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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Magnetic phase transition, magnetoresistance, and anomalous Hall effect in Ga-substituted Y Mn 6 Sn 6 with a ferromagnetic kagome lattice

The HfFe 6 Ge 6- type RMn 6 Sn 6 , a metallic system consisting of ferromagnetic kagome planes of Mn, has been recently shown to be a candidate hosting topological electronic properties. In this paper, we report magnetic and electronic properties in Y Mn 6 Sn 6-x Ga x single crystals via magnetic susceptibility, electrical transport, and single-crystal neutron diffraction measurements. We show that the magnetic ground state of Y Mn 6 Sn 6-x Ga x (0 ≤ x ≤ 0.61 ) evolves from incommensurate antiferromagnet to ferromagnet with increasing Ga substitution x, a feature which is accompanied by the decrease in magnetoresistance. Furthermore, the topological Hall effect observed in the pristine compound is absent in the Ga-substituted ones; instead, the anomalous Hall effect persists which may be associated with the Berry curvature of gapped Dirac bands near the Fermi energy. These results suggest strong correlation between electronic properties and magnetism in this topological magnet that can be readily tuned via Ga substitution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Noncoplanar magnetic order in the breathing kagome lattice compound Pb⁡(OF)⁢Cu 3 ⁢(SeO 3 ) 2⁢ (NO 3 )

We report a comprehensive study investigating the magnetic ordering properties of the quasi-two-dimensional breathing kagome lattice compound Pb⁡(OF)⁢Cu 3 ⁢(SeO 3 ) 2 ⁢(NO 3 ) using neutron diffraction and thermodynamic measurements. A long-range magnetic order emerges below the Néel temperature, T N = 18K, characterized by a propagating vector k = (0,0,1.5). Refinement analysis reveals that the copper moments display a noncoplanar arrangement, with the c components parallelly aligned within the breathing kagome plane and antiparallelly aligned between the layers. In-plane moment components form a chiral “tail-chase” structure. The magnetic order is quickly suppressed under moderate external fields applied both within and perpendicular to the kagome plane. Potential magnetic interactions that may be associated with the noncoplanar chiral magnetic structure are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Stacking Faults and Short-Range Magnetic Correlations in Single Crystal Y 5 Ru 2 O 12 : A Structure with Ru +4.5 One-Dimensional Chains

In this paper, the synthesis, structure, and physical properties of Y 5 Ru 2 O 12 , a new member of the Ln 5 B 2 O 12 class (Ln = lanthanide, B = Ru, Re, Mo) are reported. Single crystals of Y 5 Ru 2 O 12 are synthesized using a high-temperature, high-pressure (700 °C, 200 MPa) hydrothermal method. The structure features edge-sharing Ru—O—Ru 1D chains of distorted RuO6 octahedra. The YO n (n = 6 and 7) polyhedral groups bridge Ru—O—Ru chains across the unit cell by forming a complex 3D structure. The Ru—O—Ru chains contain alternating shorter and longer Ru—Ru distances along the chain. Single crystal neutron diffraction shows streak-like diffuse scattering along the H-direction, due to stacking faults. Temperature-dependent anisotropic magnetic measurements reveal a broad feature over a wide temperature range, with a maximum at ≈70 K indicating potential short-range ordering, with canted antiferromagnetic behavior below 30 K. The dominant magnetic interactions are antiferromagnetic with a large negative Weiss temperature θ ≈ –200 K. The compound has an effective magnetic moment that is anomalously large for this class of compounds (≈4 μ B Ru –1 ), and a phenomenological spin Hamiltonian describing the unusual anisotropic magnetization of Y 5 Ru 2 O 12 is proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Crystal and Magnetic Structures of Granular Powder Spinel Mn–Zn and Ni–Zn Ferrites

