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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Synthesis, magnetic and electric transport properties of mixed-valence manganites La{sub 0.5+x}Sr{sub 1.5−x}Mn{sub 0.5}Cr{sub 0.5}O{sub 4} (x=0.1, 0.2 and 0.3)

A series of mixed-valence manganites La{sub 0.5+x}Sr{sub 1.5−x}Mn{sub 0.5}Cr{sub 0.5}O{sub 4} were synthesized by sol–gel method. Rietveld profile analysis shows that the phases crystallize with tetragonal unit cell in the space group I4/mmm. The lattice parameter c increases with increase in x while a parameter almost remains constant. The Weiss constant (θ) is positive for all the phases and found to increase with increase in x. The ferromagnetic interactions becomes more dominant with increase in x due to increase in Mn{sup 3+}–O–Mn{sup 4+} and Mn{sup 3+}–O–Cr{sup 3+} double exchange interactions. Resistivity can be described by the polaron hopping and the variable range hopping model. It was found that the transport mechanism is dominated by Mott's variable range hopping (VRH) model with a decrease of Mott localization energy with increase in x, which explains the decrease of resistivity. - Graphical abstract: Unit cell structure of La{sub 0.6}Sr{sub 1.4}Mn{sub 0.5}Cr{sub 0.5}O{sub 4}. Display Omitted Research highlights: • A series of K{sub 2}NiF{sub 4}-type compounds La{sub 0.5}+xSr{sub 1.5-x}Mn{sub 0.5}Cr{sub 0.5}O{sub 4} (x=0.1, 0.2, 0.3) have been synthesized. • The calculated tolerance factors confirms the tetragonal symmetry of the proposed phases. • The lattice parameter c increases with increase in x while a parameter almost remains constant. • The Weiss constant (θ) is positive for all the phases and found to increase with increase in x • The resistivity data has been best fitted using the Mott's variable range hopping (VRH) model.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
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 OSTI2017

On the “alpha-phase” of Ca{sub 2−x}Sr{sub x}MnO{sub 4} and extending the chemistry of Sr{sub 7−y}Ca{sub y}Mn{sub 4}O{sub 15} to y>1

There has been renewed interest in the Ruddlesden-Popper phase (n=2) of composition Ca{sub n+1}Mn{sub n}O{sub 3} {sub n+1} in the light of recent research that has highlighted the nature of the improper ferroelectric ground state, which arises due to the couplings between specific combinations of MnO{sub 6} octahedral rotations and tilts. A fruitful route to control these octahedral degrees of freedom, and hence such desired physical properties, is through chemical substitution on the A–site cation i.e. Ca{sub 2−x}Sr{sub x}MnO{sub 4} for n =1, and in light of this, we have reinvestigated the chemistry of this solid solution. Here we focus on a common impurity phase observed during this synthesis which has been termed the “alpha-phase” in the literature. We show that this impurity phase is actually comprised mainly of a structure related to Sr{sub 7}Mn{sub 4}O{sub 15} but is found here with significantly higher Ca substitution than previously believed possible. Sr{sub 7}Mn{sub 4}O{sub 15} is an interesting structural type in its own right, but has been mainly overlooked to date, exhibiting interesting physics related to low dimensional magnetic ordering and dimer interactions, and we show here that the structural type is a likely candidate for exhibiting a multiferroic ground state. The prospect of being able to tune the lattice and the exchange interactions through further chemical substitution is likely to lead to a renewed interest in this material. - Graphical abstract: Extending the chemistry of Sr{sub 7−y}Ca{sub y}Mn{sub 4}O{sub 15} beyond y>1, revealing highly anisotropic cation ordering and tunable magnetic properties. - Highlights: • Chemistry of the unique structural type Sr{sub 7}Mn{sub 4}O{sub 15} is extended to high Ca concentrations. • Cation occupancy model is determined, showing highly anisotropic solubility of Ca on the 7 unique Sr crystallographic sites. • Anomalies in the magnetic susceptibility data are discussed with reference to symmetry arguments pointing towards a possible novel multiferroic mechanism in this material.

