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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Magnetic Properties of Some Tellurite Glasses

The AC magnetic susceptibility in the range 5–130 K of the tellurite glass systems: TeO{sub 2}–MnO{sub 2}–ZnO–PbO and TeO{sub 2}–MnO{sub 2}–V{sub 2}O{sub 5}–Fe{sub 2}O{sub 3} was measured and analyzed. The investigations of the AC magnetic susceptibility facilitated the determination of the molar susceptibility, paramagnetic magnetic susceptibility, paramagnetic Curie temperature, and magnetic entropy changes of the tellurite glasses. The results clarified that the temperature dependence of the magnetic susceptibility deviated from the Curie law and the increase of the small negative values of Curie temperature indicated negative interchange interactions between the antiferromagnetically coupled manganese ions within the present glass network. The magnetic moments evaluated from susceptibility measurements of the glasses show the predominance of the Mn{sup 2+} valence state than Mn{sup 3+} valence state of MnO{sub 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Ab Initio Investigation of Structural Stability and Electronic and Magnetic Properties of the Half-Heusler Alloys: MTiSb (M = Fe, Co, and Ni)

The structural, electronic, and magnetic properties of the half-Heusler MTiSb (M = Fe, Co, Ni) compounds have been examined using the first-principles calculations with the full-potential linear augmented plane wave (FP-LAPW) method within the density functional theory. The electronic structures and magnetic properties of MTiSb (M = Fe, Co, Ni) are studied. It is found that the calculated lattice parameter is in good agreement with the theoretical values. Using the general gradient approximation (GGA), we observe that for the three compounds, the α phase is more stable than β and γ phases. CoTiSb is a semiconductor, NiTiSb is a metal, while the FeTiSb compound is a half-metallic, and the total magnetic moments are 0.00085, − 0.26074, and 0.99353 μ B, respectively, which is in agreement with the Slater–Pauling rule, Furthermore, the origin for the appearance of the half-metallic band gap in the FeTiSb compound was also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Magnetic Relaxation Experiments in CNT-Based Magnetic Nanocomposite

In this work, we discuss the relaxation of the magnetic moments in a novel carbon nanotube (CNT)-based nanocomposite synthesized by using chemical vapor deposition process. The material consists of a matrix of CNT filled by Fe-based nanoparticles. This structure is seen clearly by scanning and transmission. X-ray diffraction and Raman spectroscopy are used to detect the predominant Fe{sub 3}C phase and the CNT presence in the sample, respectively. The results obtained from both hysteresis cycles, M(H), and zero field cooled-field cooled (ZFC-FC) measurements confirm that the material is characterized by both a strong ferromagnetic exchange and random magnetic anisotropy. For the first time, we have been able to fit the magnetic relaxation data, M(t), by using both the two distributions of nanoparticles data deduced from the ZFC-FC data and the temperature dependence of the magnetic anisotropy obtained from the law of approach to saturation in random magnets. .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Elastic and Magnetic Properties of Cubic Fe{sub 4}C from First-Principles

First-principles calculations, based on density functional theory (DFT), are used to study the phase stability, elastic, magnetic, and electronic properties of cubic (c)-Fe{sub 4}C. Our results show that c-Fe{sub 4}C has ferromagnetic (FM) ground state as compared with antiferromagnetic (AFM) and nonmagnetic (NM) states. To study the phase stability of c-Fe{sub 4}C, BCC Fe{sub 4}C, FCC Fe{sub 4}C, and BCC Fe{sub 16}C, where C is considered at tetrahedral and octahedral interstitial sites, are also considered. The formation energy of c-Fe{sub 4}C is smaller than BCC Fe{sub 4}C but the shear moduli of c-Fe{sub 4}C is negative in both the FM and AFM states indicating that c-Fe{sub 4}C is dynamically not stable in the magnetic (FM/AFM) state. NM state has positive shear moduli which illustrates that the instability in c-Fe{sub 4}C is due to magnetism and can lead to soft phonon modes. The calculated formation energy also shows that c-Fe{sub 4}C has higher formation energy than the FCC Fe{sub 4}C indicating no possibility of c-Fe{sub 4}C in low carbon steels at low temperature. The magnetic moment of Fe in c-Fe{sub 4}C is also sensitive to lattice deformation and the electronic structure reveals the itinerant nature of electrons responsible for metallic behavior of c-Fe{sub 4}C.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Ultrafast measurements of the interfacial spin Seebeck effect in Au and rare-earth iron-garnet bilayers

