Showing papers on "Magnetic dipole published in 2008"
TL;DR: This work proposes that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles, which would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field.
Abstract: We are familiar with elementary particles that carry either negative or positive electric charge, such as electrons and protons, but there is no evidence of elementary particles with a net magnetic charge. Magnets tend to come with inseparable north and south poles, and there are no known magnetic monopoles despite concerted efforts to find them. But an intriguing theoretical study now proposes that magnetic monopoles may exist, not as elementary particles, but as emergent particles in exotic condensed matter magnetic systems such as 'spin ice'. The theory, based on an analogy to fractional electric charges seen, for example, in quantum Hall systems in two dimensions, can explain a mysterious phase transition that has been observed experimentally in spin ice. The cover, by Alessandro Canossa, depicts a magnetic monopole (red sphere) emerging from break-up of the dipole moment (arrows) of the underlying electronic degrees of freedom in spin ice. A theoretical study proposes that magnetic monopoles may appear not as elementary but as emergent particles in complex, strongly-correlated magnetic systems such as spin ice, in analogy to fractional electric charges in quantum Hall systems. This theory explains a mysterious phase transition in spin ice that has been observed experimentally. Electrically charged particles, such as the electron, are ubiquitous. In contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches (see ref. 1 for example). We pursue an alternative strategy, namely that of realizing them not as elementary but rather as emergent particles—that is, as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics2. Here we propose that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice3,4,5: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field6,7, which is a liquid–gas transition of the magnetic monopoles. These monopoles can also be detected by other means, for example, in an experiment modelled after the Stanford magnetic monopole search8.
1,225 citations
TL;DR: It is proposed that copper(II) oxides (containing Cu2+ ions) having large magnetic superexchange interactions can be good candidates for induced-multiferroics with high Curie temperature (T(C), and ferroelectricity is demonstrated with T(C)=230 K in cupric oxide, CuO (tenorite), which is known as a starting material for the synthesis of high-T(c) (critical temperature) superconductors.
Abstract: Induced multiferroics, where ferroelectricity arises through the magnetic order, have attracted significant interest, despite maximum Curie temperatures of only 40 K. The discovery of multiferroic coupling up to 230 K in CuO therefore represents a major advance towards high-TC multiferroics. Materials that combine coupled electric and magnetic dipole order are termed ‘magnetoelectric multiferroics’1,2,3,4. In the past few years, a new class of such materials, ‘induced-multiferroics’, has been discovered5,6, wherein non-collinear spiral magnetic order breaks inversion symmetry, thus inducing ferroelectricity7,8,9. Spiral magnetic order often arises from the existence of competing magnetic interactions that reduce the ordering temperature of a more conventional collinear phase10. Hence, spiral-phase-induced ferroelectricity tends to exist only at temperatures lower than ∼40 K. Here, we propose that copper(II) oxides (containing Cu2+ ions) having large magnetic superexchange interactions11 can be good candidates for induced-multiferroics with high Curie temperature (TC). In fact, we demonstrate ferroelectricity with TC=230 K in cupric oxide, CuO (tenorite), which is known as a starting material for the synthesis of high-Tc (critical temperature) superconductors. Our result provides an important contribution to the search for high-temperature magnetoelectric multiferroics.
417 citations
TL;DR: In this paper, the magnetic anisotropy has been shown to play a decisive role in the physics of Kondo screening, and it was shown that a Kondo resonance emerges for large-spin atoms only when the magnetic aisotropic effect creates degenerate ground-state levels that are connected by the spin flip of a screening electron.
Abstract: Localized magnetic moments on surfaces can be screened through the Kondo effect by forming a correlated system with the surrounding conduction electrons. Measurements now show that the orientation of the magnetic moment’s spin relative to the surface has a decisive role in the physics of Kondo screening. In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host1. Spin S=1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin2. Larger spins are subject to the influence of magnetocrystalline anisotropy, which describes the dependence of the magnetic moment’s energy on the orientation of the spin relative to its surrounding atomic environment3,4. Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.
268 citations
TL;DR: In this article, an extended bipolar active region embedded in an oppositely directed background dipole field has all the necessary topological features required for magnetic breakout, i.e., a fan separatrix surface between the two distinct flux systems, a pair of spine fieldlines, and a true 3-dimensional coronal null point at their intersection.
