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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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DOE OSTI2022

Magnetized ICF implosions: Scaling of temperature and yield enhancement

This work investigates the impact of an applied magnetic field on the yield and hot-spot temperature of inertial confinement fusion implosions. A scaling of temperature amplification due to magnetization is shown to be in agreement with unperturbed two-dimensional (2D) extended-magnetohydrodynamic simulations. A perfectly spherical hot-spot with an axial magnetic field is predicted to have a maximum temperature amplification of 37%. However, elongation of the hot-spot along field lines raises this value by decreasing the hot-spot surface area along magnetic field lines. A scaling for yield amplification predicts that a magnetic field has the greatest benefit for low-temperature implosions; this is in agreement with simplified 1D simulations, but not 2D simulations where the hot-spot pressure can be significantly reduced by heat-flow anisotropy. Simulations including a P2 drive asymmetry then show that the magnetized yield is a maximum when the capsule drive corrects the hot-spot shape to be round at neutron bang time. An applied magnetic field is also found to be most beneficial for implosions that are more highly perturbed, exceeding the theoretical yield enhancement for symmetric hot-spots. Increasing the magnetic field strength past the value required to magnetize the electrons is beneficial due to the additional suppression of perturbations by magnetic tension.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Controlling polarization of spintronic THz emitter by remanent magnetization texture

Terahertz (THz) sciences and technologies have contributed to a rapid development of a wide range of applications and expanded the frontiers in fundamental science. Spintronic terahertz emitters offer conceptual advantages since the spin orientation in the magnetic layer can be easily controlled either by the externally applied magnetic field or by the internal magnetic field distribution determined by the specific shape of the magnetic elements. Here, we report a switchable terahertz source based on micropatterned magnetic heterostructures driven by femtosecond laser pulses. We show that the precise tunability of the polarization state is facilitated by the underlying magnetization texture of the magnetic layer that is dictated by the shape of the microstructure. These results also reveal the underlying physical mechanisms of a nonuniform magnetization state on the generation of ultrafast spin currents in the magnetic heterostructures. Our findings indicate that the emission of the linearly polarized THz waves can be switched on and off by saturating the sample using a biasing magnetic field, opening fascinating perspectives for integrated on-chip THz devices with wide-ranging potential applications.

Wu, Weipeng↗
DOE OSTI2024

Investigating instabilities in magnetized low-pressure capacitively coupled RF plasma using particle-in-cell (PIC) simulations

The effect of a uniform magnetic field on particle transport in low-pressure radio frequency (RF) capacitively coupled plasma (CCP) has been studied using a particle-in-cell model. Three distinct regimes of plasma behavior can be identified as a function of the magnetic field. In the first regime at low magnetic fields, asymmetric plasma profiles are observed within the CCP chamber due to the effect of $\overrightarrow{E}$ × $\overrightarrow{B}$ drift. As the magnetic field increases, instabilities develop and form self-organized spoke-shaped structures that are distinctly seen within the bulk plasma closer to the sheath. In this second regime, the spoke-shaped coherent structures rotate inside the plasma chamber in the -$\overrightarrow{E}$ × $\overrightarrow{B}$ direction, where $\overrightarrow{E}$ × $\overrightarrow{B}$ are the DC electric and magnetic field vectors, respectively, and the DC electric field exists in the sheath and pre-sheath regions. The spoke rotation frequency is in the megahertz range. As the magnetic field strength increases further, the rotating coherent spokes continue to exist near the sheath. The coherent structures are, however, accompanied by new small-scale incoherent structures originating and moving within the bulk plasma region away from the sheath. This is the third regime of plasma behavior. The threshold values of the magnetic field between these regimes were found not to vary with changing plasma reactor geometry (e.g., area ratio between ground and powered electrodes) or the use of an external capacitor between the RF-powered electrode and the RF source. The threshold values of the magnetic field between these regimes shift toward higher values with increasing gas pressure. Analysis of the results indicates that the rotating structures are due to the lower hybrid instability driven by density gradients and electron-neutral collisions. This paper provides guidance on the upper limit of the magnetic field for instability-free operation in low-pressure CCP-based semiconductor deposition and etch systems that use the external magnetic field for plasma uniformity control.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