We investigate picosecond spin-currents across Au/iron-garnet interfaces in response to ultrafast laser heating of the electrons in the Au film. In the picoseconds after optical heating, interfacial spin currents occur due to an interfacial temperature difference between electrons in the metal and magnons in the insulator. We report measurements of this interfacial longitudinal spin Seebeck effect between Au and rare-earth iron-garnet insulators, i.e. RE 3 Fe 5 O 12 , where RE is Y, Eu, Tb, Tm. By systematically varying the rare-earth element, we modify the total magnetic moment of the iron-garnet. We use time domain thermoreflectance (TDTR) measurements to characterize the thermal response of the bilayer to ultrafast optical heating. We use time-resolved magneto-optic Kerr effect (TR-MOKE) measurements of the Au layer to measure the time-evolution of spin accumulation in the Au film. Replacing Y with other rare-earths enhances the electron-magnon conductance G e-m at the Au iron-garnet interface by as much as a factor of three. The electron-magnon conductance does not follow the trend of either the total magnetization of the iron garnet or the magnetic moment of the rare-earth.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A First Principle Study of Half-Heusler Compounds CrTiZ (Z = P, As)

Half-Heusler compounds CrTiZ (Z = P, As) are studied in the framework of Density Functional Theory (DFT). Structural, mechanical, elastic, electronic, magnetic, and optical properties are derived using first principles. The full-potential linearized augmented plane wave (FP-LAPW) method is used for these investigations as implemented in the WIEN2k simulation package. We have found that the equilibrium lattice parameters are in the range 5.5–5.9 Å. Small band gaps of 0.5–0.6 eV in the minority spin-down states are observed. On the other hand, in the majority spin-up states, there is no band gap. Hence, the compounds are of half-metallic nature, having 100 % polarization at Fermi level. Comparatively, a higher total magnetic moment of ∼ 3μ{sub B} is found for both the compounds. Furthermore, optical properties are derived using dielectric function. Reflectivity, refractive index, conductivity and absorption coefficient are calculated. It is revealed from the imaginary part of the dielectric function that the compounds are optically metallic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Structural, Magnetic, and Magentostrictive Properties of Dy{sub 1−x}(Tb{sub 0.2}Pr{sub 0.8}){sub x}Fe{sub 1.93} (0≤ x ≤ 0.5) Compounds

The structural, magnetic, and magnetostrictive properties of light rare earth Pr-contained Dy{sub 1−x}(Tb{sub 0.2}Pr{sub 0.8}){sub x}Fe{sub 1.93} (0≤ x ≤ 0.5) compounds were investigated by means of X-ray diffraction, vibrating sample magnetometer and strain gauge technique. A single Laves phase with MgCu{sub 2}-type structure was obtained in the composition range of 0.0≤ x ≤ 0.25. The lattice parameter of the Lave phase increases with increasing x due to the large radius of Pr{sup 3+} and Tb{sup 3+}. The Curie temperature of the Laves phase decreases with increasing x, indicating weakening strength of 3d-4f coupling. The saturation magnetization decreases with increasing x, which could be ascribed to the opposite magnetic moment alignments of Pr and (Td,Dy) with Fe. The magnetostriction of Dy{sub 1−x}(Tb{sub 0.2}Pr{sub 0.8}){sub x}Fe{sub 1.93} increases with increasing x, peaks at x = 0.25 and then decreases with further increasing x.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Preference of the Ferromagnetic Ordering for the Novel Heusler Rh{sub 2}MnTi Compound

