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At least 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Magnetospheric electric fields deduced from drifting whistler paths.

Results of a study in which the amplitude of the E-W component E sub w of the convection electric field in the nightside magnetosphere has been inferred from the observed cross-L motions of whistler ducts within the plasmasphere, and several ducts distributed over 1 to 2 earth radii in L space and over plus or minus deg 15 around the longitude of the Eights, Antarctica, whistler station have been tracked simultaneously. The method appears capable of resolving fluctuations in E sub w with period T equal to approximately 15 min and rms amplitude as low as 0.05 mV/m. For variations with T greater than 1 hour the method has a sensitivity of the order of 0.01 mV/m. Three case studies are presented, two of which illustrate convection activity associated with relatively isolated substorms. In these two cases E sub w reversed from westward to eastward for a period following the decay of substorm bay activity. In the third case the substorm bay activity was prolonged, and E sub w remained westward and at enhanced levels until local dawn. Evidence was found that, at least in a limited longitudinal sector, perturbing substorm E sub w fields can penetrate deep within the plasmasphere. In two of the case studies comparisons of E sub w and the interplanetary magnetic-field theta component show evidence of a possible relation based on brief (less than or equal to 1 hour) southward excursions but not on long preceding southward events.

Carpenter, D. L.↗
NASA NTRS1982

The cometary atmosphere and its interaction with the solar wind

The present state of knowledge of cometary atmospheres and their interaction with the solar winds is assessed. Current models of the magnetospheres and ionospheres of comets are based on either an atmospheric chemistry approach, minimizing the effects of atmospheric dynamics and thermodynamics, while the other method takes the exact opposite viewpoint and roles and significance are reversed. A major difficulty in completing the models is a lack of data on the chemical composition of the cometary nucleus, although UV observations have revealed the H, C, O, and S elemental abundance ratios. It is suggested that further studies of collisional ionization by energetic electrons be performed to characterize processes in the cometary atmosphere. Hydronium may be the dominat molecule in the inner coma, if the assumption that the ionosphere is a magnetic field-free cavity separated from the solar wind by an unyielding tangential discontinuity surface is accurate. Particular observations and theoretical attention are recommended for the solar wind interaction in the intermediate range, i.e., 2.5-5 AU, when the cometary atmosphere develops.

Mendis, D. A.↗
NASA NTRS1984

Microstructure of magnetic reconnection in earth's magnetotail

The structure of heated electron reconnection events associated with magnetic substorm events in the earth's magnetotail is examined using IMP 8 spacecraft and ground-based magnetometer, plasma analyzer and spectroscopic data. Plasma, magnetic field and energetic particle data for five events are presented. Reconnection is shown to occur in two phases: preheating and heating. In preheating, lasting about 5 min, a strong tailward plasma flow appears and ends with electron heating. A 1-2 min heating phase starts with electron heating and ends with plasma sheet drop out and/or decay of the electron temperature to pre-event levels. The heating pulse is always connected with a Bx reversal at 30 earth radii tailward, where the reconnection occurs.

Bieber, J. W.↗
DOE OSTI2011

Magnetization reversal processes of epitaxial Fe{sub 3}Si films on GaAs(001)

We report magneto-optical Kerr microscopy study of the magnetic reversal behavior of Fe{sub 3}Si/GaAs(100) epitaxial thin films grown by molecular beam epitaxy. Results are described in the context of superconducting quantum interference device and Kerr effect vector magnetometry to study the magnetic switching behavior. By tuning the Fe concentration of the samples from 75% to 83%, we found that the ratio of uniaxial to cubic anisotropy constants {gamma} (=|K{sub u}/K{sub 1}|) would tremendously vary from 0.06 to 0.3. When the field was applied along the easy axis of Fe{sub 3}Si samples, a step feature was observed in the M-H loops. This feature has been resolved in the domain image by Kerr microscopy, in which we observed a two-switching behavior at the field where the step feature occurred. These can be qualitatively understood by considering the ratio {gamma}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Magnetic vortex nucleation/annihilation in artificial-ferrimagnet microdisks

The topological nature of magnetic-vortex state gives rise to peculiar magnetization reversal observed in magnetic microdisks. Interestingly, magnetostatic and exchange energies which drive this reversal can be effectively controlled in artificial ferrimagnet heterostructures composed of rare-earth and transition metals. [Py(t)/Gd(t)] 25 (t=1 or 2 nm) superlattices demonstrate a pronounced change of the magnetization and exchange stiffness in a 10–300 K temperature range as well as very small magnetic anisotropy. Due to these properties, the magnetization of cylindrical microdisks composed of these artificial ferrimagnets can be transformed from the vortex to uniformly-magnetized states in a permanent magnetic field by changing the temperature. We explored the behavior of magnetization in 1.5-µm [Py(t)/Gd(t)] 25 (t=1 or 2 nm) disks at different temperatures and magnetic fields and observed that due to the energy barrier separating vortex and uniformly-magnetized states, the vortex nucleation and annihilation occur at different temperatures. This causes the temperature dependences of the Py/Gd disks magnetization to demonstrate unique hysteretic behavior in a narrow temperature range. It was discovered that for the [Py(2 nm)/Gd(2 nm)] 25 microdisks the vortex can be metastable at a certain temperature range.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Fabrication of single-crystalline YFeO3 films with large antiferromagnetic domains

