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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Strong surface magnetic field generation in relativistic short pulse laser–plasma interaction with an applied seed magnetic field

Abstract While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field B seed can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, B gen ∼ −2 πB seed Δ x / λ 0 (in the large spot size limit), where Δ x is the target thickness and λ 0 is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a μ m-thick laser-irradiated target with a kilotesla-level seed magnetic field.

Weichman, K. (ORCID:0000000234877922)↗
DOE OSTI2016

Magnetic surface domain imaging of uncapped epitaxial FeRh(001) thin films across the temperature-induced metamagnetic transition

The surface magnetic domain structure of uncapped epitaxial FeRh/MgO(001) thin films was imaged by in-situ scanning electron microscopy with polarization analysis (SEMPA) at various temperatures between 122 and 450 K. This temperature range covers the temperature-driven antiferromagnetic-to-ferromagnetic phase transition in the body of the films that was observed in-situ by means of the more depth-sensitive magneto-optical Kerr effect. The SEMPA images confirm that the interfacial ferromagnetism coexisting with the antiferromagnetic phase inside the film is an intrinsic property of the FeRh(001) surface. Furthermore, the SEMPA data display a reduction of the in-plane magnetization occuring well above the phase transition temperature which, thus, is not related to the volume expansion at the phase transition. This observation is interpreted as a spin reorientation of the surface magnetization for which we propose a possible mechanism based on temperature-dependent tetragonal distortion due to different thermal expansion coefficients of MgO and FeRh.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Transport and Assembly of Magnetic Surface Rotors**

This minireview focuses on recent advances with surface magnetic rotors, namely field-responsive spherical or anisotropic microparticles that translate close to, or are embedded in a confining surface. The application of external magnetic modulations allows these microscopic wheels to be remotely spun and steered while also tuning their interactions and inducing assembly from a collection of disordered, moving units. With optical microscopy one can observe and characterize the complex collective phenomena that emerge in dissipative colloidal systems driven far from equilibrium by external fields. From a technological point of view, magnetic surface rotors envisage implementation into microfluidic devices, and can be used for drug delivery, mixing as well as a model system for biological active matter.

36 MATERIALS SCIENCE↗
NASA NTRS1979

Constraints on the origins of lunar magnetism from electron reflection measurements of surface magnetic fields

The paper describes a new method of detecting lunar surface magnetic fields, summarizes electron reflection measurements and correlations of surface field anomalies to moon geologic features, and discusses the constraints on the origin of lunar magnetism. Apollo 15 and 16 measurements of lunar surface magnetic fields by the electron reflection method show patches of strong surface fields distributed over the lunar surface, and a positive statistical correlation is found in lunar mare regions between the surface field strength and the geologic age of the surface. However, there is a lack of correlation of surface field with impact craters indicating that the mare does not have a strong large-scale uniform magnetization as may be expected from an ancient lunar dynamo. Fields were found in lunar highlands which imply that the rille has a strong magnetization associated with it as intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock.

Lin, R. P.↗
DOE OSTI2017

Local synthesis of 3D magnetic surfaces

Local synthesis of nested 3D toroidal magnetic surfaces is carried out on the basis of the general theory of surfaces by using magnetic coordinates (generally unknown a priori). An equilibrium magnetic surface is calculated by specifying two functions on the surface (the absolute value of the magnetic field and the distance to the nearest magnetic surface) and three parameters (the rotational transform, pressure gradient, and poloidal current). The choice of the parameters is restricted by the requirement that the surface should be closed toroidally. A method of synthesis of a closed magnetic surface is proposed when two functions—the absolute value of the magnetic field and the major radius—are specified. A set of harmonics of a new type of poloidally preudosymmetric configuration (a toroidal mirror with a large mirror ratio and small rotational transform) is obtained.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2003

Utility of Satellite Magnetic Observations for Estimating Near-Surface Magnetic Anomalies

Regional to continental scale magnetic anomaly maps are becoming increasingly available from airborne, shipborne, and terrestrial surveys. Satellite data are commonly considered to fill the coverage gaps in regional compilations of these near-surface surveys. For the near-surface Antarctic magnetic anomaly map being produced by the Antarctic Digital Magnetic Anomaly Project (ADMAP), we show that near-surface magnetic anomaly estimation is greatly enhanced by the joint inversion of the near-surface data with the satellite observations relative to the conventional technique such as minimum curvature. Orsted observations are especially advantageous relative to the Magsat data that have order-of-magnitude greater measurement errors, albeit at much lower orbital altitudes. CHAMP is observing the geomagnetic field with the same measurement accuracy as the Orsted mission, but at the lower orbital altitudes covered by Magsat. Hence, additional significant improvement in predicting near-surface magnetic anomalies can result as these CHAMP data are available. Our analysis also suggests that considerable new insights on the magnetic properties of the lithosphere may be revealed by a further order-of-magnitude improvement in the accuracy of the magnetometer measurements at minimum orbital altitude.