Rh{sub 2}MnTi is a novel magnetic material at potentially high speed in modern technologies. Rh{sub 2}MnTi is susceptible to structural distortion of the ferromagnetic tetragonal Heusler. We tried to provide additional information on the magnetic order for this material through a study by means of first-principle calculations. The numerical work on the electronic and magnetic structural parameters of Rh{sub 2}MnTi crystal was carried out within the density functional theory level as implemented in all-electron full-potential linearized augmented plane wave (FP-LAPW) method. The dependence of the elastic constants C{sub ij} for cubic structure, the aggregate elastic moduli B and G, and the related mechanical properties are investigated. The calculated density of states (DOS) and band structure for Rh{sub 2}MnTi show the absence of energy band gap in their minority-spin channel which moves away half-metallic character, the magnetic moments, and Curie temperature in both the ferromagnetic and antiferromagnetic ordering are also investigated. The dependence of Seebeck coefficient, electrical conductivity, and power factor on the Fermi level are investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Study of the Electronic Structure, Magnetic and Elastic Properties and Half-Metallic Stability on Variation of Lattice Constants for CoFeCrZ (Z = P, As, Sb) Heusler Alloys

Structural, elastic, magnetic and electronic properties of CoFeCrZ (Z = P, As, Sb) Heusler alloys in their YI-type structure have been computed by density functional theory-based WIEN2k code within generalized gradient approximation for exchange correlation functions. Values of formation energy and elastic constants verify the stability of these alloys. True half metallic ferromagnetic behaviour with 100% spin polarization and good band gap in the minority spin are obtained for all the alloys. Magnetic moment of these alloys is 4.00 μ{sub B}, which matches well with the value predicted from Slater-Pauling rule. Half-metallicity is maintained over a wide range of lattice constants making these alloys promising for spintronics device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Fast and stable deep-learning predictions of material properties for solid solution alloys

We present a novel deep learning (DL) approach to produce highly accurate predictions of macroscopic physical properties of solid solution binary alloys and magnetic systems. The major idea is to make use of the correlations between different physical properties in alloy systems to improve the prediction accuracy of neural network (NN) models. We use multitasking NN models to simultaneously predict the total energy, charge density and magnetic moment. These physical properties mutually serve as constraints during the training of the multitasking NN, resulting in more reliable DL models because multiple physics properties are correctly learned by a single model. Two binary alloys, copper–gold (CuAu) and iron–platinum (FePt), were studied. Our results show that once the multitasking NN's are trained, they can estimate the material properties for a specific configuration hundreds of times faster than first-principles density functional theory calculations while retaining comparable accuracy. We used a simple measure based on the root-mean-squared errors to quantify the quality of the NN models, and found that the inclusion of charge density and magnetic moment as physical constraints leads to more stable models that exhibit improved accuracy and reduced uncertainty for the energy predictions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Investigation of the Substituting Effect of Chromium on the Electronic Structures and the Half-Metallic Ferromagnetic Properties of BaO

The first-principle methods of density functional theory were used to investigate the doping effect of 3d chromium (Cr) impurities on the electronic properties and the ferromagnetic arrangement of (BaO) barium oxide such as Ba{sub 1−x}Cr{sub x}O compounds at concentrations x = 0.25, 0.5, and 0.75. The p–d exchange coupling between 2p states of oxygen (O) and 3d (Cr) partially filled levels induces magnetism in the Ba{sub 1−x}Cr{sub x}O compounds. The majority- and minority-spin states exhibit respectively metallic and semiconducting features, leading to a half-metallic (HM) character with 100% spin polarization for all concentrations. The Ba{sub 0.75}Cr{sub 0.25}O, Ba{sub 0.5}Cr{sub 0.5}O, and Ba{sub 0.25}Cr{sub 0.75}O compounds have integral total magnetic moments of 4 μ{sub B} per Cr atom with HM gaps of 0.055, 0.218, and 0.308 eV, respectively. These behaviors form accurate half-metallic ferromagnets and make Ba{sub 1−x}Cr{sub x}O as potential materials for possible spintronics applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural, Magnetic, and Optoelectronic Properties of CuMnSe{sub 2}-Chalcopyrite: DFT + U and Hybrid Functional Investigation

