Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Exchange interaction”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Enhanced survival of short-range magnetic correlations and frustrated interactions in R3T intermetallics

Elastic and inelastic neutron scattering and magnetization measurements have been used to study peculiarities of the magnetic state in R{sub 3}T compounds (R=Gd, Er, Tb; T=Ni, Co) below and above magnetic ordering temperatures. A pronounced non-Brillouin shape of the magnetization curves observed in the antiferromagnetic compounds Gd{sub 3}Ni and Tb{sub 3}Ni above their magnetic ordering temperatures together with earlier reported data about the retention of the magnetic contribution to the total specific heat of Gd{sub 3}T and anomalous behavior of the electrical resistivity above magnetic ordering temperatures are ascribed to the existence of short-range magnetic correlations in the wide temperature range in the paramagnetic state. The persistence of short-range magnetic order up to temperatures greater than 5-6 times the Neel temperature has been revealed by powder neutron diffraction measurements performed for Tb{sub 3}Ni and Tb{sub 3}Co. On the other hand, results from inelastic neutron scattering show that the low temperature magnetic excitations are strongly suppressed in both Tb{sub 3}Co and Er{sub 3}Co. It is suggested that the extended short-range magnetic correlations, which turn out to be an inherent feature of R{sub 3}T type compounds, are due to the layered crystal structure and to the difference between geometrically frustrated intra-layer exchange interactions and inter-layer exchange.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Transmission of spin waves in ordered FeRh epitaxial thin films

We report on B2-ordering dependence of magnetostatic surface spin waves in ferromagnetic FeRh at room temperature. Spin waves transmit over a distance longer than 21 μm in highly ordered FeRh alloys even with relatively large spin-orbit interaction. The long-range transmission likely arises from the induced Rh moments of the ordered FeRh due to ferromagnetic exchange interaction between Fe and Rh. The results indicate a potential of using FeRh in spintronic and magnonic applications by integrating with other fascinating magnetic characteristics of FeRh such as electric field induced magnetic phase transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Dimensionality driven spin-flop transition in layered iridates..

Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr{sub 3}Ir{sub 2}O{sub 7}, a significant contrast to the single layer Sr{sub 2}IrO{sub 4} with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO{sub 2} layers is due to strong competition among intra- and interlayer bond-directional pseudodipolar interactions of the spin-orbit entangled J{sub eff} = 1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Phase Transitions and the Critical Properties of the Heisenberg Model on a Body-Centered Cubic Lattice

The Monte Carlo replica technique is used to study phase transitions and the thermodynamic and critical properties of the three-dimensional Heisenberg antiferromagnetic model on the body-centered cubic lattice with the inclusion of the interaction of the nearest and the next-nearest neighbors. The studies are performed for the proportions of the values of the exchange interactions of the nearest and the next-nearest neighbors in the range of values of k [0.0, 0.6]. The phase transition character is analyzed based on the histogram method. The overall set of static critical indices is calculated in terms of the theory of the finite-dimensional scaling. It is shown that the class of the critical behavior universality of this model is conserved in the above range of values of k.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Pressure-Induced Decrease in the Curie Temperature of Gd{sub 2}Fe{sub 17}: LSDA+U Calculations

To explain the magnetic properties of advanced ferromagnetic intermetallic compounds of the R{sub 2}Fe{sub 17} (R is a rare-earth element) class, experimentalists often use the hypothesis of competition between ferromagnetic exchange and antiferromagnetic exchange between four types of the nearest iron atoms in nonequivalent lattice sites. For the rhombohedral Gd{sub 2}Fe{sub 17} ferromagnet, we calculate the magnetic moments of iron and gadolinium ions, the parameters of exchange between Fe atoms, and Curie temperature T{sub C} at a zero pressure and during hydrostatic lattice compression. The magnetic moment of the unit cell of Gd{sub 2}Fe{sub 17} is shown to decrease under pressure, and this decrease is almost completely associated with a decrease in the magnetic moments of Fe rather than Gd ions, the pressure dependence of the magnetic moments of which is weaker by an order of magnitude. In contrast to the hypothesis regarding the competition of exchange interactions between different kinds of Fe atoms, the parameters of exchange between the nearest iron atoms in different crystallographic sites are found to be positive ferromagnetic (at a zero pressure and during compression), and a ferromagnetic character of interaction is shown to remain unchanged under pressure even for Fe atoms in the so-called dumbbell sites with the nearest interatomic distances. The Curie temperature T{sub C} of Gd{sub 2}Fe{sub 17} is shown to decrease with increasing pressure. The changes in the exchange parameters and the magnetic moments of Gd{sub 2}Fe{sub 17} during compression are found to be mainly related to a change in the position of energy spectrum branches with respect to each other and the Fermi level ϵ{sub F} rather than to a change in the overlapping of wavefunctions, which play a minor role.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Evolution of Triplet Paramagnetic Centers in Diamonds Obtained by Sintering of Detonation Nanodiamonds at High Pressure and Temperature