A magnetic analog of pressure–strain interaction

We study the evolution equation for magnetic energy density for a non-relativistic magnetized plasma in the (Lagrangian) reference frame comoving with the electron bulk velocity. Analyzing the terms that arise due to the ideal electric field, namely, perpendicular electron compression and magnetic field line bending, we recast them to reveal a quantity with a functional form analogous to the often-studied pressure–strain interaction term that describes one piece of internal energy density evolution of the species in a plasma, except with the species pressure tensor replaced by the magnetic stress tensor. We dub it the “magnetic stress–strain interaction.” We discuss decompositions of the magnetic stress–strain interaction analogous to those used for pressure–strain interaction. These analogies facilitate the interpretation of the evolution of the various forms of energy in magnetized plasmas and should be useful for a wide array of applications, including magnetic reconnection, turbulence, collisionless shocks, and wave–particle interactions. We display and analyze all the terms that can change magnetic energy density in the Lagrangian reference frame of the electrons using a particle-in-cell simulation of magnetic reconnection.

Barbhuiya, M. Hasan (ORCID:0000000163301650)↗
DOE OSTI2019

A 1.2 T canted cos $θ$ dipole magnet using high-temperature superconducting CORC ® wires

In this study, REBa 2 Cu 3 O x (REBCO, RE = rare earth elements) coated conductors can carry high current in high background fields, in principle enabling dipole magnetic fields beyond 20 T. Although model accelerator magnets wound with single REBCO tapes have been successfully demonstrated, magnet technology based on high-current REBCO cables for high-field accelerator magnet applications has yet to be established. We developed a two-layer canted cosθ dipole magnet with an aperture of 70 mm using 30 m long commercial Conductor on Round Core (CORC ® ) wires. The 3.1 mm diameter CORC ® wire contained 16 commercial REBCO tapes with a 30 $μ$m thick substrate. The magnet was tested at 77 and 4.2 K. It generated a peak dipole field of 1.2 T with 4.8 kA at 4.2 K with neither thermal runaway nor training. Reasonable geometric field quality and strong magnetization-current effects with multipole decay were observed. Our work demonstrated a feasible high-temperature superconducting magnet technology as a first step toward a new accelerator magnet paradigm that will enable high-field inserts for next-generation circular colliders and stand-alone magnets that can operate over a wide temperature range for a broad range of applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Distributed fiber-optic sensing in a subscale high-temperature superconducting dipole magnet

High-temperature superconductors, such as REBa2Cu3O7−x (REBCO, RE = rare earth), are becoming pivotal for high-field magnet technology for future circular colliders and compact fusion reactors. The U.S. Magnet Development Program, in collaboration with industry, is developing REBCO magnet technology using round conductors consisting of multiple REBCO tapes. For these multi-tape cables, traditional instrumentation, such as voltage taps and resistive strain gauges, become insufficient to help measure and understand the performance-limiting factors in these model magnets. Distributed fiber-optic sensing (DFOS) is a potential solution to address this challenge. Although DFOS is well established for various applications, measuring temperature and strain in high-temperature superconducting magnets is in its infancy. Here we report the detailed implementation and test results of DFOS based on Rayleigh scattering in a subscale canted cosθ (CCT) dipole magnet using high-temperature superconducting CORC® wires. We co-wound optical fibers in each layer of the CCT magnet and compared different types of commercial fibers and mold-release agents to reduce the power attenuation in the fibers. The DFOS allowed us to measure mechanical deformation and temperature along the conductor during tests at 77 and 4.2 K. The measured strain agreed quantitively with a finite-element mechanical model of the subscale magnet. Our results indicate that DFOS can effectively identify locations of strain and temperature changes, offering unique insight into magnet performance that can advance our understanding and development of the REBCO magnet technology for high-energy physics and fusion applications.

