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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Vacancy-induced suppression of charge density wave order and its impact on magnetic order in kagome antiferromagnet FeGe

Two-dimensional (2D) kagome lattice metals are interesting because their corner sharing triangle structure enables a wide array of electronic and magnetic phenomena. Recently, post-growth annealing is shown to both suppress charge density wave (CDW) order and establish long-range CDW with the ability to cycle between states repeatedly in the kagome antiferromagnet FeGe. Here we perform transport, neutron scattering, scanning transmission electron microscopy (STEM), and muon spin rotation (μSR) experiments to unveil the microscopic mechanism of the annealing process and its impact on magneto-transport, CDW, and magnetism in FeGe. Annealing at 560 °C creates uniformly distributed Ge vacancies, preventing the formation of Ge-Ge dimers and thus CDW, while 320 °C annealing concentrates vacancies into stoichiometric FeGe regions with long-range CDW. The presence of CDW order greatly affects the anomalous Hall effect, incommensurate magnetic order, and spin-lattice coupling in FeGe, placing FeGe as the only kagome lattice material with tunable CDW and magnetic order.

critical phenomena↗
DOE OSTI2025

Structural properties and recrystallization effects in ion beam modified B20-type FeGe films

Disordered iron germanium (FeGe) has recently garnered interest as a testbed for a variety of magnetic phenomena as well as for use in magnetic memory and logic applications. This is partially owing to its ability to host skyrmions and antiskyrmions—nanoscale whirlpools of magnetic moments that could serve as information carriers in spintronic devices. In particular, a tunable skyrmion–antiskyrmion system may be created through precise control of the defect landscape in B20-phase FeGe, motivating the development of methods to systematically tune disorder in this material and understand the ensuing structural properties. To this end, we investigate a route for modifying magnetic properties in FeGe. In particular, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au 4+ ions, which creates a dispersion of amorphized regions that may preferentially host antiskyrmions at densities controlled by the irradiation fluence. To further tune the disorder landscape, we conduct a systematic electron diffraction study with in situ annealing, demonstrating the ability to recrystallize controllable fractions of the material at temperatures ranging from ~150 to 250°C. Finally, we describe the crystallization kinetics using the Johnson–Mehl–Avrami–Kolmogorov model, finding that the growth of crystalline grains is consistent with diffusion-controlled one-to-two dimensional growth with a decreasing nucleation rate.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Ultralow effective Gilbert damping and induced orbital moment in strain-engineered FeGe films with Curie temperature exceeding room temperature

We report the magnetic and magnetodynamic properties of strained epitaxial FeGe thin films on Ge(111) substrates, and confirm the generation of orbital moment in tensile-strained FeGe not seen in bulk or unstrained films. In-plane tensile strain resulted in an increase of the magnetic transition temperature to 350 K, likely the result of decreased Fe–Fe bond lengths that increase spin–orbit coupling strength. X-ray magnetic circular dichroism (XMCD) shows orbital-to-spin magnetic moment ratios of 0.18 and 0.14 for films of 18 nm and 72 nm thickness, respectively, indicative of partially quenched atomic orbitals. Finally, a very low effective Gilbert damping parameter, α eff = 0.003 ± 0.001 at room temperature was observed, suggesting that strained FeGe could be useful for spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Frustrated Magnetism in FeGe 3 O 4 with a Chiral Trillium Network

The discovery of new magnetic ground states in geometrically frustrated lattices remains a central challenge in materials science. Here, we report the synthesis, structural characterization, and frustrated magnetic properties of FeGe 3 O 4 , a newly identified compound that crystallizes in the noncentrosymmetric cubic space group P 2 1 3. In this structure, Fe atoms form an intricate double-trillium lattice with nearest-neighbor Fe−Fe distances of ∼4.2 Å, while Ge 2+ ions mediate magnetic interactions through Fe− Ge−Fe pathways. Field-dependent magnetization at 2 K shows a pronounced nonlinearity, reaching a maximum moment of 2.55(3) μ B /Fe 2+ at 70 kOe without evidence of saturation. Magnetic susceptibility, heat capacity, and neutron scattering collectively reveal the onset of short-range magnetic interactions near 5 K, with no longrange ordering detected down to 0.06 K. Specific heat measurements demonstrate strong frustration: only ∼34% of the expected magnetic entropy is recovered at 2.4 K. Taken together, these results establish FeGe 3 O 4 as a rare example of a geometrically frustrated trillium lattice magnet, offering a promising platform for exploring exotic quantum magnetic phenomena.

