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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 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Synthesis and Characterisation of Superparamagnetic MgFe{sub 2}O{sub 4} Nanoferrite

Nanocrystalline MgFe{sub 2}O{sub 4} with crystallite size of 10.4 nm was synthesised using combustion method and characterised for structural, electrical, and magnetic properties. The powder X-ray diffraction analysis reveals the single-phase nature of MgFe{sub 2}O{sub 4}. The field emission scanning electron microscopy image shows the nanosized grains. The magnetic study reveals the superparamagnetic behaviour of MgFe{sub 2}O{sub 4} with saturation magnetisation value of 14.57 emu/g. The blocking temperature is determined using low-temperature magnetic measurement which is equal to 139 K. Electrical study shows that the value of AC conductivity at 1 Hz is 1.12 × 10{sup − 8} S/cm at 303 K and increases with frequency as well as temperature. The complex impedance analysis shows the grain and grain boundary resistance equal to 6.23 × 10{sup 6} Ω and 1.27 × 10{sup 7} Ω, respectively. The complex electric modulus analysis confirms the presence of two relaxation processes in nanocrystalline MgFe{sub 2}O{sub 4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Ferromagnetic resonances in single-crystal yttrium iron garnet nanofilms fabricated by metal-organic decomposition

Tunable microwave and millimeter wave oscillators and bandpass filters with ultra-low phase noise play a critical role in electronic devices, including wireless communication, microelectronics, and quantum computing. Magnetic materials, such as yttrium iron garnet (YIG), possess ultra-low phase noise and a ferromagnetic resonance tunable up to tens of gigahertz. Here, we report structural and magnetic properties of single-crystal 60 and 130 nm-thick YIG films prepared by metal-organic decomposition epitaxy. These films, consisting of multiple homoepitaxially grown monolayers, are atomically flat and possess magnetic properties similar to those grown with liquid-phase epitaxy, pulsed laser deposition, and sputtering. Our approach does not involve expensive high-vacuum deposition systems and is a true low-cost alternative to current commercial techniques that have the potential to transform the industry.

Wang, Szu-Fan (Paul)↗
DOE OSTI2017

Emitting heterostructures with a bilayer InGaAs/GaAsSb/GaAs quantum well and a GaMnAs ferromagnetic layer

The radiative and magnetic properties of novel heterostructures with a bilayer InGaAs/GaAsSb/GaAs quantum well and a GaMnAs ferromagnetic layer are studied. The circular polarization of electroluminescent radiation is observed at temperatures from 10 to 160 K. The magnetic field dependences of the degree of circular polarization are nonlinear with a hysteresis loop at temperatures from 10 to 50 K, and they become linear at higher temperatures. The magnitude of polarization at the saturation magnetization of GaMnAs in the 2000 Oe field remains at the level of ~0.2%.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Oxygen Inhomogeneity and Reversibility in Single Crystal LaNiO{sub 3-{delta}}.

LaNiO 3− δ single crystals have been obtained via high pressure floating zone growth under 149 bar of oxygen pressure. We find a radial gradient in the magnetic properties of specimens extracted from the as-grown boule, which we correlate with the appearance of ordered oxygen vacancy structures. This radial oxygen inhomogeneity has been characterized systematically using a combination of magnetization and X-ray scattering measurements. We establish the presence of rhombohedral ( R 3 ¯ c ), oxygen stoichiometric specimens at the periphery of the boule and the presence of a dilute concentration of ordered oxygen-deficient orthorhombic La 2 Ni 2 O 5 in the center. Furthermore, we demonstrate that the as-grown, oxygen-deficient central regions of the crystal can be annealed under high oxygen pressure, without loss of crystallinity, into fully oxygenated LaNiO 3 , recovering magnetic properties that are characteristic of stoichiometric specimens from the exterior region of the crystal. Thus, single crystals of LaNiO 3− δ possess oxygen content that can be reversibly modified under oxidizing and reducing conditions.

