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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Magnetic and Electrical Properties of Hg{sub 1–x–}Mn{sub x}Fe{sub y}Te{sub 1–z}S{sub z} Crystals

Electrical and magnetic properties of the semimagnetic semiconductor Hg{sub 1–x–y}Mn{sub x}Fe{sub y}Te{sub 1–z}S{sub z} solid solutions were studied in the ranges of temperatures 77–320 K and magnetic fields 0.25–6 kOe. In the crystals under study, the effect of giant magnetoresistance was observed that reached 75% at low temperatures. This is due to the fact that charge carriers that participate in the current transfer interact with a magnetized ferromagnetic cluster subsystem (Fe–Fe–Fe) and become spin-polarized. It is these spin-polarized charge carriers that are strongly scattered on antiferromagnetic Mn–S–Mn–S and Mn–Te–Mn–Te clusters, since the magnetic moments inside the clusters and the resulting magnetic moments of these clusters are randomly oriented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Conserved momenta of a ferromagnetic soliton

Linear and angular momenta of a soliton in a ferromagnet are commonly derived through the application of Noether’s theorem. We show that these quantities exhibit unphysical behavior: they depend on the choice of a gauge potential in the spin Lagrangian and can be made arbitrary. To resolve this problem, we exploit a similarity between the dynamics of a ferromagnetic soliton and that of a charged particle in a magnetic field. For the latter, canonical momentum is also gauge-dependent and thus unphysical; the physical momentum is the generator of magnetic translations, a symmetry combining physical translations with gauge transformations. We use this analogy to unambiguously define conserved momenta for ferromagnetic solitons. General considerations are illustrated on simple models of a domain wall in a ferromagnetic chain and of a vortex in a thin film.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

History, present status, and future directions of vanadium alloys for fusion reactors

Since the need for low-activation fusion reactor materials was recognized in the mid-1980s, vanadium alloys have been researched and developed as promising candidates for blanket structural materials. Vanadium alloys are non-ferromagnetic and ductile materials and thus have advantages different from other candidates. However, since vanadium alloys are still at a premature industrial stage, research and development have been carried out not only on issues specific to fusion reactors but also on those related to industrial materials. Blankets using vanadium alloys as structural materials and liquid Li as a tritium breeder and coolant (self-cooled V/Li blankets) have unique characteristics, and research and development of issues specific to V/Li blankets have been conducted in conjunction with the development of vanadium alloys. In this paper, the progress of this research and development is reviewed, and the remaining issues are pointed out. As a result, the future direction of vanadium alloy research and development is discussed, considering recent changes in the environment surrounding fusion energy.

Liquid Li blanket↗
DOE OSTI2015

Structure and magnetism of Sr{sub 3}NiSb{sub 2}O{sub 9}

The crystal structure of the perovskite-related oxide Sr{sub 3}NiSb{sub 2}O{sub 9} has been refined from X-ray and neutron powder diffraction data; space group P2{sub 1}/n, a=5.64381(2), b=5.62299(2), c=7.95687(3) Å, β=90.014(2)°. The structure has two crystallographically-distinct six-coordinate cation sites with occupancies Sb 0.97(1), Ni 0.03 and Sb 0.36, Ni 0.64. The magnetic susceptibility has been measured over the temperature range 2≤T/K≤300. Fitting data recorded in the temperature range 150<T/K<300 to the Curie–Weiss law resulted in µ{sub eff}=3.49(1) µ{sub B} per Ni{sup 2+}, θ=−138(1) K. Ferrimagnetic coupling in the vicinity of the antisite defects is suggested to be responsible for the enhanced susceptibility observed below ~90 K. The zero-field cooled susceptibility shows a maximum at 8.7 K that is not observed in the field-cooled susceptibility. Hysteresis is observed in M(H) at 2 K but not at 7.5 K. - Graphical abstract: Antisite defects on the B sites of perovskite-related Sr{sub 3}NiSb{sub 2}O{sub 9} lead to complex magnetic behaviour. - Highlights: • Sr{sub 3}NiSb{sub 2}O{sub 9} is a monoclinic perovskite. • The magnetic behaviour of Sr{sub 3}NiSb{sub 2}O{sub 9} at around 90 K can be attributed to antisite defects. • The magnetic behaviour of Sr{sub 3}NiSb{sub 2}O{sub 9} can be related to that of the relaxor ferromagnet La{sub 3}Ni{sub 2}SbO{sub 9}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Analysis of mechanical, thermodynamic, and thermoelectric properties of ferromagnetic SrFe{sub 4}As{sub 12} skutterudite

