Showing papers on "Magnetic field published in 1987"
TL;DR: In this article, a force microscope is used to measure the magnetic force between a magnetized tip and the scanned surface, which shows promise for the high-resolution mapping of both static and dynamic magnetic fields.
Abstract: We describe a new method for imaging magnetic fields with 1000 A resolution. The technique is based on using a force microscope to measure the magnetic force between a magnetized tip and the scanned surface. The method shows promise for the high‐resolution mapping of both static and dynamic magnetic fields.
1,113 citations
18 Dec 1987
384 citations
TL;DR: In this paper, the magnetic forces involved in the interaction between a single-domain microtip and the sample surface magnetic domains were investigated and the influence of the experimental conditions on the performance of the atomic force microscope as a magnetic profiling device was discussed.
Abstract: We present a new way to observe the surface domain distribution of a magnetic sample at a submicrometer scale. This magnetic microscopy is based on the idea of measuring magnetic forces with the recently developed atomic force microscope (AFM). We study the magnetic forces involved in the interaction between a single‐domain microtip and the sample surface magnetic domains. The influence of the experimental conditions on the performance of the AFM as a magnetic profiling device is also discussed. Preliminary experimental results are reported.
330 citations
TL;DR: In this paper, it was shown that a modest increase in particle energy density can produce a substantial increase in cross-tail current, and that the increased currents carried by unaccelerated preexisting particles in the changing growth phase magnetic field can play a significant role in altering magnetic field at synchronous altitude.
Abstract: Magnetic field models are used to study electric currents that flow during growth phases and onsets of magnetospheric substorms. Large cross-tail currents between altitudes of about 7 and 10 RE are required near midnight during growth phase in order to produce the observed magnetic field perturbations at synchronous altitude. Considerations of the particle fluxes needed to carry growth phase currents showed that the required current can be carried by the drift of a particle population whose energy density is about 20 keV/cm³. This energy density frequently is present at synchronous altitude after injection events. However, the model calculations require establishment of the large currents beyond synchronous altitude during growth phase. No observations of substantial flux increases during growth phase within the region of interest could be found. As a partial explanation of this problem, we found that a modest increase in particle energy density can produce a substantial increase in cross-tail current. The increased currents carried by unaccelerated preexisting particles in the changing growth phase magnetic field can play a significant role in altering the magnetic field at synchronous altitude. This process involves a positive feedback effect, with preexisting particles carrying more cross-tail current as soon as any perturbation begins to stretch tail field lines. It is concluded that more extensive observations of the changes in particle fluxes and pitch angle distributions during growth phase are needed in the equatorial region near 8 RE. Such observations also will help determine whether the energy of the plasma that is injected near synchronous altitude during substorms primarily is introduced slowly as fluxes build up during growth phase or primarily is introduced suddenly by the local conversion of magnetic field energy to particle energy at onset. In the first case, an injection event primarily would represent the inward motion of a population which already exists at substorm onset. In the latter case, strong impulsive acceleration would be an intrinsic part of the injection process. For our calculations, substorm onset is modeled by diverting current to the ionosphere in a wedge near midnight. It is found that field lines within the wedge collapse dramatically even if only a portion of the cross-tail current is diverted. At the same time, a satellite outside the current wedge sees field lines become more taillike. It is suggested that diversion of only the electron cross-tail current to the ionosphere is enough to initiate a substorm. Ion drift is reduced substantially within the wedge as field lines become more dipolar even if the ion energy density remains large. Finally, it is noted that very strong drift shell splitting effects should be seen if cross-tail current is diverted only in a wedge near midnight.
297 citations
TL;DR: In the narrowest wires at low temperatures and for small magnetic fields, where the 1D subband splittings exceed both kBT and ℏωc, striking departures from the 2D Hall effect are observed, characterized by an unexpected low-field plateau and a precipitous, complete suppression of the Hall resistance.