Luo, Linqing↗
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↗
DOE OSTI2025

Magnetic order and physical properties of the kagome metal UNb 6 ⁢Sn 6

The 𝑅⁢𝑀 6 ⁢𝑋 6 family of materials (𝑅 = rare-earth, 𝑀 = transition metal, 𝑋 = Ga, Si, Ge, Sn) produces an array of emergent phenomena, such as charge density waves, intrinsic Hall effects, and complex magnetic order, due to its kagome net of transition metal atoms, its local-moment magnetic anisotropies, and its extensive chemical tunability. Here, in this study, we report UNb 6⁢ Sn 6 , a new “166” material containing both an actinide and a 4⁢𝑑 transition metal, along with its nonmagnetic analog ThNb6⁢Sn6, to investigate the properties of a 5⁢𝑓−4⁢𝑑 electron 166 system. UNb 6 ⁢Sn 6 crystallizes in the hexagonal 𝑃⁢6/𝑚⁢𝑚⁢𝑚 space group with a small degree of disorder due to shifts in the size of the CoSn-like cages along the 𝑐 axis. Upon cooling at zero magnetic field, the material undergoes two magnetic phase transitions at 𝑇 2 = 46K and 𝑇 N = 43K. The low-temperature, zero-field phase is an antiferromagnet with ordered uranium moments and a k = (0,0,1/2) propagation vector determined by neutron diffraction. Remarkably, with a magnetic field applied along the 𝑐 axis, five additional magnetic transitions occur, evidenced by magnetization and resistivity data, before the moment saturates at 2.62 𝜇 B /U at 2 K and ≥ 13.6T. In two magnetic phase regions, the Hall resistivity of UNb 6⁢ Sn 6 significantly deviates from the magnetization, suggesting that the phases have a large Berry curvature or a change in the Fermi surface. The unknown magnetic ordering of the field-dependent phases of UNb 6⁢ Sn 6 demonstrates the complexity of the 5⁢𝑓−4⁢𝑑 166 system and encourages further study of its properties.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Non-Fermi-liquid types of behavior associated with a magnetic quantum critical point in Sr ( Co 1 - x Ni x ) 2 As 2 single crystals

