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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 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Exceptional Radiation Absorption in a Pentagon-Based Si Allotrope

Excellent photovoltaic performance is predicted in a pentagonal covalent network of Si in a hollow structure exhibiting both thermal and dynamical stability. Consisting of a combination of sp 2 and sp 3 hybridized Si atomic orbitals, the GW0 computed band structure shows an indirect band gap near the zone edge and also a manifold of directly absorbing transitions at frequencies in the window of visible light, in distinction with conventional Si. Hydrogenation of a single sp 2 site is predicted to lead to a robust local magnetic moment. In this study, we find a low formation energy at low pressure that is compatible with other experimentally known phases, suggesting that a stable phase might be obtained.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Mechanisms of Complete Dissociation of CO 2 on Iron Clusters

Dissociation of CO 2 on iron clusters was studied by using semilocal density functional theory and basis sets of triple-zeta quality. Fe 2 , Fe 4 , and Fe 16 clusters were selected as the representative host clusters. When searching for isomers of Fe n CO 2 , n=2, 4 and 16 corresponding to carbon dioxide attachment to the host clusters, its reduction to O and CO, and to the complete dissociation, it was found that the total spin magnetic moments of the lowest energy states of the isomers are often quenched with respect to those of initial reagents Fe n +CO 2 . Dissociation pathways of the Fe 2 +CO 2 , Fe 4 +CO 2 , and Fe 16 +CO 2 reactions contain several transition states separated by the local minima states; therefore, a natural question is where do the spin flips occur? Since lifetimes of magnetically excited states were shown to be of the order of 100 fs, the search for the CO 2 dissociation pathways was performed under the assumption that magnetic deexcitation may occur at the intermediate local minima. Two dissociation pathways were obtained for each Fe n +CO 2 reaction using the gradient-based methods. It was found that the Fe 2 +CO 2 reaction is endothermic with respect to both reduction and complete dissociation of CO 2 , whereas the Fe 4 +CO 2 and Fe 16 +CO 2 reactions are exothermic to both reduction and complete dissociation of carbon dioxide. Finally, the CO 2 reduction was found to be more favorable than its complete dissociation in the Fe 4 case.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Interplay of spin-orbit coupling and hybridization in Ca 3 LiOsO 6 and Ca 3 LiRuO 6

Here, the electronic ground state of Ca 3 LiOsO 6 was recently considered within an intermediate coupling regime that revealed J = 3/2 spin-orbit entangled magnetic moments. Through inelastic neutron scattering and density functional theory we investigate the magnetic interactions and probe how the magnetism is influenced by the change in hierarchy of interactions as we move from Ca 3 LiOsO 6 (5d 3 ) to Ca 3 LiRuO 6 (4d 3 ). An alteration of the spin gap and ordered local moment is observed, however the magnetic structure, Néel temperature, and exchange interactions are unaltered. To explain this behavior it is necessary to include both spin-orbit coupling and hybridization, indicating the importance of an intermediate coupling approach when describing 5d oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Novel Theoretical Prediction of Structural Properties, Ferromagnetic Ordering, and Electronic Structure of Fe{sub 2}PbC MAX Phase

In this study, we have investigated the magnetic stability; ferromagnetic ordering; and structural, elastic, and electronic properties of Fe{sub 2}PbC compound. The calculations are performed by the use of full-potential linearized augmented plane wave method implemented in WIEN2k code. We have found that the ferromagnetic state of Fe{sub 2}PbC is more stable than non-magnetic state configuration at their equilibrium lattice parameters. The structural and elastic properties suggest that Fe{sub 2}PbC is an anisotropic material, which is stable in the hexagonal MAX phase. The electronic structures show that Fe{sub 2}PbC has metallic-covalent-ionic character. The ferromagnetic performance of Fe{sub 2}PbC is confirmed by the total magnetic moment of 4.15 μ{sub B}, where its main contribution is originated from Fe atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electronic Structure and Exchange Interactions in RNi{sub 4}Co (R = Eu, Yb) Compounds

