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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Multipole polaron roams the devil’s staircase

We report Electron–boson interaction (EBI) has an important role in the properties of materials. Electron–phonon interaction in good metals can give rise to superconductivity, where electrons flow without dissipation below a critical temperature. In ionic crystals such as organic semiconductors, the coupling of low-energy electrons (or holes) with the atomic vibrations can drive the formation of polarons — an electron dressed with a phonon cloud, with lower mobility and heavy effective mass. The same notion can be applied to strongly correlated electron systems. Electron–spin fluctuation coupling drives unconventional superconductivity in high-temperature cuprates, iron-based superconductors and heavy-fermion systems. Magnetic polarons can form around charged dopants in an otherwise antiferromagnetic background of a Mott insulator, while an electron–polaron is also possible, via electron–plasmon coupling, when a Kondo hole is introduced in a heavy-fermion material, similar to a plasmaron in doped graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Electrical Switching and Imaging of Antiferromagnetic Spins in Perovskite Epitaxial Thin Films

Electrical control of antiferromagnets is essential for the field of antiferromagnetic spintronics that promises ultrafast speed and terahertz dynamics. However, the large family of complex oxide antiferromagnets such as perovskites have been largely unexplored. Here, we demonstrate electrical switching and detection of Néel vector in LaFeO 3 epitaxial thin films with an epitaxy-enabled biaxial anisotropy, of which the switching behavior is understood by comparing to magnetic field controlled spin Hall magnetoresistance measurements. The electrical switching is corroborated by imaging of antiferromagnetic domains using X-ray Magnetic Linear Dichroism Photoemission Electron Microscopy. Electrical switching of 3-nm and 10-nm LaFeO 3 films was achieved over a wide temperature range of 25 to 300 K. This work offers an attractive platform for exploration of antiferromagnetic spintronics and other emerging phenomena such as potential insulating altermagnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Structural, electronic, and magnetic properties of nearly ideal J eff = 1/2 iridium halides

Heavy transition metal magnets with J e f f = 1 2 electronic ground states have attracted recent interest due to their penchant for hosting new classes of quantum spin liquids and superconductors. Unfortunately, model systems with ideal J e f f = 1 2 states are scarce due to the importance of noncubic local distortions in most candidate materials. In this work, we identify a family of iridium halide systems [i.e., K 2 Ir Cl 6 , K 2 Ir Br 6 , ( NH 4 ) 2 Ir Cl 6 , and Na 2 Ir Cl 6 · 6 ( H 2 O ) ] with Ir 4 + electronic ground states exhibiting extremely small deviations from the ideal J e f f = 1 2 limit. We also find ordered magnetic ground states for the three anhydrous systems, with single-crystal neutron diffraction on K 2 Ir Br 6 revealing type-I antiferromagnetism. This spin configuration is consistent with expectations for significant Kitaev exchange in a face-centered-cubic magnet. This work establishes that incorporating isolated Ir X 6 octahedra in materials, where X is a halogen ion with a low electronegativity, is an effective design principle for realizing unprecedented proximity to the pure J e f f = 1 2 state. However, at the same time, we highlight undeniable deviations from this ideal state, even in clean materials with ideal Ir X 6 octahedra as inferred from the global cubic crystal structures.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Neutron scattering investigation of proposed Kosterlitz-Thouless transitions in the triangular-lattice Ising antiferromagnet TmMgGaO 4

In this study, we use a variety of neutron scattering techniques to investigate the nature of the unusual magnetism in the compound TmMgGaO 4 . The crystal structure of this material hosts a triangular lattice of Tm spins exhibiting strong geometrical frustration. Theoretical predictions have suggested an unusual topological magnetic phase transition known as a Kosterlitz-Thouless transition at low temperature. By combining and comparing the various neutron scattering results, we have compelling evidence that such a transition indeed occurs at temperatures around 4 K in TmMgGaO 4 . This establishes TmMgGaO 4 as a valuable system to study Kosterlitz-Thouless physics in a dense spin system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Rethinking 𝛼−RuCl 3 : Parameters, models, and phase diagram

