Magnetic Field Enhancement Research Articles

Strategy for realizing magnetic field enhancement based on diffraction coupling of magnetic plasmon resonances in embedded metamaterials.

We have demonstrated a straightforward strategy to realize magnetic field enhancement through diffraction coupling of magnetic plasmon (MP) resonances by embedding the metamaterials consisting of a planar rectangular array of U-shaped metallic split-ring resonators (SRRs) into the substrate. Our method provides a more homogeneous dielectric background allowing stronger diffraction coupling of MP resonances among SRRs leading to strong suppression of the radiative damping. We observe that compared to the on-substrate metamaterials, the embedded ones lead to a narrow-band hybridized MP mode, which results from the interference between MP resonances in individual SRRs and an in-plane propagating collective surface mode arising from light diffraction. Associated with the excitation of this hybridized MP mode, a twenty-seven times enhancement of magnetic fields within the inner area of the SRRs is achieved as compared with the pure MP resonance. Moreover, we also found that besides the above requirement of homogeneous dielectric background, only a collective surface mode with its magnetic field of the same direction as the induced magnetic moment in the SRRs could mediate the excitation of such a hybridized MP mode.

ON THE ORIGIN OF HIGHLY ALFVÉNIC SLOW SOLAR WIND

Alfvénic fluctuations are a common feature in the solar wind and are found especially in the trailing edges of fast wind streams. The slow wind usually has a lower degree of Alfvénicity, being more strongly intermixed with structures of non-Alfvénic nature. In the present paper we show the first evidence in the interplanetary space of two different kinds of slow solar wind: one coming from coronal streamers or active regions and characterized by non-Alfvénic structures and the other one being highly Alfvénic and originating from the boundary of coronal holes. The Alfvénic character of fluctuations, either outward or inward, can be studied by means of the normalized cross-helicity, , which is an indicator of the alignment. The evolution of toward lower values with increasing radial distance is interpreted both as a decrease of the presence of the outward modes and as a continuous production of inward modes within those regions such as stream shears where some plasma instability is active. On the other hand, the decrease of is often related also to magnetic field and/or density enhancements which specifically act on the destruction of the alignment. In the present analysis we study the role of compressibility presenting both case studies and a statistical analysis over different phases of solar cycle 23. Our findings indicate that the presence of regions of magnetic field compression generally play a major role in the depletion of and thus in the destruction of the alignment.

Effect of magnetic field enhancement of the photocurrent in ferromagnetic metal-dielectric heterostructures SiO2(Co)/GaAs

Heterostructures of silicon dioxide films containing cobalt nanoparticles SiO2(Co) grown on GaAs substrate exhibit at room temperature high values of magnetic field enhancement of photocurrent in the vicinity and above the GaAs bandgap of ∼1.4 eV. For photon energies E above the GaAs bandgap, the photocurrent significantly increases, while the avalanche process is suppressed by the magnetic field, and the current flowing through the heterostructure decreases. The photocurrent is enhanced in the SiO2(Co 60 at. %)/GaAs heterostructure at the magnetic field H = 1.65 kOe by a factor of about ten for the photon energy E = 1.5 eV. This phenomenon is explained by a model describing electronic transitions in magnetic fields with the spin-dependent recombination process at deep impurity centers in the GaAs interface region.

Magnetic field enhancement of generation-recombination and shot noise in organic light emitting diodes

We have studied the effect of magnetic field on noise in series of 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene-based organic light emitting diodes with dominant hole injection, dominant electron injection, and balanced electron and hole injection. The noise spectra of the balanced devices revealed the generation-recombination (g-r) noise term, which we associated with bimolecular electron-hole recombination. The presence of the g-r noise term is correlated with the strong organic magnetoresistance (up to 25%) observed in the balanced devices. The noise spectra also have the shot noise contribution with the Fano factor 0.25–0.4. We found that time constant of the g-r term decreases and the magnitude of shot noise increases when magnetic field is applied. This behavior can be consistently explained within the polaron-polaron model of organic magnetoresistance. We have not found any evidence that the magnetoresistance in studied devices is affected by traps.

