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.

SourceResearch connectionFrom yearThrough year

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

At least 307 records · Page 17

All sources NASA NTRS DOE OSTI

DOE OSTI2015

Spin polarization transfer by the radical pair mechanism

In a three-site representation, we study a spin polarization transfer from radical pair spins to a nearby electron or nuclear spin. The quantum dynamics of the radical pair spins is governed by a constant exchange interaction between the radical pair spins which have different Zeeman frequencies. Radical pair spins can recombine to the singlet ground state or to lower energy triplet states. It is then shown that the coherent dynamics of the radical pair induces spin polarization on the nearby third spin in the presence of a magnetic field. The spin polarization transfer depends on the difference between Zeeman frequencies, the singlet and triplet recombination rates, and on the exchange and dipole-dipole interactions between the different spins. In particular, the sign of the polarization depends on the exchange coupling between radical pair spins and also on the difference between singlet and triplet recombination rate constants.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Magnetic properties of pseudo-ideal cubic La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3} with hole doping

The La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3} (x=0.03, 0.04, 0.05, 0.075, 0.1 and 0.15) compounds are synthesized. The cubic La{sub 0.6}Ba{sub 0.4}CoO{sub 3} has the strongest ferromagnetic (FM) double exchange interaction among all the La–Ba–Co–O compounds due to the straight Co–O–Co bond. The variation of the lattice parameter versus the calcium content exhibits a non-linear dependence and has the lowest value at x=0.05. In La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3} compounds with x≤0.05, the magnetic frustration is observed below about 190 K, and the substitution of Ca{sup 2+} for La{sup 3+} enhances the FM interaction due to the increasing double exchange through Co{sup 3+}–O{sup 2−}–Co{sup 4+} bond. The analysis based on the measured magnetization at 50 kOe and the calculated value indicates that Co{sup 3+} ions are almost equally distributed at the low spin state and the intermediate spin state, and the Co{sup 4+} ions are at the low spin state. La{sub 0.55}Ca{sub 0.05}Ba{sub 0.4}CoO{sub 3} has the lowest coercivity. As the composition deviates from x=0.05, the coercivity is enhanced. For the samples with θ{sub CW}, the substitution of Ca{sup 2+} ions enhances the antiferromagnetic (AFM) interaction, and it is strong enough to make the long range AFM interaction appear. The percentage of the FM domains is calculated, and it decreases very quickly with the Ca{sup 2+} content x≥0.05. The sample with x=0.15 is mostly composed of AFM domains, and its AFM–FM transition temperature is 120 K. Another FM interaction with a low magnetic moment at the low temperature is also observed. - Graphical abstract: The thermal magnetization at 200 Oe at the ZFC and the FC states. - Highlights: • We studied the hole doping on the magnetic properties of La{sub 0.6−x}Ca{sub x}Ba{sub 0.4}CoO{sub 3}. • With x≤0.05, Ca{sup 2+} enhances the ferromagnetic interaction. • With θ{sub CW}, Ca{sup 2+} induces long range antiferromagnetic interaction. x=0.15 is mostly AFM. • Co{sup 3+} ions are equally distributed at LS and IS state, and Co{sup 4+} are at LS state. • We analyzed the development of the magnetic interaction with the doping Ca{sup 2+}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2011

Magnetic properties of the second Mott lobe in pairing Hamiltonians

We explore the Mott insulating state of single-band bosonic pairing Hamiltonians using analytical approaches and large-scale density matrix renormalization group calculations. We focus on the second Mott lobe which exhibits a magnetic quantum phase transition in the Ising universality class. We use this feature to discuss the behavior of a range of physical observables within the framework of the one-dimensional quantum Ising model and the strongly anisotropic Heisenberg model. This includes the properties of local expectation values and correlation functions both at and away from criticality. Depending on the microscopic interactions it is possible to achieve either antiferromagnetic or ferromagnetic exchange interactions and we highlight the possibility of observing the E{sub 8} mass spectrum for the critical Ising model in a longitudinal magnetic field.

