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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Density Functional Theory Study on Electronic Structures and Magnetism for Nitrogen-Doped ZnS

Using the full-potential linearized augmented plane wave method, together with the generalized gradient approximation and modified Becke-Johnson as correlation potential, the electronic structures and magnetism for nitrogen-doped ZnS are investigated. Based on the generalized gradient approximation, calculations show nitrogen-substituting sulfur (N{sub S}) would induce the ZnS to be paramagnetic metal. As the band gap increasing to the experimental results (obtained by the modified Becke-Johnson potential), the N{sub S} defects would induce the ZnS to be a ferromagnetic metal. The total magnetic moment for ZnS supercell with single N{sub S} defect is 0.85 μ{sub B}. Positive chemical pair interactions imply that N{sub S} defects would form homogeneous distribution in ZnS host. Sulfur vacancies would give rise to the ZnS with N{sub S} defects system losing the magnetism. Moderate formation energy (0.71 eV) indicates ZnS with N{sub S} defects could be fabricated experimentally.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Epitaxy, exfoliation, and strain-induced magnetism in rippled Heusler membranes

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al 2 O 3 substrates. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. The weak Van der Waals interactions of graphene enable mechanical exfoliation to yield free-standing GdPtSb membranes, which form ripples when transferred to a flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic, measurements on rippled membranes show a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd. First-principles calculations show that the coupling to homogeneous strain is too small to induce ferromagnetism, suggesting a dominant role for strain gradients. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetism and magnetostriction) and strain gradients (flexomagnetism).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Tuning interfacial Dzyaloshinskii-Moriya interactions in thin amorphous ferrimagnetic alloys

Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMI). Further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. Tuning DMI is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMI in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the CoGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. The DMI thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. Our work opens new paths for controlling interfacial DMI in ferrimagnets to nucleate and manipulate skyrmions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Effect of lattice strain on magnetism in epitaxial YCrO 3 films

The influence of lattice strain on magnetism is studied for perovskite rare-earth chromite, YCrO 3 (YCO) thin films. A combined theoretical and experimental approach is used to demonstrate the correlation between lattice-strain and magnetic properties of YCO films. The first-principles calculations reveal that the local Cr magnetic moments and the out-of-plane magnetization linearly increase with changing the in-plane lattice strain from compressive to tensile. The experimental results agree with the theory findings showing a strong dependence of magnetization on epitaxial strain in YCO films. These results unveil the lattice-strain-controlled functionalities on perovskite rare-earth chromites and related material systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Single crystal synthesis and magnetic properties of a Shastry-Sutherland lattice compound BaNd 2 ZnS 5

In this study, single crystals of a Shastry-Sutherland magnetic semiconductor, BaNd 2 ZnS 5 , were synthesized through a high-temperature solution growth technique. Physical properties were characterized by powder and single crystal x-ray diffraction, temperature- and field-dependent magnetization, and temperature-dependent specific heat measurements. BaNd 2 ZnS 5 orders antiferromagnetically at 2.9 K, with magnetic moments primarily aligned within the $ab$ -plane. Magnetic isothermal measurements show metamagnetic transitions at ~ 15 kOe for the [110] direction and ~ 21 kOe for the [100] direction. Estimated magnetic entropy suggests a double ground state for each neodymium ion.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Magnetic Materials Research at SRNL

Introduction – What are Magnetic Materials? Magnetic materials are crystalline solids which below a certain temperature, known as the ordering temperature, display spontaneous order in their magnetic moments. Results • Enhanced thermal stability and saturation magnetization with Y substitution in Ce2Fe14B based permanent magnets • Discovered a new ferromagnetic phase transition at TC = 62.9 K in (Dy1/3Mo2/3)2AlC and validated the two previously reported low temperature magnetic phases Characterization of Magnetic Materials Raw high purity materials are melted together to synthesize a crystalline sample using an arc melter at Savannah River National Laboratory (SRNL). Use high energy ball milling to form a hard/soft magnetic composite using Y substituted Ce2Fe14B. Finish structural characterization on γ- irradiated (RE1/3Mo2/3)2AlC (RE = Rare Earth) MAX samples. Unit Cell Lattice Points