The electron paramagnetic resonance (EPR) spectra of triplet centers in detonation nanodiamonds (DNDs) and diamond single crystals of submicrometer size, synthesized from those DNDs at high pressures and temperatures, are studied. In the EPR spectra of DNDs, signals from negatively charged nitrogen- vacancy centers (NV)/sup(-) with a g factor of g{sub 1} = 4.24 and multivacancies with g{sub 2} = 4.00 are observed. The signals from (NV)/sup(-) centers disappear in the spectra of diamond single crystals, and a quintet signal with g = 4.00 is detected at the position of the signal from multivacancies. Analysis of the shape and position of the quintet’ lines showed that this ESR signal is due to the pairs of nitrogen substitution centers in diamond, separated from each other by distances not exceeding 0.7 nm, between which a strong exchange interaction takes place. A comparison of the experimental data and the simulation results allows determining the spin-Hamiltonian parameters of the exchange-coupled pairs of paramagnetic impurity nitrogen atoms.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Magnetic phase transformations and magnetotransport phenomena in La{sub 0.7}Sr{sub 0.3}Mn{sub 1–} {sub x}Co{sub x}O{sub 3} perovskite compounds

The compositions La{sub 0.7}Sr{sub 0.3}Mn{sub 1–} {sub x}Co{sub x}O{sub 3} (0.13 ≤ x ≤ 1) are studied by neutron diffraction, magnetometry, and measuring the magnetotransport properties. The substitution of cobalt ions for manganese ions is shown to decrease the magnetization and the Curie temperature from 270 K (x = 0.13) to 140 K (x = 0.33). As the cobalt ion content increases to x = 0.5, the Curie temperature increases to 190 K, the magnetization decreases, and the electrical resistivity increases. At x > 0.5, the temperature of transition into a paramagnetic state decreases to 68 K (x = 0.8) and then again increases to 225 K for the La{sub 0.7}Sr{sub 0.3}CoO{sub 3} composition. The magnetoresistive effect in the range 0.3 ≤ x ≤ 0.4 reaches 97% and decreases gradually with increasing temperature without anomalies near the Curie point. At x ≤ 0.2, the magnetoresistive effect increases near the Curie temperature. The composition at x = 0.6 is stoichiometric, and no coherent magnetic contribution to neutron scattering is detected. The magnetic properties near x ∼ 0.5 are assumed to be caused by partial ordering of Co{sup 3+} and Mn{sup 4+} ions, and the Co{sup 3+} ions can be in both low- and high-spin states. The magnetic interaction between Co{sup 3+} ions in a high-spin state and Mn{sup 4+} is predominantly ferromagnetic, and the ferromagnetic part of the exchange interactions is close to the ferromagnetic part. These data are used to plot a magnetic La{sub 0.7}Sr{sub 0.3}Mn{sub 1–} {sub x}Co{sub x}O{sub 3} phase diagram.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Evaluating the Free Energies of Solvation and Electronic Structures of Lithium‐Ion Battery Electrolytes