Some specific features of the crystal and magnetic structures of granular powder spinel-like ferrites Mn{sub 0.160}Mg{sub 0.404}Zn{sub 0.448}Fe{sub 2}O{sub 4}, Mn{sub 0.676}Zn{sub 0.227}Fe{sub 0.09}Fe{sub 2}O{sub 4}, Mn{sub 0.5792}Zn{sub 0.2597}Fe{sub 0.1612}Fe{sub 2}O{sub 4}, and Ni{sub 0.32}Zn{sub 0.68}Fe{sub 2}O{sub 4} have been studied by neutron diffraction. It has been established that the crystal structure of all the studied compounds has a cubic symmetry with space group Fd3̄m. Ferrimagnetic ordering is observed in all the studied structures. Based on the experimental data, the unit cell parameters and interatomic bond lengths of the studied compounds are determined alongside with the distribution of cations between octahedral and tetrahedral crystallographic positions in their cubic crystal structure. Corresponding average magnetic moments are calculated for different positions in their cubic structure. Some structural mechanisms of the formation of magnetic properties depending on the level of doping and the size of powder grains are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Equation of state and phase evolution of U-7.7Nb with implications for the understanding of dynamic behavior of U-Nb alloys

Here, we performed in-situ high P-T neutron diffraction experiments on U-7.7Nb. Our results show that the tetragonal γo phase is substantially more compressible than the monoclinic α" phase. As the pressure increases, this leads to increased volume collapse and stress relaxation for the reverse martensitic transformation, α" → γ°. At moderately elevated temperatures, a substantial strain-induced stabilization against transformation into a body centered cubic γ phase was observed in the γ° phase. Additionally, it was discovered that the ability to retain the metastable γ phase, formed from rapid heating of thermally reset γ°, has subtle dependence on the austenitization temperature. The present findings provide a direct test for the hypotheses proposed to explain the low-pressure shock behaviors of the U-Nb alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Residual stress and microstructure in IN718-René41 graded superalloy fabricated by laser blown directed energy deposition

Additively printed Ni-based superalloy with a compositionally graded transition from IN718 to René41 was fabricated by laser blown-powder directed energy deposition (DED/LB-M), with the goals of meeting location-specific temperature capability and reducing component cost for hot gas path turbine components. Residual stress distribution in thin wall specimens with three sets of DED build parameters in the as-built and stress-relieved states was measured by neutron diffraction. For calculating residual stress, the calculated d0 method was found to be more appropriate as stress-free reference than using the lattice spacing measured from the stress-relief heat treated specimens. Longer dwell time (lower interpass temperature), higher energy input, smaller layer thickness resulted in a higher magnitude of tensile residual stresses at edges and compressive residual stresses at center of the specimens. The residual stress results did not show a strong dependence on graded compositions, indicating that the residual stress build-up was more geometry and process dependent. Non-destructive neutron imaging based on the attenuation coefficient qualitatively visualized the compositional variation in the bulk and showed good agreement with quantitative Electron Probe Micro-Analysis (EPMA) measurement. Grain structure, texture, and residual plastic strain along the build direction were characterized by Electron Backscatter Diffraction (EBSD). Long columnar grains with (001) preferred grain orientation were dominant along the build direction. Compositional change did not show an obvious effect on the epitaxial growth of dendrites and the continuation of the columnar grains. Residual plastic strain was relatively low in the as-built specimens.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Unusual Changes of C−H Bond Lengths in Chiral Zinc Complexes Induced by Noncovalent Interactions

In order to better understand the effect of non-covalent weak interactions on molecules, we have explored a variety of weak interactions, such as improper H-bonding (HB), tetrel bonds (TBs) and halogen bonds, in fluorinated chiral zinc complexes. High resolution neutron diffraction studies revealed a methylene carbon-hydrogen bond elongation and shortening due to TB and improper HB interactions, respectively. To show the accumulative effects of multiple weak interactions on the C-H bond, three types of tetrel bonds have been carefully examined. We have also shown how C-H bond elongation can be easily offset by forming an improper HB with the H atom from this C-H bond. Non-covalent interaction and electrostatic potential analysis investigations have been used to affirm the nature of the interactions based on density functional theory (DFT) and other related calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Complex magnetic structure in Ba 5 Ru 3 O 12 with isolated Ru 3 O 12 trimer

