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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Imbalanced spin coupling in the copper hexamer compounds A 2 Cu 3 O(SO 4 ) 3 ( A 2 = Na 2 , NaK, K 2 )

The minerals A 2 Cu 3 O(SO 4 ) 3 (A 2 = Na 2 ,NaK,K 2 ) constitute quantum spin systems with copper hexamers as basic structural units. Strong intrahexamer spin couplings give rise to an effective triplet ground state. Weak interhexamer spin couplings are responsible for two-dimensional long-range magnetic order in the (b,c) plane below 3.0 < T c < 4.7K. We investigated the magnetic excitations at T = 1.5 K by inelastic neutron scattering (INS). Furthermore, the INS technique was based on the observation of wave vector dependent slices in reciprocal space in order to selectively probe the magnetic signals in different Brillouin zones with different weight. Due to the imbalance of the spin couplings, the data analysis relies on a model in which the interhexamer spin couplings are treated perturbatively on top of the exact S = 1 ground state. The interhexamer spin couplings turn out to be ferromagnetic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Reaching the equilibrium state of the frustrated triangular Ising magnet Ca 3 Co 2 O 6

Ca 3 Co 2 O 6 is a frustrated magnet consisting of a triangular arrangement of chains of Ising spins. It shows regular magnetization steps versus magnetic field every 1.2 T that are metastable with very slow dynamics. This has puzzled the community for many years and given rise to numerous potential theories. Here we approach the problem by seeking the elusive magnetic equilibrium state at T=2 K. To this end, we explore two approaches: (1) bypassing the slow dynamics produced by changing fields by instead field-cooling directly to the target temperature and (2) quantum annealing in transverse magnetic fields. While we observe no measurable effect of the quantum annealing in fields up to 7 T, which is likely due to the large Ising anisotropy of Co spins in this material, we find that for the field cooling in longitudinal fields we achieve the predicted equilibrium 1/3 magnetization. We perform Monte Carlo simulations of the ground state phase diagram and we also simulate the quantum annealing process and find good agreement between experiment and theory. Thus we present an investigation of the elusive ground state properties of the canonical frustrated triangular system Ca 3 Co 2 O 6 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Charge transport in topological graphene nanoribbons and nanoribbon heterostructures

Although it is generally accepted that structural parameters like width, shape, and edge structure crucially affect the electronic characteristics of graphene nanoribbons (GNRs), the exact relationship between geometry and charge transport remains largely unexplored. Here, in this paper, we present in situ through-transport measurements of various topological GNRs and GNR heterostructures by lifting the ribbon with the tip of a scanning tunneling microscope. At the same time, we develop a comprehensive transport model that enables us to understand various features, such as obscuring of localized states in through transport, the effect of topology on transport, as well as negative differential conductance in heterostructures with localized electronic modes. The combined experimental and theoretical efforts described in this paper serve to elucidate general charge transport phenomena in GNRs and GNR heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Ferrimagnetic spin waves in honeycomb and triangular layers of Mn{sub 3}Si{sub 2}Te{sub 6}.

A detailed analysis of the ferrimagnetic ground state of Mn 3 Si 2 Te 6 has been performed using inelastic neutron scattering. Although the proposed valence of the nominal Mn 2+ ions would have quenched orbital angular momentum, a significant exchange anisotropy exists in Mn 3 Si 2 Te 6 . This apparent exchange anisotropy is a manifestation of a weak spin-orbit coupling in the layered material. We employ a detailed simulation of the spin-wave spectrum coupling traditional refinement of dispersion parameters to image analysis techniques, while including Monte Carlo simulations of the instrumental resolution to accurately identify the exchange couplings to the third nearest neighbor. An independent validation of our results is made by comparing our final Hamiltonian to heat capacity measurements.

Sala, G.↗
DOE OSTI2024

Magnetic field induced phases and spin Hamiltonian in Cs 2 CoBr 4

Magnetic structures and spin excitations are studied across the phase diagram of the geometrically frustrated S = 3/2 quantum antiferromagnet Cs 2 CoBr 4 in magnetic fields applied along the magnetic easy axis, using neutron diffraction, inelastic neutron scattering, and terahertz absorption spectroscopy. Here, the data are analyzed, where appropriate, using extended SU(4) linear spin wave theory. A minimal magnetic Hamiltonian is proposed based on measurements in the high-field paramagnetic state. It deviates considerably from the previously considered models. Additional dilatometry experiments highlight the importance of magnetoelastic coupling in this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Field-dependent magnons in the honeycomb antiferromagnet CoTiO 3

