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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Magnetic and transport properties of Fe-doped Weyl semimetal Co 3 Sn 2 S 2

Ferromagnetic Weyl semimetal Co 3 Sn 2 S 2 has recently attracted a lot of interest on account of its large non-saturating magnetoresistance and the high carrier mobility common to topological semimetals. Moreover, the material also provides a platform for studying the interplay between the intrinsic magnetism and topological band structures, which generates a significant anomalous Hall effect. Chemical doping is the usual approach to adjust materials’ properties, and it can lead to fascinating results. Here, we present the influence of Fe-doping on the magnetic and transport properties of polycrystalline Co 3 Sn 2 S 2 . The Fe-doping suppresses the sample’s magnetic order, increases the electrical resistivity, results in a skew-scattering contribution to the anomalous Hall effect and Fe impurities cause the Kondo effect. In addition, we report on the impact of Fe-doping on the magnetoresistance of Co 3 Sn 2 S 2 , where Fe-doping leads to a dramatic decline in magnetoresistance. Furthermore, the hysteresis behavior in the low magnetic field region, detected previously in nanoflake samples only, is observed in both pure and Fe-doped Co 3 Sn 2 S 2 bulk samples. Finally, our findings serve as a reference and source of inspiration for future chemical doping research on topological semimetals.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Eshelby-twisted three-dimensional moiré superlattices

Twisted bilayers of van der Waals materials have recently attracted great attention due to their tunable, strongly correlated phenomena. In this work, we investigate the chirality-specific physics in three-dimensional moiré superlattices induced by Eshelby twist. Our direct density functional theory calculations reveal helical rotation leads to optical circular dichroism and chirality-specific nonlinear Hall effect, even though there is no magnetization or magnetic field. Both these phenomena can be reversed by changing the structural chirality. This provides a path to constructing chirality-specific materials.

2-dimensional systems↗
DOE OSTI2022

Tuning Spin-Orbit Torques Across the Phase Transition in VO 2 /NiFe Heterostructure

The emergence of spin-orbit torques as a promising approach to energy-efficient magnetic switching has generated large interest in material systems with easily and fully tunable spin-orbit torques. We report current-induced spin-orbit torques in VO 2 /NiFe heterostructures are investigated using spin-torque ferromagnetic resonance, where the VO 2 layer undergoes a prominent insulator-metal transition. A roughly twofold increase in the Gilbert damping parameter, α, with temperature is attributed to the change in the VO 2 /NiFe interface spin absorption across the VO 2 phase transition. More remarkably, a large modulation (±100%) and a sign change of the current-induced spin-orbit torque across the VO 2 phase transition suggest two competing spin-orbit torque generating mechanisms. The bulk spin Hall effect in metallic VO 2 , corroborated by the first-principles calculation of the spin Hall conductivity σ SH ≈ -10 4 ($\frac{\hbar}{e}$) Ω -1 m -1 , is verified as the main source of the spin-orbit torque in the metallic phase. The self-induced/anomalous torque in NiFe, with opposite sign and a similar magnitude to the bulk spin Hall effect in metallic VO 2 , can be the other competing mechanism that dominates as temperature decreases. For applications, the strong tunability of the torque strength and direction opens a new route to tailor spin-orbit torques of materials that undergo phase transitions for new device functionalities.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Multimodal correlative study of Hall transport and magnetic phases in Fe/Gd multilayer systems

The Fe/Gd multilayer system hosts a number of magnetic phases, such as stripe, mixed stripe and skyrmion, skyrmion lattice, and isolated skyrmions for a wide range of temperature and magnetic field. We report different Hall transport signals in a Fe/Gd system through multimodal correlative resonant soft x-ray scattering (RSXS), Hall effect, magneto-optic Kerr effect, and transmission x-ray microscopy measurements. The simultaneous nature of the RSXS and Hall transport measurements allowed us to accurately connect various features in the transport data with the specific magnetic phases. We found that the topological Hall effect (THE) shows peaks with opposite signs, which we attribute to two different mechanisms. Our multimodal correlative study indicates that the sign reversal in THE occurs when the system transforms to and from a skyrmion lattice and low density isolated skyrmion phases. We propose that the skyrmion lattice contributes to the THE through a Berry phase induced emergent magnetic field mechanism in one case, and a skew scattering mechanism corresponding to the isolated low density skyrmion state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

