing. Cooled silicon substrates were fabricated with internal water cooling channels to reduce or eliminate thermal distortion. The wide energy bandpass of this multilayer along with sagittal focusing provides the best available flux for time resolved experiments. A flux 100 times that of conventional silicon monochromators is possible and allows for a finer time resolution for the crystal growth studies on this beamline. Measured reflectivities over 60% and bandwidths of 1.3%–1.8% were obtained. Results to beam currents of 350 mA show the effectiveness of the internal cooling design and have provided x-ray fluxes of 8×1013 photons/s/mm2.
Magnetization reversal of exchange coupled CoFe/IrMn bilayers
Authors: Zhao Tong , Zhang Kunliang , Fujiwara Hideo
Published: July 2002

The magnetization reversals of circular elements of CoFe/IrMn bilayers were studied using magnetic force microscopy. Drastic changes of domain structures and the nature of the reversal processes were observed with varying thickness of the CoFe layer. The localized incoherent rotation dominates in the reversal process when the CoFe layer is relatively thin while the domain wall motion dominates when the CoFe layer is thick. The effect of the competition between the distributed interfacial ferromagnet/antiferromagnet coupling and the coupling inside the ferromagnetic layer on the nature of the reversal process of the bilayers was investigated.
Spin-filter spin valves with nano-oxide layers for high density recording heads
Authors: Al-Jibouri Abdul , Hoban M. , Lu Z. , Pan G.
Published: July 2002

A new spin-filter spin valve with nano-oxide specular layers with structure of Ta/NiFe/IrMn/CoFe/NOL1/CoFe/Cu/CoFetfl/CutCu/NOL2/Ta was deposited using a Nordiko 9606 physical vapor deposition system. The data clearly show that the magnetoresistive (MR) ratio has been significantly improved for spin valves with thinner free layers. The MR ratio remains larger than 12% even when the CoFe free layer is as thin as 1 nm. An optimized MR ratio of ∼15% was obtained when tfl was about 1.2 nm and tCu about 1.5 nm, and was a result of the balance between the increase in the electron mean free path difference and current shunting through the conducting layer. It is also found that the Cu enhancing layer can improve soft magnetic properties of the CoFe free layer due to the low atomic intermixing observed between Co and Cu. The CoFe free layer of 1–4 nm exhibited coercivity of ∼3 Oe after annealing in a static magnetic field. This kind of spin valve with a very thin soft CoFe free layer is particularly attractive for ultra high density read head applications.
Strain-dependent spin dynamics in Nd0.67Sr0.33MnO3 near the metal–insulator transition
Authors: Ren Y. H. , Zhao H. B. , Lüpke G. , Hu Y. F. , Li Qi
Published: July 2002

We report on time-resolved pump–probe measurements of spin-dependent dynamics in strained Nd0.67Sr0.33MnO3 thin films grown on three different substrates: LaAlO3 (001), SrTiO3 (001), and NdGaO3 (110). The temperature dependence of the long-lived spin-relaxation component is represented well by a power-law decay of long-range correlations, clearly showing the transition from quasilong-range ferromagnetic order to a disordered paramagnetic phase. The “disordering” temperature, TM, where the intermediate phase of quasilong-range order appears, varies according to the creation of static-distortion waves under different strain forces.
Giant magnetoimpedance in Co-based microwires at low frequencies (100 Hz–13 MHz)
Authors: Betancourt I. , Valenzuela R. , Vazquez M.
Published: July 2002

A systematic study of the complex inductance response and the giant magnetoimpedance effect of as-cast CoFeBSiMoNi microwires (30 μm diameter) as a function of frequency (100 Hz–13 MHz) and circular field amplitude (21–212 A/m rms on the wire’s surface) is presented. The microwire magnetization mechanisms are discussed in terms of complex inductance plots (in both real and imaginary parts). The analysis of the experimental results showed evidence of pinning, bulging, and displacement of circumferential domain walls. Higher relaxation frequencies together with a larger unpinning field (3–5×106 Hz and 127 A/m, respectively), compared with conventional amorphous wires, were explained in terms of the reduced dimension of the microwire. Total impedance plots as a function of a bias Hdc field showed an asymmetric character associated with an induced anisotropy during the wire fabrication.
Crystallographic stacking faults in antiferromagnetically coupled media
Authors: Zambon C. , Holloway L. , Antel W. J. , Laidler H. , Girt E. , Harkness S. D.
Published: July 2002