Magnetic Field Induced Enhancement in Thermal Conductivity and Viscosity of Stabilized Vacuum Pump Oil (VPO)—Fe<SUB>3</SUB>O<SUB>4</SUB> Magnetic Nanofluids

In this paper, an experimental investigaitons were made to analyze the thermal conductivity and viscosity of non-polar magnetic nanofluids with and without influence of magnetic field. Magnetic Fe3O4 nanoparticles were synthesized by chemical coprecipitation method and stable nanofluids were prepared by dispersing them into vacuum pump oil using sonicator without adding any surfactant. Both thermal conductivity and viscosity experiments were conducted in the volume concentrations from 0.05% to 1.0% and in the temperatures from 20 � C to 60 � C. Based on the results, without magnetic field, thermal conductivity and viscosity enhancements are 6.20% and 66%, respectively and with magnetic field of 900 G, thermal conductivity enhancement is 69%, with magnetic field of 1300 G, viscosity enhancement is 256% was observed for 1.0% of nanofluid at a temperature of 20 � C. The viscosity enhancement is more compared to thermal conductivity enhancement at same particle concentration and temperature and magnetic field. The chain-like structure of magnetic nanoparticles aggregates under magnetic field caused the enhancement in both thermal conductivity and viscosity.

Characterizing the low‐altitude magnetic belt at Venus: Complementary observations from the Pioneer Venus Orbiter and Venus Express

AbstractUsing Venus Express, Zhang et al. (2012b) identified strong magnetic field enhancements at low altitudes over the north polar region of Venus as giant flux ropes. Strong fields at low altitudes were also observed during the Pioneer Venus Orbiter mission, but at low latitudes near the subsolar and midnight regions. We examine the possibility that the Venus Express observations are not giant flux ropes, but part of a low‐altitude magnetic belt that builds up in the subsolar region, passes over the terminator, and extends to the nightside. Our analysis indicates the magnetic belt is dominantly horizontal over the dayside and gains a radial component nightward. The peak magnetic field strength of the belt and the altitude at which it peaks also varies around the planet, with the lowest altitude and strongest field strength in the subsolar region, consistent with the idea of the belt forming on the dayside. Zhang et al. (2012b) also noted the fields in the polar region had a bias in the +By direction in Venus Solar Orbital coordinates. The multifluid magnetohydrodynamic simulation we present shows an asymmetry in the plasma flow from the subsolar region to the poles due to the oxygen ion and proton mass ratio. This causes the magnetic field to preferentially accumulate in the north for a +By interplanetary magnetic field direction, providing an explanation for this bias.

Magnetic and electric hotspots with silicon nanodimers.

The study of the resonant behavior of silicon nanostructures provides a new route for achieving efficient control of both electric and magnetic components of light. We demonstrate experimentally and numerically that enhancement of localized electric and magnetic fields can be achieved in a silicon nanodimer. For the first time, we experimentally observe hotspots of the magnetic field at visible wavelengths for light polarized across the nanodimer's primary axis, using near-field scanning optical microscopy.

Broken-diabolo nanoantenna for co-enhancing and -confining optical electric and magnetic field

In this letter, we introduce a broken-diabolo nanoantenna aimed at generating electric and magnetic hot spots in the near-infrared range, thus enhancing and confining the electric and magnetic optical field simultaneously. This nanoantenna different from the well-known diabolo nanoantenna and bowtie nanoantenna in that it can accumulate a large charge density at the upper air gap and a high optical current density within the lower central junction, leading to a large electric and magnetic field enhancement near the center of the nanoantenna at the same time. Numerical simulation results on this broken-diabolo nanoantenna show that a 5-fold enhancement of electric field (compare to the electric field enhancement of a diabolo nanoantenna) is achieved at a wavelength of 2353nm, confined to a 40-by-40nm region near the center of the nanoantenna.