crystal growth↗
DOE OSTI2015

Phase diagram and magnetocaloric effects in Ni{sub 1-x}Cr{sub x}MnGe{sub 1.05}

The structural and magnetic properties of the Ni{sub 1−x}Cr{sub x}MnGe{sub 1.05} system (for x = 0, 0.035, 0.070, 0.105) have been studied by x-ray diffraction (XRD), differential scanning calorimetry (DSC), and magnetization measurements. A change in crystal structure from orthorhombic to hexagonal was observed in the XRD data at chromium concentrations of x = 0.035, 0.070, and 0.105. The values of the cell parameters and volume of the unit cell for orthorhombic and hexagonal phase were determined. It was found that the partial substitution of Cr for Ni in Ni{sub 1−x}Cr{sub x}MnGe{sub 1.05} results in a first order magnetostructural transition from antiferromagnetic to ferromagnetic (FM) at T{sub M} = 132 K for x = 0.105. A FM to paramagnetic second order transition has been observed at T{sub C} = 204 K. A magnetic entropy change of |ΔS{sub M}| = 4.5 J/kg K for ΔH = 5 T was observed in the vicinity of T{sub C} and T{sub M} for x = 0.105. The values of the latent heat (L = 2.2 J/g) and corresponding total entropy changes (ΔS{sub T} = 16 J/kg K) have been determined from DSC measurements. A concentration-dependent phase diagram of transition temperatures has been constructed using the magnetic and DSC data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

A review: applications of the phase field method in predicting microstructure and property evolution of irradiated nuclear materials

Here, complex microstructure changes occur in nuclear fuel and structural materials due to the extreme environments of intense irradiation and high temperature. This paper evaluates the role of the phase field method in predicting the microstructure evolution of irradiated nuclear materials and the impact on their mechanical, thermal, and magnetic properties. The paper starts with an overview of the important physical mechanisms of defect evolution and the significant gaps in simulating microstructure evolution in irradiated nuclear materials. Then, the phase field method is introduced as a powerful and predictive tool and its applications to microstructure and property evolution in irradiated nuclear materials are reviewed. The review shows that (1) Phase field models can correctly describe important phenomena such as spatial-dependent generation, migration, and recombination of defects, radiation-induced dissolution, the Soret effect, strong interfacial energy anisotropy, and elastic interaction; (2) The phase field method can qualitatively and quantitatively simulate two-dimensional and three-dimensional microstructure evolution, including radiation-induced segregation, second phase nucleation, void migration, void and gas bubble superlattice formation, interstitial loop evolution, hydrate formation, and grain growth, and (3) The Phase field method correctly predicts the relationships between microstructures and properties. The final section is dedicated to a discussion of the strengths and limitations of the phase field method, as applied to irradiation effects in nuclear materials.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

Post-annealing effect on the room-temperature ferromagnetism in Cu-doped ZnO thin films

In this work, we investigated the structural and magnetic properties of both as-deposited and post-annealed Cu-doped ZnO thin films for better understanding the possible mechanisms of room-temperature ferromagnetism (RT-FM) in ZnO-based diluted magnetic oxides. All of the films have a c-axis-oriented wurtzite structure and display RT-FM. X-ray photoelectron spectroscopy results showed that the incorporated Cu ions in as-deposited films are in 1+ valence state merely, while an additional 2+ valence state occurs in post-annealed films. The presence of Cu{sup 2+} state in post-annealed film accompanies a higher magnetization value than that of as-deposited film and, in particular, the magnetization curves at 10 K and 300 K of the post-annealed film separate distinctly. Since Cu{sup 1+} ion has a filled 3d band, the RT-FM in as-deposited Cu-doped ZnO thin films may stem solely from intrinsic defects, while that in post-annealed films is enhanced due to the presence of CuO crystallites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Two-dimensional chiral asymmetry in unidirectional magnetic anisotropy structures

We investigate the symmetry-breaking effects of magnetic nanostructures that present unidirectional (one-fold) magnetic anisotropy. Angular and field dependent transport and magnetic properties have been studied in two different exchange-biased systems, i.e. ferromagnetic (FM)/ antiferromagnetic (AFM) bilayer and spin-valve structures. We experimentally show the direct relationships between the magnetoresistance (MR) response and the magnetization reversal pathways for any field value and direction. We demonstrate that even though the MR signals are related to different transport phenomena, namely anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR), chiral asymmetries are found around the magnetization hard-axis direction, in both cases originated from the one-fold symmetry of the interfacial exchange coupling. Our results indicate that the chiral asymmetry of transport and magnetic behaviors are intrinsic of systems with an unidirectional contribution.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Revealing Site Occupancy in a Complex Oxide: Terbium Iron Garnet

