bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of molecular disorder in vapor-deposited thin films of aluminum tris(quinoline-8-olate) by one-dimensional Al27 NMR under magic angle spinning
Authors: Utz Marcel , Nandagopal Magesh , Mathai Mathew , Papadimitrakopoulos Fotios
Published: January 2006

Aluminum tris (quinoline-8-olate) (Alq3) is used as an electron-transport layer in organic light-emitting diodes. The material can be obtained in a wide range of different solid phases, both crystalline and amorphous, by deposition from the vapor phase or from solution under controlled conditions. While the structure of the crystalline polymorphs of Alq3 has been investigated thoroughly by x-ray diffraction as well as solid-state NMR, very little information is currently available on the amount of structural disorder in the amorphous forms of Alq3. In the present contribution, we report the use of Al27 NMR spectroscopy in the solid state under magic angle spinning to extract such information from amorphous vapor deposits of Alq3. The NMR spectra obtained from these samples exhibit different degrees of broadening, reflecting distributions of the electric-field gradient tensor at the site of the aluminum ion. These distributions can be obtained from the NMR spectra by solving the corresponding inverse problem. From these results, the magnitude of structural disorder in terms of molecular geometry has been estimated by density-functional theory calculations. It was found that the electric-field gradient anisotropy δ follows a bimodal distribution. Its majority component is centered around δ values comparable to the meridianal α crystal polymorph and has a width of about 10%, corresponding to distortions of the molecular geometry of a few degrees in the orientation of the ligands. Alq3 samples obtained at higher deposition rates exhibit higher degrees of disorder. The minor component, present at about 7%, has a much smaller anisotropy, suggesting that it may be due to the facial isomer of Alq3.
Synchronization and propagation of bursts in networks of coupled map neurons
Authors: Tanaka Gouhei , Ibarz Borja , Sanjuan Miguel A. F. , Aihara Kazuyuki
Published: January 2006

The present paper studies regular and complex spatiotemporal behaviors in networks of coupled map-based bursting oscillators. In-phase and antiphase synchronization of bursts are studied, explaining their underlying mechanisms in order to determine how network parameters separate them. Conditions for emergent bursting in the coupled system are derived from our analysis. In the region of emergence, patterns of chaotic transitions between synchronization and propagation of bursts are found. We show that they consist of transient standing and rotating waves induced by symmetry-breaking bifurcations, and can be viewed as a manifestation of the phenomenon of chaotic itinerancy.
Relativistic small-core energy-consistent pseudopotentials for the alkaline-earth elements from Ca to Ra
Authors: Lim Ivan S. , Stoll Hermann , Schwerdtfeger Peter
Published: January 2006

Small-core ten-valence electron energy-consistent scalar- and two-component relativistic pseudopotentials for the alkaline-earth elements from Ca to Ra are presented. The accuracy and reliability of these pseudopotentials are discussed in terms of their ability to reproduce all-electron calculated and experimental dipole polarizabilities and ionization potentials.
Semiempirical hybrid density functional with perturbative second-order correlation
Authors: Grimme Stefan
Published: January 2006

A new hybrid density functional for general chemistry applications is proposed. It is based on a mixing of standard generalized gradient approximations (GGAs) for exchange by Becke (B) and for correlation by Lee, Yang, and Parr (LYP) with Hartree-Fock (HF) exchange and a perturbative second-order correlation part (PT2) that is obtained from the Kohn-Sham (GGA) orbitals and eigenvalues. This virtual orbital-dependent functional contains only two global parameters that describe the mixture of HF and GGA exchange (ax) and of the PT2 and GGA correlation (c), respectively. The parameters are obtained in a least-squares-fit procedure to the G2∕97 set of heat of formations. Opposed to conventional hybrid functionals, the optimum ax is found to be quite large (53% with c=27%) which at least in part explains the success for many problematic molecular systems compared to conventional approaches. The performance of the new functional termed B2-PLYP is assessed by the G2∕97 standard benchmark set, a second test suite of atoms, molecules, and reactions that are considered as electronically very difficult (including transition-metal compounds, weakly bonded complexes, and reaction barriers) and comparisons with other hybrid functionals of GGA and meta-GGA types. According to many realistic tests, B2-PLYP can be regarded as the best general purpose density functional for molecules (e.g., a mean absolute deviation for the two test sets of only 1.8 and 3.2kcal∕mol compared to about 3 and 5kcal∕mol, respectively, for the best other density functionals). Very importantly, also the maximum and minium errors (outliers) are strongly reduced (by about 10–20kcal∕mol). Furthermore, very good results are obtained for transition state barriers but unlike previous attempts at such a good description, this definitely comes not at the expense of equilibrium properties. Preliminary calculations of the equilibrium bond lengths and harmonic vibrational frequencies for diatomic molecules and transition-metal complexes also show very promising results. The uniformity with which B2-PLYP improves for a wide range of chemical systems emphasizes the need of (virtual) orbital-dependent terms that describe nonlocal electron correlation in accurate exchange-correlation functionals. From a practical point of view, the new functional seems to be very robust and it is thus suggested as an efficient quantum chemical method of general purpose.
Theoretical study on the circular dichroism in core and valence photoelectron angular distributions of camphor enantiomers
Authors: Stener M. , Di Tommaso D. , Fronzoni G. , Decleva P. , Powis I.
Published: January 2006

