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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Thermal hysteretic behavior and negative magnetoresistance in the charge density wave material EuTe 4

EuTe 4 is a van der Waals material exhibiting a charge density wave (CDW) with a large thermal hysteresis in the resistivity and CDW gap. In this paper, we systematically study the electronic structure and transport properties of EuTe 4 using high-resolution angle-resolved photoemission spectroscopy (ARPES), magnetoresistance (MR) measurements, and scanning tunneling microscopy (STM). Here, we observe a CDW gap of ~ 200meV at low temperatures that persists up to 400 K, suggesting that the CDW transition occurs at a much higher temperature. The ARPES intensity near the Fermi level shows large thermal hysteretic behavior, consistent with the resistivity measurement. The hysteresis in the resistivity measurement does not change under a magnetic field up to 7 T, excluding the thermal magnetic hysteretic effect. Instead, the surface topography measured with STM shows surface domains with different CDW trimerization directions, which may be important for the thermal hysteretic behavior. Interestingly, we reveal a large negative MR at low temperatures that can be associated with the canting of magnetically ordered Eu spins. Our results shed light on the understanding of magnetic, transport, and electronic properties of EuTe 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Large anomalous Nernst effect and topological Nernst effect in the noncollinear antiferromagnet NdMn 2⁢ Ge 2

ThCr 2 Si 2 -type NdMn 2 Ge 2 was recently proposed to be a magnetic skyrmion candidate exhibiting the topological Hall effect. In this work, we report magnetic, electronic, and thermoelectric measurements for NdMn 2 Ge 2 single crystals. We show that, in the canted antiferromagnetic phase and the conical magnetic phase with a large out-of-plane ferromagnetic component, this system exhibits both an anomalous Nernst effect and a topological Nernst effect. Here we argue that NdMn 2 Ge 2 is a unique magnetic material, with its topological characteristics associated with the Berry phases in both real space and momentum space.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Coupled magnetostructural continuum model for multiferroic BiFeO 3

Here, a continuum approach to study magnetoelectric multiferroic BiFeO 3 (BFO) is proposed. Our modeling effort marries the ferroelectric (FE) phase field method and micromagnetic simulations to describe the entire multiferroic order parameter sector (polarization, oxygen antiphase tilts, strain, and magnetism) self-consistently on the same time- and length scale. In this paper, we discuss our choice of FE and magnetic energy terms and demonstrate benchmarks against known behavior. We parametrize the lowest order couplings of the structural distortions against previous predictions from density functional theory calculations giving access to simulations of the FE domain wall (DW) topology. This allows us to estimate the energetic hierarchy and thicknesses of the numerous structural DWs. We then extend the model to the canted antiferromagnetic order and demonstrate how the FE domain boundaries influence the resulting magnetic DWs. We also highlight some capabilities of this model by providing two examples relevant for applications. We demonstrate spin-wave transmission through the multiferroic domain boundaries which identify rectification in qualitative agreement with recent experimental observations. As a second example of application, we model fully dynamical magnetoelectric switching, where we find a sensitivity on the Gilbert damping with respect to switching pathways. We envision that this modeling effort will set the basis for further work on properties of arbitrary 3D nanostructures of BFO (and related multiferroics) at the mesoscale.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magnetic structure and magnetoelectric properties of the spin-flop phase in LiFePO 4

