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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Large Dzyaloshinskii-Moriya interaction induced by chemisorbed oxygen on a ferromagnet surface

The Dzyaloshinskii-Moriya interaction (DMI) is an antisymmetric exchange interaction that stabilizes chiral spin textures. It is induced by inversion symmetry breaking in noncentrosymmetric lattices or at interfaces. Recently, interfacial DMI has been found in magnetic layers adjacent to transition metals due to the spin-orbit coupling and at interfaces with graphene due to the Rashba effect. We report direct observation of strong DMI induced by chemisorption of oxygen on a ferromagnetic layer at room temperature. The sign of this DMI and its unexpectedly large magnitude—despite the low atomic number of oxygen—are derived by examining the oxygen coverage–dependent evolution of magnetic chirality. We find that DMI at the oxygen/ferromagnet interface is comparable to those at ferromagnet/transition metal interfaces; it has enabled direct tailoring of skyrmion’s winding number at room temperature via oxygen chemisorption. This result extends the understanding of the DMI, opening up opportunities for the chemisorption-related design of spin-orbitronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Influence of a Strong Magnetic Field on Magnetic Order in Polycrystalline Layered Cobaltites EuBaCo{sub 2 –x}O{sub 5.5 – δ}

The magnetic properties of polycrystalline EuBaCo{sub 2 –x}O{sub 5.5 – δ} (x ≤ 0.1, δ ≤ 0.14) are studied in the temperature range 2 K < T < 300 K in magnetic fields H ≤ 90 kOe. A comparison is made of the dependencies of magnetization M on temperature T and magnetic field H for polycrystalline and single crystal EuBaCo{sub 1.90}O{sub 5.36}, in which only Cu(III) ions are present. It is shown that in the magnetically ordered temperature range of magnetic order, the samples are ferrimagnetic, in which antiferromagnetic interactions between the nearest Co{sup 3+} ions with the same spins compete with the ferromagnetic exchange. With a decrease in temperature, a strong magnetic field affects the magnetic order. At T = 2 K in fields H = 50–90 kOe, all samples exhibit the properties of mictomagnetism. There are a shift in the hysteresis loops M(H) and a large difference in the values of the magnetization of the sample cooled in a magnetic field M (FC) and without a field M (ZFC) due to unidirectional magnetic anisotropy. In this case, there is no saturation of magnetization. The field dependences of magnetization taken in the cooling and heating in unequal fields (CHUF) mode indicate a metastable magnetic state. Near T = 20–30 K, an orientational transition characteristic of micromagnets takes place and a complex ferrimagnetic structure is formed. The main reasons for the manifestation of mictomagnetism properties in layered cobaltites are the presence of magnetoactive Co{sup 3+} ions in the states with intermediate and high spins at low temperatures and structural defects that affect the exchange interaction between the nearest neighbors. Nonmagnetic Co{sup 3+} ions in the low-spin state can also act as defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Thermodynamic and Magnetic Properties of the Two-Dimensional Anisotropic Ising Model with Competing Interactions

The two-dimensional anisotropic Ising model with competing interactions is studied on a square lattice by Monte Carlo methods using the Wang–Landau algorithm. The temperature dependences of the main thermodynamic and magnetic parameters are calculated. The distinctive features of temperature dependences of these parameters for different values of the competing interaction are shown. The dependences of the wave number of modulated structures on the ratio of exchange interaction parameters are plotted. The phase diagram of the model is built. The coordinates of the Lifshits point and the localization region of the phases with different wave vector values are calculated by interpolating phase boundaries of the d-iagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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↗
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 OSTI2019

Quasicubic model for metal halide perovskite nanocrystals

In this work, we present an analysis of quantum confinement of carriers and excitons, and exciton fine structure, in metal halide perovskite (MHP) nanocrystals (NCs). Starting with coupled-band k · P theory, we derive a nonparabolic effective mass model for the exciton energies in MHP NCs valid for the full size range from the strong to the weak confinement limits. We illustrate the application of the model to CsPbBr 3 NCs and compare the theory against published absorption data, finding excellent agreement. We then apply the theory of electron-hole exchange, including both short- and long-range exchange interactions, to develop a model for the exciton fine structure. We develop an analytical quasicubic model for the effect of tetragonal and orthorhombic lattice distortions on the exchange-related exciton fine structure in CsPbBr 3 , as well as some hybrid organic MHPs of recent interest, including formamidinium lead bromide (FAPbBr 3 ) and methylammonium lead iodide (MAPbI3). Testing the predictions of the quasicubic model using hybrid density functional theory (DFT) calculations, we find qualitative agreement in tetragonal MHPs but significant disagreement in the orthorhombic modifications. Moreover, the quasicubic model fails to correctly describe the exciton oscillator strength and with it the long-range exchange corrections in these systems. Introducing the effect of NC shape anisotropy and possible Rashba terms into the model, we illustrate the calculation of the exciton fine structure in CsPbBr 3 NCs based on the results of the DFT calculations and examine the effect of Rashba terms and shape anisotropy on the calculated fine structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Dynamics of Spatially Inhomogeneous Spin Polarization of Nonequilibrium Conduction Electrons in Magnetic Transitions