In the present work the photoelectron circular dichroism of camphor has been theoretically studied using B-spline and continuum multiple scattering-Xα methods, and comparisons are made with available experimental data. In general, rather large dichroism effects have been found for both valence and core (O 1s, C 1s) photoionizations. The agreement between the two calculations reported here and previous experimental measurements for core C 1s data is essentially quantitative. For valence ionization satisfactory agreement between theory and experiment has been obtained and the discrepancies have been attributed to both exchange-correlation potential limitations and the absence of response effects in the adopted formalism. The calculations predict, moreover, important features in the cross-section profiles, which have been discussed in terms of dipole-prepared continuum orbitals.
A study of the asymmetric Malkus waterwheel: The biased Lorenz equations
Authors: Mishra Aashwin Ananda , Sanghi Sanjeev
Published: January 2006

In this work, the asymmetric case of the Malkus waterwheel is studied, where the water inflow to the system is biasing the system toward stable motion in one direction, like a Pelton wheel. The governing equations of this system, when expressed in Fourier space and decoupled to form a closed set, can be mapped into a four-dimensional space where they form a quasi-Lorenz system. This set of equations is analyzed in light of analogues of the Rayleigh Bernard convection and conclusions are drawn. The properties and behavior of the equations are studied and correlated to the physical model. Phase space behavior and linear stability analysis are used for this. Spectral analysis is used as a qualitative measure of chaos. Chaotic behavior is quantified through the calculation of the Lyapunov exponents and these are further correlated to the bifurcation diagrams for a conclusive analysis of the dynamical behavior of the system.
Estimation of synchrotron radiation and limiting energy of high-energy runaway electrons in tokamak stochastic magnetic fields
Authors: Martín-Solís J. R. , Sánchez R.
Published: January 2006

The increase of synchrotron radiation power emitted by relativistic electrons moving across stochastic magnetic fields in tokamak geometry has been investigated. It will be shown that the guiding-center motion along the magnetic-field lines leads to an enhancement of the electron radiation which, in case of strong turbulence, can dominate the whole radiation process. A threshold stochastic magnetic fluctuation level, b̃>(δ⊥∕R)1∕2 (δ⊥ is the perpendicular correlation length of the magnetic-field fluctuations), has been found for turbulence-dominated radiation. The implications that these results can have when estimating the final energy that runaway electron beams confined in tokamaks can reach are also discussed.
Low-frequency sheath instability stimulated by an energetic ion component
Authors: Starodubtsev Mikhail , Kamal-Al-Hassan Md. , Ito Hiroaki , Yugami Noboru , Nishida Yasushi
Published: January 2006

Spontaneous low-frequency oscillations have been observed in the circuit of a positively biased electrode immersed in a non-Maxwellian laboratory plasma containing an energetic ion component produced by the resonant absorption of a short microwave pulse in a nonuniform plasma column. The oscillations are found to be due to an instability of the electron-rich sheath. The instability with its characteristic frequency below the ion plasma frequency is driven by the energetic ion component reflected in the sheath area. A qualitative model of the instability is suggested.
On the unphysical impact of complex absorbing potentials on the Hamiltonian and its remedy
Authors: Scheit S. , Meyer H.-D. , Moiseyev N. , Cederbaum L. S.
Published: January 2006

The introduction of complex absorbing potentials as numerical tools to stabilize or increase the efficiency of calculations based on wave-packet propagation or on eigenvalue problems has the drawback of causing a modification of the Hamilton operator of the problem. In this work the consequences of such a modification are analyzed and the corrections required in order to properly describe the original physical process are derived. As an example, the decay of excited molecular states is considered: it is shown that the standard time-independent expression for the decay spectrum loses its validity when a complex absorbing potential is introduced in the nuclear Hamilton operator of the problem. To remedy the situation, a new, very stable formula is derived and tested on relevant model studies. Numerical examples are discussed.
On the calculation of time correlation functions by potential scaling
Authors: Xing Chenyue , Andricioaei Ioan
Published: January 2006

We present and analyze a general method to calculate time correlation functions from molecular dynamics on scaled potentials for complex systems for which simulation is affected by broken ergodicity. Depending on the value of the scaling factor, correlations can be calculated for times that can be orders of magnitude longer than those accessible to direct simulations. We show that the exact value of the time correlation functions of the original system (i.e., with unscaled potential) can be obtained, in principle, using an action-reweighting scheme based on a stochastic path-integral formalism. Two tests (involving a bistable potential model and a dipeptide bond-vector orientational relaxation) are exemplified to showcase the strengths, as well as the limitations of the approach, and a procedure for the estimation of the time-dependent standard deviation error is outlined.
Generation of chaotic radiation in a driven traveling wave tube amplifier with time-delayed feedback
Authors: Marchewka Chad , Larsen Paul , Bhattacharjee Sudeep , Booske John , Sengele Sean , Ryskin Nikita , Titov Vladimir
Published: January 2006

