Showing papers on "Magnetic field published in 1994"

Theory of NMR signal behavior in magnetically inhomogeneous tissues: the static dephasing regime.

Abstract: This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo time TE is less than a known characteristic time tc for a given system, then the signal decays exponentially with an argument which depends quadratically on TE. This is equivalent to an R2* relaxation rate which is a linear function of TE. In the opposite case, when TE is greater than tc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction sigma occupied by the field-creating objects, b) magnetic field Bo or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference delta chi between the objects and the medium.

Magneto‐impedance effect in amorphous wires

Abstract: Recent experiments have discovered a giant magneto‐impedance (MI) effect in FeCoSiB amorphous wires. This effect includes a sensitive change (as much as 60%) in a high frequency wire voltage by an applied dc magnetic field and is thus a high frequency analog of giant magnetoresistance. We consider this phenomenon in terms of ac complex resistance or impedance. The giant MI effect is demonstrated to arise from a combination of a skin effect and a strong field dependence of the circumferential magnetic permeability associated with circular domain wall movements. The theoretical results agree satisfactorily with the existing experimental data.

The GEOTAIL Magnetic Field Experiment.

Abstract: The Geotail spacecraft carries a high-resolution Magnetic Field Experiment to provide magnetic field data in the frequency range below 50 Hz. This experiment includes dual fluxgate magnetometers and a search coil magnetometer. Fluxgate sensors are mounted at distances of 4 m and 6 m from the spacecraft on a deployable mast to reduce spacecraft-generated noises. Both outboard and inboard fluxgate magnetometers have 7 automatically switchable ranges from ±16 nT to ±65536 nT (full scale) and resolutions equivalent to a 15-bit A/D conversion in each range. The basic sampling rate for the A/D conversion is 128 Hz for both magnetometers

Theory of Magnetic Recording

Abstract: Preface 1. Overview 2. Review of magnetostatic fields 3. Inductive head fields 4. Medium magnetic fields 5. Playback process: general concepts, single transitions 6. Playback process: multiple transitions 7. Magnetoresistive heads 8. Record process: transition models 9. Record process: non-linearities and overwrite 10. Medium noise mechanisms: general concepts, modulation noise 11. Medium noise mechanisms: particulate noise 12. Medium noise mechanisms: transition noise References Index.

Magnetization and transport currents in thin superconducting films.

Abstract: The critical-state behavior of an infinitely long type-II superconducting thin-film strip is theoretically analyzed for an arbitrary sequence of applied transport currents and perpendicular magnetic fields. Included are solutions for applied field only, transport current only, transport current applied to a sample initially in the remanent critical state, ac applied field, ac transport current, and simultaneously applied field and transport current. The results are compared side by side with corresponding solutions for the more famililar slab geometry; there are striking differences in behavior.

Near-Field Spectroscopy of the Quantum Constituents of a Luminescent System

Abstract: Luminescent centers with sharp (<0.07 millielectron volt), spectrally distinct emission lines were imaged in a GaAs/AIGaAs quantum well by means of low-temperature near-field scanning optical microscopy. Temperature, magnetic field, and linewidth measurements establish that these centers arise from excitons laterally localized at interface fluctuations. For sufficiently narrow wells, virtually all emission originates from such centers. Near-field microscopy/spectroscopy provides a means to access energies and homogeneous line widths for the individual eigenstates of these centers, and thus opens a rich area of physics involving quantum resolved systems.

Catalysis of dynamical flavor symmetry breaking by a magnetic field in 2 +1 dimensions

Abstract: It is shown that in 2 + 1 dimensions, a constant magnetic field is a strong catalyst of dynamical flavor symmetry breaking, leading to generating a fermion dynamical mass even at the weakest attractive interaction between fermions. The effect is illustrated in the Nambu-Jona-Lasinio model in a magnetic field. The low-energy effective action in this model is derived, and the thermodynamic properties of the model are established.