The antiferromagnetic orthoferrite YFeO3 possesses fascinating magnetic properties for spintronics, such as terahertz spin dynamics, ultrafast domain wall motion, and long magnon decay length. YFeO3 belongs to a special family of antiferromagnets that show an unusually strong non-trivial Kerr response due to its weak ferromagnetism. The highly stable antiferromagnetic domains without any spontaneous spin rotation transitions below the 645 K Néel temperature may be useful for nanoscale device applications. We report the successful fabrication of high-quality twinning-free (110)-oriented YFeO3 films by pulsed laser deposition. Detailed structural and magnetic characterization revealed that the crystal structure and magnetic properties of the YFeO3 films are comparable to bulk single crystals. We show that the spin rotation under high magnetic fields follows the two-sublattice approximation model. The film surface is atomically flat with step-terrace surface morphology. A longitudinal magneto-optic Kerr (MOKE) rotation of 10 mdeg was observed at room temperature, which is consistent with earlier reports on bulk single crystals. The in-plane anisotropy of the Kerr response corresponds to the obtained magnetic anisotropy from the SQUID measurement. The large MOKE signal enables the imaging of antiferromagnetic domains and their reversal. The domain size was found to be larger than 100 μm. These high-quality YFeO3 thin films facilitate the fabrication of antiferromagnetic spintronic devices and provide a convenient platform for studying various spin-related phenomena in thin films and at interfaces.

Physics↗
DOE OSTI2015

JOINING RADIO WITH X-RAYS: A REVISED MODEL FOR SN 1993J

A joint analysis is done of the radio and X-ray observations of SN 1993J. It is argued that neither synchrotron cooling behind the forward shock nor thermal cooling behind the reverse shock is supported by observations. In order for adiabatic models to be consistent, a reinterpretation of the radius of the spatially resolved very long baseline interferometry-source (VLBI) is needed during the first few hundred days. Instead of reflecting the position of the forward shock, it is then associated with the expansion of the Rayleigh–Taylor unstable region emanating from the contact discontinuity. Although observations imply a constant ratio between the energy densities in magnetic fields and relativistic electrons, they do not appear to scale individually with the thermal energy density behind the forward shock; rather, in adiabatic models, the evolution of the magnetic field strength is best understood as scaling inversely with the supernova radius.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Velocity profile shapes in Alcator C-Mod plasmas

Abstract Toroidal rotation velocity spatial profiles ( r / a < 0.8) have been obtained from C-Mod over a wide range of operational conditions, including H-mode, I-mode, ICRF-heated L-mode and Ohmic L-mode (LOC and SOC), and in plasmas with ITBs, LH wave injection and MCFD. Peaked, flat and hollow rotation profiles have been observed. In H- and I-mode plasmas, generally with co-current peaked profiles, the peaking is correlated with temperature profile peaking, and both increase with toroidal magnetic field (decrease with ρ ∗ ). Any dependence on density peaking is unclear. For Ohmic L-mode discharges, with LOC, the velocity profiles are usually flat and most often directed co-current, while with SOC the profiles are hollow, mostly co-current at the edge and counter-current in the core. Both of these Ohmic rotation states exist with matched density and temperature profiles (and gradients), indicating that neither gradient is relevant during rotation reversals. For plasmas with LH wave injection and discharges with ITBs, the velocity profiles are hollow while the density and temperature profiles exhibit substantial peaking. Broadly speaking for all operational regimes, there is no unifying ordering of the velocity gradient with plasma parameters.