EuAuSb: An odd-parity helical variation of altermagnetism

EuAuSb is a triangular-lattice Dirac semimetal in which a topological Hall effect has been observed to develop in association with a magnetically ordered phase. Our single-crystal neutron diffraction measurements have identified an incommensurate helical order in which individual ferromagnetic Eu 2+ layers rotate in-plane by ∼ 120° from one layer to the next. An in-plane magnetic field distorts the incommensurate order, eventually leading to a first order transition to a state that is approximately commensurate and that is continuously polarized as the bulk magnetization approaches saturation. From an analysis of the magnetic diffraction intensities versus field, we find evidence for a dip in the ordered in-plane moment at the same field where the topological Hall effect is a maximum, and we propose that this is due to field-induced quantum spin fluctuations. Our electronic structure calculations yield exchange constants compatible with the helical order and show that the bands near the Fermi level lose their spin degeneracy via a mechanism similar to that in the collinear altermagnets. In conclusion, we find that, unlike the even symmetry seen in the altermagnets, the spin splitting in EuAuSb has odd-wave symmetry similar to that recently found in a number of coplanar magnetic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Single crystal growth, chemical defects, magnetic and transport properties of antiferromagnetic topological insulators ( Ge 1 – δ – x Mn x ) 2 Bi 2 Te 5 ( x ≤ 0.47 , 0.11 ≤ δ ≤ 0.20 )

Magnetic topological insulators provide a platform for emergent phenomena arising from the interplay between magnetism and band topology. Here we report the single crystal growth, crystal structure, magnetic and transport properties, as well as the neutron scattering studies of topological insulator series (Ge 1–δ–x Mn x ) 2 ⁢Bi 2 ⁢Te 5 (x ≤ 0.47, 0.11 ≤ δ ≤ 0.20). Upon doping up to x = 0.47, the lattice parameter c decreases by 0.8%, while the lattice parameter a remains nearly unchanged. Significant Ge vacancies and Ge/Bi site mixing are revealed via elemental analysis as well as refinements of the neutron and x-ray diffraction data, resulting in holes dominating the charge transport. At x = 0.47, below 10.8 K, a bilayer A-type antiferromagnetic ordered state emerges, featuring an ordered moment of 3.0(3) μ B /Mn at 5 K, with the c axis as the easy axis. Magnetization data unveils a much stronger effective interlayer antiferromagnetic exchange interaction and a much smaller uniaxial anisotropy compared to MnBi 2 ⁢Te 4 . We attribute the former to the shorter nearest-neighbor Mn-Mn interlayer superexchange path and the latter to the smaller ligand-field splitting in (Ge 1–δ–x ⁢Mn x ) 2 ⁢Bi 2 ⁢Te 5 . Finally, our study demonstrates that this series of materials holds promise for the investigation of the layer Hall effect and quantum metric nonlinear Hall effect.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Intrinsic topological Weyl phase transition induced by a magnetostructural transformation in a kagome magnet

Topological phase transitions provide a unique window into the interplay between structure, magnetism, and Weyl physics in magnetic Weyl semimetals. However, realizing an intrinsic Weyl phase transition between two distinct Weyl states near room temperature remains challenging. Here, we demonstrate that a magnetostructural transition effectively induces such a transition in the kagome magnet Mn3Ga. High-resolution neutron diffraction, magnetization characterizations and first-principles calculations reveal that Mn3Ga undergoes a chiral antiferromagnetic transition below 485 K, followed by a magnetostructural transition to a monoclinic structure with highly canted antiferromagnetic order near room temperature. These cooperative changes in lattice and magnetic symmetries reorganize Weyl nodes, driving a transition from a primary type-II Weyl state to a distinct Weyl state, accompanied by dramatic variations in the anomalous Hall effect and appearance of topological Hall effect. Our findings open a new pathway for discovering novel topological Weyl states and advancing potential spintronic applications.