Complex oxide films stabilized by epitaxial growth can exhibit large populations of point defects which have important effects on their properties. The site occupancy of pulsed laser-deposited epitaxial terbium iron garnet (TbIG) films with excess terbium (Tb) is analyzed, in which the terbium:iron (Tb:Fe)ratio is 0.86 compared to the stoichiometric value of 0.6. The magnetic properties of the TbIG are sensitive to site occupancy, exhibiting a higher compensation temperature (by 90 K) and a lower Curie temperature (by 40 K) than the bulk Tb 3 Fe 5 O 12 garnet. Data derived from X-ray core-level spectroscopy, magnetometry, and molecular field coefficient modeling are consistent with occupancy of the dodecahedral sites by Tb 3+ , the octahedral sites by Fe 3+ , Tb 3+ and vacancies, and the tetrahedral sites by Fe 3+ and vacancies. Energy dispersive X-ray spectroscopy in a scanning transmission electron microscope provides direct evidence of Tb Fe antisites. A small fraction of Fe 2+ is present, and oxygen vacancies are inferred to be present to maintain charge neutrality. In conclusion, variation of the site occupancies provides a path to considerable manipulation of the magnetic properties of epitaxial iron garnet films and other complex oxides, which readily accommodate stoichiometries not found in their bulk counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Probing Defects and Spin‐Phonon Coupling in CrSBr via Resonant Raman Scattering

Abstract Understanding the stability limitations and defect formation mechanisms in 2D magnets is essential for their utilization in spintronic and memory technologies. Here, defects in mono‐ to multilayer CrSBr are correlated with structural, vibrational, and magnetic properties. Resonant Raman scattering is used to reveal distinct vibrational defect signatures. In pristine CrSBr, it is shown that bromine atoms mediate vibrational interlayer coupling, allowing for distinguishing between surface and bulk defect modes. Environmental exposure is shown to cause drastic degradation in monolayers, with the formation of intralayer defects. This is in contrast to multilayers that predominantly show bromine surface defects. Through deliberate ion irradiation, the formation of defect modes is tuned: these are strongly polarized and resonantly enhanced, reflecting the quasi‐‐1D electronic character of CrSBr. Strikingly, pronounced signatures of spin‐phonon coupling of the intrinsic phonon modes and the ion beam‐induced defect modes are observed throughout the magnetic transition temperature. Overall, defect engineering of magnetic properties is possible, with resonant Raman spectroscopy serving as a direct fingerprint of magnetic phases and defects in CrSBr.

2D magnetic semiconductors↗
DOE OSTI2019

Effect of Annealing Treatment on Mechanical and Magnetic Softening Behaviors of Cold Rolled Interstitial-Free Steel

The recovery and recrystallization behaviors of low-carbon interstitial-free steel are investigated at different annealing temperatures (200-690 °C) through correlation between the changes in magnetic parameters (coercivity and r.m.s. voltage), hardness, tensile strength and high-angle grain boundary fractions (HAB). Magnetic parameters and tensile strength show a significant change within recovery region (200-500 °C), while the hardness variation is very minimal at this temperature region. On the other hand, the hardness is abruptly changed compared to magnetic properties at higher annealing temperature (550-640 °C). The grain boundary characteristics and microtexture evolution occurring at recrystallization regime are evaluated by orientation imaging microscopy (OIM) and orientation distribution function (ODF) through electron backscattered diffraction (EBSD). The increase in recrystallization fraction noticeably affects microtexture property, justifying with the strengthening of γ-fiber, weakening of α-fiber and disappearance of rotated cube component with increasing annealing temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Giant magnetic coercivity in orthorhombic YNi{sub 4}Si-type SmNi{sub 4}Si compound

