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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Influence of the vertex region on spin dynamics in artificial Kagome spin ice.

We present experimental and theoretical studies of spin-wave mode dynamics in artificial kagome spin ice vertices made of three identical 15-nm thick elongated Ni80Fe20 nanoislands (macrospins). We consider several possible configurations, from completely disjointed macrospins (full dipolar interelement interac-tions) to fully jointed macrospins (full dipolar-exchange interactions). Using angular-resolved magnetic field dependent broadband ferromagnetic resonance (FMR), we demonstrate the occurrence of a mode localized in the vertex region as indicated by the distinct behavior of the FMR spectra at different angles and configurations. Theoretical calculations using micromagnetic simulations support the existence, ori-gin, and behavior of this mode by interpreting it as a localized, quasi-uniform Kittel mode. Our findings pave the way for designing the most appropriate network consisting of ferromagnetic nanomagnets for specific application purposes in magnonics.

Bang, Wonbae↗
DOE OSTI2024

Detecting fractionalization in critical spin liquids using color centers

Quantum spin liquids are highly entangled ground states of insulating spin systems, in which magnetic ordering is prevented down to the lowest temperatures due to quantum fluctuations. One of the most extraordinary characteristics of quantum spin liquid phases is their ability to support fractionalized, low-energy quasiparticles known as spinons, which carry spin-1/2 but bear no charge. Relaxometry based on color centers in crystalline materials—of which nitrogen-vacancy (NV) centers in diamond are a well-explored example—provides an exciting new platform to probe the spin spectral functions of magnetic materials with both energy and momentum resolution and to search for signatures of these elusive, fractionalized excitations. In this work, we theoretically investigate the color-center relaxometry of two archetypal quantum spin liquids: the two-dimensional U(1) quantum spin liquid with a spinon Fermi surface and the spin-1/2 antiferromagnetic spin chain. The former is characterized by a metallic, spin-split ground state of mobile, interacting spinons, which closely resembles a spin-polarized Fermi liquid ground state but with neutral quasiparticles. We show that the observation of the Stoner continuum and the collective spin wave mode in the spin spectral function would provide a strong evidence for the existence of spinons and fractionalization. In one dimension, mobile spinons form a Luttinger liquid ground state. We show that the spin spectral function exhibits strong features representing the collective density and spin-wave modes, which are broadened in an algebraic fashion with an exponent characterized by the Luttinger parameter. The possibilities of measuring these collective modes and detecting the power-law decay of the spectral weight using NV relaxometry are discussed. We also examine how the transition rates are modified by marginally irrelevant operators in the Heisenberg limit. Published by the American Physical Society 2024

Takei, So (ORCID:0000000191771895)↗
DOE OSTI2022

Spin excitations in the kagome-lattice metallic antiferromagnet Fe 0.89 Co 0.11 Sn

Kagome-lattice materials have attracted tremendous interest due to the broad prospect for seeking superconductivity, quantum spin liquid states, and topological electronic structures. Among them, the transition-metal kagome lattices are high-profile objects for the combination of topological properties, rich magnetism, and multiple-orbital physics. Here we report an inelastic neutron scattering study on the spin dynamics of a kagome-lattice antiferromagnetic metal Fe 0.89 Co 0.11 Sn. Although the magnetic excitations can be observed up to ~250 meV, well-defined spin waves are only identified below ~90 meV and can be modeled using Heisenberg exchange with ferromagnetic in-plane nearest-neighbor coupling J 1 , in-plane next-nearest-neighbor coupling J 2 , and antiferromagnetic (AFM) interlayer coupling J c under linear spin-wave theory. Above ~90 meV, the spin waves enter the itinerant Stoner continuum and become highly damped particle-hole excitations. At the K point of the Brillouin zone, we reveal a possible band crossing of the spin wave, which indicates a potential Dirac magnon. Finally, our results uncover the evolution of the spin excitations from the planar AFM state to the axial AFM state in Fe 0.89 Co 0.11 Sn, solve the magnetic Hamiltonian for both states, and confirm the significant influence of the itinerant magnetism on the spin excitations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Reconfigurable spin current transmission and magnon–magnon coupling in hybrid ferrimagnetic insulators