Rice, J. E. (ORCID:0000000183195971)↗
DOE OSTI2018

Cubic hastatic order in the two-channel Kondo-Heisenberg model

Materials with non-Kramers doublet ground states naturally manifest the two-channel Kondo effect, as the valence fluctuations are from a non-Kramers doublet ground state to an excited Kramers doublet. Here, the development of a heavy Fermi liquid requires a channel-symmetry-breaking spinorial hybridization that breaks both single and double time-reversal symmetry, and is known as hastatic order. Motivated by cubic Pr-based materials with $Γ$ 3 non-Kramers ground state doublets, this paper provides a survey of cubic hastatic order using the simple two-channel Kondo-Heisenberg model. Hastatic order necessarily breaks time-reversal symmetry, but the spatial arrangement of the hybridization spinor can be either uniform (ferrohastatic) or break additional lattice symmetries (antiferrohastatic). The experimental signatures of both orders are presented in detail, and include tiny conduction electron magnetic moments. Interestingly, there can be several distinct antiferrohastatic orders with the same moment pattern that break different lattice symmetries, revealing a potential experimental route to detect the spinorial nature of the hybridization. We employ an SU( N ) fermionic mean-field treatment on square and simple cubic lattices, and examine how the nature and stability of hastatic order varies as we vary the Heisenberg coupling, conduction electron density, band degeneracies, and apply both channel and spin symmetry breaking fields. In conclusion, we find that both ferrohastatic and several types of antiferrohastatic orders are stabilized in different regions of the mean-field phase diagram, and evolve differently in strain and magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1978

Measurement and analysis of magnetic field variation during a class 2b flare

A digital analysis of high-time-resolution videomagnetograms taken during a class 2b flare that occurred at 60 deg east is reported. The data were obtained at the Big Bear Observatory and calibrated by a Mt. Wilson magnetogram. Changes of weak magnetic fields (less than 100 G) with an amplitude of 30% to 100% have been detected over 55% of the optical flare region, apparently taking place at the initial phase of the flare. Statistical considerations suggest a real flare association with most of these changes. H-alpha observations show that large changes took place over the footpoints of heavily inclined structures like penumbral fibrils, while smaller changes took place over the plage region. An apparent polarity reversal was found at the feet of erupted fibrils. Based on force-free field calculations these changes can be reasonably explained as a transformation of the current-carrying fields to potential fields which produced large changes in the field-line inclination and rotation.

Tanaka, K.↗
NASA NTRS1977

Pioneer 10, 11 observations of evolving solar wind streams and shocks beyond 1 AU

Observations between 1 and 5 AU by Pioneers 10 and 11 have led to the identification of large numbers of interplanetary shocks. Both forward and reverse shocks, which begin to develop beyond 1.5 AU and which frequently appear as shock pairs, are found to accompany solar wind streams. The number of forward shocks continues to increase out to at least 5 AU. Reverse shocks are seen less often than forward shocks and, in some instances, disappear at larger distances. There is evidence that the shocks are corotating in the solar frame, as anticipated theoretically. The evolution of solar wind streams beyond 1 AU is profoundly affected by the shocks. A thick interaction region, with large enhancements in density, temperature, field strength and fluctuation level, forms in the region originally characterized by a positive velocity gradient. The solar wind and magnetic field properties adjacent to, and within, the interaction regions have been studied to determine their qualitative behavior and characteristic changes with distance. Several interplanetary shocks generated by solar flares have also been identified and analyzed.

Smith, E. J.↗
NASA NTRS1994

Probing the antiferromagnetic long-range order with Glauber spin states

It is well known that the ground state of low-dimensional antiferromagnets deviates from Neel states due to strong quantum fluctuations. Even in the presence of long-range order, those fluctuations produce a substantial reduction of the magnetic moment from its saturation value. Numerical simulations in anisotropic antiferromagnetic chains suggest that quantum fluctuations over Neel order appear in the form of localized reversal of pairs of neighboring spins. In this paper, we propose a coherent state representation for the ground state to describe the above situation. In the one-dimensional case, our wave function corresponds to a two-mode Glauber state, when the Neel state is used as a reference, while the boson fields are associated to coherent flip of spin pairs. The coherence manifests itself through the antiferromagnetic long-range order that survives the action of quantum fluctuations. The present representation is different from the standard zero-point spin wave state, and is asymptotically exact in the limit of strong anisotropy. The fermionic version of the theory, obtained through the Jordan-Wigner transformation, is also investigated.

Cabrera, Guillermo G.↗
NASA NTRS1995

Changes in the superconducting properties of high-T(sub c) ceramics produced by applied electric fields

Effect of an electrostatic field in the electrode-insulator-superconductor system on the current-voltage characteristics of high-T(sub c) ceramics with various composition and different preparation technology has been studied at 77 K. Ceramics of Y-Ba-Cu-O (123) and Bi-Pb-Sr-Ca-Cu-O (2223) systems and also ones doped by Ag have been used. Electric field strength has been up to 140 MV/m. It has been shown that there are reversible changes in the critical current I(sub c) and in the conductivity in electric field at the currents somewhat more than I(sub c) at T is less than T(sub c), while at T is greater than T(sub c) the noticeable electric field effect has not been found. These effects are qualitatively similar in both ceramic systems. High negative and positive gate voltages result in an increase of the conductivity. The electric field effect is modified by magnetic field H. The field effect decreases with increasing magnetic field and disappears at H is greater than 30 Oe. In Y-Ba-Cu-O/Ag (10 wt. percent) ceramics the field effect is practically absent. It may be supposed that in the ceramics the field-induced effect is consistent with weak links at grain boundaries.