We report detailed magnetic, transport, heat-capacity, and neutron diffraction measurements of Ba 5 Ru 3 O 12 , a compound consisting of isolated Ru 3 O 12 trimers. We show that this system develops long-range antiferromagnetic ordering at T N ~ 60 K without structural distortion or metal-insulator-type transition, which is in sharp contrast to other Barium Ruthenate trimer systems such as 9R-BaRuO 3 and Ba 4 Ru 3 O 10 . A complex magnetic structure is revealed which is attributable to the magnetic frustration due to competing exchange interactions between Ru ions on different crystallographic sites within the Ru 3 O 12 trimer and different degree of orbital hybridization on different Ru-sites. In conclusion, we have also investigated the magnetic structure of Ba 4 Ru 3 O 10 for comparisons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Site Mixing for Engineering Magnetic Topological Insulators

The van der Waals compound, MnBi 2 Te 4 , is the first intrinsic magnetic topological insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk conductivity, and the roles of magnetic defects are still unknown. With higher concentrations of the same types of magnetic defects, the isostructural compound MnSb 2 Te 4 is a better model system for a systematic investigation of the connections among magnetism, topology, and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb 2 Te 4 . Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site mixing of MnSb2Te4 crystals depend on the growth parameters, which can lead to ≈40% of Mn sites occupied by Sb and ≈15% of Sb sites by Mn in as-grown crystals. Single-crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calculations suggest that the Mn-Sb site mixing favors a ferromagnetic interlayer coupling, consistent with experimental observation, but is detrimental to the band inversion required for a nontrivial topology. Overall, our results suggest a long-range magnetic order of Mn ions sitting on Bi sites in MnBi 2 Te 4 . The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topological layers, including the entire family of MnTe(Bi 2 Te 3 )n, its Sb analogs, and their solid solution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Finite temperature properties of uranium mononitride

Uranium mononitride (UN) is a promising nuclear fuel that combines the advantageous properties of readily used UO 2 and uranium alloys, such as high melting temperature and high uranium density, and thermal conductivity, respectively. A better understanding of UN behavior at operating temperatures can be obtained from finite temperature data, such as elastic properties. To get this information, ab initio molecular dynamics (AIMD) simulations were performed at five different temperatures using constant volume (NVT) and constant pressure (NPT) ensembles. Initially, the performance of PBE functional in reproducing experimental crystallographic properties and magnetic ordering is assessed. The finite temperature phonon dispersions are calculated using NVT simulation results, which show a softening of the phonon modes with increasing temperature. The NPT results are used to obtain the thermal expansion of UN and finite temperature electronic properties. The calculated thermal expansion is compared with our measurements using neutron diffraction. Additionally, the temperature dependent elastic properties of UN are evaluated using the strain-stress method in AIMD simulations, indicating that UN becomes softer and more compressible with increasing temperature. Also, the calculated Young’s modulus slope is in very good agreement with the experiment. The finite temperature heat capacity and electronic thermal conductivity are calculated from AIMD simulations, which are in better agreement with the experiment than the heat capacity and thermal conductivity calculated using the structures relaxed at 0K. Finally, the thermal diffusivity from AIMD has opposite temperature dependence compared to experimental results, which we argued comes from the underestimated electronic thermal conductivity.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Structural distortion and incommensurate noncollinear magnetism in EuAg 4 As 2

Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. In this work, we report the observation of structural distortion and noncollinear magnetism in layered pnictide EuAg 4 As 2 via transport, magnetization, single crystal x-ray, and neutron diffraction data.EuAg 4 As 2 single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of q 1 =±(0, 0.25, 0.5) and q 2 =±(0.25, 0, 1) emerge. Between 9–15 K, the hexagonal Eu 2+ sublattice enters an unpinned incommensurate magnetic state, with magnetic Bragg reflections pictured as circular sectors. Furthermore, below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12) and a very rare magnetic structure, which is helical along the c axis and cycloidal along the b axis with a moment of 6.4 μB/Eu 2+ . Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in EuAg 4 As 2 are revealed.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Giant Doping Response of Magnetic Anisotropy in MnTe

Developing simple ways to control spin states in spintronic devices is a crucial step towards increasing their functionality. MnTe is a room-temperature antiferromagnet with promising spintronic properties, including for thermospintronics and magnon-based devices. Here, we show that, in MnTe, less than 1% Li is sufficient to produce a dramatic spin reorientation as observed by neutron diffraction. The behavior of the 0001 magnetic Bragg peak reveals a significant reorientation of the Mn spins from planar in the pure material to almost completely axial with minimal Li doping. Temperature dependence of the magnetic peaks in Li-doped samples indicates that axial spins shift back to planar suddenly upon approaching the Néel temperature (T N = 307 K). Density functional theory calculations support the idea that a shift in the Fermi level caused by doping is responsible for switching the material between two competing magnetic ground states. These results pave the way for developing easy switching of magnetic states in functional materials such as spintronic devices and topological insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Local molecular environment drives speciation and reactivity of ion complexes in concentrated salt solution

