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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Structural, electronic, and magnetic properties of CoFeVGe-based compounds: Experiment and theory

We have carried out a combined theoretical and experimental investigation of both stoichiometric and nonstoichiometric CoFeVGe alloys. In particular, we have investigated CoFeVGe, Co 1.25 Fe 0.75 VGe, Co 0.75 Fe 1.25 VGe, and CoFe 0.75 VGe bulk alloys. Our first principles calculations suggest that all four alloys show ferromagnetic order, where CoFeVGe, Co 1.25 Fe 0.75 VGe, and Co 0.75 Fe 1.25 VGe are highly spin polarized with spin polarization values of over 80%. However, the spin polarization value of CoFe 0.75 VGe is only about 60%. We have synthesized all four samples using arc melting and high-vacuum annealing at 600 °C for 48 hours. The room temperature x-ray diffraction of these samples exhibits a cubic crystal structure with disorder. All the samples show single magnetic transitions at their Curie temperatures, where the Curie temperature and high field (3T) magnetization are 288 K and 42 emu/g; 305 K and 1.5 emu/g; 238 K and 39 emu/g; and 306 K and 35 emu/g for CoFeVGe, Co 1.25 Fe 0.75 VGe, Co 0.75 Fe 1.25 VGe, and CoFe 0.75 VGe, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Influence of Ga doping on magnetic properties, magnetocaloric effect, and electronic structure of pseudobinary GdZn 1 - x Ga x ( x = 0 - 0.1 )

Here, we explore the impact of introducing IIIA-group element Ga in place of IIB-group element Zn in binary intermetallic GdZn on its magnetic and magnetocaloric properties, as well as explicate the modified electronic band structure of the compound. The magnetic transition temperature of the compound decreases with the increase of Ga concentration in GdZn 1-x ⁢Ga x ⁢(x = 0–0.1) while the crystal structure (CsCl prototype) and lattice parameters remain unchanged. Our detailed analysis of magnetization and magnetocaloric data conclusively proves that long-ranged magnetic ordering exists in the sample, despite the magnetic interaction considerably weakening with the increase of Ga. The experimental data are rationalized using both a theoretical machine-learning model and first-principle density functional theory. The electronic band structure of GdZn is manifested with some unusual complex features which gradually diminish with Ga doping and the conventional sinusoidal feature of Ruderman-Kittel-Kasuya-Yosida-type interactions also disappears. A mean-field theory model is developed and can successfully describe the overall magnetocaloric behavior of the GdZn 1-x⁢ Ga x series of samples.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spin Polarization and Magnetic Properties of V{sub Ga}O{sub N} and V{sub Ga}O{sub N}In{sub Ga} in GaN: GGA+U Approach

Electronic structure of a defect center containing the gallium vacancy and substitutional oxygen atom at nitrogen site (V{sub Ga}O{sub N}) in zinc blende and wurtzite GaN was analyzed within GGA+U approach. The +U term was applied to d(Ga), p(N), p(O), and d(In). Neutral V{sub Ga}O{sub N} is in the stable high spin state with spin S = 1. The defect structure is strongly dependent on geometry of the defect and the charge state. Two spin structures, which arise due to two different configurations in V{sub Ga}O{sub N}, with O{sub N} either along the c-axis or in one of three equivalent tetrahedral positions in wurtzite structure were analyzed. The weak ferromagnetic coupling between centers was found. The strength of magnetic coupling is increased when there is a complex containing V{sub Ga}O{sub N} with additional substitutional indium atom at the second neighbor to vacancy gallium site (V{sub Ga}O{sub N}In{sub Ga}). Magnetic coupling between V{sub Ga}O{sub N}In{sub Ga} is ferromagnetic due to strong spin polarization of p electrons of the nearest and distant nitrogen atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Structural and Magnetic Properties of FePt/SiO{sub 2} Core/Shell Nanoparticles