The application of chaos in communications and radar offers new and interesting possibilities. This article describes investigations on the generation of chaos in a traveling wave tube (TWT) amplifier and the experimental parameters responsible for sustaining stable chaos. Chaos is generated in a TWT amplifier when it is made to operate in a highly nonlinear regime by recirculating a fraction of the TWT output power back to the input in a delayed feedback configuration. A driver wave provides a constant external force to the system making it behave like a forced nonlinear oscillator. The effects of the feedback bandwidth, intensity, and phase are described. The study illuminates the different transitions to chaos and the effect of parameters such as the frequency and intensity of the driver wave. The detuning frequency, i.e., difference frequency between the driver wave and the natural oscillation of the system, has been identified as being an important physical parameter for controlling evolution to chaos. Among the observed routes to chaos, besides the more common period doubling, a new route called loss of frequency locking occurs when the driving frequency is adjacent to a natural oscillation mode. The feedback bandwidth controls the nonlinear dynamics of the system, particularly the number of natural oscillation modes. A computational model has been developed to simulate the experiments and reasonably good agreement is obtained between them. Experiments are described that demonstrate the feasibility of chaotic communications using two TWTs, where one is operated as a driven chaotic oscillator and the other as a time-delayed, open-loop amplifier.
Density-functional theory with effective potential expressed as a mapping of the external potential: Applications to open-shell molecules
Authors: Theophilou Andreas K. , Glushkov Vitaly N.
Published: January 2006

In this paper we apply the direct-mapping density-functional theory (DFT) to open-shell systems, in order to get many-electron wave functions having the same transformation properties as the eigenstates of the exact Hamiltonians. Such a case is that of spin, where in order to get the magnetic properties, the many-particle states must be eigenstates not only of Sz but also of S2. In this theory the Kohn and Sham [Phys. Rev. A 140, 1133 (1965)] potential is expressed directly as a mapping of the external potential. The total energies of the molecules calculated were satisfactory as their relative deviations (ΔE∕E) from the exact Hartree-Fock ones were of the order of 10−4. This accuracy is much higher than that of the standard DFT in its local exchange potential approximation. This method does not need an approximate density as input, as the effective potential is derived directly from the external potential.
Computer simulations of diffusion and dynamics of short-chain polyelectrolytes
Authors: Zhou Tong , Chen Shing Bor
Published: January 2006

Brownian dynamics simulations are conducted to investigate the diffusional and dynamic properties of polyelectrolytes in dilute salt-free solutions. The polyelectrolyte molecule is represented by a bead-spring chain in a primitive model. The long-range hydrodynamic and Coulomb interactions are both taken into consideration through the Ewald summations for the first time. The major finding of our simulations is that the dependence of the long-time chain diffusivity on the Coulomb interaction strength is very different from that of the Kirkwood short-time diffusivity, which simply shows a trend nearly opposite to the chain size. When ignoring the hydrodynamic interaction (HI), the coupling effect between the chain and its counterions gives rise to a noticeable increase in the long-time diffusivity at intermediate electrostatic interaction strengths. However, the incorporation of HI suppresses this effect to a degree that one can no longer discern it. Moreover, the rotational relaxation is found to show a dependence opposite to that of the gyration radius relaxation.
Shear-wave Mach cones in a strongly coupled dusty plasma
Authors: Bose Anirban , Janaki M. S.
Published: January 2006

Shear-wave Mach cones excited in a strongly coupled dusty plasma in the fluid regime are studied using the generalized hydrodynamic model. The Mach cones are excited by a laser beam that is modeled to sweep the three-dimensional dusty plasma with a velocity that is supersonic with respect to the phase velocity of the transverse shear waves. The formation of single Mach cone structures in vorticity maps reveals that they are formed due to shear motion. It is found that an asymmetry in the wake excitation technique gives rise to certain asymmetries in the Mach cone patterns.
Solar wind interaction with the stationary dust can produce drift waves to form nonlinear structures
Authors: Saleem H.
Published: January 2006

Solar wind electrons and ions penetrating with shear flow into the stationary dust can introduce electrostatic drift wave in plasmas of cometary and planetary environments. The drift wave becomes linearly unstable in the presence of shear flow. The background current also produces shear in the static magnetic field which does not allow the Shukla-VarmaPhys. Fluids B [5, 236 (1993)] mode to exist in such a system. The vortex structures can be formed in nonlinear regime. The relevance of this investigation to space plasmas is pointed out.
Multimode switching induced by a transverse field in planar magnetic nanowires
Authors: Bryan M. T. , Atkinson D. , Allwood D. A.
Published: January 2006

We report how transverse fields affect the axial field needed to “inject” domain walls from a large Permalloy (Ni80Fe20) pad into planar nanowires of width 184nm, 303nm, 321nm, and 537nm fabricated by electron beam lithography. For the narrowest wire, different switching fields are observed under the same transverse field conditions, indicating that more than one mode or state for the domain walls may exist. In contrast, in the widest wires a transverse field causes each reversal event to occur in two stages. The different response may be attributed to the magnetostatic energy differences of domain walls in wires of different widths.
Self-similar expansion of finite-size non-quasi-neutral plasmas into vacuum: Relation to the problem of ion acceleration
Authors: Murakami M. , Basko M. M.
Published: January 2006

