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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
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 OSTI2024

A new class of single-phase multiferroics: Bismuth-based layered supercell oxide thin films—Current progress and future perspectives

Multiferroic materials, where ferroelectric and magnetic orders coexist, have ignited substantial research interest due to the achievable manipulation of magnetic orders using external electric fields, a feature that has garnered serious interest for memory storage applications. Nonetheless, naturally occurring single-phase multiferroic materials are scarce, thus constraining options for practical use. Over the last decade, bismuth-based layered supercell (LSC) oxides have emerged as novel candidates for multiferroics, catalyzing extensive investigations in this domain. Additionally, these LSC systems are known for their anisotropic structures and optical properties, making them promising for application in optics such as polarizers, beam splitters, and modulators. This thorough review explores the development and current advancements in multiferroic bismuth-based LSC materials. It covers the diverse nature of LSCs, detailing their microstructure, properties, and the mechanics of self-assembly formation. It also highlights the remarkable multifunctional characteristics of LSC-based nanocomposites, with a particular focus on their applications in electronics and optics. Moreover, this review examines the significant potential of LSCs in practical applications, particularly through their integration onto silicon and flexible substrates via heteroepitaxy and film transfer techniques. Finally, it offers insights into potential future research avenues and the broader implications of these versatile LSC materials.

Materials Science↗
DOE OSTI2023

Global quantum phase diagram and non-Abelian chiral spin liquid in a spin- 3 2 square-lattice antiferromagnet

Since strong quantum fluctuations are essential for the emergence of quantum spin liquids, there have been extensive exploration and identification of spin liquid candidates in spin-$\frac{1}{2}$ systems, while such activities are rare in higher spin systems. Here we report an example of non-Abelian chiral spin liquid emerging in a spin-$\frac{3}{2}$ Heisenberg model on a square lattice. By tuning Heisenberg exchange interaction and scalar chirality interaction, we map out a quantum phase diagram enclosing three conventional magnetic orders and a chiral spin liquid based on density-matrix renormalization group studies. The nature of the spin liquid is identified as a long-sought bosonic version of the Read-Rezayi state that supports non-Abelian Fibonacci anyonic statistics, identified by the ground state entanglement spectrum. Significantly, we establish that the non-Abelian chiral spin liquid emerges through the enlarged local degrees of freedom and enhanced quantum fluctuations near the classical phase boundaries of competing magnetic orders. Finally, our numerical discovery of an exotic quantum spin liquid in a spin-$\frac{3}{2}$ system suggests a route for discovering fractionalized quantum phases in frustrated higher spin magnetic compounds.

2-dimensional systems↗
DOE OSTI2025

Disentangling the intertwined orders in a magnetic kagome metal

Intertwined orders appear when multiple orders are strongly interacting, and kagome metals have emerged as new platforms to explore exotic phases. FeGe has been found to develop a charge density wave (CDW) order within magnetic phase, suggesting an intricate interplay of the lattice, charge, and spin degrees of freedom. Recently, postgrowth annealing has been proposed to tune the CDW order from long-range to complete suppression, offering a tuning knob for the CDW order. Here, by comparing the electronic structures of FeGe subjected to different annealing conditions and distinct CDW properties, we report spectral evolution associated with the lattice and spin degrees of freedom. We find band evolution linked to a spin density wave (SDW) order present in both samples with and without CDW order, and another evolution connected to the lattice distortions that onset with the long-range CDW order and revert with the SDW order. Our results reveal a rare competitive cooperation of the lattice, spin, and charge in FeGe.

Oh, Ji Seop↗
DOE OSTI2025

Magnetic properties of Tm 3+ in layered triangular lattices

Rare-earth chalcogenides, ARCh 2 (A = alkali metal or monovalent ion; R = rare-earth elements; Ch = O, S, and Se), have been identified as promising candidates for exploring a variety of novel frustrated quantum magnetic phenomena. Tm-based series, ATmCh 2 , where Tm 3+ ions are arranged on various frustrated geometric lattices, provide a platform for investigating the competitions among spin–orbit coupling, crystal field effects, and magnetic exchange interactions in the context of geometric frustration. In this study, we present how the crystallographic structures of newly synthesized ATmSe 2 (A = Li and Na) influence their frustrated magnetic behaviors. Both NaTmSe 2 and LiTmSe 2 adopt a delafossite-type structure with a two-dimensional (2D) triangular lattice, but LiTmSe 2 has a much shorter c-axis parameter compared to this in NaTmSe 2 . Both heat capacity and magnetic susceptibility measurements confirm the absence of long-range magnetic order in either compound. However, magnetic measurements data reveals the differences in magnetic interactions: NaTmSe 2 exhibits features of low-dimensional magnetism, likely driven by its layered crystal structure, compared to LiTmSe 2 .