The equation of the dynamics of the magnetic moment motion that has been averaged over the ensemble of nonequilibrium spin-injected electrons in a ferromagnetic transition in the presence of exchange interaction, interaction with an external electromagnetic field, and a thermostat has been obtained taking into account the spatial inhomogeneity of the charge carrier distribution. It has been shown that taking into account the spatial inhomogeneity of the charge carrier distribution in the equation of the dynamics of the magnetic moment motion allows for describing the various processes of electron transfer in the magnetic transition. The probability of quantum transitions of electrons with opposite spin directions, which determine spin relaxation in interaction with a thermostat, has been estimated. It has been shown that the anisotropy of the radiation medium is determined not only by the anisotropy of the sd-exchange tensor, but also by the additional anisotropy that is caused by the electron impulse derivatives of this tensor. The considered phenomena have a great potential for the detection of new effects and development of new devices based on them, including compact tunable radiation sources in the terahertz frequency range, which is obviously difficult to generate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electron Spin Resonance Study of Zn{sub x}Co{sub 1−x}Fe{sub 2−x}Al{sub x}O{sub 4} Nanoparticles

Zn- and Al-substituted cobalt ferrite Zn{sub x}Co{sub 1−x}Fe{sub 2−x}Al{sub x}O{sub 4} (0 ≤x≤ 0.9) with particle size in the range of 6–13 nm were synthesized by glycol-thermal reaction. The compounds were investigated by electron spin resonance (ESR) measurements. CoFe{sub 2}O{sub 4} (x = 0) shows a broad ESR signal associated with magnetic dipole-dipole interactions among particles which is highly sensitive to the Zn and Al ion concentrations. Narrower ESR signals in Zn- and Al-doped samples are attributed to the weakening of super-exchange interactions. ESR and magnetization measurements reveal a reduction in magnetization with increasing particle size. This is explained on the basis of the redistribution of Zn{sup 2+} ions in both tetrahedral (A) and octahedral (B) sites in fine powders.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Scattering solution of an interacting Hamiltonian for the electronic control of molecular spin qubits

We theoretically study how a scattered electron can entangle molecular spin qubits (MSQs). This requires solving the inelastic transport of a single electron through a scattering region described by a tight-binding interacting Hamiltonian. We accomplish this using a Green's-function solution. We can model realistic physical implementations of MSQs by parametrizing the tight-binding Hamiltonian with first-principles descriptions of magnetic anisotropy and exchange interactions. As a result, we find that, for two-MSQ systems with inversion symmetry, projective measurement of the spin degree of freedom of the scattered electron offers probabilistic control of the degree of entanglement between the MSQs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

The Imry–Ma Phase in a Nanocrystalline Ferromagnet

A possibility of appearing a disordered Imry–Ma state induced by fluctuations of the anisotropy easy axis direction in a nanocrystalline sample in the case of a weak exchange interaction between crystallites has been studied. A phase diagram of the system has been built in variables “the characteristic crystallite size–the exchange integral of the intercrystallite interaction.” The characteristic value of statistic fluctuations of the crystallographic anisotropy has been estimated, and the dependences of the coercive field on the crystallite size have been found for systems of various space dimensions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Simultaneous measurement of the exchange parameter and saturation magnetization using propagating spin waves

The exchange interaction in ferromagnetic ultra thin films is a critical parameter in magnetization-based storage and logic devices, yet the accurate measurement of it remains a challenge. While a variety of approaches are currently used to determine the exchange parameter, each has its limitations, and good agreement among them has not been achieved. To date, neutron scattering, magnetometry, Brillouin light scattering, spin-torque ferromagnetic resonance spectroscopy, and Kerr microscopy have all been used to determine the exchange parameter. Here, we present a method that exploits the wavevector selectivity of Brillouin light scattering to measure the spin wave dispersion in both the backward volume and Damon–Eshbach orientations. The exchange, saturation magnetization, and magnetic thickness are then determined by a simultaneous fit of both dispersion branches with general spin wave theory without any prior knowledge of the thickness of a magnetic “dead layer.” In this study, we demonstrate the strength of this technique for ultrathin metallic films, typical of those commonly used in industrial applications for magnetic random-access memory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Simulations of x-ray absorption spectra for CO desorbing from Ru(0001) with transition-potential and time-dependent density functional theory approaches