Coherent spin waves possess immense potential in wave-based information computation, storage, and transmission with high fidelity and ultra-low energy consumption. However, despite their seminal importance for magnonic devices, there is a paucity of both structural prototypes and theoretical frameworks that regulate the spin current transmission and magnon hybridization mediated by coherent spin waves. Here, we demonstrate reconfigurable coherent spin current transmission, as well as magnon–magnon coupling, in a hybrid ferrimagnetic heterostructure comprising epitaxial Gd 3 Fe 5 O 12 and Y 3 Fe 5 O 12 insulators. By adjusting the compensated moment in Gd 3 Fe 5 O 12 , magnon–magnon coupling was achieved and engineered with pronounced anticrossings between two Kittel modes, accompanied by divergent dissipative coupling approaching the magnetic compensation temperature of Gd 3 Fe 5 O 12 (T M,GdIG ), which were modeled by coherent spin pumping. Remarkably, we further identified, both experimentally and theoretically, a drastic variation in the coherent spin wave-mediated spin current across T M,GdIG , which manifested as a strong dependence on the relative alignment of magnetic moments. Our findings provide significant fundamental insight into the reconfiguration of coherent spin waves and offer a new route towards constructing artificial magnonic architectures.

42 ENGINEERING↗
DOE OSTI2020

Magnetic droplet solitons

Magnetic droplet solitons are dynamical magnetic textures that form due to an attractive interaction between spin waves in thin films with perpendicular magnetic anisotropy. Spin currents and the spin torques associated with these currents enable their formation as they provide a means to excite non-equilibrium spin-wave populations and compensate their decay. Recent years have seen rapid advances in experiments that realize and study magnetic droplets. Important advances include the first direct x-ray images of droplets, determination of their threshold and sustaining currents, measurement of their generation and annihilation time, and evidence for drift instabilities, which can limit their lifetime. This perspective discusses these studies and contrasts these solitons to other types of spin-current excitations, such as spin-wave bullets, and static magnetic textures, including magnetic vortices and skyrmions. Magnetic droplet solitons can also serve as current controlled microwave frequency oscillators with potential applications in neuromorphic chips as nonlinear oscillators with memory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Spin-density-wave order controlled by uniaxial stress in CeAuSb 2

We report the tetragonal heavy-fermion compound CeAuSb 2 (space group P4/nmm ) exhibits incommensurate spin-density wave (SDW) order below T N ≈ 6.5 K with the propagation vector q A = (δ A , δ A , 1/2 ). The application of uniaxial stress along the [010] direction induces a sudden change in the resistivity ratio ρ a /ρ b at a compressive strain of ε ≈ - 0.5 %. Here we use neutron scattering to show that the uniaxial stress induces a first-order transition to a SDW state with a different propagation vector (0, δ B ,1/2) with δ B = 0.25.The magnetic structure of the new (B) phase consists of Ce layers with ordered moments alternating with layers with zero moment stacked along the c axis. The ordered layers have an up-up-down-down configuration along the b axis. This is an unusual situation in which the loss of spatial inversion in a metallic system is driven by the magnetic order. We argue that the change in SDW wave vector leads to Fermi-surface reconstruction and a concomitant change in the transport properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Spin dynamics in the skyrmion-host lacunar spinel GaV 4 S 8