The electronic structure and the exchange interactions in EuNi{sub 4}Co and YbNi{sub 4}Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + U method); the variants of substitution of cobalt ion for nickel in the 3d lattice in both types of crystallographic positions 2c and 3g are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3g type. A Co ion in RNi{sub 4}Co (R = Eu, Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3d metal sublattice.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Unusual Behavior of the Coercive Field in a (CoFeB){sub x}(LiNbO{sub y}){sub 100 –x} Nanocomposite with a High Content of Magnetic Ions in an Insulating Matrix

The temperature behavior of the coercive field H{sub c}(T) and the magnetic anisotropy in (CoFeB){sub x}(LiNbO{sub y}){sub 100 –x} nanocomposite films with a ferromagnetic alloy content x = 33–48 at % near the metal–insulator transition (x{sub c} ≈ 42 at %) have been studied by the magnetometry and ferromagnetic resonance methods. The films were ensembles of CoFe granules with lateral sizes of 2–4 nm, which are highly elongated (up to 10–15 nm) in the nanocomposite growth direction and embedded in a LiNbO{sub y} matrix with a high content of magnetic Fe{sup 2+} and Co{sup 2+} ions (up to 3 × 10{sup 22} cm{sup –3}). A nonmonotonic behavior of H{sub c}(T), viz., a sharp minimum at a temperature T{sub F} ≈ 50 K close to the blocking temperature (T{sub b}{sup ∗} ≈ 70 K) of the granule magnetic moment, has been detected in samples with x < 42 at %. The effective field of the perpendicular growth anisotropy (0.4–0.8 kOe) turns out to be an order of magnitude lower than the field of the granule shape anisotropy (about 7 kOe) and increases with x. The revealed peculiarities are explained by the fact that, apart from the ferromagnetic intergranular exchange interaction in an infinite cluster, the surface anisotropy effects involving magnetic ions in a thin layer adjacent to the cluster and responsible for the surface interaction fluctuations play a big role in the investigated percolation nanocomposites, enhancing the nanocomposite demagnetization at T ≈ T{sub F} ~ T{sub b}{sup ∗}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Addition of phenylacetylene and camphor to the complex [(dpp-bian)Eu(dme){sub 2}] (dpp-bian is the 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene dianion)

The reduction of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) with an excess of europium metal in 1,2-dimethoxyethane (dme) produces a divalent europium complex with the dpp-bian dianion, [(dpp-bian)Eu(dme){sub 2}] (1). The reactions of 1 with phenyl-acetylene and camphor proceed via protonation of the diimine ligand to form the monomeric amido-amino complexes of divalent europium — [H(dpp-bian)Eu(C≡CPh)(dme){sub 2}] (2) and [H(dpp-bian)Eu(camphor)(dme){sub 2}] (3), respectively. Compounds 2 and 3 were characterized by IR spectroscopy and elemental analysis. Their molecular structures were determined by X-ray diffraction. Compounds 2 and 3 were shown to be monomeric seven-coordinate europium(II) complexes with terminal phenylethynyl and enol ligands, respectively. According to the IR spectroscopic data, the terminal ligands in complexes 2 and 3 undergo tautomerization involving backward proton transfer from the amido-amino ligand to the substrate. The magnetic moment of compound 2 (8.03 μB) remains constant in the temperature range of 4—300 К and confirms the presence of divalent europium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Theoretical Investigations of Properties of New Half-Heusler Compounds NiFeZ (Z = Si, Ge)

The half-Heusler alloys NiFeZ (Z = Si, Ge) are investigated theoretically in the general framework of Density Functional Theory (DFT). We have calculated structural, mechanical, elastic, electronic, magnetic, and optical properties using WIEN2k simulation package. To incorporate the electronic exchange-correlation energy we have used the GGA+U approximation. Both the compounds are found structurally stable, however, elastic properties show that NiFeSi is ductile while NiFeGe is brittle. Electronically, the compounds are found to be half-metals with small band gaps in the spin-down channels whereas spin-up channels are conducting. The total magnetic moments for both the compounds are ≈ 2μ{sub B}. Optical properties including dielectric function, refractive index, extinction coefficient, reflectivity, conductivity, and absorption coefficient are also calculated and analyzed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Three-dimensional magnetic interactions in quasi-two-dimensional PdAs 2 O 6