36 MATERIALS SCIENCE↗

DOE OSTI2015

A study of magnetic ordering in multiferroic hexagonal Ho{sub 1-x}Dy{sub x}MnO{sub 3}

This paper investigates the magnetic properties of Ho{sub 1-x}Dy{sub x}MnO{sub 3} by considering the inter-planar Mn{sup 3+}-O-O-Mn{sup 3+} interaction. The theoretical analysis shows that the asymmetric in-plane exchange interaction couples the in-plane and inter-planar Mn{sup 3+} spin structures via asymmetry parameter δ. This leads to the existence of both the in-plane and inter-planar ordering, which in turn restricted the allowed magnetic space groups to Γ{sub 1} and Γ{sub 4}. The experimental studies confirmed the concomitant nature of the in-plane and the inter-planar ordering at T{sub N}, T{sub SR}, and T{sub 2}. It also showed that the magnetic phase diagram is dominated by the allowed magnetic structures Γ{sub 1} and Γ{sub 4}. Furthermore, an effort is made to resolve the inconsistency regarding the T{sub SR} (32 or 40 K). The studies revealed that the antiferromagnetic inter-planar interaction is switched to the ferromagnetic interaction (40 K) upon cooling, which in turn drives the spin reorientation at 32 K. The Mn{sup 3+} spin structure is seen to be coupled to rare earth sub-lattice through the modulation of the inter-planar interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2011

Spin segregation via dynamically induced long-range interactions in a system of ultracold fermions

We investigate theoretically the time evolution of a one-dimensional system of spin-1/2 fermions in a harmonic trap after, initially, a spiral spin configuration far from equilibrium is created. We predict a spin segregation building up in time already for weak interaction under realistic experimental conditions. The effect relies on the interplay between exchange interaction and the harmonic trap, and it is found for a wide range of parameters. It can be understood as a consequence of an effective, dynamically induced long-range interaction that is derived by integrating out the rapid oscillatory dynamics in the trap.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2019

Magnetic Properties and Mössbauer Investigations of La{sub 0.67}Sr{sub 0.33}Fe{sub x}Mn{sub 1−x}O{sub 3} (x = 0 and 0.5) Perovskites

We report magnetic phase separation in pure La{sub 0.67}Sr{sub 0.33}Fe{sub x}Mn{sub 1−x}O{sub 3} (x = 0 and 0.5) ceramic samples. Rhombohedral crystal structures obtained with R3C space group show no significant changes in the lattice structures due to the close values of ionic radii of Fe{sup 3+} and Mn{sup 3+} ions. However, significant changes occur in the magnetic properties due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions because of the presence of Fe{sup 3+} substitution. The FM double exchange interactions (DE) (Mn{sup 3+}–O–Mn{sup 4+}) appears suppressed by AFM superexchange interactions (Fe{sup 3+}–O–Mn{sup 4+}, Fe{sup 3+}–O–Fe{sup 3+}, Mn{sup 4+}–O–Mn{sup 4+}). The saturation magnetization for x = 0 has the highest value of approximately 80 emu/g at 2 K, which dropped significantly to approximately 5 emu/g at 2 K for the x = 0.5 sample. The coercivity (H{sub c}) for x = 0.5 increased from 161 Oe at 300 K to 786 Oe at 2 K compared with that of x = 0 which varied from 110 to 150 Oe. A value of H{sub c}(0) = 1.5 ± 0.04 kOe was extrapolated from the model fit to the temperature dependence of H{sub c}. The temperature-dependent {sup 57}Fe Mössbauer spectroscopy measurements show the presence of Fe in two different environments that have an AFM Fe{sup 3+}–O–Mn{sup 4+} and Fe{sup 3+/2+}–O–Fe{sup 3+/2+} coordination. The unequal fraction population of Fe ions at both AFM environments seems to increase the exchange bias effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Finite-Temperature Quantum Matter with Rydberg or Molecule Synthetic Dimensions

Synthetic-dimension platforms offer unique pathways for engineering quantum matter. We compute the phase diagram of a many-body system of ultracold atoms (or polar molecules) with a set of Rydberg states (or rotational states) as a synthetic dimension, where the particles are arranged in real space in optical microtrap arrays and interact via dipole-dipole exchange interaction. Using mean-field theory, we find three ordered phases—two are localized in the synthetic dimension, predicted as zero-temperature ground states, and one is a delocalized phase. We characterize them by identifying the spontaneously broken discrete symmetries of the Hamiltonian. We also compute the phase diagram as a function of temperature and interaction strength for both signs of the interaction. For system sizes with more than six synthetic sites and attractive interactions, we find that the thermal phase transitions can be first or second order, which leads to a tricritical point on the phase boundary. Furthermore, by examining the dependence of the tricritical point and other special points of the phase boundary on the synthetic dimension size, we shed light on the physics for thermodynamically large synthetic dimension.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2023

Temperature dependence of magnetic anisotropy and magnetoelasticity from classical spin-lattice calculations

