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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Magnetic field induced a b -plane rotation of the Eu magnetic moments in trigonal EuMg 2 Bi 2 and EuMg 2 Sb 2 single crystals below their Néel temperatures

The thermodynamic and electronic-transport properties of trigonal EuMg 2 Bi 2 in ab-plane magnetic fields H x and the A-type antiferromagnetic structure have recently been reported. The Eu magnetic moments with spin S = 7/2 remain locked in the ab plane up to and above the ab-plane critical field $H^{c}_{x}$ = 27.5 kOe at which the Eu moments become parallel to H x . Here additional measurements at low fields are reported that reveal a new spin-reorientation transition at a field H c1 ≈ 465 Oe where the Eu moments remain in the ab plane but become perpendicular to H x . At higher fields, the moments cant toward the field resulting in M ∝ H x up to $H^{c}_{x}$. Similar results are reported from measurements of the magnetic properties of EuMg 2 Sb 2 single crystals, where H c1 ≈ 220 Oe is found. Theory is formulated that models the low-field magnetic behavior of both materials, and the associated anisotropies are calculated. Further, the ab-plane trigonal anisotropy in EuMg 2 Sb 2 is found to be significantly smaller than in EuMg 2 Bi 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Enhancement of charge-neutral fermionic excitations near the spin-flop transition in the magnetic Kondo material YbIr 3 Si 7

The new Kondo material YbIr 3 Si 7 , similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. We performed 29 Si-NMR on YbIr 3 Si 7 to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the c axis, a single NMR peak in the paramagnetic state splits into three peaks below TN. In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the c-axis magnetic field is interpreted as a spin-flop transition, at which the magnetic moments oriented along the c axis antiferromagnetic (AFM-I) phase are rotated to the ab plane with a ferromagnetic component along the c-axis (AFM-II phase). In the vicinity of the spin-flop magnetic field H M , the nuclear spin-lattice relaxation rate 1/T 1 was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of 1/T 1 versus the c-axis magnetic field suggests that the charge-neutral fermions in YbIr 3 Si 7 are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Spin-orbital order and excitons in magnetoresistive HoBi

The magnetism of the rocksalt fcc rare-earth monopnictide HoBi, a candidate topological material with extreme magnetoresistance, is investigated. From the Ho 3+ non-Kramers J=8 spin-orbital multiplet, the cubic crystal electric field yields six nearly degenerate low-energy levels. These constitute an anisotropic magnetic moment with a Jahn-Teller-like coupling to the lattice. In the cubic phase for T>T N =5.72(1)K, the paramagnetic neutron scattering is centered at k=($\frac{1}{2}$$\frac{1}{2}$$\frac{1}{2}$) and was fit to dominant antiferromagnetic interactions between Ho spins separated by {100} and ferromagnetic interactions between spins displaced by {$\frac{1}{2}$ $\frac{1}{2}$ 0}. For T < T N , a type-II AFM long-range order with k=($\frac{1}{2}$$\frac{1}{2}$$\frac{1}{2}$) develops along with a tetragonal lattice distortion. While neutron diffraction from a multidomain sample cannot unambiguously determine the spin orientation within a domain, the bulk magnetization, structural distortion, and our measurements of the magnetic excitations all show the easy axis coincides with the tetragonal axis. Finally, the weakly dispersive excitons for T < T N can be accounted for by a spin Hamiltonian that includes the crystal electric field and exchange interactions within the random phase approximation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Plaquette valence bond state in the spin-$\frac{1}{2}$ J 1 –J 2 XY model on the square lattice

Here, we studied the ground-state phase diagram of spin-1/2 J 1 –J 2 XY model on the square lattice with first-neighbor J 1 and second-neighbor J 2 antiferromagnetic interactions using both infinite density matrix renormalization group (iDMRG) and DMRG approaches. We show that a plaquette valence bond phase is realized in an intermediate region 0.50 ≤ J 2 /J 1 < 0.54 between a Néel magnetic ordered phase at J 2 /J 1 < 0.50 and a stripy magnetic ordered phase at J 2 /J 1 ≥ 0.54 . The plaquette valence bond phase is characterized by finite dimer orders in both the horizontal and vertical directions. Contrary to the spin-1/2 J 1 –J 2 Heisenberg model, we do not find numerical evidence for a quantum spin liquid phase in the J 1 –J 2 XY model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Moiré Kanamori-Hubbard model for transition metal dichalcogenide homobilayers