Smirnov, B. I.↗
DOE OSTI2015

The Q weak experimental apparatus

The Jefferson Lab Q weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was created for this experiment to meet the technical challenges presented by the smallest and most precise e → p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 μA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Møller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8° and 11.6° were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q 2 =0.025 GeV 2 was observed using dedicated low-current ( ~ 100 pA ) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2010

Plasma expansion and impedance collapse in a foil-less diode for a klystronlike relativistic backward wave oscillator

Klystronlike relativistic backward wave oscillator (RBWO) can produce microwave power exceeding 5 GW with a high efficiency larger than 40%. In the experiment of klystronlike RBWO, for about 1 MV peak diode voltages, increasing magnetic field from 1.43 to 1.89 T slowed the impedance collapse until it was suppressed completely. The introduction of a stainless steel obstructing ring aggravated the impedance collapse, whereas replacing the stainless steel obstructing ring with a flat stainless steel provided a more stable impedance variation during the pulse duration. These impedance collapses did not affect microwave generation seriously and may be attributed to the radial expansion of cathode plasma initialing from the cathode shank so that part of reverse currents were collected at the anode wall, contributing to the measured diode current. On the other hand, it was found that microwave generation shot-to-shot reproducibility was closely related to the diode impedance variation. When there was no or very low microwave measured, diode impedance collapse appeared at the latter of the pulse. The microwave generation shot-to-shot reproducibility was improved greatly after the electron collector was enlarged on radius with 1 mm. A possible explanation is that the anode plasma produced from electron collector expands axially and enters the diode region at a very high velocity of several mm/ns. The movement of the anode plasma in the beam-wave interaction region affects the microwave generation, which reduces the microwave power during the whole pulse duration significantly.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Strain dependence of antiferromagnetic interface coupling in La 0.7 Sr 0.3 MnO 3 /SrRuO 3 superlattices

We have investigated the magnetic response of La 0.7 Sr 0.3 MnO 3 /SrRuO 3 superlattices to biaxial in-plane strain applied in situ. Superlattices grown on piezoelectric substrates of 0.72PbMg 1/3 Nb 2/3 O 3 -0.28PbTiO 3 (001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu H-0(AF) = 1.8 T is found to change by mu(0)Delta H-AF/Delta epsilon similar to -520 mT %(-1) under reversible biaxial strain Delta epsilon at 80 K in a [La 0.7 Sr 0.3 MnO 3 (22 angstrom)/SrRuO 3 (55 angstrom)] 15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers, which are under as-grown tensile strain, leading to a larger net coupling of SrRuO 3 layers at the interface. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface for the strength of the apparent antiferromagnetic coupling. We discuss our results in the framework of available models.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Interaction-Driven Spontaneous Quantum Hall Effect on a Kagome Lattice

Topological states of matter have been widely studied as being driven by an external magnetic field, intrinsic spin-orbital coupling, or magnetic doping. With this work, we unveil an interaction-driven spontaneous quantum Hall effect (a Chern insulator) emerging in an extended fermion-Hubbard model on a kagome lattice, based on a state-of-the-art density-matrix renormalization group on cylinder geometry and an exact diagonalization in torus geometry.We first demonstrate that the proposed model exhibits an incompressible liquid phase with doublet degenerate ground states as time-reversal partners. The explicit spontaneous time-reversal symmetry breaking is determined by emergent uniform circulating loop currents between nearest neighbors. Moreover, the fingerprint topological nature of the ground state is characterized by quantized Hall conductance. Hence, we identify the liquid phase as a quantum Hall phase, which provides a “proof-of-principle” demonstration of the interaction-driven topological phase in a topologically trivial noninteracting band.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1985

Radiative and reconnection instabilities - Compressible and viscous effects

Filaments and flares are prominent indicators of the magnetic fields of solar activity. These instability phenomena arise from the influence of weak transport effects (radiation and resistivity, respectively) on coronal magnetodynamics and energy flow. It has been shown that the filament and flare (tearing or reconnection) mechanisms are resistively coupled in sheared magnetic fields of the kind existing in active regions. The present paper expands this treatment to include the effects of compressibility and viscosity, which are most prominent at short wavelengths. The results show that compressibility affects the radiative mode, including a modest increase of its growth rate, and that viscosity modifies the tearing mode, partially through a decrease of its growth rate. A comprehensive discussion of the mode structures and flows is presented. The strongest effect found is a reversal, at very long wavelengths, of the radiative cooling of the resistive interior layer of the tearing mode, caused by compressional heating.

Tachi, T.↗