Abstract: W present the extension of the magnetic breakout model for CME initiation to a fully 3-dimensional, spherical geometry. Given the increased complexity of the dynamic magnetic field interactions in 3-dimensions, we first present a summary of the well known axisymmetric breakout scenario in terms of the topological evolution associated with the various phases of the eruptive process. In this context, we discuss the completely analogous topological evolution during the magnetic breakout CME initiation process in the simplest 3-dimensional multipolar system. We show that an extended bipolar active region embedded in an oppositely directed background dipole field has all the necessary topological features required for magnetic breakout, i.e. a fan separatrix surface between the two distinct flux systems, a pair of spine fieldlines, and a true 3-dimensional coronal null point at their intersection. We then present the results of a numerical MHD simulation of this 3-dimensional system where boundary shearing flows introduce free magnetic energy, eventually leading to a fast magnetic breakout CME. The eruptive flare reconnection facilitates the rapid conversion of this stored free magnetic energy into kinetic energy and the associated acceleration causes the erupting field and plasma structure to reach an asymptotic eruption velocity of greater than or approx. equal to 1100 km/s over an approx.15 minute time period. The simulation results are discussed using the topological insight developed to interpret the various phases of the eruption and the complex, dynamic, and interacting magnetic field structures.
263 citations
TL;DR: In this paper, the authors examined the additional dynamical eects of stellar rotation in the (still) 2-D axisymmetric case of an aligned dipole surface eld In addition to the magnetic connement parameter introduced in Paper I, the stellar rotation was characterized in terms of a parameter W Vrot=Vorb (the ratio of the equatorial surface rotation speed to orbital speed).
Abstract: Building upon our previous MHD simulation study of magnetic channeling in radiatively driven stellar winds, we examine here the additional dynamical eects of stellar rotation in the (still) 2-D axisymmetric case of an aligned dipole surface eld In addition to the magnetic connement parameter introduced in Paper I, we characterize the stellar rotation in terms of a parameter W Vrot=Vorb (the ratio of the equatorial surface rotation speed to orbital speed), examining specically models with moderately strong rotation W = 025 and 05, and comparing these to analogous non-rotating cases Dening the associated Alfv en radius RA 1=4 R and Kepler
227 citations
TL;DR: It is shown that, unlike any other material, the formation of magnetic moments can be controlled by an electric field effect.
Abstract: We examine the conditions necessary for the presence of localized magnetic moments on adatoms with inner shell electrons in graphene. We show that the low density of states at the Dirac point, and the anomalous broadening of the adatom electronic level, lead to the formation of magnetic moments for arbitrarily small local charging energy. As a result, we obtain an anomalous scaling of the boundary separating magnetic and nonmagnetic states. We show that, unlike any other material, the formation of magnetic moments can be controlled by an electric field effect.
213 citations
TL;DR: During its first flyby of Mercury, the MESSENGER spacecraft measured the planet's near-equatorial magnetic field, consistent to within an estimated uncertainty of 10% with that observed near the equator by Mariner 10.
Abstract: During its first flyby of Mercury, the MESSENGER spacecraft measured the planet's near-equatorial magnetic field. The field strength is consistent to within an estimated uncertainty of 10% with that observed near the equator by Mariner 10. Centered dipole solutions yield a southward planetary moment of 230 to 290 nanotesla RM3 (where RM is Mercury's mean radius) tilted between 5 degrees and 12 degrees from the rotation axis. Multipole solutions yield non-dipolar contributions of 22% to 52% of the dipole field magnitude. Magnetopause and tail currents account for part of the high-order field, and plasma pressure effects may explain the remainder, so that a pure centered dipole cannot be ruled out.
193 citations
TL;DR: The significance of magnetic dipole interactions in degenerate 87Rb F=1 spinor gases is demonstrated and the reduction of dipolar interactions by a sequence of rf pulses results in a suppression of the modulated phase, thereby confirming the role of dipolars in this process.
