Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “EXCHANGE INTERACTIONS”

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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
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 OSTI2017

Heterospin biradicals based on new piperidineoxyl-substituted 3,6-di-tert-butyl-o-benzoquinone

A nucleophilic addition reaction of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (OH-TEMPO) to 3,6-di-tert-butyl-o-benzoquinone was used to obtain a new sterically hindered o-benzoquinone (1) containing 2,2,6,6-tetramethylpiperidineoxyl functional group, which was characterized by IR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction. A one-electron reduction of 1 with potassium and thallium is an efficient method for the generation of earlier unknown heterospin biradicals 5a and 5b, respectively, containing nitroxide and o-semiquinone radical centers. Analysis of the hyperfine structure of the ESR spectra of biradicals 5a and 5b in solution showed that they belong to the group of heterospin biradicals with strong (J >> a) and fast exchange interaction between the radical centers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Novel bimetallic thiocyanate-bridged Cu(II)-Hg(II) compounds-synthesis, X-Ray studies and magnetic properties

Seven novel heterobimetallic Cu/Hg polymers based on thiocyanate bridges have been synthesised and characterised by means of IR, EPR, magnetic measurements and single crystal X-Ray. Three of them, [Cu(pzH){sub 4}Hg(SCN){sub 4}]{sub n} (1) [Cu(indH){sub 4}Hg(SCN){sub 4}]{sub n} (2) and [Cu(ampy){sub 2}Hg(SCN){sub 4}]{sub n} (3), have one-dimensional coordination structure. Two compounds [Cu(pzH){sub 2}Hg(SCN){sub 4}]{sub n} (4) and [Cu(abzimH)Hg(SCN){sub 4}]{sub n} (5) form two-dimensional nets, whereas the complexes [Cu(pyCN){sub 2}Hg(SCN){sub 4}]{sub n} (6) and [Cu(pyCH(OH)(OMe)){sub 2}Hg(SCN){sub 4}]{sub n} (7) are three-dimensional coordination polymers. The chains of 1 are connected by the intermolecular N-H Bullet Bullet Bullet N hydrogen bonds to the three dimensional net. In 2 the N-H Bullet Bullet Bullet S hydrogen bonds link the polymeric chains to the two dimensional layer extending along crystallographic (0 0 1) plane. The polymeric chains of compound 3 are joined by the intermolecular N-H Bullet Bullet Bullet N and N-H Bullet Bullet Bullet S hydrogen bonds to the three dimensional net. The polymeric layers of 4 are connected by the intermolecular N-H Bullet Bullet Bullet N hydrogen bonds to the three dimensional net. - Graphical abstract: Novel bimetallic thiocyanate-bridged Cu(II)-Hg(II) compound-synthesis,X-Ray studies and magnetic properties. Highlights: Black-Right-Pointing-Pointer Novel heterobimetallic Cu/Hg coordination polymers were synthesised. Black-Right-Pointing-Pointer The multidimensional structures have been proved by single X-ray analysIs. Black-Right-Pointing-Pointer A variation in the crystalline architectures was observed depending on auxiliary ligands. Black-Right-Pointing-Pointer Magnetic measurements indicate weak exchange interaction between Cu(II) in the crystal lattices below 10 K.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ab Initio Study of the Structural, Magnetic, Electronic, and Thermodynamic Properties of Pd{sub 2}MnZ (Z = Ga, Ge, As) Heusler Alloys

The results of examination of the structural, magnetic, electronic, and thermodynamic properties of Pd{sub 2}MnZ (Z = Ga, Ge, As) Heusler alloys obtained in ab initio and Monte Carlo modeling are presented. It is demonstrated that a stable martensitic state is possible for Pd{sub 2}MnGa and Pd{sub 2}MnAs alloys. The equilibrium lattice parameter increases in the considered series of alloys with the number of valence electrons per atom (e/a). The Curie temperature of Pd{sub 2}MnZ (Z = Ga, Ge, As) alloys is determined using the calculated parameters of exchange interaction and total magnetic moments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Reentrant spin glass ordering in an Fe-based bulk metallic glass

We report the results of the complex susceptibility, temperature, and field dependence of DC magnetization and the nonequilibrium dynamics of a bulk metallic glass Fe{sub 40}Co{sub 8}Cr{sub 15}Mo{sub 14}C{sub 15}B{sub 6}Er{sub 2}. Solid indication of the coexistence of reentrant spin glass (SG) and ferromagnetic orderings is determined from both DC magnetization and AC susceptibility under different DC fields. Dynamics scaling of AC susceptibility indicates critical slowing down to a reentrant SG state with a static transition temperature T{sub s} = ∼17.8 K and a dynamic exponent zv = ∼7.3. The SG nature is further corroborated from chaos and memory effects, magnetic hysteresis, and aging behavior. We discuss the results in terms of the competition among random magnetic anisotropy and exchange interactions and compare them with simulation predictions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