Synchrotron x-ray scattering has been used to examine stacking faults in the constituent layers of an antiferromagnetically coupled (AFC) media film. By varying the x-ray incident angle, we have varied the x-ray penetration depth and, hence, the layer under examination. Three films were studied, one consisting of a full AFC media structure and the other two consisting of a single magnetic layer with the thicknesses of the constituent layers in the AFC medium. The stacking faults in the bottom magnetic layer were investigated using the single layer film. The stacking fault density in the top AFC media layer was measured using careful depth profiling to ensure that the penetration depth of the x rays remained within the top layer. We were unable to estimate the stacking faults in the bottom layer film but the stacking fault densities are constant at approximately 5% for the top layer of the AFC media and the top layer single film within a relatively large error of 3%.
Spin excitations in [La1−xCaxMnO3] in the mixed-phase region
Authors: Stumpe L. , Kirby B. , Kaiser H. , Rhyne J. J. , Mitchell J. F.
Published: July 2002

The magnetic excitations and the ferromagnetic order parameter have been studied by neutron scattering in a series of the manganese-based CMR perovskites [La1−xCaxMnO3] (x=0.46, 0.48, 0.50) near the metallic ferromagnetic to insulating antiferromagnetic phase. Well-defined ferromagnetic spin waves were detected for the x=0.46 and x=0.48 compositions. From the measurements of the order parameter, only the x=0.48 sample showed conclusive evidence of a coexistence of an antiferromagnetic phase with the ferromagnetic phase. For this composition, hysteresis was observed in the spin wave intensity but not in the spin stiffness parameter. This effect indicates that the ferromagnetic exchange is not perturbed by the antiferromagnetic ordering. No measurable ferromagnetic magnetization was found in the x=0.50 sample; thus no spin waves could be detected. The results indicate that the onset of the antiferromagnetism upon hole doping for the series occurs in a narrow region of x below the x=0.50 phase boundary.
Magneto-optical observation of the latchup effect in CMOS ICs induced by uniform and local irradiation
Authors: Egorov Andrey N. , Lebedev Sergey V.
Published: July 2002

A new application of garnet films with high Faraday rotation—the first direct observation of latchup effect in serially produced complementary metal-oxide-semiconductor integrated circuits (CMOS ICs)—is reported. The visualization method based on the formation of straight domain walls inside magneto-optical films along electrical current path in ICs is used. The influence of spatially uniform or focused laser irradiation on the latchup configuration and thresholds is discussed.
Wave functions of Andreev bound states in superconductor/normal-metal/superconductor junctions
Authors: Nakayama Akiyoshi
Published: July 2002

In this article the wave functions of Andreev bound states in superconductor/normal-metal/superconductor (SNS) junctions have been studied. Discrete energy levels of the Andreev bound states have been obtained as a function of the phase difference and as a function of the normal-metal thickness of a SNS junction. Concrete form of the two-component wave functions of Andreev bound states has been described. Using special orthogonal group SO(3), new representation of the two-component wave functions has also been proposed.
Effect of Pd addition on the magnetic characteristics of amorphous CoNbZr films
Authors: Min B. K. , Song J. S. , Kim H. S. , Jeong S. J.
Published: July 2002

The goal of the present paper is to investigate the phase stability and soft magnetic properties of amorphous CoNbZr films when Pd is added. The CoNbZrPd films deposited on Si wafers exhibited amorphous structures independent of the amount of Pd. On the addition of 4.34% Pd, excellent soft magnetic characteristics of the films were observed with a coercive force of 0.54 Oe and an anisotropy field of 11 Oe, whereas a coercive force of 1 Oe and an anisotropy field of 3.5 Oe were shown in the film without the addition of Pd. The increased anisotropy field and low coercive force of the films may be attributed to the occupancy of Pd in preferred sites parallel to an external magnetic field applied during the deposition process. A permeability of about 1100 was constant at operation frequencies ranging up to 100 MHz, which can be explained by the Landau–Lifshitz formula.
Sr effects on anomalous Pr ordering in PrBa2Cu3O7 system
Authors: Luo H. M. , Ding S. Y. , Liang B.
Published: July 2002

