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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Effect of adatom deposition on surface magnetism and exchange coupling parameter in (0001) SmCo{sub 5} slabs

First principles calculations are performed on 3d-transition metal atom deposited (0001) surface of SmCo{sub 5} to understand the magnetic properties and the improvement of Curie temperature (T{sub c}). Various atomic sites are examined to identify the energetically feasible adsorption of adatom and it is found that the void site of Co-rich (0001) SmCo{sub 5} surface is the most favourable one to deposit. The surface magnetic moments of various adatom deposited SmCo{sub 5} surfaces are larger than the clean surface except for Cu and Zn. Eventually, the surface exchange coupling of clean and adatom deposited surface is found to increase for Mn, Fe, Co, Ni, and Cu deposited surfaces and this improvement results in the increase in T{sub c} of SmCo{sub 5} slab.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Pressure-induced metallization and loss of surface magnetism in FeSi

Single-crystalline FeSi samples with a conducting surface state were studied under high pressure and magnetic field by means of electrical resistance measurements to explore how the bulk semiconducting state and the surface state are tuned by the application of pressure. Here, we found that the energy gap associated with the semiconducting bulk phase begins to close abruptly at a critical pressure of ~10 GPa and the bulk material becomes metallic with no obvious sign of any emergent phases or non-Fermi liquid behavior in temperature dependent electrical resistance in the neighborhood of the critical pressure above 3 K. Moreover, the metallic phase appears to remain at near-ambient pressure upon release of the pressure. Interestingly, the hysteresis in the electrical resistance vs magnetic field curve associated with the magnetically ordered conducting surface state decreases with pressure and vanishes at the critical pressure, while the slope of the electrical resistance vs magnetic field curve, which has a negative value for pressure below the critical pressure, decreases in magnitude with pressure and changes sign at the critical pressure. Thus the conducting surface state and the corresponding two-dimensional magnetic order collapse at the critical pressure where the energy gap of the bulk material starts to close abruptly, revealing the connection between the conducting surface state and the semiconducting bulk state in FeSi.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Possible surface magnetism in the topological Kondo insulator candidate FeSi

Our group has previously reported the existence of a conducting surface state (CSS) in FeSi, a candidate for a d-electron topological Kondo insulator (TKI), at low temperature. In this paper, we present the electrical transport properties of single crystals of FeSi studied in the phase space of temperature (T), magnetic field (B), and the angle (θ) between the electrical current and B. The normalized T-dependent electrical resistance (R) of a successively thinned FeSi crystal provides further confirmation of the existence of a CSS. We report that, in the CSS, the magnetoresistance (MR) exhibits a hysteresis loop bounded within ±0.5 T, suggesting two-dimensional magnetic ordering. The hysteretic MR is asymmetric in B and anisotropic with respect to θ. Further exploration of R(θ) at a fixed field of 9 T reveals an initial progressive rotation of the axis for twofold rotational symmetry from 2 to 10 K, then a stabilized axis for twofold symmetry until, at T > 40 K, the anisotropy vanishes, coincident with the disappearance of the CSS. Furthermore, these observations point to a possible magnetically ordered surface state that has been reported in similar systems such as FeSi nanofilms and bulk SmB 6 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effects of magnetic drift tangential to magnetic surfaces on neoclassical transport in non-axisymmetric plasmas

In evaluating neoclassical transport by radially local simulations, the magnetic drift tangential to a flux surface is usually ignored in order to keep the phase-space volume conservation. In this paper, effect of the tangential magnetic drift on the local neoclassical transport is investigated. To retain the effect of the tangential magnetic drift in the local treatment of neoclassical transport, a new local formulation for the drift kinetic simulation is developed. The compressibility of the phase-space volume caused by the tangential magnetic drift is regarded as a source term for the drift kinetic equation, which is solved by using a two-weight δf Monte Carlo method for non-Hamiltonian system [G. Hu and J. A. Krommes, Phys. Plasmas 1, 863 (1994)]. It is demonstrated that the effect of the drift is negligible for the neoclassical transport in tokamaks. In non-axisymmetric systems, however, the tangential magnetic drift substantially changes the dependence of the neoclassical transport on the radial electric field E{sub r}. The peaked behavior of the neoclassical radial fluxes around E{sub r }={sub }0 observed in conventional local neoclassical transport simulations is removed by taking the tangential magnetic drift into account.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Surface magnetism and proximity effects in hexaboride thin films

Here, the unexpected ferromagnetism in alkaline hexaborides has attracted extensive research efforts. The exceptionally high Curie temperature sustained by very low density of magnetic moments is yet to be clearly understood. Here, we report the magnetic properties studied in CaB 6 and SrB 6 thin films and their heterostructures with SmB 6 grown by pulsed laser deposition. A coexistence of ferromagnetic and superparamagnetic orders with 2D-type magnetic anisotropy was observed in CaB 6 and SrB 6 . Boron deficiency and closely correlated local oxidization were found at the film surface that contacts air and its interface with the oxide substrate. These results suggest that the magnetism observed is likely generated by two thin surface/interface layers with high concentration of boron vacancies which are energetically stabled by oxygen cation. Additionally, clear suppression of the superconductivity proximity effect in SmB 6 film was observed when SmB 6 was grown on top of the magnetic CaB 6 . Such results demonstrate the interesting perspective of engineering multifunctional heterostructures by coupling the surface magnetic layer with the diversity of other hexaboride materials.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Superconductivity Behavior in Epitaxial TiN Films Points to Surface Magnetic Disorder

