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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Transient electrically detected magnetic resonance spectroscopy applied to organic solar cells

The influence of light-induced paramagnetic states on the photocurrent generated by polymer:fullerene solar cells is studied using spin-sensitive techniques in combination with laser-flash excitation. For this purpose, we developed a setup that allows for simultaneous detection of transient electron paramagnetic resonance as well as transient electrically detected magnetic resonance (trEDMR) signals from fully processed and encapsulated solar cells. Combining both techniques provides a direct link between photoinduced triplet excitons, charge transfer states, and free charge carriers as well as their influence on the photocurrent generated by organic photovoltaic devices. Our results obtained from solar cells based on poly(3-hexylthiophene) as electron donor and a fullerene-based electron acceptor show that the resonant signals observed in low-temperature (T = 80 K) trEDMR spectra can be attributed to positive polarons in the polymer as well as negative polarons in the fullerene phase, indicating that both centers are involved in spin-dependent processes that directly influence the photocurrent.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Synthesis, characterization and magnetic properties of ultrafine Co{sub 3}O{sub 4} octahedra

Octahedral Co{sub 3}O{sub 4} nanoparticles were synthesised in an aqueous ammonia solution using hexagonal β-Co(OH){sub 2} nanoplates as starting materials. Electron microscopy analysis indicates that the Co{sub 3}O{sub 4} particles have diameters of 20-40 nm and adopt a well-crystallized cubic spinel structure. The octahedral habit was verified by high angle annular dark field imaging. High resolution electron microscopy results revealed that the long axis of the octahedral Co{sub 3}O{sub 4} nanoparticles coincides with crystallographic <111> direction and the facets are the (111) planes. Magnetization measurements reveal antiferromagnetic ordering below 10 K, with a paramagnetic Curie temperature of 3 K and a paramagnetic susceptibility that is double that expected for high-spin Co{sup 2+}. The results show that a substantial fraction of the B-site Co{sup 3+} in the nanoparticles is in a high-spin state.

36 MATERIALS SCIENCE↗
DOE OSTI2015

W-band EPR studies of high-spin nitrenes with large spin-orbit contribution to zero-field splitting

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6-tribromo-5-nitrenobenzene (T1) has D{sub T} = 1.369 cm{sup −1}, E{sub T} = 0.093 cm{sup −1}, and g = 2.0033, quintet 1-azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows D{sub Q} = − 0.306 cm{sup −1}, E{sub Q} = 0.0137 cm{sup −1}, and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has D{sub S} = − 0.203 cm{sup −1}, E{sub S} = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the “heavy atom effect.” This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Influence of temperature on spin polarization dynamics in dilute nitride semiconductors—Role of nonparamagnetic centers

We report theoretical studies of spin polarization dynamics in dilute nitride semiconductors. We develop a commonly used rate equation model [Lagarde et al., Phys. Status Solidi A 204, 208 (2007) and Kunold et al. Phys. Rev. B 83, 165202 (2011)] to take into account the influence of shallow localizing states on the temperature dependence of spin polarization dynamics and a spin filtering effect. Presented investigations show that the experimentally observed temperature dependence of a spin polarization lifetime in dilute nitrides can be related to the electron capture process by shallow localizing states without paramagnetic properties. This process reduces the efficiency of spin filtering effect by deep paramagnetic centers, especially at low temperatures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Study of Ho-doped Bi{sub 2}Te{sub 3} topological insulator thin films

Breaking time-reversal symmetry through magnetic doping of topological insulators has been identified as a key strategy for unlocking exotic physical states. Here, we report the growth of Bi{sub 2}Te{sub 3} thin films doped with the highest magnetic moment element Ho. Diffraction studies demonstrate high quality films for up to 21% Ho incorporation. Superconducting quantum interference device magnetometry reveals paramagnetism down to 2 K with an effective magnetic moment of ∼5 μ{sub B}/Ho. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact with Ho doping, consistent with the material's paramagnetic state. The large saturation moment achieved makes these films useful for incorporation into heterostructures, whereby magnetic order can be introduced via interfacial coupling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Gd-doped BaSnO{sub 3}: A transparent conducting oxide with localized magnetic moments