RuCl 3 was likely the first ever deliberately synthesized ruthenium compound, following the discovery of the 44 Ru element in 1844. For a long time it was known as an oxidation catalyst, with its physical properties being discrepant and confusing, until a decade ago when its allotropic form 𝛼−RuCl 3 rose to exceptional prominence. This “rediscovery” of 𝛼−RuCl 3 has not only reshaped the hunt for a material manifestation of the Kitaev spin liquid, but it has opened the floodgates of theoretical and experimental research in the many unusual phases and excitations that the anisotropic-exchange magnets as a class of compounds have to offer. Given its importance for the field of Kitaev materials, it is astonishing that the low-energy spin model that describes this compound and its possible proximity to the much-desired spin-liquid state is still a subject of significant debate ten years later. In the present study, we argue that the existing key phenomenological observations put strong natural constraints on the effective microscopic spin model of 𝛼−RuCl 3 , and specifically on its spin-orbit-induced anisotropic-exchange parameters that are responsible for the nontrivial physical properties of this material. These constraints allow one to focus on the relevant region of the multidimensional phase diagram of the 𝛼−RuCl 3 model, suggest an intuitive description of it via a different parametrization of the exchange matrix, offer a unifying view on the earlier assessments of its parameters, and bring closer together several approaches to the derivation of anisotropic-exchange models. We explore extended phase diagrams relevant to the 𝛼−RuCl 3 parameter space using quasiclassical, Luttinger-Tisza, exact diagonalization, and density-matrix renormalization-group methods, demonstrating a remarkably close quantitative accord between them on the general structure and hierarchy of the phases, with the zigzag, ferromagnetic, and incommensurate phases that are proximate to each other. As a result, one of the highlights is the detailed agreement on the nature of the incommensurate phases that realize two distinct counterrotating helical states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Synthesis, structure and canted antiferromagnetism of layered cobalt hydroxide sorbate

Layered hybrid organic–inorganic compounds of the transition metal hydroxides, in which part of the hydroxide ions are exchanged for organic ligands that act as spacers of the 2-dimensional inorganic layers, elicit new or enhanced properties. The interlayer ligands enable fine-tuning of the physicochemical properties of the metal hydroxide layers, including their magnetism. Specifically, within the Co(OH) 2 family of transition ion hydroxides, this approach has been leveraged to stabilize antiferromagnetic and ferrimagnetic ground states for pillared and layered complexes, respectively. Here, in this work, we further investigate the effects of changing dimensionality, local Co coordination, and interlayer spacing on the magnetic ground state of brucite-like β-Co(OH) layered complexes. We report the hydrothermal synthesis of a new crystalline material, Co(OH)(sorb), where the inorganic layers are spaced by monotopic, doubly unsaturated sorbate ligands (sorb = [C 6 H 7 O 2 ] – ). The material was structurally characterized using powder X-ray and neutron diffraction measurements and the crystal structure was solved ab initio from powder diffraction data and refined using the Rietveld method. The magnetic susceptibility and magnetization measurements reveal canted antiferromagnetic ordering with T N = 41.7 K. This report augments our understanding of tuning magnetism through dimensionality in layered complexes and represents a step forward in the design of new 2D layered hybrid compounds.