We investigate the magnetic structure and magnetoelectric(ME) effect in the high-field phase of the antiferromagnet LiFePO 4 above the critical field of 31 T. A neutron diffraction study in pulsed magnetic fields reveals the propagation vector to be q = 0 for the high-field magnetic structure. Pulsed-field electric polarization measurements show that, at the critical field, the low-field off-diagonal ME coupling α ab is partially suppressed, and the diagonal element α bb emerges. These results are consistent with a spin-flop transition where the spin direction changes from primarily being along the easy b axis below the transition to being along a above. The persistence of off-diagonal ME tensor elements above the critical field suggests a lowering of the magnetic point-group symmetry and hence a more complex magnetic structure in the high-field phase. In addition, neutron diffraction measurements in low magnetic fields show no observable field-induced spin canting, which indicates a negligible Dzyaloshinskii-Moriya interaction. The observed spin-flop field supports the Hamiltonian recently deduced from inelastic neutron studies and indicates that the system is less frustrated and with a larger single-ion anisotropy than originally thought. Our results demonstrate the effectiveness of combining pulsed-field neutron diffraction and electric polarization measurements to elucidate the magnetic structures and symmetries at the highest attainable field strengths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Field-dependent magnons in the honeycomb antiferromagnet CoTiO 3

We report field-dependent high-resolution inelastic neutron scattering (INS) measurements on the honeycomb lattice magnet, CoTiO 3 , to study the evolution of its magnon excitations across a spin reorientation transition driven by an in-plane magnetic field. By carrying out elastic neutron scattering in a magnetic field, we show that the sample transitions from a collinear antiferromagnetic state with multiple magnetic domains at a low field to a monodomain state with a canted magnetic structure at a high field. Concurrent with this transition, we observed significant changes in both the energy and the width of the zone center magnon peak. The observed width change was argued to be consistent with an unusual zero-field state with extended domain walls. On the other hand, the magnon spectra near the K point of the Brillouin zone boundary were found to be largely insensitive to the changes in the ordered moment directions and the domain configuration. We argue that this observation is difficult to explain within the framework of the bond-dependent model proposed in a recent INS study [M. Elliot et al., Nat. Commun. 12, 3936 (2021)]. Our paper therefore calls for alternative explanations for the observed K-point gap in CoTiO 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetic properties of a staggered S = 1 chain with an alternating single-ion anisotropy direction

Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states that are sensitive to the single-ion-anisotropy (SIA) energy ( D ), and intrachain ( J 0 ) and interchain ( J 1 , 2 ′ ) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of Ni ( pyrimidine ) ( H 2 O ) 2 ( NO 3 ) 2 , a chain compound where the tilting of Ni octahedra leads to a twofold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at T N = 2.3 K and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find J 0 = 5.107 ( 7 ) K , D = 2.79 ( 1 ) K , J 1 ′ = 0.00 ( 5 ) K , J 2 ′ = 0.18 ( 3 ) K , and a rhombic anisotropy energy E = 0.19 ( 9 ) K . Mean-field modeling reveals that the ground state structure hosts spin canting of ϕ ≈ 6 . 5 ∘ , which is not detectable above the noise floor of the elastic neutron diffraction data. Monte Carlo simulation of the powder-averaged magnetization, M ( H ) , is then used to confirm these Hamiltonian parameters, while single-crystal M ( H ) simulations provide insight into features observed in the data. Published by the American Physical Society 2025