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 OSTI2014

Anti-site disorder and improved functionality of Mn₂NiX (X = Al, Ga, In, Sn) inverse Heusler alloys

Recent first-principles calculations have predicted Mn₂NiX (X = Al, Ga, In, Sn) alloys to be magnetic shape memory alloys. Moreover, experiments on Mn₂NiGa and Mn₂NiSn suggest that the alloys deviate from the perfect inverse Heusler arrangement and that there is chemical disorder at the sublattices with tetrahedral symmetry. In this work, we investigate the effects of such chemical disorder on phase stabilities and magnetic properties using first-principles electronic structure methods. We find that except Mn₂NiAl, all other alloys show signatures of martensitic transformations in presence of anti-site disorder at the sublattices with tetrahedral symmetry. This improves the possibilities of realizing martensitic transformations at relatively low fields and the possibilities of obtaining significantly large inverse magneto-caloric effects, in comparison to perfect inverse Heusler arrangement of atoms. We analyze the origin of such improvements in functional properties by investigating electronic structures and magnetic exchange interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Phase transitions in a two-dimensional antiferromagnetic Potts model on a triangular lattice with next-nearest neighbor interactions

Phase transitions (PTs) and frustrations in two-dimensional structures described by a three-vertex antiferromagnetic Potts model on a triangular lattice are investigated by the Monte Carlo method with regard to nearest and next-nearest neighbors with interaction constants J{sub 1} and J{sub 2}, respectively. PTs in these models are analyzed for the ratio r = J{sub 2}/J{sub 1} of next-nearest to nearest exchange interaction constants in the interval |r| = 0–1.0. On the basis of the analysis of the low-temperature entropy, the density of states function of the system, and the fourth-order Binder cumulants, it is shown that a Potts model with interaction constants J{sub 1} < 0 and J{sub 2} < 0 exhibits a first-order PT in the range of 0 ⩽ r < 0.2, whereas, in the interval 0.2 ⩽ r ⩽ 1.0, frustrations arise in the system. At the same time, for J{sub 1} > 0 and J{sub 2} < 0, frustrations arise in the range 0.5 < |r| < 1.0, while, in the interval 0 ⩽ |r| ⩽ 1/3, the model exhibits a second-order PT.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Fragment transition density method to calculate electronic coupling for excitation energy transfer

A general approach, the Fragment Transition Density (FTD) scheme, is introduced to estimate electronic coupling for excitation energy transfer in a molecular system. Within this method, the excitation energies and transition densities of the system are used to derive the coupling matrix element. The scheme allows one to treat systems where exciton donor and acceptor are close together and their exchange interaction and orbital overlap are significant. The FTD method can be applied in combination with any quantum mechanical approach to treat excited states of general nature including single-, double-, and higher excitations. Using FTD approach, we derive excitonic couplings for several systems computed with the CIS, TD DFT and MS-CASPT2 methods. In particular, it is shown that the estimated coupling values in DNA π-stacks are strongly affected by the short-range electronic interaction of adjacent nucleobases.

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

Optically induced spin wave dynamics in [Co/Pd]{sub 8} antidot lattices with perpendicular magnetic anisotropy

We present an all-optical time-resolved measurement of spin wave (SW) dynamics in a series of antidot lattices based on [Co(0.75 nm)/Pd(0.9 nm)]{sub 8} multilayer (ML) systems with perpendicular magnetic anisotropy. The spectra depend significantly on the areal density of the antidots. The observed SW modes are qualitatively reproduced by the plane wave method. The interesting results found in our measurements and calculations at small lattice constants can be attributed to the increase of areal density of the shells with modified magnetic properties probably due to distortion of the regular ML structure by the Ga ion bombardment and to increased coupling between localized modes. We propose and discuss the possible mechanisms for this coupling including exchange interaction, tunnelling, and dipolar interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Room temperature ferromagnetism in liquid-phase pulsed laser ablation synthesized nanoparticles of nonmagnetic oxides