Yang, Tsung-Han [ORNL] (ORCID:000000030186302X)↗
DOE OSTI2023

Energy gap of topological surface states in proximity to a magnetic insulator

Abstract Topological surface-states can acquire an energy gap when time-reversal symmetry is broken by interfacing with a magnetic insulator. This gap has yet to be measured. Such topological-magnetic insulator heterostructures can host a quantized anomalous Hall effect and can allow the control of the magnetic state of the insulator in a spintronic device. In this work, we observe the energy gap of topological surface-states in proximity to a magnetic insulator using magnetooptical Landau level spectroscopy. We measure Pb 1-x Sn x Se–EuSe heterostructures grown by molecular beam epitaxy exhibiting a record mobility and low Fermi energy. Through temperature dependent measurements and theoretical calculations, we show this gap is likely due to quantum confinement and conclude that the magnetic proximity effect is weak in this system. This weakness is disadvantageous for the realization of the quantum anomalous Hall effect, but favorable for spintronic devices which require the preservation of spin-momentum locking at the Fermi level.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Ferroelectric Semimetals with α-Bi/SnSe van der Waals Heterostructures and Their Topological Currents

We show that proximity effects can be utilized to engineer van der Waals heterostructures (vdWHs) displaying semimetallic spin-ferroelectricity locking, where ferroelectricity and semimetallic spin states are confined to different layers, but are correlated by means of proximity effects. Our findings are supported by first principles calculations involving α-Bi/SnSe bilayers. We show that such systems support ferroelectrically switchable nonlinear anomalous Hall effect originating from large Berry curvature dipoles as well as direct and inverse spin Hall effects with giant bulk spin-charge interconversion efficiencies. In conclusion, the giant efficiencies are consequences of the proximity-induced semimetallic nature of low energy electron states, which are shown to behave as two-dimensional pseudo-Weyl fermions by means of symmetry analysis and first principles calculations as well as direct angle-resolved photoemission spectroscopy measurements.

2-dimensional systems↗
DOE OSTI2024

Even–Odd Layer-Dependent Exchange Bias Effect in MnBi 2 Te 4 Chern Insulator Devices

Magnetic topological materials with coexisting magnetism and nontrivial band structures exhibit many novel quantum phenomena, including the quantum anomalous Hall effect, the axion insulator state, and the Weyl semimetal phase. As a stoichiometric layered antiferromagnetic topological insulator, thin films of MnBi 2 Te 4 show fascinating even–odd layer-dependent physics. Here, in this work, we fabricate a series of thin-flake MnBi 2 Te 4 devices using stencil masks and observe the Chern insulator state at high magnetic fields. Upon magnetic field training, a large exchange bias effect is observed in odd but not in even septuple layer (SL) devices. Through theoretical calculations, we attribute the even–odd layer-dependent exchange bias effect to the contrasting surface and bulk magnetic properties of MnBi 2 Te 4 devices. Our findings reveal the microscopic magnetic configuration of MnBi 2 Te 4 thin flakes and highlight the challenges in replicating the zero magnetic field quantum anomalous Hall effect in odd SL MnBi 2 Te 4 devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Current-Driven Magnetization Reversal in Orbital Chern Insulators

Graphene multilayers with flat moiré minibands can exhibit the quantized anomalous Hall effect due to the combined influence of spontaneous valley polarization and topologically nontrivial valley-projected bands. The sign of the Hall effect in these Chern insulators can be reversed either by applying an external magnetic field, or by driving a transport current through the system. Here we propose a current-driven mechanism whereby reversal occurs along lines in the (current I , magnetic-field B ) control parameter space with slope d I / d B = ( e / h ) M A M ( 1 – γ 2 ) / γ , where M is the magnetization, A M is the moiré unit cell area, and γ < 1 is the ratio of the chemical potential difference between valleys along a domain wall to the electrical bias e V .

