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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 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Resistivity and magnetoresistance properties of R 2 NiSi 3 (R = Gd, Dy, Ho, Er, Tm) compounds

The resistivity and magnetoresistance behaviour of the hexagonal intermetallic compounds R 2 NiSi 3 (R = Gd, Dy, Ho, Er, and Tm) are reported here. All the studied polycrystalline compounds exhibit metallic behavior along with additional magnetic anomalies at low temperatures. A well-defined resistivity minima is observed in Gd 2 NiSi 3 and Dy 2 Ni 0.87 Si 2.95 at a temperature much higher than their respective magnetic transition temperatures. The anomaly has been ascribed to the charge carrier localization caused by magnetic precursor effect. Magnetic field induced crossover from positive to negative magnetoresistance (MR) behavior associated with antiferromagnetic ground state is evidenced for Gd 2 NiSi 3 and Er 2 NiSi 3 in the low temperature region. Although Tm 2 Ni 0.93 Si 2.93 does not exhibit any long range magnetic order down to 2 K, a sudden drop in resistivity behavior is observed below ~10 K. Furthermore, presence of short range magnetic correlation observed in a wide temperature range, much beyond their respective magnetic ordering temperatures, has been argued to be responsible for achieving finite negative MR for all the compounds. Additionally, a subtle resemblance between the observed transport anomalies and the magnetic properties of these systems has been discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Iridium valence variation and carrier sign tuning in ( Ca , Ba ) x La 2 – x CuIrO 6 double perovskites

Here, we report the structure and properties of ( Ca , Ba ) x La 2 – x CuIrO 6 . The rock-salt double perovskite structure goes through a P 1 ¯ to P 2 1 / n phase transition with increasing Ba content but not with Ca content. In both cases, the Ir ion is oxidized from 4 + to 5 + with increasing substitution. Transport and magnetic properties measurements reveal that all compositions are insulating with hysteretic spin-freezing magnetic behavior. Seebeck coefficient measurements reveal a large thermopower change from + 176 μ V /K in La 2 CuIrO 6 to – 223 μ V /K with increasing Ba content in Ba x La 2 – x CuIrO 6 . This behavior is modeled well by the Heikes formula for correlated hopping conduction. ( Ca , Ba ) x La 2 – x CuIrO 6 provides a model system for investigations of other systems with similar carrier sign tuning, with applications toward semiconductors and thermoelectric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetic and electrocatalytic properties of transition metal doped MoS 2 nanocrystals

In this paper, the magnetic and electrocatalytic properties of hydrothermally grown transition metal doped (10% of Co, Ni, Fe, and Mn) 2H-MoS 2 nanocrystals (NCs) with a particle size 25–30 nm are reported. The pristine 2H-MoS 2 NCs showed a mixture of canted anti-ferromagnetic and ferromagnetic behavior. While Co, Ni, and Fe doped MoS 2 NCs revealed room temperature ferromagnetism, Mn doped MoS 2 NCs showed room temperature paramagnetism, predominantly. The ground state of all the materials is found to be canted-antiferromagnetic phase. To study electrocatalytic performance for hydrogen evolution reaction, polarization curves were measured for undoped and the doped MoS 2 NCs. At the overpotential of η = –300 mV, the current densities, listed from greatest to least, are FeMoS 2 , CoMoS 2 , MoS 2 , NiMoS 2 , and MnMoS2, and the order of catalytic activity found from Tafel slopes is CoMoS2 > MoS 2 > NiMoS 2 > FeMoS 2 > MnMoS 2 . The increasing number of catalytically active sites in Co doped MoS 2 NCs might be responsible for their superior electrocatalytic activity. In conclusion, the present results show that the magnetic order-disorder behavior and catalytic activity can be modulated by choosing the suitable dopants in NCs of 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Promising Graphene-Like Half-Metallic Nanosheets TM-InSe (TM = Mn, Fe, and Co) Induced by TM Adsorption