The disorder-order phase transition of FePt/SiO{sub 2} core/shell nanoparticles (NPs) induced by rapid thermal annealing was investigated. The FePt/SiO{sub 2} NPs were synthesized using FePt cores as seeds. For as-prepared FePt/SiO{sub 2} NPs, the average size of the FePt core and the thickness of the SiO{sub 2} shell are about 4.8 and 1.2 nm respectively, and the FePt NPs exhibit a disordered fcc structure. The FePt core NPs show obvious phase transformation from a disordered to an ordered structure after they were annealed at 600 °C for 2 min. Moreover, increasing annealing time can promote effectively the formation of the ordered fct-FePt phase. The relatively large coercivity H{sub c} is obtained for FePt NPs with a thicker SiO{sub 2} shell. The H{sub c} decreases firstly with decreasing temperature from 300 to 100 K and then increases from 100 to 10 K, which is ascribed to the coexistence of magnetically hard fct-FePt and magnetically soft fcc-FePt in the sample, and the incomplete exchange coupling between the hard and soft magnetic phases.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Synthesis, structure and magnetic properties of Ba 3 M 2 Ge 4 O 14 (M = Mn and Fe): Quasi-one-dimensional zigzag chain compounds

A series of trivalent transition metal germanates with a general formula of A 3 M 2 Ge 4 O 14 ( A = Sr, Ba, M = Mn and Fe) was synthesized using a high-temperature (580 °C) hydrothermal technique. Compound I , Ba 3 Mn 2 Ge 4 O 14 , is a new structure type that is characterized by one-dimensional Mn–O–Mn chains made from edge-sharing highly distorted Mn 3+ O 6 units, and exhibits antiferromagnetic behavior below 15 K with field induced metamagnetic transitions. The A 3 Fe 2 Ge 4 O 14 ( A = Sr and Ba, II and III, respectively) series belongs to the well-known langasite structure type, crystallizing in space group P 321. In A 3 Fe 2 Ge 4 O 14 , there is one octahedral Fe 3+ site and one mixed-occupancy Fe 3+ /Ge 4+ site that form the one-dimensional substructure. The magnetic susceptibility data of Ba 3 Fe 2 Ge 4 O 14 reveals a much simpler magnetic structure with an antiferromagnetic transition near 5.5 K. The Jahn-Teller distortion of Mn 3+ appears to be influential in the phase formation, leading to significantly different structures for these Mn 3+ and Fe 3+ compounds with similar chemical formulas.

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

Structural, Bulk Permittivity, and Magnetic Properties of Lead-Free Electronic Material: Ba{sub 1}Bi{sub 1}Cu{sub 1}Fe{sub 1}Ni{sub 1}Ti{sub 3}O{sub 12}

In this report, Ba{sub 1}Bi{sub 1}Cu{sub 1}Fe{sub 1}Ni{sub 1}Ti{sub 3}O{sub 12} (BBCFNTO termed further) ceramics were synthesized using a conventional ceramic processing route. The formation of multiple phases has been confirmed from by the XRD pattern at room temperature. The surface micrograph indicates the uniform distribution of grains with distinct grain boundary. The co-relation between the impedance and dielectric parameters was realized by using a phase sensitive meter over a good temperature and frequency range. The temperature- and frequency-dependent dielectric properties are linked to the conduction mechanism. The dielectric constant (K) and loss (tanδ) are increased sharply at high-temperature region, which is expected to be the onset of dipolar relaxation phenomena. The contribution of grain boundary (GBs) and grain (Gs) effects can be identified using the complex impedance spectroscopy. This synthesized material is investigated to model multifunctional devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Emergence of the isotropic Kitaev honeycomb lattice $α$–RuCl 3 and its magnetic properties

Here, we present a comprehensive investigation of the crystal and magnetic structures of the van der Waals antiferromagnet $α$–RuCl 3 using single crystal x-ray and neutron diffraction. The crystal structure at room temperature is a monoclinic (C2/m). However, with decreasing temperature, a remarkable first-order structural phase transition is observed, leading to the emergence of a rhombohedral ($R\bar3$) structure characterized by three-fold rotational symmetry forming an isotropic honeycomb lattice. On further cooling, a zigzag-type antiferromagnetic order develops below $T_{\textrm{N}} = 6\sim6.6$ K. The critical exponent of the magnetic order parameter was determined to be $\beta = 0.11(1)$, which is close to the two-dimensional Ising model. Additionally, the angular dependence of the magnetic critical field of the zigzag antiferromagnetic order for the polarized ferromagnetic phase reveals a six-fold rotational symmetry within the ab–plane. These findings reflect the symmetry associated with the Ising-like bond-dependent Kitaev spin interactions and underscore the universality of the Kitaev interaction-dominated antiferromagnetic system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Simple Hydrothermal Synthesis for Mesoporous Pebble-Like NiCo{sub 2}O{sub 4} Nanostructures and Their Attractive Magnetic Properties