Abstract: We report the first observation of the complete quenching of the Hall effect in a one-dimensional conductor. In our narrowest wires at low temperatures and for small magnetic fields, where the 1D subband splittings exceed both kBT and ℏωc, we observe striking departures from the 2D Hall effect, characterized by an unexpected low-field plateau and a precipitous, complete suppression of the Hall resistance. We believe these to be unambiguous manifestations of one-dimensional electrical transport; they appear to provide a direct measure of the number of quantum conduction channels that participate.
292 citations
TL;DR: The classical and quantum mechanics of a charged particle moving on the hyperbolic plane in a constant magnetic field are discussed in this article, where the underlying SL(2,R) symmetry leads to a general description of various possible trajectories.
284 citations
TL;DR: In this article, an efficient numerical technique is presented for the calculation of induced electric currents on coupled wires and multiconductor bundles placed in an arbitrary shaped cavity and excited by an external incident plane wave.
Abstract: An efficient numerical technique is presented for the calculation of induced electric currents on coupled wires and multiconductor bundles placed in an arbitrary shaped cavity and excited by an external incident plane wave. The method is based upon the finite-difference time-domain (FD-TD) formulation. The concept of equivalent radius is used to replace wire bundles with single wires in the FD-TD model. Then, the radius of the equivalent wire is accounted by a modified FD-TD time-stepping expression (based on a Faraday's law contour-path formulation) for the looping magnetic fields adjacent to the wire. FD-TD computed fields at a virtual surface fully enclosing the equivalent wire are then obtained, permitting calculation of the currents on the wires of the original bundle using a standard electric field integral equation (EFIE). Substantial analytical and experimental validations are reported for both time-harmonic and broad-band excitations of wires in free space and in a high- Q metal cavity.
273 citations
TL;DR: In this article, it was shown that the zero temperature conductivity tensor is given by σij=(e2/2πh)∑keijkGk, where \vecG is a reciprocal-lattice-vector of the potential.
Abstract: If a three-dimensional semimetal or doped semiconductor is placed in a sufficiently strong magnetic field, then a change in its transport properties will occur. If the electron-impurity interaction is dominant, then the magnetic field will produce localization of the electron wavefunctions, sometimes described as magnetic freezeout. If the electron-electron interaction is more important, then some type of collective transition may occur. Spin-density waves, charge-density waves, valley-density waves, excitonic insulators, and Wigner crystallization have been proposed to occur under various circumstances. As a generalization to three-dimensions of the integral quantized Hall effect, we show that for electrons in periodic or quasiperiodic potential, when the Fermi level lies in an energy gap, the zero temperature conductivity tensor is given by σij=(e2/2πh)∑keijkGk, where \vecG is a reciprocal-lattice-vector of the potential. We discuss the effect of impurities and dislocations on this result.
239 citations
TL;DR: In this paper, the authors considered a general field of electromagnetic waves of a single frequency and identified the salient structurally stable features of the three-dimensional pattern of polarization, which is applicable even when the constituent plane waves are travelling in all directions.
Abstract: The paper considers a general field of electromagnetic waves of a single frequency and identifies the salient structurally stable features of the three-dimensional pattern of polarization. The approach is geometrical rather than analytical, and it differs from previous treatments of this kind by being applicable even when the constituent plane waves are travelling in all directions. Lines and surfaces exist where the electric or magnetic vibration ellipse is singular. The field is divided into right-handed and left-handed regions by \`T surfaces', the electric and magnetic T surfaces not being coincident. Lying in the T surfaces are \`L$^T$ lines' where the vibration is linear, and cutting through the T surfaces are `C$^T$ lines' where the vibration is circular. Both kinds of lines are surrounded by characteristic patterns of vibration ellipses, which provide a singularity index, $\pm$ 1 for L$^T$ and $\pm \frac{1}{2}$ for C$^T$. The analysis is applicable in a cavity, but a loss-free resonating cavity represents a highly degenerate case.
224 citations
TL;DR: In this article, the authors presented a technique for interpreting magnetic field-dependent Hall data at magnetic fields below the range at which Shubnikov-de Haas oscillations occur, in which the maximum carrier density or maximum conductivity is determined as a continuous function of mobility.