Several half-metallic graphene-like nanosheets, namely transition metal (TM)-adsorbed monolayers (TM-InSe, TM = Mn, Fe, and Co), are predicted by the first-principles calculations. Their structural, electric, and magnetic properties are studied in detail. The adsorption energies of these nanosheets are negative so that they have stable adsorption structures, indicating that it is possible to prepare them in experiments. The pristine InSe nanosheet is a typical semiconductor, but TM-InSe nanosheets are conductive. They are possible half-metallic 2D materials. Calculated total magnetic moments of TM-InSe nanosheets are about 5.18, 3.93, and 1.28 μ{sub B} for TM = Mn, Fe, and Co, respectively. Their conductibility, half-metallicity, and magnetic moments come mainly from adsorbed TM ions, partially from In and Se ions. For Mn-InSe and Fe-InSe nanosheets, TM-3d4s and Se-4p{sub z} electrons enter into the molecular orbitals in high-spin sequence, but for the Co-InSe nanosheet, electrons enter into the molecular orbitals in lowspin sequence. Electronic structures of the Mn-InSe, Fe-InSe and Co-InSe nanosheets are σ{sup 1} ↑ σ{sup 1} ↓ d{sup 5} ↑ d{sup 1} ↓ σ*{sup 1} ↑, σ{sup 1} ↑ σ{sup 1} ↓ d{sup 5} ↑ d{sup 2} ↓ σ*{sup 1} ↑, and σ{sup 1} ↑ σ{sup 1} ↓ d{sup 5} ↑ d{sup 4} ↓, respectively. The total moments 5.00, 4.00, and 1.00 μ{sub B} of TM-InSe nanosheets based on these electronic structures are basically in accordance with calculated values, respectively.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Ultrafast dynamics in van der Waals heterostructures

Van der Waals heterostructures are synthetic quantum materials composed of stacks of atomically thin two-dimensional (2D) layers. Because the electrons in the atomically thin 2D layers are exposed to layer-to-layer coupling, the properties of van der Waals heterostructures are defined not only by the constituent monolayers, but also by the interactions between the layers. Many fascinating electrical, optical and magnetic properties have recently been reported in different types of van der Waals heterostructures. In this Review, we focus on unique excited-state dynamics in transition metal dichalcogenide (TMDC) heterostructures. TMDC monolayers are the most widely studied 2D semiconductors, featuring prominent exciton states and accessibility to the valley degree of freedom. Many TMDC heterostructures are characterized by a staggered band alignment. This band alignment has profound effects on the evolution of the excited states in heterostructures, including ultrafast charge transfer between the layers, the formation of interlayer excitons, and the existence of long-lived spin and valley polarization in resident carriers. Furthermore we review recent experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures, extending from timescales of femtoseconds to microseconds, and comment on the relevance of these effects for potential applications in optoelectronic, valleytronic and spintronic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Recent progress of MnBi 2 Te 4 epitaxial thin films as a platform for realising the quantum anomalous Hall effect

Since the first realisation of the quantum anomalous Hall effect (QAHE) in a dilute magnetic-doped topological insulator thin film in 2013, the quantisation temperature has been limited to less than 1 K due to magnetic disorder in dilute magnetic systems. With magnetic moments ordered into the crystal lattice, the intrinsic magnetic topological insulator MnBi 2 Te 4 has the potential to eliminate or significantly reduce magnetic disorder and improve the quantisation temperature. Surprisingly, to date, the QAHE has yet to be observed in molecular beam epitaxy (MBE)-grown MnBi 2 Te 4 thin films at zero magnetic field, and what leads to the difficulty in quantisation is still an active research area. Although bulk MnBi 2 Te 4 and exfoliated flakes have been well studied, revealing both the QAHE and axion insulator phases, experimental progress on MBE thin films has been slower. Furthermore, understanding how the breakdown of the QAHE occurs in MnBi 2 Te 4 thin films and finding solutions that will enable mass-produced millimetre-size QAHE devices operating at elevated temperatures are required. In this mini-review, we will summarise recent studies on the electronic and magnetic properties of MBE MnBi 2 Te 4 thin films and discuss mechanisms that could explain the failure of the QAHE from the aspects of defects, electronic structure, magnetic order, and consequences of their delicate interplay. Finally, we propose several strategies for realising the QAHE at elevated temperatures in MnBi 2 Te 4 thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Single-ion anisotropy is necessary and appropriate to study the magnetic behavior of ${\mathrm{Tb}}^{3+}$ moments with ${J}_{\text{eff}}=\frac{1}{2}$ on the honeycomb lattice in ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$