The mesoporous pebble-like NiCo{sub 2}O{sub 4} nanostructures were smoothly obtained via a simple hydrothermal route and the succeeding annealing treatment. Various techniques, including XRD, SEM, TEM, Raman spectra, FTIR, and MH curves have been used to study the obtained products. The obtained results exhibited that the morphology of spinel NiCo{sub 2}O{sub 4} could be controlled by adjusting the content of cetyltrimethylammonium bromide (CTAB). The morphology of NiCo{sub 2}O{sub 4} nanostructures transformed from a 300-nm popcorn-like to 30–40-nm pebble-like nanostructure by changing the content of CTAB. Compared with the popcorn-like NiCo{sub 2}O{sub 4} nanostructures, the pebble-like samples showed effective blue shifting of Raman spectroscopy, which might be due to the nano-sized effect of NiCo{sub 2}O{sub 4} particles. Finally, vibration sample magnetometer (VSM) measurements indicated the samples transformed from weak ferromagnetic (popcorn-like) to paramagnetic (pebble-like) due to small particle size.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of Laser Fluence on Magnetic Properties of Thin Mn{sub x}Si{sub 1 –} {sub x} (x ≈ 0.5) Films Prepared by Pulsed Laser Deposition

Abstract—The effect of laser pulse energy E on the possibility of forming of a homogeneous “high-temperature” ferromagnetic phase in Mn{sub x}Si{sub 1 –} {sub x} (x ≈ 0.5) alloy films grown by pulsed laser deposition onto an Al{sub 2}O{sub 3} (0001) substrate has been studied. The high-temperature phase with manganese concentration x ≈ 0.53 and the Curie temperature T{sub C} ~ 200–300 K is shown to form near the substrate at the initial stage of the film growth. In this case, high values E ≥ 6.8 J/cm{sup 2} favor the stabilization of this phase over all film thickness, while low values E = 2.6–5.7 J/cm{sup 2} lead to a decrease in the manganese concentration in the upper film layer and the formation of additional “low–temperature” phase with T{sub C} ≈ 30–50 K provided by silicides MnSi and Mn{sub 4}Si{sub 7} crystallites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The Effect of the Composition of a Carrier Gas during the Growth of a Mn delta-Layer on the Electrical and Magnetic Properties of GaAs Structures

The effect of the arsine content (from 0 to 8 μmol), supplied to a reactor together with hydrogen, on the formation of delta-doped gallium-arsenide structures by the pulsed laser sputtering of a Mn target is investigated. It is found that an arsine molar fraction of ~2.5 μmol in the reactor atmosphere during the formation of a Mn delta-doped GaAs layer allows one to obtain single-crystal epitaxial structures with the lowest layer resistance and a ferromagnetic-paramagnetic phase transition temperature of about 40 K. Increasing the arsine content to 8 μmol or the absence of arsine in a hydrogen flow leads to a significant increase in the layer resistance in the temperature range below 150 K and a decrease in the Curie temperature. In the first case, this may be due to partial compensation of the hole conductivity by donor-type defects formed as a result of an excess of arsenic on the growing surface (arsenic atoms in Ga positions or in interstitial positions). In the second case, when no arsine is supplied to the reactor, the growth surface is likely to be enriched with gallium atoms, which makes the incorporation of Mn into the Ga sublattice difficult.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Spectral and Magnetic Properties of Er{sup 3+} and Yb{sup 3+} Ions in Y{sub 2}Ti{sub 2}O{sub 7} Crystals with a Pyrochlore Structure