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

Competing itinerant and local spin interactions in kagome metal FeGe

The combination of a geometrically frustrated lattice, and similar energy scales between degrees of freedom endows two-dimensional Kagome metals with a rich array of quantum phases and renders them ideal for studying strong electron correlations and band topology. The Kagome metal, FeGe is a noted example of this, exhibiting A-type collinear antiferromagnetic (AFM) order at T N ≈ 400 K, then establishes a charge density wave (CDW) phase coupled with AFM ordered moment below T CDW ≈ 110 K, and finally forms a c-axis double cone AFM structure around T Canting ≈ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure of FeGe at temperatures well above T Canting and T CDW that merge into gapped commensurate spin waves from the A-type AFM order. Commensurate spin waves follow the Bose factor and fit the Heisenberg Hamiltonian, while the incommensurate spin excitations, emerging below T N where AFM order is commensurate, start to deviate from the Bose factor around T CDW , and peaks at T Canting . This is consistent with a critical scattering of a second order magnetic phase transition with decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Frustrated charge density wave and quasi-long-range bond-orientational order in the magnetic kagome FeGe

The intrinsic frustrated nature of a kagome lattice is amenable to the realization of exotic phases of matter, such as quantum spin liquids or spin ices, and the multiple-q charge density waves (CDW) in the kagome metals. Despite intense efforts to understand the mechanism driving the electronic modulations, its origin is still unknown and obscured by competing interactions and intertwined orders. Here, we identify a dimerization-driven 2D hexagonal charge-diffuse precursor in the antiferromagnetic kagome metal FeGe and demonstrate that the fraction of dimerized/undimerized states is the relevant order parameter of the multiple-q CDW of a continuous phase transition. The pretransitional charge fluctuations with propagation vector q = q M at T CDW < T < T*(125 K) are anisotropic, hence holding a quasi-long-range bond-orientational order. The broken translational symmetry emerges from the anisotropic diffuse precursor, akin to the Ising scenario of antiferromagnetic triangular lattices. The temperature and momentum dependence of the critical scattering show parallels to the stacked hexatic B-phases reported in liquid crystals and transient states of CDWs and highlight the key role of the topological defect-mediated melting of the CDW in FeGe.

Subires, D. [Donostia International Physics Center↗
DOE OSTI2023

Magnetism and charge density wave order in kagome FeGe

Electron correlations often lead to emergent orders in quantum materials, and one example is the kagome lattice materials where topological states exist in the presence of strong correlations between electrons. This arises from the features of the electronic band structure that are associated with the kagome lattice geometry: flat bands induced by destructive interference of the electronic wavefunctions, topological Dirac crossings, and a pair of van Hove singularities. Various correlated electronic phases have been discovered in kagome lattice materials, including magnetism, charge density waves, nematicity, and superconductivity. Recently, a charge-density wave was discovered in the magnetic kagome FeGe, providing a platform for understanding the interplay between charge order and magnetism in kagome materials. Here, we observe all three electronic signatures of the kagome lattice in FeGe using angle-resolved photoemission spectroscopy. The presence of van Hove singularities near the Fermi level is driven by the underlying magnetic exchange splitting. Furthermore, we show spectral evidence for the charge-density wave as gaps near the Fermi level. Furthermore, our observations point to the magnetic interaction-driven band modification resulting in the formation of the charge-density wave, and indicate an intertwined connection between the emergent magnetism and charge order in this moderately-correlated kagome metal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Inducing a tunable skyrmion-antiskyrmion system through ion beam modification of FeGe films

Abstract Skyrmions and antiskyrmions are nanoscale swirling textures of magnetic moments formed by chiral interactions between atomic spins in magnetic noncentrosymmetric materials and multilayer films with broken inversion symmetry. These quasiparticles are of interest for use as information carriers in next-generation, low-energy spintronic applications. To develop skyrmion-based memory and logic, we must understand skyrmion-defect interactions with two main goals—determining how skyrmions navigate intrinsic material defects and determining how to engineer disorder for optimal device operation. Here, we introduce a tunable means of creating a skyrmion-antiskyrmion system by engineering the disorder landscape in FeGe using ion irradiation. Specifically, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au 4+ ions at varying fluences, inducing amorphous regions within the crystalline matrix. Using low-temperature electrical transport and magnetization measurements, we observe a strong topological Hall effect with a double-peak feature that serves as a signature of skyrmions and antiskyrmions. These results are a step towards the development of information storage devices that use skyrmions and antiskyrmions as storage bits, and our system may serve as a testbed for theoretically predicted phenomena in skyrmion-antiskyrmion crystals.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Frustration-induced diffusive scattering anomaly and dimension change in FeGe 2