By developing models of increasing complexity, we show that a model without single-ion anisotropy (SIA) cannot explain the magnetic properties of J eff = 1 / 2 Tb 3+ moments in the orthorhombically distorted, honeycomb material Tb 2 Ir 3 Ga 9 . In four different models for the magnetization of a single honeycomb layer, the only sources of anisotropy are symmetric exchange interactions J nαβ = J nβα along three different bonds n , an anisotropic $\underline{g}$ tensor, and a Dzyalloshinskii-Moriya interaction (asymmetric exchange) that produces the observed canted moment along b . With 21 parameters, the best such model yields χ 2 = 0.065 , which is substantially smaller than χ 2 = 0.112 obtained using a Heisenberg model containing six parameters including easy-axis anisotropy. However, models without SIA fail to reproduce the linear dependence of the magnetization with a field perpendicular to the Ising axis while predicting a saturation magnetization that is far too low. Due to the complex crystal-field environments, we argue that SIA is necessary to study low-symmetry, three-dimensional J eff = 1 / 2 materials containing Tb 3+ ions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Magnetic dichroism in the Kondo insulator SmB 6

Samarium hexaboride (${\mathrm{SmB}}_{6}$) is a purported topological Kondo insulator, with theory predicting that the experimentally observed metallic surface states manifest from a topologically nontrivial insulating bulk band structure. The insulating bulk itself is driven by strong correlations, and both bulk and surface are known to host compelling magnetic and electronic phenomena. We employed here x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Sm ${M}_{4,5}$ edges to probe the surface and bulk magnetic properties of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ within ${\mathrm{SmB}}_{6}$. We observed an unexpected antialignment to the applied field of the ${\mathrm{Sm}}^{3+}$ magnetic dipole moment below $T=75$ K and of the total orbital moment of samarium below 30 K. The total bulk magnetization at 2 K is, however, positive and driven by ${\mathrm{Sm}}^{2+}$ Van Vleck susceptibility as well as 1% paramagnetic impurities with ${{\mu}}_{\mathrm{eff}}=5.2(1){{\mu}}_{\mathrm{B}}$. This indicates the diamagneticlike ${\mathrm{Sm}}^{3+}$ magnetism is only a portion of the net magnetization, partially offsetting the response of paramagnetic impurities known within the bulk.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Competing magnetic states in multiferroic BaYFeO 4 : A high magnetic field study

Spin-driven ferroelectricity phenomena have drawn great interest in the scientific community due to potential application in spintronics and their complex physical mechanisms. A noticeable example of this is multiferroic BaYFeO 4 that exhibits an unconventional magnetoelectric (ME) coupling due to the uncorrelated behavior of the ferroelectric and cycloidal states under an applied magnetic field. To shed more light on this spin-driven ME effect, a high-quality sample of BaYFeO 4 was synthesized by a standard solid-state reaction method, and its high-field (up to 9 T) magnetic properties have been systematically investigated by means of magnetometry, magnetocaloric effect, and Mössbauer measurements over a wide temperature range (5–400 K). In addition, its crystal and magnetic structures have been studied using x-ray and neutron powder diffraction. Results obtained in this work indicate that Fe spins form a long-range spin density wave (SDW) antiferromagnetic (AFM) order at $T_{\text{N1}}$ ~ 50K, which transforms into the cycloidal AFM order at $T_{\text{N2}}$ ~ 35K. A spin-glass-like state emerges below $T^\ast$ ~ 17K, and coexists with the long-range cycloidal AFM one in this temperature range. Magnetocaloric and Mössbauer measurements consistently confirm the robustness of both the long-range SDWand cycloidal AFM orders under applied magnetic fields up to 6 T, whereas the spin-glass state is converted into the ferromagnetic (FM) state when the applied magnetic field exceeds 1 T. These findings pinpoint the fact that the magnetic field evolution of spin correlations from the AFM to FM character in the spin-glass state is responsible for the magnetic field dependence of ferroelectricity in BaYFeO 4 .

36 MATERIALS SCIENCE↗
DOE OSTI2025

Experiments in 3D Printing Electric Motors

This paper catalogues a series of experiments we conducted to explore how to 3D print a DC electric motor. The individual parts of the electric motor were 3D printed but assembled by hand. First, we focused on a rotor with soft magnetic properties, for which we adopted ProtoPasta TM , which is a commercial off-the-shelf PLA filament incorporating iron particles. Second, we focused on the stator permanent magnets, which were 3D printed through binder jetting. Third, we focused on the wire coils, for which we adopted a form of laminated object manufacture of copper wire. The chief challenge was in 3D printing the coils, because the winding density is crucial to the performance of the motor. We have demonstrated that DC electric motors can be 3D printed and assembled into a functional system. Although the performance was poor due to the wiring problem, we showed that the other 3D printing processes were consistent with high performance. Nevertheless, we demonstrated the principle of 3D printing electric motors.