Angular momentum transport and particle acceleration during magnetorotational instability in a kinetic accretion disk.

Angular momentum transport and particle acceleration during the magnetorotational instability (MRI) in a collisionless accretion disk are investigated using three-dimensional particle-in-cell simulation. We show that the kinetic MRI can provide not only high-energy particle acceleration but also enhancement of angular momentum transport. We find that the plasma pressure anisotropy inside the channel flow with p(∥)>p(⊥) induced by active magnetic reconnection suppresses the onset of subsequent reconnection, which, in turn, leads to high-magnetic-field saturation and enhancement of the Maxwell stress tensor of angular momentum transport. Meanwhile, during the quiescent stage of reconnection, the plasma isotropization progresses in the channel flow and the anisotropic plasma with p(⊥)>p(∥) due to the dynamo action of MRI outside the channel flow contribute to rapid reconnection and strong particle acceleration. This efficient particle acceleration and enhanced angular momentum transport in a collisionless accretion disk may explain the origin of high-energy particles observed around massive black holes.

Vortex beams with strong longitudinally polarized magnetic field and their generation by using metasurfaces

A novel method of generation and synthesis of azimuthally E-polarized vortex beams is presented. Along the beam axis such beams have a strong longitudinally polarized magnetic field where ideally there is no electric field. We show how these beams can be constructed through the interference of Laguerre–Gaussian beams carrying orbital angular momentum (OAM), and then quantify the longitudinal magnetic field of such beams. As an example, we present a metasurface made of double-split ring slot pairs and report a good agreement between simulated and analytical results. Both a high magnetic-to-electric-field contrast ratio and a magnetic field enhancement are achieved. We also investigate the metasurface physical constraints to convert a linearly polarized beam into an azimuthally E-polarized beam and characterize the performance of magnetic field enhancement and electric field suppression of a realistic metasurface. These findings are potentially useful for novel optical spectroscopy related to magnetic dipolar transitions and for optical manipulation of particles with spin and OAM.

Experimental demonstration of ultrasensitive sensing with terahertz metamaterial absorbers: A comparison with the metasurfaces

Planar metasurfaces and plasmonic resonators have shown great promise for sensing applications across the electromagnetic domain ranging from the microwaves to the optical frequencies. However, these sensors suffer from lower figure of merit and sensitivity due to the radiative and the non-radiative loss channels in the plasmonic metamaterial systems. We demonstrate a metamaterial absorber based ultrasensitive sensing scheme at the terahertz frequencies with significantly enhanced sensitivity and an order of magnitude higher figure of merit compared to planar metasurfaces. Magnetic and electric resonant field enhancement in the impedance matched absorber cavity enables stronger interaction with the dielectric analyte. This finding opens up opportunities for perfect metamaterial absorbers to be applied as efficient sensors in the finger print region of the electromagnetic spectrum with several organic, explosive, and bio-molecules that have unique spectral signature at the terahertz frequencies.

A possible mechanism of the enhancement and maintenance of the shear magnetic field component in the current sheet of the Earth’s magnetotail

The influence of the shear magnetic field component, which is directed along the electric current in the current sheet (CS) of the Earth’s magnetotail and enhanced near the neutral plane of the CS, on the nonadiabatic dynamics of ions interacting with the CS is studied. The results of simulation of the nonadiabatic ion motion in the prescribed magnetic configuration similar to that observed in the magnetotail CS by the CLUSTER spacecraft demonstrated that, in the presence of some initial shear magnetic field, the north-south asymmetry in the ion reflection/refraction in the CS is observed. This asymmetry leads to the formation of an additional current system formed by the oppositely directed electric currents flowing in the northern and southern parts of the plasma sheet in the planes tangential to the CS plane and in the direction perpendicular to the direction of the electric current in the CS. The formation of this current system perhaps is responsible for the enhancement and further maintenance of the shear magnetic field near the neutral plane of the CS. The CS structure and ion dynamics observed in 17 intervals of the CS crossings by the CLUSTER spacecraft is analyzed. In these intervals, the shear magnetic field was increased near the neutral plane of the CS, so that the bell-shaped spatial distribution of this field across the CS plane was observed. The results of the present analysis confirm the suggested scenario of the enhancement of the shear magnetic field near the neutral plane of the CS due to the peculiarities of the nonadiabatic ion dynamics.

