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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Magnetopause Reconnection as Influenced by the Dipole Tilt Under Southward IMF Conditions: Hybrid Simulation and MMS Observation

Abstract Using a three‐dimensional (3‐D) global‐scale hybrid code, the Magnetospheric Multiscale (MMS) reconnection event around 02:13 UT on 18 November 2015, highlighted in the Geospace Environment Modeling (GEM) Dayside Kinetic Challenge, is simulated, in which the interplanetary magnetic field (IMF) points southward and the geomagnetic field has a −27° dipole tilt angle. Strong southward plasma jets are found near the magnetopause as a result of the dayside reconnection. Our results indicate that the subsolar magnetopause reconnection X line shifts from the subsolar point toward the Northern Hemisphere due to the effect of the tilted geomagnetic dipole angle, consistent with the MMS observation. Subsequently, the reconnection X lines or sites and reconnection flux ropes above the equator propagate northward along the magnetopause. The formation and global distribution of the X lines and the structure of the magnetopause reconnection are investigated in detail with the simulation. Mirror mode waves are also found in the middle of the magnetosheath downstream of the quasi‐perpendicular shock where the plasma properties are consistent with the mirror instability condition. As a special outcome of the GEM challenge event, the spatial and temporal variations in reconnection, the electromagnetic power spectra, and the associated D‐shaped ion velocity distributions in the simulated reconnection event are compared with the MMS observation.

Guo, Zhifang↗
DOE OSTI2016

Magnetic curvature effects on plasma interchange turbulence

The magnetic curvature effects on plasma interchange turbulence and transport in the Z-pinch and dipole-like systems are explored with two-fluid global simulations. By comparing the transport levels in the systems with a different magnetic curvature, we show that the interchange-mode driven transport strongly depends on the magnetic geometry. For the system with large magnetic curvature, the pressure and density profiles are strongly peaked in a marginally stable state and the nonlinear evolution of interchange modes produces the global convective cells in the azimuthal direction, which lead to the low level of turbulent convective transport.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Enabling room temperature ferromagnetism in monolayer MoS2 via in situ iron-doping

Abstract Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS 2 monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS 2 crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS 2 monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS 2 . Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS 2 monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2011

MAGNETIC INTERACTIONS IN PRE-MAIN-SEQUENCE BINARIES

Young stars typically have strong magnetic fields, so that the magnetospheres of newly formed close binaries can interact, dissipate energy, and produce synchrotron radiation. The V773 Tau A binary system, a pair of T Tauri stars with a 51 day orbit, displays such a signature, with peak emission taking place near periastron. This paper proposes that the observed emission arises from the change in energy stored in the composite magnetic field of the system. We model the fields using the leading order (dipole) components and show that this picture is consistent with current observations. In this model, the observed radiation accounts for a fraction of the available energy of interaction between the magnetic fields from the two stars. Assuming antisymmetry, we compute the interaction energy E{sub int} as a function of the stellar radii, the stellar magnetic field strengths, the binary semimajor axis, and orbital eccentricity, all of which can be measured independently of the synchrotron radiation. The variability in time and energetics of the synchrotron radiation depend on the details of the annihilation of magnetic fields through reconnection events, which generate electric fields that accelerate charged particles, and how those charged particles, especially fast electrons, are removed from the interaction region. However, the major qualitative features are well described by the background changes in the global magnetic configuration driven by the orbital motion. The theory can be tested by observing a collection of pre-main-sequence binary systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Tuning spin-orbit coupling towards enhancing photocurrent in hybrid organic-inorganic perovskites by using mixed organic cations

The presence of the heavy elements leads to strong spin-orbit coupling (SOC) in hybrid organic-inorganic perovskites (HOIPs), which plays an important role in the photovoltaic performance of HOIPs by changing the populations between bright and dark states. The organic cation is a critical composition and affects SOC via the interaction with the inorganic cage. In this work, we use magneto-photocurrent measurement to explore the SOC effect upon using mixed organic cations (methylammonium (MA) and formamidinium (FA)) with different dipole moments in Pb based perovskites. Magneto-photocurrent measurements showed that the internal magnetic parameter B0 is decreased from 402.41 mT to 180.18 mT and 104.58 mT with decreasing the MA/FA ratio. This provides evidence that changing the internal dipole moment by using mixed organic cations can essentially change the SOC in HOIPs. Simultaneously, the photocurrent is increased from 21.46 mA/cm 2 to 22.60 mA/cm 2 and 23.08 mA/cm 2 when the MA/FA ratio is changed from 1:0 to 0.9:0.1 and 0.7:0.3. Above all, our results indicate that the change in SOC by different organic cations can also be a factor affecting the performance of perovskite solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Self-interaction correction schemes for non-collinear spin-density-functional theory