The influence of defects on magnetic properties of fcc-Pu

The influence of vacancies and interstitial atoms on magnetism in Pu is considered in the framework of the density functional theory. The crystal structure relaxation arising due to various types of defects is calculated using the molecular dynamics method with a modified embedded atom model. The local density approximation with explicit inclusion of Coulomb and spin-orbit interactions is applied in matrix invariant form to describe correlation effects in Pu with these types of defects. The calculations show that both vacancies and interstitials give rise to local moments in the f-shell of Pu in good agreement with experimental data for aged Pu. Magnetism appears due to the destruction of a delicate balance between spin-orbit and exchange interactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Magnetic studies of cobalt doped barium hexaferrite nanoparticles prepared by modified sol-gel method

M-type barium hexaferrite (BaFe{sub 12}O{sub 19}) and cobalt doped barium hexaferrite (BaFe{sub 11}CoO{sub 19}) nanopowders were synthesized by modified sol-gel auto-combustion technique and were annealed at 900°C in air for 4 hours. The annealed powders were studied in the present work and X-ray diffraction studies showed pure phase formation after annealing. The average grain size in the nanopowder sample was decreased after doping. Magnetization value of 60 emu/g was observed at 300 K for the barium hexaferrite and was reduced to 54 emu/g after doping. The coercivity of 5586 Oe was observed at 300 K for the undoped sample and was found to be decreased in the doped sample. As the measurement temperature was decreased from 300 K to 60 K, magnetization value was increased in both the samples compared to those at 300 K. The coercivity of the undoped sample was found to decrease whereas it was increased for the doped sample at 60 K. The observed magnetic properties may be understood on the basis of modified exchange interaction and anisotropy in the doped sample compared to that of pure barium hexaferrite.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Fano-type coupling of a bound paramagnetic state with 2D continuum

We analyze an effect of a bound impurity state located at a tunnel distance from a quantum well (QW). The study is focused on the resonance case when the bound state energy lies within the continuum of the QW states. Using the developed theory we calculate spin polarization of 2D holes induced by paramagnetic (Mn) delta-layer in the vicinity of the QW and indirect exchange interaction between two impurities located at a tunnel distance from electron gas.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Electrical and Magnetic Properties of Hafnium Disulfide Intercalated with Iron Atoms

Simultaneous study of the dependences of the structural parameters, electrical, and magnetic properties of hafnium disulfide intercalated iron atoms in the dependence on the intercalate concentration and temperature has been performed for the first time. The temperature dependences of the electrical resistance are shown to exhibit the activation character with the activation energies characteristic of impurity conduction. The effective magnetic moments of iron ions in Fe{sub x}HfS{sub 2} is found to be significantly lesser than the values of free iron ions and to decrease as the iron content increases. The character of the temperature dependences of the effective magnetic moments and negative values of the paramagnetic Curie temperatures indicate possible interactions of the antiferromagnetic type between intercalated atoms. However, the dependences of the magnetization on field for Fe{sub 0.33}HfS{sub 2} and Fe{sub 0.5}HfS{sub 2} obtained at T = 2 K demonstrate the hysteresis phenomenon characteristic of the ferromagnetic state. The results are discussed assuming the existence of hybridization 3d electron states of intercalated iron atoms with the electronic states of HfS{sub 2} matrices and the competition of various exchange interaction.

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 OSTI2013

Magnetic field induced discontinuous spin reorientation in ErFeO{sub 3} single crystal

The spin reorientation of ErFeO{sub 3} that spontaneously occurs at low temperature has been previously determined to be a process involving the continuous rotation of Fe{sup 3+} spins. In this work, the dynamic process of spin reorientation in ErFeO{sub 3} single crystal has been investigated by AC susceptibility measurements at various frequencies and static magnetic fields. Interestingly, two completely discontinuous steps are induced by a relatively large static magnetic field due to the variation in the magnetic anisotropy during this process. It provides deeper insights into the intriguing magnetic exchange interactions which dominate the sophisticated magnetic phase transitions in the orthoferrite systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Improvement of saturation magnetization of Fe nanoparticles by post-annealing in a hydrogen gas atmosphere

Fe nanoparticles (NPs) were synthesized by the thermal decomposition of Fe(CO){sub 5} and then post-annealing in a hydrogen gas atmosphere to produce highly monodisperse Fe NPs with high saturation magnetization (M{sub s}). The as-synthesized pre-anneal Fe NPs had an expanded α-Fe structure and M{sub s} was only 39% of that for bulk Fe because of the low crystallinity and the inclusion of a surfactant. Post-annealing of the Fe NPs in a hydrogen gas atmosphere at 200 °C improved the crystallinity of the Fe NPs from an amorphous-like structure to a body centered cubic (bcc) structure without any lattice expansion. This result indicates that hydrogen gas plays a significant role in improvement of the crystallinity of Fe NPs. Accompanying the improvement in crystallinity, M{sub s} for the Fe NPs increased from 86 to 190 emu/g{sub net} at 300 K, the values of which include the weight of surfactant. This enhanced M{sub s} is almost the same as that of bulk Fe (218 emu/{sub Fe}). It was concluded that the crystallinity has a significant influence on the M{sub s} of the Fe NPs because long-range ordering of the lattice can maintain strong direct exchange interactions between Fe atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