In order to make clear the Ba-site Sr substitution effect, we measured the PrBa2−xSrxCu3−yAlyO7 single crystals, and PrBa2−xSrxCu2.7Mo0.3O7 polycrystalline samples by magnetic susceptibility. It is found that, Sr doping causes a change in the crystal field interaction at the Pr site and therefore a change in the magnetic anisotropy for crystals. Sr increases the TN(Pr) slightly first in the range of 0⩽x⩽0.2, then decreases slightly in 0.2<x<1, and decreases abruptly after x⩾1. No TN(Pr) was observed for x⩾1.5. The results are discussed based on the variation of hybridization between Pr ions and O 2pπ of the CuO2 planes.
Dynamics of dilute vortices in various high-TC thin films
Authors: Åkerman Johan J. , Rao K. V.
Published: July 2002

We study the dynamics of dilute vortices in c-axis oriented Y-123, Hg-, and Tl-1212 thin films, as well as a-axis oriented Hg-1212 thin films. Results on the Hg- and Tl-1212 thin films indicate that dislocation-mediated plastic flux creep of single vortices dominates at low temperatures and fields. As the temperature (or the field) is increased, increasing vortex–vortex interactions promote a collective behavior, characterized by elastic creep with a nonzero μ exponent. In some samples, effects of thermally assisted quantum creep are visible up to 45 K. In Y-123 thin films, creep is collective down to the lowest temperatures and fields investigated, while quantum creep persists up to 10–11 K.
Structure and superconductivity of Dy(Ba2−yLay)Cu3Oz (0⩽y⩽0.5) system
Authors: Joshi Amish G. , Malik S. K. , Yelon W. B.
Published: July 2002

Structural and superconducting properties of single-phase La-substituted compounds of the formula Dy(Ba2−yLay)Cu3Oz (0.0⩽y⩽0.5) have been investigated by neutron diffraction, electrical resistivity, and magnetization measurements. The compound DyBa2Cu3O7−δ (orthorhombic, space group Pmmm) exhibits superconductivity with transition temperature, Tc, of about 90 K. Rietveld analysis of the neutron diffraction data on La containing compounds reveals that the orthorhombicity decreases with increasing La concentration and the compounds become tetragonal for higher La content. The overall oxygen content increases as divalent Ba is replaced by trivalent La and approximately compensates for the difference in the valence state of these two ions. The superconducting transition temperature, obtained from low field magnetization and/or four-probe resistivity measurements, decreases from ∼90 K for y=0.0 to ∼34 K for y=0.5. The depression in Tc in Dy(Ba2−yLay)Cu3Oz system is attributed to hole filling/localization caused by the rare earth substitution at the Ba site.
Angular dependence of electromagnetic torque in a bulk high-temperature superconductor
Authors: Postrekhin Y. V. , Ma Ki , Ye Hong , Chu Wei-Kan
Published: July 2002

The angular dependence of the magnetic torque has been measured in cubic shaped YBaCuO samples. The bulk sample was prepared by a seeded-melt growth process in which a small NdBaCuO seed was used to induce domain growth of YBaCuO. The sample had cubic shape so that its edges coincided with basic crystallographic directions. The measurements were conducted in static and dynamic modes. Under static mode, the angular dependencies of the torque have been obtained in a series of zero field cooled magnetic hysteresis measurements. Between each measurement at a specific angle, the sample was warmed up above critical temperature. Under dynamic mode, the magnetic field was increased up to a certain point and then the sample was turned around its axis continuously. It was found that the static torque was a sinusoidal function of the double angle between the external field and one of basic directions of the crystal structure. It also exhibited prominent anisotropy in relation to the crystal structure of the high-temperature superconductor. The dynamic torque-angle relation is not only a result of the anisotropic structure of the superconductor but also a result of the strong intrinsic pinning of this material. Based on Kogan’s theory for the anisotropic superconductors and the Bean model approach, a physical model has been developed to describe the angular dependence of the magnetic torque.
Flux pinning force in Nb thin films with periodic vortex pinning arrays
Authors: Horng Lance , Wu J. C. , Wu T. C. , Lee S. F.
Published: July 2002

