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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Giant room temperature anomalous Hall effect and tunable topology in a ferromagnetic topological semimetal Co 2 MnAl

Weyl semimetals exhibit unusual surface states and anomalous transport phenomena. It is hard to manipulate the band structure topology of specific Weyl materials. Topological transport phenomena usually appear at very low temperatures, which sets challenges for applications. In this work, we demonstrate the band topology modification via a weak magnetic field in a ferromagnetic Weyl semimetal candidate, Co 2 MnAl, at room temperature. We observe a tunable, giant anomalous Hall effect (AHE) induced by the transition involving Weyl points and nodal rings. The AHE conductivity is as large as that of a 3D quantum AHE, with the Hall angle (Θ H ) reaching a record value (tan Θ H = 0.21) at the room temperature among magnetic conductors. Furthermore, we propose a material recipe to generate large AHE by gaping nodal rings without requiring Weyl points. Our work reveals an intrinsically magnetic platform to explore the interplay between magnetic dynamics and topological physics for developing spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Third harmonic characterization of antiferromagnetic heterostructures

Abstract Electrical switching of antiferromagnets is an exciting recent development in spintronics, which promises active antiferromagnetic devices with high speed and low energy cost. In this emerging field, there is an active debate about the mechanisms of current-driven switching of antiferromagnets. For heavy-metal/ferromagnet systems, harmonic characterization is a powerful tool to quantify current-induced spin-orbit torques and spin Seebeck effect and elucidate current-induced switching. However, harmonic measurement of spin-orbit torques has never been verified in antiferromagnetic heterostructures. Here, we report harmonic measurements in Pt/ α -Fe 2 O 3 bilayers, which are explained by our modeling of higher-order harmonic voltages. As compared with ferromagnetic heterostructures where all current-induced effects appear in the second harmonic signals, the damping-like torque and thermally-induced magnetoelastic effect contributions in Pt/ α -Fe 2 O 3 emerge in the third harmonic voltage. Our results provide a new path to probe the current-induced magnetization dynamics in antiferromagnets, promoting the application of antiferromagnetic spintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Towards understanding the magnetic properties of the breathing pyrochlore compound Ba3Yb2Zn5O11through single-crystal studies

Abstract Ba 3 Yb 2 Zn 5 O 11 is exceptional among breathing pyrochlore compounds for being in the nearly-decoupled limit where inter-tetrahedron interactions are weak, hosting isolated clusters or molecular magnet-like tetrahedra of magnetic ytterbium (Yb 3+ ) ions. In this work, we present the study carried out on single-crystal samples of the breathing pyrochlore Ba 3 Yb 2 Zn 5 O 11 , using a variety of magnetometry and neutron scattering techniques along with theoretical modeling. We employ inelastic neutron scattering to investigate the magnetic dynamics as a function of applied field (with respect to both magnitude and direction) down to a temperature of 70 mK, where inelastic scattering reveals dispersionless bands of excitations as found in earlier powder sample studies, in good agreement with a single-tetrahedron model. However, diffuse neutron scattering at zero field and dc-susceptibility at finite field exhibit features suggesting the presence of excitations at low-energy that are not captured by the single tetrahedron model. Analysis of the local structure down to 2 K via pair distribution function analysis finds no evidence of structural disorder. We conclude that effects beyond the single tetrahedron model are important in describing the low-energy, low-temperature physics of Ba 3 Yb 2 Zn 5 O 11 , but their nature remains undetermined.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Probing magnon–magnon coupling in exchange coupled Y 3 Fe 5 O 12 /Permalloy bilayers with magneto-optical effects

We demonstrate the magnetically-induced transparency (MIT) effect in Y 3 Fe 5 O 12 (YIG)/Permalloy (Py) coupled bilayers. The measurement is achieved via a heterodyne detection of the coupled magnetization dynamics using a single wavelength that probes the magneto-optical Kerr and Faraday effects of Py and YIG, respectively. Clear features of the MIT effect are evident from the deeply modulated ferromagnetic resonance of Py due to the perpendicular-standing-spin-wave of YIG. We develop a phenomenological model that nicely reproduces the experimental results including the induced amplitude and phase evolution caused by the magnon–magnon coupling. Our work offers a new route towards studying phase-resolved spin dynamics and hybrid magnonic systems