Bretana, Alex↗
NASA NTRS1973

Analysis of Mossbauer Fe-57 absorption in an F.C.C. Fe-Co-V alloy

Mossbauer Fe-57 absorption spectra of superparamagnetic F.C.C. Fe sub .34 Co sub .52 V sub 14 (Vicalloy II) have been analyzed using a cluster model and magnetic resonance relaxation theory. The number and magnetic moment of the clusters were determined from magnetization data. A least squares fit to the spectrum at 4.2 K gives sub Hhyp = 175. kG, substantially greater than the value 55 kG inferred just from the observed broadening of the single line spectrum. From this broadening an estimate of 40 kG is obtained for the width of the distribution in Hhyp at 4.2 K arising from compositional fluctuations sub and small cluster effects.

Bambakidis, G.↗
NASA NTRS1991

Makeup and uses of a basic magnet laboratory for characterizing high-temperature permanent magnets

A set of instrumentation for making basic magnetic measurements was assembled in order to characterize high intrinsic coercivity, rare earth permanent magnets with respect to short term demagnetization resistance and long term aging at temperatures up to 300 C. The major specialized components of this set consist of a 13 T peak field, capacitor discharge pulse magnetizer; a 10 in. pole size, variable gap electromagnet; a temperature controlled oven equipped with iron cobalt pole piece extensions and a removable paddle that carries the magnetization and field sensing coils; associated electronic integrators; and sensor standards for field intensity H and magnetic moment M calibration. A 1 cm cubic magnet sample, carried by the paddle, fits snugly between the pole piece extensions within the electrically heated aluminum oven, where fields up to 3.2 T can be applied by the electromagnet at temperatures up to 300 C. A sample set of demagnetization data for the high energy Sm2Co17 type of magnet is given for temperatures up to 300 C. These data are reduced to the temperature dependence of the M-H knee field and of the field for a given magnetic induction swing, and they are interpreted to show the limits of safe operation.

Niedra, Janis M.↗
DOE OSTI2018

Ground State Structure of BaFeO{sub 3}: Density Functional Theory Calculations

Using density functional theory calculations, the ground state structure of BaFeO{sub 3} (BFO) is investigated with local spin density approximation (LSDA). Cubic, tetragonal, orthorhombic, and rhombohedral types BFO are considered to calculate the formation enthalpy. The formation enthalpies reveal that cubic is the most stable structure of BFO. Small energy difference between the cubic and tetragonal suggests a possible tetragonal BFO. Ferromagnetic (FM) and anitiferromagnetic (AFM) coupling between the Fe atoms show that all the striochmetric BFO are FM. The energy difference between FM and AFM shows room temperature ferromagnetism in cubic BFO in agreement with the experimental work. The LSDA-calculated electronic structures are metallic in all studied crystallographic phases of BFO. Calculations including the Hubbard potential U, i.e. LSDA + U, show that all phases of BFO are half-metallic consistent with the integer magnetic moments. The presence of half-metallicity is discussed in terms of electronic band structures of BFO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Specificities of Spin Wave Focusing in EuO and EuS Crystals

Specificities of spin wave focusing in EuO and EuS crystals are studied. It is shown that spin wave focusing is absent in the long-wave approximation: it is observed only for wave vectors in the second half of the Brillouin zone (aq≳ π, where a is the lattice constant and q is the magnitude of the wave vector). It was found that, for EuO and EuS crystals, the directions of spin wave focusing differ due to the different signs of the exchange integrals responsible for the exchange interaction of the magnetic moment with the second neighbors. The directions along which a magnon caustic is formed are determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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↗