Abstract: A novel technique is presented for interpreting magnetic field‐dependent Hall data at magnetic fields below the range at which Shubnikov–de Haas oscillations occur. The technique generates a ‘‘mobility spectrum’’ in which the maximum carrier density or maximum conductivity is determined as a continuous function of mobility. Examples of the use of the technique with synthetic data as well as data from HgCdTe and GaAs/AlGaAs samples are provided. Other uses of the procedure, including measurement of Fermi surface shapes and direct measurement of the distribution of relaxation times, are discussed.
223 citations
Book•
01 Jul 1987
TL;DR: In this paper, the authors present a review of magnetic separation techniques, including dry low-intensity magnetic separators, wet high-intensity high-gradient magnetometers and superconducting magnetometers.
Abstract: Introduction. I. The Physical Principles of Magnetic Separation. Principles of magnetic beneficiation. Magnetic units. Magnetic properties of minerals. Generation of magnetic field in magnetic separator. Measurement of magnetic properties. II. Review of Magnetic Separation Techniques. Dry low-intensity magnetic separators. Wet low-intensity magnetic separators. High-intensity magnetic separators. Wet high-intensity high-gradient magnetic separators. Superconducting magnetic separators. Laboratory magnetic separators. III. Theory of High-Gradient Magnetic Separation. Theory of single-wire particle capture. Theory of capture in multi-wire matrices. Particle capture by ferromagnetic spheres. Particle capture in a real matrix. Magnetic flocculation. Open-gradient magnetic separation. IV. Practical Aspects of Magnetic Separation. Selection of magnetic separation technique. Dry magnetic separation. Wet magnetic separation. Magnetic field. Matrix. Flow velocity. Pulp density. Feed rate. Rinse and flush water. The effect of particle size. The speed of rotation. Pre-treatment of pulp. The selectivity of separation. Comparison of wet high-gradient magnetic separators. Magnetic flocculation. V. Industrial Applications of High-Gradient Magnetic Separation. Minerals treatment. The magnetic beneficiation of coal. The magnetic beneficiation of fly ash. The application of HGMS to waste water treatment. The application of HGMS in nuclear industry. Magnetic filtration of gases. VI. The Economics of Magnetic Separation. Energy costs. The cost of magnetic separators and their economic comparison. The economics of mineral treatment by magnetic separation. Appendices: List of symbols. List of abbreviations. Values of physical constants. Conversion of units. Definitions of derived units. List of selected companies manufacturing magnets, magnetic separators, magnetic measuring instruments and matrices. Bibliography. Subject Index.
TL;DR: In this article, the authors show that the breakdown of quasi-separability in the quantal problem is closely related to the disappearance of regular orbits in the classical problem, and they conclude that magnetized Rydberg atoms lend themselves as ideal objects in which to study, theoretically and experimentally, manifestations of quantum stochasticity.
Abstract: We review recent results of investigations of hydrogen-like systems at magnetic field strengths where the Lorentz forces are comparable to, or larger than, the Coulomb binding forces. This situation is realized for low-lying states at field strengths typical of magnetic white dwarfs and neutron stars, while for Rydberg states already laboratory field strengths are sufficient. We discuss the wavelength spectrum of the hydrogen atom in magnetic fields of arbitrary strength, and describe in which way the spectroscopy of "stationary lines", which appear in this spectrum, has made possible the detection of the largest magnetic field strength ever found in a white dwarf star to date. For Rydberg states in strong laboratory fields we perform a quantitative comparison between experimental and theoretical spectra, and demonstrate that symptoms of "quantum stochasticity" are recovered in the spectra of magnetized Rydberg atoms. In particular we point out that the breakdown of quasi-separability in the quantal problem is closely related to the disappearance of regular orbits in the classical problem. We conclude that magnetized Rydberg atoms lend themselves as ideal objects in which to study, theoretically and experimentally, manifestations of quantum stochasticity.
