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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Thin-Film Three-Layer Fe/Poly(diphenylene phthalide)/Fe Systems: Structural Properties and Behavior in Magnetic Fields

The structural and magnetic properties of thin-film Fe/poly(diphenylene phthalide) (PDP)/Fe systems are studied, along with the behavior of these systems in magnetic fields. The mean surface roughness of the studied samples is around 5–8 nm, and local near-surface magnetic properties show variation within 10%. The samples are characterized by two-step hysteresis loops, the step size depending on the thickness of the polymer layer and the difference in the thickness between the magnetic layers. The results are explained by the effects the exchange interaction between the magnetic layers, mediated by the PDP interlayer, has on the behavior of our samples in a magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Study on the Properties of Novel Soft Magnetic Composites of FeSiAl

In this paper, we compounded the FeSiAl powder with a novel amorphous inorganic insulation agent (AIIA). Silicone resin (3 wt.%) was used as organic insulating agent, and the inorganic insulating agent was Fe{sub 83.5}Si{sub 7.5}B{sub 5}Cr{sub 4} amorphous powder (0, 1, 2, 3, and 4 wt.%). Amorphous powder was prepared by passivation pretreatment before compounding. The results showed that the addition of amorphous inorganic insulating agent will significantly affect the soft magnetic properties of FeSiAl powders. Compared with the pure FeSiAl powder having only silicone resin the saturation magnetization of composite powders slightly increased (by 0.1–4.9%). When AIIA content was 2 wt.%, the sample had the highest initial permeability, which increased by 14.6%. At the same time, the core loss was also reduced by 15.7% (B{sub m} = 5 mT, f = 1000 kHz) when AIIA content was also 2 wt.%.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Semiconductor Behavior and Room Temperature Ferromagnetism in e-Beam Evaporated Co/TiO{sub 2} Multilayer Thin Films

The Co/TiO{sub 2} multilayer thin films have been deposited by e-beam evaporation method on glass substrates in vacuum and annealed in air at 773 K for 1 h. The crystal structure, surface morphology, optical properties, electrical properties and magnetic properties of Co/TiO{sub 2} multilayer thin films have been systematically investigated. The results showed that the particle sizes were significantly reduced when the samples are annealed and the enlargement of particle size occurred when the samples thickness is increased. The spectroscopic analysis exhibited enhanced transmittance and higher optical band gap of annealed sample than the as-deposited one and it was decreased with sample thickness. The resistivity measurement confirmed resistivity decreament with temperature. Furthermore, by investigating the magnetic properties, room temperature ferromagnetism was observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Helimagnetism in MnBi 2 Se 4 Driven by Spin-Frustrating Interactions Between Antiferromagnetic Chains

We report the magnetic properties and magnetic structure determination for a linear-chain antiferromagnet, MnBi 2 Se 4 . The crystal structure of this material contains chains of edge-sharing MnSe 6 octahedra separated by Bi atoms. The magnetic behavior is dominated by intrachain antiferromagnetic (AFM) interactions, as demonstrated by the negative Weiss constant of –74 K obtained by the Curie–Weiss fit of the paramagnetic susceptibility measured along the easy-axis magnetization direction. The relative shift of adjacent chains by one-half of the chain period causes spin frustration due to interchain AFM coupling, which leads to AFM ordering at TN = 15 K. Neutron diffraction studies reveal that the AFM ordered state exhibits an incommensurate helimagnetic structure with the propagation vector k = (0, 0.356, 0). The Mn moments are arranged perpendicular to the chain propagation direction (the crystallographic b axis), and the turn angle around the helix is 128°. The magnetic properties of MnBi2Se4 are discussed in comparison to other linear-chain antiferromagnets based on ternary mixed-metal halides and chalcogenides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Thermal Decomposition Synthesis of MgFe{sub 2}O{sub 4} Nanoparticles for Magnetic Hyperthermia

In this work, the effects of the thermal decomposition temperature and time on the structure, cation distributions, magnetic properties, and AC magnetically induced heating characteristics of MgFe{sub 2}O{sub 4} nanoparticles were investigated. The structure and cation distributions between the tetrahedral and octahedral sites determined by X-ray diffraction method showed single-phase MgFe{sub 2}O{sub 4} nanoparticles had a partially inverse structure. The inversion coefficient increased using high reaction temperature and time. The square-like shape and small magnesium ferrite nanoparticles with narrow particle size distribution were synthesized at high reaction temperatures, as observed by transmission electron microscopy. Magnetic properties of MgFe{sub 2}O{sub 4} nanoparticles studied by vibrating sample magnetometry showed the ferrimagnetic characteristics with the highest saturation magnetization of 24 emu/g at a reaction temperature of 300 °C for 60 min. Furthermore, the AC magnetically induced heating characteristics of MgFe{sub 2}O{sub 4} nanoparticles were correlated to the saturation magnetization and coercivity.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS2013