3D printed motor↗
DOE OSTI2021

Effect of Polymer Removal on the Morphology and Phase of the Nanoparticles in All-Inorganic Heterostructures Synthesized via Two-Step Polymer Infiltration

Polymer templates play an essential role in the robust infiltration-based synthesis of functional multicomponent heterostructures with controlled structure, porosity, and composition. Such heterostructures are be used as hybrid organic–inorganic composites or as all-inorganic systems once the polymer templates are removed. Using iron oxide/alumina heterostructures formed by two-step infiltration of polystyrene-block-polyvinyl pyridine block copolymer with iron and aluminum precursors from the solution and vapor-phases, respectively, we show that the phase and morphology of iron oxide nanoparticles dramatically depend on the approach used to remove the polymer. We demonstrate that thermal and plasma oxidative treatments result in iron oxide nanoparticles with either solid or hollow morphologies, respectively, that lead to different magnetic properties of the resulting materials. Our study extends the boundaries of structure manipulations in multicomponent heterostructures synthesized using polymer infiltration synthesis, and hence their properties.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
NASA NTRS1989

Characterizations of electrical properties of highT(sub c) superconducting materials

The automated data acquisition system developed in the Space Science Laboratory at Marshall Space Flight Center for measuring electrical properties of high temperature superconductors is described. The acquisition system, consisting of a computer and computer-controlled hardware, allows large numbers of voltage, current, temperature, and magnetic measurements to be performed on bulk and thin film samples. Typical results are shown characterizing transition temperature (T sub c), critical current density (J sub c), and magnetic properties of bulk high T(sub c) materials as a function of temperature.

Sisk, R. C.↗
NASA NTRS2012

Photospheric Magnetic Evolution in the WHI Active Regions

Sequences of line-of-sight (LOS) magnetograms recorded by the Michelson Doppler Imager are used to quantitatively characterize photospheric magnetic structure and evolution in three active regions that rotated across the Sun s disk during the Whole Heliosphere Interval (WHI), in an attempt to relate the photospheric magnetic properties of these active regions to flares and coronal mass ejections (CMEs). Several approaches are used in our analysis, on scales ranging from whole active regions, to magnetic features, to supergranular scales, and, finally, to individual pixels. We calculated several parameterizations of magnetic structure and evolution that have previously been associated with flare and CME activity, including total unsigned magnetic flux, magnetic flux near polarity-inversion lines, amount of canceled flux, the "proxy Poynting flux," and helicity flux. To catalog flare events, we used flare lists derived from both GOES and RHESSI observations. By most such measures, AR 10988 should have been the most flare- and CME-productive active region, and AR 10989 the least. Observations, however, were not consistent with this expectation: ARs 10988 and 10989 produced similar numbers of flares, and AR 10989 also produced a few CMEs. These results highlight present limitations of statistics-based flare and CME forecasting tools that rely upon line-of-sight photospheric magnetic data alone.

Welsch, B. T.↗
DOE OSTI2019

Correlation Between Structural, Magnetic and Dielectric Properties of Microwave Sintered Ni-Zn-Al Nanoferrites

A series of diamagnetic aluminum (Al{sup 3+}) substituted Ni-Zn Nanoferrites have been synthesized using sol-gel auto combustion route. Structural, magnetic, and dielectric properties were systematically studied and reported with respect to Al{sup 3+} substitution in host Fe{sup 3+} ions. X-ray diffraction (XRD) and infrared spectroscopy (IR) measurements confirm the presence of Al{sup 3+} ions at both A and B sites. Cation distribution proposed from XRD and IR correlates with magnetic and dielectric results. It is observed that Al{sup 3+} ions are distributed in to tetrahedral and octahedral sub-lattices. The saturation magnetization varies between 58.5 and 44.2 emu/g with increasing Al{sup 3+} substitution. This decrease in magnetization is ascribed to presence of the non-collinear spin (canted spin) structure in octahedral sub-lattice. Dielectric constant decreased and a significant improvement in AC resistivity is observed with Al{sup 3+} substitution. The observed variations are accounted on the basis of cation distribution in spinel ferrite.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Transient gap generation in BaFe2As2 driven by coherent lattice vibrations