A new self-similar solution is presented which describes nonrelativistic expansion of a finite plasma mass into vacuum with a full account of charge separation effects. The solution exists only when the ratio Λ=R∕λD of the plasma scale length R to the Debye length λD is invariant, i.e., under the condition Te(t)∝[ne(t)]1−2∕ν, where ν=1, 2, and 3 corresponds, respectively, to the planar, cylindrical, and spherical geometries. For Λ⪢1 the position of the ion front and the maximum energy Ei,max of accelerated ions are calculated analytically: in particular, for ν=3 one finds Ei,max=2ZTe0W(Λ2∕2), where Te0 is the initial electron temperature, Z is the ion charge, and W is the Lambert W function. It is argued that, when properly formulated, the results for Ei,max can be applied more generally than the self-similar solution itself. Generalization to a two-temperature electron system reveals the conditions under which the high-energy tail of accelerated ions is determined solely by the hot-electron population.
“Ultrasonic stars” for time-reversal focusing using induced cavitation bubbles
Authors: Pernot Mathieu , Montaldo Gabriel , Tanter Mickael , Fink Mathias
Published: January 2006

Ultrasound waves can be focused by multichannel arrays through heterogeneous media using a time-reversal focusing method. In this method, it is required that a reference signal be either sent by a small active source embedded in the medium or backscattered by a strong scatterer acting as a passive source. The potential of this method in ultrasonic medical imaging has been already envisioned for aberration corrections. However, in many practical situations it is not possible to insert an active source in the medium or to rely on the presence of a unique strong scatterer in order to generate the reference signal. Analogous to the field of adaptive optics in astronomy, we propose here to create artificial “ultrasonic stars” in the body. The trick consists of first creating a bubble inside the medium using a section of the ultrasonic array. Due to cavitation, the bubble generates a spherical wave that propagates through a heterogeneous medium to the ultrasound array. The time-reversal method is then applied to the ultrasonic wave received by the array. This technique is experimentally validated for aberrations corrections in tissue mimicking phantoms.
Failure mechanism of AlN nanocaps used to protect rare earth-implanted GaN during high temperature annealing
Authors: Nogales E. , Martin R. W. , O’Donnell K. P. , Lorenz K. , Alves E. , Ruffenach S. , Briot O.
Published: January 2006

The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300°C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1–2μm which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.
Nuclear-Spin Exchange Interaction in Solid 3He
Authors: Meyer Horst
Published: January 2006

Solid 3He has an unusually large nuclear spin-spin exchange interaction, which is due to the large zero-point motion of the atoms. This exchange can be described by a Hamiltonian of the form ℋ = −ħJ I1 · I2. The parameter J is strongly dependent on the density, decreasing from ħJ/kB = 2×10−3°K at V = 24.6 cm3/mole to ħJ/kB = 4×10−5 °K at V = 19.5 cm3/mole in the bcc phase. Hence the exchange is large in comparison with the energy of dipolar interaction Edip/kB = 2×10−8 °K. There is some evidence that the exchange is antiferromagnetic. One can calculate that at the highest stable molar volume, the transition to the ordered state should occur at about 1.8×10−3 °K. In the still more dense hcp phase, the exchange continues to decrease with molar volume.

The experimental evidence for this exchange comes from nuclear magnetic relaxation measurements taken in several laboratories. This paper presents a review of longitudinal and transverse relaxation data as well as diffusion data and their interpretation in terms of exchange. The consequences of exchange on some thermodynamic properties is then discussed.
Gyrotropic Birefringence—Phenomenological Theory
Authors: Hornreich R. M.
Published: January 2006

A phenomenological theory of gyrotropic birefringence is presented. Since the magnetic classes in which gyrotropic birefringence is allowed are among those in which the magnetoelectric effect may occur, the constitutive relations between the complex amplitudes of the fields are so written as to incorporate both these effects simultaneously. All induced effects are then combined in a suitably renormalized electric dipole moment. For the case of lossless media, the gyrotropic birefringence property tensor has 18 linearly independent components before crystalline symmetry considerations are introduced. It is shown that a physical basis for these 18 independent quantities may be found in electric quadrupole and magnetoelectric effects, with the former contributing ten independent quantities and the latter eight. In particular, the compounds Cr2O3 and MnTiO3, in which an experimental observation of gyrotropic birefringence may be possible, are considered. Finally, the closely related effect of natural optical activity is discussed and correlated with the point of view presented here.
Pressure Dependence of the Magnetization of Invar and Silectron from 30–450 kbar
Authors: Graham R. A.
Published: January 2006

Measurements of the change in saturation magnetization of Invar and Silectron subjected to shock-wave compression from 30 to 450 kbar show that Invar exhibits a constant coefficient of saturation magnetization change with pressure, Ms−1(dMs/dp) and that Silectron experiences a pressure-induced transition to a nonferromagnetic phase. Shock waves are generated in tape-wound core samples by projectile impact techniques which allow experiments in small pressure increments over a wide range in pressure. The Invar measurements give a value for Ms−1(dMs/dp) of −1.3×10−2 kbar−1 up to a magnetization change of 90% of the saturation magnetization. This value is the same as that obtained in previous static measurements to 5 kbar. Measurements on Silectron (grain-oriented 3% Si-97% Fe) cores show a change in magnetization beginning at 150 kbar which is the pressure at which a first-order transition has been detected from previous shock-wave pressure-volume measurements. The present measurements indicate that the high-pressure phase of Silectron is nonferromagnetic and show that a mixed-phase region extends to a pressure of 225 kbar. These shock-wave measurements cover a pressure range which is about two orders of magnitude greater than that used previously in static magnetization vs pressure measurements.
Magnetic Domains of Cobalt Single Crystals at Elevated Temperatures
Authors: Hubert A.
Published: January 2006