Crystal field theory↗
DOE OSTI2023

Possible surface magnetism in the topological Kondo insulator candidate FeSi

Our group has previously reported the existence of a conducting surface state (CSS) in FeSi, a candidate for a d-electron topological Kondo insulator (TKI), at low temperature. In this paper, we present the electrical transport properties of single crystals of FeSi studied in the phase space of temperature (T), magnetic field (B), and the angle (θ) between the electrical current and B. The normalized T-dependent electrical resistance (R) of a successively thinned FeSi crystal provides further confirmation of the existence of a CSS. We report that, in the CSS, the magnetoresistance (MR) exhibits a hysteresis loop bounded within ±0.5 T, suggesting two-dimensional magnetic ordering. The hysteretic MR is asymmetric in B and anisotropic with respect to θ. Further exploration of R(θ) at a fixed field of 9 T reveals an initial progressive rotation of the axis for twofold rotational symmetry from 2 to 10 K, then a stabilized axis for twofold symmetry until, at T > 40 K, the anisotropy vanishes, coincident with the disappearance of the CSS. Furthermore, these observations point to a possible magnetically ordered surface state that has been reported in similar systems such as FeSi nanofilms and bulk SmB 6 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Higher Dimensionality in the Mg–Co–B System: Synthesis and Structure of Incommensurate Composite Mg 1+ε Co 4 B 4

Guided by high-temperature in situ X-ray diffraction, the discovery and synthesis of Mg 1+ε Co 4 B 4 (ε ≈ 0.272) using a MgH 2 hydride precursor is reported, along with a detailed crystal structure description and measurement of magnetic properties. The mismatch in lattice periodicities between Mg and Co–B substructures places Mg 1+ε Co 4 B 4 in the family of incommensurate composite crystals and prompted structural refinement in a (3 + 1)-dimensional model. The structure of Mg 1+ε Co 4 B 4 (P4 2 /ncm(00γ)s00s, a = 6.75847(7) Å, c = 3.94007(8) Å, q = (0, 0, 1.2721(3))) was refined from neutron powder diffraction and high-resolution powder X-ray diffraction data and confirmed by scanning transmission electron microscopy and electron diffraction. Mg 1+ε Co 4 B 4 is isostructural to Nd 1+ε Fe 4 B 4 and several related ternary borides with 0.07 ≤ ε ≤ 0.17, with Mg occupying the rare-earth site. Satellite reflections in the electron diffraction patterns hinted at positional modulation of the transition metal–boron substructure by Mg atoms, but this could not be refined from the neutron or X-ray diffraction data. Low-temperature magnetic measurements show no indications of long-range magnetic ordering or superconductivity down to 5 K. DFT calculations confirmed the absence of a magnetically ordered ground state and the stability of a 5:4 supercell (ε = 0.25) relative to the fully commensurate structure. Neutron diffraction and synthesis from elemental Mg demonstrated that Mg 1+ε Co 4 B 4 is not a hydrogen-stabilized phase. Mg 1+ε Co 4 B 4 represents the second compound reported in the Mg–Co–B system and the first superspace symmetry model of a Nd 1+ε Fe 4 B 4 -type incommensurate composite compound refined from powder diffraction data.

chemical structure↗
DOE OSTI2023

Tuning the band topology of GdSb by epitaxial strain

Rare-earth monopnictide (RE-V) semimetal crystals subjected to hydrostatic pressure have shown interesting trends in magnetoresistance, magnetic ordering, and superconductivity, with theory predicting pressure-induced band inversion. Yet, thus far, there have been no direct experimental reports of interchanged band order in RE-Vs due to strain. This work studies the evolution of band topology in biaxially strained GdSb(001) epitaxial films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). As biaxial strain is tuned from tensile to compressive strain, the gap between the hole and the electron bands dispersed along [001] decreases. The conduction and valence band shifts seen in DFT and ARPES measurements are explained by a tight-binding model that accounts for the orbital symmetry of each band. Finally, we discuss the effect of biaxial strain on carrier compensation and magnetic ordering temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Spin-Charge-Lattice Coupling across the Charge Density Wave Transition in a Kagome Lattice Antiferromagnet