We report field-dependent high-resolution inelastic neutron scattering (INS) measurements on the honeycomb lattice magnet, CoTiO 3 , to study the evolution of its magnon excitations across a spin reorientation transition driven by an in-plane magnetic field. By carrying out elastic neutron scattering in a magnetic field, we show that the sample transitions from a collinear antiferromagnetic state with multiple magnetic domains at a low field to a monodomain state with a canted magnetic structure at a high field. Concurrent with this transition, we observed significant changes in both the energy and the width of the zone center magnon peak. The observed width change was argued to be consistent with an unusual zero-field state with extended domain walls. On the other hand, the magnon spectra near the K point of the Brillouin zone boundary were found to be largely insensitive to the changes in the ordered moment directions and the domain configuration. We argue that this observation is difficult to explain within the framework of the bond-dependent model proposed in a recent INS study [M. Elliot et al., Nat. Commun. 12, 3936 (2021)]. Our paper therefore calls for alternative explanations for the observed K-point gap in CoTiO 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magnon gap tuning in lithium-doped MnTe

Manganese telluride (MnTe) is a prospective platform for ultrafast carrier dynamics, spin-based thermoelectrics, and magnon-drag transport due to its unique electronic and magnetic properties. We use inelastic neutron scattering to study both pure and lithium-doped MnTe, focusing on the influence of doping in opening a magnon gap. We use neutron powder diffraction to determine critical exponents for the phase transition in both pure and Li-doped MnTe and complement this information with muon spin rotation/relaxation. In conclusion, the opening of the magnon gap and spin reorientation in Li-doped MnTe is mainly due to increased magnetic anisotropy along the [001] axis, a feature not present in pure MnTe.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Spin-flop coupling at L⁢a 0.5 ⁢S⁢r 0.5 ⁢Fe⁢O 3 /L⁢a 0.7⁢ S⁢r 0.3 ⁢Mn⁢O 3 interfaces

Antiferromagnetic (AFM) spintronics offer several benefits compared to their ferromagnetic (FM) counterparts, such as high storage capacity and faster processing speed, however, difficulties in manipulating and detecting the AFM moments impede their implementation. Spin-flop coupling, the interfacial perpendicular coupling between FM and AFM moments, can be utilized to control the orientation of AFM moments with the application of moderate magnetic fields on the scale of tenths of a Tesla. Here, in this work, epitaxial bilayers of AFM La 0.5 Sr 0.5 FeO 3 (LSFO)/FM La 0.7 Sr 0.3 MnO 3 (LSMO) with fixed LSMO thickness (~85 u.c.) and LSFO thicknesses varying from 10 to 50 u.c. were investigated to determine the effect of Sr doping and La 1-x Sr x FeO 3 magnetocrystalline anisotropy on the strength of spin-flop coupling. X-ray magnetic linear dichroism demonstrated that the spin-flop coupling strength decreased with increasing LSFO layer thickness, persisting at a thickness of 50 u.c. (~20 nm). Furthermore, photoemission electron microscopy revealed a domain-by-domain correlation between the FM and AFM domains consistent with the perpendicular orientation dictated by spin-flop coupling. These results demonstrate that LSFO/LSMO bilayers have the potential to serve as a model materials system for AFM spin transport measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Understanding Reentrance in Frustrated Magnets: The Case of the Er 2 Sn 2 O 7 Pyrochlore

Reentrance, the return of a system from an ordered phase to a previously encountered less-ordered one as a controlled parameter is continuously varied, is a recurring theme found in disparate physical systems, yet its microscopic cause is often not investigated thoroughly. Here, through detailed characterization and theoretical modeling, we uncover the microscopic mechanism behind reentrance in the strongly frustrated pyrochlore antiferromagnet Er 2 Sn 2 O 7 . We use single crystal heat capacity measurements to expose that Er 2 Sn 2 O 7 exhibits multiple instances of reentrance in its magnetic field B vs temperature T phase diagram for magnetic fields along three cubic high symmetry directions. Through classical Monte Carlo simulations, mean field theory, and classical linear spin-wave expansions, we argue that the origins of the multiple occurrences of reentrance observed in Er 2 Sn 2 O 7 are linked to soft modes. These soft modes arise from phase competition and enhance thermal fluctuations that entropically stabilize a specific ordered phase, resulting in an increased transition temperature for certain field values and thus the reentrant behavior. Our work represents a detailed examination into the mechanisms responsible for reentrance in a frustrated magnet and may serve as a template for the interpretation of reentrant phenomena in other physical systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Tuning the interlayer coupling in La 0.7 Sr 0.3 Mn 0.95 Ru 0.05 O 3 / LaNiO 3 multilayers with perpendicular magnetic anisotropy