We extend some of the well-established self-interaction correction (SIC) schemes of density-functional theory—the Perdew–Zunger SIC and the average-density SIC—to the case of systems with noncollinear magnetism. Our proposed SIC schemes are tested on a set of molecules and metallic clusters in combination with the widely used local spin-density approximation. As expected from the collinear SIC, we show that the averaged-density SIC works well for improving ionization energies but fails to improve more subtle quantities like the dipole moments of polar molecules. We investigate the exchange-correlation magnetic field produced by our extension of the Perdew–Zunger SIC, showing that it is not aligned with the local total magnetization, thus producing an exchange-correlation torque.

Chemistry↗
DOE OSTI2011

A Project to Design and Build the Magnets for a New Test Beamline, the ATF2, at KEK

In order to achieve the high luminosity required at the proposed International Linear Collider (ILC), it is critical to focus the beams to nanometer size with the ILC Beam Delivery System, and to maintain the beams collisions with a nanometer-scale stability. To establish the technologies associated with this ultra-high precision beam handling, a special beamline has been designed and built as an extension of the existing extraction beamline of the Accelerator Test Facility at KEK, Japan. The ATF provides an adequate ultra-low emittance electron beam that is comparable to the ILC requirements; the ATF2 mimics the ILC final focus system to create a tightly focused, stable beam. There are 37 magnets in the ATF2, 29 quadrupoles, 5 sextupoles and 3 bends. These magnets had to be acquired in a short time and at minimum cost, which led to various acquisition strategies; but nevertheless they had to meet strict requirements on integrated strength, physical dimensions, compatibility with existing magnet movers and beam position monitors, mechanical stability and field stability and quality. This paper will describe how 2 styles of quadrupoles, 2 styles of sextupoles, one dipole style and their supports were designed, fabricated, refurbished or modified, measured and aligned by a small team of engineers from 3 continents.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

THE INTERSTELLAR MAGNETIC FIELD CLOSE TO THE SUN. II

The magnetic field in the local interstellar medium (ISM) provides a key indicator of the galactic environment of the Sun and influences the shape of the heliosphere. We have studied the interstellar magnetic field (ISMF) in the solar vicinity using polarized starlight for stars within 40 pc of the Sun and 90 Degree-Sign of the heliosphere nose. In Frisch et al. (Paper I), we developed a method for determining the local ISMF direction by finding the best match to a group of interstellar polarization position angles obtained toward nearby stars, based on the assumption that the polarization is parallel to the ISMF. In this paper, we extend the analysis by utilizing weighted fits to the position angles and by including new observations acquired for this study. We find that the local ISMF is pointed toward the galactic coordinates l, b =47 Degree-Sign {+-} 20 Degree-Sign , 25 Degree-Sign {+-} 20 Degree-Sign . This direction is close to the direction of the ISMF that shapes the heliosphere, l, b =33 Degree-Sign {+-} 4 Degree-Sign , 55 Degree-Sign {+-} 4 Degree-Sign , as traced by the center of the 'Ribbon' of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX) mission. Both the magnetic field direction and the kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. A nearby ordered component of the local ISMF has been identified in the region l Almost-Equal-To 0 Degree-Sign {yields} 80 Degree-Sign and b Almost-Equal-To 0 Degree-Sign {yields} 30 Degree-Sign , where PlanetPol data show a distance-dependent increase of polarization strength. The ordered component extends to within 8 pc of the Sun and implies a weak curvature in the nearby ISMF of {approx}0.{sup 0}25 pc{sup -1}. This conclusion is conditioned on the small sample of stars available for defining this rotation. Variations from the ordered component suggest a turbulent component of {approx}23 Degree-Sign . The ordered component and standard relations between polarization, color excess, and H{sup o} column density predict a reasonable increase of N(H) with distance in the local ISM. The similarity of the ISMF directions traced by the polarizations, the IBEX Ribbon, and pulsars inside the Local Bubble in the third galactic quadrant suggest that the ISMF is relatively uniform over spatial scales of 8-200 pc and is more similar to interarm than spiral-arm magnetic fields. The ISMF direction from the polarization data is also consistent with small-scale spatial asymmetries detected in GeV-TeV cosmic rays with a galactic origin. The peculiar geometrical relation found earlier between the cosmic microwave background dipole moment, the heliosphere nose, and the ISMF direction is supported by this study. The interstellar radiation field at {approx}975 A does not appear to play a role in grain alignment for the low-density ISM studied here.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Two-dimensional NMR measurement and point dipole model prediction of paramagnetic shift tensors in solids

