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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 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Fabrication of MgAl{sub 2}O{sub 4} tunnel barrier by radio frequency-sputtering method and magnetoresistance effect through it with Fe or Fe{sub 4}N ferromagnetic electrode

Spinel MgAl{sub 2}O{sub 4} thin films were deposited on MgO single-crystal substrates and epitaxial Fe (or Fe{sub 4}N) thin films by RF-sputtering from a ceramic target. Epitaxial relationship was confirmed by X-ray diffraction analysis between the crystalline spinel MgAl{sub 2}O{sub 4} films and the respective substrate and underlayers, while no diffraction peak was observed from the films deposited on amorphous substrates. Spin-valve type magnetic tunnel junctions (MTJs) with a stacking structure of Fe [Fe{sub 4}N]/MgAl{sub 2}O{sub 4}/CoFeB/Ru/Fe/MnIr exhibited normal [inverse] tunnel magnetoresistance (TMR) effect, reflecting the sign of spin polarization of Fe [Fe{sub 4}N]. The maximum magnitude of the TMR ratio obtained for the Fe-based and Fe{sub 4}N-based MTJs was 67% and 18%, respectively. The resistance area product values of the MTJs were significantly larger than the reported values for the MTJs with a post-oxidized spinel MgAl{sub 2}O{sub 4} barrier.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The electronic structure of homogeneous ferromagnetic (Ga, Mn)N epitaxial films

X-ray Absorption Fine Structure (XAFS) techniques, namely, X-ray Near Edge Structure (XANES), Extended XAFS (EXAFS), and Anomalous X-ray Diffraction (AXRD) were used to investigate the local atomic and electronic structure of (Ga, Mn)N magnetic layers with Mn concentrations of up to 10% grown by Molecular Beam Epitaxy. The XANES and AXRD analysis prove the Mn incorporation on substitutional GaN lattice sites. EXAFS results indicate the good quality of the structure under examination, although 0.5 nitride atom vacancies were found. The Wien2k code was applied to interpret the XANES spectra quantitatively, i.e., to determine the electronic structure of the Mn atoms. It was shown that accounting for the core-hole effect is necessary to reconstruct effectively the XANES spectra. Conducted charge density analysis based on DFT calculations identified the valency of Mn atom to be of 2.4+.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Metastable bcc phase formation in 3d ferromagnetic transition metal thin films sputter-deposited on GaAs(100) substrates

Co{sub 100−x}Fe{sub x} and Ni{sub 100−y}Fe{sub y} (at. %, x = 0–30, y = 0–60) films of 10 nm thickness are prepared on GaAs(100) substrates at room temperature by using a radio-frequency magnetron sputtering system. The detailed growth behavior is investigated by in-situ reflection high-energy electron diffraction. (100)-oriented Co and Ni single-crystals with metastable bcc structure are formed in the early stage of film growth, where the metastable structure is stabilized through hetero-epitaxial growth. With increasing the thickness up to 2 nm, the Co and the Ni films start to transform into more stable hcp and fcc structures through atomic displacements parallel to bcc(110) slide planes, respectively. The stability of bcc phase is improved by adding a small volume of Fe atoms into a Co film. The critical thickness of bcc phase formation is thicker than 10 nm for Co{sub 100−x}Fe{sub x} films with x ≥ 10. On the contrary, the stability of bcc phase for Ni-Fe system is less than that for Co-Fe system. The critical thicknesses for Ni{sub 100−y}Fe{sub y} films with y = 20, 40, and 60 are 1, 3, and 5 nm, respectively. The Co{sub 100−x}Fe{sub x} single-crystal films with metastable bcc structure formed on GaAs(100) substrates show in-plane uniaxial magnetic anisotropies with the easy direction along GaAs[011], similar to the case of Fe film epitaxially grown on GaAs(100) substrate. A Co{sub 100−x}Fe{sub x} film with higher Fe content shows a higher saturation magnetization and a lower coercivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ferromagnetic tunnel contacts to graphene: Contact resistance and spin signal

We report spin transport in CVD graphene-based lateral spin valves using different magnetic contacts. We compared the spin signal amplitude measured on devices where the cobalt layer is directly in contact with the graphene to the one obtained using tunnel contacts. Although a sizeable spin signal (up to ∼2 Ω) is obtained with direct contacts, the signal is strongly enhanced (∼400 Ω) by inserting a tunnel barrier. In addition, we studied the resistance-area product (R.A) of a variety of contacts on CVD graphene. In particular, we compared the R.A products of alumina and magnesium oxide tunnel barriers grown by sputtering deposition of aluminum or magnesium and subsequent natural oxidation under pure oxygen atmosphere or by plasma. When using an alumina tunnel barrier on CVD graphene, the R.A product is high and exhibits a large dispersion. This dispersion can be highly reduced by using a magnesium oxide tunnel barrier, as for the R.A value. This study gives insight in the material quest for reproducible and efficient spin injection in CVD graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Determination of the Schottky barrier height of ferromagnetic contacts to few-layer phosphorene