Flux pinning force has been investigated in superconducting Nb thin films with square arrays of artificial flux pinning centers. These pinning centers due to the structural corrugation in the Nb thin film were created. In the mixed state, the magnetoresistance minima and critical current density maxima to be separated at constant field intervals corresponding to the matching fields were found. The temperature dependence of critical current density can be fitted to an expression (1−T/TC0)n near superconducting transition temperature. The n values at applied field equal matching fields (H=H1,H2,H3…) are 5/2, in contrast to 3/2 at half integer matching fields. It indicates that the dynamics of interstitial vortices at higher matching fields is similar to those at the first matching field. It can be believed that the collective flux pinning force stabilized by the vortices on pinning centers indirectly prevents interstitial vortices from flowing.
Magnetic and magneto-caloric properties of Tb5Ge2Si2
Authors: Tegus O. , Dagula O. , Brück E. , Zhang L. , de Boer F. R. , Buschow K. H. J.
Published: July 2002

We have studied the magnetic properties and magneto-caloric effect in the compound Tb5Ge2Si2 by means of magnetic measurements. The compound Tb5Ge2Si2 is a canted antiferromagnet with Gd5Ge2Si2-type monoclinic structure. The ordering temperature is about 76 K. Above the ordering temperature, a field-induced first-order phase transition was observed in the magnetization curves with increasing and decreasing fields. The magnetic entropy changes have been calculated from the magnetization data based on the thermodynamic Maxwell relation. The character of the magnetic entropy changes, depending on the field and temperature, is similar to that in the Gd5Ge2Si2 compound. There is hardly any influence of the magnetic anisotropy on the magneto-caloric effects in Tb5Ge2Si2.
Magnetization distribution and coercivity in multigrain materials
Authors: Paul D. I. , Quattrochi D. J.
Published: July 2002

The effect of the interface or defect region between two ferromagnetic grains or variants on the dynamics of the magnetization process has been analyzed analytically. The grains can have arbitrary crystalline orientations with respect to each other and to the defect region. The composition of the defect region can also vary with respect to the host material. Boundary conditions are imposed on the direction of magnetization both at the far edges of the grains and at the two interfaces of the defect region. The sum of the exchange, anisotropy, and Zeeman energies is minimized and the resulting Euler equations are solved. The magnetization direction distribution throughout the defect region is determined as a function of the external magnetic field. It is shown that the magnetization rotation across this region deviates further from the Bloch wall form as the applied magnetic field increases. The coercivity as a function of defect width and difference in anisotropy orientations has also been determined for various applied fields along one of the crystalline axes. It is found that, for given defect widths, as the difference in orientation between the two grains increases (from 0° to 90°), the coercivity increases. We find that the dominant effect for the coercivity is the difference in magnetic anisotropy orientations between the two grains. Only when this difference is small does the change in chemical composition at the interface assume importance.
Analysis of instabilities in nonlinear Landau–Lifshitz–Gilbert dynamics under circularly polarized fields
Authors: Bertotti G. , Mayergoyz I. D. , Serpico C.
Published: July 2002

The stability of large spatially uniform magnetization motions driven by circularly polarized fields is studied within the framework of the Landau–Lifshitz–Gilbert equation. It is demonstrated that the occurrence of spin-wave instability of large scale magnetization dynamics may depend on history of its formation.
Size and relative Co layer thickness dependence of magnetization flop in magnetic tunnel junctions exchange biased by Co/Ru/Co synthetic antiferromagnets
Authors: Uhm Y. R. , Shin K. H. , Lim S. H.
Published: July 2002

Computer simulation is used to examine the size and relative Co layer thickness dependence of the magnetization flop in magnetic tunnel junctions with the structure NiFe(I)(7.5 nm)/AlOx (0.7 nm)/Co(I)(y nm)/Ru(0.7 nm)/Co(II)(7−y nm)/FeMn(10 nm). The flop angle, which was obtained at a typical sputtering condition from an energy minimization, increases with the increase of y. The unidirectional pinning reduces the magnetization flop and its effect is greater when the Co thickness difference is smaller due to a larger energy contribution from the pinning term. Magnetoresistance hysteresis loops of “as-sputtered” samples, which were subjected to the magnetization flop, are found to vary significantly with the relative Co layer thickness, the cell size, and the magnitude of the unidirectional pinning.
Properties of the native oxide in metal/native oxide multilayers (invited)
Authors: Beach G. S. D. , Berkowitz A. E. , Harris V. G. , Parker F. T. , Ramadurai B. , Smith David J.
Published: July 2002