36 MATERIALS SCIENCE↗
DOE OSTI2025

Determining spin-dependent light dark matter rates from neutron scattering

The scattering and absorption rates of light dark matter with electron spin-dependent interactions depend on the target’s spin response. We show how this response is encoded by the target’s dynamical magnetic susceptibility, which can be measured using neutron scattering. We directly use existing neutron scattering data to compute the dark matter scattering rate in a candidate target material, finding close agreement with the previous first-principles calculation at MeV dark matter masses. Complementary experiments and measurements can extend the reach of this technique to other dark matter models and masses, and identify promising target materials for future experiments.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Theory of tunneling between two-dimensional electron layers driven by spin pumping: Adiabatic regime and beyond

Tunneling spectroscopy between parallel two-dimensional (2D) electronic systems provides a powerful method to probe the underlying electronic properties by measuring tunneling conductance. Here, in this work, we present a theoretical framework for spin transport in 2D-to-2D tunneling systems, driven by spin pumping. This theory applies to a vertical heterostructure where two layers of metallic 2D electron systems are separated by an insulating barrier, with one layer exchange coupled to a magnetic layer driven at resonance. Utilizing a nonperturbative Floquet-Keldysh formalism, we derive general expressions for the tunneling spin and charge currents across a broad range of driving frequencies, extending beyond the traditional adiabatic pumping regime. At low frequencies, we obtain analytical results that recover the known behaviors in the adiabatic regime. However, at higher frequencies, our numerical findings reveal significant deviations in the dependence of spin and charge currents on both frequency and precession angle. This work offers fresh insights into the role of magnetization dynamics in tunneling transport, opening up new avenues for exploring nonadiabatic spin pumping phenomena.

Green's function methods↗
DOE OSTI2021

ac Sensing Using Nitrogen-Vacancy Centers in a Diamond Anvil Cell up to 6 GPa

Nitrogen-vacancy color centers in diamond have attracted broad attention as quantum sensors for both static and dynamic magnetic, electrical, strain, and thermal fields and are particularly attractive for quantum sensing under pressure in diamond anvil cells. Optically based nuclear magnetic resonance may be possible at pressures greater than a few gigapascals and offers an attractive alternative to conventional Faraday-induction-based detection. In this work, we present ac sensing results and demonstrate synchronized readout up to 6 GPa but find that the sensitivity is reduced due to inhomogeneities of the microwave field and pressure within the sample space. These experiments enable the possibility of all-optical high resolution magnetic resonance of nanoliter sample volumes at high pressures.

58 GEOSCIENCES↗
DOE OSTI2021

Superconductivity-enhanced spin pumping: Role of Andreev resonances

Here we describe a simple hybrid superconductor–ferromagnetic-insulator structure manifesting spin-resolved Andreev bound states in which dynamic magnetization is employed to probe spin related physics. We show that, at low bias and below T c , the transfer of spin angular momentum pumped by an externally driven ferromagnetic insulator is greatly affected by the formation of spin-resolved Andreev bound states. Our results indicate that these bound states capture the essential physics of condensate-facilitated spin flow. For finite thicknesses of the superconducting layer, comparable to the coherence length, resonant Andreev bound states render highly transmitting subgap spin transport channels. We point out that the resonant enhancement of the subgap transport channels establishes a prototype Fabry-Pérot resonator for spin pumping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Anticollinear order and degeneracy lifting in square lattice antiferromagnet LaSrCrO 4

In this work, we report the static and dynamic magnetic properties of LaSrCrO 4 , a seemingly canonical spin-3/2 square-lattice antiferromagnet that exhibits frustration between magnetic layers—owing to their AB stacking—and offers a rare testbed to investigate accidental-degeneracy lifting in magnetism. Neutron diffraction experiments on single-crystal samples uncover a remarkable anticollinear magnetic order below T N = 170 K characterized by a Néel arrangement of the spins within each layer and an orthogonal arrangement between adjacent layers. To understand the origin of this unusual magnetic structure, we analyze the spin-wave excitation spectrum by means of inelastic neutron scattering and bulk measurements. A spectral gap of 0.5 meV, along with a spin-flop transition at 3.2 T, reflect the energy scale associated with the degeneracy-lifting. A minimal model to explain these observations requires both a positive biquadratic interlayer exchange and dipolar interactions, both of which are on the order of 10 –4 meV, only a few parts per million of the dominant exchange interaction J 1 ≈11 meV. These results provide direct evidence for the selection of a noncollinear magnetic structure by the combined effect of two distinct degeneracy lifting interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Low-energy electronic interactions in ferrimagnetic Sr 2 CrReO 6 thin films