In ferromagnetic oxide epitaxial multilayers, magnetic properties and interlayer coupling are determined by a variety of factors. Beyond the contribution of interlayer exchange coupling, strain and interfacial effects, such as structural reconstructions or charge transfer, play significant roles, resulting in complex magnetic behavior. In this study, the interlayer coupling of ferromagnetic La 0.7 ⁢Sr 0.3⁢ Mn 0.95 ⁢Ru 0.05 ⁢O 3 (LSMRO) layers (8 nm thick) was investigated, when separated by epitaxial spacers of paramagnetic metallic LaNiO 3 (LNO), in stacks exhibiting perpendicular magnetic anisotropy. By varying the thickness of the spacer, it was found that the coupling between two LSMRO layers changes from antiferromagnetic (with a 4-unit-cell-thick LNO spacer) to ferromagnetic (with a 6-unit-cell-thick LNO spacer). For multilayers comprising five LSMRO layers and a 4-unit-cell-thick LNO spacer, the antiferromagnetic coupling was preserved. However, the effective magnetic anisotropy changed, causing the magnetization to cant more towards the in-plane direction. Further, this behavior was corroborated by X-ray magnetic circular dichroism (XMCD) investigations at the Mn and Ni L 3 edges. The XMCD results indicated that the 4-unit-cell-thick LNO spacer in the multilayer become magnetically ordered, closely following the magnetization of adjacent LSMRO layers.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Quantum stabilization of unexpected ordered phases on the honeycomb lattice

In this work, I discuss and showcase the utility of the method called minimally-augmented spin-wave theory (MAGSWT) as a relatively simple semi-analytical approach to the phase diagrams of quantum magnets. It complements numerical methods by providing physical insight into which states are competitive and by yielding approximate phase boundaries that agree well with much more numerically intensive calculations. This approach enabled me to construct the faithful phase diagrams of two paradigmatic honeycomb-lattice models in the quantum S=1/2 limit, the J 1 –J 3 FM-AF and J 1 –J 2 AF models, for the collinear quantum phases that replace or extend the classical ones. The results are in good qualitative and semi-quantitative agreement with state-of-the-art numerical studies for these models, correctly capturing the emergence of unexpected quantum phases and the suppression of classically favored spiral orders by quantum fluctuations. This study provides a much-needed important guidance to the ongoing theoretical and experimental searches of the unconventional quantum states. This work will be of significant and timely interest to both theorists and experimentalists in the field of quantum magnetism, broadly defined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetic transition behavior in epitaxial Fe 47 Rh 47 Pd 6 films

The properties of Fe–Rh–Pd epitaxial thin films grown on MgO(001) were studied as a function of growth temperature. Films grown above 400°C exhibit a first-order antiferromagnetic to ferromagnetic magnetic phase transition with a transition temperature that decreases as the growth temperature is increased. The chemical order parameter computed from the ratio of intensities of the (001) and (002) diffraction peaks is nearly independent of the growth temperature, while the lattice constants change slightly. A comparison of our structural, magnetic, and electrical transport results with first-principle-based calculations as well as literature results indicates that the transition temperature of Fe–Rh-based alloy films depends sensitively on the lattice parameters and is of electronic origin. The transition temperature and its width can be tuned over a wide range by controlling the crystal structure via growth conditions or postdeposition annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Board on Earth Sciences and Resources and Its Activities

The National Academies’ Board on Earth Sciences and Resources (BESR) and its standing committees provide an ongoing forum for advancing the understanding and communication of Earth sciences and resource topics, including emerging topics and innovative techniques. BESR activities help to provide evidence-based information to members of the executive and legislative branches of the federal government, the private sector, states and tribes, academia, non-governmental organizations, and the public to support decision-making. BESR and its standing Committee on Solid Earth Geophysics (COSEG), also supported by this award, fulfill this role through development and administration of consensus studies, as well as workshops and other convening activities related to the Earth sciences; overseeing selected activities of the Board’s standing committees, such as disciplinary meetings and webinars; and communicating, sharing information, and providing opportunities for interaction and exchange among technical and non-technical stakeholders. The core support received from DOE helps BESR and COSEG maintain a central body of volunteer experts and National Academies staff who can respond to pressing needs and requests from federal sponsors and other members of the Earth science community, to maintain the health and relevance of the Earth sciences discipline, and to provide the Earth science community with a privileged interface to the government to support scientific and engineering advances and decision making related to Earth sciences and engineering. Initiation and oversight of Earth science activities at the National Academies is an enduring function of the BESR and COSG that helps to ensure development and completion of projects and activities that are responsive to the needs of sponsors and the broader Earth science and research enterprise.

58 GEOSCIENCES↗