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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Calculation of nuclear spin-spin coupling constants using frozen density embedding

We present a method for a subsystem-based calculation of indirect nuclear spin-spin coupling tensors within the framework of current-spin-density-functional theory. Our approach is based on the frozen-density embedding scheme within density-functional theory and extends a previously reported subsystem-based approach for the calculation of nuclear magnetic resonance shielding tensors to magnetic fields which couple not only to orbital but also spin degrees of freedom. This leads to a formulation in which the electron density, the induced paramagnetic current, and the induced spin-magnetization density are calculated separately for the individual subsystems. This is particularly useful for the inclusion of environmental effects in the calculation of nuclear spin-spin coupling constants. Neglecting the induced paramagnetic current and spin-magnetization density in the environment due to the magnetic moments of the coupled nuclei leads to a very efficient method in which the computationally expensive response calculation has to be performed only for the subsystem of interest. We show that this approach leads to very good results for the calculation of solvent-induced shifts of nuclear spin-spin coupling constants in hydrogen-bonded systems. Also for systems with stronger interactions, frozen-density embedding performs remarkably well, given the approximate nature of currently available functionals for the non-additive kinetic energy. As an example we show results for methylmercury halides which exhibit an exceptionally large shift of the one-bond coupling constants between {sup 199}Hg and {sup 13}C upon coordination of dimethylsulfoxide solvent molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Phase diagram and magnetocaloric effects in Ni{sub 50}Mn{sub 35}(In{sub 1−x}Cr{sub x}){sub 15} and (Mn{sub 1−x}Cr{sub x})NiGe{sub 1.05} alloys

The magnetocaloric and thermomagnetic properties of Ni{sub 50}Mn{sub 35}(In{sub 1−x}Cr{sub x}){sub 15} and (Mn{sub 1−x}Cr{sub x}) NiGe{sub 1.05} systems for 0 ≤ x ≤ 0.105 and 0 ≤ x ≤ 0.1, respectively, have been studied by x-ray diffraction, differential scanning calorimetry, and magnetization measurements. Partial substitution of Cr for Mn in (Mn{sub 1−x}Cr{sub x})NiGe{sub 1.05} results in a first order magnetostructural transition from a hexagonal paramagnetic to an orthorhombic paramagnetic phase near T{sub M} ∼ 380 K (for x = 0.07). Partial substitution of Cr for In in Ni{sub 50}Mn{sub 35}(In{sub 1−x}Cr{sub x}){sub 15} shifts the magnetostructural transition to a higher temperature (T = T{sub M} ∼ 450 K) for x = 0.1. Large magnetic entropy changes of ΔS = −12 (J/(kgK)) and ΔS = −11 (J/(kgK)), both for a magnetic field change of 5 T, were observed in the vicinity of T{sub M} for (Mn{sub 1−x}Cr{sub x})NiGe{sub 1.05} and Ni{sub 50}Mn{sub 35}(In{sub 1−x}Cr{sub x}){sub 15}, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Neutral nitrogen acceptors in ZnO: The {sup 67}Zn hyperfine interactions

Electron paramagnetic resonance (EPR) is used to characterize the {sup 67}Zn hyperfine interactions associated with neutral nitrogen acceptors in zinc oxide. Data are obtained from an n-type bulk crystal grown by the seeded chemical vapor transport method. Singly ionized nitrogen acceptors (N{sup −}) initially present in the crystal are converted to their paramagnetic neutral charge state (N{sup 0}) during exposure at low temperature to 442 or 633 nm laser light. The EPR signals from these N{sup 0} acceptors are best observed near 5 K. Nitrogen substitutes for oxygen ions and has four nearest-neighbor cations. The zinc ion along the [0001] direction is referred to as an axial neighbor and the three equivalent zinc ions in the basal plane are referred to as nonaxial neighbors. For axial neighbors, the {sup 67}Zn hyperfine parameters are A{sub ‖} = 37.0 MHz and A{sub ⊥} = 8.4 MHz with the unique direction being [0001]. For nonaxial neighbors, the {sup 67}Zn parameters are A{sub 1} = 14.5 MHz, A{sub 2} = 18.3 MHz, and A{sub 3} = 20.5 MHz with A{sub 3} along a [101{sup ¯}0] direction (i.e., in the basal plane toward the nitrogen) and A{sub 2} along the [0001] direction. These {sup 67}Zn results and the related {sup 14}N hyperfine parameters provide information about the distribution of unpaired spin density at substitutional neutral nitrogen acceptors in ZnO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Antiferromagnetic ordering in NdAuGe compound