Magnetic frustration, arising from the competition of exchange interactions, has received great attention because of its relevance to exotic quantum phenomena in materials. In the current work, we report an unusual checkerboard-shaped scattering anomaly in FeGe 2 , far from the known incommensurate magnetic satellite peaks, by inelastic neutron scattering. More surprisingly, such phenomenon appears as spin dynamics at low temperature, but it becomes prominent above Néel transition as elastic scattering. Here, a model Hamiltonian that includes an intraplane next-nearest neighbor was proposed and such anomaly is attributed to the near-perfect magnetic frustration and the emergence of unexpected two-dimensional magnetic order in the quasi-one-dimensional FeGe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Symmetry Breaking and Ascending in the Magnetic Kagome Metal FeGe

Spontaneous symmetry breaking—the phenomenon in which an infinitesimal perturbation can cause the system to break the underlying symmetry—is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher-temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature T N ≈ 400 K and a charge density wave (CDW) transition at T CDW ≈ 110 K , followed by a spin-canting transition at around 60 K. In the paramagnetic state at 460 K, we confirm that the crystal structure is indeed a hexagonal kagome lattice. On cooling to around T N , the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10 − 4 and the associated splitting of the double-degenerate phonon mode of the pristine kagome lattice. Upon further cooling to T CDW , the kagome lattice shows a small negative thermal expansion, and the crystal structure gradually becomes more symmetric upon further cooling. A tendency of increasing the crystalline symmetry upon cooling is unusual; it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter and hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system, and it sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant. Published by the American Physical Society 2024

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Materials Data on Sc(FeGe)6 by Materials Project

Sc(FeGe)6 crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Sc is bonded to twelve equivalent Fe and eight Ge atoms to form distorted face-sharing ScFe12Ge8 hexagonal bipyramids. All Sc–Fe bond lengths are 3.24 Å. There are two shorter (2.76 Å) and six longer (2.93 Å) Sc–Ge bond lengths. Fe is bonded in a 12-coordinate geometry to two equivalent Sc, four equivalent Fe, and six Ge atoms. All Fe–Fe bond lengths are 2.53 Å. There are a spread of Fe–Ge bond distances ranging from 2.49–2.64 Å. There are three inequivalent Ge sites. In the first Ge site, Ge is bonded in a 6-coordinate geometry to six equivalent Fe atoms. In the second Ge site, Ge is bonded in a 12-coordinate geometry to three equivalent Sc and six equivalent Fe atoms. In the third Ge site, Ge is bonded in a 8-coordinate geometry to one Sc, six equivalent Fe, and one Ge atom. The Ge–Ge bond length is 2.56 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Nb(FeGe)6 by Materials Project

Nb(FeGe)6 crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Nb is bonded to eight Ge atoms to form distorted edge-sharing NbGe8 hexagonal bipyramids. There are two shorter (2.70 Å) and six longer (2.88 Å) Nb–Ge bond lengths. Fe is bonded in a 12-coordinate geometry to four equivalent Fe and six Ge atoms. All Fe–Fe bond lengths are 2.50 Å. There are four shorter (2.47 Å) and two longer (2.59 Å) Fe–Ge bond lengths. There are three inequivalent Ge sites. In the first Ge site, Ge is bonded in a 8-coordinate geometry to one Nb, six equivalent Fe, and one Ge atom. The Ge–Ge bond length is 2.61 Å. In the second Ge site, Ge is bonded in a 6-coordinate geometry to six equivalent Fe atoms. In the third Ge site, Ge is bonded in a 12-coordinate geometry to three equivalent Nb and six equivalent Fe atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Th(FeGe)2 by Materials Project

Th(FeGe)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Th is bonded in a 8-coordinate geometry to eight equivalent Fe and eight equivalent Ge atoms. All Th–Fe bond lengths are 3.21 Å. All Th–Ge bond lengths are 3.21 Å. Fe is bonded in a 4-coordinate geometry to four equivalent Th and four equivalent Ge atoms. All Fe–Ge bond lengths are 2.35 Å. Ge is bonded in a 9-coordinate geometry to four equivalent Th and four equivalent Fe atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on FeGe by Materials Project

FeGe crystallizes in the cubic P2_13 space group. The structure is three-dimensional. Fe is bonded in a 7-coordinate geometry to seven equivalent Ge atoms. There are a spread of Fe–Ge bond distances ranging from 2.38–2.62 Å. Ge is bonded in a 7-coordinate geometry to seven equivalent Fe atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on FeGe by Materials Project