The speciation and reactivity of aqueous salt solutions are important for a wide variety of applications. However, application of this information is inhibited by broad disagreement about composition, mechanisms, and structure in concentrated solutions especially. Here, neutron diffraction with isotopic substitution measurements on aqueous zinc chloride solutions are used to calibrate molecular dynamics simulations that include effective electronic polarization. This allows us to probe the origin of speciation and reactivity of zinc chloride-water ion complexes, ZnCl x (H 2 O) y 2-x (x ≤ 4 and y ≤ 6), by comparing the reactivity of species in concentrated (4.5 m) and dilute (0.01 m) conditions. Within the concentrated solution, it is found that the extended solvation environment is dominated by solvent-separated ion complexes whose presence increases the free energy of activation for interconversion of species while simultaneously enhancing their thermodynamic stability. This concentration-dependent reactivity and stability suggests that other reactions, such as the nucleation of solid phases, will also be affected.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Effect of High-Density Nanoparticles on Recrystallization and Texture Evolution in Ferritic Alloys

Ferritic alloys are important for nuclear reactor applications due to their microstructural stability, corrosion resistance, and favorable mechanical properties. Nanostructured ferritic alloys having a high density of Y-Ti-O rich nano-oxides (NOs < 5 nm) are found to be extremely stable at high temperatures up to ~1100 °C. This study serves to understand the effect of a high density of nano-particles on texture evolution and recrystallization mechanisms in ferritic alloys of 14YWT (14Cr-3W-0.4Ti-0.21Y-Fe wt %) having a high density of nano-particles and dispersion-free FeCrAl (13Cr-5.2Al-0.05Y-2Mo-0.2Si-1Nb wt %). In order to investigate the recrystallization mechanisms in these alloys, neutron diffraction, electron backscattered diffraction, and in situ and ex situ transmission electron microscopy have been utilized. It has been observed that even though the deformation textures of both the 14YWT and FeCrAl alloys evolved similarly, resulting in either the formation (in FeCrAl alloy) or increase (in 14YWT) in γ-fiber texture, the texture evolution during recrystallization is different. While FeCrAl alloy keeps its γ-fiber texture after recrystallization, 14YWT samples develop a ε-fiber as a result of annealing at 1100 °C, which can be attributed to the existence of NOs. In situ transmission electron microscopy annealing experiments on 14YWT show the combination and growth of the lamellar grains rather than nucleation; however, the recrystallization and growth kinetics are slower due to NOs compared to FeCrAl.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Deformation Behavior of a Double Soaked Medium Manganese Steel with Varied Martensite Strength

The effects of athermal martensite on yielding behavior and strain partitioning during deformation is explored using in situ neutron diffraction for a 0.14C–7.14Mn medium manganese steel. Utilizing a novel heat treatment, termed double soaking, samples with similar microstructural composition and varied athermal martensite strength and microstructural characteristics, which composed the bulk of the matrix phase, were characterized. It was found that the addition of either as-quenched or tempered athermal martensite led to an improvement in mechanical properties as compared to a ferrite plus austenite medium manganese steel, although the yielding and work hardening behavior were highly dependent upon the martensite characteristics. Specifically, athermal martensite was found to promote continuous yielding and improve the work hardening rate during deformation. The results of this study are particularly relevant when considering the effect of post-processing thermal heat treatments, such as tempering or elevated temperature service environments, on the mechanical properties of medium manganese steels containing athermal martensite.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Elastic behavior of binary and ternary refractory multi-principal-element alloys