In this work we analyze the evolution of the normal and superconducting properties of epitaxial TiN films, characterized by high Ioffe-Regel parameter values, as a function of the film thickness. As the film thickness decreases, we observe an increase of the residual resistivity, that becomes dominated by diffusive surface scattering for d ≤ 20 nm . At the same time, a substantial thickness-dependent reduction of the superconducting critical temperature is observed compared to the bulk TiN value. In such high-quality material films, this effect can be explained by a weak magnetic disorder residing in the surface layer with a characteristic magnetic defect density of approximately 10 12 cm –2 . Our results suggest that surface magnetic disorder is generally present in oxidized TiN films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Adsorption-Induced Surface Magnetism

We report the emergence of adsorption-induced magnetism from heterohelicene molecules on a nonmagnetic Cu(100) surface. Spin-polarized low-energy electron microscopy measurements reveal spin-dependent electron reflectivity for enantiopure 7,12,17-trioxa[11]helicene (TO[11]H) monolayers, indicating the formation of a spin-polarized state localized in the topmost copper layer. Control experiments on clean Cu(100) and TO[11]H on highly oriented pyrolytic graphite show no such effect, excluding artifacts and chirality-induced spin selectivity as origins. Spin-polarized density functional theory calculations with hybrid functionals attribute the magnetism to strong chemisorption, which induces hybridization between the molecular HOMO and copper s- and d-states, driving asymmetric spin-polarized charge redistribution at the interface. An extended Newns-Anderson-Grimley model incorporating on-site Coulomb repulsion in Cu d-orbitals reproduces the emergence of interfacial spin polarization above a threshold interaction strength, highlighting the key roles of hybridization parameters and Coulomb correlation. These findings reveal a mechanism for inducing magnetism at molecule-metal interfaces without inherently magnetic components, offering avenues for engineering spin-polarized states in organic-inorganic hybrid systems.

Hubbard model↗
DOE OSTI2022

Surface magnetic anisotropy-mediated spin Hall magnetoresistance and spin Seebeck effects in a YIG/Pt heterostructure

The role of magnon-phonon coupling in the low-temperature behavior of the spin Seebeck effect (SSE) in YIG/Pt has been puzzling for more than a decade. Here, to elucidate the origin of the anomalous peak around 80 K, we investigate the temperature evolution of SSE, spin Hall magnetoresistance (SMR), and magnetic anisotropy in the same YIG/Pt heterostructure. We find that these effects, along with magnetic damping, show the peaks at the same temperature (~80 K). This simultaneous occurrence, where no heat is applied in the case of SMR, rules out the phonon-magnon drag related origin of SSE in the YIG/Pt system. We further show that the intrinsic surface anisotropy behavior in YIG is responsible for controlling the SSE, SMR, and magnetic damping in the YIG/Pt structure. Our findings not only help to understand these effects fundamentally but also provide an effective way for improving them by manipulating the surface magnetic anisotropy for spin caloritronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The transport characteristics of passing fast ions produced by nonlocal overlapping of drift island surfaces and magnetic island surfaces

The structure of the drift-island surface of passing fast ions (PFIs) is investigated in the presence of the resonant interaction with a magnetic island. Two overlapping regions of the drift-island surface and the magnetic island surface are found, one corresponding to local overlapping region and the other to non-local one. Here, the word “nonlocal” denotes that the resonances in the core plasma can have effects on the PFIs near the plasma boundary, while the “local” represents that the PFIs just near the resonant location are influenced. The nonlocal overlapping constructs a transport path along which the PFIs can become losses. There are three kinds of drift-island surfaces to join in forming the transport paths. A pitch angle region, which is called pitch angle gap, is found near the plasma boundary, where the drift-island surface cannot be formed and few PFIs are lost. The pitch-angle selective features of PFI losses are obtained by analyzing the three kinds of drift-island surfaces. The coupling between the crowd drift island surfaces and the collision can induce the prompt losses of PFIs and rapidly slowing down of PFI energy. The time of the prompt losses and the slowing down rate are calculated. Qualitatively, the theoretical results are in well agreement with the experimental observations in ASDEX Upgrade [M. García-Muñoz et al., Nucl. Fusion 47, L10 (2007)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

THE NONPOTENTIALITY OF CORONAE OF SOLAR ACTIVE REGIONS, THE DYNAMICS OF THE SURFACE MAGNETIC FIELD, AND THE POTENTIAL FOR LARGE FLARES