Here in the lacunar spinel GaV 4 S 8 , the interplay of spin, charge, and orbital degrees of freedom produces a rich phase diagram that includes an unusual Néel-type skyrmion phase composed of molecular spins. To provide insight into the interactions underlying this complex phase diagram, we study the spin excitations in GaV 4 S 8 through inelastic neutron scattering measurements on polycrystalline and single crystal samples. Using linear spin wave theory, we describe the spin wave excitations using a model where V 4 clusters decorate an fcc lattice. The effective cluster model includes a ferromagnetic interaction and a weaker antisymmetric Dzyaloshinskii-Moriya interaction between the neighboring molecular spins. Our work clarifies the spin interactions in GaV 4 S 8 and supports the picture of interacting molecular clusters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Incommensurate spin density wave and magnetocaloric effect in the metallic triangular lattice HoAl 2 Ge 2

Here, we report the magnetic structure and the magnetocaloric effect (MCE) of the ternary compound HoAl 2 Ge 2 with a trigonal CaAl 2 Si 2 -type crystal structure. A neutron powder diffraction experiment reveals that HoAl 2 Ge 2 exhibits an incommensurate spin density wave (SDW) with a propagation vector k=(0.23,0,0.06). The special arrangement of magnetic moments in HoAl 2 Ge 2 induces interesting physical phenomena and large magnetocaloric effects. The rise in resistivity at low temperatures indicates the effect of the SDW state in the electronic transport. The maximum magnetic-entropy change is –16.1J/kg K under a magnetic field change of 0–70 kOe for an isotropic HoAl 2 Ge 2 powder and it increases to –17.9J/kg K for a single crystal when the magnetic field (H) is applied parallel to the ab plane. A large rotating magnetic-entropy change of –5.1J/kg K for H=20 kOe in a HoAl 2 Ge 2 single crystal is obtained, which is closely associated to the magnetic anisotropy of the SDW order and its response to the external magnetic field. We discuss the large MCE in terms of the field-induced metamagnetic transition from the incommensurate SDW order to the ferromagnetic order. Our study establishes the triangular lattice R Al 2 Ge 2 (R=rare-earth elements) as a unique family of compounds to explore the existence of the incommensurate spin density waves and the correlated physical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Spin wavepackets in the Kagome ferromagnet Fe 3 Sn 2 : Propagation and precursors

The propagation of spin waves in magnetically ordered systems has emerged as a potential means to shuttle quantum information over large distances. Conventionally, the arrival time of a spin wavepacket at a distance, d , is assumed to be determined by its group velocity, v g . Here, we report time-resolved optical measurements of wavepacket propagation in the Kagome ferromagnet Fe 3 Sn 2 that demonstrate the arrival of spin information at times significantly less than d / v g . We show that this spin wave “precursor” originates from the interaction of light with the unusual spectrum of magnetostatic modes in Fe 3 Sn 2 . Related effects may have far-reaching consequences toward realizing long-range, ultrafast spin wave transport in both ferromagnetic and antiferromagnetic systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Magnetic field effects in an octupolar quantum spin liquid candidate

Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to an external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here, in this study, we use neutron scattering to show that the application of a large external magnetic field to Ce 2 Zr 2 O 7 , an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wave vectors in Ce 2 Sn 2 O 7 pyrochlore, and its absence in Ce 2 Zr 2 O 7 . Therefore, our results identify Ce 2 Zr 2 O 7 as a strong candidate for an octupolar U(1) QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Assessing spin-density wave formation in La 3 Ni 2 O 7 from electronic structure calculations

Here, we employ correlated density-functional theory methods (DFT + Hubbard U) to investigate the spin-density wave state of the bilayer Ruddlesden-Popper (RP) nickelate La 3 Ni 2 O 7 which becomes superconducting under pressure. We predict that the ground state of this bilayer RP material has traits of both the double spin-stripe and the single spin-charge stripe phases proposed in the literature as it corresponds to in-plane up/up′/down/down′ diagonal stripes with up/down being high spin (formally Ni 2+ : d 8 ), and up′ /down′ being low spin (formally Ni 3+ : d 7 ). The main feature of this solution (that is insulating even at U = 0) is the dominant role of the d x 2 −y 2 bands around the Fermi level, which would become doped with the introduction of electrons via oxygen vacancies. In spite of the similarity with cuprates in terms of the dominant role of d x 2 −y 2 bands, some differences are apparent in the magnetic ground state of La 3 Ni 2 O 7 : the antiferromagnetic out-of-plane coupling within the bilayer (linked to the d z 2 orbitals forming a spin-singlet-like configuration) is found to be the dominant one while in-plane interactions are reduced due to the stripe order of the ground state. With pressure, this striped magnetic ground state remains similar in nature but the increase in bandwidth quickly transitions La 3 Ni 2 O 7 into a metallic state with all the activity close to the Fermi level involving, to a large extent, d x 2 −y 2 orbitals. This is reminiscent of the cuprates and may provide key insights into how superconductivity arises in this material under pressure.