TL;DR: In this paper, the intrinsic nucleation and instability fields of uniaxial single domain particles are determined including the effects of the second anistropy constant and of oblique applied magnetic fields.
TL;DR: In this paper, anisotropic upper critical field of single crystal samples of orthorhombic YBa2Cu3Ox was studied and the authors reported that the critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor and is highest when the field is oriented perpendicular to the c-axis.
Abstract: We report a study of anisotropic upper critical field of single crystal samples of orthorhombic YBa2Cu3Ox. The critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor, and is highest when the field is oriented perpendicular to the c-axis. For this field direction, the zero resistance state persists up to 86 K in a magnetic field of 90 kOe, which is lower by only 5 K than the zero resistance temperature in the absence of magnetic field. The anisotropy Hc2⊥/Hc2// is about 2 at 90 K, 5 at 86.5 K and tends to increase with decreasing temperature.
TL;DR: A quantum Boltzmann equation for electron transport in electric and magnetic fields including all many-body effects is derived in this article, and a solution in both d.c and a.c. electric fields is given for electrons in simple metals.
TL;DR: The physical principles relevant to magnetometry are presented and models for intracellular particle motion driven by thermal, elastic, or cellular forces are suggested and described.
TL;DR: It is clear that the value assumed for the momentum diffusivity strongly affects the interpretation of recent probe measurements, and it is argued that the present results offer a more reliable basis for this interpretation.
Abstract: A one‐dimensional fluid theory of Langmuir probe operation in strong magnetic fields is presented. Cross‐field diffusion of ions both into and out of the the collection region is consistently accounted for, in effect taking momentum and particle diffusivity to be equal. The results differ by significant factors from previous analyses, which did not account for outward diffusion but in effect set momentum diffusivity to zero. The differences are especially large when parallel flow of the external plasma is present. It is thus clear that the value assumed for the momentum diffusivity strongly affects the interpretation of recent probe measurements. It is argued that the present results offer a more reliable basis for this interpretation.
Patent•
10 Mar 1987
TL;DR: In this article, the authors described a radio frequency magnetic field with a magnetic field direction substantially perpendicular to both the longitudinal axis and to the static field direction for nuclear magnetic resonance sensing.
Abstract: Nuclear magnetic resonance sensing apparatus includes one or more magnets (10) operative to generate a static magnets (10) define a longitudinal axis (8) the static magnetic field in a region (9) remote therefrom containing materials sought to be analyzed. The one or more magnetic field having a static field direction (12) substantially perpendicular to the longitudinal axis and having generally uniform amplitude along the azimuth with respect to the longitudinal axis in the remote region (9). Apparatus (16) generates a radio frequency magnetic field in the remote region (9) for exciting nuclei of the material sought to be analyzed and has a radio frequency magnetic field direction substantially perpendicular to both the longitudinal axis (8) and to the static field direction (12). Apparatus (26) receives nuclear magnetic resonance signals from the excited nuclei and provides an output indication of the properties of the material sought to be analyzed. Methods for nuclear magnetic resonance sensing are also described and claimed.
TL;DR: It is shown that in ''heavy-fermion'' superconductors a new vortex state can occur characterized by the existence of half magnetic flux quanta.
Abstract: It is shown that in ``heavy-fermion'' superconductors a new vortex state can occur characterized by the existence of half magnetic flux quanta. Vortices in polycrystals should exist even in the absence of an externally applied magnetic field. The internal structure of the vortices is also investigated.
TL;DR: A quantitative theory is developed which shows that each classical closed electron orbit which begins and ends near the nucleus contributes an oscillatory term to the average oscillator strength.
Abstract: Measurements of the absorption spectrum near the ionization threshold for an atom in a strong magnetic field showed that the spectrum is a superposition of many oscillatory terms (``quasi-Landau oscillations''). We have developed a quantitative theory which shows that each classical closed electron orbit which begins and ends near the nucleus contributes an oscillatory term to the average oscillator strength. The theory gives new understanding of the behavior under combined Coulomb and Lorentz forces, and it elucidates the roles of isolated closed orbits in chaotic systems. The first results of this theory are shown to be in good agreement with experimental results.