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

Magnetoelectric effect arising from a field-induced pseudo Jahn-Teller distortion in a rare-earth magnet

Magnetoelectric materials are attractive for several applications, including actuators, switches, and magnetic field sensors. Typical mechanisms for achieving a strong magnetoelectric coupling are rooted in transition metal magnetism. In sharp contrast, here we identify CsEr(MoO 4 ) 2 as a magnetoelectric material without magnetic transition metal ions, thus ensuring that the Er ions play a key role in achieving this interesting property. Our detailed study includes measurements of the structural, magnetic, and magnetoelectric properties of this material. Bulk characterization and neutron powder diffraction show no evidence for structural phase transitions down to 0.3 K and therefore CsEr(MoO 4 ) 2 maintains the room temperature P2/c space group over a wide temperature range without external magnetic field. These same measurements also identify collinear antiferromagnetic ordering of the Er 3+ moments below T N =0.87K. Complementary dielectric constant and pyroelectric current measurements reveal that a ferroelectric phase with a maximum polarization P~0.6 nC/cm 2 emerges when applying a modest external magnetic field, which indicates that this material has a strong magnetoelectric coupling. Finally, we argue that the magnetoelectric coupling in this system arises from a pseudo Jahn-Teller distortion induced by the magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Magnetic structure of triangular lattice compound Tb2Ni0.90Si2.94

AlB 2 -type ternary intermetallic compound Tb 2 Ni 0.90 Si 2.94 (space group P 6=mmm, hP 3, No. 191) was reported to exhibit spin freezing behaviour of the ferromagnetic clusters present in the system below T f = 9:9 K, along with the presence of spatially limited antiferromagnetic phase. In this work, on the basis of variable temperature zero-field neutron diffraction measurements, we have shown that the antiferromagnetic phase transition occurs for the compound below T N ~13 K. Neutron diffraction study indicates ab-plane non-collinear sine-modulated antiferromagnetic ordering of the system with wave vectors of k 1 = [±1/6, ±1/6, 0] and k 2 = [±1/3, ±1/3, 0] down to 1.7 K. The weak and diffuse nature of the magnetic Bragg peaks along with limited coherence length further confirm the short-range nature of the antiferromagnetic phase in this compound.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Synthesis and magnetic properties of the Shastry-Sutherland family R 2 Be 2 SiO 7 ( R = Nd , Sm , Gd-Yb )

Compounds forming the quasi-two-dimensional Shastry-Sutherland lattice (SSL) have attracted significant experimental and theoretical attention in the field of frustrated magnetism. This is primarily due to their realization of an exactly soluble J 1 –J 2 orthogonal dimer model capable of hosting magnetic order, dimer singlet, and plaquette singlet phases in zero applied field and their complex magnetic phase diagrams with fractional magnetization plateaus and possible superfluid and supersolid phases found between the plateau states. The discovery and characterization of SSL compounds based on rare-earth magnetic ions provide a direct route to study the stability and properties of these exotic magnetic phases in systems with a variety of different magnetic anisotropies. In this paper, we discuss the synthesis and magnetic characterization of polycrystalline samples of the R 2 ⁢Be 2 ⁢SiO 7 family, where R=Nd, Sm, and Gd-Yb. All family members crystallize in the space group $P\bar{⁢4}⁢2_1⁢m$ (113) and show no signs of long-range magnetic order above 2 K, except for R=Tb which orders antiferromagnetically at 2.6 K.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Flat bands in the CoSn-type compounds

Quantum interference on the kagome lattice generates electronic bands with narrow bandwidth, called flat bands. Crystal structures incorporating this lattice can host strong electron correlations with nonstandard ingredients, but only if these bands lie at the Fermi level. In the six compounds with the CoSn structure type (FeGe, FeSn, CoSn, NiIn, RhPb, and PtTl) the transition metals form a kagome lattice. The two iron variants are robust antiferromagnets so we focus on the latter four and investigate their thermodynamic and transport properties. We consider these results and calculated band structures to locate and characterize the flat bands in these materials. Finally, we propose that CoSn and RhPb deserve the community's attention for exploring flat-band physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Exciton-driven antiferromagnetic metal in a correlated van der Waals insulator