Magnetic properties, magnetocaloric effect and heat capacity of the YNi{sub 4}Si-type SmNi{sub 4}Si compound have been investigated. SmNi{sub 4}Si exhibits ferromagnetic transition at 17 K. Below ~9 K, the magnetic isotherms of SmNi{sub 4}Si show metamagnetic-like behavior with critical field of 20 kOe at 5 K. Heat capacity measurements of SmNi{sub 4}Si show the electronic heat capacity coefficient γ=94 mJ/(mol K{sup 2}), phonon coefficient β=0.35 mJ/(mol K{sup 4}) and Debye temperature T{sub D}=310 K. The magnetocaloric effect of SmNi{sub 4}Si is calculated in terms of isothermal magnetic entropy change which is obtained by the isothermal magnetization (ΔS{sub m}{sup magn}) and heat capacity measurements (ΔS{sub m}{sup heat}). Both ΔS{sub m}{sup magn} and ΔS{sub m}{sup heat} reach a maximum of –1.0 J/kg K at 17 K for a field change of 50 kOe and they show positive values for a field change of 20 kOe at ~9 K. Below 15 K, SmNi{sub 4}Si shows large magnetic hysteresis with considerable remanence. At 5 K it exhibits giant coercive field of 58 kOe in an applied field of 90 kOe. - Highlights: • YNi{sub 4}Si-type SmNi{sub 4}Si has Curie point at 17 K. • ΔS{sub m}{sup magn} of SmNi{sub 4}Si reaches value of –1.0 J/kg K in 0–50 kOe near Curie point. • ΔS{sub m}{sup magn} of SmNi{sub 4}Si shows value of –0.4 and +0.8 J/kg K at 17 and 9 K in 0–20 kOe. • SmNi{sub 4}Si shows electronic heat capacity the 94 mJ/(mol K{sup 2}) and Debye T{sub D}=310 K. • SmNi{sub 4}Si shows huge magnetic hysteresis with coercive field of 58 kOe at 5 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effects of crystallite size on the structure and magnetism of ferrihydrite

The structure and magnetic properties of nano-sized (1.6 to 4.4 nm) ferrihydrite samples are systematically investigated through a combination of X-ray diffraction (XRD), X-ray pair distribution function (PDF), X-ray absorption spectroscopy (XAS) and magnetic analyses. The XRD, PDF and Fe K-edge XAS data of the ferrihydrite samples are all fitted well with the Michel ferrihydrite model, indicating similar local-, medium- and long-range ordered structures. PDF and XAS fitting results indicate that, with increasing crystallite size, the average coordination numbers of Fe–Fe and the unit cell parameter c increase, while Fe2 and Fe3 vacancies and the unit cell parameter a decrease. Mössbauer results indicate that the surface layer is relatively disordered, which might have been caused by the random distribution of Fe vacancies. These results support Hiemstra's surface-depletion model in terms of the location of disorder and the variations of Fe2 and Fe3 occupancies with size. Magnetic data indicate that the ferrihydrite samples show antiferromagnetism superimposed with a ferromagnetic-like moment at lower temperatures (100 K and 10 K), but ferrihydrite is paramagnetic at room temperature. In addition, both the magnetization and coercivity decrease with increasing ferrihydrite crystallite size due to strong surface effects in fine-grained ferrihydrites. Smaller ferrihydrite samples show less magnetic hyperfine splitting and a lower unblocking temperature (T B ) than larger samples. The dependence of magnetic properties on grain size for nano-sized ferrihydrite provides a practical way to determine the crystallite size of ferrihydrite quantitatively in natural environments or artificial systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Emergence of exchange bias and giant coercive field enhancement by internal magnetic frustration in La 0.67 Sr 0.33 MnO 3 thin films

We have studied the influence of controlled defects on the magnetic properties of La 0.67 Sr 0.33 MnO 3 (LSMO) thin films. We used 100 nm thick epitaxial LSMO films, which were grown on SrTiO 3 substrates and introduced stoichiometry changes and defects using oxygen ion irradiation through nanoporous alumina masks. Oxygen irradiation through the mask creates cascades of defects in spatially-modulated regions in the plane of the LSMO film. The magnetic properties of the samples were compared before and after the irradiation. We have found that irradiation reduces the ferromagnetic ordering temperature, decreases the total magnetization, enhances the coercivity, and induces exchange bias below 50 K. The coercivity enhancement is dramatically higher below 50 K and can be associated with exchange bias. These results can be explained by the formation of Mn-rich antiferromagnetic or ferrimagnetic phases within the bombarded regions that are exchange coupled to the non-irradiated - ferromagnetic host.