By means of the enhanced magneto-optic Kerr effect the magnetic-domain structure of cobalt single crystals is investigated in the temperature range from 20° to 400°C. Cobalt is of special interest for the theory of domain structures, since the strongly temperature-dependent anisotropy constant K1 induces uniaxial anisotropy at low temperatures, and conical and planar anisotropy at higher temperatures. While the domain patterns in the uniaxial case are now rather well understood, the continuous variation of K1 gives an access to the understanding of the structures of the conical and the planar anisotropy. The observed domain patterns are analyzed and an interpretation of the main features, especially of those which are connected with internal stresses, is given.
Temperature Dependence of the Generalized Susceptibility of Nickel
Authors: Lowde R. D. , Windsor C. G.
Published: January 2006

The extension to a range of temperatures of our previous work (at 1.6 TC) on fitting the susceptibility of the free-electron gas to the measured magnetic neutron scattering from a crystal of 60Ni indicates that the exchange enhancement is virtually constant at about a factor of three from 1.05 TC to 2.3 TC.
Neutron Paramagnetic Scattering from Holmium and Erbium
Authors: Holden T. M. , Powell B. M. , Woods A. D. B.
Published: January 2006

The energy distribution of neutrons scattered by polycrystalline holmium and erbium and by a single crystal of holmium, in the paramagnetic phase, has been studied by time-of-flight and triple-axis-spectrometer techniques at 296°K. The distributions exhibit some dependence on momentum transfer and are not of a Gaussian form. The widths of the distributions for holmium and erbium are similar. For polycrystalline holmium at 4πsinθ/λ = 0.5 Å−1 the full width at half-maximum of the energy distribution is ∼2.5 meV, which is three times greater than the largest crystal-field transition predicted on the basis of a simple interpretation of the magnetic-susceptibility data. Experiments on the holmium crystal indicate little difference between the widths of the distributions measured along the a and c axes. None of the experiments reveal the presence of discrete crystal field levels.
Magnetic Moments and Hyperfine Interactions in Carbon-Stabilized Fe5Si3
Authors: Johnson C. E. , Forsyth J. B. , Lander G. H. , Brown P. J.
Published: January 2006

Powder samples of the intermetallic compound Fe5Si3 which are stable at room temperature have been prepared by the addition of small quantities of carbon. They have been studied by x-ray, Mössbauer, and neutron-diffraction techniques. The structure remains essentially of the D88 type with 4FeI and 6FeII atoms in the hexagonal unit cell. The material is ferromagnetic with a Curie temperature of 112°±2°C. The Mössbauer spectra show a complex hyperfine pattern which is sensibly constant between 77° and 4.2°K and which can be analyzed as a superposition of a set of six sharp lines corresponding to a hyperfine field of 137 kOe on a more diffuse spectrum corresponding to a large hyperfine field. An estimate of the areas under two spectra is consistent with the smaller field being associated with FeI. The absorption by FeII nuclei shows that they are not all equivalent and have hyperfine fields ranging between 180 and 242 kOe. The FeI spectra show an axially symmetric quadrupole splitting of 12e2Qq = +0.35 mm/sec.

Neutron diffraction measurements at 4.2°, 100°, 290°K, and 150°C confirm that the spin direction is parallel to the haxagonal axis, and show that the magnetic moments are FeI = 1.05±0.15 μB, FeII = 1.55± 0.15 μB.
Nuclear Spin-Lattice Relaxation in the Antiferromagnets CoCl2·2H2O and FeCl2·2H2O
Authors: Daniel A. C. , Bevan A. W. , Mahler R. J.
Published: January 2006

Spin-lattice relaxation of protons in CoCl2·2H2O and FeCl2·2H2O has been measured over a temperature range of 1.3° to 12°K in zero external magnetic field. The temperature dependence of the relaxation rate exhibits two relaxation processes, neither of which is adequately characterized by the T and T8 laws previously reported for CuCl2·2H2O. Additional T11 and AT2 exp(− TAE/T) dependences are required to fit our data.
Exchange Enhancement of Spin Susceptibility in Strongly Paramagnetic Transition Metal Alloys : Breakdown of the Rigid-Band Model
Authors: Jensen M. A.
Published: January 2006

The behavior of the magnetic susceptibility and the electronic specific heat of alloys of Ir with the other nearly filled d-band elements indicates that a rigid band model cannot be used to describe the behavior of the susceptibility. It appears that the large-susceptibility enhancement in these metals in mainly due to large intra-atomic interactions which, e.g., are large on Pt ions in pure Pt and remain large when a Pt ion is placed in Ir.
On the Magnetostriction of Au2Mn at High Magnetic Field
Authors: Hirone Tokutaro , Kamigaki Kazuo , Kaneko Takejiro , Kazama Noriaki
Published: January 2006