The compound SrCo 2 As 2 with the body-centered tetragonal ThCr 2 Si 2 structure is known to remain paramagnetic down to a temperature T = 0.05 K, but inelastic neutron scattering studies have shown that both ferromagnetic (FM) and antiferromagnetic (AFM) fluctuations occur in single crystals. Thus it is of interest to study how the magnetism evolves on doping SrCo 2 As 2 . Previous work on polycrystalline samples of Sr (Co 1-x Ni x ) 2 As 2 indicated the development of AFM order for 0 < x ≲ 0.3. Here we studied single crystals of Sr(Co 1-x Ni x ) 2 As 2 for 0 < x ≤ 1 and confirmed the occurrence of AFM order which we deduce to have a c -axis helix structure. We also find evidence for an unusual composition-induced magnetic quantum critical point at x ≈ 0.3 where non-Fermi-liquid types of behavior were revealed by heat capacity and electrical resisitivity measurements at low T. Electron-doped Sr (Co 1-x Ni x ) 2 As 2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi 2 As 2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD) at room temperature, and magnetic susceptibility χ (H, T), isothermal magnetization M (H, T), heat capacity Cp (H, T), and electrical resistivity ρ (H, T) measurements versus applied magnetic field H and T. The XRD studies show that the system undergoes a continuous structural crossover from the uncollapsed-tetragonal (ucT) structure to the collapsed tetragonal (cT) structure with increasing Ni doping. The χ(T) data show that SrCo 2 As 2 exhibits an AFM ground state almost immediately upon Ni doping on the Co site. Ab initio electronic-structure calculations for x = 0 and 0.15 indicate that a flat band with a peak in the density of states just above the Fermi energy is responsible for this initial magnetic-ordering behavior on Ni doping. The AFM ordering is observed in the range 0.013 ≤ x ≤ 0.25 with the ordered moments aligned in the ab plane and with a maximum ordering temperature T N = 26.5 K at x = 0.10. The Curie-Weiss-like T dependence of χ in the paramagnetic (PM) state indicates dominant FM interactions. The behavior of the anisotropic susceptibilities below T N suggests a planar helical magnetic ground state with a composition-dependent pitch based on a local-moment molecular-field-theory model, with FM interactions in the ab plane and weaker AFM interactions along the helix c axis. However, the small ordered (saturation) moments ~0.1 μ B per transition metal atom, where μ B is the Bohr magneton, and the values of the Rhodes-Wohlfarth ratio indicate that the magnetism is itinerant. The high-field M (H) isotherms and the low-field χ-1 ( T > T N ) data were successfully analyzed within the framework of Takahashi's theory of FM spin fluctuations. The C p (T) at low T exhibits Fermi-liquid behavior for 0 ≤ x ≤ 0.15 , whereas an evolution to a logarithmic non-Fermi-liquid (NFL) behavior is found for x = 0.2 to 0.3. The logarithmic dependence is suppressed in an applied magnetic field. The low- T ρ ( H = 0 , T ) data show a T 2 dependence for 0 ≤ x ≤ 0.20 and a power-law dependence ρ (H = 0, T) = ρ 0 + AT n with n < 2 for x = 0.20 and 0.30. The exponent n shows a notable field dependence, suggesting both doping- and magnetic-field-tuned quantum critical phenomena. These low- T NFL types of behavior observed in the C p and ρ measurements are most evident near the quantum critical concentration x ≈ 0.3 at which a T = 0 composition-induced transition from the AFM phase to the PM phase occurs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Noncollinear magnetic structure and anisotropic magnetoelastic coupling in cobalt pyrovanadate $\mathrm{Co_2V_2O_7}$

$\mathrm{Co_2V_2O_7}$ was recently reported to exhibit remarkable magnetic-field-induced magnetization plateaus and ferroelectricity, but its magnetic ground state remains ambiguous. Magnetometry measurements and time-of-flight neutron powder diffraction (NPD) have been employed to study the structural and magnetic properties of $\mathrm{Co_2V_2O_7}$, which includes two nonequivalent Co sites. Upon cooling below the Néel temperature $T_N = 6.0(2)$ K, we observe magnetic Bragg peaks at 2 K in NPD, which indicates the formation of long-range magnetic order of $\mathrm{Co^{2+}}$ moments. After symmetry analysis and magnetic structure refinement, we demonstrate that $\mathrm{Co_2V_2O_7}$ possesses a complicated noncollinear magnetic ground state with Co moments mainly located in the b-c plane and forming a noncollinear spin-chain-like structure along the c -axis. The ab initio calculations demonstrate that the noncollinear magnetic structure is more stable than various ferromagnetic states at low temperature. The noncollinear magnetic structure with a canted ↑↑↓↓ spin configuration is considered to be the origin of magnetoelectric coupling in$\mathrm{Co_2V_2O_7}$ because the inequivalent exchange striction induced by the spin-exchange interaction between the neighboring spins could be the driving force of ferroelectricity. Finally, it is also found that the deviation of lattice parameters a and b is opposite below $T_N$, while the lattice parameter c and $β$ stay almost constant below $T_N$, evidencing the anisotropic magnetoelastic coupling in $\mathrm{Co_2V_2O_7}$.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Complex magnetic order in the decorated spin-chain system Rb 2 Mn 3 ( MoO 4 ) 3 ( OH ) 2