Millimeter-sized PdAs 2 O 6 single crystals are grown using the vapor transport technique. The magnetic order at T N = 140 K is studied by measuring magnetic properties, specific heat, and neutron single crystal diffraction. The anisotropic magnetic susceptibility and a metamagnetic transition observed in magnetic fields above 20 kOe suggest that the magnetic moment lies in the ab plane, consistent with the magnetic structure determined by neutron single crystal diffraction. Below 140 K, Pd 2+ ions order ferromagnetically in the ab plane but antiferromagnetically along the crystallographic c axis. The ordered moment is refined to be 2.09(2) μ B /Pd 2+ using the fitted magnetic form factor of Pd 2+ . A weak λ-type anomaly around T N was observed in specific heat and the magnetic entropy change across T N is 1.72 J mol -1 K. This small entropy change and the temperature dependence of the magnetic susceptibility support the presence of short range correlations in a wide temperature range T N 2 O 6 suggests that the magnetic interactions in PdAs 2 O 6 are dominated by Pd-(O-$As\atop{As}$-O)-Pd super-superexchange and three dimensional despite the quasi-two-dimensional arrangement of magnetic ions. Finally, the comparison with NiAs 2 O 6 suggests that increasing covalency of isostructural compounds is an effective approach to design and to discover new materials with higher magnetic order temperatures in the localized regime.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Thermodynamics of spin ice in staggered and direct (along the [111] axis) fields in the cluster approximation

We have analyzed the low-temperature thermodynamic properties of spin ice in the staggered and direct (acting along the [111] axis) fields for rare-earth oxides with the chalcolamprite structure and general formula Re{sub 2}{sup 3+}Me{sub 2}{sup 4+}O{sub 7}{sup 2-}. Calculations have been performed in the cluster approximation. The results have been compared with experimental temperature dependences of heat capacity and entropy for Dy{sub 2}Ti{sub 2}O{sub 7} compound for different values of the external field in the [111] direction. The experimental data and calculated results have also been compared for the Pr{sub 2}Ru{sub 2}O{sub 7} compound with the antiferromagnetic ordering of magnetic moments of ruthenium ions, which gives rise to the staggered field acting on the system of rare-earth ions. The calculated temperature dependences of heat capacity and entropy are in good agreement with experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Ab Initio Study of Half-Heusler Compounds MnVZ (Z = P, As, Sb)

Ab initio study of the half-Heusler compounds MnVZ (Z = P, As, Sb) is performed in the framework of Density Functional Theory (DFT). We have investigated their structural, elastic, mechanical, electronic, magnetic, and optical properties using the full potential linearly augmented plane wave (FP-LAPW) method. The compounds have stable structure with lattice constant in the range 4.90–5.75 Å. The correlation effects are incorporated by performing the GGA + U calculations. The compounds are ductile in nature as indicated by their elastic properties like Cauchy pressure (CP), Pugh’s and Poisson’s ratios. All the three compounds MnVZ (Z = P, As, Sb) are half-metals with small indirect band gaps in the spin-down channels of the values 0.74, 0.30, and 1.29 eV respectively at Γ −X symmetry points. These compounds possess relatively small total magnetic moments of about ≈ 1μ{sub B}. The analysis of optical properties reveals that these compounds are more active at small and high energies and show negligible activity at intermediate energies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Role of the Cluster Structure of Amorphous Fe{sub 67}Cr{sub 18}B{sub 15} Alloy in Magnetism and in the Changing of Electron Scattering Mechanisms under the Influence of Ion (Ar{sup +}) Irradiation