The magnetostriction of the intermetallic antiferromagnet Au2Mn was measured up to a magnetic field intensity of 80 kOe in the temperature range from room temperature to He point. Both the longitudinal and transversal magnetostriction curves consist of (1) a fairly complicated part in the low magnetic field range, (2) followed by a contraction of the order of 200×10−6, then (3) a weak contraction proportional to the magnetic field intensity at high fields. Above 200°K, slight expansion can be seen, in the case of longitudinal effect, between states (2) and (3). The present compound Au2Mn has a tetragonal lattice with a magnetic structure of screw arrangement. The observed magnetostriction can be explained by assuming a superposition of (1) magnetostriction due to the rotation of the magnetic moment in the c plane, (2) that due to its rotation toward the c axis, (3) exchange magnetostriction due to the change of screw angle during the course of magnetisation, and (4) forced magnetostriction due to the change of the moment caused by the action of the magnetic field.
Magnetoelastic Coupling Constants of Rare Earth Ions from Optical-Spectroscopic Data
Authors: Comstock R. L. , Buchanan R. A. , White R. L.
Published: January 2006

Measurements are presented of the shifts with uniaxial pressure of the exchange split F7/22(Γ7) levels of Ce3+ in YIG. By assuming that the pressure shifts of the exchange splittings scale as the unshifted splittings, the Ce3+ excited-state data is used to predict the shifts in the F7/22(Γ7) ground state of Yb3+ in YIG. The Yb3+ shifts are used to predict low-temperature magnetoelastic coupling constants of Yb3+ in YIG. The predicted constants are in good agreement with those previously measured using a strain gauge.
Dependence of Magnetostriction and Coercivity on Process Variables for Rolled 4–79 Molybdenum Permalloy Tape
Authors: Vanskike L. L. , Chin G. Y.
Published: January 2006

In certain device applications, it is necessary to obtain cold-rolled Permalloy tape having a high squareness ratio, a very low magnetostriction, and a low coercive force. These properties have been achieved in 4–79 molybdenum Permalloy by discrete variations in processing. A wire is first annealed at a selected diameter, and then cold-drawn to a final diameter followed by roll-flattening. Increasing the anneal temperature (in the 650°–950°C range) and the extent of cold drawing (prior to flattening) by increasing the anneal diameter, generally results in lower tape Hc. However, as the extent of cold-drawing is increased beyond about 75% area reduction, increased anneal temperatures cause a slight increase in tape Hc. Further, the discrete process variations bring about an optimum two-component tape texture (〈111〉+〈100〉 along tape axis) for zero magnetostriction. However, this optimum texture also depends critically on composition. Compositional deviations of 1% (well within commerical tolerances for 4–79 molybdenum Permalloy) have been found unacceptable in device applications.
Spin-Structure Determination in Single Crystals of Some Chromites
Authors: Tsushima T. , Kino Y. , Funahashi S.
Published: January 2006

Single crystals of oxygen chromites of Mn, Co, Ni, and Zn were successfully grown, and measurements of magnetization, torque, and magnetic resonance on some of them were done at low temperatures in various crystallographic directions. In MnCr2O4, the direction of easy magnetization at 4.2°K is [110], and the maximum torque in the (110) plane at this temperature is of order of magnitude of a few times 104 erg/cm3. The anisotropy field is 2×103 Oe. Temperature variations of the magnetization and the magnetic resonance frequency are discussed in terms of a change in spin configuration. In CoCr2O4, the easy direction lies along [100], and there is a maximum in the resonance field with temperature, which corresponds to the dip in the magnetization curve on powder specimen. Maximum torque in the (110) plane at 4.2°K is 1×105erg/cm3. Temperature variations of the magnetization and resonance field are correlated with neutron-diffraction data. NiCr2O4 has its easy direction along [111], which agrees with the previous result. No anisotropy was observed in the magnetic susceptibility of ZnCr2O4.
Magnetic Structure of MnSeO4
Authors: Fuess H. , Will G.
Published: January 2006

The antiferromagnetic structure of anhydrous manganese selenate MnSeO4 has been determined from neutron-diffraction powder data. The magnetic structure is collinear with A (+ − − +) ordering on the 000, 0012, 121212, and 12120 sites in the Pbnm orthorhombic cell. The Mn2+ moment was found to be 4.9±0.1 Bohr magnetons, with the spin orientation parallel to the a axis of the unit cell.
Electrical Transport in FeCr2S4-CuCr2S4 Spinels
Authors: Haacke G. , Beegle L. C.
Published: January 2006

Thermoelectric power α, electrical conductivity σ, and Hall effect have been studied for single crystals of the Fe1−xCuxCr2S4 system. While FeCr2S4 is a p-type semiconductor, substitution of Fe by Cu causes n-type conduction between x∼0.2 and x∼0.5 in the temperature range 4.2°–550°K. For x>0.5, p-type conductivity is observed. An explanation of the experiments is given on the basis of a three-band model.
Mismatch-induced recrystallization of giant magneto-resistance multilayer systems
Authors: Vovk Vitaliy , Schmitz Guido , Hütten Andreas
Published: January 2006

In Py∕Cu (Py=Ni81Fe19),Py∕Cu∕Co∕Cu and also in binary Cu∕Co layer systems, a recrystallization is induced by annealing at about 400 °C, which leads to a transformation of the crystallographic orientation from a [111] to a [200] texture. In this paper it is demonstrated that the observed recrystallization is driven by the elastic energy induced by the lattice mismatch between the thin film materials. The NixFe1−x∕Cu-system is chosen for the study, since the lattice parameter of the NixFe1−x alloy may be suitably controlled by its iron content. Indeed, an increase of the iron content has a dramatic influence on the recrystallization probability.
Magnetic Anisotropy due to Dynamical Jahn-Teller Effect in dε1 and dε5 Ions
Authors: Kamimura H. , Mizuhashi S.
Published: January 2006