In the present work, the electronic, mechanical, thermal and thermoelectric properties of Sr-filled skutterudite have been investigated in the framework of density functional theory. SrFe{sub 4}As{sub 12} shows a ferromagnet character with a magnetic moment of 1.76 μ{sub B} at an optimized lattice parameter of 8.38 Å. The effect of high-pressures on thermodynamic properties viz. thermal expansion coefficient, specific heat capacity and Grüneisen parameter is also carried out by employing the quasi-harmonic Debye model. Temperature dependence of Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit is observed up to 800 K. The lattice thermal conductivity decreases with temperature due to rattling behavior of the filler atom. SrFe{sub 4}As{sub 12} exhibits the high value of the figure of merit equal to 0.76 at 800 K. The present results clearly signify that SrFe{sub 4}As{sub 12} may be a good choice for thermoelectric applications due to its high-power factor and figure of merit.

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

Structural and magnetic properties of La2Ni1−xCoxMnO6 compounds

Highlights: • Monoclinic La2Ni1−xCoxMnO6 samples have been prepared using Sol-gel technique. • Rietveld analysis shows an octahedrally distorted double perovskite structure. • The nature of FM interaction and TC depend on the B-site ions. • The short range FM ordering above TC is induced by antisite disorders. • The existence of any spin glass like phase in such system is discarded. - Abstract: Single phase monoclinic La{sub 2}Ni{sub 1−x}Co{sub x}MnO{sub 6} (x = 0.2, 0.4, 0.8) compounds, henceforth, referred as LNC{sub x}MO (x = 0.2, 0.4, 0.8) have been prepared by Sol–gel technique. The structural study and redox titration suggest that the samples are both La and O deficient. The generated atomic models using Rietveld refined structural parameters show octahedral distortion in the double perovskite structure. X-ray photoelectron spectroscopy (XPS) study has been used to determine the predominant valence states of Ni, Mn and Co ions. LNC{sub x}MO (x = 0.2, 0.4) are found to be Heisenberg ferromagnets whereas LNC{sub 0.8}MO is a phase segregated system. In LNC{sub 0.4}MO and LNC{sub 0.8}MO, at temperatures above T{sub C}, χ{sup −1}(T) curve shows the characteristics of Griffiths phase. The study of ac susceptibility and isothermal remanent magnetization (IRM) discard the possibility of any glassy state at low temperatures. The antiphase boundaries (APBs), antisite disorders (ASDs) and oxygen vacancies play important roles in this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Voltage control of magnetism in Fe3-xGeTe2/In2Se3 van der Waals ferromagnetic/ferroelectric heterostructures

Abstract We investigate the voltage control of magnetism in a van der Waals (vdW) heterostructure device consisting of two distinct vdW materials, the ferromagnetic Fe 3- x GeTe 2 and the ferroelectric In 2 Se 3 . It is observed that gate voltages applied to the Fe 3- x GeTe 2 /In 2 Se 3 heterostructure device modulate the magnetic properties of Fe 3- x GeTe 2 with significant decrease in coercive field for both positive and negative voltages. Raman spectroscopy on the heterostructure device shows voltage-dependent increase in the in-plane In 2 Se 3 and Fe 3- x GeTe 2 lattice constants for both voltage polarities. Thus, the voltage-dependent decrease in the Fe 3- x GeTe 2 coercive field, regardless of the gate voltage polarity, can be attributed to the presence of in-plane tensile strain. This is supported by density functional theory calculations showing tensile-strain-induced reduction of the magnetocrystalline anisotropy, which in turn decreases the coercive field. Our results demonstrate an effective method to realize low-power voltage-controlled vdW spintronic devices utilizing the magnetoelectric effect in vdW ferromagnetic/ferroelectric heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Strongly correlated Chern insulators in magic-angle twisted bilayer graphene

Interactions between electrons and the topology of their energy bands can create unusual quantum phases of matter. Most topological electronic phases appear in systems with weak electron–electron interactions. The instances in which topological phases emerge only as a result of strong interactions are rare and mostly limited to those realized in intense magnetic fields. The discovery of flat electronic bands with topological character in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for strongly correlated topological phases. Here we introduce a local spectroscopic technique using a scanning tunnelling microscope to detect a sequence of topological insulators in MATBG with Chern numbers C = ±1, ±2 and ±3, which form near filling factors of ±3, ±2 and ±1 electrons per moiré unit cell, respectively, and are stabilized by modest magnetic fields. One of the phases detected here (C = +1) was previously observed when the sublattice symmetry of MATBG was intentionally broken by a hexagonal boron nitride substrate, with interactions having a secondary role. We also demonstrate that strong electron–electron interactions alone can produce not only the previously observed phase, but also other unexpected Chern insulating phases in MATBG. The full sequence of phases that we observe can be understood by postulating that strong correlations favour breaking time-reversal symmetry to form Chern insulators that are stabilized by weak magnetic fields. Our findings illustrate that many-body correlations can create topological phases in moiré systems beyond those anticipated from weakly interacting models.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Large negative magnetoresistance in reactive sputtered polycrystalline GdN{sub x} films