We have synthesized transparent, conducting, paramagnetic stannate thin films via rare-earth doping of BaSnO{sub 3}. Gd{sup 3+} (4f{sup 7}) substitution on the Ba{sup 2+} site results in optical transparency in the visible regime, low resistivities, and high electron mobilities, along with a significant magnetic moment. Pulsed laser deposition was used to stabilize epitaxial Ba{sub 0.96}Gd{sub 0.04}SnO{sub 3} thin films on (001) SrTiO{sub 3} substrates, and compared with Ba{sub 0.96}La{sub 0.04}SnO{sub 3} and undoped BaSnO{sub 3} thin films. Gd as well as La doping schemes result in electron mobilities at room temperature that exceed those of conventional complex oxides, with values as high as 60 cm{sup 2}/V·s (n = 2.5 × 10{sup 20 }cm{sup −3}) and 30 cm{sup 2}/V·s (n = 1 × 10{sup 20 }cm{sup −3}) for La and Gd doping, respectively. The resistivity shows little temperature dependence across a broad temperature range, indicating that in both types of films the transport is not dominated by phonon scattering. Gd-doped BaSnO{sub 3} films have a strong magnetic moment of ∼7 μ{sub B}/Gd ion. Such an optically transparent conductor with localized magnetic moments may unlock opportunities for multifunctional devices in the design of next-generation displays and photovoltaics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Atomic scale understanding of magnetic properties in Ni{sub 50}Fe{sub 35}Co{sub 15}

Mössbauer spectroscopic studies have been carried out at different temperatures across ferromagnetic to paramagnetic transition in Ni{sub 50}Fe{sub 35}Co{sub 15} and the evolution of hyperfine parameters such as centre shift and magnetic hyperfine fields with temperature has been studied. Mössbauer spectrum obtained at 300 K in Ni{sub 50}Fe{sub 35}Co{sub 15} exhibiting fcc crystal structure is a six line pattern with the mean value of the hyperfine field close to 33 Tesla. Ferromagnetic to paramagnetic transition has been observed to occur in this system around 895 K matching with that of magnetization results. Debye temperature of this nickel rich alloy is deduced to be around 470 K matching with that of Ni. Effect of prolonged annealing at 750 K on the magnetic property is also investigated with respect to the thermal stability of the alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Phase transitions and magnetocaloric and transport properties in off-stoichiometric GdNi{sub 2}Mn{sub x}

The structural, magnetic, magnetocaloric, transport, and magnetoresistance properties of the rare-earth intermetallic compounds GdNi{sub 2}Mn{sub x} (0.5 ≤ x ≤ 1.5) have been studied. The compounds with x = 0.5 and 0.6 crystallize in the cubic MgCu{sub 2} type phase, whereas samples with x ≥ 0.8 form a mixed MgCu{sub 2} and rhombohedral PuNi{sub 3} phase. A second order magnetic phase transition from a ferromagnetic to paramagnetic state was observed near the Curie temperature (T{sub C}). The GdNi{sub 2}Mn{sub x} (0.5 ≤ x ≤ 1.5) compounds order in a ferrimagnetic structure in the ground state. The largest observed values of magnetic entropy changes (at T{sub C} for ΔH = 5T) were 3.9, 3.5, and 3.1 J/kg K for x = 0.5, 0.6, and 0.8, respectively. The respective relative values of the cooling power were 395, 483, and 220 J/kg. These values are greater than some well-known prototype magnetocaloric materials such as Gd (400 J/kg) and Gd{sub 5}Si{sub 2}Ge{sub 2} (240 J/kg). Analysis of the resistivity data showed a T{sup 2} dependence at low temperatures, suggesting strong electron-phonon interactions, whereas at higher temperatures s-d scattering was dominated by the electron-phonon contribution, resulting in a slow increase in resistivity. Magnetoresistance values of ∼−1.1% were found for x = 0.5 near T{sub C}, and −7% for x = 1.5 near T = 80 K.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Properties of Gd{sub 2}O{sub 3} nanoparticles studied by hyperfine interactions and magnetization measurements

The magnetic behavior of Gd{sub 2}O{sub 3} nanoparticles, produced by thermal decomposition method and subsequently annealed at different temperatures, was investigated by magnetization measurements and, at an atomic level, by perturbed γ − γ angular correlation (PAC) spectroscopy measuring hyperfine interactions at {sup 111}In({sup 111}Cd) probe nuclei. Nanoparticle structure, size and shape were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). Magnetization measurements were carried out to characterize the paramagnetic behavior of the samples. XRD results show that all samples crystallize in the cubic-C form of the bixbyite structure with space group Ia3. TEM images showed that particles annealed at 873 K present particles with highly homogeneous sizes in the range from 5 nm to 10 nm and those annealed at 1273 K show particles with quite different sizes from 5 nm to 100 nm, with a wide size distribution. PAC and magnetization results show that samples annealed at 873 and 1273 K are paramagnetic. Magnetization measurements show no indication of blocking temperatures for all samples down to 2 K and the presence of antiferromagnetic correlations.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Ti{sub 3}CrCu{sub 4}: A possible 2-D ferromagnetic spin fluctuating system