Effects of Jahn-Teller distortions on the E2 ground state of dε1 and dε5 ions with tetragonal symmetry are investigated. For C4 symmetry the electronic system in the E state interacts only with a nondegenerate B-vibration mode to remove its electronic degeneracy. The resulting vibronic energy levels are all doubly degenerate, the same as in the absence of Jahn-Teller effect. The obtained results are applied to interpreting the anisotropy of g factors of the low-spin ferrihaemoglobin compounds and it is concluded that a combined action of the dynamical Jahn-Teller effect and the rhombic crystal field may be important in producing the large anisotropy within the haem plane.
Multiple-Phonon Interactions in Ultrasonic Paramagnetic Resonance of Uranium Ions in Calcium Fluoride
Authors: Wetsel Grover C.
Published: January 2006

The method of ultrasonic paramagnetic resonance (UPR) has been used to study direct spin-lattice interactions between tetravalent uranium ions in sites of trigonal symmetry and the host lattice, CaF2. Intense absorption and dispersion of 9.0-GHz longitudinal elastic waves propagated along the [111] direction of calcium fluoride were observed when an external magnetic field was adjusted to the spin-resonance value B1, characteristic of the absorption of one phonon by each paramagnetic ion. Resonant phonon absorption of progressively lower intensity was observed at multiples of two, three, four, and five of the one-phonon resonant field, B1. The absorption was measured as a function of magnetic field, angle between B and the symmetry axis of the paramagnetic ion, ultrasonic intensity, ultrasonic pulsewidth, and temperature in the range 1.5° to 4.2°K. Analysis of the results indicates that two processes are extant: (1) the generation of phonons at harmonics of the applied frequency due to the anharmonic lattice interaction and the UPR absorption of these phonons, and (2) multiple-phonon UPR absorption by each paramagnetic ion.
Anomalous Tunneling in Normal Metal Junctions using Ferromagnetic Films
Authors: Isin A. , Christopher J. E. , Coleman R. V.
Published: January 2006

Measurements of tunnel current flow through normal metal junctions made with the ferromagnetic metals Fe, Ni, and Co have been carried out in the range 1°–300°K. The insulating layer has been an oxide of the above metals or aluminum oxide and the second metal either aluminum or silver. In the case of Fe-FeOx-Al a giant zero-bias anomaly corresponding to a resistance maximum has been observed at helium temperatures and this persists up to room temperature. Co-CoOx-Al also shows a similar but smaller anomaly while Ni-NiOx-Al shows an anomalous behavior of reverse sign. Data on the dynamic resistance as a function of voltage and temperature are presented in this paper. The effects of magnetic fields up to 50 kOe have also been studied.
Thermal-Expansion Anomaly of MnCL2 · 4H2O near its Néel Point
Authors: Gonano R. , Philp J. W. , Adams E. D.
Published: January 2006

We have observed a λ anomaly in the thermal expansion of MnCl2 · 4H2O due to antiferromagnetic ordering at 1.622°K. The linear thermal expansion coefficient, α, was measured along three mutually perpendicular directions between 0.4° and 4.2°K. The length of the sample determined the plate spacing of a capacitor, so that length changes of 0.05 Å could be measured using a capacitance bridge. Temperature resolution was about three microdegrees, with points taken at 40 μ°K intervals near the transition.

The expansion coefficient along the a axis. αa, is negative, while αb and αc′, are positive. The temperature dependences of αa, αb, αc′, and the specific heat are similar but not identical, in agreement with theoretical expectations. Near the transition, for 10−4< | 1 − T/TN | <10−1, the singularity in α appears to be logarithmic for T < TN, and of the form | 1− T/TN |−p with p≃0.35 for T> TN.
A Richter-Type Aftereffect in Thin Iron Films
Authors: Lambeck M.
Published: January 2006

The magnetization reversal of thin iron films at 300° and 80°K was observed by means of the Kerr magneto-optical effect. It was found that the coercive force of a wall increases during the time the wall is fixed in one place of the film. This leads to the conclusion that the dynamics of magnetization reversal is influenced by a Richter-type aftereffect. A new model of wall motion is proposed.
Low-Temperature Susceptibility in Dilute Systems of Magnetic Impurities in Noble Metals
Authors: Dreyfus B. , Souletie J. , Tholence J. L. , Tournier R.
Published: January 2006

Together with already published results on the Cu-Mn system, we present new measurements at very low temperatures on Au-Fe and Cu-Fe alloys. The classical model of the molecular field distribution p(H, T) gives a good description of the specific heat and magnetic properties of Cu-Mn alloys down to 200 ppm and Au-Fe alloys down to 1000 ppm. The abrupt fall at very low concentrations of the initial susceptibility of Au-Fe and Cu-Fe alloys is a good evidence in favor of a spin-compensated state as suggested by recent theories.
Magnetic Ordering in Dilute Cd-Mn Alloys
Authors: Hedgcock F. T. , Mahajan S. N. , Rizzuto C.
Published: January 2006