The contribution of clusters of different sizes to magnetism and the switching of electron scattering mechanisms in amorphous Fe{sub 67}Cr{sub 18}B{sub 15} alloy during ion Ar{sup +} irradiation is studied. The cluster magnetism is found to be related to the presence of clusters of the following two types: large α-(Fe, Cr) clusters of size D = 150–250 Å and small (D = 40–80 Å) clusters in a random intercluster medium. The generation of small ferromagnetic and antiferromagnetic clusters during ion irradiation leads to the formation of cluster glass, which affects the electrical properties of the alloy and causes a magnetic frustration. The temperature dependence of the barrier height is shown to characterize the magnetic state of the alloy in low fields. On the whole, the temperature dependence of the order parameter is a universal characteristic of the system. The temperature dependence of resistivity of initial alloys in the temperature range 98–300 K (ρ(T) ∝ T{sup 2}) is determined by electron scattering by quantum defects, and the transition into a ferromagnetic state is revealed when the derivative ∂ρ/∂T ∝ T is analyzed. The increase in resistivity and the relation ρ ∝ T{sup 1/2} in strongly inhomogeneous samples after irradiation at a dose Φ = 1.5 × 10{sup 18} ions/cm{sup 2} are caused by weak localization effects, and the transition to a ferromagnetic state becomes obvious when the derivative ∂ρ/∂T ∝ T{sup –1/2} is considered. Irradiation by fluence Φ = 3 × 10{sup 18} ions/cm2 induces a giant (twofold) increase in the alloy density, restores the ferromagnetism of large clusters, decreases the resistivity by 37%, and restores the relation ρ(T) ∝ T{sup 2}, which results from the overlapping of the irradiation-induced small clusters when their concentration increases and from an increase in the alloy density. The overlapping of clusters lowers the barrier height and decreases the sensitivity of the alloy to an applied field. The relation ρ(T) ∝ T{sup 2} is valid for the entire temperature range T = 2–300 K because of the partial screening of the magnetic moments of large clusters by a medium having the properties of cluster glass.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Features of Spin Waves Focusing in Ferromagnets

Abstract—Features of focusing spin waves in ferromagnets with magnetic moment exchange interaction with the closest neighbors and second neighbors are explored. It is shown that in the long-wave approximation, no spin wave focusing occurs: it is observed only in the wavevectors in the second half of the Brillouin zone (at aq ≥ π/2, where a is the lattice constant, q is the value of wavevector). Additionally, it is shown that magnons in such system are focused along directions [110], and are defocused in directions [100]. It is found that the external magnetic field and the magnetic anisotropy field do not cause changes in directions of magnon focusing. When the parameter of exchange interaction with the second neighbors near directions [110] and [111] is negative, magnons form a caustic, near which the intensity of the spin-wave field surges.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Dynamical resilience to disorder: The dilute Hubbard model on the Lieb lattice

In itinerant systems, electron-electron interactions may lead to the formation of local magnetic moments and their effective exchange coupling, which in turn gives rise to long-range magnetic order. Therefore, when moment formation is weakened, such as in the single-band Hubbard model on a square lattice with the on-site repulsion being randomly switched off on a fraction $\textit{x}$ of sites, magnetic order is suppressed beyond some critical $x_c$, which was found to lie below the classical percolation threshold $x^{\text{(perc,sq)}}_c$. Here we study dilute magnetism in flat band systems, namely, in the Hubbard model on a “Lieb” lattice. Interestingly, we show that magnetic order persists to x almost twice as large as the classical percolation threshold for the lattice, thus emphasizing the central role of electron itinerancy to the magnetic response. Further, the analysis of the orbital-resolved order parameters reveals that the contribution of the fourfold coordinated $\textit{“d”}$ sites to magnetism is dramatically affected by dilution, while the localized $\textit{“p”}$ states of the flat band provide the dominant contribution to long-range correlations. We also examine the transport properties, which suggest the existence of an insulator-to-metal transition in the same range of the critical magnetic dilution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Electromagnetic moments of 215,217 Bi: Probing shell evolution beyond N = 126

