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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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Static and dynamical properties of the spin-$\frac{5}{2}$ nearly ideal triangular lattice antiferromagnet $\mathrm{Ba_3}$ $\mathrm{MnSb_2}$ $\mathrm{O_9}$

Here, we study the ground state and spin excitations in Ba 3 MnSb 2 O 9 , an easy-plane S = 5/2 triangular lattice antiferromagnet. By combining single-crystal neutron scattering, electric spin resonance (ESR), and spin wave calculations, we determine the frustrated quasi-two-dimensional spin Hamiltonian parameters describing the material. While the material has a slight monoclinic structural distortion, which could allow for isosceles-triangular exchanges and biaxial anisotropy by symmetry, we observe no deviation from the behavior expected for spin waves in the in-plane $120$° state. Even the easy-plane anisotropy is so small that it can only be detected by ESR in our study. In conjunction with the quasi-two-dimensionality, our study establishes that Ba 3 MnSb 2 O 9 is a nearly ideal triangular lattice antiferromagnet with the quasiclassical spin S = 5/2, which suggests that it has the potential for an experimental study of Z- or Z 2 -vortex excitations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Tailoring physical properties of crystals through synthetic temperature control: A case study for new polymorphic NbFeT e 2 phases

Growth parameters play a significant role in the crystal quality and physical properties of layered materials. Here we present a case study on a van der Waals magnetic NbFeTe 2 material. Two different types of polymorphic NbFeTe 2 phases, synthesized at different temperatures, display significantly different behaviors in crystal symmetry, electronic structure, electrical transport, and magnetism. While the phase synthesized at low temperature showing behavior consistent with previous reports, the new phase synthesized at high temperature, has completely different physical properties, such as metallic resistivity, long-range ferromagnetic order, anomalous Hall effect, negative magnetoresistance, and distinct electronic structures. Neutron diffraction reveals out-of-plane ferromagnetism below 70 K, consistent with the electrical transport and magnetic susceptibility studies. Finally, our work suggests that simply tuning synthetic parameters in a controlled manner could be an effective route to alter the physical properties of existing materials potentially unlocking new states of matter, or even discovering new materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Electronic structure and magnetic tendencies of trilayer La 4 Ni 3 O 10 under pressure: Structural transition, molecular orbitals, and layer differentiation

Motivated by the recent observation of superconductivity in the pressurized trilayer Ruddlesden-Popper (RP) nickelate La 4 Ni 3 O 10 , we explore its structural, electronic, and magnetic properties as a function of hydrostatic pressure from first-principles calculations. We find that an orthorhombic (monoclinic)-to-tetragonal transition under pressure takes place concomitantly with the onset of superconductivity. The electronic structure of La 4 Ni 3 O 10 can be understood using a molecular trimer basis wherein n molecular subbands arise as the d z 2 orbitals hybridize strongly along the c axis within the trilayer. The magnetic tendencies indicate that the ground state at ambient pressure is formed by nonmagnetic inner planes and stripe-ordered outer planes that are antiferromagnetically coupled along the c axis, resulting in an unusual ↑, 0, ↓ stacking that is consistent with the spin density wave model previously suggested by neutron diffraction. Such a state is destabilized at the pressure where superconductivity arises. Despite the presence of d z 2 states at the Fermi level, the d x 2 –y 2 orbitals also play a key role in the electronic structure of La4Ni3O10. Finally, this active role of the d x 2 –y 2 states in the low-energy physics of the trilayer RP nickelate, together with the distinct electronic behavior of the inner and outer planes, resembles the physics of multilayer cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magnetic structure and Ising-like antiferromagnetism in the bilayer triangular lattice compound NdZnPO

Here, the complex interplay of spin frustration and quantum fluctuations in low-dimensional quantum materials leads to a variety of intriguing phenomena. This research focuses on a detailed analysis of the magnetic behavior exhibited by NdZnPO, a bilayer spin-1/2 triangular lattice antiferromagnet. The investigation employs magnetization, specific heat, and powder neutron scattering measurements. At zero field, a long-range magnetic order is observed at T N = 1.64 K. Powder neutron diffraction experiments show the Ising-like magnetic moments along the c-axis, revealing a stripe like magnetic structure with magnetic propagation vector (1/2, 0, 1/2). Application of a magnetic field along the c-axis suppresses the antiferromagnetic order, leading to a fully polarized ferromagnetic state above B c = 4.5 T. This transition is accompanied by notable enhancements in the nuclear Schottky contribution. Moreover, the absence of spin frustration and expected field-induced plateau-like phases are remarkable observations. Detailed calculations of magnetic dipolar interactions revealed complex couplings reminiscent of a honeycomb lattice, suggesting the potential emergence of Kitaev-like physics within this system. This comprehensive study of the magnetic properties of NdZnPO highlights unresolved intricacies, underscoring the imperative for further exploration to unveil the underlying governing mechanisms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Unraveling the magnetic ground state in the alkali-metal lanthanide oxide Na 2 ⁢Pr⁡O 3

