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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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Size and surface effects on the magnetism of magnetite and maghemite nanoparticles

The size effects of magnetite and maghemite nanoparticles on their magnetic properties (magnetic moment, Curie temperature, blocking temperature, etc.) have been investigated. Magnetic separation and centrifugation of an aqueous solution of nanoparticles were used for their separation into fractions; their sizes were measured by atomic force microscopy, dynamic light scattering, and electron microscopy. A change in the size leads to a change in the Curie temperature and magnetic moment per formula unit. Both native nanoparticles and those covered with a bioresorbable layer have been considered. The magnetic properties have been calculated by the Monte Carlo method for the classical Heisenberg model with various bulk and surface magnetic moments.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Probing the localization of magnetic dichroism by atomic-size astigmatic and vortex electron beams

We report localization of a magnetic dichroic signal on atomic columns in electron magnetic circular dichroism (EMCD), probed by beam distorted by four-fold astigmatism and electron vortex beam. With astigmatic probe, magnetic signal to noise ratio can be enhanced by blocking the intensity from the central part of probe. However, the simulations show that for atomic resolution magnetic measurements, vortex beam is a more effective probe, with much higher magnetic signal to noise ratio. For all considered beam shapes, the optimal SNR constrains the signal detection at low collection angles of approximately 6–8 mrad. Irrespective of the material thickness, the magnetic signal remains strongly localized within the probed atomic column with vortex beam, whereas for astigmatic probes, the magnetic signal originates mostly from the nearest neighbor atomic columns. Due to excellent signal localization at probing individual atomic columns, vortex beams are predicted to be a strong candidate for studying the crystal site specific magnetic properties, magnetic properties at interfaces, or magnetism arising from individual atomic impurities.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A novel cryogenic magnetic refrigerant metal-organic framework based on 1D gadolinium(III) chain

A metal-organic framework (MOF) based on gadolinium ion (Gd{sup 3+}) and tricarboxylate ligand, [Gd(BTPCA)(H{sub 2}O)]·2DMF·3H{sub 2}O (Gd-BTPCA) (H{sub 3}BTPCA =1,1′,1′-(benzene-1,3,5-triyl)tripiperidine-4-carboxylic acid; DMF=dimethylformamide), was synthesized and structurally characterized. The adjacent Gd{sup 3+} ions are intraconnected by the carboxylate groups of the BTPCA{sup 3-} ligands to form a 1D Gd{sup 3+} ion chain. The 1D Gd{sup 3+} ion chains are interconnected by the BTPCA{sup 3-} ligands, giving rise to a 3D framework with 1D open channel. The magnetic studies indicate that Gd-BTPCA exhibits weak ferromagnetic interactions, and acts as a cryogenic magnetic refrigerant having the magnetic entropy change (−ΔS{sub m}) of 20.40 J kg{sup −1} K{sup −1} for ΔH =7 T at 3 K. - Graphical abstract: A 1D gadolinium(III) chains-based metal-organic framework performed ferromagnetic coupling on the magnetic property. Magnetic investigation reveals that Gd-BTPCA exhibits the entropy change (−ΔS{sub m}) of 20.40 J kg{sup −1} K{sup −1} for ΔH =7 T at 3 K. - Highlights: • The MOF based on gadolinium ion and tricarboxylate ligand was synthesized. • This MOF is connected with 1D Gd{sup 3+} ions chain and the carboxylate groups of BTPCA{sup 3-} ligands. • The magnetic studies indicate that the MOF exhibits the weak ferromagnetic interactions. • Magnetic investigation reveals that the MOF exhibits the high entropy change.