The desorption of a carbon monoxide molecule from a Ru(0001) surface was studied by means of X-ray Absorption Spectra (XAS) computed with Transition Potential (TP-DFT) and Time Dependent (TD-DFT) DFT methods. By unraveling the evolution of the CO electronic structure upon desorption, we observed that at 2.3 Å from the surface, the CO molecule has already predominantly gas-phase character. While C 1s XAS is quite insensitive to changes in the C–O bond length, the O 1s excitation is very sensitive with the π* coming down in energy upon CO bond stretching, which competes with the increase in orbital energy due to the repulsive interaction with the metallic surface. We show in a systematic way that the TP-DFT method can describe the XAS rather well at the endpoints (chemisorbed and gas phase) but is affected by artificial charge transfer and/or incorrect spin treatment in the transition region in cases like CO, where there are low-lying π* orbitals and large exchange interactions between the core 1s and valence-acceptor π* orbitals. As an alternative, we demonstrate by comparing with experimental data that a linear response approach using TD-DFT employing common exchange-correlation functionals and finite-size clusters can yield a good description of the spectral evolution of the 1s → π* transition with correct spin and gas-to-chemisorbed chemical shifts in good agreement with experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Role of Ni concentration on structural and magnetic properties of inverse spinel Ferrite

Highlights: • Ni doped Co{sub 1-x}Mn{sub x}Fe{sub 2}O{sub 4} (0≤x≤0.1) nanoparticles have been synthesized with an average particle size in the range 15–20 nm. • Lattice parameter decreases with increase in Ni concentration in CoFe{sub 2}O{sub 4} nanoparticles. • XRD and FTIR analysis confirmed the formation of cubic spinel ferrite structure. • Room temperature and low temperature magnetic properties of described system have been measured. - Abstract: In present work, Nickel (Ni) substituted Mn-Co ferrite nanoparticles, with doping content varying from 0 to 0.1, have been synthesized. The inverse spinel structure of doped cobalt ferrite has been claimed by X-ray diffractometer with average particle size of 20 nm. The decrease in lattice parameter and increase in X-ray density has been found with increase in Ni concentration in ferrite structure. Fourier transform infrared spectroscopy analysis showed two vibrational frequency bands (υ{sub 1} and υ{sub 2}) for octahedral and tetrahedral sites depicting the formation of spinel ferrite. In addition, Transmission electron microscopy analyses were carried out to investigate particle size, which indicates that TEM and XRD analyses are in agreement. Magnetic analyses depict that coercivity value increases from 635 Oe to 841 Oe in Ni doped Mn-Co ferrite. Exchange interactions between Ni and Mn{sub 0.2}Co{sub 0.8-x}Fe{sub 2}O{sub 4} result for enlargement in coercivity and shift in hysteresis loop towards positive direction along the field axis. The increment in magnetic coercive field and exchange bias has been observed as temperature decreases. Enhanced magnetic properties at low temperature are attributed to magneto-crystalline anisotropy and surface effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Spontaneous Valley Polarization of Interacting Carriers in a Monolayer Semiconductor

Here, we report magnetoabsorption spectroscopy of gated WSe 2 monolayers in high magnetic fields up to 60 T. When doped with a 2D Fermi sea of mobile holes, well-resolved sequences of optical transitions are observed in both σ ± circular polarizations, which unambiguously and separately indicate the number of filled Landau levels (LLs) in both K and K ' valleys. This reveals the interaction-enhanced valley Zeeman energy, which is found to be highly tunable with hole density p . We exploit this tunability to align the LLs in K and K ' , and find that the 2D hole gas becomes unstable against small changes in LL filling and can spontaneously valley polarize. These results cannot be understood within a single-particle picture, highlighting the importance of exchange interactions in determining the ground state of 2D carriers in monolayer semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magnetic interactions and excitations in SrMnSb 2

The magnetic interactions in the antiferromagnetic (AFM) Dirac semimetal candidate SrMnSb 2 are investigated using ab initio linear response theory and inelastic neutron scattering (INS). Our calculations reveal that the first two nearest in-plane couplings (J 1 and J 2 ) are both AFM in nature, indicating a significant degree of spin frustration, which aligns with experimental observations. The orbital resolution of exchange interactions shows that J 1 and J 2 are dominated by direct and superexchange, respectively. In a broader context, a rigid-band model suggests that electron doping fills the minority spin channel and results in a decrease in the AFM coupling strength for both J 1 and J 2 . To better compare with INS measurements, we calculate the spin-wave spectra within a linear spin-wave theory, utilizing the computed exchange parameters. Although the calculated spin-wave spectra somewhat overestimate the magnon bandwidth, they exhibit overall good agreement with measurements from INS experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Theoretical simulation of a signal for a scheme of an atomic spin gyroscope with optical detection

An operation scheme of an atomic spin gyroscope with optical detection is described, in which two noble gas isotopes, {sup 129} Xe and {sup 131} Xe, are used as carriers of the spin of an atomic nucleus. Based on this scheme, we construct a theoretical model of the dynamics of the spin polarisation of an alkali metal and the magnetisation of a noble gas under conditions of spin-exchange interaction and optical pumping. This model allows one to simulate the pumping and signal readout in the gyroscope scheme. The time dependences of this signal are plotted, the analysis of which shows that the signal has a sophisticated complex structure. Thus, the problem arises of demodulating this signal based on the improvement of demodulation methods and algorithms. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