Multilayers composed of thin CoxFe100−x layers, each exposed successively to oxygen to form oxide surfaces, were studied to determine the structural and magnetic properties of the native oxide. Transmission electron microscopy and x-ray reflectivity provided microstructural characterization, while extended x-ray absorption fine structure and Mössbauer spectroscopy were used as local probes of the structural, chemical, and magnetic environments of the constituent atoms. The results of these analyses combined with magnetometry show an onset of magnetism in the oxide with decreasing x, near x=90.
Single-pole-type head with multicharged surfaces for ultrahigh density recording
Authors: Takahashi S. , Yamakawa K. , Ouchi K.
Published: July 2002

A multicharged surfaces single-pole-type (MCS-SPT) head whose pole tip is formed from two steps of a wide cross section and a narrow one is proposed in order to improve writability of the conventional SPT head. A field from the wide tip can modify a recording field of the narrow tip in terms of not only the field strength but also the gradient. The field strength of the MCS-SPT head combined with a soft backlayer (SBL) of a double-layered medium was calculated by using a surface charge method. The superimposed net field strength becomes high by increasing the wide tip width and can largely surpass the saturation flux density (BS) of the head material like an electromagnet. The allowable maximum field of the wide tip should be determined by a negative nucleation field (HN) of a medium. A time decay of medium magnetization can not be observed for a dc field less than HN but can be more for that than HN. It has been confirmed also by a finite element method that the 100 nm track-wide MCS-SPT head combined with SBL separated by 30 nm from the narrow tip whose BS was 19 kG can generate the recording field strength of 23 kOe.
Enhanced thermally induced stress effect on an ultrathin gate oxide
Authors: Su Jiann-Liang , Hong Chao-Chi , Hwu Jenn-Gwo
Published: July 2002

The effects of thermal stress on the electrical characteristics of metal–oxide–semiconductor diodes with oxides in an ultrathin regime were studied. By centering a quartz ring as a heat sink beneath the silicon wafer, the introduced temperature gradient results in a corresponding hat-like shape thickness distribution for an oxide grown on the wafer with a rapid thermal processing system. The enhanced exterior tensile and compressive thermal stresses due to introduced temperature gradient make the oxides exhibit less and more substrate injection saturation current Jsat, respectively, in comparison to control oxides. Their flatband voltage VFB data also clearly show the dependency of effective charge number density Neff on exterior thermal stress. A stress distribution model is proposed to explain the observation. Co-60 irradiation was also performed on the stressed samples to observe this stress extent by examining the variation of electrical characteristics. It was found that an oxide grown on a wafer in exterior compression exhibited better radiation hardness than one in tension.
Characterization of magnetoimpedance on polycrystalline and amorphous chromium oxides bilayered thin films
Authors: Fu C. M. , Lai C. J. , Hsu H. S. , Chao Y. C. , Huang J. C. A. , Wu C.-C. , Shyu S.-G.
Published: July 2002

The impedance of chemical vapor deposited CrO2 and Cr2O3 bilayered thin films, composed of polycrystalline and amorphous structure, have been systematically studied in function of frequency and temperature. In the polycrystalline-CrO2/amorphous-Cr2O3 bilayer, the real part of impedance at low frequency (f<300 kHz) demonstrates a sharp transition at temperature around 330 K, with a specific feature of positive temperature coefficient, similar to the variation of dc resistance occurs at ferroelectric-paraelectric transition in the BaTiO3 ceramics. In contrast, the imaginary part of impedance, at frequency f>300 kHz, shows a characteristic of negative temperature coefficient. Further analysis of the frequency dependence of the impedance shows the contribution from the dynamics of both the dielectric and magnetic dipoles in the layers. Comparison of polycrystalline-CrO2 and amorphous-Cr2O3 single layer with the CrO2/Cr2O3 bilayer is discussed.
Optical parameters of a magnetized space-charge neutral group IV semiconductor
Authors: Sharma Giriraj , Ghosh S.
Published: July 2002

