Development of a Dispersion Interferometer for Low Electron Density Measurements in Magnetically Insulated Transmission Lines on Z.
Abstract not provided.
Hines, Nathan↗
SEARCH · Engineering Papers
Results for “MAGNETIC DISPERSION”
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Phonons, magnons, and lattice thermal transport in antiferromagnetic semiconductor MnTe
The antiferromagnetic semiconductor MnTe has recently attracted attention for spintronics and high-performance thermoelectric applications. However, little is known about its vibrational and thermal transport properties and how these might relate to the electronic and magnetic structure, particularly as related to 3d Mn orbital correlations. Here, we calculate a physically justified Coulomb correlation parameter within the DFT+U framework. We couple this framework with the Heisenberg Hamiltonian and first-principles Boltzmann transport to understand the magnetic, vibrational, and phonon thermal transport properties of MnTe. We also perform inelastic neutron and nuclear inelastic x-ray scattering measurements of the total and partial phonon density of states, respectively. Very good agreement is obtained with the measured and calculated phonon density of states, and with available measurements for the band gap, local magnetic moments, Néel temperature, magnon dispersion, thermal conductivity, and phonon dispersion. Here, this study demonstrates that the vibrational and magnetic degrees of freedom are not strongly coupled in MnTe, and provides a more comprehensive picture of this technologically promising material.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
A super-resolution technique to analyze single-crystal inelastic neutron scattering measurements using direct-geometry chopper spectrometers
Direct-geometry time-of-flight chopper neutron spectroscopy is instrumental in studying dynamics in liquid, powder, and single crystal systems. We report here that real-space techniques in optical imagery can be adapted to obtain reciprocal-space super resolution dispersion for phonon or magnetic excitations from single-crystal neutron spectroscopy measurements. The procedure to reconstruct super-resolution energy dispersion of excitations relies on an accurate determination of the momentum and energy-dependent point spread function and a dispersion correction technique inspired by an image disparity calculation technique commonly used in stereo imaging. Here by, applying these methods to spinwave dispersion data from a virtual neutron experiment demonstrates ~5-fold improvement over nominal energy resolution.
47 OTHER INSTRUMENTATION↗
ON ELECTRON-SCALE WHISTLER TURBULENCE IN THE SOLAR WIND
For the first time, the dispersion relation for turbulence magnetic field fluctuations in the solar wind is determined directly on small scales of the order of the electron inertial length, using four-point magnetometer observations from the Magnetospheric Multiscale mission. The data are analyzed using the high-resolution adaptive wave telescope technique. Small-scale solar wind turbulence is primarily composed of highly obliquely propagating waves, with dispersion consistent with that of the whistler mode.
79 ASTRONOMY AND ASTROPHYSICS↗
Comment on “Generalized dispersion relation for electron Bernstein waves in a non-Maxwellian magnetized anisotropic plasma” [Phys. Plasmas 17, 102114 (2010)]
In a recent paper [Deeba et al., Phys. Plasmas 17, 102114 (2010)], a generalized dielectric constant for the electron Bernstein waves using non-Maxwellian distribution functions was derived in a collisionless, uniform magnetized plasma. Using the Neumann series expansion for the products of Bessel functions, Deeba, Ahmad, and Murtaza derived the dispersion relations for both kappa and the generalized (r, q) distributions in a straightforward manner. However, their results are questionable, since the Neumann series expansion for the products of Bessel functions is valid for small argument, while for perpendicular propagation, it is necessary to evaluate special integrands for small as well as large arguments.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
The dispersive evolution of charged-particle bunches in random magnetic fields
Shortly after a strongly anisotropic beam of charged particles is injected along a guiding magnetic field on which is superimposed a small random conponent, the particle density can be represented by a Gaussian profile whose center moves with the coherent velocity and whose width increases with time at a rate controlled by the coefficient of dispersion. Both parameters depend upon the mean free path, which characterizes scattering by the random fields, and the focusing length, which characterizes spatial variations of the guiding field. These dependencies are known explicitly for the coherent velocity. Formulae for coefficient of dispersion are available only in the limits of very weak and very strong focusing. A new expression for coefficient of dispersion, which spans this gap, is presented.