Intrinsic Room Temperature Ferromagnetism (RTF) has been observed in undoped/uncapped zinc oxide and titanium dioxide spherical nanoparticles (NPs) obtained by a purely green approach of liquid phase pulsed laser ablation of corresponding metal targets in pure water. Saturation magnetization values observed for zinc oxide (average size, 9 ± 1.2 nm) and titanium dioxide (average size, 4.4 ± 0.3 nm) NPs are 62.37 and 42.17 memu/g, respectively, which are several orders of magnitude larger than those of previous reports. In contrast to the previous works, no postprocessing treatments or surface modification is required to induce ferromagnetism in the case of present communication. The most important result, related to the field of intrinsic ferromagnetism in nonmagnetic materials, is the observation of size dependent ferromagnetism. Degree of ferromagnetism in titanium dioxide increases with the increase in particle size, while it is reverse for zinc oxide. Surface and volume defects play significant roles for the origin of RTF in zinc oxide and titanium dioxide NPs, respectively. Single ionized oxygen and neutral zinc vacancies in zinc oxide and oxygen and neutral/ionized titanium vacancies in titanium dioxide are considered as predominant defect centres responsible for observed ferromagnetism. It is expected that origin of ferromagnetism is a consequence of exchange interactions between localized electron spin moments resulting from point defects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effects of Rashba Spin-Orbit Coupling on the Anisotropic Magneto Resistance in Domain Wall

The present paper, based on semi-classical Boltzmann equation, aims to investigate the effects of Rashba and Dresselhaus spin orbit interaction and impurities on domain wall anisotropic magneto resistance. It has been shown that the mentioned effects play a remarkable role in anisotropic magneto resistance of electron current in domain walls. It was also concluded that while an increase in Rashba coupling strength can effectively enhance anisotropic magneto resistance of the domain wall, an increase in the wave-vector and exchange interaction leads to their decrease.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Study on isotropic Heisenberg interaction for the realization of SWAP {sup ±α} gates

It is known that nonlocal two-qubit gates are geometrically represented by tetrahedron called as Weyl chamber. Two edges of the Weyl chamber are formed by SWAP{sup ±α} family gates with 0 ≤ α ≤ 1. In this work SWAP{sup ±α} are being realized as two spin system with isotropic Heisenberg exchange interaction. The real parameter α is shown to be the function of duration and strength of interaction. Entanglement of the states generated by these two families of gates is studied with concurrence. Significance of time scale in realizing CNOT using SWAP{sup ±1/2} is highlighted.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
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 OSTI2018

Susceptibility Critical Exponent of a 1D Ising Ring-Type Ferromagnetic

Using the Monte Carlo method, critical behavior of the one-dimensional ferromagnetic Ising model has been investigated with allowance for the interaction of the second and third neighbors and four-particle interaction. The obtained results on the critical temperature were compared with the critical temperature of the quasi-one-dimensional Ising magnetic [(CH{sub 3}){sub 3}NH] · FeCl{sub 3} · 2H{sub 2}O and with the magnitude of the exchange interaction J/k{sub B} = 17.4 K. Within the scope of the finite-dimensional scaling theory, the critical susceptibility exponent has been calculated. It has been shown that values of the susceptibility exponent for the one-dimensional Ising model with periodic boundary conditions are considerably less than the known values of the exponents for three-dimensional systems. The critical susceptibility exponent strongly depends on energy parameters; namely, it decreases with an increase in them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phase Diagrams of a Ferrimagnetic Superlattice in a Transverse Field: a Quantum Monte Carlo Simulation

In this work, quantum Monte Carlo simulation based on the Trotter-Suzuki formalism has been used to study the phase diagrams of a superlattice formed by two ferromagnetic slabs A and B with an antiferromagnetic interfacial coupling in a Transverse field. The effect of the transverse field Ω, the exchange coupling J{sub A}, and the thickness of the two slabs A and B on the phase diagrams of the system have been investigated. A number of interesting phenomena have been found. In particular, the superlattice can exhibit a compensation temperature T{sub k} or a compensation transverse field Ω{sub k} for certain ranges of the exchange interactions. The dependence of the critical and compensation behaviors on the slabs thicknesses has been also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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↗