Using a hydrodynamic model of semiconductor plasmas, we report an analytical investigation of hot-carrier induced nonlinearity and its impact on the optical parameters (refractive index and absorption coefficient) of a magnetized, space-charge neutral group IV semiconductor. The carrier heating by the pump is assumed to induce nonlinearity in the medium through momentum transfer collision frequency ν of the carriers and space-charge neutrality of the medium. For the linear optical parameters (nl, al), both electrons and holes are found to contribute resonantly at high frequency of the pump (ω0≈ωce). For the nonlinear optical parameters (n2, a2), the holes contribute significantly in the low pump frequency regime (ω0≈ωch,) whereas in the high pump frequency regime (ω0≈ωce), both kinds of carriers contribute resonantly in which a much larger contribution comes from the electrons.
Dynamic Kerr imaging of soft underlayers for perpendicular recording applications (invited)
Authors: Wolfson Jason T. , Bain James A. , Khizroev Sakhrat , Litvinov Dmitri
Published: July 2002

Time-resolved Kerr imaging was used to examine the magnetization dynamics of soft underlayers (SULs) within perpendicular magnetic recording media. Two media configurations were examined with permalloy SULs of 20 and 200 nm thickness. The media in both of these samples were 14 nm thick Co/Pt multilayer films separated from the soft underlayer by a thin exchange decoupling layer of Ta. For comparison, two equivalent SUL samples were examined with no media layer. The media were examined while driven at 80 MHz by one of two types of magnetic recording heads: a 2.5 μm track width longitudinal head, and a 1 μm track width perpendicular recording head. The rise time (10%–90%) of the fields from the heads was ∼1.4 ns. It was observed in the SUL films without media, that the film switching speeds were similar to that of the driving head, for both longitudinal and perpendicular heads. The switching speed of the SUL films with a recording layer was slower due to influence of the recording layer on the magnetic properties of the SUL. In the 200 nm thick SUL-only film, the integration of the normal magnetization at the back surface of the medium indicates that approximately half of the flux generated by the write head passes out through the back of the film. The switching behavior seen in the films with media were significantly different from the films without media. The switching seen on the back surface of the 20 nm SUL with media was confined to the area directly over the pole tips. For the 200 nm with media, the switching was more active past the track edges than directly over the pole tips.
Measured vector magnetization of magnetic particle tape
Authors: Kahler G. R. , Della Torre E.
Published: July 2002

This paper presents two-dimensional vector measurements of major hysteresis loops of magnetic particle tape measured with a vibrating sample magnetometer. The measurements, presented in magnet and sample coordinates, may be used to evaluate how well the computations of a vector model fit the corresponding measured data. The measurement process is described, and the characteristics of the measurements are discussed.
Monte Carlo study of surface-frustrated Heisenberg thin films with magnetoelastic coupling: An off-lattice model
Authors: Ngo V. T. , Diep H. T.
Published: July 2002

We show extensive Monte Carlo (MC) results on properties of thin films made of stacked XY triangular layers of atoms bearing Heisenberg spins. The film surfaces are frustrated due to antiferromagnetic in-plane interactions, while interior layers are ferromagnetic. We have included both the cohesive and magnetic interactions between neighboring atoms. These simultaneous magnetic and cohesive interactions are supposed to be the Lennard-Jones potential. Due to these interactions, the magnetic atoms move around their equilibrium positions (off-lattice model). We have carried MC calculations with varying surface antiferromagnetic exchange interaction. Depending on parameters the surface spin configuration can be noncollinear and film surfaces become magnetically disordered at a temperature lower than that of interior layers. Detailed results on other properties are shown and discussed.
Electronic structure of MgCNi3−xTMx (TM=Cu,Co,Mn)
Authors: Wang J. L. , Xu Y. , Zeng Z. , Zheng Q. Q. , Lin H. Q.
Published: July 2002

The effects on the electronic structure and magnetic property of the Ni site substitution in nonoxide perovskite superconductor MgCNi3 are studied by using density functional calculations. A sharp high peak of Ni d density of states just lies below the Fermi energy in pure MgCNi3. Co-doping results in the onset of ferromagnetism at x=0.083. However, the little energy gain as well as small magnetic moment per atom indicate the weak itinerant ferromagnetism. The strong ferromagnetic spin fluctuation in MgCNi3−xCox leads to quench bulk superconductivity, while the Mn atom forms a local magnetic moment of 1.06 μB at x=0.042 which suppresses superconductivity more seriously than Co does in the low doping case. Cu doping can be well accounted for by the rigid band model, which reduces the density of states at the Fermi level and the Tc.
Influence of Pr on the magnetic structure of Er
Authors: Moze O. , Kockelmann W. , Wu Y. L. , Pecharsky A. O. , Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