Abstract Iron-based superconductors provide a rich platform to investigate the interplay between unconventional superconductivity, nematicity, and magnetism. The electronic structure and the magnetic properties of iron-based superconductors are highly sensitive to the pnictogen height. Coherent excitation of the A1g phonon by femtosecond laser directly modulates the pnictogen height, which has been used to control the physical properties of iron-based superconductors. Previous studies show that the driven A1g phonon resulted in a transient increase of the pnictogen height in BaFe2As2, favoring an enhanced Fe magnetic moment. However, there are no direct observations on either the enhanced Fe magnetic moments or the enhanced spin-density wave (SDW) gap. Here, we use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFe2As2 in the A1g phonon-driven state. Below the SDW transition temperature, we observe a transient gap generation at early-time delays. A similar transient feature is observed in the normal state up to room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Inhomogeneous magnetic ordered state and evolution of magnetic fluctuations in Sr⁢(Co 1–x ⁢Ni x ) 2 ⁢P 2 revealed by 31 P NMR

SrCo 2 ⁢P 2 with a tetragonal structure is known to be a Stoner-enhanced Pauli paramagnetic metal being nearly ferromagnetic. Recently J. Schmidt et al. [Phys. Rev. B 108, 174415 (2023)] reported that a ferromagnetic ordered state is actually induced by a small Ni substitution for Co of x = 0.02 in Sr⁢(Co 1–x ⁢Ni x ) 2 ⁢P 2 where an antiferromagnetic ordered phase also appears by further Ni substitution with x = 0.06–0.35. Here, in this work, using nuclear magnetic resonance (NMR) measurements on 31 P nuclei, we have investigated how the magnetic properties change by the Ni substitution in Sr⁢(Co 1–x ⁢Ni x ) 2 ⁢P 2 from a microscopic point of view, especially focusing on the evolution of magnetic fluctuations with the Ni substitution and the characterization of the magnetically ordered states. The temperature dependencies of the 31 P spin-lattice relaxation rate divided by temperature (1/T 1 ⁢T) and Knight shift (K) for SrCo 2 ⁢P 2 are reasonably explained by a model where a double-peak structure for the density of states near the Fermi energy is assumed. Based on a Korringa ratio analysis using the T 1 and K data, ferromagnetic spin fluctuations are found to dominate in the ferromagnetic Sr⁢(Co 1–x ⁢Ni x ) 2 ⁢P 2 as well as the antiferromagnets where no clear antiferromagnetic fluctuations are observed. We also found the distribution of the ordered Co moments in the magnetically ordered states from the analysis of the 31 P-NMR spectra exhibiting a characteristic rectangular-like shape.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Fermi surface of the flat-band intermetallics A P d 3 ( A = Pb , Sn )

The intermetallic phases APd 3 (A = Pb, Sn) were recently predicted to host an unconventional combination of unique electronic structure features, namely, flat bands near the Fermi energy coexisting with topologically protected surface states at the Gamma point. These features each could independently produce alternative electronic states, including electronically or magnetically ordered states coexisting with unconventional edge dominated transport and a significantly large thermopower coexisting with topological characteristics. To explore these expectations, we report the synthesis, structural/chemical characterization, electrical and thermal transport properties, magnetic torque (up to 45 T), and Fermi surface mapping for single crystals produced using the Czochralski technique. X-ray diffraction and scanning transmission electron microscope measurements establish the absence of defects, while small measured values of the thermopower indicate that the Fermi level is located away from the flat-band region. The electronic properties are further clarified by the topography of the Fermi surfaces, measured through the de Haas–van Alphen effect. We find that the Fermi levels are placed at higher energy values than the original ones resulting from the density functional theory calculations, 54 meV higher for PbPd 3 and 68 meV higher for SnPd 3 . Furthermore, the molten flux method was used to synthesize PbPd 3 , yielding nearly identical Fermi surfaces between the specimens grown using different synthesis techniques, indicating the robustness of the Fermi level position. According to the density functional theory calculations, the flat band is mainly formed by the 4d bands of Pd. Thus, we propose monovalent doping on the Pb/Sn site as a viable approach to accessing the flat band while maintaining the unique band structure features of these compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