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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Pseudospin-lattice coupling and electric control of the square-lattice iridate Sr2IrO4

Sr 2 IrO 4 is an archetypal spin-orbit-coupled Mott insulator and has been extensively studied in part because of a wide range of predicted states. Limited experimental characterization of these states thus far brings to light the extraordinary susceptibility of the physical properties to the lattice, particularly, the Ir-O-Ir bond angle. Here, we report a microscopic rotation of the IrO 6 octahedra below 50 K measured by single crystal neutron diffraction. Overall, this sharp lattice anomaly provides keys to understanding the anomalous low-temperature physics and a direct confirmation of a crucial role that the Ir-O-Ir bond angle plays in determining the ground state. Indeed, as also demonstrated in this study, applied electric current readily weakens the antiferromagnetic order via the straightening of the Ir-O-Ir bond angle, highlighting that even slight change in the local structure can disproportionately affect the physical properties in the spin-orbit-coupled system.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Deterministic Current–Induced Perpendicular Switching in Epitaxial Co/Pt Layers without an External Field

Current–induced spin–orbit torques (SOTs) have emerged as a powerful tool to control magnetic elements and non–uniform magnetic textures such as domain walls and skyrmions. SOT–induced switching of perpendicular magnetization generally requires an external field to break the rotational symmetry of the spin–orbit effective fields responsible for the deterministic reversal. The proposed mechanisms to eliminate this requirement often rely on complex multilayer structures that necessitate laborious optimization in the material and spin transport properties, making them less attractive for applications. Herein, current–induced, external field–free switching of an epitaxial MgO/Pt/Co trilayer with an extremely large perpendicular anisotropy in excess of 3 Tesla is reported. It is found that switching occurs due to the interplay of strong SOTs, local anisotropy fluctuations, and the Dzyaloshinkii–Moriya interaction inherent to this epitaxial system. Finally, given that these layers constitute the base stack of a magnetic tunnel junction, this switching mechanism offers the most technologically viable path toward devices such as field–free SOT–based magnetic random–access memories.

36 MATERIALS SCIENCE↗
NASA NTRS1977

Coupling of mid-latitude spread-F between conjugate stations

It is shown that very marked differences occur between the spread F incidence at two stations (Puerto Rico and Port Stanley) in opposite hemispheres with similar magnetic coordinates and that these differences correspond to differences in the local ionospheres. It is deduced from this that electric field coupling of the local and conjugate ionosphere does not play a dominant role in the build-up or maintenance of the irregularity structure manifesting itself as spread F on ionograms.

Zinchenko, G. N.↗
NASA NTRS1984

Current carriers for the field-aligned current system

The Dynamics Explorer satellites have returned a large body of data containing high resolution magnetometer measurements and distributions of charged particles of all but thermal electrons. From these data the relations of the field aligned currents to particle precipitation structures were investigated and the charge carriers were identified. Results include the following: (1) during very quiet periods, field aligned currents exist primarily as fine structure; (2) during onset of substorms, region 1 and region 2 become clearly evident but contain significant structure; (3) as magnetic activity subsides, current regions become less distinct, and structure becomes more dominant; (4) the distribution of the upward currents derived from magnetometer data and calculated from suprathermal electron data agree remarkably well in shape but not necessarily in magnitude; (5) at all local times, 5 eV electrons seldom carry most of the upward current; (6) except for the accelerated inverted-V electrons, the dominant upward current carriers which are measured are below 500 eV and are distributed in energy; (7) dusk upward currents (region 1) are associated with the boundary plasma sheet; and (8) suprathermal electron bursts are important current carrying structures.

Hoffman, R. A.↗
NASA NTRS1985

Current carriers for the field-aligned current system

The Dynamics Explorer satellites have returned a large body of data containing high resolution magnetometer measurements and distributions of charged particles of all but thermal electrons. From these data the relations of the field aligned currents to particle precipitation structures were investigated and the charge carriers were identified. Results include the following: (1) during very quiet periods, field aligned currents exist primarily as fine structure; (2) during onset of substorms, region 1 and region 2 become clearly evident but contain significant structure; (3) as magnetic activity subsides, current regions become less distinct, and structure becomes more dominant; (4) the distribution of the upward currents derived from magnetometer data and calculated from suprathermal electron data agree remarkably well in shape but not necessarily in magnitude; (5) at all local times, 5 eV electrons seldom carry most of the upward current; (6) except for the accelerated inverted-V electrons, the dominant upward current carriers which are masured are below 500 eV and are distributed in energy; (7) dusk upward currents (region 1) are associated with the boundary plasma sheet; and (8) suprathermal electron bursts are important current carrying structures.