Time-of-flight neutron diffraction has been used to determine the temperature dependence of the magnetic structure of three binary hexagonal Er–Pr alloys, Er90Pr10, Er80Pr20, and Er60Pr40. In overall agreement with magneto-thermal measurements performed on these compounds, the addition of Pr initially increases the Curie temperature and decreases the Néel temperature, observed at 20 K and 86 K, respectively, for pure Er. The neutron diffraction data for Er90Pr10, however, also clearly show that a ferromagnetic phase, with moments parallel to the c-axis, coexists with modulated structure components, with increasing temperature up to 35 K, above which a pure sine modulated structure sets in up to the Néel temperature at approximately 65 K. A similar behavior is observed for Er80Pr20, where the sine modulated phase is observed to disappear at 48 K. In sharp contrast, only one magnetic phase, identified as ferromagnetic with moments parallel to the c-axis, is observed for Er60Pr40 from low temperatures up to the Curie point at 35 K. The propagation vectors of the sine modulated phases are found to be temperature dependent.
Plasma confining potential self-consistently formed by local electron cyclotron resonance in a well-shaped magnetic field
Authors: Kaneko T. , Hatakeyama R. , Sato N. , Ishiguro S.
Published: July 2002

It is clearly demonstrated by means of two-and-a-half-dimensional particle simulations that a plasma potential structure is created by local electron cyclotron resonance (ECR) in a plasma flow along a well-shaped magnetic field. The potential structure consists of a negative potential dip and a positive potential hump (plug/barrier potential) around the ECR point at the magnetic-well bottom. In addition, the simulation points out that a spatial deviation of the electron- and ion-density profiles is generated by the ECR, resulting in the formation of the plug/barrier potential to self-consistently compensate it. According to an analysis of the ion energy distribution function parallel to the magnetic field in the upstream region, the height of the plug potential is found to reach the value equivalent to the ion energy corresponding to the high-energy tail component, plugging most of the ions not to pass through the region of the magnetic variation.
Direct measurement of coherent ultrahigh wakefields excited by intense ultrashort laser pulses in a gas-jet plasma
Authors: Kotaki Hideyuki , Kando Masaki , Oketa Takatsugu , Masuda Shinichi , Koga James K. , Kondo Shuji , Kanazawa Shuhei , Yokoyama Takashi , Matoba Toru , Nakajima Kazuhisa
Published: July 2002

The coherent wakefield excited by 2 TW, 50 fs laser pulses in a gas-jet plasma around 1018 cm−3 is measured with a time-resolved frequency domain interferometer. The density distribution of the helium gas is measured with a time-resolved Mach–Zehnder interferometer to search for the optimum laser focus position and timing in the gas jet. The results show an accelerating wakefield excitation of 20 GeV/m with good coherency, which is useful for ultrahigh gradient particle acceleration in a compact system. This is the first time-resolved measurement of laser wakefield excitation in a gas-jet plasma.
Observation of the growth mode of TiN during magnetron sputtering using synchrotron radiation
Authors: Bøttiger J. , Chevallier J. , Petersen J. H. , Schell N. , Matz W. , Mücklich A.
Published: July 2002

The heteroepitaxial growth of TiN on MgO(001), deposited by reactive magnetron sputtering, has been studied in situ. Using real-time specular x-ray reflectivity, layer-by-layer growth was observed, with the surface roughening decreasing with an increase in the deposition temperature. Higher temperatures also resulted in lower growth rates. The film thickness was measured with specular x-ray reflectivity. Using off-plane Bragg–Brentano as well as grazing incidence in-plane wide angle scattering, the pseudomorphic growth of TiN to the underlying MgO(001) was established. Transmission electron microscopy reveals atomic planes passing through the MgO–TiN boundary, thus confirming heteroepitaxial growth.
On the beam cross section of a high-gain Compton free-electron laser
Authors: Seo Y. , Choi E. H.
Published: July 2002

Investigated is the beam cross-section matter in a high-gain Compton free-electron laser. The growth characteristics of the radiation eigenmodes are analyzed, treating the equilibrium beam radius as an independent parameter.