Design Issues for Using Magnetic Materials in Radiation Environments at Elevated Temperature

One of the challenges of designing motors and alternators for use in nuclear powered space missions is accounting for the effects of radiation. Terrestrial reactor power plants use distance and shielding to minimize radiation damage but space missions must economize volume and mass. Past studies have shown that sufficiently high radiation levels can affect the magnetic response of hard and soft magnetic materials. Theoretical models explaining the radiation-induced degradation have been proposed but not verified. This paper reviews the literature and explains the cumulative effects of temperature, magnetic-load, and radiation-level on the magnetic properties of component materials. Magnetic property degradation is very specific to alloy choice and processing history, since magnetic properties are very much entwined with specific chemistry and microstructural features. However, there is basic theoretical as well as supportive experimental evidence that the negative impact to magnetic properties will be minimal if the bulk temperature of the material is less than fifty percent of the Curie temperature, the radiation flux is low, and the demagnetization field is small. Keywords: Magnets, Permanent Magnets, Power Converters, Nuclear Electric Power Generation, Radiation Tolerance.

Bowman, Cheryl L.↗
DOE OSTI2024

Robustness of magnetocrystalline anisotropy and coercivity in Fe–Co–B

As a potential candidate for critical chemical element free permanent magnet, (Fe 0.7 Co 0.3 ) 2 B, shows promising intrinsic magnetic properties, including large magnetization, high Curie temperature and moderate magnetocrystalline anisotropy (MCA). Still, the coercivity that has been achieved is relatively low. To understand and improve coercivity, here we study the robustness of MCA related to the fluctuation of chemical composition and the structural distortion and its effect on coercivity in (Fe 0.7 Co 0.3 ) 2 B from first principles DFT (density functional theory) calculation and micromagnetic simulation. A fluctuation of cobalt content of 10%, or 1% isotropic lattice strain, or 2% tetragonal distortion reduces MCA energy up to 30%. By considering randomly distributed local MCA energy reduction up to 30%, the calculated coercivity of anisotropic bulk magnets reduced from 3.6 kOe to 2.2 kOe. Rational microstructure design, such as grain size refinement and development of strong crystal texture of MCA easy-axis (i.e., degree of alignment for (Fe 0.7 Co 0.3 ) 2 B grains), can enhance coercivity and mitigate the effect of local MCA reduction. In addition to acceptable intrinsic magnetic properties, the promising hard magnetic phase needs good MCA robustness to facilitate the development of practical magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Theory of thermal properties of magnetic materials with unknown entropy

Theoretical approaches to study thermal properties of magnetic materials typically require accurate models of magnetic interactions in order to define the entropy. Here we introduce a complementary approach for examining thermal properties in magnetic systems where an accepted model for such interactions does not exist. In place of a specific model for magnetic interactions, the approach integrates measurements of temperature dependent magnetization of the studied material into a first principles computational scheme. The approach calculates system pressure from thermally disordered microstates that properly incorporate vibrational and spin subsystems at each temperature as well as the coupling between these subsystems. We apply the approach to calculate phonon modes and to investigate the anomalously low thermal expansion of the classical Invar alloy, Fe 0.65 Ni 0.35 . Here, the calculated phonon dispersions for Invar are in excellent agreement with measured data. The Invar thermal expansion is shown to remain small between 50 K and room temperature, consistent with the experimentally observed low thermal expansion value in this same temperature range. This anomalously small thermal expansion is directly connected to a small positive contribution from lattice thermal disorder that is nearly canceled by a large negative magnetic disorder contribution. By contrast, calculations for bcc Fe show a much larger thermal expansion, consistent with experiment, which is dominated by a large contribution from lattice thermal disorder that is reduced only slightly by a small negative contribution from that of magnetism. These findings give insights into the unusual nature of magnetism and spin-lattice coupling in Invar and Fe. In addition, they give promising preliminary support to the presented new methodology as a complementary way to investigate thermal properties of magnetic materials. The success achieved on Invar and Fe motivates future testing of the approach on other magnetic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Controlling Noncollinear Ferromagnetism in van der Waals Metal–Organic Magnets