2-dimensional systems↗
DOE OSTI2025

Harnessing ferro-valleytricity in pentalayer rhombohedral graphene for memory and compute

Two-dimensional materials with multiple degrees of freedom, including spin, valleys, and orbitals, open up an exciting avenue for engineering multifunctional devices. Beyond spintronics, these degrees of freedom can lead to novel quantum effects such as valley-dependent Hall effects and orbital magnetism, which could revolutionize next-generation electronics. However, achieving independent control over valley polarization and orbital magnetism has been a challenge due to the need for large electric fields. A recent breakthrough involving pentalayer rhombohedral graphene has demonstrated the ability to individually manipulate anomalous Hall signals and orbital magnetic hysteresis, forming what is known as a valley-magnetic quartet. Here, we leverage the electrically tunable ferro-valleytricity of pentalayer rhombohedral graphene to develop nonvolatile memory and in-memory computation applications. We propose an architecture for a dense, scalable, and selector-less nonvolatile memory array that harnesses the electrically tunable ferro-valleytricity. In our designed array architecture, nondestructive read and write operations are conducted by sensing the valley state through two different pairs of terminals, allowing for independent optimization of read/write peripheral circuits. The power consumption of our PRG-based array is remarkably low, with only ∼6 nW required per write operation and ∼2.3 nW per read operation per cell. This consumption is orders of magnitude lower than that of the majority of state-of-the-art cryogenic memories. Additionally, we engineer in-memory computation by implementing majority logic operations within our proposed nonvolatile memory array without modifying the peripheral circuitry. In conclusion, our framework presents a promising pathway toward achieving ultra-dense cryogenic memory and in-memory computation capabilities.

2D materials↗
NASA NTRS1982

Magnetic field control

A torque control for an electromechanical torquing device of a type where a variable clearance occurs between a rotor and field is described. A Hall effect device senses the field present, which would vary as a function of spacing between field and rotor. The output of the Hall effect device controls the power applied to the field so as to provide a well defined field and thus a controlled torque to the rotor which is well defined.

Haeussermann, W.↗
NASA NTRS2017

Thermal Analysis of the X3 Hall Thruster

Electric propulsion has generated significant interest recently as a mass efficient thrust option for deep space missions. This increased interest has drawn a renewed focus on improving the efficiency of electric propulsion thrusters, specifically Hall thrusters. Hall thrusters are a specific subset of ion engines that accelerate charged particles using a Hall current. Recent research into the 12.5 kW HERMeS (Hall Effect Rocket with Magnetic Shielding) for use on the proposed Asteroid Redirect Robotic Mission (ARRM) has shown the desire to use these devices on flight missions outside of earth orbit. This paper will focus on another Hall thruster, the X3. The X3 is a 100 kW class Hall effect thruster used for laboratory technology development. What makes the X3 unique, relative to more contemporary Hall thrusters beyond the order of magnitude increase in discharge power, is that the X3 is a 3 channel, nested Hall thruster. Common Hall thrusters typically have one annular channel where the plasma is constrained and used to charge the fuel and accelerated. The X3’s 3 channels are nested in a concentric format, with each channel fitting in a single plane. This presents very unique thermal modeling challenges with respect to quantifying the impact of plasma thermal loading and cross talk between the power dissipating components for each channel. This work will discuss the development of the X3 thermal model and the efforts to validate it with experimental data. Furthermore, some of the unique challenges that appear when trying to model high power components will be discussed. The X3 represents a challenging and interesting example of the issues affecting thermal modeling of high power electric propulsion thrusters which will become more prevalent as their use becomes more common. II