MAGNETOHYDRODYNAMIC MODELING OF SOLAR SYSTEM PROCESSES ON GEODESIC GRIDS

This report describes a new magnetohydrodynamic numerical model based on a hexagonal spherical geodesic grid. The model is designed to simulate astrophysical flows of partially ionized plasmas around a central compact object, such as a star or a planet with a magnetic field. The geodesic grid, produced by a recursive subdivision of a base platonic solid (an icosahedron), is free from control volume singularities inherent in spherical polar grids. Multiple populations of plasma and neutral particles, coupled via charge-exchange interactions, can be simulated simultaneously with this model. Our numerical scheme uses piecewise linear reconstruction on a surface of a sphere in a local two-dimensional 'Cartesian' frame. The code employs Haarten-Lax-van-Leer-type approximate Riemann solvers and includes facilities to control the divergence of the magnetic field and maintain pressure positivity. Several test solutions are discussed, including a problem of an interaction between the solar wind and the local interstellar medium, and a simulation of Earth's magnetosphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Local lattice distortions and magnetic properties of CdCr{sub 2}Se{sub 4−x}S{sub x}

Interplay between structural disorder and magnetic interaction is investigated here for a multiferroic candidate material, CdCr{sub 2}Se{sub 4}. Ferromagnetic order in CdCr{sub 2}Se{sub 4} sets in below T{sub C} ∼ 130 K as a result of competition between the direct Cr-Cr spin coupling and the near neighbour Cr-Se-Cr exchange interactions. Hence, a small change in the crystal structure is expected to drastically affect its magnetic order. In this report, local lattice distortions within the overall cubic symmetry were brought about by replacing a small percentage of Se by isovalent S. Detailed crystal structure study using EXAFS and Raman Spectroscopy reflects the presence of local distortions within the overall cubic symmetry. Contrary to the expectation, magnetic properties of the substituted compositions do not show any drastic changes. Though, a signature of spin-phonon coupling is present across the magnetic ordering temperature. No structural phase transition occurs within the investigated temperature range of 80–300 K.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
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 OSTI2018

Dynamic Magnetic Features of the Mixed Ising System on the Bilayer Square Lattice

In order to study the dynamic magnetic features of the mixed spin-1 and spin-5/2 Ising system, we have used the mean-field theory (MFT) based on the Glauber-type stochastic dynamics on the bilayer square lattice (BSL). By employing the Master equation, dynamic equations describing the behavior of the system are derived. The average order parameters and dynamic order parameters are studied for different values of exchange interactions, oscillating magnetic field, and temperature. The complete dynamic phase diagrams (DPDs) are obtained for the ferromagnetic/ferromagnetic (FM/FM), ferromagnetic/antiferromagnetic (FM/AFM), and antiferromagnetic/antiferromagnetic (AFM/AFM) interactions in the different planes, and the role of the interaction in the Hamiltonian on the DPDs is explored. It is found that the DPDs display richer dynamic behavior for this mixed Ising system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effects of Cr doping on the magnetic properties of multiferroic YMnO{sub 3}

We have synthesized a series of YMn{sub 1−x}Cr{sub x}O{sub 3} (0 ≤ x ≤ 0.1) samples and study the effect of Cr-doping on their magnetic properties. The magnetic characterization indicates that with increasing Cr-content up to 0.1, the antiferromagnetic (AFM) transition temperature increases from 73 to 89 K. Our experiment results also indicate that the Cr-doped samples exhibit the characteristics of spin-glass state at low temperature. Moreover, the magnetic hysteresis curves of the doped samples show a weak ferromagnetic (FM) behavior. It is found that the spin-glass state of the Cr-doped samples is due to the competition between AFM superexchange and FM double-exchange interaction, induced by the Cr doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigation of A-site La substituted BaTi{sub 0.96}Mn{sub 0.04}O{sub 3} ceramics: Searching for ferromagnetic origin

Ba{sub 1-x}La{sub x}Ti{sub 0.96}Mn{sub 0.04}O{sub 3} ceramics (x = 0, 3%, 7%) were prepared by a modified Pechini method and the structure, morphology, magnetic and electric properties have been investigated. X-ray diffraction analysis reveals La{sup 3+} ions could suppress the formation of hexagonal phase effectively. The cell parameter c decreases firstly and then increases with the increase of La content, which indicates that La ions have been incorporated into the lattice of BaTi{sub 0.96}Mn{sub 0.04}O{sub 3}, and that Mn ions transform from high valence to low valence. The magnetization, polarization and leakage currents are also influenced by La doping greatly. Detailed analysis of magnetic variation with structural change attributes the room temperature ferromagnetism to double exchange interactions between Mn{sup 3+} and Mn{sup 4+} ions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