FeGe crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Fe is bonded in a 10-coordinate geometry to four equivalent Fe and six Ge atoms. All Fe–Fe bond lengths are 2.49 Å. There are four shorter (2.48 Å) and two longer (2.49 Å) Fe–Ge bond lengths. There are two inequivalent Ge sites. In the first Ge site, Ge is bonded in a 6-coordinate geometry to six equivalent Fe atoms. In the second Ge site, Ge is bonded in a hexagonal planar geometry to six equivalent Fe atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on FeGe by Materials Project

FeGe crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are three inequivalent Fe sites. In the first Fe site, Fe is bonded in a 6-coordinate geometry to four Fe and six Ge atoms. There are two shorter (2.45 Å) and two longer (2.64 Å) Fe–Fe bond lengths. There are four shorter (2.47 Å) and two longer (2.49 Å) Fe–Ge bond lengths. In the second Fe site, Fe is bonded in a 6-coordinate geometry to four Fe and six Ge atoms. Both Fe–Fe bond lengths are 2.68 Å. There are two shorter (2.41 Å) and four longer (2.43 Å) Fe–Ge bond lengths. In the third Fe site, Fe is bonded in a 11-coordinate geometry to four Fe and seven Ge atoms. Both Fe–Fe bond lengths are 2.48 Å. There are a spread of Fe–Ge bond distances ranging from 2.44–2.79 Å. There are two inequivalent Ge sites. In the first Ge site, Ge is bonded in a 6-coordinate geometry to six Fe and two equivalent Ge atoms. Both Ge–Ge bond lengths are 2.73 Å. In the second Ge site, Ge is bonded in a 7-coordinate geometry to seven Fe atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Tm9(FeGe)10 by Materials Project

Tm9(FeGe)10 crystallizes in the orthorhombic Immm space group. The structure is three-dimensional. there are four inequivalent Tm sites. In the first Tm site, Tm is bonded in a 6-coordinate geometry to twelve Fe and two equivalent Ge atoms. There are four shorter (2.98 Å) and eight longer (3.19 Å) Tm–Fe bond lengths. Both Tm–Ge bond lengths are 2.91 Å. In the second Tm site, Tm is bonded in a 2-coordinate geometry to four equivalent Fe and six Ge atoms. All Tm–Fe bond lengths are 3.13 Å. There are a spread of Tm–Ge bond distances ranging from 2.87–3.21 Å. In the third Tm site, Tm is bonded in a 7-coordinate geometry to five Fe and six Ge atoms. There are one shorter (2.94 Å) and four longer (3.10 Å) Tm–Fe bond lengths. There are a spread of Tm–Ge bond distances ranging from 2.81–3.05 Å. In the fourth Tm site, Tm is bonded in a 7-coordinate geometry to four equivalent Fe and seven Ge atoms. There are two shorter (3.08 Å) and two longer (3.25 Å) Tm–Fe bond lengths. There are a spread of Tm–Ge bond distances ranging from 2.91–3.06 Å. There are two inequivalent Fe sites. In the first Fe site, Fe is bonded in a 12-coordinate geometry to three Tm, five Fe, and two equivalent Ge atoms. There are four shorter (2.46 Å) and one longer (2.60 Å) Fe–Fe bond lengths. Both Fe–Ge bond lengths are 2.62 Å. In the second Fe site, Fe is bonded in a 12-coordinate geometry to five Tm, three Fe, and four Ge atoms. There are one shorter (2.60 Å) and one longer (2.76 Å) Fe–Fe bond lengths. There are a spread of Fe–Ge bond distances ranging from 2.46–2.55 Å. There are four inequivalent Ge sites. In the first Ge site, Ge is bonded in a 10-coordinate geometry to eight Tm and two equivalent Ge atoms. There are one shorter (2.66 Å) and one longer (2.70 Å) Ge–Ge bond lengths. In the second Ge site, Ge is bonded in a 10-coordinate geometry to four Tm and six Fe atoms. In the third Ge site, Ge is bonded in a 9-coordinate geometry to five Tm and four equivalent Fe atoms. In the fourth Ge site, Ge is bonded in a 9-coordinate geometry to five Tm and four equivalent Fe atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Tm(FeGe)2 by Materials Project

Tm(FeGe)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Tm is bonded in a 8-coordinate geometry to eight equivalent Fe and eight equivalent Ge atoms. All Tm–Fe bond lengths are 3.22 Å. All Tm–Ge bond lengths are 3.07 Å. Fe is bonded to four equivalent Tm and four equivalent Ge atoms to form a mixture of distorted face, edge, and corner-sharing FeTm4Ge4 tetrahedra. All Fe–Ge bond lengths are 2.35 Å. Ge is bonded in a 9-coordinate geometry to four equivalent Tm, four equivalent Fe, and one Ge atom. The Ge–Ge bond length is 2.56 Å.

36 MATERIALS SCIENCE↗