Ab initio and continuum model studies predicted that the Γ valley transition metal dichalcogenide (TMD) homobilayers could simulate the conventional multiorbital Hubbard model on the moiré honeycomb lattice. Here, in this work, we perform the Wannierization starting from the continuum model and show that a more general moiré Kanamori-Hubbard model emerges, beyond the extensively studied standard multiorbital Hubbard model, which can be used to investigate the many-body physics in the Γ valley TMD homobilayers. Using the unrestricted Hartree-Fock and Lanczos techniques, we study these half-filled multiorbital moiré bands. By constructing the phase diagrams we predict the presence of an antiferromagnetic state and in addition we found unexpected and dominant states, such as a S = 1 ferromagnetic insulator and a charge density wave state. Our theoretical predictions made using this model can be tested in future experiments on the Γ valley TMD homobilayers.

2-dimensional systems↗
DOE OSTI2023

Disorder-induced excitation continuum in a spin- 1 2 cobaltate on a triangular lattice

A spin-1/2 triangular-lattice antiferromagnet is a prototypical frustrated quantum magnet, which exhibits remarkable quantum many-body effects that arise from the synergy between geometric spin frustration and quantum fluctuations. It can host quantum frustrated magnetic topological phenomena such as quantum spin liquid (QSL) states, highlighted by the presence of fractionalized quasiparticles within a continuum of magnetic excitations. Here, in this work, we use neutron scattering to study CoZnMo 3 O 8 , which has a triangular lattice of J eff = 1/2 Co 2+ ions with octahedral coordination. We found a wave-vector-dependent excitation continuum at low energy that disappears with increasing temperature. Although these excitations are reminiscent of a spin excitation continuum in a QSL state, their presence in CoZnMo 3 O 8 originates from magnetic intersite disorder-induced dynamic spin states with peculiar excitations. Our results, therefore, provide direct experimental evidence for the presence of a disorder-induced spin excitation continuum.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Magnetic states of graphene proximitized Kitaev materials

Single-layer α-ruthenium trichloride (α–RuCl 3 ) has been proposed as a potential quantum spin liquid. Graphene/RuCl 3 heterobilayers have been extensively studied with a focus on the large interlayer electron transfer that dopes both materials. Here we examine the interplay between the competing magnetic state of RuCl 3 layer and graphene electronic properties. We perform self-consistent Hartree-Fock calculations on a Hubbard-Kanamori model of the 4d 5 t 2g electrons of α–RuCl 3 and confirm that out-of-plane ferromagnetic and zigzag antiferromagnetic states are energetically competitive. We show that the influence of hybridization between graphene and RuCl 3 bands is strongly sensitive to the magnetic configuration of RuCl 3 and the relative orientations of the two layers. We argue that strong hybridization leads to graphene magnetoresistance and that it may tilt the balance between closely competing magnetic states. Furthermore, Our analysis can be applied to any van der Waals heterobilayer system with weak interlayer hybridization and allows for arbitrary lattice constant mismatch and relative orientation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Spin wave Hamiltonian and anomalous scattering in NiPS 3

Here, we report a comprehensive spin wave analysis of the semiconducting honeycomb van der Waal antiferromagnet NiPS 3 . Using single-crystal inelastic neutron scattering, we map out the full Brillouin zone and fit the observed modes to a spin wave model with rigorously defined uncertainty. We find that the third-nearest-neighbor exchange J 3 dominates the Hamiltonian, a feature which we fully account for by ab initio density functional theory calculations. We also quantify the degree to which the threefold rotation symmetry is broken and account for the Q = 0 excitations observed in other measurements, yielding a spin exchange model which is consistent across multiple experimental probes. We also identify a strongly reduced static ordered moment and reduced low-energy intensity relative to the linear spin wave calculations, signaling unexplained features in the magnetism which requires going beyond the linear spin wave approximation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Quantitative investigation of the short-range magnetic correlations in the candidate quantum spin liquid NaYbO 2