Phosphorene, the 2D analogue of black phosphorus, is a promising material for studying spin transport due to its low spin-orbit coupling and its ½ nuclear spin, which could allow the study of hyperfine effects. In this work, the properties of permalloy (Py) and cobalt (Co) contacts to few-layer phosphorene are presented. The Schottky barrier height was extracted and determined as a function of gate bias. Flat-band barrier heights, relative to the valence band edge, of 110 meV and 200 meV were determined for Py and Co, respectively. These results are important for future studies of spin transport in phosphorene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Phase transitions in continuum ferromagnets with unbounded spins

States of thermal equilibrium of an infinite system of interacting particles in ℝ{sup d} are studied. The particles bear “unbounded” spins with a given symmetric a priori distribution. The interaction between the particles is pairwise and splits into position-position and spin-spin parts. The position-position part is described by a superstable potential, and the spin-spin part is attractive and of finite range. Thermodynamic states of the system are defined as tempered Gibbs measures on the space of marked configurations. It is proved that the set of such measures contains at least two elements if the activity is big enough.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Experimental determination of next-nearest-neighbor interlayer exchange coupling in ferromagnetic GaMnAs/GaAs:Be multilayers

We report direct experimental determination of next-nearest-neighbor (NNN) interlayer exchange coupling (IEC) in antiferromagnetically coupled GaMnAs/GaAs:Be multilayers. Magnetoresistance in such multilayers shows step-like transitions as a function of applied magnetic field that corresponds to abrupt changes of spin configuration. By adjusting the field range, one obtains minor hysteresis loops that allow one to determine spin configurations occurring in the multilayer, which in turn can be used to obtain the ratio of NNN IEC to NN IEC. By using this method on a series of GaMnAs/GaAs:Be with different numbers of GaMnAs layers, we obtain this ratio to be 0.23, in good agreement with theoretical predictions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Magnetic microstructure and magnetic properties of uniaxial itinerant ferromagnet Fe 3 GeTe 2

Here, magnetic force microscopy was used to observe the magnetic microstructure of Fe 3 GeTe 2 at 4 K on the (001) surface. The surface magnetic structure consists of a two-phase domain branching pattern that is characteristic for highly uniaxial magnets in the plane perpendicular to the magnetic easy axis. The average surface magnetic domain width D s = 1.3 μm determined from this pattern, in combination with intrinsic properties calculated from bulk magnetization data (the saturation magnetization M s = 376 emu/cm 3 and the uniaxial magnetocrystalline anisotropy constant K u = 1.46 × 10 7 erg/cm 3 ), was used to determine the following micromagnetic parameters for Fe 3 GeTe 2 from phenomenological models: the domain wall energy γ w = 4.7 erg/cm 2 , the domain wall thickness δ w = 2.5 nm, the exchange stiffness constant A ex = 0.95 × 10 –7 erg/cm, the exchange length l ex = 2.3 nm, and the critical single domain particle diameter d c = 470 nm.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Robust giant anomalous Nernst effect in polycrystalline nodal web ferromagnets

The transverse thermoelectric effect based on the anomalous Nernst effect (ANE) has attracted attention, especially for thermoelectric and spintronic applications. Fe3X (X = Ga, Al) is known to exhibit a large ANE at room temperature owing to the topological electronic band structure so-called nodal web. Here, we systematically investigate ANE in the polycrystalline Fe3Ga1−xAlx and Fe3Al1−xSix. Despite significant chemical substitutions, a very robust feature of the large ANE −Syx∼ 5.7 μ V/K is found for Fe3Ga1−xAlx (0≲x≲0.6), exhibiting a striking x independent nodal web contribution. Since aluminum is more low-cost and abundant than gallium, our results indicate Fe3Ga1−xAlx (x∼0.6) should be suitable for large-scale thermoelectric applications.

Physics↗
DOE OSTI2020

Element- and Time-Resolved Measurements of Spin Dynamics Using X-ray Detected Ferromagnetic Resonance

Although XFMR experiments have been carried out at a number of synchrotron radiation facilities since 2004, only a few facilities currently have XFMR setups available for user operation. Here in this review, we mainly focus on recent developments and work that has been carried out using time-resolved XFMR at beamline 4.0.2 of the Advanced Light Source and at beamline I10 of the Diamond Light Source. The experimental setup and the sample holder used at beamline I10 and beamline 4.0.2, respectively, are shown in Figure 1. We will first introduce the working principle of XFMR and then provide examples of the capabilities of this technique. Finally, we will briefly describe an outlook for the future development of and new opportunities for XFMR.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