Earl, J. A.↗
Role of transients in the sustainability of solar coronal plumes
We report on the role of small-scale, transient magnetic activity in the formation and evolution of solar coronal plumes. Three plumes within equatorial coronal holes are analyzed over the span of several days based on the Solar Dynamic Observatory (SDO)/Atmospheric Imaging Assembly 171 Å and 193 Å images and SDO/Helioseismic and Magnetic Imager line-of-sight magnetograms. The focus is on the role of transient structures at the footpoints in sustaining coronal plumes for relatively long periods of time (i.e., several days). The appearance of plumes is a gradual and lengthy process. In some cases, the initial stages of plume formation are marked by the appearance of pillar-like structures whose footpoints are the sources of transient brightenings. In addition to nominal jets occurring prior to and during the development of plumes, the data show that a large number of small jets (i.e., {sup j}etlets{sup )} and plume transient bright points (PTBPs) occur on timescales of tens of seconds to a few minutes. These features are the result of quasi-random cancellations of fragmented and diffuse minority magnetic polarity with the dominant unipolar magnetic field concentration over an extended period of time. They unambiguously reflect a highly dynamical evolution at the footpoints and are seemingly the main energy source for plumes. This suggests a tendency for plumes to be dependent on the occurrence of transients (i.e., jetlets, and PTBPs) resulting from low-rate magnetic reconnection. The decay phase of plumes is characterized by gradual fainting and multiple rejuvenations as a result of the dispersal of the unipolar magnetic concentration and its precipitation into multiple magnetic centers.
79 ASTRONOMY AND ASTROPHYSICS↗
Small amplitude nonlinear electron acoustic solitary waves in weakly magnetized plasma
Nonlinear propagation of electron acoustic waves in homogeneous, dispersive plasma medium with two temperature electron species is studied in presence of externally applied magnetic field. The linear dispersion relation is found to be modified by the externally applied magnetic field. Lagrangian transformation technique is applied to carry out nonlinear analysis. For small amplitude limit, a modified KdV equation is obtained, the modification arising due to presence of magnetic field. For weakly magnetized plasma, the modified KdV equation possesses stable solitary solutions with speed and amplitude increasing temporally. The solutions are valid upto some finite time period beyond which the nonlinear wave tends to wave breaking.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Electron temperature gradient mode instability and stationary vortices with elliptic and circular boundary conditions in non-Maxwellian plasmas
Linear and nonlinear dynamics of electron temperature gradient mode along with parallel electron dynamics is investigated by considering hydrodynamic electrons and non-Maxwellian ions. It is noticed that the growth rate of η{sub e}-mode driven linear instability decreases by increasing the value of spectral index and increases by reducing the ion/electron temperature ratio along the magnetic field lines. The eigen mode dispersion relation is also found in the ballooning mode limit. Stationary solutions in the form of dipolar vortices are obtained for both circular and elliptic boundary conditions. It is shown that the dynamics of both circular and elliptic vortices changes with the inclusion of inhomogeneity and non-Maxwellian effects.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Exchange bias magnetism in films of NiFe/(Ni,Fe)O nanocrystallite dispersions
Ni{sub 3}Fe/(Ni,Fe)O thin films having a nanocrystallite dispersion morphology were prepared by a reactive ion beam-assisted deposition technique. The crystallite sizes of these dispersion-based films were observed to decrease from 8.4 ± 0.3 nm to 3.4 ± 0.3 nm as the deposition flow-rate increased from 2.78% to 7.89% O{sub 2}/Ar. Thin film composition was determined using selective area electron diffraction images and Multislice simulations. Through a detailed analysis of high resolution transmission electron microscopy images, the nanocrystallites were determined to be Ni{sub 3}Fe (a ferromagnet), NiO, and FeO (both antiferromagnets). It was determined that the interfacial molar Ni{sub 3}Fe ratio in the nanocrystallite dispersions increased slightly at first, then decreased as the oxygen content was increased; at 7.89% O{sub 2}/Ar, the interfacial molar ratio was essentially zero (only NiO and FeO remained). For nanocrystallite dispersion films grown with O{sub 2}/Ar flow-rate greater than 7.89%, no interfacial (intermixed) Ni{sub 3}Fe phase was detected, which resulted in no measurable exchange bias. Comparing the exchange bias field between the nanocrystallite dispersion films at 5 K, we observed a decrease in the magnitude of the exchange bias field as the nanocrystallite size decreased. The exchange bias coupling for all samples measured set in at essentially the same temperature (i.e., the exchange bias blocking temperature). Since the ferromagnetic/anti-ferromagnetic (FM/AFM) contact area in the nanocrystallite dispersion films increased as the nanocrystallite size decreased, the increase in the magnitude of the exchange bias could be attributed to larger regions of defects (vacancies and bond distortions) which occupied a significant portion of the FM/AFM interfaces in the nanocrystallite dispersion films.