36 MATERIALS SCIENCE↗
DOE OSTI2018

An Investigation on Magnetic-Interacting Fe{sub 3}O{sub 4} Nanoparticles Prepared by Electrochemical Synthesis Method

Naked, polyvinylchloride (PVC)-coated, and polyethylene glycol (PEG)-coated Fe{sub 3}O{sub 4} nanoparticles (NPs) with an equivalent average particle size of 10 nm were prepared through an electrochemical synthesis method. The structural and magnetic properties were investigated systematically. The X-ray diffraction study coupled with the Rietveld refinement of XRD data showed that samples are crystallized in cubic spinel structure with space group referred to as Fd3m. The nature of DC magnetization versus field M (H) plot for sample resembles qualitatively with ferromagnetic (FM) systems. The AC-susceptibility data obtained at different frequencies confirmed the presence of a frequency-dependent freezing temperature. Based on the interparticle interaction strength and phenomenological models (Néel–Brown, Vogel–Fulcher, and critical slowing down), the magnetic dynamic behavior of nanoparticles was characterized. By fitting the experimental data with mentioned models, the possibility existence of superparamagnetic and a spin-glass state at low temperatures was proposed for the samples. Also, the obtained values from fitting performance showed a coating effect on interparticle interaction for PVC- and PEG-coated Fe{sub 3}O{sub 4} nanoparticles.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

NMR study of the paramagnetic state of low-dimensional magnets LiCu{sub 2}O{sub 2} and NaCu{sub 2}O{sub 2}

A comprehensive NMR study of the magnetic properties of single crystal LiCu{sub 2}O{sub 2} (LCO) and NaCu{sub 2}O{sub 2} (NCO) is carried out in the paramagnetic region of the compounds for various orientations of single crystals in an external magnetic field. The values of the electric-field gradient (EFG) tensor, as well as the dipole and transferred hyperfine magnetic fields for {sup 63,65}Cu, {sup 7}Li, and {sup 23}Na nuclei are determined. The results are compared with the data obtained in previous NMR studies of the magnetically ordered state of LCO/NCO cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Magnetism and magnetocrystalline anisotropy in single-layer PtSe{sub 2}: Interplay between strain and vacancy

The electronic and magnetic properties of the newly synthesized single-layer (1 L) transition-metal dichalcogenide (TMD) PtSe{sub 2} are studied by first-principles calculations. We find the strain or selenium vacancy (V{sub Se}) alone cannot induce the magnetism. However, an interplay between strain and V{sub Se} leads to the magnetism due to the breaking of Pt-Pt metallic bonds. Different from the case of 1 L-MoS{sub 2} with V{sub S}, the defective 1 L-PtSe{sub 2} has the spatially extended spin density, which is responsible for the obtained long range ferromagnetic coupling. Moreover, the 1 L-PtSe{sub 2} with V{sub Se} undergoes a spin reorientation transition from out-of-plane to in-plane magnetization, accompanying a maximum magnetocrystalline anisotropy energy of ∼9–10.6 meV/V{sub Se}. These results indicate the strain not only can effectively tune the magnetism but also can manipulate the magnetization direction of 1 L-TMDs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Ab Initio Study of Magnetism in Nonmetal Adsorption on Arsenene Monolayer

The structural, electronic, and magnetic properties of adsorbed arsenene monolayer with a series of nonmetal atoms, including B, C, F, N, and O, are systematically investigated using first-principles calculations. Nonmagnetic behavior shows in the B-, F-, N-, and O-adsorbed systems. The magnetism has been observed in the case of C, which originated from the 2p electrons of C and its neighboring As 4p states. Meanwhile, these nonmagnetic elements can induce impurity channels around Fermi level, which reduce band gap of the systems. It implies that the B-, F-, N-, and O-adsorbed arsenene tune into narrow gap semiconductors without magnetic character. The X-adsorbed (X = B, C, F, N, and O) arsenene systems are promising candidates for spintronic devices in the future.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