Ti{sub 3}CrCu{sub 4} is a new ternary compound which crystallizes in the tetragonal Ti{sub 3}Pd{sub 5} structure type. The Cr atoms form square nets in the a-b plane (a = 3.124 Å) which are separated by an unusually large distance c = 11.228 Å along the tetragonal axis, thus forming a -2-D Cr-sublattice. The paramagnetic susceptibility is characterized by a low effective moment, μ{sub eff} = 1.1 μ{sub B}, a low paramagnetic Curie temperature θ{sub P} (below 7 K) and a temperature independent χ{sub 0} = 6.7 x 10{sup −4} emu/mol. The magnetization at 1.8 K increases rapidly with field nearly saturating to 0.2 μ{sub B}/f.u. The zero field heat capacity C/T shows an upturn below 7 K (∼190 mJ/mol K{sup 2} at ∼0.1K) which is suppressed in applied magnetic fields and interpreted as suggesting the presence of spin fluctuations. The resistivity at low temperatures shows non-Fermi liquid behavior. Overall, the experimental data thus reveal an unusual magnetic state in Ti{sub 3}CrCu{sub 4}, which likely has its origin in the layered nature of the Cr sub-lattice and ferromagnetic spin fluctuations. Density functional theoretical calculations reveal a sharp Cr density of states peak just above the Fermi level, indicating the propensity of Ti{sub 3}CrCu{sub 4} to become magnetic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Theoretical and experimental studies of the molecular orbital bonding coefficients for Cu{sup 2+} ion in cesium hydrogen oxalate single crystals

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu{sup 2+} ions in cesium hydrogen oxalate single crystals have been investigated at room temperature. The spin-Hamiltonian parameters (g and A), have been determined. Crystalline field around the Cu{sup 2+} ion is almost axially symmetric. The results show a single paramagnetic site which confirms the triclinic crystal symmetry. Molecular orbital bonding coefficients are studied from the EPR and optical data. Theoretical octahedral field parameter and the tetragonal field parameters have been evaluated from the superposition model. Using these parameters, various bonding parameters are analyzed and the nature of bonding in the complex is discussed. The theoretical results are supported by experimental results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Inverse and normal magnetocaloric effects in LaFe{sub 12}B{sub 6}

Intrinsic magnetic properties and magnetocaloric effect were studied for LaFe{sub 12}B{sub 6} itinerant-electron system, which presents an antiferromagnetic ground state below 36 K. For certain magnetic fields values, LaFe{sub 12}B{sub 6} exhibits a sequence of two successive magnetic transitions: an antiferromagnetic-ferromagnetic (AFM-FM) transition at low temperature followed by a ferromagnetic-paramagnetic transition, leading to normal and inverse magnetocaloric effects, respectively. At finite temperatures, both antiferromagnetic (AFM) and paramagnetic states can be transformed into a ferromagnetic (FM) state via a field-induced metamagnetic transition accompanied with a huge magnetic hysteresis. Moreover, we reveal that, at low temperatures, the magnetization displays abrupt jumps across the first-order AFM-FM transition, giving rise to an unusual and unique staircase-like behavior. LaFe{sub 12}B{sub 6} exhibits both normal magnetocaloric effect around the Curie temperature and large inverse magnetocaloric effect around the AFM-FM transition temperature; for μ{sub 0}ΔH = 7 T, ΔS{sub M} is found to be −6.8 and 19 J kg{sup −1} K{sup −1} around 38 and 8 K, respectively.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Strain development during the phase transition of La(Fe,Mn,Si){sub 13}H{sub z}

We use powder X-ray diffraction to evaluate the temperature dependence of the crystalline properties during the magnetic phase transition of La(Fe,Mn,Si){sub 13}H{sub z} as a function of the Fe/Mn/Si ratio. Both the paramagnetic and ferromagnetic phases were observed as peak overlaps in the patterns around the Curie temperature (T{sub C}) occurring continuously in a temperature range of about 5 K around T{sub C.} Using the Williamson-Hall method, we evaluate the strain developing in the crystallites during the transition and find that it is associated with the growth of the paramagnetic phase as the transition occurs. Based on our measurements and microstructure analyses, we propose that cracking during the phase transition is due to or aggravated by the small content of a La-rich phase.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spin transport, magnetoresistance, and electrically detected magnetic resonance in amorphous hydrogenated silicon nitride