Hofer, Richard Jr.↗
DOE OSTI2020

Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice

Studies of two-dimensional electron systems in a strong magnetic field revealed the quantum Hall effect, a topological state of matter featuring a finite Chern number C and chiral edge states. Haldane later theorized that Chern insulators with integer quantum Hall effects could appear in lattice models with complex hopping parameters even at zero magnetic field. The ABC-trilayer graphene/hexagonal boron nitride (ABC-TLG/hBN) moiré superlattice provides an attractive platform with which to explore Chern insulators because it features nearly flat moiré minibands with a valley-dependent, electrically tunable Chern number. In this work, we report the experimental observation of a correlated Chern insulator in an ABC-TLG/hBN moiré superlattice. We show that reversing the direction of the applied vertical electric field switches the moiré minibands of ABC-TLG/hBN between zero and finite Chern numbers, as revealed by large changes in magneto-transport behaviour. For topological hole minibands tuned to have a finite Chern number, we focus on quarter filling, corresponding to one hole per moiré unit cell. The Hall resistance is well quantized at h/2e 2 (where h is Planck’s constant and e is the charge on the electron), which implies C = 2, for a magnetic field exceeding 0.4 tesla. The correlated Chern insulator is ferromagnetic, exhibiting substantial magnetic hysteresis and a large anomalous Hall signal at zero magnetic field. Finally, our discovery of a C = 2 Chern insulator at zero magnetic field should open up opportunities for discovering correlated topological states, possibly with topological excitations, in nearly flat and topologically nontrivial moiré minibands.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Direct evidence from high-field magnetotransport for a dramatic change of quasiparticle character in van der Waals ferromagnet Fe 3−𝑥 ⁢GeTe 2

Magnetometry and magnetoresistance (MR) data taken on the van der Waals ferromagnet Fe 3−𝑥 ⁢GeTe 2 (FGT) reveal three distinct contributions to the MR: a linear negative component, a contribution from closed Fermi-surface orbits, and an enhancement proportional to the square of the applied magnetic field which is linked to a noncoplanar spin arrangement. Contrary to earlier studies on FGT, by accounting for the field dependence of the anomalous Hall effect, we find that the ordinary Hall coefficient decreases markedly below 80 K, indicating a significant change in character of the electrons and holes on the Fermi-surface at this temperature. The resulting altered ground state eventually causes the Hall coefficient to reverse sign at 35 K. Our Hall data support the proposal that Kondo-lattice behavior develops in this 𝑑-electron material below 80 K. Additional evidence comes from the negative linear component of the MR, which arises from electron-magnon scattering with an atypical temperature dependence attributable to the onset of Kondo screening.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Electrical readout of topological spin order in ultrathin insulators with small magnetization

Probing magnetic order in insulators with small magnetization is particularly challenging in the ultrathin limit, where magnetometry and diffraction lose sensitivity. Here, in this study, we show that topological spin order with vanishingly small magnetization can be electrically detected through an interfacial topological Hall effect in heavy-metal/magnetic-insulator heterostructures. Using Pt coupled to the canted antiferromagnetic insulator hexagonal LuFe⁢O 3 (h-LuFe⁢O 3 ) we observe an unusually large and robust Hall response arising from the transfer of real-space spin topology across the interface via magnetic proximity effect (MPE). This interfacial signal enables detection of magnetic order in h-LuFe⁢O 3 with an extremely small net magnetization (0.025 𝜇 B /Fe), corresponding to Hall-conductivity-magnetization ratio ≈ 2 V −1 , which is one to two orders of magnitude large than other MPE-induced Hall effects and anomalous Hall effects of conducting magnets. This high sensitivity allows detection of magnetic order in h-LuFe⁢O 3 down to a thickness of only 1.5 unit cells. Our results establish an interfacial Hall-based approach for electrically probing topological spin structures in ultrathin insulators and the related interfaces, enabling access to magnetic order in regimes where conventional probes fail.

Topological Hall Effect↗
NASA NTRS1991

Magnet/Hall-Effect Random-Access Memory

In proposed magnet/Hall-effect random-access memory (MHRAM), bits of data stored magnetically in Perm-alloy (or equivalent)-film memory elements and read out by using Hall-effect sensors to detect magnetization. Value of each bit represented by polarity of magnetization. Retains data for indefinite time or until data rewritten. Speed of Hall-effect sensors in MHRAM results in readout times of about 100 nanoseconds. Other characteristics include high immunity to ionizing radiation and storage densities of order 10(Sup6)bits/cm(Sup 2) or more.

Wu, Jiin-Chuan↗