Polycrystalline ferromagnetic GdN{sub x} films were fabricated at different N{sub 2} flow rates (f{sub N2}) to modify N-vacancy concentration so as to study its influence on electrotransport. Metal-semiconductor transition appears at Curie temperature (T{sub C}) of {approx}40 K. Temperature-dependent magnetoresistance (MR) shows a peak at T{sub C}. The films at f{sub N2} = 5, 10, 15, and 20 sccm show MR of -38%, -42%, -46%, and -86% at 5 K and 50 kOe, respectively. Above 15 K, MR is from colossal MR and from both colossal and tunneling MR below 15 K. The enhanced MR at f{sub N2} = 20 sccm is attributed to large spin polarization of half-metallicity in GdN{sub x} with low N vacancies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Angular dependence of anisotropic magnetoresistance in magnetic systems

Anisotropic magnetoresistance (AMR), whose physical origin is attributed to the combination of spin dependent scattering and spin orbital coupling (SOC), usually displays simple angular dependence for polycrystalline ferromagnetic metals. By including generic spin dependent scattering and spin Hall (SH) terms in the Ohm's law, we explicitly show that various magneto-transport phenomena such as anomalous Hall (AH), SH, planar Hall (PH) and AMR could be quantitatively related for bulk polycrystalline ferromagnetic metals. We also discuss how AMR angular dependence is affected by the presence of interfacial SOC in magnetic layered structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Deep Co penetration and spin-polarization of C{sub 60} molecules at hybridized Co-C{sub 60} interfaces

We used near-edge x-ray absorption fine structure spectroscopy to identify the interplays at organic semiconductor-ferromagnet interfaces. When monitoring the L-edge intensity of 0.36 nm Co depositing on C{sub 60} films of various thicknesses, we detected weaker Co signal from structures with larger C{sub 60} thicknesses. Having determined that the electron mean escape depth in C{sub 60} is 4.9 nm, further model analysis indicates that the decline of spectral intensity is due to deep penetration of Co clusters. Finally, C K-edge spectra reveal clear evidences of orbital hybridization between Co and C{sub 60} as well as a visible dichroic effect at 125 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Controlling magnetic anisotropy in La{sub 0.7}Sr{sub 0.3}MnO{sub 3} nanostructures

We have developed a chlorine based dry etching process for nanopatterning the ferromagnetic oxide La{sub 0.7}Sr{sub 0.3}MnO{sub 3} (LSMO). Large arrays of millions of identical structures have been fabricated from thin LSMO films by electron-beam lithography and reactive ion etching. SQUID magnetometry demonstrates that patterned nanostructures with lateral dimensions down to 100 nm retain their full magnetization and the Curie temperature of the bulk layer. In addition, their shape anisotropy is sufficient to overcome the crystalline anisotropy of the bulk. High resolution scanning transmission electron microscopy shows that crystallinity is preserved even at the edges of the nanostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Local magnetoresistance through Si and its bias voltage dependence in ferromagnet/MgO/silicon-on-insulator lateral spin valves

Local magnetoresistance (MR) through silicon (Si) and its bias voltage (V{sub bias}) (bias current (I{sub bias})) dependence in ferromagnet (FM)/MgO/silicon-on-insulator lateral spin valves are investigated. From the experimental measurements, we find that the local-MR through Si increases with increasing V{sub bias}. This anomalous increase of local-MR as a function of V{sub bias} can be understood by considering the standard drift-diffusion theory improved by taking into account the difference in the interface resistances and first order quantum effect between FM/MgO/Si (source) and Si/MgO/FM (drain) interfaces. The interface resistance dependence on experimentally obtained local-MR ratios also agrees with the improved standard spin diffusion theory. These results indicate that experimentally observed local-MR is certainly related to the spin signal through the Si bulk band.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Development and intrinsic properties of hexagonal ferromagnetic (Zr,Ti)Fe{sub 2}