Vaidya, S. (ORCID:000000033975497X)↗
DOE OSTI2025

Structural and magnetic properties of CoTeMo⁢O 6

Here, we have conducted a comprehensive investigation into the magnetic properties of the chiral multiferroic material CoTeMo⁢O 6 . In contrast with the previous claim of canted antiferromagnetic order with ferromagnetic components [Y. Doi et al., J. Solid State Chem. 182, 3232 (2009)], our investigation reveals an antiferromagnetic ground state with compensated moments, providing an interesting platform for exploring exotic material properties. Through careful measurements of magnetization under a series of applied fields, we demonstrate that there exist two sequential field-induced magnetic transitions in CoTeMo⁢O 6 , with one occurring at 𝐻 𝑐⁢1 =460 Oe along the 𝑎 axis, and the other at 𝐻 𝑐⁢2 =1.16 T with the field along the 𝑏 axis. The values of 𝐻 𝑐⁢1 and 𝐻 𝑐⁢2 exhibit strong angular dependence and diverge with different rates as the applied field is rotated 90 ° within the 𝑎⁢𝑏 plane. This reflects the distinct nature of these transitions, which is further supported by the different critical behavior of 𝐻 𝑐⁢1 and 𝐻 𝑐⁢2 , characterized by the values of 𝛾, in the function of 𝐻 𝑐 =𝐻 0 ⁡(1−𝑇/𝑇 𝑐 ) 𝛾 . Furthermore, we have demonstrated that there exist structural and magnetic twin domains in CoTeMo⁢O 6 that strongly affect the experimental measurement of their macroscopic properties. Intriguingly, these twin domains can be related to the orthorhombicity/chirality of the crystal structure with the space group 𝑃⁢2 1 ⁢2 1⁢ 2. We further explored the magnetic and structural domains with uniaxial pressure and polarized light microscopy. Our results suggest that CoTeMo⁢O 6 could be used as a unique platform for investigating the intriguing physics involving intertwined degrees of freedom. The tunability of the underlying domain distribution and its strong anisotropy could also be useful for developing functional devices and applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Bayesian Optimization of a Free-Electron Laser

The Linac Coherent Light Source X-ray free-electron laser is a complex scienti c apparatus which changes con gurations multiple times per day, necessitating fast tuning strategies to reduce setup time for successive experiments. To this end, we employ a Bayesian approach to maximizing X- ray laser pulse energy by controlling groups of quadrupole magnets. A Gaussian process model provides probabilistic predictions for the machine response with respect to control parameters, enabling a balance of exploration and exploitation in the search for the global optimum. We show that the model parameters can be learned from archived scans, and correlations between devices can be extracted from the beam transport. Furthermore, the result is a sample-e cient optimization routine, combining both historical data and knowledge of accelerator physics to signi cantly outperform existing optimizers.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Magnetization Process of Atacamite: A Case of Weakly Coupled S = 1/2 Sawtooth Chains

We present a combined experimental and theoretical study of the mineral atacamite Cu 2 Cl(OH) 3 . Density-functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Here, magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateaulike magnetization at about M sat /2. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.

1-dimensional spin chains↗
DOE OSTI2022

Observation of Vector Spin Seebeck Effect in a Noncollinear Antiferromagnet

Spintronic phenomena to date have been established in magnets with collinear moments, where the spin injection through the spin Seebeck effect (SSE) is always along the out-of-plane direction. Here, we report the observation of a vector SSE in a noncollinear antiferromagnet (AF) LuFeO 3 , where temperature gradient along the out-of-plane and also the in-plane directions can both inject a pure spin current and generate a voltage in the heavy metal via the inverse spin Hall effect (ISHE). We show that the thermovoltages are due to the magnetization from canted spins in LuFeO 3 . Furthermore, in contrast to the challenges of generating, manipulating, and detecting spin current in collinear AFs, the vector SSE in LuFeO 3 is readily viable in zero magnetic field and can be controlled by a small magnetic field of about 150 Oe at room temperature. Furthermore, the noncollinear AFs expand new realms for exploring spin phenomena and provide a new route to low-field antiferromagnetic spin caloritronics and magnonics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Improved Particle Confinement with Resonant Magnetic Perturbations in DIII-D Tokamak H-Mode Plasmas

Experiments on the DIII-D tokamak have identi fied a novel regime in which applied resonant magnetic perturbations (RMPs) increase the particle con finement and overall performance. This work details a robust range of counter-current rotation over which RMPs cause this density pump-in effect for high con finement (H-mode) plasmas. The pump-in is shown to be caused by a reduction of the turbulent transport and to be correlated with a change in the sign of the induced neoclassical transport. Furthermore, this novel reversal of the RMP induced transport has the potential to signifi cantly improve reactor relevant, three-dimensional magnetic con finement scenarios.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Observation of Skewed Electromagnetic Wakefields in an Asymmetric Structure Driven by Flat Electron Bunches