Hoffman, R. A.↗
DOE OSTI2023

Thickness dependent itinerant ferromagnetism in ultrathin Ba-doped SrRu O 3 films

The electronic and magnetic properties in AB⁢O 3 perovskite oxides are extremely sensitive to lattice structure, but also to dimensionality, such as the thickness in thin film-form. Here, we report the thickness-dependent electromagnetic properties of ultrathin epitaxially stabilized Sr 1-x ⁢Ba x ⁢Ru⁢O 3 (x = 0.08, 0.2) thin films on a SrTi⁢O 3 (001) substrate. The results reveal that the barium doping (0.08 ≤ x ≤ 0.20) reduces Ru⁢O 6 orthorhombic distortions existing in SrRu⁢O 3 and induces a tetragonal distortion, as evidenced by out-of-plane lattice expansion. A metal-to-insulator transition, accompanied by a ferromagnetic to nonmagnetic transition occurs with reducing film thickness from 10 to 3 unit cells for both x = 0.08 and 0.2, regardless of the doping level. The results suggest that the effects of compositional vacancies and surface/interface contributions introduced via dimensional confinement are more dominant than A-site chemical disorder or structural distortion for the loss of metallicity and ferromagnetism in ultra-thin epitaxial films.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Coherent fluctuations in a partially magnetized small magnetron discharge operating on molecular nitrogen

We report experimental data on rotating plasma disturbances in a quasi-steady direct-current magnetron discharge operating with molecular nitrogen at pressures of 27, 40, 67, and 133 Pa. Experiments with a segmented anode measuring the local current flow reveal up to 12 coherent structures (“spokes”) propagating in the +E×B and −E×B azimuthal directions. The current–voltage characteristics are studied and found to follow a negative resistance profile at the lower discharge currents. The existence of coherent structures and the number of spokes observed depend on the discharge current and operating pressure. Lower pressures are more favorable to the retrograde −E×B spoke rotation, which is observed in the 27, 40, and 67 Pa discharges. The results are interpreted with a gradient-driven drift wave model, and using the dispersion relation, we show that the propagation direction depends on the orientation of the local electric field within the plasma. Using an electron fluid model, we show how experimental conditions can lead to field reversals that generate −E×B spoke rotation in the lower pressure discharges.

Physics↗
DOE OSTI2023

Destabilizing effects of edge infernal components on resistive wall modes in advanced tokamak scenarios

The stability of the n = 1 resistive wall modes (RWMs) dominated by the pressure-driven infernal components is investigated using the ideal magnetohydrodynamics (MHD) code AEGIS for the advanced tokamak scenarios. Here, n is the toroidal mode number. In the advanced tokamak scenarios, due to the large fraction of bootstrap current contribution, the profile of safety factor q is deeply reversed in magnetic shear in the central core region and locally flattened within the edge pedestal. Consequently, the pressure-driven infernal components develop in the corresponding flat-q regions of both core and edge. However, the edge infernal components dominate the n = 1 RWM structure and lead to lower β N limits for the advanced tokamak scenarios. In the framework of ideal MHD, the edge rotation is found the most critical to the stabilization due to the dominant influence of the edge infernal components, which should be maintained sufficiently large in magnitude and range in order for the rotation alone to fully suppress the n = 1 RWM in typical advanced tokamak scenarios.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Multiple switch-off slow shock solutions

A new electromagnetic quasi-neutral time-dependent implicit hybrid model for simulating the slow-mode switch-off shock is developed, in which the electrons are treated as a finite-mass adiabatic fluid (gamma sub e = 5/3), whose motion along the shock normal provides local charge neutrality. The model is used to examine the effects of various boundary conditions applied during shock formation. It is shown that, for each upstream condition, two distinctly different slow-shock structures can be obtained: a low-shock structure with a trailing magnetic wave (TMW) and a slow-shock structure without a TMW, depending strongly on the boundary conditions for the electric and magnetic fields at the physical piston which generates the shock. The results indicate that the slow-shock structure with a TMW is nonlinearly unstable with respect to changes in the upstream magnetic field, suggesting that the downstream structure of slow shocks need not be associated with a TMW.