Van der Waals (vdW) magnets both allow exploration of fundamental 2D physics and offer a route toward exploiting magnetism in next generation information technology, but vdW magnets with complex, noncollinear spin textures are currently rare. We report here the syntheses, crystal structures, magnetic properties and magnetic ground states of four bulk vdW metal–organic magnets (MOMs): FeCl 2 (pym), FeCl 2 (btd), NiCl 2 (pym), and NiCl 2 (btd), pym = pyrimidine and btd = 2,1,3-benzothiadiazole. Using a combination of neutron diffraction and bulk magnetometry we show that these materials are noncollinear magnets. Although only NiCl 2 (btd) has a ferromagnetic ground state, we demonstrate that low-field hysteretic metamagnetic transitions produce states with net magnetization in zero-field and high coercivities for FeCl 2 (pym) and NiCl 2 (pym). By combining our bulk magnetic data with diffuse scattering analysis and broken-symmetry density-functional calculations, we probe the magnetic superexchange interactions, which when combined with symmetry analysis allow us to suggest design principles for future noncollinear vdW MOMs. These materials, if delaminated, would prove an interesting new family of 2D magnets.

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

Antiferromagnetic excitonic insulator state in Sr3Ir2O7

Abstract Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr 3 Ir 2 O 7 . By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr 3 Ir 2 O 7 indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1999

Titanomagnetite-bearing Palagonitic Dust as an Analogue for Magnetic Aeolian Dust on Mars

The Mars Pathfinder magnetic properties experiment included two Magnet Arrays (MAs) which consist of five permanent magnets that have different strengths. Each magnet is a cylindrical and ring magnet arranged in a "bulls-eye" pattern beneath a thin surface layer. The design of the MA permits estimation of magnetic properties (saturation magnetization and magnetic susceptibility) of adhering material, principally by the number of magnets that have adhering material and the extent each magnet is saturated. The two MAs passively sample aeolian dust. Additional information is contained in the original extended abstract.

Morris, Richard V.↗
DOE OSTI2022

Exchange bias and interface-related effects in two-dimensional van der Waals magnetic heterostructures: Open questions and perspectives

The exchange bias (EB) effect is known as a fundamentally and technologically important magnetic property of a magnetic bilayer film. It is manifested as a horizontal shift in a magnetic hysteresis loop of a film subject to cooling in the presence of a magnetic field. The EB effect in van der Waals (vdW) heterostructures offers a novel approach for tuning the magnetic properties of the newly discovered single-layer magnets, as well as adds a new impetus to magnetic vdW heterostructures. Indeed, intriguing EB effects have recently been reported in a variety of low-dimensional vdW magnetic systems ranging from a weakly interlayer-coupled vdW magnet (e.g., Fe 3 GeTe 2 ) to a bilayer composed of two different magnetic vdW materials (e.g., Fe 3 GeTe 2 /CrCl 3 , Fe 3 GeTe 2 /FePS 3 , Fe 3 GeTe 2 /MnPS 3 , Fe 3 GeTe 2 /CrSe, Fe 3 GeTe 2 /CrOCl, Fe 3 GeTe 2 /CoPc, Fe 5 GeTe 2 /FePS 3 ), to bilayers of two different vdW defective magnets (e.g., VSe 2 /MoS 2 ), or to metallic ferromagnet/vdW defective magnet interfaces (e.g., Fe/MoS 2 ). Despite their huge potential in spintronic device applications, the physical origins of the observed EB effects have remained elusive to researchers. Here, we present here a critical review of the EB effect and associated phenomena such as magnetic proximity (MP) in various vdW heterostructure systems and propose approaches to addressing some of the emerging fundamental questions.

2D magnets↗
NASA NTRS1993

Magnetic field properties of the intermediate transition of the Venus ionosheath

Results of a statistical analysis of the magnetic properties of the intermediate transition of the Venus ionosheath based on the magnetic field data of the Pioneer Venus Orbiter are reported. It is found that the transition is often characterized by a substantial decrease in magnetic field intensity and an accompanying rotation to a direction that is more closely aligned with the sun-Venus axis. In other cases, the 30 kHz bursts occur outside a region of enhanced magnetic field reminiscent of the magnetic barrier external to the Venus dayside ionopause. The rotation of the magnetic field toward the sun-Venus line associated with the 30 kHz bursts is also evident here, but generally the conditions change more gradually. Possible mechanisms of these variations and ionospheric plasma expansion are examined.