Abstract Collective excitations of bound electron-hole pairs—known as excitons—are ubiquitous in condensed matter, emerging in systems as diverse as band semiconductors, molecular crystals, and proteins. Recently, their existence in strongly correlated electron materials has attracted increasing interest due to the excitons’ unique coupling to spin and orbital degrees of freedom. The non-equilibrium driving of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phases beyond thermodynamic equilibrium. Here, we achieve this in the van der Waals correlated insulator NiPS 3 by photoexciting its newly discovered spin–orbit-entangled excitons that arise from Zhang-Rice states. By monitoring the time evolution of the terahertz conductivity, we observe the coexistence of itinerant carriers produced by exciton dissociation and a long-wavelength antiferromagnetic magnon that coherently precesses in time. These results demonstrate the emergence of a transient metallic state that preserves long-range antiferromagnetism, a phase that cannot be reached by simply tuning the temperature. More broadly, our findings open an avenue toward the exciton-mediated optical manipulation of magnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Magnetic order and physical properties of the kagome metal UNb 6 ⁢Sn 6

The 𝑅⁢𝑀 6 ⁢𝑋 6 family of materials (𝑅 = rare-earth, 𝑀 = transition metal, 𝑋 = Ga, Si, Ge, Sn) produces an array of emergent phenomena, such as charge density waves, intrinsic Hall effects, and complex magnetic order, due to its kagome net of transition metal atoms, its local-moment magnetic anisotropies, and its extensive chemical tunability. Here, in this study, we report UNb 6⁢ Sn 6 , a new “166” material containing both an actinide and a 4⁢𝑑 transition metal, along with its nonmagnetic analog ThNb6⁢Sn6, to investigate the properties of a 5⁢𝑓−4⁢𝑑 electron 166 system. UNb 6 ⁢Sn 6 crystallizes in the hexagonal 𝑃⁢6/𝑚⁢𝑚⁢𝑚 space group with a small degree of disorder due to shifts in the size of the CoSn-like cages along the 𝑐 axis. Upon cooling at zero magnetic field, the material undergoes two magnetic phase transitions at 𝑇 2 = 46K and 𝑇 N = 43K. The low-temperature, zero-field phase is an antiferromagnet with ordered uranium moments and a k = (0,0,1/2) propagation vector determined by neutron diffraction. Remarkably, with a magnetic field applied along the 𝑐 axis, five additional magnetic transitions occur, evidenced by magnetization and resistivity data, before the moment saturates at 2.62 𝜇 B /U at 2 K and ≥ 13.6T. In two magnetic phase regions, the Hall resistivity of UNb 6⁢ Sn 6 significantly deviates from the magnetization, suggesting that the phases have a large Berry curvature or a change in the Fermi surface. The unknown magnetic ordering of the field-dependent phases of UNb 6⁢ Sn 6 demonstrates the complexity of the 5⁢𝑓−4⁢𝑑 166 system and encourages further study of its properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Bond valence sum analysis of pyrochlore oxides including the novel dielectric Te 6+ pyrochlores: A Bi M TeO 7-y ( A = Cd, Ca; M = Cr, Ga, Sc, In, Fe)

Bond valence sum analysis is a powerful tool used in evaluating and validating crystal structures; especially when those structures are complex in nature. The pyrochlore structure type is versatile in not only the unique bonding that it exhibits, but also in the properties that results from the structure. Here this paper aims to center the discussion of evaluating the pyrochlore structure using the bond valence sum method. In this study, novel quaternary pyrochlores with a general stoichiometry of ABiMTeO 7 (A = Cd, Ca; M = Cr, Ga, Sc, In, Fe) were synthesized and characterized for their structural, magnetic, and dielectric properties. Two representative compounds within this series of pyrochlores, BiCaFeTeO7 and BiCdFeTeO 7 , were structurally characterized utilizing a combination of high-resolution synchrotron X-ray diffraction and neutron diffraction revealing oxygen deficient pyrochlore systems which were off from the expected stoichiometry with respect to the M site. The A site of both pyrochlores were found to be moved off-center from the expected 16d site to the 96h displaced position at a magnitude of 0.25 Å and 0.22 Å for the Bi/Ca and Bi/Cd systems, respectively. These structures were evaluated using the bond valence sum method and compared with trends in the literature. The properties are also reported for the Bi/Ca system for the first time, showing relatively high dielectric constants with a low dielectric loss which are primarily independent of frequency and temperature. The magnetic measurements for the Bi/Ca system for the magnetic substitutions reveal a paramagnet and antiferromagnetic properties for the Fe and Cr analogs, respectively. The novel BiCaMTeO 7 quaternary pyrochlore system shows great promise as an emerging dielectric material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Double-domed temperature-pressure phase diagram of CePd 3 S 4