Flares and eruptions from solar active regions (ARs) are associated with atmospheric electrical currents accompanying distortions of the coronal field away from a lowest-energy potential state. In order to better understand the origin of these currents and their role in M- and X-class flares, I review all AR observations made with Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager and SDO/Atmospheric Imaging Assembly from 2010 May through 2014 October within ≈40° from the disk center. I select the roughly 4% of all regions that display a distinctly nonpotential coronal configuration in loops with a length comparable to the scale of the AR, and all that emit GOES X-class flares. The data for 41 regions confirm, with a single exception, that strong-field, high-gradient polarity inversion lines (SHILs) created during emergence of magnetic flux into, and related displacement within, pre-existing ARs are associated with X-class flares. Obvious nonpotentiality in the AR-scale loops occurs in six of ten selected regions with X-class flares, all with relatively long SHILs along their primary polarity inversion line, or with a long internal filament there. Nonpotentiality can exist in ARs well past the flux-emergence phase, often with reduced or absent flaring. I conclude that the dynamics of the flux involved in the compact SHILs is of pre-eminent importance for the large-flare potential of ARs within the next day, but that their associated currents may not reveal themselves in AR-scale nonpotentiality. In contrast, AR-scale nonpotentiality, which can persist for many days, may inform us about the eruption potential other than those from SHILs which is almost never associated with X-class flaring.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Solar Coronal Loops Associated with Small-scale Mixed Polarity Surface Magnetic Fields

How and where are coronal loops rooted in the solar lower atmosphere? The details of the magnetic environment and its evolution at the footpoints of coronal loops are crucial to understanding the processes of mass and energy supply to the solar corona. To address the above question, we use high-resolution line-of-sight magnetic field data from the Imaging Magnetograph eXperiment instrument on the Sunrise balloon-borne observatory and coronal observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory of an emerging active region. We find that the coronal loops are often rooted at the locations with minor small-scale but persistent opposite-polarity magnetic elements very close to the larger dominant polarity. These opposite-polarity small-scale elements continually interact with the dominant polarity underlying the coronal loop through flux cancellation. At these locations we detect small inverse Y-shaped jets in chromospheric Ca ii H images obtained from the Sunrise Filter Imager during the flux cancellation. Our results indicate that magnetic flux cancellation and reconnection at the base of coronal loops due to mixed polarity fields might be a crucial feature for the supply of mass and energy into the corona.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

High spatial resolution measurements of surface magnetic fields of the lunar frontside

Using 0.5 keV electron reflection measurements from real time tracking of the Apollo 15 and 16 Subsatellites the surface fields were mapped on the lunar frontside. Although the surface coverage is poor except near the Apollo 16 Subsatellite track, over 100 distinct regions of greater than 0.2 square degrees area with maximum surface field strength equal to or greater than 12 nT were identified. Preliminary contour maps of two of the largest regions and the Reiner Gamma region observed by the orbiting magnetometer are presented.

Lin, R. P.↗
NASA NTRS1978

The utility of surface magnetic field measurements in the MAGSAT program

To take full advantage of the global, vector, survey by the Magsat satellite, and international program of augmented surface measurements was proposed. For secular variation and upper mantle conductivity the proposed measurements are global. The repeat station measurements for secular variation should be occupied at 2-3 year intervals. A special observing period in November and December of 1979 is proposed during which simultaneous, continuous, global measurements for upper mantle conductivity studies are to be gathered. Finally, it is recommended that the networks in operation during the IMS extend their operation through the Fall of 1980 to provide correlative data useful for high latitude disturbance studies and for crustal conductivity studies.

Langel, R. A.↗
NASA NTRS1981

Surface magnetic fields during the solar activity cycle

The behavior of the magnetic field of the sun as measured in the Fe I 5250 A line is summarized. A latitudinal distribution of the fields observed over 13.5 yr is presented, and episodic formation of the polar fields (about 6 gauss) from a poleward drift originating at the sunspots is noted. Weak magnetic fields (-2 to +2) reach a maximum two years before the maximum in the average field. The total flux remains constant from minimum to maximum, with strong magnetic flux exhibiting an equatorward drift; both strong and new flux appear mostly around sunspots. The appearance of new flux implies the decay of flux elsewhere on the sun, because of the measured constancy of total flux; total replacement of the surface flux can occur within ten days. Field annihilation is concentrated in the active latitudes, where strong opposite polarity fields occur close together.

Howard, R.↗
DOE OSTI2018

Self-generated surface magnetic fields inhibit laser-driven sheath acceleration of high-energy protons

High-intensity lasers interacting with solid foils produce copious numbers of relativistic electrons, which in turn create strong sheath electric fields around the target. The proton beams accelerated in such fields have remarkable properties, enabling ultrafast radiography of plasma phenomena or isochoric heating of dense materials. In view of longer-term multidisciplinary purposes (e.g., spallation neutron sources or cancer therapy), the current challenge is to achieve proton energies well in excess of 100 MeV, which is commonly thought to be possible by raising the on-target laser intensity. Here we present experimental and numerical results demonstrating that magnetostatic fields self-generated on the target surface may pose a fundamental limit to sheath-driven ion acceleration for high enough laser intensities. Those fields can be strong enough (~10 5 T at laser intensities ~10 21 W cm –2 ) to magnetize the sheath electrons and deflect protons off the accelerating region, hence degrading the maximum energy the latter can acquire.

43 PARTICLE ACCELERATORS↗