Understanding spin and lattice excitations in a metallic magnetic ordered system forms the basis to unveil the magnetic and lattice exchange couplings and their interactions with itinerant electrons. Kagome lattice antiferromagnet FeGe is interesting because it displays a rare charge density wave (CDW) deep inside the antiferromagnetic ordered phase that interacts with the magnetic order. Here, we use neutron scattering to study the evolution of spin and lattice excitations across the CDW transition 𝑇 CDW in FeGe. While spin excitations below ∼100 meV can be well described by spin waves of a spin-1 Heisenberg Hamiltonian, spin excitations at higher energies are centered around the Brillouin zone boundary and extend up to ∼180 meV consistent with quasiparticle excitations across spin-polarized electron-hole Fermi surfaces. Furthermore, 𝑐-axis spin wave dispersion and Fe-Ge optical phonon modes show a clear hardening below 𝑇 CDW due to spin-charge-lattice coupling but with no evidence of a phonon Kohn anomaly. By comparing our experimental results with density functional theory calculations in absolute units, we conclude that FeGe is a Hund’s metal in the intermediate correlated regime where magnetism has contributions from both itinerant and localized electrons arising from spin polarized electronic bands near the Fermi level.

charge density waves↗
DOE OSTI2020

First-principles study of magnetism and electric field effects in 2D systems

This review article provides a bird's-eye view of what first-principles based methods can contribute to next-generation device design and simulation. After a brief overview of methods and capabilities in the area, the authors focus on published work by their group since 2015 and current work on CrI3. The authors introduce both single- and dual-gate models in the framework of density functional theory and the constrained random phase approximation in estimating the Hubbard U for 2D systems vs their 3D counterparts. A wide range of systems, including graphene-based heterogeneous systems, transition metal dichalcogenides, and topological insulators, and a rich array of physical phenomena, including the macroscopic origin of polarization, field effects on magnetic order, interface state resonance induced peak in transmission coefficients, spin filtration, etc., are covered. For CrI3, the authors present their new results on bilayer systems such as the interplay between stacking and magnetic order, pressure dependence, and electric field induced magnetic phase transitions. The authors find that a bare bilayer CrI3, graphene|bilayer CrI3|graphene, hexagonal boron nitride (h-BN)|bilayer CrI3|h-BN, and h-BN|bilayer CrI3|graphene all have a different response at high field, while at small field, the difference is small except for graphene|bilayer CrI3|graphene. The authors conclude with discussion of some ongoing work and work planned in the near future, with the inclusion of further method development and applications.

Cheng, Hai-Ping (ORCID:0000000159901725)↗
DOE OSTI2024

Structural phase transition, s ± -wave pairing, and magnetic stripe order in bilayered superconductor La 3 Ni 2 O 7 under pressure

Motivated by the recently discovered high-T c superconductor La 3 Ni 2 O 7 , we comprehensively study this system using density functional theory and random phase approximation calculations. At low pressures, the Amam phase is stable, containing the Y 2- mode distortion from the Fmmm phase, while the Fmmm phase is unstable. Because of small differences in enthalpy and a considerable Y 2- mode amplitude, the two phases may coexist in the range between 10.6 and 14 GPa, beyond which the Fmmm phase dominates. In addition, the magnetic stripe-type spin order with wavevector (π, 0) was stable at the intermediate region. Pairing is induced in the s ± -wave channel due to partial nesting between the M = (π, π) centered pockets and portions of the Fermi surface centered at the X = (π, 0) and Y = (0, π) points. This resembles results for iron-based superconductors but has a fundamental difference with iron pnictides and selenides. Moreover, our present efforts also suggest La 3 Ni 2 O 7 is qualitatively different from infinite-layer nickelates and cuprate superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Inverse Lieb materials: altermagnetism and more

The Lieb lattice, originally proposed for cuprate superconductors, has gained new attention in the emerging field of altermagnetism as a minimal analytical model for the latter. While the inverse Lieb lattice (ILL) was once considered purely theoretical, several materials with this motif have recently been discovered. Its unique geometry supports complex magnetic orders driven by geometric frustration, offering high tunability. In this work, we provide comprehensive insights into ILL magnetic phases and establish guidelines for identifying altermagnets. Using a Heisenberg model, we first construct phase diagrams to elucidate the mechanisms underlying experimental magnetic phases. We then bridge theory and experiment via density functional theory (DFT) calculations on existing ILL compounds, finding results consistent with experimental data. Notably, we identified a trend linking d-shell filling to magnetic order, where d x (x≤5) configurations show a propensity for altermagnetism. We also highlight Sr 2 CrO 2 Cr 2 OAs 2 as a promising metallic altermagnet with highly anisotropic J 2 exchange and a high Néel temperature (~600 K). Finally, our magnon spectra calculations confirm that chiral splittings correlate directly with anisotropies between inequivalent J 2 interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2024