A new two-dimensional Nuclear Magnetic Resonance (NMR) experiment to separate and correlate the first-order quadrupolar and chemical/paramagnetic shift interactions is described. This experiment, which we call the shifting-d echo experiment, allows a more precise determination of tensor principal components values and their relative orientation. It is designed using the recently introduced symmetry pathway concept. A comparison of the shifting-d experiment with earlier proposed methods is presented and experimentally illustrated in the case of {sup 2}H (I = 1) paramagnetic shift and quadrupolar tensors of CuCl{sub 2}⋅2D{sub 2}O. The benefits of the shifting-d echo experiment over other methods are a factor of two improvement in sensitivity and the suppression of major artifacts. From the 2D lineshape analysis of the shifting-d spectrum, the {sup 2}H quadrupolar coupling parameters are 〈C{sub q}〉 = 118.1 kHz and 〈η{sub q}〉 = 0.88, and the {sup 2}H paramagnetic shift tensor anisotropy parameters are 〈ζ{sub P}〉 = − 152.5 ppm and 〈η{sub P}〉 = 0.91. The orientation of the quadrupolar coupling principal axis system (PAS) relative to the paramagnetic shift anisotropy principal axis system is given by (α,β,γ)=((π)/2 ,(π)/2 ,0). Using a simple ligand hopping model, the tensor parameters in the absence of exchange are estimated. On the basis of this analysis, the instantaneous principal components and orientation of the quadrupolar coupling are found to be in excellent agreement with previous measurements. A new point dipole model for predicting the paramagnetic shift tensor is proposed yielding significantly better agreement than previously used models. In the new model, the dipoles are displaced from nuclei at positions associated with high electron density in the singly occupied molecular orbital predicted from ligand field theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Linear and nonlinear studies of velocity shear driven three dimensional electron-magnetohydrodynamics instability

The study of electron velocity shear driven instability in electron magnetohydrodynamics (EMHD) regime in three dimensions has been carried out. It is well known that the instability is non-local in the plane defined by the flow direction and that of the shear, which is the usual Kelvin-Helmholtz mode, often termed as the sausage mode in the context of EMHD. On the other hand, a local instability with perturbations in the plane defined by the shear and the magnetic field direction exists which is termed as kink mode. The interplay of these two modes for simple sheared flow case as well as that when an external magnetic field exists has been studied extensively in the present manuscript in both linear and nonlinear regimes. Finally, these instability processes have been investigated for the exact 2D dipole solutions of EMHD equations [M. B. Isichenko and A. N. Marnachev, Sov. Phys. JETP 66, 702 (1987)] for which the electron flow velocity is sheared. It has been shown that dipoles are very robust and stable against the sausage mode as the unstable wavelengths are typically longer than the dipole size. However, we observe that they do get destabilized by the local kink mode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Two examples of in-flight spin flippers

In precise experiments with polarized beam it’s very often appear a necessity to change beam polarization on opposite. If such operation does not change other beam parameters, it helps to avoid or minimize some systematic errors. It is especially important in experiments, where spin dependent effect is small enough. This paper describes two set of equipments, that make spin flip for extracted beams. In both cases, these devices are absolutely distinct, because they are appropriate for different particles and at different energy range. The first of them is intended for future muon (g-2) experiment, which is under preparation now at JPARC. Here, the muon spin flip will be done by chain of electrostatic and magnetic bends at the kinetic energy 340 keV. A beam matching is provided by a number of short solenoids. The other flipper (or Siberian snake) will rotate spin of protons or antiprotons, which come from Λ-meson decay with the energy up to 40 GeV. This experiment (no. 24) is planed at IHEP, Protvino. In this case, two superconducting helical magnets with opposite helicities and magnetic field 4.5 T will be used. To correct beam trajectory, additional dipole correctors are required.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Multipacting optimization of a 750 MHz rf dipole