Abstract: Helical spin textures in a 87Rb F=1 spinor Bose-Einstein condensate are found to decay spontaneously toward a spatially modulated structure of spin domains. The formation of this modulated phase is ascribed to magnetic dipolar interactions that energetically favor the short-wavelength domains over the long-wavelength spin helix. The reduction of dipolar interactions by a sequence of rf pulses results in a suppression of the modulated phase, thereby confirming the role of dipolar interactions in this process. This study demonstrates the significance of magnetic dipole interactions in degenerate 87Rb F=1 spinor gases.
181 citations
TL;DR: The physical concept and performance analysis of all-dielectric metamaterials are presented in this article, where a full wave finite difference time domain technique is applied to comprehensively obtain the physical insights of dielectric resonators.
Abstract: In this paper, physical concept and performance analysis of all-dielectric metamaterials are presented. Metamaterials with desired material parameters $(\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},\ifmmode\pm\else\textpm\fi{}\ensuremath{\mu})$ are developed by creating electric and magnetic resonant modes. Dielectric disk and spherical particle resonators are considered as the great candidates for establishing the dipole moments (metamaterial alphabet). A full wave finite difference time domain technique is applied to comprehensively obtain the physical insights of dielectric resonators. Near-field patterns are plotted to illustrate the development of electric and magnetic dipole fields. Geometric-polarization control of the dipole moments allows $\ensuremath{\epsilon}$ and $\ensuremath{\mu}$ to be tailored to the application of interest. All-dielectric double negative metamaterials are designed. Engineering concerns, such as loss reduction and bandwidth enhancement are investigated.
174 citations
TL;DR: In this article, a formalism was developed to model magnetically deformed stars, using both realistic equations of state and field configurations, and the internal magnetic field was determined consistent with a given neutron star model.
Abstract: Rotating deformed neutron stars are important potential sources for ground-based gravitational wave interferometers such as LIGO, GEO600 and VIRGO. One mechanism that may lead to significant non-asymmetries is the internal magnetic field. It is well known that a magnetic star will not be spherical and, if the magnetic axis is not aligned with the spin axis, the deformation will lead to the emission of gravitational waves. The aim of this paper is to develop a formalism that would allow us to model magnetically deformed stars, using both realistic equations of state and field configurations. As a first step, we consider a set of simplified model problems. Focusing on dipolar fields, we determine the internal magnetic field which is consistent with a given neutron star model and calculate the associated deformation. We discuss the relevance of our results for current gravitational wave detectors and future prospects.
154 citations
TL;DR: In this article, the authors investigated the three-dimensional structure of the pulsar magnetosphere through time-dependent numerical simulations of a magnetic dipole that is set in rotation, and developed their own Eulerian finite difference time domain numerical solver of force-free electrodynamics and implemented the technique of nonreflecting and absorbing outer boundaries.
Abstract: We investigate the three-dimensional structure of the pulsar magnetosphere through time-dependent numerical simulations of a magnetic dipole that is set in rotation. We developed our own Eulerian finite difference time domain numerical solver of force-free electrodynamics and implemented the technique of non-reflecting and absorbing outer boundaries. This allows us to run our simulations for many stellar rotations, and thus claim with confidence that we have reached a steady state. A quasi-stationary corotating pattern is established, in agreement with previous numerical solutions. We discuss the prospects of our code for future high-resolution investigations of dissipation, particle acceleration, and temporal variability.
TL;DR: In this paper, the authors re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star.
Abstract: Aims. We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods. New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipole field enters the resistive accretion disc, whose structure is self-consistently computed. Results. The analytical criterion derived allows to predict a priori the position of the truncation radius from a non perturbative accretion disc model. Accretion funnels are found to be robust features which occur below the co-rotation radius, where the stellar poloidal magnetic pressure becomes both at equipartition with the disc thermal pressure and is comparable to the disc poloidal ram pressure. We confirm the results of Romanova et al. (2002, ApJ, 578, 420) and find accretion funnels for stellar dipole fields as low as 140 G in the low accretion rate limit of 10(-9) M-circle dot yr(-1). With our present numerical setup with no disc magnetic field, we found no evidence of winds, neither disc driven nor X-winds, and the star is only spun up by its interaction with the disc. Conclusions. Weak dipole fields, similar in magnitude to those observed, lead to the development of accretion funnel flows in weakly accreting T Tauri stars. However, the higher accretion observed for most T Tauri stars ((M)overdot similar to 10(-8) M-circle dot yr(-1)) requires either larger stellar field strength and/or different magnetic topologies to allow for magnetospheric accretion.