TL;DR: In this paper, it was found that there is a strong correlation between the electron properties and the sector structure of the magnetic field, and that anomalous scattering increases significantly toward sector boundaries for all energies up to several hundred eV.
Abstract: Variations of electron distribution functions in the solar wind have been investigated using the electron data observed aboard Helios 2 during the first four months of its mission in 1976 in the distance range between 0.3 and 1 AU. In particular, variations across the sector structure of the interplanetary magnetic field and across the plasma stream structures have been studied. It has been found that there is a strong correlation between the electron properties and the sector structure of the magnetic field. Within the interior of magnetic sectors the electron distribution functions are extremely anisotropic and skewed with respect to the magnetic field direction at high particle energies. Toward sector boundaries the electron distribution functions become less anisotropic and less skewed. Right at sector boundaries the electrons are relatively cool, and their distributions are nearly isotropic often showing a slight bidirectional anisotropy. These observations have been interpreted to indicate that scattering of electrons with energies above 100 eV is weak within the interior of magnetic sectors but anomalous scattering increases drastically toward sector boundaries for all energies in the halo regime up to several hundred eV. There is evidence for closed magnetic field structures, probably occurring frequently in the solar wind, where the magnetic field lines should usually be connected to the sun outside sector boundaries but may or may not be disconnected from the sun at sector boundaries.
TL;DR: In this article, the global electrodynamics of a viscous resistive accretion disk around a Schwarzschild black hole with a force-free plasma outside of the disk is worked out, and a global condition is derived which imposes an upper bound on the reaction of the accretion luminosity which can be carried away by the jets.
Abstract: The global electrodynamics of a viscous resistive accretion disk around a Schwarzschild black hole with a force-free plasma outside of the disk is worked out The magnetic field in the disk is assumed to include a well-ordered component The magnetic field and fluid dynamics of the disk are tested, simplifying the induction equation and solving for the flux function and the toroidal magnetic field A Green's function method is used to obtain far-field solutions of the basic electromagnetic field equation for the plasma outside of the disk The solutions are found to include self-collimated electromagnetic jets The overall energy conservation for the disk-jet system is considered, and a global condition is derived which imposes an upper bound on the reaction of the accretion luminosity which can be carried away by the jets 36 references
TL;DR: In this paper, a moment solution is presented for the problem of transverse magnetic scattering from homogeneous dielectric cylinders, which uses fictitious filamentary currents to simulate both the field scattered by the cylinder and the field inside the cylinder, and in turn point-match the continuity conditions for the tangential components of the electric and magnetic fields across the cylinder surface.
Abstract: A moment solution is presented for the problem of transverse magnetic (TM) scattering from homogeneous dielectric cylinders. The moment solution uses fictitious filamentary currents to simulate both the field scattered by the cylinder and the field inside the cylinder and in turn point-matches the continuity conditions for the tangential components of the electric and magnetic fields across the cylinder surface. The procedure is simple to execute and is general in that cylinders of arbitrary shape and complex permittivity can be handled effectively. Metallic cylinders are treated as reduced cases of the general procedure. Results are given and compared with available analytic solutions, which demonstrate the very good performance of the procedure.
15 Feb 1987
TL;DR: In this paper, the idea of a molecular property surface in which a property (such as the dipole moment) is expressed as a function of nuclear coordinates R I is extended to cover properties odd under time-reversal by including the nuclear velocities ·R I as variables.
Abstract: The familiar idea of a molecular property surface in which a property (such as the dipole moment) is expressed as a function of nuclear coordinates R I , is extended to cover properties odd under time-reversal by including the nuclear velocities ·R I as variables. One such property is the magnetic dipole moment m . This approach relates magnetic dipole vibrational transition moments to derivatives (∂ m / ∂ ·R I ) which may be calculated within the clamped-nucleus approximation, and used in the theory of vibrational circular dichroism. The same derivatives describe the electric field at a nucleus induced by an external dynamic magnetic field, leading to a physically transparent description of VCD. Sum rules relate these derivatives to the molecular paramagnetizability.