Giant magneto‐impedance and magneto‐inductive effects in amorphous alloys (invited)

Abstract: Recent experiments have discovered giant and sensitive magneto‐impedance and magneto‐inductive effects in FeCoSiB amorphous wires. These effects include a sensitive change in an ac wire voltage with the application of a small dc longitudinal magnetic field. At low frequencies (1–10 kHz) the inductive voltage drops by 50% for a field of 2 Oe (25%/Oe) reflecting a strong field dependence of the circumferential permeability. At higher frequencies (0.1–10 MHz) when the skin effect is essential, the amplitude of the total wire voltage decreases by 40%–60% for fields of 3–10 Oe (about 10%/Oe). These effects exhibit no hysteresis for the variation of an applied field and can be obtained even in wires of 1 mm length and a few micrometer diameter. These characteristics are very useful to constitute a highly sensitive microsensor head to detect local fields of the order of 10−5 Oe. In this paper, we review recently obtained experimental results on magneto‐inductive and magneto‐impedance effects and present a detail...

Principles of Traveling Wave Tubes

Abstract: Static fields produced by electrons electron motion in static electric fields influence of magnetic field on electron motion cathodes electron guns electron beams beam-gap interactions electron bunching theory of travelling-wave interaction wave velocities and dispersion helix TWTs coupled cavity TWTs depressed collectors noise nonlinearities and distortion breakdown and protection reliability.

Does a Non-Magnetic Solar Chromosphere Exist?

Abstract: Enhanced chromospheric emission which corresponds to an outwardly increasing semiempirical temperature structure can be produced by wave motion without any increase in the mean gas temperture. Hence, the sun may not have a classical chromosphere in magnetic field free internetwork regions. Other significant differences between the properties of dynamic and static atmospheres should be considered when analyzing chromospheric observations.

Local shear instabilities in weakly ionized, weakly magnetized disks

Abstract: We extend the analysis of axisymmetric magnetic shear instabilities from ideal magnetohydrodynamic (MHD) flows to weakly ionized plasmas with coupling between ions and neutrals caused by collisions, ionization, and recombination. As part of the analysis, we derive the single-fluid MHD dispersion relation without invoking the Boussinesq approximation. This work expands the range of applications of these instabilities from fully ionized accretion disks to molecular disks in galaxies and, with somewhat more uncertainty, to protostellar disks. Instability generally requires the angular velocity to decrease outward, the magnetic field strengths to be subthermal, and the ions and neutrals to be sufficiently well coupled. If ionization and recombination processes can be neglected on an orbital timescale, adequate coupling is achieved when the collision frequency of a given neutral with the ions exceeds the local epicyclic freqency. When ionization equilibrium is maintained on an orbital timescale, a new feature is present in the disk dynamics: in contrast to a single-fluid system, subthermal azimuthal fields can affect the axisymmetric stability of weakly ionized two-fluid systems. We discuss the underlying causes for this behavior. Azimuthal fields tend to be stabilizing under these circumstances, and good coupling between the neutrals and ions requires the collision frequency to exceed the epicyclic frequency by a potentially large secant factor related to the magnetic field geometry. When the instability is present, subthermal azimuthal fields may also reduce the growth rate unless the collision frequency is high, but this is important only if the field strengths are very subthermal and/or the azimuthal field is the dominant field component. We briefly discuss our results in the context of the Galactic center circumnuclear disk, and suggest that the shear instability might be present there, and be responsible for the observed turbulent motions.

Helical magnetic fields in filaments

Abstract: For both even and odd-numbered solar cycles, right-hand heliform filaments predominate at middle and high latitudes in the northern hemisphere while left-handed ones predominate in the south. This recent discovery has prompted a re-examination of past measurements of magnetic fields in prominences. This re-examination indicates that Rust (1967), in his interpretation of solar cycle 20 measurements in terms of the Kippenhahn-Schluter model, and Leroy, Bommier, and Sahal-Brechot (1984), in their interpretation of solar cycle 21 measurements in terms of the Kuperus-Raadu model were both misled by the global pattern of helicity. While the original magnetic field measurements are consistent with the new results about heliform magnetic fields in filaments, neither of the well-known classes of two-dimensional models can produce both the proper axial field direction and the observed pattern of helicity. A global, subsurface velocity pattern that would twist the fields before emergence as filaments seems to be required. In this paper a twisted-flux-rope model consistent with the new understanding of filament fields is presented. The model is based on a constant-α solution of the magnetostatic equations, where electric current densityj(r) =α B(r). The model filament has dimensions in general agreement with observations. It is shown to be stable if the length is less than 140 000 km to 1,400 000 km, depending on the value ofα. The model also provides a new explanation of eruptive prominences and for the origin of the entrained material.