04 May 2008
TL;DR: In this paper, the contribution of electric quadrupole resonance in optical metamaterials through numerical simulation is studied, and the individual contributions can be determined by angle-resolved scattering spectroscopy.
Abstract: Contribution of electric quadrupole resonance is studied in optical metamaterials through numerical simulation. For nanostructures, its radiation is often comparable to that from magnetic dipole. Their individual contributions can be determined by angle-resolved scattering spectroscopy.
TL;DR: In this paper, the formation of stable equilibria from turbulent initial conditions is studied. But the authors focus on the conditions under which more complex, non-axisymmetric configurations can form.
Abstract: In previous work, stable approximately axisymmetric equilibrium configurations for magnetic stars were found by numerical simulation. Here, I investigate the conditions under which more complex, non-axisymmetric configurations can form. I present numerical simulations of the formation of stable equilibria from turbulent initial conditions and demonstrate the existence of non-axisymmetric equilibria consisting of twisted flux tubes lying horizontally below the surface of the star, meandering around the star in random patterns. Whether such a non-axisymmetric equilibrium or a simple axisymmetric equilibrium forms depends on the radial profile of the strength of the initial magnetic field. The results could explain observations of non-dipolar fields on stars such as the B0.2 main-sequence star r Sco or the pulsar 1E 1207.4-5209. The secular evolution of these equilibria due to Ohmic and buoyancy processes is also examined.
TL;DR: In this article, the authors studied the magnetic excitations of a square lattice spin-ice recently produced in an artificial form, as an array of nanoscale magnets, and found magnetic monopole-like excitations effectively interacting by means of the usual Coulombic plus a linear confining potential.
Abstract: We study the magnetic excitations of a square lattice spin-ice recently produced in an artificial form, as an array of nanoscale magnets. Our analysis, based upon the dipolar interaction between the nanomagnetic islands, correctly reproduces the ground-state observed experimentally. In addition, we find magnetic monopole-like excitations effectively interacting by means of the usual Coulombic plus a linear confining potential, the latter being related to a string-like excitation binding the monopoles pairs, what indicates that the fractionalization of magnetic dipoles may not be so easy in two dimensions. These findings contrast this material with the three-dimensional analogue, where such monopoles experience only the Coulombic interaction. We discuss, however, two entropic effects that affect the monopole interactions: firstly, the string configurational entropy may loose the string tension and then, free magnetic monopoles should also be found in lower dimensional spin ices; secondly, in contrast to the string configurational entropy, an entropically driven Coulomb force, which increases with temperature, has the opposite effect of confining the magnetic defects.
TL;DR: In this article, a double-lined spectroscopic binary system, HD 200775, was studied to determine if its magnetic topology is similar to that of the main sequence magnetic stars.
Abstract: The origin of the magnetic fields observed in some intermedia te mass and high mass main sequence stars is still a matter of vigorous debate. The favo ured hypothesis is a fossil field origin, in which the observed fields are the condensed remnan ts of magnetic fields present in the original molecular cloud from which the stars formed. According to this theory a few percent of the PMS Herbig Ae/Be star should be magnetic with a magnetic topology similar to that of main sequence intermediate-mass stars. After our recent discovery of four magnetic Herbig stars, we have decided to study in detail one of them, HD 200775, to determine if its magnetic topology is similar to that of the main sequence magnetic stars. With this aim, we monitored this star in Stokes I and V over more than two years, using the new spectropolarimeters ESPaDOnS at CFHT, and Narval at TBL. By analysing the intensity spectrum we find that HD 200775 is a double-lined spectroscopic binary system, whose secondary seems similar, in temperature, to the primary. We have carefully compared the observed spectrum to a synthetic one, and we found no evidence of abundance anomalies in its spectrum. We infer the luminosity ratio of the components from the Stokes I profiles. Then, using the temperature and luminosity of HD 20 0775 found in the literature, we estimate the age, the mass and the radius of both components from their HR diagram positions. From our measurements of the radial velocities of both stars we determine the ephemeris and the orbital parameters of the system. A Stokes V Zeeman signature is clearly visible in most of the Least Square Deconvolution profiles and varies on a timescale on the order of one day. We ha ve fitted the 30 profiles simultaneously, using a χ 2 minimisation method, with a centered and a decentered-dipole model. The best-fit model is obtained with a reducedχ 2 = 1.0 and provides a rotation period of 4.3281±0.0010 d, an inclination angle of 60±11 ◦ , and a magnetic obliquity angleβ = 125±8 ◦ . The polar strength of the magnetic dipole field is 1000 ± 150 G, which is decentered by 0.05± 0.04 R∗ from the center of the star. The derived magnetic field model i s qualitatively identical to those commonly observed in the Ap/Bp stars. Our determination of the inclination of the rotation axis le ads to a radius of the primary which is smaller than that derived from the HR diagram position. This can be explained by a larger intrinsic luminosity of the secondary relative to th e primary, due to a larger circumstellar extinction of the secondary relative to the primary.