We reveal in this study the fundamental low-energy landscape in the ferrimagnetic Sr 2 CrReO 6 double perovskite and describe the underlying mechanisms responsible for the three low-energy excitations below 1.4 eV. Here, based on resonant inelastic x-ray scattering and magnetic dynamics calculations, and experiments collected from both Sr 2 CrReO 6 powders and epitaxially strained thin films, we reveal a strong competition between spin-orbit coupling, Hund’s coupling, and the strain-induced tetragonal crystal field. Here, we also demonstrate that a spin-flip process is at the origin of the lowest excitation at 200 meV, and we bring insights into the predicted presence of orbital ordering in this material. We study the nature of the magnons through a combination of ab initio and spin-wave theory calculations, and show that two nondegenerate magnon bands exist and are dominated either by rhenium or chromium spins. The rhenium band is found to be flat at about 200 meV (±25 meV) through X-L-W-U high-symmetry points and is dispersive toward Γ.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Suppressed superexchange interactions in the cuprates by bond-stretching oxygen phonons

We study a multiorbital Hubbard–Su-Schrieffer-Heeger model for the one-dimensional (1D) corner-shared cuprates in the adiabatic and nonadiabatic limits using exact diagonalization and determinant quantum Monte Carlo. At half filling and in the adiabatic limit, lattice dimerization can be achieved only over a narrow range of couplings slightly below a critical coupling g c . Beyond this value, the sign of the effective hopping changes, and the lattice becomes unstable. Strong lattice fluctuations replace the dimerization state in the nonadiabatic case. We also examine the model's temperature-dependent uniform and dynamical magnetic susceptibilities and compare them to the results of an effective spin-1/2 Heisenberg model. In doing so, we demonstrate that lattice fluctuations induced by the e-ph interaction suppress the effective superexchange interaction when g < g c . Furthermore, our results elucidate the effect of bond-stretching phonons in the parent cuprate compounds in general and are particularly relevant to 1D cuprates, where strong e-ph interactions have recently been inferred.

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 OSTI2020

Protecting Spin Coherence in a Tunable Heisenberg Model

In this work, using an ensemble of atoms in an optical cavity, we engineer a family of nonlocal Heisenberg Hamiltonians with continuously tunable anisotropy of the spin-spin couplings. Therefore, we thus gain access to a rich phase diagram, including a paramagnetic-to-ferromagnetic Ising phase transition that manifests as a diverging magnetic susceptibility at the critical point. The susceptibility displays a symmetry between Ising interactions and XY (spin-exchange) interactions of the opposite sign, which is indicative of the spatially extended atomic system behaving as a single collective spin. Images of the magnetization dynamics show that spin-exchange interactions protect the coherence of the collective spin, even against inhomogeneous fields that completely dephase the noninteracting and Ising systems. Our results underscore prospects for harnessing spin-exchange interactions to enhance the robustness of spin squeezing protocols.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Antiferromagnetic V 2 ⁢O 3 based exchange coupling

Vanadium sesquioxide (V 2 ⁢O 3 ) is a strongly correlated electronic material that famously undergoes a triple coupled first-order transition where it transitions from a paramagnetic metal with a rhombohedral structure at high temperature to an antiferromagnetic insulator with a monoclinic structure. While several studies have used one of both electronic and structural transitions to control the properties of a heterostructure, evidence of magnetic coupling has notoriously yet to be found. In this paper, we report on a robust magnetic coupling between the antiferromagnetic (AFM) V 2 ⁢O 3 and ferromagnetic (FM) Permalloy (Py) layers that results in a significant exchange bias and strain-induced coercivity enhancement. We provide a temperature and angle-dependent study of magnetic properties, which clearly indicates exchange bias at the AFM/FM interface that appears at the onset of the metal-insulator transition. The magnitude of the exchange bias is strongest when the field is applied along the [001] V 2 ⁢O 3 crystallographic orientation which corresponds to an AFM spin configuration on the [110] surface. Here, this opens the door to designing and implementing novel functionalities in transition metal oxide-based computing using the connection between magnetism and the metal-insulator transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Slow ferromagnetic fluctuations in the kagome metal Sc 3 ⁢ Mn 3 ⁢Al 7 ⁢ Si 5 revealed by 27 Al NMR