Abstract Adaptive biasing force molecular dynamics simulations and density functional theory calculations were performed to understand the interaction of Li + with pure carbonates and ethylene carbonate (EC)‐based binary mixtures. The most favorable Li carbonate cluster configurations obtained from molecular dynamics simulations were subjected to detailed structural and thermochemistry calculations on the basis of the M06‐2X/6‐311++G(d,p) level of theory. We report the ranking of these electrolytes on the basis of the free energies of Li‐ion solvation in carbonates and EC‐based mixtures. A strong local tetrahedral order involving four carbonates around the Li + was seen in the first solvation shell. Thermochemistry calculations revealed that the enthalpy of solvation and the Gibbs free energy of solvation of the Li + ion with carbonates are negative and suggested the ion–carbonate complexation process to be exothermic and spontaneous. Natural bond orbital analysis indicated that Li + interacts with the lone pairs of electrons on the carbonyl oxygen atom in the primary solvation sphere. These interactions lead to an increase in the carbonyl (C=O) bond lengths, as evidenced by a redshift in the vibrational frequencies [ν(C=O)] and a decrease in the electron density values at the C=O bond critical points in the primary solvation sphere. Quantum theory of atoms in molecules, localized molecular orbital energy decomposition analysis (LMO‐EDA), and noncovalent interaction plots revealed the electrostatic nature of the Li + ion interactions with the carbonyl oxygen atoms in these complexes. On the basis of LMO‐EDA, the strongest attractive interaction in these complexes was found to be the electrostatic interaction followed by polarization, dispersion, and exchange interactions. Overall, our calculations predicted EC and a binary mixture of EC/dimethyl carbonate to be appropriate electrolytes for Li‐ion batteries, which complies with experiments and other theoretical results.

Shakourian‐Fard, Mehdi↗
DOE OSTI2013

Watermelon-like iron nanoparticles: Cr doping effect on magnetism and magnetization interaction reversal

Chromium (Cr) forms a solid solution with iron (Fe) lattice when doped in core-shell iron -iron oxide nanocluster (NC) and shows a mixed phase of sigma (σ) FeCr and bcc Fe. The Cr dopant affects heavily the magnetization and magnetic reversal process, and causes the hysteresis loop to shrink near the zero field axis. Dramatic transformation happens from dipolar interaction (0 at. % Cr) to strong exchange interaction (8 at. % of Cr) is confirmed from the Henkel plot and delta M plot, and is explained by a water-melon model of core-shell NC system.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Quantum transport in ferromagnetic graphene: Role of Berry curvature

The magnetic effects in ferromagnetic graphene basically depend on the principle of exchange interaction when ferromagntism is induced by depositing an insulator layer on graphene. Here we deal with the consequences of non-uniformity in the exchange coupling strength of the ferromagnetic graphene. We discuss how the in- homogeneity in the coordinate and momentum of the exchange vector field can provide interesting results in the conductivity analysis of the ferromagnetic graphene. Our analysis is based on the Kubo formalism of quantum transport.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1981

Quantum chemical calculation of the equilibrium structures of small metal atom clusters

A decomposition of the molecular energy is presented that is motivated by the atom superposition and electron delocalization physical model of chemical binding. The energy appears in physically transparent form consisting of a classical electrostatic interaction, a zero order two electron exchange interaction, a relaxation energy, and the atomic energies. Detailed formulae are derived in zero and first order of approximation. The formulation extends beyond first order to any chosen level of approximation leading, in principle, to the exact energy. The structure of this energy decomposition lends itself to the fullest utilization of the solutions to the atomic sub problems to simplify the calculation of the molecular energy. If nonlinear relaxation effects remain minor, the molecular energy calculation requires at most the calculation of two center, two electron integrals. This scheme thus affords the prospects of substantially reducing the computational effort required for the calculation of molecular energies.

Kahn, L. R.↗
NASA NTRS1985

Studies of electron-polyatomic-molecule collisions Applications to e-CH4

The first application of the Schwinger multichannel formulation to low-energy electron collisions with a nonlinear polyatomic target is reported. Integral and differential cross sections are obtained for e-CH4 collisions from 3 to 20 eV at the static-plus-exchange interaction level. In these studies, the exchange potential is directly evaluated and not approximated by local models. An interesting feature of the small-angle differential cross section is ascribed to polarization effects and not reproduced at the static-plus-exchange level. These differential cross sections are found to be in reasonable agreement with existing measurements at 7.5 eV and higher energies.

Lima, M. A. P.↗
DOE OSTI2016

Magnetic nanoscopic correlations in the crossover between a superspin glass and a superferromagnet

Collective behaviors in which the magnetic response depends not only on the individual constituents but also on their interactions are an area of active research. We have produced a paradigmatic system where DC magnetron sputtered Fe{sub x}Ag{sub 100–x} (x = 15, 35) nanogranular films exhibit a crossover between a superspin glass (SSG) state and a superferromagnetism (SFM), where direct exchange interactions overcome the frustration. The systems have been studied by non-linear susceptibility (NLS) and small angle neutron scattering (SANS). The NLS measurements were carried out between 2 and 300 K, in the absence of a biasing magnetic field, with frequencies spanning two decades. These measurements shed light on the complex nature of the interactions and the intricate relationship between direct exchange and long range magnetic interactions. The use of SANS allows us to estimate qualitatively the lengthscale of the magnetic correlations, and therefore identify a clear difference between the collective “supermagnetic” states (i.e., SSG and SFM) while establishing links between the structure and the magnetic interactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