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17 Sep 2008TL;DR: In this article, it is made possible to restrict the variation of the oscillation frequency of a spin torque oscillator placed in the vicinity of the recording magnetic pole by applying a magnetic field to the oscillator.
Abstract: It is made possible to restrict the variation of the oscillation frequency of a spin torque oscillator placed in the vicinity of the recording magnetic pole. A magnetic head includes: a recording magnetic pole to generate a recording magnetic field; a spin torque oscillator formed in the vicinity of the recording magnetic pole; and a magnetic field applying unit configured to apply a magnetic field to the spin torque oscillator. The magnetic field applied to the spin torque oscillator by the magnetic field applying unit is perpendicular to a recording magnetic field generated from the recording magnetic pole.
TL;DR: An analytical description for plane-wave propagation in metamaterials is presented in this article, which follows the usual approach for describing light propagation in homogeneous media on the basis of Maxwell's equations, although applied to a medium composed of metallic nanostructures.
Abstract: An analytical description for plane-wave propagation in metamaterials is presented. It follows the usual approach for describing light propagation in homogeneous media on the basis of Maxwell's equations, although applied to a medium composed of metallic nanostructures. Here, as an example, these nanostructures are double (or cut) wires. In the present approach it is assumed that the carriers perform collective oscillations in a single wire. These oscillations are coupled to those in the adjacent wire; thus, the internal carrier dynamics may be described by a coupled-oscillator model. The multipole expansion technique is used to account for the electric and magnetic dipole as well as the electric quadrupole moments of these carrier oscillations within the nanostructure. It turns out that the symmetric normal mode is related to the electric dipole moment whereas the antisymmetric normal mode evokes simultaneously a magnetic dipole and an electric quadrupole moment. It is shown how effective permittivity and permeability can be derived from analytical expressions for the dispersion relation, the magnetization, and the electric displacement field. The results of the analytical model are compared with rigorous simulations of Maxwell's equations yielding the limitations and the domain of applicability of the proposed model.
TL;DR: In this paper, a phenomenological approach based on the Onsager reciprocity principle is developed, which allows us to capture the effect of spin dephasing and make a connection to the well studied problem of current-driven magnetic dynamics.
Abstract: Spin-polarized electron transport driven by inhomogeneous magnetic dynamics is discussed in the limit of a large exchange coupling. Electron spins rigidly following the time-dependent magnetic profile experience spin-dependent fictitious electric and magnetic fields. We show that the electric field acquires important corrections due to spin dephasing, when one relaxes the spin-projection approximation. Furthermore, spin-flip scattering between the spin bands needs to be taken into account in order to calculate voltages and spin accumulations induced by the magnetic dynamics. A phenomenological approach based on the Onsager reciprocity principle is developed, which allows us to capture the effect of spin dephasing and make a connection to the well studied problem of current-driven magnetic dynamics. A number of results that recently appeared in the literature are related and generalized.
TL;DR: Wiese et al. as mentioned in this paper presented the first in a series of updates to a critical compilation published in 1969 [W. L. Wiese, M. W. Smith, and B. M. Miles, Atomic Transition Probabilities, Vol. II: Sodium through Calcium, NSRDS-NBS Vol. 2 (U.S. GPO, Washington, D.C., 1969).