Static and dynamical magnetic and electronic properties of the kagome metal Sc 3 ⁢Mn 3 ⁢ Al 7⁢ Si 5 have been investigated by 27 Al nuclear magnetic resonance (NMR) measurements. Two distinct 27 Al -NMR signals with two different values of quadrupolar frequencies of 𝜈 Q = 1.55(2) and 1.07(2) MHz are observed, which are assigned to Al(1) and Al(2), respectively. From the detailed NMR spectrum measurements under three different magnetic field directions and the density functional theory calculations, the principal axes of the electric field gradient for each Al site have been determined. The temperature dependence of Knight shift (𝐾) shows a similar temperature dependence of the DC magnetic susceptibility 𝜒 except for the low-temperature region below ∼50 K where 𝐾 is almost constant while 𝜒 keeps increasing, which suggests that the increase in 𝜒 at low temperatures is not intrinsic. 27 Al spin-lattice relaxation rate divided by temperature (1/𝑇 1 ⁢𝑇) is found to be constant, confirming the metallic state of Sc 3 ⁢Mn 3 ⁢ Al 7⁢ Si 5 from a microscopic point of view. Based on a Korringa ratio analysis using the 𝑇 1 and 𝐾 data, ferromagnetic fluctuations are found to dominate in Sc 3 ⁢Mn 3 ⁢ Al 7⁢ Si 5 . In conclusion, these fluctuations are suggested to be very slow with frequencies on the order of kilohertz or lower.

Ding, Qing-Ping [Ames Laboratory (AMES), Ames, IA ↗
DOE OSTI2026

Tunable magnetic excitations in the honeycomb antiferromagnet (Co,Ni)⁢TiO3

The solid solution of the honeycomb antiferromagnet (AFM) Co0.5⁢Ni0.5⁢TiO3 (CNTO) was synthesized by mixing a 1:1 molar ratio of CoTiO3 (CTO) and NiTiO3 (NTO). Powder neutron scattering was used to determine the structure and magnetic spectrum, where the nuclear and magnetic structures resemble those of the end members. The Néel temperature (T𝑁 = 27 ± 1 K), the ordered magnetic moment (M = 2.29 ± 0.03 𝜇𝐵 per magnetic ion), and lattice parameters are intermediate between those of the two. From inelastic neutron scattering, it is observed that the magnon density of states (MDOS) extends up to 13 meV, and the interaction can be described by an XXZ-type magnetic Hamiltonian. Above 15 meV, spin-orbit excitons (SOEs) are observed similarly to those in CTO, but with modified spin-orbit crystal-field splitting because of the substitution of Ni at Co sites, and the disorder induced by the Ni substitution modifies the spin-orbit crystal field. Thus, the magnetic dynamics of the CNTO cannot be described as a simple average of the two parent compounds.

Rathnayaka, Srimal [University of Virginia, Charlo↗
DOE OSTI2024

A galactic approach to neutron scattering science

Neutron scattering science is leading to significant advances in our understanding of materials and will be key to solving many of the challenges that society is facing today. Improvements in scientific instruments are actually making it more difficult to analyze and interpret the results of experiments due to the vast increases in the volume and complexity of data being produced and the associated computational requirements for processing that data. New approaches to enable scientists to leverage computational resources are required, and Oak Ridge National Laboratory (ORNL) has been at the forefront of developing these technologies. We recently completed the design and initial implementation of a neutrons data interpretation platform that allows seamless access to the computational resources provided by ORNL. For the first time, we have demonstrated that this platform can be used for advanced data analysis of correlated quantum materials by utilizing the world's most powerful computer system, Frontier. In particular, we have shown the end-to-end execution of the DCA++ code to determine the dynamic magnetic spin susceptibility χ(q, ω) for a single-band Hubbard model with Coulomb repulsion U/t = 8 in units of the nearest-neighbor hopping amplitude t and an electron density of n = 0.65. The following work describes the architecture, design, and implementation of the platform and how we constructed a correlated quantum materials analysis workflow to demonstrate the viability of this system to produce scientific results.

97 MATHEMATICS AND COMPUTING↗