LaBollita, Harrison↗
DOE OSTI2024

Intertwined charge and spin density waves in a topological kagome material

Using neutrons and x rays we show the topological kagome antiferromagnet Mn 3 Sn for T < 285 K forms a homogeneous spin and charge ordered state comprising a longitudinally polarized spin density wave with wave vector k β = k β c ̂ , a helical modulated version of the room temperature antichiral magnetic order with k χ = k χ c ̂ , and charge density waves with wave vectors 2 k β , 2 k χ , and k β + k χ . Though k χ and k β coincide for 200 K < T < 230 K , they exhibit distinct continuous T dependencies before locking to commensurate values of k β = 1 12 c * and k χ = 5 48 c * at low T . Density functional theory indicates this complex modulated state may be associated with the nesting of Fermi surfaces from correlated flat kagome bands, which host Weyl nodes that are annihilated as it forms. Published by the American Physical Society 2024

Chen, Y. (ORCID:0000000177169179)↗
DOE OSTI2021

Neutron scattering study of magnetic anisotropy in the tetragonal antiferromagnet Bi 2 CuO 4

In this work, we present a comprehensive study of magnon excitations in the tetragonal easy-plane antiferromagnet Bi 2 CuO 4 using inelastic neutron scattering and spin-wave analyses. The nature of low-energy magnons, and hence the anisotropy in this material, has been controversial. We show unambiguously that the low-energy magnon spectrum consists of a gapped and a gapless mode, which we attribute to out-of-plane and in-plane spin fluctuations, respectively. We modeled the observed magnon spectrum using linear spin-wave analysis of a minimal anisotropic spin model motivated by the lattice symmetry. By studying the magnetic field dependence of the (1,0,0) Bragg peak intensity and the in-plane magnon intensity, we observed a spin-flop transition in the ab plane at ~0.4 T, which directly indicates the existence of a small in-plane anisotropy that is classically forbidden. It is only by taking into account magnon zero-point fluctuations beyond the linear spin-wave approximation that we could explain this in-plane anisotropy and its magnitude, the latter of which is deduced from the critical field of the spin-flop transition. The microscopic origins of the observed anisotropic interactions are also discussed. We found that our data are inconsistent with a large Dzyaloshinskii-Moriya interaction, which suggests a potential departure of Bi 2 CuO 4 from the conventional theories of magnetic anisotropy for other cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Block orbital-selective Mott insulators: A spin excitation analysis

We present a comprehensive study of the spin excitations—as measured by the dynamical spin structure factor S(q,ω)—of the so-called block-magnetic state of low-dimensional orbital-selective Mott insulators. We realize this state via both a multi-orbital Hubbard model and a generalized Kondo-Heisenberg Hamiltonian. Due to various competing energy scales present in the models, the system develops periodic ferromagnetic islands of various shapes and sizes, which are antiferromagnetically coupled. The 2×2 particular case was already found experimentally in the ladder material BaFe2Se3 that becomes superconducting under pressure. Here we discuss the electronic density as well as Hubbard and Hund coupling dependence of S(q,ω) using density matrix renormalization group method. Several interesting features were identified: (1) An acoustic (dispersive spin-wave) mode develops. (2) The spin-wave bandwidth establishes a new energy scale that is strongly dependent on the size of the magnetic island and becomes abnormally small for large clusters. (3) Optical (dispersionless spin excitation) modes are present for all block states studied here. In addition, a variety of phenomenological spin Hamiltonians have been investigated but none matches entirely our results that were obtained primarily at intermediate Hubbard U strengths. Our comprehensive analysis provides theoretical guidance and motivation to crystal growers to search for appropriate candidate materials to realize the block states, and to neutron scattering experimentalists to confirm the exotic dynamical magnetic properties unveiled here, with a rich mixture of acoustic and optical features.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Effects of exchange distortion and spin rotation in the magnetic Kagome Lattice