Abstract: This compilation is the first in a series of updates to a critical compilation published in 1969 [W. L. Wiese, M. W. Smith, and B. M. Miles, Atomic Transition Probabilities, Vol. II: Sodium through Calcium, NSRDS-NBS Vol. 2 (U.S. GPO, Washington, D.C., 1969)]. Atomic transition probabilities have been critically evaluated and compiled for about 11 400 spectral lines of sodium and magnesium (nuclear charge Z=11–12, respectively). The cited values and their estimated uncertainties are based on our consideration of all available theoretical and experimental literature sources. All ionization stages (except for hydrogenic) are covered, and the data are presented in separate tables for each atom and ion. Separate listings are given for “allowed” (electric dipole) transitions, on the one hand, and for “forbidden” (magnetic dipole plus electric and magnetic quadrupole) transitions, on the other. In each spectrum, lines are grouped into multiplets which are arranged in order of ascending lower and upper-level ene...
TL;DR: In this paper, an analytical technique is proposed to describe the magnetophoretic transport of magnetic microspheres under an imposed magnetic field when there is a pressure-driven or electroosmotic flow through a microchannel.
TL;DR: In this paper, the authors investigated the influence of the quadrupole component on the shape of the hotspots on rotating stars with complex magnetic fields using full 3D magnetohydrodynamic (MHD) simulations.
Abstract: Disc accretion to rotating stars with complex magnetic fields is investigated using full 3D magnetohydrodynamic (MHD) simulations. The studied magnetic configurations include superpositions of misaligned dipole and quadrupole fields and off-centre dipoles. The simulations show that when the quadrupole component is comparable to the dipole component, the magnetic field has a complex structure with three major magnetic poles on the surface of the star and three sets of loops of field lines connecting them. A significant amount of matter flows to the quadrupole 'belt', forming a ring-like hotspot on the star. If the maximum strength of the magnetic field on the star is fixed, then we observe that the mass accretion rate, the torque on the star and the area covered by hotspots are several times smaller in the quadrupole-dominant cases than in the pure dipole cases. The influence of the quadrupole component on the shape of the hotspots becomes noticeable when the ratio of the quadrupole and dipole field strengths Bq /B d ≥ 0.5. It becomes dominant in determining the shape of the hotspots when Bq /B d ≥ 1. We conclude that if the quadrupole component is larger than the dipole one, then the shape of the hotspots is determined by the quadrupole field component. In the case of an off-centre dipole field, most of the matter flows through a one-armed accretion stream, forming a large hotspot on the surface, with a second much smaller secondary spot. The light curves may have simple, sinusoidal shapes, thus mimicking stars with pure dipole fields. Or, they may be complex and unusual. In some cases, the light curves may be indicators of a complex field, in particular if the inclination angle is known independently. We also note that in the case of complex fields, magnetospheric gaps are often not empty, and this may be important for the survival of close-in exosolar planets.
01 Sep 2008
TL;DR: In this article, the authors analyzed more than 1000 HST/Advanced Camera for Survey images of the ultraviolet auroral emissions appearing in the northern hemisphere of Jupiter and found that the VIP4 magnetic field model is in relatively good agreement with the observed footprint of Io.
Abstract: [1] We analyze more than 1000 HST/Advanced Camera for Survey images of the ultraviolet auroral emissions appearing in the northern hemisphere of Jupiter. The auroral footprints of Io, Europa, and Ganymede form individual footpaths, which are fitted with three reference contours. The satellite footprints provide a convenient mapping between the northern Jovian ionosphere and the equatorial plane in the middle magnetosphere, independent of any magnetic field model. The VIP4 magnetic field model is in relatively good agreement with the observed footprint of Io. However, in the auroral kink sector, between the 80° and 150° System III meridians, the model significantly departs from the observation. One possible way to improve the agreement between the VIP4 model and the observed footprints is to include a magnetic anomaly. We suggest that this anomaly is characterized by a weakening of the surface magnetic field in the kink sector and by an added localized tilted dipole field. This dipole rotates with the planet at a depth of 0.245 RJ below the surface, and its magnitude is set to ∼1% of Jupiter's dipole moment. The anomaly has a very limited influence on the magnetic field intensity in the equatorial plane between the orbits of Io and Ganymede. However, it is sufficient to bend the field lines near the high-latitude atmosphere and to reproduce the observed satellite ultraviolet footpaths. JUNO's in situ measurements will determine the structure of Jupiter's magnetic field in detail to expand on these results.