Vu, H. X.↗
NASA NTRS1995

Four large-scale field-aligned current systmes in the dayside high-latitude region

A system of four current sheets of large-scale field-aligned currents (FACs) was discovered in the data set of simultaneous Viking and Defense Meteorological Satellire Program-F7 (DMSP-F7) crossing of the dayside high-latitude region. This paper reports four examples of this system that were observed in the prenoon sector. The flow polarities of FACs are upward, downward, upward, and downward, from equatorward to poleward. The lowest-latitude upward current is flowing mostly in the central plasma sheet (CPS) precipitation region, often overlapping with the boundary plasma sheet (BPS) at its poleward edge, andis interpreted as a region 2 current. The pair of downward and upward FACs in the middle of te structure are collocated with structured electron precipitation. The precipitation of high-energy (greater than 1 keV) electrons is more intense in the lower-latitude downward current sheet. The highest-latitude downward flowing current sheet is located in a weak, low-energy particle precipitation region, suggesting that this current is flowing on open field lines. Simulaneous observations in the postnoon local time sector reveal the standard three-sheet structure of FACs, sometimes described as region 2, region 1, and mantle (referred to the midday region O) currents. A high correlation was found between the occurrence of the four FAC sheet structure and negative interplanetary magnetic field (IMF) B(sub Y). We discuss the FAC structurein terms of three types of convection cells: the merging, viscous, andlobe cells. During strongly negative IMF B(sub Y), two convection reversals exist in the prenoon sector; one is inside the viscous cell, and the other is between the viscous cell and the lobe cell. This structure of convection flow is supported by the Viking electric field and auroral UV image data. Based on the convection pattern, the four FAC sheet structure is interpreted as the latitude overlap of midday and morning FAC systems. We suggest that the for-current sheet structure is common in a certain prenoon localtime sector during strongly negative IMF B(sub Y).

Ohtani, S.↗
NASA NTRS2012

Longitudinal Differences of Ionospheric Vertical Density Distribution and Equatorial Electrodynamics

Accurate estimation of global vertical distribution of ionospheric and plasmaspheric density as a function of local time, season, and magnetic activity is required to improve the operation of space-based navigation and communication systems. The vertical density distribution, especially at low and equatorial latitudes, is governed by the equatorial electrodynamics that produces a vertical driving force. The vertical structure of the equatorial density distribution can be observed by using tomographic reconstruction techniques on ground-based global positioning system (GPS) total electron content (TEC). Similarly, the vertical drift, which is one of the driving mechanisms that govern equatorial electrodynamics and strongly affect the structure and dynamics of the ionosphere in the low/midlatitude region, can be estimated using ground magnetometer observations. We present tomographically reconstructed density distribution and the corresponding vertical drifts at two different longitudes: the East African and west South American sectors. Chains of GPS stations in the east African and west South American longitudinal sectors, covering the equatorial anomaly region of meridian approx. 37 deg and 290 deg E, respectively, are used to reconstruct the vertical density distribution. Similarly, magnetometer sites of African Meridian B-field Education and Research (AMBER) and INTERMAGNET for the east African sector and South American Meridional B-field Array (SAMBA) and Low Latitude Ionospheric Sensor Network (LISN) are used to estimate the vertical drift velocity at two distinct longitudes. The comparison between the reconstructed and Jicamarca Incoherent Scatter Radar (ISR) measured density profiles shows excellent agreement, demonstrating the usefulness of tomographic reconstruction technique in providing the vertical density distribution at different longitudes. Similarly, the comparison between magnetometer estimated vertical drift and other independent drift observation, such as from VEFI onboard Communication/Navigation Outage Forecasting System (C/NOFS) satellite and JULIA radar, is equally promising. The observations at different longitudes suggest that the vertical drift velocities and the vertical density distribution have significant longitudinal differences; especially the equatorial anomaly peaks expand to higher latitudes more in American sector than the African sector, indicating that the vertical drift in the American sector is stronger than the African sector.

total electron content↗
DOE OSTI2022

Entanglement between a muon spin and $I>\frac{1}{2}$ nuclear spins

We report on the first example of quantum coherence between the spins of muons and quadrupolar nuclei. We observe this effect in vanadium intermetallic compounds which adopt the A15 crystal structure, and whose members include all technologically dominant superconductors. The entangled states are extremely sensitive to the local structural and electronic environments through the electric field gradient at the quadrupolar nuclei. This case-study demonstrates that positive muons can be used as a quantum sensing tool to probe also structural and charge related phenomena in materials, even in the absence of magnetic order.