Here, a comprehensive set of muon spin spectroscopy and neutron scattering measurements supported by ab initio and model Hamiltonian simulations have been used to investigate the magnetic ground state of Na 2 ⁢PrO 3 . μSR reveals a Néel antiferromagnetic order below T N ~ 4.9K, with a small static magnetic moment m static ≤ 0.22 μ B /Pr collinearly aligned along the c axis. Inelastic neutron measurements reveal the full spectrum of crystal field excitations and confirm that the Pr 4+ ground-state wave function deviates significantly from the Γ 7 limit that is relevant to the Kitaev model. Single- and two-magnon excitations are observed in the ordered state below T N =4.6K and are well described by nonlinear spin wave theory from the Néel state using a magnetic Hamiltonian with Heisenberg exchange J=1 meV and symmetric anisotropic exchange Γ/J=0.1, corresponding to an XY model. Intense two magnon excitations are accounted for by g-factor anisotropy g z /g ± = 1.29. A fluctuating moment δm 2 = 0.57 ⁢(22) ⁢μ$^2_B$/Pr extracted from the energy and momentum integrated inelastic neutron signal is reduced from expectations for a local J = 1/2 moment with average g factor g avg ≈ 1.1. Together, the results demonstrate that the small moment in Na 2 ⁢PrO 3 arises from crystal field and covalency effects and the material does not exhibit significant quantum fluctuations.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Anomalous temperature-dependent magnetization in the nearly collinear antiferromagnet Y 2 Co 3

Y 2 Co 3 is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Here, previous investigations via bulk magnetization measurements suggested a complex non-collinear magnetic behavior, with magnetic moments primarily anti-aligned along the b axis and some canting towards the ac plane. In this study, we report the magnetic structure of Y 2 Co 3 to be an A-type AFM structure with ferromagnetic (FM) interactions within the distorted kagome plane and an inter-plane antiferromagnetic interaction, as determined by single crystal neutron diffraction. The magnetic moments align along the b axis, with minimal canting towards the c axis, at odds with the previous interpretation of bulk magnetization measurements. The magnetic moments on the two distinct Co sites are (0, -0.68, 0) μB and (0, 1.25, 0.07) μ B . We attribute the previously reported ”non-collinear” behavior to the considerable temperature dependence of itinerant AFM exchange interactions, induced by thermal contraction along the b axis. Additionally, our examination of lattice constants through pressure studies reveals compensating effects on FM and AFM interactions, resulting in negligible pressure dependence of T N .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Charge density waves and pinning by lattice anisotropy in 214 cuprates

The detection of static charge density waves (CDWs) in La 2-x ⁢Sr x ⁢CuO 4 (LSCO) with x ~ 0.12 at relatively high temperatures has raised the question of what lattice feature pins the CDWs. Some recent structural studies have concluded that some form of monoclinic distortion, indicated by the appearance of certain weak Bragg peaks (type M peaks) at otherwise forbidden positions, are responsible for CDW pinning. Here, as a test of this idea, we present neutron diffraction results for a single crystal of La 2-x ⁢Ba x CuO 4 (LBCO) with x = 1/8, which is known to undergo two structural transitions on cooling, from high-temperature tetragonal (HTT) to low-temperature orthorhombic (LTO) near 240 K, involving a collective tilt pattern of the corner-sharing CuO 6 octahedra, and from LTO to low-temperature tetragonal (LTT) near 56 K, involving a new tilt pattern and the appearance of intensity at peaks of type T. We observe both type M and type T peaks in the LTT phase, while the type M peaks (but not type T) are still present in the LTO phase. Given that CDW order is observed only in the LTT phase of LBCO, it is apparent that the in-plane Cu-O bond anisotropy associated with the octahedral tilt pattern is responsible for charge pinning. We point out that evidence for a similar, but weaker, bond anisotropy has been observed previously in LSCO and should be responsible for CDW pinning there. In the case of LBCO, the monoclinic distortion may help to explain previously reported magneto-optical evidence for gyrotropic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Spin-reorientation transitions in the Cairo pentagonal magnet Bi 4 Fe 5 O 13 F

Here, we show that interlayer spins play a dual role in the Cairo pentagonal magnet Bi 4 Fe 5 O 13 F, on one hand mediating the three-dimensional magnetic order, and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and Mössbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but Bi 4 Fe 5 O 13 F is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer Fe 3+ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Magnetic order, hysteresis, and phase coexistence in magnetoelectric LiCoPO 4