Crab crossing schemes have been proposed to re-instate luminosity degradation due to crossing angles at the interaction points in next generation colliders to avoid the use of sharp bending magnets and their resulting large synchrotron radiation generation, highly undessirable in the detector region. The rf dipole has been considered for a different set of applications in several machines, both rings and linear colliders. We present in this paper a study of the effects on the multipacting levels and location depending on geometrical variations on the design for a crabbing/deflecting application in a high current (3/0.5 A), high repetition (750 MHz) electron/proton collider, as a matter to provide a comparison point for similar applications of rf dipoles.

Delayen, Jean R.↗
DOE OSTI2011

Development and Fabrication of Nb3Sn Rutherford Cable for the 11 T DS Dipole Demonstration Model

Fermilab and CERN started the development of 11 T 11-m long Nb{sub 3}Sn dipoles to replace few regular LHC NbTi dipoles and free space for cold collimators in LHC DS areas. An important step in the design of these magnets is the development of the high aspect ratio Nb{sub 3}Sn cable to achieve the nominal field of 11 T at the nominal LHC operating current of 11.85 kA with 20% margin. The keystoned cables 14.7 mm wide with and without a stainless steel core were made out of hard Cu wires and Nb{sub 3}Sn RRP strand 0.7 mm nominal diameter. The cable optimization process was aimed at achieving both mechanical stability and minimal damage to the delicate internal architecture of the Restacked-Rod-Process (RRP) Nb{sub 3}Sn strands with 127 restack design to be used in the magnet short models. Each cable was characterized electrically for transport properties degradation at high field and for low field stability, and metallographically for internal damage.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Magnetic Tilt Effect on Externally Driven Electromagnetic Ion Cyclotron (EMIC) Waves

Abstract We examine coupling of fluctuations in the solar wind with electromagnetic ion cyclotron (EMIC) waves in the magnetosphere using an advanced full‐wave simulation code, Petra‐M. Dipole tilt dramatically affects the coupling process. While very little wave power can reach the inner magnetosphere without tilt effects, a tilted dipole field dramatically increases the efficiency of the coupling process. Solar wind fluctuations incident at high magnetic latitude effectively reaches the ground along the field line and mode‐convert to linearly polarized field‐aligned propagating waves at the Alfvén and IIH resonances. Therefore, solar wind compressions efficiently drive linearly polarized EMIC waves when the dipole angle is tilted toward or away from the Sun‐Earth direction.

58 GEOSCIENCES↗
DOE OSTI2019

Hydrodynamic description of long-distance spin transport through noncollinear magnetization states: Role of dispersion, nonlinearity, and damping

Nonlocal compensation of magnetic damping by spin injection has been theoretically shown to display dynamic, noncollinear magnetization states that carry spin currents over micrometer distances. Such states can be generically referred to as dissipative exchange flows (DEFs) because spatially diffusing spin currents are established by the mutual exchange torque exerted by neighboring spins. Analytical studies to date have been limited to the weak spin injection assumption whereby the equation of motion for the magnetization is mapped to hydrodynamic equations describing spin flow and then linearized. In this work, we analytically and numerically study easy-plane ferromagnetic channels subject to spin injection of arbitrary strength at one extremum under a unified hydrodynamic framework. We propose that DEFs generally exhibit a nonlinear profile along the channel accompanied by a nonlinear frequency tunability. At large injection strengths, we fully characterize a magnetization state we call a contact-soliton DEF (CS-DEF) composed of a stationary soliton at the injection site, which smoothly transitions into a DEF and exhibits a negative frequency tunability. The transition between a DEF and a CS-DEF occurs at the maximum precessional frequency and coincides with the Landau criterion: a subsonic to supersonic flow transition. Leveraging the hydraulic-electrical analogy, the current-voltage characteristics of a nonlinear DEF circuit are presented. Micromagnetic simulations of nanowires that include magnetocrystalline anisotropy and nonlocal dipole fields are in qualitative agreement with the analytical results. The magnetization states found here along with their characteristic profile and spectral features provide quantitative guidelines to pursue an experimental demonstration of DEFs in ferromagnetic materials and establish a unified description for long-distance spin transport.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Ferromagnetism out of charge fluctuation of strongly correlated electrons in κ -(BEDT-TTF) 2 Hg(SCN) 2 Br