Magnetic field dragging in accretion discs

Abstract: We consider a thin accretion disc of half-thickness H, vertically threaded by a magnetic field. The field is due to contributions from both the disc current and an external current (giving rise to a uniform external field). We derive an integro-differential equation for the evolution of the magnetic field, subject to magnetic diffusivity η and disc accretion with radial velocity v r . The evolution equation is solved numerically, and a steady state is reached. The evolution equation depends upon a single, dimensionless parameter D=2η/(3H|v r |)=(R/H)(η/ν), where the latter equality holds for a viscous disc having viscosity ν. At the disc surface, field lines are bent by angle i from the vertical, such that tan i=1.52D −1

Resolving the 180-degree ambiguity in vector magnetic field measurements: The 'minimum' energy solution

Abstract: I present a robust algorithm that resolves the 180-deg ambiguity in measurements of the solar vector magnetic field. The technique simultaneously minimizes both the divergence of the magnetic field and the electric current density using a simulated annealing algorithm. This results in the field orientation with approximately minimum free energy. The technique is well-founded physically and is simple to implement.

The influence of a mean magnetic field on three- dimensional magnetohydrodynamic turbulence

Abstract: Building on results from two-dimensional magnetohydrodynamic (MHD) turbulence (Shebalin, Matthaeus & Montgomery 1983), the development of anisotropic states from initially isotropic ones is investigated numerically for fully three-dimensional incompressible MHD turbulence. It is found that when an external d.c. magnetic field (B0) is imposed on viscous and resistive MHD systems, excitations are preferentially transferred to modes with wavevectors perpendicular to B0). The anisotropy increases with increasing mechanical and magnetic Reynolds numbers, and also with increasing wavenumber. The tendency of B0 to inhibit development of turbulence is also examined.

The geoelectrical methods in geophysical exploration

Abstract: Part 1 Electrical methods in geophysics: in the beginning the mind set and the future. Part 2 General concepts of electromagnetic field behaviour: Maxwell's equations diffusing electromagnetic fields static electromagnetic fields quasi-stationary electromagnetic fields. Part 3 Properties of rocks and minerals: properties and units properties in a parametric sense properties in an existential sense geoelectrical mesostructures and megastructures electrical properties of the atmosphere and the ocean. Part 4 Electromagnetic environment of planet Earth: Earth currents of external origin industrial noise atmospheric electric sources mechanically generated electric fields extraneous sources of electric fields electromagnetic fields generated by the oceans. Part 5 Direct current and induced polarization methods: point and dipole sources on a uniform Earth electric fields in one-dimensional Earth structures fields of a point source for two- and three-dimensional structures vertical electric sounding and apparent resistivity induced polarization magnetic field measurements. Part 6 Natural-field electromagnetic methods: the Tikhonov-Cagniard approach linearity of relationships between components of the magnetotelluric field electromagnetic fields in a horizontally homogenous medius electromagnetic fields in horizontally inhomogeneous media magnetotelluric and magnetovariational survey methods data reduction interpretation of MT data interpretation of MVP and GDS data. Part 7 Controlled source electromagnetic methods: principles of controlled source electromagnetic methods electromagnetic sounding electromagnetic profiling. Part 8 Modelling and simulation: exact analytical solutions for special cases thin sheet models the integral equation method finite difference modelling finite element method physical and analog modelling other approaches to modelling. Part 9 Insensitivity and ambiguity: insensitivity for a laterally uniform Earth ambiguity for a laterally uniform Earth. Part 10 Practical aspects of data acquisition: sources detection of electric and magnetic fields sampling filtering survey design. Part 11 Interpretation: rules, intuition and pragmatic interpretation pseudo-inversion of soundings by transformation inversion continuation of electromagnetic fields electromagnetic migration comprehensive three-dimensional interpretation. Part 12 Other platforms, other methodologies: airborne electromagnetic (AEM) methods marine electrical surveys bore hole assisted methods spontaneous polarization (SP) method ground penetrating radar (GPR) the piezoelectric method (PEP). Part 13 A baker's dozen of case histories: minerals exploration - Que River Deposit, Tasmania, Australia physical volcanology - Oshima volcano, Japan groundwater exploration - Apodi Valley, Rio Grande do Norte, Brazil. (Part Contents).