The compound NdAuGe was investigated by means of electrical resistivity, ρ(T), magnetic susceptibility, χ(T), magnetization, σ(μ{sub 0}H), and specific heat, C{sub p}(T), measurements. Powder X-ray diffraction studies confirm a hexagonal LiGaGe-type structure with space group P6{sub 3}mc (No. 186). ρ(T) data show normal metallic behaviour and a tendency toward saturation at higher temperatures. The low temperature ρ(T) data indicate a phase transition around 3.8 K. The low field dc χ(T) data show an antiferromagnetic anomaly associated with a Néel temperature at T{sub N} = 3.7 K close to the phase transition observed in ρ(T) results. At higher temperatures, χ(T) follows the paramagnetic Curie-Weiss behaviour with an effective magnetic moment μ{sub eff}=3.546(4) μ{sub B} and a paramagnetic Weiss temperature of θ{sub p}=−6.1(4) K. The value obtained for μ{sub eff} is close to the value of 3.62 μ{sub B} expected for the free Nd{sup 3+}-ion. σ(μ{sub 0}H) shows a linear behaviour with applied field up to 3 T with an evidence of metamagnetic behaviour above 3 T. C{sub p}(T) confirms the magnetic phase transition at T{sub N} = 3.4 K. The 4f-electron specific heat indicates a Schottky-type anomaly around 16.5 K with energy splitting Δ{sub 1}=25.8(4) K and Δ{sub 2}=50.7(4) K of the Nd{sup 3+} (J = 9/2) multiplet, that are associated with, respectively, the first and second excited states of the Nd{sup 3+}-ion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural phase transitions of robust insulating Bi{sub 1-x}La{sub x}Fe{sub 1-y}Ti{sub y}O{sub 3} multiferroics

In contrast to leaky BiFeO{sub 3}, of which two structural phase transitions of cycloidal modulated antiferromagnetic-paramagnetic and ferroelectric-paraelectric are observed below 850 °C, chemical co-substitution to form Bi{sub 1−x}La{sub x}Fe{sub 1−y}Ti{sub y}O{sub 3} ternary solid solution makes these multiferroic ceramics become robust insulating low dielectric loss and exhibit rich structural phase transitions. Differential thermal analysis and temperature-dependent X-ray diffraction measurements probe four first-order structural phase transitions, e.g., T{sub H} = 305 °C, T{sub N} = 365 °C, T{sub C} = 810 °C, and T{sub S} = 830 °C observed in the Bi{sub 0.98}La{sub 0.02}Fe{sub 0.99}Ti{sub 0.01}O{sub 3} system, which are reasonably attributed to Brazovskii-type cycloidal modulated antiferromagnetic-helimagnetic (at T{sub H}) and helimagnetic-paramagnetic (at T{sub N}) magnetic phase transitions, ferroelectric-paraelectric (at T{sub C}) and rhombohedral-cubic (at T{sub S}) structural phase transitions, respectively. Magnetic phase transition temperatures change a little but ferroelectric and lattice structural phase transition temperatures decrease gradually with increasing co-substitution up to composition-induced rhombohedral-(pseudo-)cubic structural phase boundary. The first-order nature of magnetic phase transition and emergence of helimagnetic phase were attributed to Ti{sup 3+} d{sup 1} magnetic disorder distribution in the Fe{sup 3+} d{sup 5}-O-Fe{sup 3+} d{sup 5} chains, while the first-order nature becomes weak with increasing co-substitution, owing to decreased ferroelectric rhombohedral lattice distortion. An intermediate rhombohedral paraelectric phase is discovered intervening between ferroelectric rhombohedral and paraelectric cubic phase, of which the temperature range defined by difference between T{sub C} and T{sub S} increases with increasing co-substitution. It was found that T{sub C} and T{sub S} are able to be predicted quantitatively by reduced mass of unit cell. These findings enrich our understanding of ferroic phase transitions and advance designing novel high temperature multiferroic compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves

We report solid state {sup 13}C and {sup 1}H nuclear magnetic resonance (NMR) experiments with magic-angle spinning (MAS) on frozen solutions containing nitroxide-based paramagnetic dopants that indicate significant perturbations of nuclear spin polarizations without microwave irradiation. At temperatures near 25 K, {sup 1}H and cross-polarized {sup 13}C NMR signals from {sup 15}N,{sup 13}C-labeled L-alanine in trinitroxide-doped glycerol/water are reduced by factors as large as six compared to signals from samples without nitroxide doping. Without MAS or at temperatures near 100 K, differences between signals with and without nitroxide doping are much smaller. We attribute most of the reduction of NMR signals under MAS near 25 K to nuclear spin depolarization through the cross-effect dynamic nuclear polarization mechanism, in which three-spin flips drive nuclear polarizations toward equilibrium with spin polarization differences between electron pairs. When T{sub 1e} is sufficiently long relative to the MAS rotation period, the distribution of electron spin polarization across the nitroxide electron paramagnetic resonance lineshape can be very different from the corresponding distribution in a static sample at thermal equilibrium, leading to the observed effects. We describe three-spin and 3000-spin calculations that qualitatively reproduce the experimental observations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Spin dynamics simulation of electron spin relaxation in Ni{sup 2+}(aq)