TL;DR: Effets des instabilites dynamiques et resistives sur la redistribution magnetique du moment angulaire a l'interieur d'une etoile.
Abstract: Effets des instabilites dynamiques et resistives sur la redistribution magnetique du moment angulaire a l'interieur d'une etoile
TL;DR: Aspherical harmonic model of the magnetic field of Uranus was obtained from the Voyager 2 encounter observations using generalized inverse techniques which allow partial solutions to complex (underdetermined) problems as mentioned in this paper.
Abstract: Aspherical harmonic model of the planetary magnetic field of Uranus is obtained from the Voyager 2 encounter observations using generalized inverse techniques which allow partial solutions to complex (underdetermined) problems. The Goddard Space Flight Center 'Q3' model is characterized by a large dipole tilt (58.6 deg) relative to the rotation axis, a dipole moment of 0.228 G R(Uranus radii cubed) and an unusually large quadrupole moment. Characteristics of this complex model magnetic field are illustrated using contour maps of the field on the planet's surface and discussed in the context of possible dynamo generation in the relatively poorly conducting 'ice' mantle.
Patent•
23 Sep 1987TL;DR: In this paper, the magnetic field intensity was selected to be in the range from 25 gausses to 35 gusses, and the intensity of the applied magnetic field was set to be 1.5 times or more the applied RF power, which caused electron cyclotron resonance to occur at the frequency f of the RF power.
Abstract: A plasma processing apparatus performs various plasma processings of a substrate having a large area in a semiconductor element manufacturing process, by using highly excited plasma generated at a low pressure under the application of RF power and a magnetic field. In this plasma processing apparatus, a gas is introduced into a vacuum chamber to be used as an ion source, RF power is applied to two electrodes having respective surfaces opposite to each other through the gas to thereby generate the plasma in the vacuum chamber, and a magnetic field is applied to the plasma from a magnetic field source arranged at a predetermined position. The intensity of the applied magnetic field is set to be 1.5 times or more the magnetic field intensity which causes electron cyclotron resonance to occur at the frequency f of the applied RF power. Particularly, when the frequency f of the RF power is 13.56 MHz, the magnetic field intensity is selected to be in the range from 25 gausses to 35 gausses.
TL;DR: In this paper, a double-barrier resonant tunneling structure is proposed to describe the experimental currentvoltage curves and the space-charge formation, both in the electrodes and in the well.
Abstract: The standard model of double-barrier resonant tunneling structures inadequately describes the experimental current-voltage curves. When applied to experiments on resonant tunneling in quantizing magnetic fields, it leads to incorrect values of the electron effective mass in the well. We show that the space-charge formation, both in the electrodes and in the well, not considered previously, is, in fact, very important. We present a model which takes into account these effects. The calculations, based on our model, are in good agreement with the experimental results.
TL;DR: An efficient and systematic treatment of classical and neoclassical transport in all regimes of collisionality is formulated that permits toroidal rotation speeds on the order of the ion thermal speed for arbitrary aspect ratio, cross section, and poloidal magnetic field strength as mentioned in this paper.
Abstract: An efficient and systematic treatment of classical and neoclassical transport in all regimes of collisionality is formulated that permits toroidal rotation speeds on the order of the ion thermal speed for arbitrary aspect ratio, cross section, and poloidal magnetic field strength. A more convenient, but somewhat unconventional, form of the reduced kinetic equation is derived that is shown to extend the previous form by properly retaining electric field modifications. The generalized kinetic description is exploited to evaluate explicitly the radial fluxes of toroidal angular momentum and energy in a pure plasma via a variational formulation. The specific results obtained in the Pfirsch–Schluter regime are substantially more general than previous evaluations; also, significant improvements are made in the banana regime.