Plasma processing apparatus

Abstract: In a plasma processing apparatus, a second microwave guiding unit has at least one vacuum waveguide with a dielectric window and an opening being formed on the side of the microwave introducing hole. The vacuum waveguide is arranged at a position where an external magnetic field applied from an external magnetic field applying unit is stronger than an ECR condition, and causes the microwave guided from a first microwave guiding unit to propagate through the dielectric window in a direction perpendicular to the external magnetic field such that the electric field of the microwave is parallel to the external magnetic field applied to the second microwave guiding unit by the external magnetic field applying unit. The dielectric window is arranged at a position at which at least a portion of the dielectric window cannot been seen directly from the microwave introducing hole. The propagating direction of the microwave is changed at a position immediately above the plasma chamber, at which the external magnetic field strength is higher than the ECR condition, thereby introducing, through the opening, the microwave to the microwave introducing hole along the external magnetic field, whereby converting a raw material in the plasma chamber into plasma by electron cyclotron resonance (ECR).

Orientational dependence of the exchange biasing in molecular‐beam‐epitaxy‐grown Ni80Fe20/Fe50Mn50 bilayers (invited)

Abstract: The exchange biasing field (Heb) and coercive field (Hc) of molecular‐beam‐epitaxy‐grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001], and [110] orientations have been investigated by longitudinal Kerr effect measurements. Ni80Fe20 and Fe50Mn50 were deposited as orthogonal wedge‐shaped layers on single‐crystal Cu substrates in a magnetic field, enabling the study of the thickness dependence of Heb and Hc on a single sample for each orientation. A strong dependence of Heb and Hc on the growth orientation is observed. The results are interpreted in terms of the observed noncollinear spin structure of the antiferromagnet and a comparison is given with the predictions from recent theoretical models.

Permanent Magnet Materials and their Application

Abstract: 1 Fundamentals of magnetism 2 Permanent magnet processes 3 Thermal stability 4 Magnetic circuit design 5 Magnetic field analysis 6 Magnetizing and testing 7 Applications

Vector magnetic field changes associated with X-class flares

Abstract: We present high-resolution transverse and longitudinal magnetic field measurements bracketing five X-class solar flares. We show that the magnetic shear, defined as the angular difference between the measured field and calculated potential field, actually increases after all of these flares. In each case, the shear is shown to increase along a substantial portion of the magnetic neutral line. For two of the cases, we have excellent time resolution, on the order of several minutes, and we demonstrate that the shear increase is impulsive. We briefly discuss the theoretical implications of our results.

Force detection of nuclear magnetic resonance.

Abstract: Micromechanical sensing of magnetic force was used to detect nuclear magnetic resonance with exceptional sensitivity and spatial resolution. With a 900 angstrom thick silicon nitride cantilever capable of detecting subfemtonewton forces, a single shot sensitivity of 1.6 x 1013 protons was achieved for an ammonium nitrate sample mounted on the cantilever. A nearby millimeter-size iron particle produced a 600 tesla per meter magnetic field gradient, resulting in a spatial resolution of 2.6 micrometers in one dimension. These results suggest that magnetic force sensing is a viable approach for enhancing the sensitivity and spatial resolution of nuclear magnetic resonance microimaging.