TL;DR: The effective fragment potential (EFP) method is used to solvate an ab initio water molecule to predict the dipole moments for various cluster sizes and localized charge distributions are used in conjunction with localized molecular orbitals to interpret the Dipole moment enhancement.
Abstract: The dipole moment of the gas phase water monomer is 1.85 D. When solvated in bulk water, the dipole moment of an individual water molecule is observed to be enhanced to the much larger value of 2.9 ± 0.6 D. To understand the origin of this dipole moment enhancement, the effective fragment potential (EFP) method is used to solvate an ab initio water molecule to predict the dipole moments for various cluster sizes. The dipole moment as a function of cluster size, nH2O, is investigated [for n = 6–20 (even n), 26, 32, 41, and 50]. Localized charge distributions are used in conjunction with localized molecular orbitals to interpret the dipole moment enhancement. These calculations suggest that the enhancement of the dipole moment originates from the decrease of the angle between the dipole vectors of the lone pairs on oxygen as the number of hydrogen bonds to that oxygen increases. Thus, the decreased angle, and the consequent increase in water dipole moment, is most likely to occur in environments with a larg...
TL;DR: Calculations based on the Mie theory are performed to determine the locally enhanced electric fields due to whispering-gallery mode resonances for dielectric microspheres, with emphasis on electromagnetic "hot spots" that are located along the wavevector direction on the surface of the sphere.
Abstract: Calculations based on the Mie theory are performed to determine the locally enhanced electric fields due to whispering-gallery mode resonances for dielectric microspheres, with emphasis on electromagnetic "hot spots" that are located along the wavevector direction on the surface of the sphere. The local electric field enhancement associated with these hot spots is used to determine the surface enhanced Raman scattering enhancement factors for a molecule, here treated as a classical dipole, located near the surface of the sphere. Both incident and Raman emission enhancements are calculated accurately using an extension of the Mie theory that includes interaction of the Raman dipole field with the sphere. The enhancement factors are calculated for dielectric spheres in vacuum with a refractive index of 1.9 and radii of 5, 10, and 20 microm and for wavelengths that span the visible spectrum. Maximum Raman scattering enhancement factors on the order of 10(3)-10(4) are found at locations slightly off the propagation axis when the incident excitation but not the Stokes-shifted radiation is coincident with a whispering-gallery mode resonance. The enhancement factors are found to vary inversely with the resonance width, and this determines the influence of the mode number and order on the results. Additional calculations are performed for the case where the Stokes-shifted radiation is also on-resonance and Raman enhancement factors as large as 10(8) are found. These enhancement factors are typically a factor of 10(2) smaller than would be obtained from /E/4 enhancement estimates, as enhancement of the Raman dipole emission is significantly reduced compared to the local field enhancement for micron size particles or larger. Conditions under which single-molecule or few-molecule measurements are feasible are identified.
TL;DR: In this article, a kind of magneto-electric (ME) optics has been observed in non-centrosymmetric ferromagnetic materials, which can be explained from the microscopic point of view by the interference of an electric dipole process with a magnetic dipole (M1) or electric quadrupole (E2) process.
Abstract: Non-centrosymmetric ferromagnetic materials can show optical and x-ray responses unique to magneto-electrics For example, the refractive index and absorption coefficient may be dependent on the directions of light (x-ray) propagation k and magnetization M, and these dependences can be termed directional birefringence and directional dichroism, respectively Such a kind of magneto-electric (ME) optics has recently been observed in GaFeO3 and CuB2O4 The ME optics can be explained from the microscopic point of view by the interference of an electric dipole (E1) process with a magnetic dipole (M1) or electric quadrupole (E2) process at non-centrosymmetric magnetic sites Even in magnetic crystals with global inversion symmetry, the E1–E2 interference can occur at magnetic sites with local inversion breaking, causing unique x-ray diffraction Such ME-effect-related x-ray diffraction has been found in GaFeO3 and Fe3O4 Artificial breaking of space inversion by means of currently used techniques such as lithography can be applied to the exploration of multiferroics, as a way of investigating ME optics Relatively large ME optical signals at room temperature have been successfully observed in diffraction from such tailor-made multiferroics
TL;DR: The exact two-body calculation is presented and a procedure to include the higher multipoles arising in a pairwise interaction into a dipole model is described, which can be integrated with any dipole or higher-multipole calculation.