Kim, Hyung Rae↗
DOE OSTI2022

Entangled core/shell magnetic structure driven by surface magnetic symmetry-breaking in Cr 2 O 3 nanoparticles

Bulk Cr 2 O 3 is an antiferromagnetic (AFM) oxide that exhibits the magnetoelectric effect at room temperature, with neither spontaneous magnetization nor net electric polarization. These physical properties stem from a subtle competition between exchange and crystal field interactions. In this article, we exploit the symmetry breaking at the surface of Cr 2 O 3 nanoparticles for unbalancing this delicate physical equilibrium. The emerging weak ferromagnetic signal we observe persists up to near room temperature (≈ 270 K) at which the antiferromagnetic order disappears. In addition, an exchange-bias effect, that rapidly decreases on heating from low temperature up to 30 K, is resistant to thermal disorder above 200 K. In conclusion, our findings point to the possible formation of an entangled core/shell magnetic structure, where pinned uncompensated spins at the shell are randomly distributed in a low-temperature spin-glass ordering, with low net magnetic moment and an ordering temperature governed by the AFM Néel temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

The rapid destruction of toroidal magnetic surfaces

The operation of ITER will require reliable simulations in order to avoid major damage to the device from disruptions. Disruptions are the sudden breakup of magnetic surfaces across the plasma volume—a fast magnetic reconnection. This reconnection can be caused by the growth of perturbations outside of the plasma core causing an ideal perturbation to the core. This causes an increasing ratio of the maximum to the minimum separation, Δmax/Δmin, between neighboring magnetic surfaces. Magnetic reconnection becomes a dominant process when magnetic field lines can quickly interchange connections over a spatial scale ar. This occurs when Δmax/Δmin≳ar/Δd, where Δd is the scale over which non-ideal effects make magnetic field lines indistinguishable. Traditional reconnection theory is fundamentally different. It is a study of the steady-state cancelation of oppositely directed magnetic field components across a thin layer. During more than sixty years, mathematical implications of Faraday's law have been derived that clarify and constrain the physics of fast magnetic reconnection. These are reviewed because they are not commonly known but are needed to understand and to place in context how an ideal magnetic evolution can cause reconnection to quickly become a dominant process no matter how small Δd/ar may be.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Surface magnetism in Fe 3 GeTe 2 van der Waals ferromagnet

The surface magnetization of Fe 3 GeTe 2 was examined by low-energy electron microscopy (LEEM) using an off-normal incidence electron beam. We found that the 180o domain walls are of Bloch type. Temperature-dependent LEEM measurements yield a surface magnetization with a surface critical exponent β1 = 0.79±0.02. This result is consistent with surface magnetism in the 3D semi-infinite Heisenberg (β1 = 0.84±0.01) or Ising (β1 = 0.78±0.02) models, which is distinctly different from the bulk exponent (β = 0.34±0.07). Further, the measurements reveal the power of LEEM with a tilted beam to determine magnetic domain structure in quantum materials, without need for the use of spin-polarized electrons. Single crystal diffraction measurements reveal inversion symmetry-breaking weak peaks and yield space group P-6m2. This Fe site defect-derived loss of inversion symmetry enables the formation of skyrmions in this Fe 3 GeTe 2 crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effects of solar surface magnetic fields on the time–distance analysis of solar subsurface meridional flows