Here, we present a neutron diffraction study of NaYbO 2 , a candidate quantum spin-liquid (QSL) compound hosting a geometrically frustrated triangular lattice of magnetic Yb 3+ ions. We observe diffuse magnetic scattering that persists to at least 20 K, demonstrating the presence of short-range magnetic correlations in this system up to a relatively high-energy scale. Using a reverse Monte Carlo and magnetic pair distribution function analysis, we confirm the predominant antiferromagnetic nature of these correlations and show that the diffuse scattering data can be well described by noninteracting layers of Heisenberg or XY spins on the triangular lattice. We rule out Ising spins and short-range-ordered stripe or 120° phases as candidate ground states of NaYbO 2 . These results are consistent with a possible QSL ground state in NaYbO 2 and showcase the benefit of a combined reciprocal- and real-space analysis of materials with short-range magnetic correlations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

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

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

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

High-order harmonics and the reverse of the squaring up process in the triangular-lattice magnet HoPdAl 4 ⁢Ge 2

We uncover high-order harmonics and the reverse of the squaring-up process, in terms of analyzing the evolution of magnetic orders in a centrosymmetric layered triangular-lattice magnet HoPdAl 4 ⁢Ge 2 , based on a detailed study of the crystal structure, magnetic susceptibility, magnetization, heat capacity, and magnetic structure. Temperature dependencies of magnetic susceptibility and heat capacity show two magnetic transitions at T N = 10.5 K and T t = 5.5 K. Below T N , Ho 3+ spins order antiferromagnetically as a transverse spin-density wave with the propagation vector k 1 = (001.5 – δ) with δ ≈ 0.18. Upon further cooling through T t , the high-order harmonics with k n = (001.5 – nδ), n = 3, 5, 7 develop, suggesting a squaring-up process. It is surprising that the squaring-up process does not continue down to 0 K but reverses the trend below 3 K. Magnetic-field-induced metastable transitions were observed in M⁡(H) curves with the fields applied both parallel and perpendicular to the triangular-lattice plane. Neutron-diffraction results suggest that the magnetization process in HoPdAl 4 ⁢Ge 2 involves the conversion of k n = (001.5 – n⁢δ) with n = 1, 3, 5, 7, to a ferromagnetic k F = (000) component. It is worth noting that the already weak seventh harmonic magnetic peak is enhanced by applying a small magnetic field in the ab plane at 1.5 K or warming up to 3 K, accompanied by a slight decrease of δ.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Effects of Na deficiency on spin dynamics in the Mott insulating Na 4−x Ir 3 O 8

Here, we report on Raman scattering and 23 Na nuclear magnetic resonance (NMR) investigations of j eff = 1/2 hyperkagome antiferromagnet Na 4−x Ir 3 O 8 , which lies in the Mott insulating state. Our Raman scattering experiments unveil remarkable parallels between the magnetic excitations of the pristine Na 4 Ir 3 O 8 and the Kitaev honeycomb material Na 2 IrO 3 , which are characterized by dominant fractional spinon excitations. In the case of moderate Na-deficient Na 4−x Ir 3 O 8 , however, a substantial suppression of the magnetic excitations is observed, alluding to the notable influence of charge fluctuations on spin dynamics. In addition, our site-specific 23 Na NMR measurements offer further insights into the spin dynamics when a minor concentration of holes is introduced into a spin-orbit coupled Mott insulator. Specifically, the spin-lattice relaxation rate 1/ T 1 reveals the emergence of pseudogaplike correlations at the Na(2) site, alongside a critical slowing down behavior at the Na(1) and Na(3) sites. These findings showcase the intricate interplay between itinerant holes and magnetic correlations in a spin-liquid-like background.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

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

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

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magnetic, charge, and bond order in the two-dimensional Su-Schrieffer-Heeger-Holstein model

Most nonperturbative numerical studies of electron-phonon interactions focus on model Hamiltonians where the electrons interact with a phonon branch via a single type of microscopic mechanism. Two commonly explored couplings in this context are the Holstein and Su-Schrieffer-Heeger (SSH) interactions, which describe phonons modulating the on-site energy and intersite electron hopping, respectively. Many materials, however, have multiple phonon branches that can each interact with electronic degrees of freedom in different ways. We present here a determinant quantum Monte Carlo study of the half-filled two-dimensional (bond) SSH-Holstein Hamiltonian, where electrons couple to different phonon branches via either the Holstein or SSH mechanism. As a result, we map the model's phase diagram and determine the nature of the transitions between charge-density wave, bond-order wave, and antiferromagnetic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Temperature-dependent spin dynamics in Cr 2 O 3