The macroscopic magnetic properties and microscopic magnetic structure of Rb 2 Mn 3 (MoO 4 ) 3 (OH) 2 (space group Pn ma) are investigated by magnetization, heat capacity, and single-crystal neutron diffraction measurements. The compound's crystal structure contains bond-alternating [Mn 3 O 11 ] ∞ chains along the b axis, formed by isosceles triangles of Mn ions occupying two crystallographically nonequivalent sites (the Mn1 site on the base and Mn2 site on the vertex). These chains are only weakly linked to each other by nonmagnetic oxyanions. Both superconducting quantum interference device magnetometry and neutron diffraction experiments show two successive magnetic transitions as a function of temperature. On cooling, it transitions from a paramagnetic phase into an incommensurate phase below 4.5 K with a magnetic wave vector near k 1 = ( 0 , 0.46 , 0 ). An additional commensurate antiferromagnetically ordered component arises with k 2 = ( 0, 0, 0), forming a complex magnetic structure below 3.5 K with two different propagation vectors of different stars. On further cooling, the incommensurate wave vector undergoes a lock-in transition below 2.3 K. The experimental results suggest that the magnetic superspace group is Pnma. 1' (0b0) s0ss for the single- k incommensurate phase and is Pn' m a ( 0 b 0 ) 00 s for the two- k magnetic phase. We propose a simplified magnetic structure model taking into account the major ordered contributions, where the commensurate k 2 defines the ordering of the c-axis component of the Mn1 magnetic moment, while the incommensurate k 1 describes the ordering of the ab -plane components of both Mn1 and Mn2 moments into elliptical cycloids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Manipulating anomalous Hall antiferromagnets with magnetic fields

The symmetry considerations that imply a nonzero anomalous Hall effect (AHE) in certain noncollinear antiferromagnets also imply both nonzero orbital magnetization and a net spin magnetization. We have explicitly evaluated the orbital magnetizations of several anomalous Hall effect antiferromagnets and find that they tend to dominate over spin magnetizations, especially so when spin-orbit interactions are weak. Because of the greater relative importance of orbital magnetization, the coupling between magnetic order and an external magnetic field is unusual. Finally, we explain how magnetic fields can be used to manipulate magnetic configurations in these systems, pointing in particular to the important role played by the response of orbital magnetization to the Zeeman-like spin exchange fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Suppression of superconducting parameters by correlated quasi-two-dimensional magnetic fluctuations

In this work, we consider a clean layered magnetic superconductor in which a continuous magnetic transition takes place inside a superconducting state. We assume that the exchange interaction between superconducting and magnetic subsystems is weak so that superconductivity is not destroyed at the magnetic transition. A representative example of such material is Rb Eu Fe 4 As 4 . We investigate the suppression of the superconducting gap and superfluid density by correlated magnetic fluctuations in the vicinity of the magnetic transition. The influence of nonuniform exchange field on superconducting parameters is very sensitive to the relation between the magnetic correlation length ξ h and superconducting coherence length ξ s defining the ‘scattering’ ( ξ h < ξ s ) and ‘smooth’ ( ξ h > ξ s ) regimes. As a small uniform exchange field does not affect the superconducting gap and superfluid density at zero temperature, smoothening of the spatial variations of the exchange field reduces its effects on these parameters. We develop a quantitative description of this ‘scattering-to-smooth’ crossover for the case of quasi-two-dimensional magnetic fluctuations realized in Rb Eu Fe 4 As 4 . Since the magnetic-scattering energy scale is comparable with the gap in the crossover region, the standard quasiclassical approximation is not applicable and full microscopic treatment is required. We find that the corrections to both the gap and superfluid density increase proportionally to ξ h until it remains much smaller than ξ s . In the opposite limit, when the correlation length exceeds the coherence length both parameters have much weaker dependence on ξ h . Moreover, the gap correction may decrease with increasing of ξ h in the immediate vicinity of the magnetic transition if it is located at temperature much lower than the superconducting transition. We also find that the crossover between the two regimes is unexpectedly broad: The standard scattering approximation becomes sufficient only when ξ h is substantially smaller than ξ s .