Cubic Y{sub 2}Ti{sub 2}O{sub 7} single crystals doped with Er{sup 3+} or Yb{sup 3+} ions have been studied by the methods of electron spin resonance (ESR) and selective laser spectroscopy. ESR spectra exhibit signals from rare-earth ions that substitute for yttrium ions in sites with local trigonal symmetry. The g tensor components are determined. The results of optical investigations indicate that impurity centers of several types are formed; the sublevel energies of the ground and excited multiplets of these centers are found. Among the great variety of detected optical centers, the centers that dominate in the formation of ESR spectra are discriminated. An analysis of the experimental data using the exchange-charge model have made it possible to determine the sets of parameters of the crystal field for Er{sup 3+} and Yb{sup 3+} ions substituting Y{sup 3+} ions in regular crystallographic sites in pyrochlore Y{sub 2}Ti{sub 2}O{sub 7}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Light-Induced Decoupling of Electronic and Magnetic Properties in Manganites

The strongly correlated material La 0.7 ⁢Sr 0.3 ⁢Mn⁢O 3 (LSMO) exhibits metal-to-insulator and magnetic transition near room temperature. Although the physical properties of LSMO can be manipulated by strain, chemical doping, temperature, or magnetic field, they often require large external stimuli. Further, to include additional flexibility and tunability, we developed a hybrid optoelectronic heterostructure that uses photocarrier injection from cadmium sulfide (Cd⁢ S) to an LSMO layer to change its electrical conductivity. LSMO exhibits no significant optical response; however, the Cd ⁢S/LSMO heterostructures show an enhanced conductivity, with a resistance drop of about 37%, at the transition temperature under light stimuli. This enhanced conductivity in response to light is comparable to the effect of a 9 T magnetic field in pure LSMO. Surprisingly, the optical and magnetic responses of Cd⁢ S/LSMO heterostructures are decoupled and exhibit different effects when both stimuli are applied. This unexpected behavior shows that heterostructuring strongly correlated oxides may require a new understanding of the coupling of physical properties across the transitions and provide the means to implement new functionalities.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Pressure-tuning of the electronic and magnetic properties of EuPt 2 Si 2

Using the element and orbital selectivity of x-ray absorption spectroscopy at Eu and Pt L 3 edges we investigate the effects of the lattice contraction, induced by temperature and external pressure, on the magnetic and electronic properties of the EuPt 2 Si 2 system. Our findings point to a clear relationship between the volume and the Eu valence in this material. From XANES experiments as a function of pressure we show that the Europium valence tends to stabilize at 3+ for pressures up to 27 GPa. The XMCD results demonstrate that pressure induced valence change of the Europium ion leads to a suppression of the magnetic ordering of the material. Finally, altogether our results provide direct evidence that Eu-4f /5d electronic hybridization effects underlie the mechanism that regulates the valence and magnetic ordering in this material.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Electrical and Magnetic Properties of FeGa 3 with Antisite Disorder

We investigated the electronic and magnetic ground states of the cage compound FeGa 3 under the presence of Fe antisite disorder (namely as Fe 0.258 Ga 0.742 ) by x-ray diffraction (XRD), Mössbauer spectroscopy (MS), magnetization and electrical resistivity. Our results show that approximately 4.5 (5)% Fe antisite occupying one of the two non-equivalent Ga sites changes significantly the electronic and magnetic ground states of the pristine compound. We uncover that Fe 0.258 Ga 0.742 undergoes two magnetic transitions, T 1 ≈ 60 K and T 2 ≈ 15 K, the latter associated with the suppression of semiconducting behavior. We focus our discussion on the role that in-gap impurity states and spin disorder play across semiconducting-metal and paramagnetic-magnetic transitions.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Temperature-dependent magnetic properties of individual glass spherules, Apollo 11, 12, and 14 lunar samples.

Magnetic susceptibility of 11 glass spherules from the Apollo 14 lunar fines have been measured from room temperature to 4 K. Data taken at room temperature, 77 K, and 4.2 K, show that the soft saturation magnetization was temperature independent. In the temperature range 300 to 77 K the temperature-dependent component of the magnetic susceptibility obeys the Curie law. Susceptibility measurements on these same specimens and in addition 14 similar spherules from the Apollo 11 and 12 mission show a Curie-Weiss relation at temperatures less than 77 K with a Weiss temperature of 3-7 degrees in contrast to 2-3 degrees found for tektites and synthetic glasses of tektite composition. A proposed model and a theoretical expression closely predict the variation of the susceptibility of the glass spherules with temperature.

Thorpe, A. N.↗