The work presented in this manuscript is a study of structural, magnetic, and optoelectronic properties of CuMnSe{sub 2}-chalcopyrite by FP-(L)APW + lo method using semilocal and hybrid functional. Structural properties such as cell parameters, bulk modulus, and its pressure derivative, as well as the cohesive energy and total energy of the unit cell were determined for the three magnetic phases (AFM, FM, and NM) from which it has been found that CuMnSe{sub 2}-chalcopyrite is ferromagnetic. The studied elastic properties confirm the mechanical stability of CuMnSe{sub 2} in its chalcopyrite structure. Electronic properties, such as the band gap energy, density of states, and charge density, and magnetic properties, such as Hubbard term estimation, magnetic moment, and polarization of the CuMnSe{sub 2}, have been predicted by several methods (GGA-PBEsol + U{sub eff}, GGA-PBEsol-(mBJ) + U{sub eff}, and hybrid functional). Optical properties such as the analysis of the dielectric function and the prediction of the refractive index and birefringence variations were also studied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Half Metallic Ferromagnetic Character in ZnXP{sub 2} (X = Ge, Si) Chalcopyrites Doped with Mn

The electronic and the magnetic properties of ZnXP{sub 2} (X = Ge, Si) were studied using the Korring-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA). The total and partial density of state (DOS) are computed for different Mn concentrations. The total magnetic moment, FM and DLM energies, and their variation as well as Curie temperature are also given. It is shown that the substitution of X cations (X = Ge, Si) by Mn atoms in ZnXP{sub 2} chalcopyrites leads to half metallic ferromagnetic character with double exchange mechanism. Thus, the critical temperature can be controlled by varying the concentration of manganese impurity.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Strongly coupled rotational band in Mg 33

The “island of inversion” at N≈20 for the neon, sodium, and magnesium isotopes has long been an area of interest both experimentally and theoretically due to the subtle competition between 0p-0h and np-nh configurations leading to deformed shapes. However, the presence of rotational band structures, which are fingerprints of deformed shapes, have only recently been observed in this region. In this work, we report on a measurement of the low-lying level structure of Mg33 populated by a two-stage projectile fragmentation reaction and studied with the Gamma Ray Energy Tracking In-Beam Nuclear Array (GRETINA). The experimental level energies, ground-state magnetic moment, intrinsic quadrupole moment, and γ-ray intensities show good agreement with the strong-coupling limit of a rotational model.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Magnetic Properties of a Layered Cobaltite Sr{sub 1–x}Y{sub x}CoO{sub 3–δ} (x = 0.1)

The structure, the magnetic and magnetotransport properties of perovskite Sr{sub 0.9}Y{sub 0.1}CoO{sub 2.63} have been studied. The sample is shown to have a two-phase structure. The main phase has a tetragonally distorted unit cell and is described by space group I4/mmm. The broadening of the reflections with indices corresponding to doubling unit cell parameter c indicates the absence of the rigorous translation symmetry along axis c. The existence of the broadened superstructure reflection observed in the diffraction pattern at small angles at temperature lower than 400 K is explained by the existence of the monoclinic phase whose content is significantly lower than that of the tetragonal phase, but is dominant in the Sr{sub 0.8}Y{sub 0.2}CoO{sub 3–δ} composition. The spontaneous magnetization appears as the monoclinic phase forms. The magnetic structure is mainly G-type antiferromagnetic with magnetic moments 1.5μ{sub B} in the layers of CoO{sub 6} octahedra and 2μ{sub B} in the anion-deficit CoO{sub 4+γ} layers. The conduction of the Sr{sub 0.9}Y{sub 0.1}CoO{sub 2.63} composition has a semiconducting character. The magnetoresistance is 57% in a field of 14 T at a temperature of 5 K and strongly decreases with the temperature increase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Search for CPT and Lorentz invariance violation in the muon $g -2$ experiment at Fermilab