The nuclear properties of bismuth isotopes (Z = 83) , with just one valence proton above the closed spherical shell at (Z = 82) , are expected to be governed by a single unpaired proton. However, already in semimagic 209 Bi (Z = 83, N = 126) , , the magnetic moment (μ) strongly deviates from the single-particle Schmidt value. A near linear decrease in μ with the increase of N after the N = 126 magic number was observed up to N = 130 . In order to test whether this trend is kept at N > 130 and to reveal the underlying mechanisms, an investigation of 215,217 Bi (N = 132, 134) has been undertaken. The magnetic dipole and electric quadrupole moments of the I π = 9/2 - nuclear ground states in these isotopes have been measured for the first time using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). It has been shown that the linearly decreasing trend of μ( 209,211,213 Bi g ) is broken in 215,217 Bi with a nearly constant value of μ observed. Experimental data have been compared to calculations in the framework of the configuration-interaction shell model with the monopole-based universal V MU +LS interaction. The peculiarities in the behavior of μ(Bi, 9/2 - ) with increasing neutron number are explained as being due to the shell evolution, change of the neutron orbitals occupancies and strong configuration mixing beyond N = 130 . Also, the difference in the μ trends for bismuth (Z = 83) and astatine (Z = 85) isotopes with N > 126 are reproduced by the shell-model calculations. It is shown that monopole interaction plays noticeable role in the description of the peculiarities of the μ behaviour. Additionally, the extension of the application of the V MU interaction to the μ isotopic trends for heavy nuclei is important for further study of the capabilities of this promising version of the shell-model calculations.

Dipole magnetic moments↗
DOE OSTI2019

Investigation on Physical Properties of Sn-Modified Cubic Cu{sub 2}O Nanostructures

Cubic Cu{sub 2−x} Sn{sub x}O (0 ≤ x ≤ 0.4) nanostructures have been developed with the solvothermal method. X-ray diffraction data analysis confirms the formation of a pristine Cu{sub 2}O phase below x=0.3 Sn{sup 2+} substitution, and SnO-related phases appear in 0.3 substitute sample indicating phase segregation starts at x≥0.3. The magnetization versus applied field (M − H) curves have been determined using a vibrating sample magnetometer at 300 K which reveals that ferromagnetic behavior of synthesized nanomaterials is reliant on the substituent concentration and enhanced gradually with its addition in the material. Photoluminescence spectra show that ferromagnetism originates from the induced magnetic moment in defects created as cation vacancies in the host material. Beside these, the dielectric measurements were performed at various frequencies (20 Hz–1 MHz) and in the temperature range of 100 to 350 K.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Electronic Structures and Ferromagnetic Properties of 3d (Cr)-Doped BaSe Barium Selenide

In this study, we have employed the first-principle methods based on spin-polarized density functional theory to investigate the structural parameters, the electronic structures, and the half-metallic ferromagnetic behavior of chromium (Cr)-doped barium selenide (BaSe) such as Ba{sub 1 − x}Cr{sub x}Se at concentrations x = 0.25, 0.5, and 0.75. The exchange and correlation potential is described by the generalized gradient approximation of Wu and Cohen (GGA-WC). The calculated structural parameters of BaSe are in good agreement with theoretical data. Our findings reveal that the p-d exchange coupling is ferromagnetic for Ba{sub 0.75}Cr{sub 0.25}Se and Ba{sub 0.5}Cr{sub 0.5}S, but it becomes anti-ferromagnetic for Ba{sub 0.25}Cr{sub 0.75}S. The electronic structures exhibit that the Ba{sub 1 − x}Cr{sub x}Se materials for all concentrations are half-metallic ferromagnets with spin polarization of 100% and total magnetic moment per Cr atom of 4 μ{sub B}. Therefore, the Ba{sub 1 − x}Cr{sub x}Se compounds are suitable candidates for possible spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Cyclotron resonance features in a three-dimensional topological insulators

The specific features of the cyclotron resonance in topological insulators of the Bi{sub 2}Te{sub 3} family are analyzed. The energy spectrum of electron excitation on the surface of these compounds in a magnetic field has the C{sub 3V} symmetry. With the use of quasiclassical equations of motion, which take into account the Berry curvature and magnetic moment, it is found that the cyclotron frequency spectrum contains additional harmonics due to spectrum anisotropy. An analysis of these harmonics makes it possible to reveal the contribution from the topological characteristics to the observables.

36 MATERIALS SCIENCE↗