In this study, we examine the effect of distorted triangular magnetic interactions in the kagome lattice. Using a Holstein-Primakoff expansion, we determine the analytical solutions for classical energies and the spin-wave modes for various magnetic configurations. By understanding the magnetic phase diagram, we characterize the changes in the spin waves and examine the spin distortions of the ferromagnetic, antiferrimagnetic, and 120° phases that are produced by variable exchange interactions and lead to various noncollinear phases, which provides a deeper understanding of the magnetic fingerprints of these configurations for experimental characterization and identification.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Designed Spin‐Texture‐Lattice to Control Anisotropic Magnon Transport in Antiferromagnets

Abstract Spin waves in magnetic materials are promising information carriers for future computing technologies due to their ultra‐low energy dissipation and long coherence length. Antiferromagnets are strong candidate materials due, in part, to their stability to external fields and larger group velocities. Multiferroic antiferromagnets, such as BiFeO 3 (BFO), have an additional degree of freedom stemming from magnetoelectric coupling, allowing for control of the magnetic structure, and thus spin waves, with the electric field. Unfortunately, spin‐wave propagation in BFO is not well understood due to the complexity of the magnetic structure. In this work, long‐range spin transport is explored within an epitaxially engineered, electrically tunable, 1D magnonic crystal. A striking anisotropy is discovered in the spin transport parallel and perpendicular to the 1D crystal axis. Multiscale theory and simulation suggest that this preferential magnon conduction emerges from a combination of a population imbalance in its dispersion, as well as anisotropic structural scattering. This work provides a pathway to electrically reconfigurable magnonic crystals in antiferromagnets.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Generic character of charge and spin density waves in superconducting cuprates

Charge density waves (CDWs) have been observed in nearly all families of copper-oxide superconductors. But the behavior of these phases across different families has been perplexing. In La-based cuprates, the CDW wavevector is an increasing function of doping, exhibiting the so-called Yamada behavior, while in Y- and Bi-based materials the behavior is the opposite. Here, we report a combined resonant soft X-ray scattering (RSXS) and neutron scattering study of charge and spin density waves in isotopically enriched La 1.8–x Eu 0.2 Sr x CuO 4 over a range of doping 0.07 ≤ x ≤ 0.20. In this work, we find that the CDW amplitude is temperature independent and develops well above experimentally accessible temperatures. Further, the CDW wavevector shows a nonmonotonic temperature dependence, exhibiting Yamada behavior at low temperature with a sudden change occurring near the spin ordering temperature. We describe these observations using a Landau–Ginzburg theory for an incommensurate CDW in a metallic system with a finite charge compressibility and spin-CDW coupling. Extrapolating to high temperature, where the CDW amplitude is small and spin order is absent, our analysis predicts a decreasing wavevector with doping, similar to Y and Bi cuprates. Our study suggests that CDW order in all families of cuprates forms by a common mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnons and magnetic fluctuations in atomically thin MnBi 2 Te 4

Electron band topology is combined with intrinsic magnetic orders in MnBi 2 Te 4 , leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin fluctuations in atomically thin MnBi 2 Te 4 flakes using Raman spectroscopy. In a two-septuple layer with non-trivial topology, magnon characteristics evolve as an external magnetic field tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. We also find magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers.

36 MATERIALS SCIENCE↗