Cr 2 ⁢O 3 emerges as a prominent candidate material for spintronics and magnetoelectronic applications. However, a comprehensive understanding of its temperature-dependent spin dynamics remains elusive, impeding the engineering of novel spintronics based on this material. Here, we delve into this through a combination of inelastic neutron scattering experiments and atomistic simulations. Our results unveiled the emergence of paramagnons above and below 𝑇 𝑁 . We demonstrated a significant softening of linear magnons upon heating in the antiferromagnetic state. Further analysis revealed that this softening primarily originated from four-magnon interactions, while thermal expansion played a minor role.

antiferromagnetism↗
DOE OSTI2025

Mapping the three-dimensional fermiology of the triangular lattice magnet EuAg 4 Sb 2

In this paper, we report the temperature-field phase diagram as well as present a comprehensive study of the electronic structure and three-dimensional fermiology of the triangular-lattice magnet EuAg 4 ⁢Sb 2 , utilizing quantum oscillation measurements, angle-resolved photoemission spectroscopy, and first-principles calculations. The complex magnetic phase diagram of EuAg 4 ⁢Sb 2 highlights many transitions through nontrivial AFM states. Shubnikov-de Haas and de Haas-van Alphen oscillations were observed in the polarized ferromagnetic state of EuAg 4 ⁢Sb 2 , revealing three pairs of distinct spin-split frequency branches with small effective masses. A comparison of the angle-dependent oscillation data with first-principles calculations in the ferromagnetic state and angle-resolved photoemission spectra shows good agreement, identifying tubular hole pockets and hourglass-shaped hole pockets at the Brillouin zone center, as well as diamond-shaped electron pockets at the zone boundary. As the temperature increases, the frequency branches of the tiny hourglass pockets evolve into a more cylindrical shape, while the larger pockets remain unchanged. This highlights that variations in exchange splitting, driven by changes in the magnetic moment, primarily impact the small Fermi pockets without significantly altering the overall band structure. As a result, this is consistent with first-principles calculations, which show minimal changes near the Fermi level across ferromagnetic and simple antiferromagnetic states or under varying on-site Coulomb repulsion.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetic Damping Modulation in IrMn 3 / Ni 80 Fe 20 via the Magnetic Spin Hall Effect

Noncollinear antiferromagnets can have additional spin Hall effects due to the net chirality of their magnetic spin structure, which provides for more complex spin-transport phenomena compared to ordinary nonmagnetic materials. Here we investigated how ferromagnetic resonance of permalloy (Ni 80 Fe 20 ) is modulated by spin Hall effects in adjacent epitaxial IrMn 3 films. We observe a large dc modulation of the ferromagnetic resonance linewidth for currents applied along the [001] IrMn 3 direction. Furthermore, this very strong angular dependence of spin-orbit torques from dc currents through the bilayers can be explained by the magnetic spin Hall effect where IrMn 3 provides novel pathways for modulating magnetization dynamics electrically.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Magnetic Weyl Semimetallic Phase in Thin Films of Eu 2 Ir 2 O 7

The interplay between electronic interactions and strong spin-orbit coupling is expected to create a plethora of fascinating correlated topological states of quantum matter. Of particular interest are magnetic Weyl semimetals originally proposed in the pyrochlore iridates, which are only expected to reveal their topological nature in thin film form. To date, however, direct experimental demonstrations of these exotic phases remain elusive, due to the lack of usable single crystals and the insufficient quality of available films. Here, in this study, we report on the discovery of signatures for the long-sought magnetic Weyl semimetallic phase in (111)-oriented Eu 2 Ir 2 O 7 high-quality epitaxial thin films. We observed an intrinsic anomalous Hall effect with colossal coercivity but vanishing net magnetization, which emerges right below the onset of a peculiar magnetic phase with all-in-all-out (AIAO) antiferromagnetic ordering. The anomalous Hall conductivity obtained experimentally is consistent with the theoretical prediction, likely arising from the nonzero Berry curvature emanated by Weyl node pairs near the Fermi level that act as sources and sinks of Berry flux, activated by broken cubic crystal symmetry at the top and bottom terminations of the thin film.

36 MATERIALS SCIENCE↗