The solar meridional flow in the deep convection zone can be measured with time–distance analysis. The difference between northward and southward acoustic wave travel times relates to the speed of meridional flow in the solar convection zone. We study the effects of surface magnetic field on the measured travel time difference in time–distance analysis by comparing the results using data with and without removing surface magnetic regions. Two results are significantly different if the field strength threshold used to remove magnetic regions is small enough, such as 50 G. The difference represents the surface magnetic effects. This difference strongly correlates with the sunspot number. The range of travel distance in this study is 7°–75°, corresponding to a depth range of 0.54−0.96 R{sub ⊙}. The difference depends on the travel time distance, and is greatest for the travel distance of 11{sup ∘}−20{sup ∘}. The study with different field strength thresholds indicates that a threshold of about 50 G can remove most surface magnetic effects. The measured surface magnetic effects can be explained by an effective downflow inside magnetic regions. These signals are not related to the large-scale meridional flow; they need to be considered in the measured travel time difference even when activity is low, if the travel time difference is used to infer meridional flow signals.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Surface magnetic field strengths: New tests of magnetoconvective models of M dwarfs

Precision modeling of M dwarfs has become worthwhile in recent years due to the increasingly precise values of masses and radii which can be obtained from eclipsing binary studies. In a recent paper, Torres has identified four prime M dwarf pairs with the most precise empirical determinations of masses and radii. The measured radii are consistently larger than standard stellar models predict by several percent. These four systems potentially provide the most challenging tests of precision evolutionary models of cool dwarfs at the present time. We have previously modeled M dwarfs in the context of a criterion due to Gough and Tayler in which magnetic fields inhibit the onset of convection according to a physics-based prescription. In the present paper, we apply our magnetoconvective approach to the four prime systems in the Torres list. Going a step beyond what we have already modeled in CM Dra (one of the four Torres systems), we note that new constraints on magnetoconvective models of M dwarfs are now available from empirical estimates of magnetic field strengths on the surfaces of these stars. In the present paper, we consider how well our magnetoconvective models succeed when confronted with this new test of surface magnetic field strengths. Among the systems listed by Torres, we find that plausible magnetic models work well for CM Dra, YY Gem, and CU Cnc. (The fourth system in Torres's list does not yet have enough information to warrant magnetic modeling.) Our magnetoconvection models of CM Dra, YY Gem, and CU Cnc yield predictions of the magnetic fluxes on the stellar surface which are consistent with the observed correlation between magnetic flux and X-ray luminosity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Topological surface magnetism and Néel vector control in a magnetoelectric antiferromagnet

Antiferromagnetic states with no stray magnetic fields can enable high-density ultra-fast spintronic technologies. However, the detection and control of antiferromagnetic Néel vectors remain challenging. Linear magnetoelectric antiferromagnets (LMAs) may provide new pathways, but applying simultaneous electric and magnetic fields, necessary to control Néel vectors in LMAs, is cumbersome and impractical for most applications. Herein, we show that Cr 2 O 3 , a prototypical room-temperature LMA, carries a topologically-protected surface magnetism in all surfaces, which stems from intrinsic surface electric fields due to band bending, combined with the bulk linear magnetoelectricity. Consequently, bulk Néel vectors with zero bulk magnetization can be simply tuned by magnetic fields through controlling the magnetizations associated with the surface magnetism. Our results imply that the surface magnetizations discovered in Cr 2 O 3 should be also present in all LMAs.

36 MATERIALS SCIENCE↗
DOE OSTI2015

CONSTRAINTS ON THE SURFACE MAGNETIC FIELDS AND AGE OF A COOL HYPERGIANT: XMM-NEWTON X-RAY OBSERVATIONS OF VY CMa

The complex circumstellar ejecta of highly evolved, cool hypergiants are indicative of multiple, asymmetric mass-loss events. To explore whether such episodic, non-isotropic mass loss may be driven by surface magnetic activity, we have observed the archetypical cool hypergiant VY CMa with the XMM-Newton X-ray satellite observatory. The hypergiant itself is not detected in these observations. From the upper limit on the X-ray flux from VY CMa at the time of our observations (F {sub X,} {sub UL} ≈ 8 × 10{sup –14} erg cm{sup –2} s{sup –1}, corresponding to log L{sub X} /L {sub bol} ≤ –8), we estimate an average surface magnetic field strength fB ≤ 2 × 10{sup –3} G (where f is the filling factor of magnetically active surface regions). These X-ray results for VY CMa represent the most stringent constraints to date on the magnetic field strength near the surface of a hypergiant. VY CMa's mass loss is episodic, however, and the hypergiant may have been in a state of low surface magnetic activity during the XMM observations. The XMM observations also yield detections of more than 100 X-ray sources within ∼15' of VY CMa, roughly 50 of which have near-infrared counterparts. Analysis of X-ray hardness ratios and IR colors indicates that some of these field sources may be young, late-type stars associated with VY CMa, its adjacent molecular cloud complex, and the young cluster NGC 2362. Further study of the VY CMa field is warranted, given the potential to ascertain the evolutionary timescale of this enigmatic, massive star.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1988