Solar Magnetic Fields: Polarized Radiation Diagnostics

Abstract: Preface. 1. Solar Magnetism -- an Overview. 2. Theory of Polarized Radiation. 3. Interaction of Matter with Radiation. 4. Radiative Transfer without Scattering. 5. Classical Scattering and the Hanle Effect. 6. Non-LTE Radiative Transfer: Phenomenological Treatment. 7. Introduction to Quantum Field Theory of Polarized Radiative Transfer. 8. Multi-Level Radiative Transfer with Coherence Effects. 9. Rayleigh and Raman Scattering. 10. Collisions, Partial Redistribution, and Turbulent Magnetic Fields. 11. Solutions of the Polarized Transfer Equation. 12. Diagnostics of Small-Scale Magnetic Fields. 13. Instrumentation for Solar Polarimetry. References. Symbol Index. Subject Index.

Thin superconductors in a perpendicular magnetic ac field: General formulation and strip geometry.

Abstract: The sheet current, electric field, and penetrating magnetic field in response to an applied perpendicular ac magnetic field are calculated for a thin type-II superconducting strip characterized completely by its sheet resistivity, which may be either nonlinear and frequency independent or linear, complex, and frequency dependent. The general formulation is given for the linear or nonlinear response of a strip and a circular disk in perpendicular time-varying magnetic field. An elegant and rapid numerical method is presented which solves this, in general, nonlinear one-dimensional integrodifferential equation with high precision on a personal computer and which accounts for the facts that the integral kernel has a logarithmic singularity and the sheet current for nearly ideal shielding (occurring at short times or high frequencies or for strong pinning of flux lines) has a one-over-square-root singularity near the specimen edges. As examples the linear Ohmic response of the strip to a sudden change of the applied field and to an ac field are given; Ohmic response is realized during flux flow or thermally activated flux flow. The complex magnetic susceptibility and the ac losses of the Ohmic strip are computed and approximated by simple expressions. This work completes the calculation of dissipation peaks in vibrating superconductors caused by various diffusion modes of the flux lines.

Magnetically driven jets and winds: Exact solutions

Abstract: We present a general class of self-similar solutions of the full set of MHD equations that include matter flow, electromagnetic fields, pressure, and gravity. The solutions represent axisymmetric, time-independent, nonrelativistic, ideal, magnetohydrodynamic, collimated outflows (jet and winds) from magnetized accretion disks around compact objects. The magnetic field extracts angular momentum from the disk, accelerates the outflows perpedicular to the disk, and provides collimation at large distances. The terminal outflow velocities are of the order of or greater than the rotational velocity of the disk at the base of the flow. When a nonzero electric current flows along the jet, the outflow radius oscillates with axial distance, whereas when the total electric current is zero (with the return current flowing across the jet's cross section), the outflow radius increase to a maximum and then decreases. The method can also be applied to relativistic outflows.

A new experimental limit on neutron-antineutron oscillations

Abstract: The experimental search for neutron-antineutron oscillations has been completed at the ILL high flux reactor at Grenoble. A neutron beam of intensity 1011n s−1 was propagated for a timet≅0.1 s in vacuum in a region shielded against the external magnetic field. No antineutron was detected in 2.4·107 s running time. The lower limit\(\tau _{n\bar n} \mathbin{\lower.3ex\hbox{$\buildrel>\over{\smash{\scriptstyle=}\vphantom{_x}}$}} 0.86 \cdot 10^8 \) s for\(n\bar n\) transitions was established at 90% C.L.

Weak localization in chaotic versus nonchaotic cavities: A striking difference in the line shape.

Abstract: We report experimental evidence that chaotic and nonchaotic scattering through ballistic cavities display distinct signatures in quantum transport. In the case of nonchaotic cavities, we observe a linear decrease in the average resistance with magnetic field which contrasts markedly with a Lorentzian behavior for a chaotic cavity. This difference in line shape of the weak-localization peak is related to the differing distribution of areas enclosed by electron trajectories. In addition, periodic oscillations are observed which are probably associated with the Aharonov-Bohm effect through a periodic orbit within the cavities.