The ability to quantitatively predict and analyze the rate of electron spin relaxation of open-shell systems is important for electron paramagnetic resonance and paramagnetic nuclear magnetic resonance spectroscopies. We present a combined molecular dynamics (MD), quantum chemistry (QC), and spin dynamics simulation method for calculating such spin relaxation rates. The method is based on the sampling of a MD trajectory by QC calculations, to produce instantaneous parameters of the spin Hamiltonian used, in turn, to numerically solve the Liouville-von Neumann equation for the time evolution of the spin density matrix. We demonstrate the approach by simulating the relaxation of electron spin in an aqueous solution of Ni{sup 2+} ion. The spin-lattice (T{sub 1}) and spin-spin (T{sub 2}) relaxation rates are extracted directly from the simulations of the time dependence of the longitudinal and transverse magnetization, respectively. Good agreement with the available, indirectly obtained experimental data is obtained by our method.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Influence of the electron spin resonance saturation on the power sensitivity of cryogenic sapphire resonators

Here, we study the paramagnetic ions behavior in presence of a strong microwave electromagnetic field sustained inside a cryogenic sapphire whispering gallery mode resonator. The high frequency measurement resolution that can be now achieved by comparing two Cryogenic Sapphire Oscillators (CSOs) permit to observe clearly the non-linearity of the resonator power sensitivity. These observations that, in turn, allow us to optimize the CSO operation are well explained by the electron spin resonance saturation of the paramagnetic impurities contained in the sapphire crystal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Large dielectric permittivity and possible correlation between magnetic and dielectric properties in bulk BaFeO{sub 3−δ}

We report structural, magnetic, and dielectric properties of oxygen deficient hexagonal BaFeO{sub 3−δ}. A large dielectric permittivity comparable to that of other semiconducting oxides is observed in BaFeO{sub 3−δ}. Magnetization measurements indicate magnetic inhomogeneity and the system shows a paramagnetic to antiferromagnetic transition at ∼160 K. Remarkably, the temperature, at which paramagnetic to antiferromagnetic transition occurs, around this temperature, a huge drop in the dissipation factor takes place and resistivity shoots up; this indicates the possible correlation among magnetic and dielectric properties. First principle simulations reveal that some of these behaviors may be explained in terms of many body electron correlation effect in the presence of oxygen vacancy present in BaFeO{sub 3−δ} indicating its importance in both fundamental science as well as in applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of particle size distribution on the structure, hyperfine, and magnetic properties of Ni{sub 0.5}Zn{sub 0.5}Fe{sub 2}O{sub 4} nanopowders

Ni{sub 0.5}Zn{sub 0.5}Fe{sub 2}O{sub 4} nano powders were synthesized by an auto combustion method and then heat treated at different temperatures in air for a fixed time. As a consequence, a distribution in particle size and strain was incorporated within the specimens, as estimated from the Rietveld refinement analysis of the powder x-ray diffraction data. The changes in the microstructure and crystal structure parameters were carefully extracted through the refinement analysis. Thermal annealing causes increment in the dispersion and mean of the size distribution. Reallocation of cations in the lattice sites occur as a consequence of the heat treatment which was manifested in their altered unit cell length (a), r.m.s. strain (〈ε{sup 2}〉{sup 1/2}), oxygen positional parameter (u), metal-oxygen bond lengths (R{sub OA} and R{sub OB}), and the band positions (ν{sub 1}and ν{sub 2}) in the vibrational spectroscopy. We also investigate the hyperfine and magnetic properties of the samples using different instrumental techniques (with different operating time scales) like Mössbauer spectroscopy, electron paramagnetic resonance spectroscopy, and superconducting quantum interference device magnetometry. Results show that the effect of particle size distribution was manifested in their hyperfine field distribution profile, paramagnetic resonance spectra, and magnetic anisotropy energy distribution profile. Co-existence of superparamagnetic and ferrimagnetic phase was recorded at room temperature in the samples when annealed at lower temperature. However, with increase in annealing temperature, the nature of the size distribution changes and ferrimagnetic ordering predominates for the larger size nanoparticles. Thus, the effect of particle size distribution on the structural, hyperfine, and magnetic properties of various Ni{sub 0.5}Zn{sub 0.5}Fe{sub 2}O{sub 4} nanoparticles was investigated herein which hitherto has not been discussed in the literature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural, magnetic, and dielectric studies of the Aurivillius compounds SrBi{sub 5}Ti{sub 4}MnO{sub 18} and SrBi{sub 5}Ti{sub 4}Mn{sub 0.5}Co{sub 0.5}O{sub 18}