Bonfà, Pietro↗
DOE OSTI2021

Pressure-induced spin crossover in a Fe 78 Si 9 B 13 metallic glass

The pressure effect on structures and properties of a Fe 78 Si 9 B 13 metallic glass was investigated by in situ high-pressure synchrotron Fe Kβ x-ray emission spectroscopy and x-ray diffraction, and electrical resistivity measurements up to ~51 GPa. The study reveals a reversible and continuous pressure-induced high- to low-spin crossover of Fe atoms in an amorphous structure. The changes of the local spin moment can be scaled to match its average atomic distance shrinkage very well during compression. The crossover of electronic spin states in the Fe 78 Si 9 B 13 metallic glass resembles that of typical crystalline Fe-bearing materials but without a sharp atomic volume collapse and an abrupt electrical resistivity jump. Furthermore, these findings could help guide applications of Fe-based metallic glasses as a soft ferromagnetic material at extreme conditions and also improve our understanding of magnetism and coupling of its changes with disordered atomic structures and other properties in metallic glasses.

36 MATERIALS SCIENCE↗
NASA NTRS1982

DE-2 cusp observations - Role of plasma instabilities in topside ionospheric heating and density fluctuations

Observations by the low altitude Dynamics Explorer satellite (DE-2) in the polar cusp show the ionospheric plasma electron temperature and the ratio of electron to ion temperature to be increased, and the electron density fluctuations to be enhanced. Also, downward fluxes of energetic eletrons and ions increase in the cusp, and the magnetic field structure are consistent with the existence of a field-aligned current. Simultaneously, there is characteristic broadband electrostatic noise (BEN) with amplitudes of 1-10 mV/m, peaking in the cusp but extending into the polar cap. These emissions range from far below the local 0(+) gyrofrequency F sub O(+) to the vicinity of the proton gyrofrequency, but below the oxygen lower hybrid frequency. The BEN observations are compared to the predictions of several theories. The amplitude of these waves is shown to be far too small to contribute significantly to the observed ionospheric heating or density fluctuations by local wave-particle interactions. Rather, the observed spatial variations are attributed to nonlocal field aligned heating processes and reflect the nonuniformity of the magnetosheath plasma's penetration into the ionosphere.

Curtis, S. A.↗
NASA NTRS2012

Shocklets, SLAMS, and Field-Aligned Ion Beams in the Terrestrial Foreshock

We present Wind spacecraft observations of ion distributions showing field- aligned beams (FABs) and large-amplitude magnetic fluctuations composed of a series of shocklets and short large-amplitude magnetic structures (SLAMS). The FABs are found to have T(sub k) approx 80-850 eV, V(sub b)/V(sub sw) approx 1.3-2.4, T(sub perpendicular,b)/T(sub paralell,b) approx 1-8, and n(sub b)/n(sub o) approx 0.2-11%. Saturation amplitudes for ion/ion resonant and non-resonant instabilities are too small to explain the observed SLAMS amplitudes. We show two examples where groups of SLAMS can act like a local quasi-perpendicular shock reflecting ions to produce the FABs, a scenario distinct from the more-common production at the quasi-perpendicular bow shock. The SLAMS exhibit a foot-like magnetic enhancement with a leading magnetosonic whistler train, consistent with previous observations. Strong ion and electron heating are observed within the series of shocklets and SLAMS with temperatures increasing by factors approx > 5 and approx >3, respectively. Both the core and halo electron components show strong perpendicular heating inside the feature.