The low-temperature relative electrical resistance and magnetic susceptibility of a series of Cd-Mn alloys containing up to 0.38 at.% Mn are reported. The magnetic susceptibility indicates an antiferromagnetic ordering which is believed responsible for the observed suppression of the Kondo spin-scattering term in the electrical resistance at temperatures below the resistance maximum.
Hydromagnetic Instabilities in the Dense Plasma Focus
Authors: Comisar G. G.
Published: January 2006

Coaxial discharges of the Mather type owe their performance to the rapid collapse of a noncylindrical current sheath into a dense plasma focus. The lifetime of the resulting fountainlike pinch column appears to be governed by the formation of sausage instabilities; however, typical estimates of the buildup time for m = 0 hydromagnetic instabilities in cylindrical columns are at least 5–10 times shorter than the observed lifetimes (∼50 nsec). In order to study the effect of pinch curvature a hydromagnetic stability analysis is performed in terms of the Bernstein energy principle expressed in curvilinear coordinates. A necessary stability condition for curved pinches with no axial magnetic fields turns out to be the presence of a delta-function current singularity on the axis, in accordance with hard-core pinch measurements of Colgate. For noncylindrical, axisymmetric pinches of finite curvature and large aspect ratio the m = 0 instability is found to grow about 10 times slower than the corresponding cylindrical pinch case.
Adiabaticity of Charged-Particle Trajectories in a Cusped Magnetic Field
Authors: Leffel Claude S. , Gray Ernest P.
Published: January 2006

The region of adiabatic charged-particle motion for a spindle-cusp field is determined on the basis of confinement times both by numerical integration of the particle orbits and by an experiment utilizing electrons. If the critical flux line dividing the adiabatic and nonadiabatic regions is expressed in Grad's form, ψc = (aλ0)3, where λ0 is the cyclotron radius of the particle corresponding to its total velocity, the numerical analysis yields a value for the constant a of 3.5–4.2 and the experiment yields a = 4.36. It is further shown that the transition zone between the regions of adiabatic and nonadiabatic motion is about two-thirds of an electron cyclotron radius in width, and that the relative field variation encountered during one cyclotron period by an electron whose guiding center is located in this zone is between 11 and 19%. A random-walk model of particle motion, based on the near noncorrelation between fluctuations of the magnetic moment on successive traversals of the field, is used to calculate the effect on particle lifetime of the location of the first reflection point, and thereby explain the apparently excessive random scatter of the data.
Microwave Scattering and Noise Emission from Afterglow Plasmas in a Magnetic Field
Authors: Blum F. A. , Bauer L. O. , Gould R. W. , Stenzel R. L.
Published: January 2006

The microwave reflection and noise emission (extraordinary mode) from cylindrical rare-gas (He, Ne, Ar) afterglow plasmas in an axial magnetic field is described. Reflection and noise emission are measured as a function of magnetic field near electron cyclotron resonance (ω ≈ ωc) with electron density as a parameter (ωp < ω). A broad peak, which shifts to lower values of ωc/ω) as electron density increases, is observed for (ωc/ω) ≤ 1. For all values of electron density a second sharp peak is found very close to cyclotron resonance in reflection measurements. This peak does not occur in the emission data. Calculations of reflection and emission using a theoretical model consisting of a one-dimensional, cold plasma slab with nonuniform electron density yield results in qualitative agreement with the observations. Both the experimental and theoretical results suggest that the broad, density-dependent peak involves resonance effects at the upper hybrid frequency (ωh2 = ωc2 + ωp2) of the plasma.
Amplitude Dispersion and Nonlinear Instability of Whistlers
Authors: Tam Christopher K. W.
Published: January 2006

The propagation of small but finite amplitude plane “whistlers” in a cold plasma imbedded in a uniform magnetic field is studied. The usual frequency dispersion relation of linear theory is extended to include the effect of amplitude dispersion. The present analysis is based on a perturbation procedure in which the frequency, wavenumber, and amplitude relation is represented by a power series in the energy density or the square of the amplitude of the wave, i.e., ω = ω0(k) + ω2(k) a2 + ω4(k) a4 + …. It is found that the effect of amplitude dispersion is to cause the “whistlers” to undergo nonlinear instability if the wave vector k is not parallel to the direction of the undisturbed magnetic field. For plane whistlers which propagate along the mean magnetic field lines it is noted that the linear solution is also an exact solution of the two component cold plasma equations.
Stability of a Plane Relativistic E Layer in a Plasma with Imbedded Shearing Magnetic Fields
Authors: Zien Hwa M. , Lessen Martin
Published: January 2006

The linear, normal mode instability of a plane relativistic (E-layer) jet is investigated. The surrounding plasma is infinitely conducting, and there are shearing magnetic fields imbedded parallel to the plane of the jet and directed perpendicular to the direction of the jet flow. The basic equations are derived from the special theory of relativity. It is found that an increase of the equilibrium energy density of the jet has a stabilizing effect at large disturbance wavenumbers. Increases in the intensity of the shearing magnetic fields and in the jet flow Mach number also have a stabilizing effect at moderate wavenumbers. Unstable, locally supersonic disturbances of outgoing waves relative to the surrounding plasma are found. These radiating waves may be of use in heating the surrounding plasma.