We have successfully synthesized the Aurivillius compounds SrBi{sub 5}Ti{sub 4}MnO{sub 18} and SrBi{sub 5}Ti{sub 4}Mn{sub 0.5}Co{sub 0.5}O{sub 18} using a modified Pechini method. Both samples have an orthorhombic structure with the space group B2cb. The valence state of Mn is suggested to be +3 and the doped Co ions exist in the form of Co{sup 2+} and Co{sup 3+} based on the results of x-ray photoelectron spectroscopy. The sample SrBi{sub 5}Ti{sub 4}MnO{sub 18} exhibits a dominant paramagnetic state with the existence of superparamagnetic state as evidenced by the electron paramagnetic resonance results, whereas SrBi{sub 5}Ti{sub 4}Mn{sub 0.5}Co{sub 0.5}O{sub 18} undergoes a ferrimagnetic transition at 161 K originating from the antiferromagnetic coupling of Co-based and Mn-based sublattices, and a ferromagnetic transition at 45 K arising from the Mn{sup 3+}-O-Co{sup 3+} (low spin) interaction. The sample SrBi{sub 5}Ti{sub 4}Mn{sub 0.5}Co{sub 0.5}O{sub 18} exhibits two dielectric anomalies. One corresponds to a relaxor-like dielectric relaxation which follows the Vogel-Fulcher function and the other dielectric relaxation obeys the Arrhenius law arising from the collective motion of oxygen vacancies. In addition, the sample SrBi{sub 5}Ti{sub 4}Mn{sub 0.5}Co{sub 0.5}O{sub 18} exhibits a magnetodielectric effect caused by the Maxwell-Wagner effect because of the conductivity of the sample. This is demonstrated by the fact that the activation energy in dielectric loss process is close to that for dc conductivity and the magnetodielectric effect is sensitive to the measured frequency.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Strong spin-lattice coupling in CrSiTe{sub 3}

CrSiTe{sub 3} has attracted recent interest as a candidate single-layer ferromagnetic semiconductor, but relatively little is known about the bulk properties of this material. Here, we report single-crystal X-ray diffraction, magnetic properties, thermal conductivity, vibrational, and optical spectroscopies and compare our findings with complementary electronic structure and lattice dynamics principles calculations. The high temperature paramagnetic phase is characterized by strong spin-lattice interactions that give rise to glassy behavior, negative thermal expansion, and an optical response that reveals that CrSiTe{sub 3} is an indirect gap semiconductor with indirect and direct band gaps at 0.4 and 1.2 eV, respectively. Measurements of the phonons across the 33 K ferromagnetic transition provide additional evidence for strong coupling between the magnetic and lattice degrees of freedom. The Si-Te stretching and Te displacement modes are sensitive to the magnetic ordering transition, a finding that we discuss in terms of the superexchange mechanism. Spin-lattice coupling constants are also extracted.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Spectroscopic studies of Cr{sup 3+} ions doped in poly(vinylalcohol) complexed polyethylene glycol polymer films

Polymer films of Poly(vinylalcohol) (PVA) complexed with Polyethylene glycol (PEG) with different dopant concentrations of Cr{sup 3+} ions are prepared by solution cast technique. Electron paramagnetic resonance (EPR), Optical absorption and FT-IR studies have been carried out on the polymer films. The EPR spectra of the entire samples exhibit resonance signal at g ≈1.97 which is attributed to the isolated Cr{sup 3+} pairs. The temperature variation EPR studies show that the population of spin-levels participating in the resonance decreases with an increase in temperature, which is in accordance with the Boltzmann Law. The paramagnetic susceptibilities (X) have been calculated from the EPR data at different temperatures. The linewidth of the g ≈1.97 resonance signal has been found to be decreasing with an increase in temperature, which confirms the pairing mechanism between Cr{sup 3+} ions. The Optical absorption spectrum of chromium ions in (PVA+PEG) polymer films exhibits three bands, corresponding to the d-d transitions {sup 4}A{sub 2g}(F)→{sup 4}T{sub 1g}(F), {sup 4}A{sub 2g}(F)→{sup 4}T{sub 2g}(F) and {sup 4}A{sub 2g}(F)→{sup 2}T{sub 1g}(G), in the order of decreasing energy. The crystal field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. From the ultraviolet absorption edges, Optical band gap (E{sub opt}) and Urbach (ΔE) energies are evaluated. FT-IR spectrum exhibits few bands which are attributed to O-H, CH, C=C and C=O groups of stretching and bending vibrations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Phase evolution and magnetic properties of HoIn{sub 3−x}Co{sub x}