Wilson, L. B.↗
DOE OSTI2023

ORNL_AISD_NiPt

This dataset describes the nickel-platinum (NiPt) solid solution binary alloy, where the two constituent elements nickel (Ni) and platinum (Pt) are randomly placed on the face centered cubic (FCC) crystal structure, with the lattice constant of 3.840 angstroms. The dataset comprises data for three different sizes of the crystal structure: 256 atoms, 864 atoms, and 2,048 atoms, each of which contains 1900 configurations. For each size of the crystal structure, the data set was generated for concentrations ranging from 0at% of Pt to 100at% of Pt in the NiPt binary system, with increasing the concentration of Pt in the system every 5at%. For each one of the chemical compositions, 100 random configurations were generated, each with a different random seed. Each of the output files contains the mass, type, atomic coordinates, energy per atom, and forces in x, y, and z directions respectively. For each atomic configuration, the output was collected every 150 steps during the minimization stage and every 1000 steps during the replica exchange stage. Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [1], which is a molecular dynamics code, was used to generate data for NiPt alloy. The simulation used the interatomic potential for NiPt binary system MEAM_LAMMPS_KimSeolJi_2017_PtNi__MO_020840179467_001 [3] from the OpenKIM library (Open Knowledgebase of Interatomic Models) [2]. This potential was developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM). The simulation process begins with the generation of the random NiPt structure and follows with the short minimization and replica exchange simulation. The minimization procedure adjusts atomic coordinates and performs energy minimization, which typically leads to a local potential energy minimum. The method used for the minimization was the conjugate gradient algorithm. A short replica exchange (parallel tempering) simulation involves four replicas (ensembles) of a system and follows the minimization stage. Multiple snapshots of the configuration were collected during the minimization and replica exchange stages. NiPt alloy is interesting due to its magnetic and charge transfer properties [4]. The data is provided in three compressed zipped folders: atoms256.zip, atoms864.zip, atoms2048.zip Each zipped folder contains the data that describes crystals of size 256 atoms, 864 atoms, and 2,048 atoms respectively. Each one of the three zipped folders contains the data structured in the following way: -Ni_ground_state.cfg --> atomic configuration for the pure nickel -Pt_ground_state.cfg --> atomic configuration for the pure platinum -Pt#_filtered --> folders containing atomic configurations for #at% concentration of platinum. The folder contains 100 atomic configurations, each saved in a subfolder. Each subfolder named config* is associated with a specific atomic configuration. Each of these subfolders contains files with .cfg format, corresponding to outputs for each atomic configuration The total number of atomic configurations contained in atoms256.zip is 65,046. The total number of atomic configurations contained in atoms864.zip is 63,936. The total number of atomic configurations contained in atoms2048.zip is 61,997. The total number of atomic configurations spanned by the entire dataset is 190,979. References [1] https://www.lammps.org/ [2] https://openkim.org/ [3] https://openkim.org/id/MEAM_LAMMPS_KimSeolJi_2017_PtNi__MO_020840179467_001 [4] El-Gendy, Ahmed A. and Hampel, Silke and Büccchner, Bernd and Klingeler, Rüdiger, Tuneable magnetic properties of carbon-shielded NiPt-nanoalloys, RSC Adv., volume 6, issue 57, pages 52427-52433, 2016, The Royal Society of Chemistry, doi:10.1039/C6RA05910D

36 MATERIALS SCIENCE↗
DOE OSTI2021

Mass enhancement in 3 d and s - p perovskites from symmetry breaking

In some d-electron oxides, the measured effective mass m* exptl has long been known to be significantly larger than the model effective mass m* model deduced from mean-field band theory, i.e., m* exptl =βm* model , where β>1 is the “mass-enhancement” or “mass-renormalization” factor. Previous applications of density functional theory (DFT), based on a symmetry-restricted structure with the smallest number of possible magnetic, orbital, and structural degrees of freedom, missed such mass enhancement. This fact has been taken as evidence of strong electronic correlation, often described via the symmetry-restricted dynamic mean-field approach of the many-body theory, being the exclusive enabling physics. This paper uses instead a static density functional approach that does not restrict positional or spin degrees of freedom (symmetry-broken structures). This approach analyzes the contributions of different symmetry-broken modalities to mass enhancement for a few nominally highly correlated d-electron perovskites as well as the nominally uncorrelated, closed-shell s-p bonding perovskites. It shows that the energy-lowering symmetry-broken spin effects (e.g., nonzero local moment in the paramagnetic phase) and structural effects (e.g., atomic displacement) as described in mean-field DFT already manifest mass enhancement for both electrons and holes in a range of d-electron perovskites SrVO 3 , SrTiO 3 , BaTiO 3 , and LaMnO 3 , as well as p-electron perovskites CsPbI 3 and SrBiO 3 , including both metals (SrVO 3 ) and insulators (the rest). This is revealed only when enlarged unit cells of the same parent global symmetry, which are large enough to allow for symmetry-breaking distortions and concomitant variations in spin order, are explored for their ability to lower the total energy. Positional symmetry breaking that leads to mass enhancement includes octahedral rotation in halide perovskites such as CsPbI 3 , Jahn-Teller-like Q${+}\atop{2}$ distortion in LaMnO 3 , and bond disproportionation in SrBiO 3 , while magnetic symmetry breaking resulting in mass enhancement includes the formation of a distribution of local moments in SrVO3 that averages to zero in the paramagnetic phase. Not all symmetry breaking leads to significant mass enhancement, e.g., the rather small octahedral rotations in the nearly perfectly cubic SrTiO 3 cause negligible mass enhancement, as do the paraelectric displacements in BaTiO 3 . In principle, by ergodicity, the two descriptions, i.e., the symmetry-restricted dynamic approach with a single time-fluctuating magnetic moment and the symmetry-broken mean-field approach with a static distribution of spatially fluctuated local moments, are not mutually exclusive but are a choice of representation and consequently, a choice of computational efficiency. In approximate implementations, the symmetry-broken mean-field approach appears to remove much of what was strong correlation in dynamically correlated symmetry-restricted solutions, leaving smaller (“weak”) residual correlation with respect to the exact solution.

36 MATERIALS SCIENCE↗