Here, the magnetic phase diagram of magnetoelectric LiCoPO 4 is established using neutron diffraction and magnetometry in fields up to 25.9T applied along the crystallographic b axis. For fields greater than 11.9T, the magnetic unit cell triples in size with propagation vector Q = (0,1/3,0). A magnetized elliptic cycloid is formed with spins in the (b,c) plane and the major axis oriented along b. Such a structure allows for the magnetoelectric effect with an electric polarization along c induced by magnetic fields applied along b. Intriguingly, additional ordering vectors Q ≈ (0,1/4,0) and Q ≈ (0,1/2,0) appear for increasing fields in the hysteresis region below the transition field. Traces of this behavior are also observed in the magnetization. A simple model based on a mean-field approach is proposed to explain these additional ordering vectors. In the field interval 20.5–21.0T, the propagation vector Q = (0,1/3,0) remains but the spins orient differently compared to the cycloid phase. Furthermore, above 21.0T and up until saturation, a commensurate magnetic structure exists with a ferromagnetic component along b and an antiferromagnetic component along

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Structural and magnetic transitions in spinel FeM n 2 O 4 single crystals

Materials that form the spinel structure are known to exhibit geometric frustration, which can lead to magnetic frustration as well. Through magnetization and neutron diffraction measurements, we find that FeMn 2 O 4 undergoes one structural and two magnetic transitions. The structural transition occurs at T s ~595K from cubic at high temperatures to tetragonal at low temperatures. Here, two magnetic transitions are ferrimagnetic at T FI–1 ~373K and T FI–2 ~50K, respectively. Further investigation of the specific heat, thermal conductivity, and Seebeck coefficient confirms both magnetic transitions. Of particular interest is that there is a significant magnetic contribution to the low-temperature specific heat and thermal conductivity, providing a unique system to study heat transport by magnetic excitations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Reduction of the ordered magnetic moment and its relationship to Kondo coherence in Ce 1-x La x Cu 2 Ge 2

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series Ce 1-x La x Cu 2 Ge 2 due to nonmagnetic dilution by La are revealed through neutron diffraction results for x = 0:20, 0:40, 0:75, and 0:85. Magnetic Bragg peaks are found for 0:20 ≤ x ≤ 0:75, and both the Neel temperature, T N , and the ordered magnetic moment per Ce μ linearly decrease with increasing x. The reduction in μ points to strong hybridization of the increasingly diluted Ce 4f electrons, and we find a remarkable quadratic dependence of μ on the Kondo-coherence temperature. Here, we discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory, and show that Ce 1-x La x Cu 2 Ge 2 provides an exceptional opportunity to quantitatively study the multiple magnetic interactions in a Kondo lattice.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Antiferromagnetic order in CaK ( Fe 1 – x Ni x ) 4 As 4 and its interplay with superconductivity

The magnetic order in CaK(Fe 1–x Ni x ) 4 As 4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated with the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe 2 As 2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, noncollinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. Lastly, this supports the notion that both collinear and noncollinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anisotropic antiferromagnetic order in the spin-orbit coupled trigonal-lattice Ca 2 Sr 2 IrO 6

Here, we used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate Ca 2 Sr 2 IrO 6 . The crystal structure is determined to be R ¯3 with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below T N = 13.1 K. The magnetic wave vector is identified as (0,0.5,1) with ferromagnetic coupling along the a axis and antiferromagnetic correlation along the b axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt 34° toward the c axis. The ordered moment is 0.66(3) μ B /Ir, larger than other iridates where iridium ions form corner- or edge-sharing IrO 6 octahedral networks. The tilting angle is reduced to ≈19° when a magnetic field of 4.9 T is applied along the c axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap ~0.5 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Dipolar-octupolar Ising antiferromagnetism in Sm 2 Ti 2 O 7 : A moment fragmentation candidate

Over the past two decades, the magnetic ground states of all rare-earth titanate pyrochlores have been extensively studied, with the exception of Sm 2 Ti 2 O 7 . This is, in large part, due to the very high absorption cross section of naturally occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically enriched single crystal of Sm 2 Ti 2 O 7 . Using inelastic neutron scattering, we determine that the crystal field ground state for Sm3+ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm 2 Ti 2 O 7 orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7)μ B below TN=0.35 K, consistent with expectations for antiferromagnetically coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. In conclusion, the combination of the dipolar-octupolar nature of the Sm 3+ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Thermal evolution of quasi-one-dimensional spin correlations within the anisotropic triangular lattice of α - NaMnO 2