CePd 3 S 4 exhibits interplay between ferromagnetism (FM), quadrupolar order, and the Kondo effect at low temperatures with a FM transition temperature that is much higher than the value expected from the de Gennes scaling of the heavier RPd 3 S 4 compounds. In this work, we investigated the electrical transport and magnetic properties of CePd 3 S 4 under pressure up through 12 GPa so as to better understand the interplay between electronic and magnetic phases in this material. Our findings show that the low pressure FM state is suddenly replaced by a new magnetically ordered phase that is most likely antiferromagnetic that spans from ~ 7 GPa to ~ 11 GPa. Whereas this could be described as an example of avoided quantum criticality, given that clear changes in resistance and Hall data are detected near 6.3 GPa for all temperatures below 300 K, it is also possible that the change in ground state is a response to a pressure induced change in structure. Furthermore, the lack of any discernible change in the pressure dependence of the room temperature unit cell parameter/volume across this whole pressure range suggests that this change in structure is either more subtle than could be detected by our measurements (i.e. development of weak, new wave vector) or the transition is electronic (such as a Lifshitz transition).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnon thermal conductivity in multiferroics with spin cycloids

Multiferroic materials, characterized by the occurrence of two or more ferroic properties, hold potential in future technological applications and also exhibit intriguing phenomena caused by the interplay of multiple orders. One such example is the formation of spin-cycloid structures within multiferroic materials, which we investigate in this work by focusing on their magnon excitations and transport based on a general multiferroic Hamiltonian with an antiferromagnetic order. More specifically, we identify the ground state and explore the dynamics of magnon modes, revealing distinct in-plane and out-of-plane modes with anisotropic dispersion relations. The magnon modes include a massless excitation, known as the Goldstone boson, originating from the spontaneous breaking of the translational symmetry by the formation of the cycloid structures. By employing the Boltzmann transport formalism, the magnonic thermal conductivity with spin cycloids and low-temperature anisotropic behaviors is discussed. This work provides pathways to envision the spin-textured multiferroics, which may serve as a fertile ground to look for novel thermal and spin transport with the rich interplay of quasiparticles such as magnons and phonons.

Park, Hyeon Woo↗
DOE OSTI2024

Unraveling the origin of antiferromagnetic coupling at YIG/permalloy interface

We investigate the structural and electronic origin of antiferromagnetic (AFM) coupling in the yttrium iron garnet (YIG) and permalloy (Ni 80 ⁢Fe 20 , Py) bilayer system at the atomic level. Ferromagnetic resonance (FMR) spectra reveal unique hybrid modes in samples prepared with surface ion milling, indicative of antiferromagnetic exchange coupling at the YIG/Py interface. Using atomic resolution scanning transmission electron microscopy (STEM), we found that AFM coupling appears at the YIG/Py interface of the tetrahedral YIG surface formed with ion milling. Here, the STEM measurements suggest that the interfacial AFM coupling is predominantly driven by an oxygen-mediated superexchange coupling mechanism, which is confirmed by the density-functional theory (DFT) calculations to be energetically favorable. Thus, the combined experimental and theoretical results reveal the critical role of interfacial atomic structure in determining the type of magnetic coupling in a YIG/ferromagnet heterostructure, and prove that the interfacial structure can be experimentally tuned by surface ion milling.

36 MATERIALS SCIENCE↗