Electromagnetic Field Enhancement in Small Gaps: A Rigorous Mathematical Theory

This paper is concerned with the field enhancement when an electromagnetic wave passes through subwavelength metallic gaps. We focus on a particular configuration when there is extreme scale difference between the wavelength of the incident wave, the thickness of metal films, and the size of gap apertures. Based upon a rigorous study of the perfect electrical conductor model for the transverse magnetic ploarization, we show that enormous electric field enhancement occurs inside the gap if the gap size is sufficiently small. (There is no electromagnetic field enhancement for the transverse electric polarization case, of which the mathematical results are less interesting and they are not presented in this paper.) Furthermore, when the gap size approaches zero, there is a limit on the ultimate enhancement strength, which is proportional to the ratio between the wavelength of the incident wave and the thickness of the metal film. On other hand, it is demonstrated that there is no significant magnetic field enhancement inside the gap.

Sudden impulse observations in the dayside magnetosphere by THEMIS

AbstractWe present a study of the magnetospheric response to interplanetary shocks. We show eight events with simultaneous observations of sudden impulses in the dayside magnetosphere and interplanetary shocks in the solar wind. The spacecraft measurements in the equatorial plane, even those very close to the Earth, can be interpreted in terms of the vortices predicted by previous studies employing global MHD models. In fact, these vortices are velocity oscillations with the properties of Alfvén waves. The amplitude and frequency of the oscillations depend on radial distance from the Earth. The amplitude of the velocity perturbations decreases with increase of the density and magnetic field magnitude, but the velocity amplitude shows no dependence on magnitude of the solar wind dynamic pressure change attending the interplanetary shocks. The oscillations are observed both in the outer magnetosphere and the plasmasphere, but they become less sinusoidal near the plasmapause, i.e., in the region with a large‐density gradient. The amplitude of the magnetic field enhancement in the sudden impulses also depends on the radial distance. The MHD simulations successfully predict the amplitudes of magnetic field increase and the first cycles of the velocity oscillations in these events.

Zeeman splitting of laser-driven soft x-ray laser line by the enhancement of magnetic field in plasma

We observed a laser-driven plasma soft x-ray laser (SXRL) line at the wavelength of 18.895 nm from the nickel-like molybdenum ions under the existence of external magnetic field. In this experiment, 4 mm-diameter magnetic coil driven by an electrical pulsed power supply provided the external magnetic field of 15 T along the direction of the plasma column, and we could obtain the left-handed circular and right-handed circular polarization components separately. The experimental Zeeman splitting indicated that the strength of the magnetic field was enhanced by a factor of more than 10 than that of the applied field, which implied that the compression of magnetic field occurred in the dynamics of intense laser-plasma interaction.

The Effect of the External Magnetic Field on the Wide Band Gap Polymer Conductivity

The polydiphenylenephtalide (PDP) films of submicron thickness can reversibly switch their conductivity from dielectric to quasi-metallic state with changing their boundary conditions. The intermediate conductivity state “semi-conducting” can also be obtained. The metallic conductivity is characterized by quasi-one-dimensional conductive channels with diameters of 10–100 nm. PDP molecule contains only C, H, and O atoms. The behavior of PDP films out-of-plane conductivity with in-plane magnetic field is studied with current-voltage characteristics (CVC) method. In dielectric state, the films have some volumetric conductivity, so the magnetic field suppresses the conductivity, as it must be because of the Hall effect. When the boundary conditions are changed to provide the “semi-conductive” state, some quasi-one-dimensional conducting channels are partially switching on. So the Hall effect is being suppressed in these sectors, and the previously trapped electrons become free just enlarging conductivity with the magnetic field enhancement. When the film is shifted to quasi-metallic state with linear CVC, the processes of enlarging the conductivity with the magnetic field enhancement become prevailing.