Lunar surface magnetic field concentrations antipodal to young large impact basins

Planetary electron reflection magnetometry data of the Apollo 15 and 16 subsatellites indicate the presence of major regions of strong surface magnetic fields near the antipodes of four large, young, ringed impact basins. While mechanisms yielding surface magnetic field concentrations antipodal to impact basins are not yet clear, the present results' indicated period of strong lunar magnetic fields, at between about 3.85 and 3.6 aeons, is consistent with lunar paleomagnetic sample data. The origin of the strong surface magnetic fields is discussed.

Lin, R. P.↗
NASA NTRS1992

Constraints on the surface magnetic fields of hot stars with winds

The present study discusses several constraints on the surface magnetic fields of rotating stars with winds. It is shown that there are two allowed ranges for the strengths of surface radial magnetic fields, which are called the 'strong field' and 'weak field' ranges. Attention is given to rotating hot stars with winds and weaker surface magnetic fields. Constraints on rotation and magnetic fields are derived in relation to the extreme case when the equatorial rotation speed approaches the critical rotation. For the O-type main-sequence star 9 Sgr and the B supergiant star Zeta-1 Sco, the upper bound for a weak field is found to be of order 1 G, which is consistent with recent interpretations of radio observations. For a Wolf-Rayet star, with stellar parameters similar to those of CV Ser, it is found to be about 20 G, at faster rotation speeds. From the requirement of hydrostatic equilibrium in the interior, a condition is derived for a star to be an 'extreme magnetic rotator', which is similar to the Eddington limit for stars with radiative forces.

Maheswaran, Murugesapillai↗
DOE OSTI2018

Fracture Strength and Surface Magnetic Flux Uniformity in an Isotropic Polymer-Bonded Ring-Shaped Nd-Fe-B Magnet

Ring-shaped (RS) polymer-bonded magnets were produced from a mixture of an isotropic nanocrystalline Nd-Fe-B powder with a nominal composition of 59.8 wt.% Fe, 29 wt.% Nd, 5 wt.% Zr, 4.8 wt.% Co and 1.4 wt.% B and a variety of epoxy resins as binders, using a compaction-molding technique. The morphology and average particle size of the powders are determined by an SEM. The magnetic properties of the magnets were measured using a permeameter. Magnetic flux on the surface of the magnets was determined by a Gauss meter. The mechanical properties of the RS specimens were determined using a tensile fracture strength (FS) test. The effects of polymer type and amount, hardener amount, applied pressing pressure and curing temperature and time on FS were investigated. The experimental results showed that at optimal conditions of 8 wt.% solid epoxy, 3.5 wt.% solid hardener, a pressing pressure of 900 MPa and a curing time of 8 h at 170 °C, a maximum tensile strength of ~ 36 MPa was achieved. Surface magnetic flux uniformity in optimal mechanical conditions was also found to be optimum at around ± 40 G, which is one of the best flux uniformities in bonded magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Direct computation of magnetic surfaces in Boozer coordinates and coil optimization for quasisymmetry

We propose a new method to compute magnetic surfaces that are parametrized in Boozer coordinates for vacuum magnetic fields. We also propose a measure for quasisymmetry on the computed surfaces and use it to design coils that generate a magnetic field that is quasisymmetric on those surfaces. The rotational transform of the field and complexity measures for the coils are also controlled in the design problem. Using an adjoint approach, we are able to obtain analytic derivatives for this optimization problem, yielding an efficient gradient-based algorithm. Starting from an initial coil set that presents nested magnetic surfaces for a large fraction of the volume, our method converges rapidly to coil systems generating fields with excellent quasisymmetry and low particle losses. In particular for low complexity coils, we are able to significantly improve the performance compared with coils obtained from the standard two-stage approach, e.g. reduce losses of fusion-produced alpha particles born at half-radius from $17.7\,\%$ to $6.6\,\%$ . We also demonstrate 16-coil configurations with alpha loss ${<}1\,\%$ and neoclassical transport magnitude $\epsilon _{\text {eff}}^{3/2}$ less than approximately $5\times 10^{-9}$ .

Physics↗