We report on a study of spin transport via electrically detected magnetic resonance (EDMR) and near-zero field magnetoresistance (MR) in silicon nitride films. Silicon nitrides have long been important materials in solid state electronics. Although electronic transport in these materials is not well understood, electron paramagnetic resonance studies have identified a single dominating paramagnetic defect and have also provided physical and chemical descriptions of the defects, called K centers. Our EDMR and MR measurements clearly link the near-zero field MR response to the K centers and also indicate that K center energy levels are approximately 3.1 eV above the a-SiN:H valence band edge. In addition, our results suggest an approach for the study of defect mediated spin-transport in inorganic amorphous insulators via variable electric field and variable frequency EDMR and MR which may be widely applicable.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Magnetic cooling close to a quantum phase transition—The case of Er{sub 2}Ti{sub 2}O{sub 7}

Magnetic cooling, first introduced in the late twenties of last century, has regained considerable interest recently as a cost-efficient and easy-to-handle alternative to {sup 3}He-based refrigeration techniques. Especially, adiabatic demagnetization of paramagnets—the standard materials for magnetic refrigeration—has become indispensable for the present space applications. To match the growing demand for increasing the efficiency in these applications, a new concept for magnetic cooling based on many-body effects around a quantum-critical-point has been introduced and successfully tested [B. Wolf et al., Proc. Natl. Acad. Sci. U.S.A. 108, 6862 (2011)]. By extending this concept to three-dimensional magnetic systems, we present here the magnetothermal response of the cubic pyrochlore material Er{sub 2}Ti{sub 2}O{sub 7} in the vicinity of its B-induced quantum-critical point which is located around 1.5 T. We discuss performance characteristics such as the range of operation, the efficiency, and the hold time. These figures are compared with those of state-of-the-art paramagnetic coolants and with other quantum-critical systems which differ by the dimensionality of the magnetic interactions and the degree of frustration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Low-temperature spin Seebeck effect in nonmagnetic vanadium dioxide

The spin Seebeck effect (SSE) is sensitive to thermally driven magnetic excitations in magnetic insulators. Vanadium dioxide in its insulating low-temperature phase is expected to lack magnetic degrees of freedom, as vanadium atoms are thought to form singlets upon dimerization of the vanadium chains. Instead, we find a paramagnetic SSE response in V⁢O 2 films that grows as the temperature decreases below 50 K. The field and temperature-dependent SSE voltage is qualitatively consistent with a general model of paramagnetic SSE response and inconsistent with triplet spin transport. Quantitative estimates find a spin Seebeck coefficient comparable in magnitude to that observed in strongly magnetic materials. Finally, the microscopic nature of the magnetic excitations in V⁢O 2 requires further examination.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Anisotropic Melting of Frustrated Ising Antiferromagnets

Magnetic frustrations and dimensionality play an important role in determining the nature of the magnetic long-range order and how it melts at temperatures above the ordering transition T N . In this Letter, we use large-scale Monte Carlo simulations to study these phenomena in a class of frustrated Ising spin models in two spatial dimensions. Here we find that the melting of the magnetic long-range order into an isotropic gaslike paramagnet proceeds via an intermediate stage where the classical spins remain anisotropically correlated. This correlated paramagnet exists in a temperature range T N < T < T * , whose width increases as magnetic frustrations grow. This intermediate phase is typically characterized by short-range correlations; however, the two-dimensional nature of the model allows for an additional exotic feature—formation of an incommensurate liquidlike phase with algebraically decaying spin correlations. The two-stage melting of magnetic order is generic and pertinent to many frustrated quasi-2D magnets with large (essentially classical) spins.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Magnetism of a relativistic degenerate electron gas in a strong magnetic field

The magnetization and magnetic susceptibility of a degenerate electron gas in a strong magnetic field in which electrons are located on the ground Landau level and the electron gas has the properties of a nonlinear paramagnet have been calculated. The paradoxical properties of the electron gas under these conditions-a decrease in the magnetization with the field and an increase in the magnetization with the temperature-have been revealed. It has been shown that matter under the corresponding conditions of neutron stars is a paramagnet with a magnetic susceptibility of {chi} {approx} 0.001.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