The Muon g-2 experiment at Fermilab (E989) aims to measure the anomalous magnetic moment of the muon, $a_{\mu}= (g-2)/2$, to a groundbreaking precision of $140$ ppb, obtaining a near four-fold increase in precision over the previous experiment, E821, at the Brookhaven National Laboratory (BNL). The value of $a_{\mu}$ from BNL currently differs from the Standard Model prediction by $\sim 3.7$ standard deviations, suggesting the potential for new physics and therefore, motivating a new experiment.Because the theory predicts this number with high precision, testing the g-factor through experiment provides a stringent test of the SM and can suggest physics beyond the Standard Model. The goal of the Fermilab Muon $g-2$ experiment is to increase the statistical precision by more than a factor of 20 and reduce systematic errors by a factor of 3. By measuring muon precession rate ($\omega_a$) in an external magnetic field, the anomalous magnetic moment will be calculated. This is an incredibly challenging experiment with a unique opportunity to provide new insight into nature. \\ The $g-2$ data also provides a great opportunity for setting the most stringent limits on some of the Standard Model Extension CPT Lorentz violating (LV) parameters in the muon sector. One of the CPT and Lorentz violating signatures that we can look for using $g-2$ data is a sidereal variation of $\omega_a(t)$. Extensive simulation studies confirm that the sensitivity regarding the sidereal varation roughly scales with $\omega_a$ uncertainty. Hence, the $g-2$ experiment at FNAL should be able to reach limits of $\sim 5\times10^{-25}$ GeV. Because the CPT and LV analyses are essentially studies of variations in $\omega_a$ as a function of time and charge, performing an $\omega_a$ analysis sets the stage for the CPT and LV measurement. This dissertation focuses on the methodology of a fully functioning framework and analyzing the Fermilab Muon $g - 2$ Run 2 data containing $\sim 11$ billion events above an energy threshold of $1.7$~GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Measurement of the anomalous precession frequency in the Muon $g-2$ experiment at Fermilab

The muon anomalous magnetic moment, $a_\mu=\frac{g-2}{2}$, is a low energy observable that can be both measured and computed with high precision, therefore it can provide an important test of the Standard Model and it is a sensitive probe for new physics. In particular, any discrepancy between the experimental value and the theoretical prediction can be due to Beyond Standard Model physics contribution. The E989 Muon $g-2$ Experiment at Fermilab aims to measure $a_\mu$ with a precision of 140 parts per billion, four time more precisely than the previous experiment at Brookhaven National Laboratory (BNL). E989 seeks to either resolve or confirm the observed discrepancy of $3.7\sigma$ between the Standard Model value and the experimental one. The $a_\mu$ measurement requires a precise determination of both the muon spin anomalous precession frequency and the average magnetic field seen by the muons as they circulate in a storage ring. The anomalous precession frequency measurement is b ased on the time distribution of high-energy decay positrons observed by 24 electromagnetic calorimeters placed around the inner circumference of the ring, while the magnetic field is constantly monitored by NMR probes. Last spring, E989 has published a new measurement of $a_\mu$ from the 2018 data-taking campaign, confirming the previous BNL value with comparable precision, bringing the discrepancy with the theoretical value to $4.2\sigma$. The candidate worked on the measurement of the anomalous precession frequency and on the estimate of the related systematic uncertainties of the 2018 data, giving an important contribution to the data analysis. The result presented in chapters 4 and 5 of this dissertation was included, among three other analyses, to determine the estimate of the anomalous magnetic moment by the Muon g-2 experiment. This work will present the anomalous precession frequency analysis technique and its result, the related systematics and finally the latest $a_\mu$ result.

Sorbara, Matteo↗
DOE OSTI2017

Study of Electronic, Mechanical, Magnetic, and Optical Properties of Mg{sub 0.75}TM{sub 0.25}S/Se (TM = Fe, Co, Ni): A First Principle Approach

A systematic study of the magnetism, mechanical, and optical behaviors of Mg{sub 0.75}TM{sub 0.25}S/Se (TM = Fe, Co, Ni) ordered alloys was conducted using the full-potential linearized augmented plane wave plus local orbital method. The ferromagnetic (FM) and antiferromagnetic (AFM) ground state energy differences and enthalpy of formation scrutinized their dynamical stability in the FM state. The elastic parameters decided their mechanical stability, ductile behavior, and partial ionic and covalent characters. Moreover, the calculated band structures and density of states revealed the type and origin of magnetism in terms of exchange splitting energies and exchange constants. The Fe- and Ni-doped MgS/Se alloys exhibited half metallic ferromagnetic (HMF) characteristics, while the Co-doped MgS/Se alloy depicted ferromagnetic semiconducting characters. A reduction in the magnetic moments of the transition metals was observed compared with their free space value, which is due to strong p-d hybridization. Furthermore, the optical spectrum showed the maximum light absorbed in the visible and in the UV region of the electromagnetic spectrum, which makes them potential candidates for optoelectronic and spintronic device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