We perform magnetic susceptibility and magnetic torque measurements on the organic κ-(BEDT-TTF) 2 Hg(SCN) 2 Br, which is recently suggested to host an exotic quantum dipole-liquid in its low-temperature insulating phase. Below the metal-insulator (MI) transition temperature, the magnetic susceptibility follows a Curie–Weiss law with a positive Curie–Weiss temperature, and a particular M∝ $\sqrt{H}$ curve is observed. The emergent ferromagnetically interacting spins amount to about 1/6 of the full spin moment of localized charges. Taking account of the possible inhomogeneous quasi-charge-order that forms a dipole-liquid, we construct a model of antiferromagnetically interacting spin chains in two adjacent charge-ordered domains, which are coupled via fluctuating charges on a Mott-dimer at the boundary. We find that the charge fluctuations can draw a weak ferromagnetic moment out of the spin singlet domains.

36 MATERIALS SCIENCE↗
DOE OSTI2013

From spin induced ferroelectricity to spin and dipolar glass in a triangular lattice: The CuCr{sub 1−x}V{sub x}O{sub 2} (0≤x≤0.5) delafossite

The change from antiferromagnetism induced ferroelectricity to spin glass ferroelectric relaxor has been studied along the CuCr{sub 1−x}V{sub x}O{sub 2} (0≤x≤0.5) solid solution of polycrystalline samples. As x increases from CuCrO{sub 2} (x=0) to CuCr{sub 0.82}V{sub 0.18}O{sub 2}, it is found that the Néel temperature decreases from ∼24 K down to ∼13 K. This progressive weakening of the antiferromagnetism of CuCrO{sub 2} induces a rapid decrease of the spin induced ferroelectricity with polarization values going from ∼44 μC/m{sup 2} down to ∼1.5 μC/m{sup 2} for x=0.04 and x=0.08, respectively. Beyond x=0.18 (0.20≤x≤0.50), ac-magnetic susceptibility and magnetization measurements evidence a spin glass state while dielectric permittivity and polarization measurements point towards a relaxor behaviour. This shows that competing magnetic interactions in delafossites are an efficient way to transform a spin induced magnetoelectric into a multiglass (spin and dipolar) state. - Graphical abstract: The P(T) curves evidencing the aging effect on polarization in CuCr{sub 0.5}V{sub 0.5}O{sub 2}: E=135 kV/m is applied during cooling at different temperatures. The P values and the inflection point of the transition depend on the poling temperature suggesting a relaxor behaviour. This effect related to the spin glass state is not observed for the lowest vanadium content. - Highlights: • Samples of the CuCr{sub 1−x}V{sub x}O{sub 2} series have been studied. • The V content increase induces a change from antiferromagnetism to spin glass. • A behavior characterisitic of a spin and dipole glass is demonstrated. • The ferroelectricity is shown to go from spin induced to relaxor. • Competing magnetic interactions are efficient way to generate multiglass state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Coupling between magnon and ligand field excitations in magnetoelectric Tb3Fe5O12 garnet

The spectra of far-infrared transmission in Tb{sub 3}Fe{sub 5}O{sub 12} magnetoelectric single crystals have been studied in the range between 15 and 100 cm{sup -1}, in magnetic fields up to 10 T, and for temperatures between 5 and 150 K. We attribute some of the observed infrared-active excitations to electric dipole transitions between ligand-field split states of Tb{sup 3+} ions. Anticrossing between the magnetic exchange excitation and the ligand-field transition occurs at the temperature between 60 and 80 K. The corresponding coupling energy for this interaction is 6 cm{sup -1}. Temperature-induced softening of the hybrid IR excitation correlates with the increase in the static dielectric constant. We discuss the possibility for hybrid excitations of magnons and ligand-field states and their possible connection to the magnetoelectric effect in Tb{sub 3}Fe{sub 5}O{sub 12}.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