Relativistic charged -particle beams that generate intense longitudinal fields in accelerating structures also inherently couple to transverse modes. The effects of this coupling may lead to beam breakup instability and thus must be countered to preserve beam quality in applications such as linear colliders. Beams with highly asymmetric transverse sizes (flat beams) have been shown to suppress the initial instability in slab -symmetric structures. However, as the coupling to transverse modes remains, this solution serves only to delay instability. In order to understand the hazards of transverse coupling in such a case, we describe here an experiment characterizing the transverse effects on a flat beam, traversing near a planar dielectric lined structure. Further, the measurements reveal the emergence of a previously unobserved skew-quadrupolelike interaction when the beam is canted transversely, which is not present when the flat beam travels parallel to the dielectric surface. We deploy a multipole field fitting algorithm to reconstruct the projected transverse wakefields from the data. We generate the effective kick vector map using a simple two -particle theoretical model, with particle -in -cell simulations used to provide further insight for realistic particle distributions.

43 PARTICLE ACCELERATORS↗
DOE OSTI2025

Large Spin-Orbit Torque in 𝑎-Plane 𝛼−Fe 2 ⁢O 3 /Pt Bilayers

Realization of efficient spin-orbit torque switching of the Neel vector in insulating antiferromagnets is a challenge, often complicated by spurious effects. Quantifying the spin-orbit torques in antiferromagnet or heavy metal heterostructures is an important first step toward this goal. Here, we employ magneto-optic techniques to study damping-like spin-orbit torque (DL-SOT) in a-plane α-Fe 2 O 3 (hematite) with a Pt spin-orbit overlayer. We find that the DL-SOT efficiency is 2 orders of magnitude larger than reported in c- and r-plane hematite/Pt using harmonic Hall techniques. The large magnitude of DL-SOT is supported by direct imaging of current-induced motion of antiferromagnetic domains that happens at moderate current densities. Furthermore, our study introduces a new method for quantifying spin-orbit torque in antiferromagnets with a small canted moment and identifies a-plane α-Fe 2 O 3 as a promising candidate to realize efficient SOT switching.

Antiferromagnetism↗
DOE OSTI2020

Evolution of noncollinear magnetism in magnetocaloric MnPtGa

MnPtGa crystallizes in the hexagonal Ni 2 In structure type in space group P6 3 /mmc and has been reported to display a ferromagnetic Curie temperature near 220K. Here we find a transition near T c = 236 K to a ferromagnetic state, albeit with a reduced moment from what is expected for collinear ordering. The peak magnetocaloric entropy change was determined to be ΔS M = –1.9 J kg –1 K –1 for an applied magnetic field of H = 5 T at the ferromagnetic ordering temperature. Magnetostructural coupling manifests as a change in the slope of the thermal expansion coefficients of the c lattice parameter near T c , with a negative spontaneous volume magnetostriction; ω = –300 ppm at 190K. In this work, neutron powder diffraction studies of the magnetic ground state reveal an evolution in complexity as temperature decreases: from a ferromagnet, to a canted antiferromagnet, to the eventual formation of a spin-density-wave state at low temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Complex magnetic properties in the mixed 4 f - 5 d double perovskite iridates X 2 ZnIrO 6 ( X = Nd , Sm , Eu , Gd )

Here in this paper, we report on the synthesis and magnetic properties of a series of double perovskites Ln 2 ZnIrO 6 with Ln = Nd, Sm, Eu, Gd. These compounds present examples of the rare case of double perovskites (general formula A 2 BB'O6) with a magnetic 4ƒ-ion on the A site in combination with the strongly spin-orbit coupled 5d transition-metal ion Ir 4+ on the B sublattice. We discuss the impact of different rare earths on the macroscopic magnetic properties. Gd 2 ZnIrO 6 and Eu 2 ZnIrO 6 show ferrimagnetic or canted antiferromagnetic order below T N = 23 and 12 K, respectively. Sm 2 ZnIrO 6 orders antiferromagnetically at T N = 13 K. Nd 2 ZnIrO 6 exhibits more complex magnetic properties with a strong field dependence ranging from a two-step spin reorientation at μ 0 H = 0.01 T to an antiferromagnetic ground state at intermediate external fields to a spin-flop phase for 0H ≥ 4 T. This unique behavior suggests an interesting interplay between Nd 3+ and Ir 4+ . To further shed light on the relevant magnetic interaction, the zero-field magnetic ground state of Nd 2 ZnIrO 6 is examined via neutron powder diffraction. In general, the magnetic properties of Ln 2 ZnIrO 6 hint towards a substantial contribution of the rare-earth ions and a significant correlation between 5d and 4ƒ magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetic order and spin liquid behavior in [Mo 3 ] 11+ molecular magnets