Here we present a classical molecular-spin dynamics (MSD) methodology that enables accurate computations of the temperature dependence of the magnetocrystalline anisotropy as well as magnetoelastic properties of magnetic materials. The nonmagnetic interactions are accounted for by a spectral neighbor analysis potential (SNAP) machine-learned interatomic potential, whereas the magnetoelastic contributions are accounted for using a combination of an extended Heisenberg Hamiltonian and a Néel pair interaction model, representing both the exchange interaction and spin-orbit-coupling effects, respectively. All magnetoelastic potential components are parameterized using a combination of first-principles and experimental data. Our framework is applied to the α phase of iron. Initial testing of our MSD model is done using a 0 K parametrization of the Néel interaction model. After this, we examine how individual Néel parameters impact the $B$ 1 and $B$ 2 magnetostrictive coefficients using a moment-independent δ sensitivity analysis. The results from this study are then used to initialize a genetic algorithm optimization which explores the Néel parameter phase space and tries to minimize the error in the B 1 and B 2 magnetostrictive coefficients in the range of 0–1200 K. Our results show that while both the 0 K and genetic algorithm optimized parametrization provide good experimental agreement for $B$ 1 and $B$ 2 , only the genetic algorithm optimized results can capture the second peak in the $B$ 1 magnetostrictive coefficient which occurs near approximately 800 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

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 OSTI2014

Spin-phonon interactions of multiferroic Bi{sub 4}Ti{sub 3}O{sub 12}-BiFeO{sub 3} ceramics: Low-temperature Raman scattering and infrared reflectance spectra investigations

Optical phonons of multiferroic Bi{sub 4}Ti{sub 3}O{sub 12}-BiFeO{sub 3} ceramic have been investigated by low temperature Raman scattering and infrared reflectance spectra. Anomalies at about 85 K can be observed from the temperature dependence of the Raman and infrared modes, which arise from spin-phonon interaction during antiferromagnetic to paramagnetic phase transition. It was found that the change of exchange interaction in magnetic phase transition can be induced by Fe-O-Fe octahedral tilting driven from the A-site atoms. Moreover, ferroelectricity-related displacement of Bismuth atoms suggests the coupling of magnetic and ferroelectric orders.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Nuclear symmetry energy in terms of single-nucleon potential and its effect on the proton fraction of β-stable npeμ matter

Momentum and density dependence of single-nucleon potential u{sub τ} (k, ρ, β) is analyzed using a density dependent finite range effective interaction of the Yukawa form. Depending on the choice of the strength parameters of exchange interaction, two different trends of the momentum dependence of nuclear symmetry potential are noticed which lead to two opposite types of neutron and proton effective mass splitting. The 2nd-order and 4th-order symmetry energy of isospin asymmetric nuclear matter are expressed analytically in terms of the single-nucleon potential. Two distinct behavior of the density dependence of 2nd-order and 4th-order symmetry energy are observed depending on neutron and proton effective mass splitting. It is also found that the 4th-order symmetry energy has a significant contribution towards the proton fraction of β-stable npeμ matter at high densities.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2015

Magnetic and electrode properties, structure and phase relations of the layered triangular-lattice tellurate Li{sub 4}NiTeO{sub 6}

We examined the magnetic properties of layered oxide Li{sub 4}NiTeO{sub 6} by magnetic susceptibility, magnetization and ESR measurements and density functional calculations, and characterized phase relations, crystal structure and electrochemical properties of Li{sub 4}NiTeO{sub 6}. The magnetization and ESR data indicate the absence of a long-range magnetic order down to 1.8 K, and the magnetic susceptibility data the presence of dominant antiferromagnetic interactions. These observations are well accounted for by density functional calculations, which show that the spin exchanges of the LiNiTeO{sub 6} layers in Li{sub 4}NiTeO{sub 6} are strongly spin frustrated. The electrochemical charging of Li{sub 4}NiTeO{sub 6} takes place at constant potential of ca. 4.2 V vs. Li/Li{sup +} indicating two-phase process as confirmed by X-rays. The starting phase is only partially recovered on discharge due to side reactions. - Graphical abstract: No long-range magnetic order due to frustration in 2D triangular lattice antiferromagnet Li{sub 4}NiTeO{sub 6}. - Highlights: • Li{sub 4}NiTeO{sub 6} is 2D triangular lattice magnet with no long-range order down to 1.8 K. • Intralayer exchange interactions are antiferromagnetic and strongly spin frustrated. • The electrochemical Li extraction proceeds in a two-phase mode at 4.2 V vs. Li/Li{sup +}. • The electrochemical charge–discharge is only partially reversible. • Li{sub 2}O–NiO{sub y}–TeO{sub x} phase relations are reported; Li{sub 4}NiTeO{sub 6} is essentially stoichiometric.