Elastic properties are essential to the mechanical performance of materials, and therefore, can be tuned to design high-performance materials. In this study, the elastic constants of the equiatomic ternary refractory multi-principal-element alloys, NbTiV, and MoNbV, were investigated, using in-situ neutron diffraction and first-principles calculations. The experimentally measured and theoretically predicted elastic constants show a good agreement. The alloying effect of adding Ti and Mo into NbV base alloy on the elastic constants is studied. Particularly, adding Ti elements into the NbV alloy results in the increase of the Zener anisotropy ratio from 0.59 to 0.99, leading to the formation of the elastically isotropic NbTiV alloy, while the addition of Mo decreases the anisotropy ratio to 0.52. Pugh’s ratio (B/G), Cauchy pressure (C 12 -C 44 ), and Poisson’s ratio (ν) are used to predict the brittle/ductile nature of the studied alloys, which is consistent with the mechanical results. The present work provides valuable insights into the design of ductile and strong refractory high-entropy alloys by tuning the elastic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Frustration-mediated crossover from long-range to short-range magnetic ordering in Y 1 – x L u x BaC o 4 O 7

The R⁢BaC⁢o 4⁢ O 7 system is a prototype geometrically frustrated magnet in which kagome planes and triangular layers of Co-O tetrahedra interleave. For R=Y, an antiferromagnetic ground state is realized due to a frustration-breaking trigonal-orthorhombic phase transition. For R = Lu, however, a long-range ordered state has rarely, if ever, been reported despite a similar symmetry-breaking transition, albeit at a significantly lower temperature. To explore this dichotomy, we present a comprehensive magnetic and structural phase diagram for Y 1-x L⁢u x BaC⁢o 4 ⁢O 7 , established through complementary neutron diffraction and magnetization measurements. Our results outline the phase evolution of the nuclear structures in response to changes in composition and temperature. Further, the temperature of the trigonal (P⁢31⁢c) to orthorhombic (Pbn⁢2 1 ) transition, T s⁢1 , decreases monotonically with increasing Lu content from 310 K for x = 0.0 to 110 K for x = 1.0. In Lu-rich compositions (0.7 ≤ x ≤ 1.0), first-order structural transitions are observed with coexisting and competing orthorhombic Pbn⁢2 1 and metastable monoclinic Cc phases. For the magnetically ordered Y-rich compositions, T- and x-dependent refinements of the magnetic structure reveal an antiferromagnetic “ribbonlike” arrangement of Co spin pairs in both the triangular and the kagome layers. A gradual suppression of long-range magnetic order is observed with increasing the Lu content, accompanied by the development of short-range magnetic correlations present in all the samples.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Observation of a Large Magnetic Anisotropy and a Field-Induced Magnetic State in SrCo(VO 4 )(OH): A Structure with a Quasi One-Dimensional Magnetic Chain

A new member of the descloizite family, a cobalt vanadate, SrCo(VO 4 )(OH), has been synthesized as large single crystals using high-temperature and high-pressure hydrothermal methods. SrCo(VO 4 )(OH) crystallizes in the orthorhombic crystal system in space group P 2 1 2 1 2 1 with the following unit cell parameters: a = 6.0157(2) Å, b = 7.645(2) Å, c = 9.291(3) Å, V = 427.29(2) Å 3 , and Z = 4. It contains one-dimensional Co–O–Co chains of edge-sharing CoO 6 octahedra along the a -axis connected to each other via VO 4 tetrahedra along the b -axis forming a three-dimensional structure. The magnetic susceptibility of SrCo(VO 4 )(OH) indicates an antiferromagnetic transition at 10 K as well as unusually large spin orbit coupling. Single-crystal magnetic measurements in all three main crystallographic directions displayed a significant anisotropy in both temperature- and field-dependent data. Single-crystal neutron diffraction at 4 K was used to characterize the magnetically ordered state. The Co 2+ magnetic spins are arranged in a staggered configuration along the chain direction, with a canting angle that follows the tipping of the CoO 6 octahedra. The net magnetization along the chain direction, resulting in ferromagnetic coupling of the a -axis spin components in each chain, is compensated by an antiferromagnetic interaction between nearest neighbor chains. A metamagnetic transition appears in the isothermal magnetization data at 2 K along the chain direction, which seems to correspond to a co-alignment of the spin directions of the nearest neighbor chain. We propose a phenomenological spin Hamiltonian that describes the canted spin configuration of the ground state and the metamagnetic transition in SrCo(VO 4 )(OH).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