Nanocrystalline Ti{sub 0.75}Zr{sub 0.25}Fe{sub 2+x} (x = 0-0.4) and Ti{sub 0.75−y}B{sub y}Zr{sub 0.25}Fe{sub 2.4} (y = 0-0.35) with high saturation magnetization have been fabricated by the melt-spinning technique. Nanocrystalline Ti{sub 0.75}Zr{sub 0.25}Fe{sub 2+x} consists of the hexagonal C14 Laves phase (Ti,Zr)Fe{sub 2}. Fe addition decreases the lattice parameter a and shrinks the cell volume. The antiferromagnetic Fe-Fe interactions may decrease with the increase of x, leading to a significant enhancement of saturation polarization (J{sub s}) and Curie temperature (T{sub c}). The magnetocrystalline anisotropy constant K also increases with increasing x. Excessive Fe addition (x > 0.25) may induce structural disorder which lowers the J{sub s} and T{sub c}. Nanocrystalline Ti{sub 0.75−y}B{sub y}Zr{sub 0.25}Fe{sub 2.4} is composed of hexagonal (Ti,Zr)Fe{sub 2} and Fe-rich amorphous phases with relatively high J{sub s}. The lattice parameters a, c and cell volume V are almost unchanged with the increase of y for y ≥ 0.16. Simultaneously, the T{sub c} of (Ti,Zr)Fe{sub 2} remains unchanged, indicating that B does not enter this lattice but takes part in forming the amorphous phase, in good agreement with the X-ray diffraction results. The volume fraction of the amorphous phase increases with the increase of B content and results in a large enhancement of J{sub s} up to 10.8 kG. Further B addition (y > 0.30) decreases J{sub s}, possibly due to the decrease of the J{sub s} of the amorphous phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Nonreciprocal dispersion of spin waves in ferromagnetic thin films covered with a finite-conductivity metal

We study the effect of one-side metallization of a uniform ferromagnetic thin film on its spin-wave dispersion relation in the Damon–Eshbach geometry. Due to the finite conductivity of the metallic cover layer on the ferromagnetic film, the spin-wave dispersion relation may be nonreciprocal only in a limited wave-vector range. We provide an approximate analytical solution for the spin-wave frequency, discuss its validity, and compare it with numerical results. The dispersion is analyzed systematically by varying the parameters of the ferromagnetic film, the metal cover layer and the value of the external magnetic field. The conclusions drawn from this analysis allow us to define a structure based on a 30 nm thick CoFeB film with an experimentally accessible nonreciprocal dispersion relation in a relatively wide wave-vector range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Emergent spin electromagnetism induced by magnetization textures in the presence of spin-orbit interaction (invited)

Emergent electromagnetic field which couples to electron's spin in ferromagnetic metals is theoretically studied. Rashba spin-orbit interaction induces spin electromagnetic field which is in the linear order in gradient of magnetization texture. The Rashba-induced effective electric and magnetic fields satisfy in the absence of spin relaxation the Maxwell's equations as in the charge-based electromagnetism. When spin relaxation is taken into account besides spin dynamics, a monopole current emerges generating spin motive force via the Faraday's induction law. The monopole is expected to play an important role in spin-charge conversion and in the integration of spintronics into electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Twofold Andreev reflections and coherent transport in ferromagnetic semiconductor/d-wave superconductor/ferromagnetic semiconductor double tunneling junctions

Coherent transport in a ferromagnetic semiconductor (FS)/d-wave superconductor (SC)/FS structure with (110) interfaces is studied by extending Bogoliubov-de Gennes equation into eight components, in which the interband coupling of heavy and light hole bands in the FS, the strengths of potential scattering at the interfaces, and the mismatches in the effective mass and Fermi vector between the FS and SC are taken into account. Twofold Andreev reflections exist due to the existence of two bands in the FS, in which the incident hole and the two Andreev-reflected electrons, belonging to the different spin subbands, form twofold spin-singlet pairing states near the FS/SC interface. It is shown that due to the interplay of the SC with unconventional d-wave pairing symmetry and FS, the differential conductance and tunneling magnetoresistance exhibit an abundant dependence on not only the interband coupling in the FS but also the strengths of potential scattering at the interfaces. More importantly, the properties are found to be quite different from those in the FS/s-wave SC/FS structure with conventional pairing symmetry for the SC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Unusual multiferroicity in Cu{sub 3}Nb{sub 2}O{sub 8}

Cu{sub 3}Nb{sub 2}O{sub 8} is an anti-ferromagnetic - ferroelectric material in which the non-collinear spin alignment leads to breaking of spatial inversion symmetry. We confirm that the ferroelectricity and magnetic anomaly both occur at T{sub N1} = 25K. The emergence of ferroelectricity due to complex spin ordering is investigated by the temperature dependent low temperature measurements of magnetization, dielectric constant and electric polarization. The microscopic origin to explain magneto-electric coupling in this compound is based on a model based on ferroaxial coupling rather than inverse Dzyalonshkii-Moria interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