Molecular magnets based on [Mo 3 ] 11+ units with one unpaired electron per trimer have attracted recent interest due to the identification of quantum spin liquid candidacy in some family members. Here, we present comprehensive measurements on polycrystalline samples of ZnScMo 3 O 8 , MgScMo 3 O 8 , and Na 3 Sc 2 Mo 5 O 16 with the same Mo 3 O 13 magnetic building blocks. In this work, the crystal structures are characterized with x-ray or neutron powder diffraction and the magnetic ground states are determined by performing ac and dc susceptibility, specific heat, neutron powder diffraction, and muon spin relaxation measurements. Our work indicates that ZnScMo 3 O 8 and MgScMo 3 O 8 have ferromagnetic Curie-Weiss temperatures of 18.5 and 11.9 K, ordered ground states with net moments (low-moment ferromagnetism or canted antiferromagnetism), and zero field ordering temperatures of $T_c$ = 6 K and $T_c$ < 2 K, respectively. On the other hand, Na 3 Sc 2 Mo 5 O 16 hosts a dynamical magnetic ground state with no evidence for magnetic ordering or spin freezing down to 20 mK despite an antiferromagnetic Curie-Weiss temperature of –36.2 K, and therefore is a candidate for quantum spin liquid behavior. By comparing the present results to past work on the same family of materials, we construct a phase diagram which illustrates that the magnetic ground states of these Mo-based molecular magnets are very sensitive to small changes in the nearest-neighbor Mo-Mo distance.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Stripe helical magnetism and two regimes of anomalous Hall effect in NdAlGe

Here, we report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate spin density wave (T inc =6.8 K) whose spins are predominantly pointing along the out-of-plane direction and have a small helical spin canting of 3°. The spin density wave has a long wavelength of ≈35 nm and transitions to a commensurate ferrimagnetic state below T com =5.1K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are, respectively, governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our paper suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAlX, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Current-induced switching of thin film α - Fe 2 O 3 devices imaged using a scanning single-spin microscope

Electrical switching of Néel order in an antiferromagnetic insulator is desirable as a basis for memory applications. Unlike electrically driven switching of ferromagnetic order via spin-orbit torques, electrical switching of antiferromagnetic order remains poorly understood. Here we investigate the low-field magnetic properties of 30-nm-thick, c-axis-oriented α-Fe 2 ⁢O 3 Hall devices using a diamond nitrogen-vacancy center scanning microscope. Using the canted moment of α-Fe 2⁢ O 3 as a magnetic handle on its Néel vector, we apply a saturating in-plane magnetic field to create a known initial state before letting the state relax in low field for magnetic imaging. Here, we repeat this procedure for different in-plane orientations of the initialization field. We find that the magnetic field images are characterized by stronger magnetic textures for fields along [$\overline{11}$20] and 11$\overline{2}$0, suggesting that despite the expected 3-fold magnetocrystalline anisotropy, our α-Fe 2 ⁢O 3 thin films have an overall in-plane uniaxial anisotropy. We also study current-induced switching of the magnetic order in α-Fe 2 ⁢O 3 . We find that the fraction of the device that switches depends on the current pulse duration, amplitude, and direction relative to the initialization field.

36 MATERIALS SCIENCE↗