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

3d-Metal Induced Magnetic Ordering on U(IV) Atoms as a Route toward U(IV) Magnetic Materials

Uranium(IV) 5f 2 magnetism is dominated by a transition from a triplet to a singlet ground state at low temperatures. For the first time, we achieved magnetic ordering of U(IV) atoms in a complex fluoride through the incorporation of 3d transition metal cations. This new route allowed us to obtain an unprecedented series of U(IV) ferrimagnetic materials of the new composition Cs 2 MU 3 F 16 (M = Mn 2+ , Co 2+ , and Ni 2+ ), which were comprehensively characterized with respect to their structural and magnetic properties. Magnetic susceptibility measurements revealed ferromagnetic-like phase transitions at temperatures of ~14.0, 3.5, and 4.8 K for M = Mn 2+ , Co 2+ , and Ni 2+ , respectively. The transition is not observed when the magnetic M cations are replaced by a diamagnetic cation, Zn 2+ . Neutron diffraction measurements revealed the magnetic moments of 0.91(6)–1.97(3) μ B on the U atoms, which are only partially compensated by antiparallel moments of 1.53(14)–3.26(5) μ B on the 3 d cations. This arrangement promotes suppression of the transition to a diamagnetic ground state characteristic of U(IV), and in doing so, induces magnetic ordering on uranium via 3 d –5 f exchange coupling.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
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 OSTI2018

Half-Metallic Ferrimagnetic Characteristics of Co{sub 2}YZ (Z = P, As, Sb, and Bi) New Full-Heusler Alloys: a DFT Study

The full-Heusler alloys Co{sub 2}YZ (Z = P, As, Sb, and Bi) were investigated by using the state-of-the-art full-potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) method within the framework of density functional theory (DFT), to understand the structural, electronic, and magnetic properties of these new Heusler alloys. The generalized gradient approximation (GGA) was chosen to define the exchange-correlation potential for studying the structural properties, whereas electronic and magnetic properties were developed by using the GGA + U scheme, where U is the Hubbard term, which explains the Coulomb repulsion and was applied to describe the “d” electrons. The structural properties indicated that the ferromagnetic states of these compounds are energetically more favorable than the paramagnetic states. Through results on the electronic structures (band structures and densities of states), it is found that all these alloys exhibit a complete half-metallic nature, with half-metallic gaps (E{sub HM}) of 0.632, 0.605, 1.223, and 0.922 eV for Co{sub 2}YP, Co{sub 2}YAs, Co{sub 2}YSb, and Co{sub 2}YBi alloys, respectively. The magnetic properties indicated that the total spin magnetic moments of these Heusler alloys obey the Slater-Pauling rule around value 2 µ{sub B} approximately. The local magnetic moments of Co and Y transition elements have opposite signs with different intensities, thus revealing the ferromagnetic character. Our study of both electronic and magnetic properties demonstrates in detail that Co{sub 2}YZ (Z = P, As, Sb, and Bi) full-Heusler alloys are complete half-metallic ferrimagnetic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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↗
DOE OSTI2018

Evolution of magnetic and orbital properties in the magnetically diluted A -site spinel Cu 1 – x Zn x Rh 2 O 4

In frustrated spinel antiferromagnets, dilution with nonmagnetic ions can be a powerful strategy for probing unconventional spin states or uncovering interesting phenomena. Here, we present x-ray, neutron scattering, and thermodynamic studies of the effects of magnetic dilution of the tetragonally distorted A -site spinel antiferromagnet, CuRh 2 O 4 , with nonmagnetic Zn 2+ ions. Our data confirm the helical spin order recently identified at low temperatures in this material, and further demonstrate a systematic suppression of the associated Néel temperature with increasing site dilution towards a continuous transition with critical doping of x spin ~0.44. Interestingly, this critical doping is demonstrably distinct from a second structural critical point at x JT ~0.6, which is consistent with the suppression of orbital order on the A site through a classical percolative mechanism. In conclusion, this anomalously low value for x spin is confirmed via multiple measurements, and is inconsistent with predictions of classical percolation theory, suggesting that the spin transition in this material is driven by an enhancement of preexisting spin fluctuations with weak dilution.

36 MATERIALS SCIENCE↗