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 OSTI2014

Magnetic and transport properties of cobalt doped La₀.₇Sr₀.₃MnO₃

The magnetic and transport properties of La₀.₇Sr₀.₃Mn{sub 1–x}CoxO₃ (0≤x≤1) samples were systematically studied. A magnetic phase diagram was constructed according to the observed results. At the two ends (x ≤ 0.3 and x>0.9), the samples exhibit the conventional ferromagnetism with the metallic conducting behavior, which can be interpreted in terms of the double exchange coupling between Mn (Co) ions. As for the intermediate Co-doping region (0.3<x≤0.9), the samples enter a mixed magnetic phase state, which displays the characteristics of the glassy states. According to the frequency-dependent results from the ac susceptibility, a representative frequency sensitivity factor K was calculated to be ~0.1 for the typical La₀.₇Sr₀.₃Mn₀.₆Co₀.₄O₃ sample, corresponding to the coexistence of superparamagnetic-like free spins and the reentrant spin glass. The results of the Curie-Weiss fitting of the dc magnetization suggest that the Co³⁺ ions are in the intermediate spin state with the increase of Co-doping level (x ≥ 0.3), while the Co⁴⁺ ions show the spin state transition from the high spin state to the intermediate spin state at x=0.6. The temperature dependence of resistivity shows a metal-insulator transition character for the intermediate Co-doping samples, which can be well fitted by the three dimensional variable-range-hopping model in the whole temperature range. These facts suggest that the double exchange interaction in the perovskite La₀.₇Sr₀.₃MnO₃ is destroyed by the B site doping, due to the strong competition between the double exchange and superexchange interactions in Co/Mn ions as well as the frustration of spins in the system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Thin-film topological insulator-ferromagnet heterostructures for terahertz detection

An atomically thin topological insulator is investigated theoretically for long-wavelength photodetection when it interacts with a magnetic material. Through the coupling between top and bottom surfaces as well as the exchange interaction with the proximate ferromagnet, the distribution of optically excited carriers exhibits unique patterns that depend sensitively on the frequency of the incoming light. This effect results in the generation of strong nonzero photocurrent, leading potentially to room-temperature detection of far-infrared/THz radiation with the advantage of low noise and fast response. The ease of frequency tuning by an external electrical bias offers an added versatility in the realistic implementation.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Magnetic interactions in a quasi-one-dimensional antiferromagnet Cu(H{sub 2}O){sub 2}(en)SO{sub 4}

A theoretical ab-initio investigation of exchange interaction between Cu atoms in an insulating antiferromagnet Cu(H{sub 2}O){sub 2}(en)SO{sub 4}, en = C{sub 2}H{sub 8}N{sub 2}, is reported. While the previous experimental studies described the system's magnetism to be quasi-two-dimensional, our results, based on a mapping of the system onto an effective Heisenberg model, rather support a quasi-one-dimensional character with the exchange coupling between the Cu atoms being propagated mainly along a zigzag line lying in the crystal's bc plane and connecting the Cu atoms through the N atoms. Further, the direction of magnetic moments on the Cu atoms is suggested to be nearly along the crystal's a axis. A check of the change in the exchange constants induced either by external pressure or by various values of U in the GGA + U approximation is made. Finally, based on experimental values of positions of broad maxima in magnetic-susceptibility and specific-heat curves and using theoretical expressions available in the literature a relevant value of the U parameter and related expected value of the electronic gap are estimated to be about 5 eV and 2 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Magnetic and dielectric properties of one-dimensional array of S = 1/2 linear trimer system Na{sub 2}Cu{sub 3}Ge{sub 4}O{sub 12}

Magnetic susceptibility χ, specific heat C, capacitance C{sub p}, and {sup 23}Na-NMR measurements have been carried out on polycrystalline samples of quantum spin linear trimer system Na{sub 2}Cu{sub 3}Ge{sub 4}O{sub 12}, which has the one-dimensional array of Cu{sub 3}O{sub 8} trimers formed of edge-sharing three CuO{sub 4} square planes. The exchange interactions between the Cu{sup 2+} (S = 1/2) spins have been determined by analyzing χ-T and C-T curves. By employing the isolated S = 1/2 Heisenberg trimer model above 70 K, the nearest-neighbor exchange couplings J{sub 1} and the second-neighbor one J{sub 2} in trimer have been evaluated to J{sub 1}/k{sub B} = 30 ± 20 K (antiferromagnetic) and J{sub 2}/k{sub B} = 340 ± 20 K. At low temperature region, two spins of the edge in the Cu{sub 3}O{sub 8} trimers form a nonmagnetic singlet by strong antiferromagnetic interaction J{sub 2}, and the spin left in the center of the Cu{sub 3}O{sub 8} trimer forms one-dimensional chains by the exchange interaction J{sub 3} between the trimers. By employing the S = 1/2 uniform Heisenberg chain model below 70 K, we have evaluated to J{sub 3}/k{sub B} = 18 ± 1 K. The mechanism of multiferroic behavior at T{sub c} = 2 K is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