50 MeV, Li{sup 3+} - ion irradiation effect on magnetic ordering of Y{sup 3+} - substituted yttrium iron garnet

This communication presents the effect of non-magnetic Y{sup 3+} ions substitution for magnetic Fe{sup 3+} ions and 50 MeV, Li{sup 3+} ion irradiation (fluence: 5 × 10{sup 13} ions/cm{sup 2}) on magnetic ordering and Neel temperature of Y{sub 3+x}Fe{sub 5-x}O{sub 12} (x = 0.0, 0.2, 0.4 and 0.6) garnet system, studied by means of X-ray powder diffractometry and thermal variation of low field (0.5 Oe) ac susceptibility measurements. The un-irradiated compositions exhibit normal ferrimagnetic behavior with decrease in transition temperature (T{sub N}) on increasing Y{sup 3+}-concentration (x). The irradiated counterparts are characterized by tailing effect indicative of non-uniform effect of irradiation and lower value of T{sub N}. The results have been discussed based on the weakening of magnetic exchange interactions and cumulative effect of redistribution of cations and fractional creation of localized paramagnetic centers resulting from swift heavy ion irradiation. The Neel temperatures and exchange integrals have been calculated theoretically.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic Properties of a Mixed Spin-3/2 and Spin-1/2 Ising Nanowire with Nearest and Next-Nearest Neighbour Interactions

The magnetic properties and the hysteresis behaviours of a mixed spin-3/2 and spin-1/2 cubic Ising nanowire have been studied by the use of the Monte Carlo simulations. To investigate the effects of the exchange interaction (J{sub A}), the crystal field (Δ{sub A}) and the nanowire size (N), we have plotted the variation of the magnetizations (m{sub A} and m{sub B}) of the two sublattices and the total magnetization (M{sub T}) with temperature for different values of J{sub A}, Δ{sub A} and N. The corresponding phase diagrams and the hysteresis loops behaviour are also presented and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Specific features of magnetic structure formation in orbitally degenerate BiMnO{sub 3} manganite

The orbital structure and magnetic ordering of the Jahn-Teller multiferroic BiMnO{sub 3} manganite have been theoretically studied. It is shown that the orbital structure depends not only on the nearest-neighbor oxygen environment of manganese ions, but also on their next-to-nearest neighbors. The orbital structure significantly influences the magnetic order that forms as a result of competition between ferromagnetic and antiferromagnetic exchange interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Critical behavior in Ni 2 MnGa and Ni 2 Mn 0.85 Cu 0.15 Ga

The critical behaviors of polycrystalline Ni 2 MnGa and Ni 2 Mn 0.85 Cu 0.15 Ga have been examined through high-resolution bulk magnetization measurements. The critical exponents, β and γ, were derived from modified Arrott plots using the Kouvel-Fisher method. The values of the extracted critical exponents satisfied the scaling equation of state and associated exponent relations, indicating self-consistency of the extracted values. In Ni2MnGa, the critical exponents (β = 0.401 ± 0.003, γ = 1.27 ± 0.02) indicate a deviation from the 3D-Heisenberg values toward the mean-field values, likely due to the presence of long-range Ruderman-Kittel-Kasuya-Yosida interactions. However, the critical exponents of Ni2Mn0.85Cu0.15Ga (β = 0.389 ± 0.004, γ =1.39 ± 0.02) are closer to the 3D-Heisenberg values. This indicates a weakening of the long-range exchange interactions due to the substitution of Cu in the Mn site.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2014

Effect of lattice deformation on exchange coupling constants in Cr{sub 2}O{sub 3}

We studied lattice deformation effect on exchange interaction in the corundum-type Cr{sub 2}O{sub 3} theoretically. First-principles electronic structure calculations were performed to evaluate the total energy and exchange coupling constants of Cr{sub 2}O{sub 3} under lattice deformation. We found that a few percent elastic deformation is expected via misfit strain and that the first- and second-nearest neighbor exchange coupling constants of Cr{sub 2}O{sub 3} strongly depend on the lattice deformation. These results imply a possibility for improving the thermal stability of Cr{sub 2}O{sub 3} based magnetoelectric devices by lattice deformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