Magnetic order on the spatially anisotropic triangular lattice of α-NaMnO 2 is studied via neutron diffraction measurements. The transition into a commensurate, collinear antiferromagnetic ground state with k=(0.5,0.5,0) was found to occur below T N =22 K. Above this temperature, the transition is preceded by the formation of a coexisting, short-range ordered, incommensurate state below TIC=45 K whose two-dimensional propagation vector evolves toward k=(0.5,0.5) as the temperature approaches T N . At high temperatures (T>T IC ), quasielastic scattering reveals one-dimensional spin correlations along the nearest-neighbor Mn-Mn “chain direction” of the MnO 6 planes. Here, our data are consistent with the predictions of a mean-field model of Ising-like spins on an anisotropic triangular lattice, as well as the predominantly one-dimensional Heisenberg spin Hamiltonian reported for this material.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Gradual enhancement of stripe-type antiferromagnetism in the spin-ladder material BaFe 2 S 3 under pressure

We report the recent discovery of pressure-induced superconductivity in BaFe 2 S 3 and BaFe 2 Se 3 has generated considerable interest in the 123-type iron chalcogenides. The structure of these materials features iron atoms arranged in quasi-one-dimensional ladders with either stripe-type or block-type antiferromagnetism at ambient pressure, constituting a new parent state for iron-based superconductivity in need of detailed study. Here, we report pressure-dependent neutron diffraction and muon spin relaxation/rotation measurements combined with first-principles calculations to investigate the structural, magnetic, and electronic properties of BaFe 2 S 3 under pressure. The experimental results reveal a gradual enhancement of the stripe-type ordering temperature with increasing pressure up to 2.6 GPa and no observable change in the size of the ordered moment. The ab initio calculations suggest that the magnetism is highly sensitive to the Fe-S bond lengths and angles, clarifying discrepancies with previously published results. In contrast to our experimental observations, the calculations predict a reduction of the ordered moment with pressure. In conclusion, we suggest that the robustness of the stripe-type antiferromagnetism is due to strong electron correlations not fully considered in the calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Neutron scattering investigation of rhenium orbital ordering in the 3 d – 5 d double perovskite Ca 2 FeReO 6

Here, we have carried out inelastic neutron scattering experiments to study magnetic excitations in the ordered double perovskite Ca 2 FeReO 6 . We have found a well-defined magnon mode with a bandwidth of ~50 meV below the ferrimagnetic ordering temperature ( T c ~ 520 K), similar to the previously studied Ba 2 FeReO 6 . The spin excitation is gapless for most temperatures within the magnetically ordered phase. However, a spin gap of ~10 meV opens up below ~150 K, which is well below the magnetic ordering temperature but coincides with a previously reported metal-insulator transition and onset of structural distortion. The observed temperature dependence of the spin gap provides strong evidence for ordering of Re orbitals at ~150 K, in accordance with an earlier proposal put forward by K. Oikawa et al. based on neutron diffraction as well as recent theoretical work by Lee and Marianetti. The presence of separate orbital and magnetic ordering in Ca 2 FeReO 6 suggests weak coupling between spin and orbital degrees of freedom and hints at a subdominant role played by spin-orbit coupling in describing its magnetism. In addition, we observed only one well-defined magnon band near the magnetic zone boundary, which is incompatible with simple ferrimagnetic spin waves arising from Fe and Re local moments but suggests a strong damping of the Re magnon mode.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

U 2 Ni 2 Sn and the origin of magnetic anisotropy in uranium compounds

Here, U 2 Ni 2 Sn is a member of a large family of intermetallic compounds with the tetragonal Mo 2 FeB 2 crystal structure. It orders antiferromagnetically at 25 K with propagation vector $q = (0, 0, \frac{1}{2})$. Magnetization, magnetoacoustic, and neutron-diffraction experiments on a single crystal provide evidence that the uranium moments align parallel to the $c$ axis with the anisotropy energy of ≈170 K, indicating that U 2 Ni 2 Sn can be classified as an Ising system. The results are at variance with previous studies on polycrystals, which indicated different magnetic structure, and which were incompatible with the $5f - 5f$ two-ion anisotropy model dominant in most U band systems. High-field magnetization studies exhibit a weak linear response for fields along the basal plane up to the highest field applied (60 T), while the $c$-axis magnetization curve exhibits three metamagnetic transitions at approximately 30, 39, and 50 T. The U magnetic moments of $0.87 μ_B$, the low magnetic entropy, and the enhanced Sommerfeld coefficient $γ$ = 187 mJ/mol f.u.K 2 suggest that U 2 Ni 2 Sn can be classified as an itinerant antiferromagnet with strong electron-electron correlations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