36 MATERIALS SCIENCE↗
High-performance magnetic carbon materials in dye removal from aqueous solutions
To obtain a novel adsorbent with excellent adsorption capacity and convenient magnetic separation property, magnetic activated semi-coke was prepared by KOH activation method and further modified by FeCl{sub 3}. The surface morphology, physical structure, chemical properties and textural characteristics of unmodified semi-coke, KOH-modified semi-coke and magnetic activated semi-coke were characterized by scanning electron microscopy, X-ray powder diffraction, N{sub 2} adsorption-desorption measurement, and electronic differential system. The adsorption characteristics of the magnetic activated semi-coke were explored for the removal of methyl orang (MO), methylene blue (MB), congo red (CR), acid fuchsin (AF), and rhodamine B (RB) from aqueous solution. The effects of adsorption parameters, including adsorbent dosage, pH and contact time, were investigated by comparing the adsorption properties of the magnetic activated semi-coke to RB. The result showed that the magnetic activated semi-coke displayed excellent dispersion, convenient separation and high adsorption capacity. The adsorption experiment data indicated that the pseudosecond order model and the Langmuir model could well explain the adsorption processes of RB on the magnetic activated semi-coke, and the maximum adsorption capacity (q{sub m}) was 526.32 mg/g. The values of thermodynamic parameters (ΔG°, ΔH° and ΔS°) indicated that the adsorption process depended on the temperature of the aqueous phase, and it was spontaneous and exothermic in nature. As the addition of the magnetic activated semi-coke, the color of the solution significantly faded. Subsequently, fast aggregation of the magnetic activated semi-coke from their homogeneous dispersion in the presence of an external magnetic field could be happened. So, the magnetic activated semi-coke displayed excellent dispersion, convenient separation and high adsorption capacity. - Graphical abstract: As the addition of the magnetic activated semi-coke, the color of the solution significantly faded. Subsequently, fast aggregation of the magnetic activated semi-coke from their homogeneous dispersion in the presence of an external magnetic field could be happened. So, the magnetic activated semi-coke displayed excellent dispersion, convenient separation and high adsorption capacity. Display Omitted.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Effect of the Ce Content on the Magnetic Properties and Microstructure of CeCo{sub 5}-based Sintered Bulk Magnets
CeCo{sub 5}-based magnets have recently attracted much attention due to their moderate magnetic performance and low cost. Nevertheless, there have been few studies on the effects of Ce content on the magnetic properties and microstructures of CeCo{sub 5}-based magnets. In response to this, the magnetic properties of sintered bulk magnets with nominal compositions of Ce(Co{sub 0.73}Cu{sub 0.135}Fe{sub 0.135}){sub z} (z = 4.95, 5.15, 5.35, and 5.55), prepared by the conventional powder metallurgy method, were investigated here. Based on experimental findings, it was shown that Ce(Co{sub 0.73}Cu{sub 0.135}Fe{sub 0.135}){sub 5.15} sintered bulk magnets had comprehensive magnetic properties—maximum energy product of 80 kJ m{sup − 3} (10 MGOe) and intrinsic coercivity (H{sub cj}) of 452 kA m{sup − 1} (5.69 kOe)—superior to those previously reported by us. For z = 5.35 and 5.55, due to the presence of the minor Ce{sub 2}Co{sub 17} phase (which has a Curie temperature (T{sub c}) < 20 °C), magnets had low H{sub cj} values. Based on x-ray diffraction and scanning electron microscopy observations, it was suggested that the volume fraction of the 1:5 matrix phase was the main factor determining the H{sub cj} of CeCo{sub 5}-based sintered bulk magnets obtained with different Ce contents. Furthermore, the importance of the dispersion characteristics of the Ce{sub 2}O{sub 3} phase within the matrix was emphasized. Uniform dispersion of the Ce{sub 2}O{sub 3} phase can significantly improve the overall magnetic performance of CeCo{sub 5}-based magnets.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Modified dust ion-acoustic surface waves in a semi-bounded magnetized plasma containing the rotating dust grains
The dispersion relation for modified dust ion-acoustic surface waves in the magnetized dusty plasma containing the rotating dust grains is derived, and the effects of magnetic field configuration on the resonant growth rate are investigated. We present the results that the resonant growth rates of the wave would increase with the ratio of ion plasma frequency to cyclotron frequency as well as with the increase of wave number for the case of perpendicular magnetic field configuration when the ion plasma frequency is greater than the dust rotation frequency. For the parallel magnetic field configuration, we find that the instability occurs only for some limited ranges of the wave number and the ratio of ion plasma frequency to cyclotron frequency. The resonant growth rate is found to decrease with the increase of the wave number. The influence of dust rotational frequency on the instability is also discussed.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Synthesis of α-Fe{sub 2}O{sub 3}-functionalised graphene oxide nanocomposite by a facile low temperature method and study of its magnetic and hyperfine properties
Highlights: • Synthesis of reduced graphene oxide (rGO)/α-Fe{sub 2}O{sub 3} nanocomposites (NCs) by sol–gel method. • Samples exhibited ferromagnetism originated due to surface canted spins. • rGO enhanced the value of saturation magnetization and coercivity of the bare nanoparticle. • RKKY coupling between the isolated rGO spins and the magnetic nanoparticles. - Abstract: Reduced graphene oxide/α-Fe{sub 2}O{sub 3} nanocomposites were synthesized by a simple sol–gel method. Initially, the samples were investigated by X-ray diffraction, high resolution transmission electron microscopy and Raman spectroscopy. High resolution transmission electron micrographs indicate that the magnetic nanoparticles are well dispersed in the reduced graphene oxide sheet-like structure. Fourier transform infrared spectroscopy studies corroborate the results obtained by X-ray diffraction. Temperature as well as field dependent magnetic measurements revealed ferromagnetism in the pristine α-Fe{sub 2}O{sub 3} as well as in the nanocomposite sample that is attributed to surface canted spins and defects present in the samples. The enhanced value of coercivity in case of the nanocomposite sample could be ascribed to the Ruderman–Kittel–Kasuya–Yosida type of interaction between the isolated reduced graphene oxide spin and the magnetic ions. Presence of super paramagnetic doublet as well as sextet patterns related to hematite phase is confirmed by the {sup 57}Fe-Mössbauer spectroscopy.