TL;DR: The model indicates that dipolar interaction can be compensated by a proper choice of the scattering length and that the magnetic dipole interaction should not represent an obstacle for atom interferometry with Bose-Einstein condensates with a tunable interaction.
Abstract: We study the role played by the magnetic dipole interaction in the decoherence of a lattice-based interferometer that employs an alkali Bose-Einstein condensate with a tunable scattering length. The different behavior we observe for two different orientations of the dipoles gives us evidence of the anisotropic character of the interaction. The experiment is correctly reproduced by a model we develop only if the long-range interaction between different lattice sites is taken into account. Our model indicates that dipolar interaction can be compensated by a proper choice of the scattering length and that the magnetic dipole interaction should not represent an obstacle for atom interferometry with Bose-Einstein condensates with a tunable interaction.
TL;DR: In this article, the authors measured the microwave signals produced by spin-torque-driven magnetization dynamics excited by direct currents in patterned magnetic multilayer devices at room temperature as a function of the angle of a magnetic field applied in the sample plane.
Abstract: We measure the microwave signals produced by spin-torque-driven magnetization dynamics excited by direct currents in patterned magnetic multilayer devices at room temperature as a function of the angle of a magnetic field applied in the sample plane. We find strong variations in the frequency linewidth of the signals, with a decrease by more than a factor of 20 as the field is rotated from the magnetic easy axis to the in-plane hard axis. Based on micromagnetic simulations, we identify these variations as due to a transition from spatially incoherent to coherent precession.
TL;DR: In this paper, the authors report on the recently developed technique of magnetic-field-dependent small-angle neutron scattering (SANS), with attention to bulk ferromagnets exhibiting random magnetic anisotropy.
Abstract: We report on the recently developed technique of magnetic-field-dependent small-angle neutron scattering (SANS), with attention to bulk ferromagnets exhibiting random magnetic anisotropy. In these materials, the various magnetic anisotropy fields (magnetocrystalline, magnetoelastic, and/or magnetostatic in origin) perturb the perfectly parallel spin alignment of the idealized ferromagnetic state. By varying the applied magnetic field, one can control one of the ordering terms which competes with the above-mentioned perturbing fields. Experiments which explore the ensuing reaction of the magnetization will therefore provide information not only on the field-dependent spin structure but, importantly, on the underlying magnetic interaction terms. This strategy, which underlies conventional studies of hysteresis loops in magnetometry, is here combined with magnetic SANS. While magnetometry generally records only a single scalar quantity, the integral magnetization, SANS provides access to a vastly richer data set, the Fourier spectrum of the response of the spin system as a function of the magnitude and orientation of the wave vector. The required data-analysis procedures have recently been established, and experiments on a number of magnetic materials, mostly nanocrystalline or nanocomposite metals, have been reported. Here, we summarize the theory of magnetic-field-dependent SANS along with the underlying description of random anisotropy magnets by micromagnetic theory. We review experiments which have explored the magnetic interaction parameters, the value of the exchange-stiffness constant as well as the Fourier components of the magnetic anisotropy field and of the magnetostatic stray field. A model-independent approach, based on the experimental autocorrelation function of the spin misalignment, provides access to the characteristic length of the spin misalignment. The field dependence of this quantity is in quantitative agreement with the predictions of micromagnetic theory. Experiments on nanocomposite ferromagnets reveal that the jump of the magnetization at internal phase boundaries leads to a significant magnetostatic perturbing field, with an unusual 'clover-leaf' SANS pattern as the experimental signature. Furthermore, SANS experiments have been used to monitor the orientation of magnetic domains as well as the length scale of intradomain spin misalignment.