36 MATERIALS SCIENCE↗
DOE OSTI2023

Anisotropic magnetism and electronic structure of trigonal EuAl 2 Ge 2 single crystals

Understanding the interplay between magnetic and electronic degrees of freedom is of profound recent interest in different Eu-based magnetic topological materials. Here, we studied the magnetic and electronic properties of the layered Zintl-phase compound EuAl 2 Ge 2 crystallizing in the trigonal CaAl 2 Si 2 -type structure. We report zero-field neutron diffraction, temperature T - and magnetic-field H-dependent magnetic susceptibility χ(T, H), isothermal magnetization M(T, H), heat capacity C p (T, H), and electrical resistivity ρ(T, H) measurements, together with T-dependent angle-resolved photoemission spectroscopy (ARPES) measurements complemented with first-principle calculations. EuAl 2 Ge 2 undergoes second-order A-type antiferromagnetic (AFM) ordering below T N = 27.5(5) K, with the Eu moments (Eu 2+ , S = 7/2) aligned ferromagnetically in the ab plane while these layers are stacked antiferromagnetically along the c axis. The critical fields at which all moments become parallel to the field are 37.5(5) and 52.5(5) kOe for H ∥ ab and H ∥ c, respectively. The H = 0 magnetic structure consists of trigonal AFM domains associated with ab-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is also evident at T = 2 K below the critical field H c1 = 2.5(1) kOe. The ρ(T) measurements reveal metallic behavior transforming into a slight resistivity increase on cooling towards T N . A pronounced loss of spin-disorder scattering is observed below T N . The ARPES results show that EuAl 2 Ge 2 is metallic both above and below T N , and the Fermi surface is anisotropic with two hole pockets at the zone center and one small electron pocket at each M point. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the c axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below T N . The observed electronic structure is well reproduced by first-principle calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with Z 2 topological numbers 1;(000)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Quantum magnetism in the frustrated square lattice oxyhalides YbBi 2 IO 4 and YbBi 2 ClO 4

Square-lattice systems offer a direct route for realizing two-dimensional (2D) quantum magnetism with frustration induced by competing interactions. In this work, the square lattice materials YbBi 2 IO 4 and YbBi 2 ClO 4 were investigated using a combination of magnetization and specific-heat measurements on polycrystalline samples. Specific-heat measurements provide evidence for long-range magnetic order below $T$ N = 0.21 K (0.25 K) for YbBi 2 IO 4 (YbBi 2 ClO 4 ). On the other hand, a rather broad maximum is found in the temperature-dependent magnetic susceptibility, located at $T$ max = 0.33 K (0.38 K) in YbBi 2 IO 4 (YbBi 2 ClO 4 ), consistent with the quasi-2D magnetism expected for the large separation between the magnetic layers. Estimation of the magnetic entropy supports the expected Kramers' doublet ground state for Yb 3+ and the observed paramagnetic behavior is consistent with a well-isolated doublet. Roughly two-thirds of the entropy is consumed above $T$ N , due to a combination of the quasi-2D behavior and magnetic frustration. The impact of frustration is examined from the viewpoint of a simplified $J$ 1 -$J$ 2 square lattice model, which is frustrated for antiferromagnetic interactions. Specifically, a high-temperature series expansion analysis of the temperature-dependent specific-heat and magnetization data yields $J$ 2 /$J$ 1 = 0.30 (=0.23) for YbBi 2 IO 4 (YbBi 2 ClO 4 ). In conclusion, this simplified analysis suggests strong frustration that should promote significant quantum fluctuations in these compounds, and thus motivates future work on the static and dynamic magnetic properties of these materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Magnetic Eddy Viscosity of Mean Sheared Flows in Two-Dimensional Magnetohydrodynamics