The phase evolution of HoIn{sub 3−x}Co{sub x} exhibits the phase transformations from the original HoIn{sub 3} with cubic structure to the intermediated HoCo{sub 2}In with orthorhombic structure and to the final HoCo{sub 3} with rhombohedral structure. The corresponding magnetic states are found to closely associate with the structural phase transformation of HoIn{sub 3−x}Co{sub x}.With increasing Co contents, HoIn{sub 3−x}Co{sub x} are paramagnetic in region of x < 0.6, mixture of paramagnetic and ferromagnetic in range of 0.6 < x < 1.8, and ferromagnetic in range of 3 > x > 2.4 at room temperature, respectively. The field dependence of magnetization curves follows an equation of M=M{sub s}(1−a/H )+χH, where M{sub s} and χ are corresponding to the saturation moment and susceptibility; a is a parameter related to the external field. The fitting results show that the susceptibility increases with increasing Co contents and M{sub s} depends upon the Co content as well. It implies that the evolution of magnetic phases in HoIn{sub 3−x}Co{sub x} might originate from the change of exchange interaction from the one between the isolated Co spins and the surrounding conduction electrons in Co-poor regime to the one between Co-Co pairs mediated by the conduction electrons in Co-rich regime.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Physical properties and electronic structure of a new barium titanate suboxide Ba{sub 1+δ}Ti{sub 13−δ}O{sub 12} (δ = 0.11)

The structure, transport, thermodynamic properties, x-ray absorption spectra (XAS), and electronic structure of a new barium titanate suboxide, Ba{sub 1+δ}Ti{sub 13−δ}O{sub 12} (δ = 0.11), are reported. It is a paramagnetic poor metal with hole carriers dominating the transport. Fermi liquid behavior appears at low temperature. The oxidization state of Ti obtained by the XAS is consistent with the metallic Ti{sup 2+} state. Local density approximation band structure calculations reveal the material is near the Van Hove singularity. The pseudogap behavior in the Ti-d band and the strong hybridization between the Ti-d and O-p orbitals reflect the characteristics of the building blocks of the Ti{sub 13} semi-cluster and the TiO{sub 4} quasi-squares, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Spontaneous formation of suboxidic coordination around Co in ferromagnetic rutile Ti 0.95 Co 0.5 O 2 film

To evaluate local atomic structures around Co in high temperature diluted ferromagnetic semiconductor Co-doped TiO 2 , x-ray fluorescence holography and x-ray absorption fine structure experiments were carried out on rutile paramagnetic Ti 0.99 Co 0.01 O 2 and ferromagnetic Ti 0.95 Co 0.05 O 2 films. The Co atoms in the Ti 0.99 Co 0.01 O 2 simply substituted for Ti sites in the rutile structure, whereas a suboxidic arrangement of CoO 2 Ti 4 formed around Co in the Ti 0.95 Co 0.05 O 2 films. A theoretical investigation based on a series of first-principles calculations indicated the stability of the aggregated suboxidic clusters in the rutile TiO 2 , supporting our hypothesis for the formation of suboxidic coordination in the highly Co-doped sample. As a result, the suboxidic coordination may be the source of strong exchange interaction, resulting in the high Curie temperature in Co-doped TiO 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Copper ESEEM and HYSCORE through ultra-wideband chirp EPR spectroscopy

The main limitation of pulse electron paramagnetic resonance (EPR) spectroscopy is its narrow excitation bandwidth. Ultra-wideband (UWB) excitation with frequency-swept chirp pulses over several hundreds of megahertz overcomes this drawback. This allows to excite electron spin echo envelope modulation (ESEEM) from paramagnetic copper centers in crystals, whereas up to now, only ESEEM of ligand nuclei like protons or nitrogens at lower frequencies could be detected. ESEEM spectra are recorded as two-dimensional correlation experiments, since the full digitization of the electron spin echo provides an additional Fourier transform EPR dimension. Thus, UWB hyperfine-sublevel correlation experiments generate a novel three-dimensional EPR-correlated nuclear modulation spectrum.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