36 MATERIALS SCIENCE↗
Kelvin–Helmholtz instability under stabilizing parallel magnetic field in nonhomogeneous compressible MHD flows
We study the Kelvin–Helmholtz instability (KHI) for the general case of a compressible, nonhomogeneous, magnetized plasma flow. The study is limited to a vortex sheet interface with an imposed parallel magnetic field. We introduce a new formalism based on a convective Mach number M c , a convective Alfvénic Mach number M Ac , and a total convective Mach number that combines the two. We derive an analytic expression of the KHI growth rate for a homogeneous flow (i.e., zero Atwood number, A=0) that converges toward both the expression for unmagnetized compressible flow and Chandrasekhar's expression for magnetized incompressible flow. Otherwise, the dispersion relation is solved numerically and allows deriving general stability diagrams of magnetized KHI for the triplet (A, M c , β −plasma) parameters. We show these parameters uniquely define all configurations for a parallel magnetic field. We also construct diagrams with respect to the convective Alfvénic Mach number, the β − plasma parameter, or the magnetic field showing which magnetic field strength is required for stabilizing a given shear flow. The theoretical growth rates are compared with 18 simulations made with the GAMERA code, currently used for 3D magnetospheric simulations. Finally, we apply our results to the analysis of a past KHI experiment performed at the OMEGA laser facility, showing linear theory succeeds to provide accurate estimates of the growth rate at early times. We further discuss how our results can inform future experiments in the high-Mach magnetized regime at the National Ignition Facility. Possible limitations of the study due to resistive, mixing, or turbulence effects are discussed.
compressible flows↗
Theory of gradient drift instabilities in low-temperature, partially magnetised plasmas
A fluid dispersion theory in partially magnetised plasmas is analysed to examine the conditions under which large-wavelength modes develop in Penning-type configurations, that is, where an electric field is imposed perpendicular to a homogeneous magnetic field. The fluid dispersion relation assuming a slab geometry shows that two types of low-frequency, gradient drift instabilities occur in the direction of the E×B and diamagnetic drifts. One type of instability, observed when the equilibrium electric field and plasma density gradient are in the same direction, is similar to the classic modified Simon–Hoh instability. A second instability is found for conditions in which (i) the diamagnetic drift is in the direction opposite to the E×B drift and (ii) the magnitude of the diamagnetic drift is sufficiently larger than the electron thermal speed. The present fluid dispersion theory suggests that the rotating spokes driven by such fluid instabilities propagate in the same direction as the diamagnetic drift, which can be in the same direction as or opposite to the E×B drift, depending on the plasma conditions. This finding may account for the observation, in some plasma devices, of the rotation of large-scale structures in both the E×B and -E×B directions.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Wave propagation in pulsar magnetospheres - Dispersion relations and normal modes of plasmas in superstrong magnetic fields
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Arons, J.↗
Exchange bias effect and glassy-like behavior of EuCrO{sub 3} and CeCrO{sub 3} nano-powders
The magnetic properties of nano-sized EuCrO{sub 3} and CeCrO{sub 3} powders, synthesized by a solution combustion method, were investigated using DC/AC magnetization measurements. An exchange bias effect, magnetization irreversibility and AC susceptibility dispersion in these samples provided evidence for the presence of the spin disorder magnetic phase. The exchange bias phenomenon, which is assigned to the exchange coupling between the glassy-like shell and canted antiferromagnetic core, showed the opposite sign in EuCrO{sub 3} and CeCrO{sub 3} at low temperatures, suggesting different exchange interactions at the interfaces in these compounds. We also observed a sign reversal of exchange bias in CeCrO{sub 3} at different temperatures.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