First-principles study of the magneto-Raman effect in van der Waals layered magnets

Magneto-Raman spectroscopy has been used to study spin-phonon coupling in two-dimensional (2D) magnets. Raman spectra of CrI 3 show a strong dependence on the magnetic order within a layer and between the layers. Here we carry out the first systematic theoretical investigation of the magneto-Raman effect in 2D magnets by performing density functional theory calculations and developing a generalized polarizability model. Our first-principles simulations well reproduce experimental Raman spectra of CrI 3 with different magnetic states. The model reveals how the change of spin orientation in each layer is coupled to the layer’s vibration to induce or eliminate the spin-dependent anti-symmetric off-diagonal terms in the Raman tensor for altering the selection rules. We also uncover that the correlation between phonon modes and magnetic orders is a universal phenomenon, which should exist in other phonon modes and 2D magnets. Our predictive simulations and modeling are expected to guide the research in 2D magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Role of magnetic and structural symmetry breaking in forming the Mott insulating gap in Nb 3 ⁢Cl 8

The α-phase of the gapped insulator Nb 3 Cl 8 has recently emerged as the long-sought critical testing bed for examining the importance of strong interelectronic correlation vs symmetry breaking in understanding insulation of such Mott compounds. Structural symmetry breaking detected by density functional theory (DFT) energy lowering (such as dimer formation, disproportionation, or Jahn-Teller distortions) explains insulation in both d-electron Mott-like systems and in non-d-electron cases without recourse to strong correlation. Yet, in Nb 3 Cl 8 , structural symmetry breaking alone (viz. formation of Nb trimers) fails to explain insulation, leading instead to a partially occupied metallic flat band, in contrast with experimental observations. We examine the role of magnetic symmetry breaking, noting that Nb 3 Cl 8 is an observed paramagnet (not an antiferromagnet), thus potentially carrying also short-range ordered magnetic moments. Describing the latter as a polymorphous distribution of nonzero local moments with total zero net magnetization is demonstrated to lower the DFT total energy, while gapping the system without recourse to strong correlation or long-range magnetic order. This suggests that degeneracy removal by symmetry breaking in mean-field-like approaches—either structural, or magnetic, or both—can reduce or eliminate the need for strong correlation, allowing the use of DFT for such Mott systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Understanding and Tuning Magnetism in Layered Ising‐Type Antiferromagnet FePSe 3 for Potential 2D Magnet

Abstract Recent developments in 2D magnetic materials have motivated the search for new van der Waals magnetic materials, especially Ising‐type magnets with strong magnetic anisotropy. Fe‐based M P X 3 ( M = transition metal, X = chalcogen) compounds such as FePS 3 and FePSe 3 both exhibit an Ising‐type magnetic order, but FePSe 3 receives much less attention compared to FePS 3 . This work focuses on establishing the strategy to engineer magnetic anisotropy and exchange interactions in this less‐explored compound. Through chalcogen and metal substitutions, the magnetic anisotropy is found to be immune against S substitution for Se whereas tunable only with heavy Mn substitution for Fe. In particular, Mn substitution leads to a continuous rotation of magnetic moments from the out‐of‐plane direction toward the in‐plane. Furthermore, the magnetic ordering temperature displays non‐monotonic doping dependence for both chalcogen and metal substitutions but due to different mechanisms. These findings provide deeper insight into the Ising‐type magnetism in this important van der Waals material, shedding light on the study of other Ising‐type magnetic systems as well as discovering novel 2D magnets for potential applications in spintronics.