Effect of plasma density profile inhomogeneity on the self-generated magnetic field

The photon orbital and/or spin angular-momentum absorption in absorbing plasma medium causes the circular motion of electrons and generate the azimuthal component of current density. The axial magnetic field generation due to the azimuthal component of generated current density is known as inverse Faraday effect. In this paper, the axial magnetic field generation in an absorbing plasma with ramped density by the propagation of linear polarized Laguerre-Gaussian beam is taken into consideration. It is shown that the positive slope of the initial electron density causes a considerable enhancement of self-generated axial magnetic field. The magnetic field enhancement depends on the twisted light vortex charge number. The rotation angle of the linearly polarized light which can be employed for measurement of the generated magnetic field, is calculated.

Analysis of double-step response to an interplanetary shock in the dayside magnetosphere

Abstract. We present an analysis of double-step magnetic field enhancement caused by interplanetary (IP) shock impacts on the Earth's magnetosphere. The structures were observed by the GOES-8, 10, 11, and 12 spacecraft in the dayside geostationary orbit, particularly during northward interplanetary magnetic field (IMF) conditions. The double-step structures, similar to what is observed in the ground horizontal magnetic field (H) component at low and mid latitudes, were observed preferentially on the dayside. Structures observed around 12–15 magnetic local time (MLT) displayed the steepest initial enhancement step, followed by a magnetic field strength decrease before the second enhancement step. At other dayside MLTs of the geostationary orbit, the initial response was smoother, and no decrease was observed before the second step. We suggest that this MLT asymmetry in the decrease of the total magnetic field is caused by the pushing of the plasmaspheric ions over the geostationary orbit due to the magnetospheric compression.

Asymmetric braking and dawnward deflection of dipolarization fronts: Effects of ion reflection

AbstractDipolarization fronts (DFs), earthward propagating structures in the Earth's magnetotail with sharp enhancements of the northward magnetic field, can reflect and accelerate ions in the ambient plasma sheet. The ion reflection and acceleration process, which generates earthward flows ahead of the DF, also imposes a dynamic pressure on the DF to decelerate its earthward motion. It has been shown that the ion reflection process is not symmetric, with stronger ion accelerations at the evening side of the DF than at its morning side, which implies dawn‐dusk asymmetric reaction of the ambient plasma and consequently dawnward deflection of DFs. In this paper, we examine this scenario in detail, by carrying out statistical studies based on Time History of Events and Macroscale Interactions during Substorms observations from 2008 to 2011. We demonstrate the important role of the ion reflection process in the longstanding problems regarding DF evolution and bursty flow braking in the near‐Earth plasma sheet.

Influence of high magnetic field on the luminescence of Eu3+-doped glass ceramics

Rare earth (RE) doped materials have been widely exploited as the intriguing electronic configuration of RE ions offers diverse functionalities from optics to magnetism. However, the coupling of magnetism with photoluminescence (PL) in such materials has been rarely reported in spite of its fundamental significance. In the present paper, the effect of high pulsed magnetic field on the photoluminescence intensity of Eu3+-doped nano-glass-ceramics has been investigated. In our experiment, Eu-doped oxyfluoride glass and glass ceramic were prepared by the conventional melt-quenching process and controlled heat treatment. The results demonstrate that the integrated PL intensity of Eu3+ decreases with the enhancement of magnetic field, which can be interpreted in terms of cooperation effect of Zeeman splitting and magnetic field induced change in site symmetry. Furthermore, as a result of Zeeman splitting, both blue and red shift in the emission peaks of Eu3+ can be observed, and this effect becomes more prominent with the increase of magnetic field. Possible mechanisms associated with the observed magneto-optical behaviors are suggested. The results of the present paper may open a new gate for modulation of luminescence by magnetic field and remote optical detection of magnetic field.