Perez-De-tejada, H.↗
NASA NTRS2011

Magnetic Clouds at/near the 2007 - 2009 Solar Minimum: Frequency of Occurrence and Some Unusual Properties

Magnetic clouds (MCs) have been identified for the period 2007 2009 (at/near the recent solar minimum) from Wind data, then confirmed through MC parameter fitting using a force-free model. A dramatic increase in the frequency of occurrence of these events took place from the two early years of 2007 (with five MCs) and 2008 (one MC) compared to 2009 (12 MCs). This pattern approximately mirrors the occurrence-frequency profile that was observed over a three-year interval 12 years earlier, with eight events in 1995, four in 1996, and 17 in 1997, but decreased overall by a factor of 0.62 in number. However, the average estimated axial field strength taken over all of the 18 events of 2007 - 2009 (called the "recent period" here) was only 11.0 nT, whereas |BO| for the 29 events of 1995 - 1997 (called the "earlier period" ) was 16.5 nT. This 33% average drop in |BO| is more or less consistent with the decreased three-year average interplanetary magnetic field intensity between these two periods, which shows a 23% drop. In the earlier period, the MCs were clearly of mixed types but predominantly of the South-to-North type, whereas those in the recent period are almost exclusively the North-to-South type; this change is consistent with global solar field changes predicted by Bothmer and Rust (Geophys. Monogr. Ser. 99, 139, 1997). As we have argued in earlier work (Lepping and Wu, J. Geophys. Res. 112, A10103, 2007), this change should make it possible to carry out (accurate short-term) magnetic storm forecasting by predicting the latter part of an MC from the earlier part, using a good MC parameter-fitting model with real-time data from a spacecraft at L1, for example. The recent set s average duration is 15.2 hours, which is a 27% decrease compared to that of the earlier set, which had an average duration of 20.9 hours. In fact, all physical aspects of the recent MC set are shown to drop with respect to the earlier set; e.g., as well as the average internal magnetic field drop, the recent set had a somewhat low average speed of 379 km/s (5% drop), and the average diameter had a 24% drop. Hence, compared to the earlier set, the recent set consists of events that are smaller, slightly slower, and weaker in every respect (and fewer in number), but in a relative sense the two three-year sets have similar frequency-of-occurrence profiles. It is also interesting that the two sets have almost the same average axial inclinations, i.e., axial latitude approx. = 31deg (in GSE). These MC characteristics are compared to relevant solar features and their changes. A preliminary assessment of the statistics on possible shocks and pressure pulses upstream of these recent MCs yields the following: About 28% of the MCs, at most, had shocks, and 33% had shocks and/or pressure pulses. These are low values, since typically the percentage of cases with shocks is about 50%, and the percentage with shocks and/or pressure pulses is usually about 75%.

Lepping. R. P.↗
DOE OSTI2019

Effect of V{sub Zn}/V{sub O} on Stability, Magnetism, and Electronic Characteristic of Oxygen Ions for Li-Doped ZnO

The effects of Zn/O vacancy (V{sub Zn}/V{sub O}) and different proportions of Li/V{sub Zn} on the magnetism of Li-doped ZnO are analyzed through first-principle calculation by using generalized gradient approximation+ U (GGA + U) under density functional theory. Results reveal that Li-doped ZnO with V{sub Zn} can realize ferromagnetic long-range order and has a Curie temperature above room temperature. Under different proportions of Li/V{sub Zn} (i.e., 1:1, 1:2, and 2:2) in ZnO (2 × 2 × 4), the doping system containing 2Li/2V{sub Zn} (Zn{sub 28}Li{sub 2}O{sub 32}) shows the greatest magnetic moment and the smallest differential charge density. These characteristics pave the way for enhancing the magnetic properties of dilute magnetic semiconductors. The oxygen atoms in Zn{sub 28}Li{sub 2}O{sub 32} show acceptor and donor characteristics and exist in the forms of itinerant electrons (O{sup 1−}) and local electrons (O{sup 2−}), which have different effects on Zn{sub 28}Li{sub 2}O{sub 32} magnetism. The spin-polarization double-exchange effect among the unpaired itinerant electron (O{sup 1−}) orbit, local electron (O{sup 2−}) orbit, and unpaired Zn-3d electron orbit is the origin of magnetism for Li-doped ZnO with V{sub Zn}. By contrast, the doping systems of Li-doped ZnO with V{sub O} are nonmagnetic, rendering such systems inapplicable.

36 MATERIALS SCIENCE↗