2D magnet↗
DOE OSTI2020

Ultrafast Spin Crossover in a Room-Temperature Ferrimagnet: Element-Specific Spin Dynamics in Photoexcited Cobalt Ferrite

Transition metal complexes capable of photoinduced spin crossover have been greatly investigated because of their potential to enable ultrafast optical control of information processing. Yet, any real application of photoswitchable molecules requires that spin crossover be paired with additional functionality such as long-range magnetic order. Important advances combining these functions are notably reported for a number of bimetallic Prussian Blue analogues; however, to date, PBA-based magnetic photoswitches can only operate below 150 K due to loss of magnetic order. In contrast, cobalt ferrite is a ferrimagnetic semiconductor with a Curie temperature of 790 K and extremely favorable magnetic properties by comparison to state-of-the-art PBAs. The mixed valence electronic structure of cobalt ferrite is reminiscent of cobalt–iron PBA, which is a well-known photoswitch. To investigate the potential for photoswitching in this material, we employ transient XUV spectroscopy to probe charge and spin dynamics with element-specific resolution on the femtosecond time scale. Results show that 400 nm light excites a metal-to-metal charge transfer transition, which drives the crossover of high-spin Co 2+ to low-spin Co 3+ with a time constant of 405 ± 29 fs and an internal quantum efficiency of unity. This result establishes the existence of efficient photoswitching in a new class of robust ferrimagnetic spinel ferrites.

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

Resistivity and magnetoresistance properties of R 2 NiSi 3 (R = Gd, Dy, Ho, Er, Tm) compounds

The resistivity and magnetoresistance behaviour of the hexagonal intermetallic compounds R 2 NiSi 3 (R = Gd, Dy, Ho, Er, and Tm) are reported here. All the studied polycrystalline compounds exhibit metallic behavior along with additional magnetic anomalies at low temperatures. A well-defined resistivity minima is observed in Gd 2 NiSi 3 and Dy 2 Ni 0.87 Si 2.95 at a temperature much higher than their respective magnetic transition temperatures. The anomaly has been ascribed to the charge carrier localization caused by magnetic precursor effect. Magnetic field induced crossover from positive to negative magnetoresistance (MR) behavior associated with antiferromagnetic ground state is evidenced for Gd 2 NiSi 3 and Er 2 NiSi 3 in the low temperature region. Although Tm 2 Ni 0.93 Si 2.93 does not exhibit any long range magnetic order down to 2 K, a sudden drop in resistivity behavior is observed below ~10 K. Furthermore, presence of short range magnetic correlation observed in a wide temperature range, much beyond their respective magnetic ordering temperatures, has been argued to be responsible for achieving finite negative MR for all the compounds. Additionally, a subtle resemblance between the observed transport anomalies and the magnetic properties of these systems has been discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Cation Adaptive Structures Based on Manganese Cyanide Prussian Blue Analogues: Application of Powder Diffraction Data to Solve Complex, Unprecedented Stoichiometries and New Structure Types

Abstract A Mn(II) salt and A + CN − under anaerobic conditions react to form 2‐D and 3‐D extended structured compounds of A m Mn II n (CN) m+2n stoichiometry. Here, the creation and characterization of this large family of compounds, for example AMn II 3 (CN) 7 , A 2 Mn II 3 (CN) 8 , A 2 Mn II 5 (CN) 12 , A 3 Mn II 5 (CN) 13 , and A 2 Mn II [Mn II (CN) 6 ], where A represents alkali and tetraalkylammonium cations, is reviewed. Cs 2 Mn II [Mn II (CN) 6 ] has the typical Prussian blue face centered cubic unit cell. However, the other alkali salts are monoclinic or rhombohedral. This is in accord with smaller alkali cation radii creating void space that is minimized by increasing the van der Waals stabilization energy by reducing ∠Mn−N≡C, which, strengthens the magnetic coupling and increases the magnetic ordering temperatures. This is attributed to the non‐rigidity of the framework structure due the significant ionic character associated with the high‐spin Mn II sites. For larger tetraalkylammonium cations, the high‐spin Mn sites lack sufficient electrostatic A + ⋅⋅⋅NC stabilization and form unexpected 4‐ and 5‐coordinated Mn sites within a flexible, extended framework around the cation; hence, the size, shape, and charge of the cation dictate the unprecedented stoichio‐metry and unpredictable cation adaptive structures. Antiferromagnetic coupling between adjacent Mn II sites leads to ferrimagnetic ordering, but in some cases antiferromagnetic coupling of ferrimagnetic layers are compensated and synthetic antiferromagnets are observed. The magnetic ordering temperatures for ferrimagnetic A 2 Mn II [Mn II (CN) 6 ] with both octahedral high‐ and low‐spin Mn II sites increase with decreasing ∠Mn−N≡C. The crystal structures for all of the extended structured materials were obtained by powder diffraction.

Chemistry↗