Magnetic induction in magnetohydrodynamic fluids at magnetic Reynolds number (Rm) less than 1 has long been known to cause magnetic drag. Here we show that when Rm $\gg 1$ and the fluid is in a hydrodynamic-dominated regime in which the magnetic energy is much smaller than the kinetic energy, induction due to a mean shear flow leads to a magnetic eddy viscosity. The magnetic viscosity is derived from simple physical arguments, where a coherent response due to shear flow builds up in the magnetic field until decorrelated by turbulent motion. The dynamic viscosity coefficient is approximately $(B^2_p/2_{μ_0}) τ_{corr}$, the poloidal magnetic energy density multiplied by the correlation time. We confirm the magnetic eddy viscosity through numerical simulations of two-dimensional incompressible magnetohydrodynamics. We also consider the three-dimensional case, and in cylindrical or spherical geometry, theoretical considerations similarly point to a nonzero viscosity whenever there is differential rotation. Hence, these results serve as a dynamical generalization of Ferraro's law of isorotation. The magnetic eddy viscosity leads to transport of angular momentum and may be of importance to zonal flows in astrophysical domains such as the interior of some gas giants.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Magnetic behavior and spin-lattice coupling in cleavable van der Waals layered CrCl 3 crystals

CrCl 3 is a layered insulator that undergoes a crystallographic phase transition below room temperature and orders antiferromagnetically at low temperature. Weak van der Waals bonding between the layers and ferromagnetic in-plane magnetic order make it a promising material for obtaining atomically thin magnets and creating van der Waals heterostructures. In this work we have grown crystals of CrCl 3 , revisited the structural and thermodynamic properties of the bulk material, and explored mechanical exfoliation of the crystals. We find two distinct anomalies in the heat capacity at 14 and 17 K confirming that the magnetic order develops in two stages on cooling, with ferromagnetic correlations forming before long-range antiferromagnetic order develops between them. This scenario is supported by magnetization data. A magnetic phase diagram is constructed from the heat capacity and magnetization results. We also find an anomaly in the magnetic susceptibility at the crystallographic phase transition, indicating some coupling between the magnetism and the lattice. First-principles calculations accounting for van der Waals interactions also indicate spin-lattice coupling, and find multiple nearly degenerate crystallographic and magnetic structures consistent with the experimental observations. Lastly, we demonstrate that monolayer and few-layer CrCl 3 specimens can be produced from the bulk crystals by exfoliation, providing a path for the study of heterostructures and magnetism in ultrathin crystals down to the monolayer limit.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Design and Prototyping of a Novel Toroidal Magnet System for MOLLER Experiment at Jefferson Lab

The Thomas Jefferson National Accelerator Facility (JLab) has designed a unique spectrometer system to measure the weak interaction between electrons. The experiment— Measurement of Lepton-Lepton Electroweak Reaction (MOLLER)—requires leveraging the recent 12 GeV electron beam upgrade and will run in JLab for three years. Focusing the signal for the MOLLER experiment requires five water-cooled toroidal magnets, each with unique geometry and with 7-fold symmetry. The five magnets operate in a vacuum and provide the magnetic field required to separate the incident beam electrons scattered from the target electrons (Møller scattering) and protons (elastic e-p scattering) in a liquid hydrogen target. The conceptual design was developed by the MOLLER Collaboration and was given to JLab in the form of amp turns and physical location, with additional physics requirements. This article presents prototyping of the coils and magnet support system and discusses the lessons learned during the process along with the plans for full magnet testing and installation. The JLab Magnet Group along with the MOLLER Collaboration developed the specification document that includes keep out zones to design the set of magnets. JLab contracted the design of the first toroid magnet (TM0) of the magnet system to Massachusetts Institute of Technology. The other four toroid magnets (TM1 through TM4) have been designed by JLab and are in the process of fabrication and assembly. Prototype coils of TM1-TM4 were fabricated by Everson-Tesla Incorporated, PA (USA). Finally, this article presents the unique challenges of the design, alignment, high current density, operating range, high radiation dose, and vacuum environment.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