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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

(Ga,Fe)Sb: A p-type ferromagnetic semiconductor

A p-type ferromagnetic semiconductor (Ga{sub 1−x},Fe{sub x})Sb (x = 3.9%–13.7%) has been grown by low-temperature molecular beam epitaxy (MBE) on GaAs(001) substrates. Reflection high energy electron diffraction patterns during the MBE growth and X-ray diffraction spectra indicate that (Ga,Fe)Sb layers have the zinc-blende crystal structure without any other crystallographic phase of precipitates. Magnetic circular dichroism (MCD) spectroscopy characterizations indicate that (Ga,Fe)Sb has the zinc-blende band structure with spin-splitting induced by s,p-d exchange interactions. The magnetic field dependence of the MCD intensity and anomalous Hall resistance of (Ga,Fe)Sb show clear hysteresis, demonstrating the presence of ferromagnetic order. The Curie temperature (T{sub C}) increases with increasing x and reaches 140 K at x = 13.7%. The crystal structure analyses, magneto-transport, and magneto-optical properties indicate that (Ga,Fe)Sb is an intrinsic ferromagnetic semiconductor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Monte Carlo Study of the Core-Shell Phase Transitions in Nanostructure Systems

We present in this work the effects of the external magnetic field, the exchange anisotropy, the coupling exchange interaction and the temperature on the magnetic properties of the nanostructure systems. More exactly, the effect of the core-shell spin Ising systems has been also examined. A number of favorable phenomena such as the wetting magnetic and the layering transitions have been investigated. The Monte Carlo simulations have been used to investigate the behavior of these nanostructure systems and to obtain the interesting results presented in the form of the phase diagrams.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetooptical study of Zeeman effect in Mn modulation-doped InAs/InGaAs/InAlAs quantum well structures

We report on a magneto-photoluminescence (PL) study of Zeeman effect in Mn modulation-doped InAs/InGaAs/InAlAs quantum wells (QW). Two PL lines corresponding to the radiative recombination of photoelectrons with free and bound-on-Mn holes have been observed. In the presence of a magnetic field applied in the Faraday geometry, both lines split into two circularly polarized components. While temperature and magnetic field dependence of the splitting are well described by the Brillouin function, providing an evidence for exchange interaction with spin polarized manganese ions, the value of the splitting exceeds by two orders of magnitude the value of the giant Zeeman splitting estimated for the average Mn density in QW obtained by the secondary ion mass spectroscopy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Screening properties of multiply connected ferromagnet–superconductor hybrid structures

Superconducting phase transition temperature T{sub c} of a ferromagnet/superconductor (SF) hybrid structure consisting of a hollow superconducting (S) cylinder (shell) with the central part (core) filled with a ferromagnetic (F) metal has been analyzed on the basis of linearized Usadel equations. It has been shown that the proximity effect between the S and F metals, as well as the exchange interaction, may induce an inhomogeneous superconducting state with Δ ~ exp(iLθ + ipz), which is characterized by nonzero circulation of phase L and wavenumber p describing the Larkin–Ovchinnikov–Fulde–Ferrell (LOFF) instability along the cylinder axis. The transitions between the states with different values of L and p, which are accompanied by a nonmonotonic dependence of superconducting transition temperature T{sub c} and effective magnetic field penetration depth Λ into the SF structure on the characteristic size of the ferromagnetic region, have been investigated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Inverse spin Hall effect induced by spin pumping into semiconducting ZnO

The inverse spin Hall effect (ISHE) of n-type semiconductor ZnO thin films with weak spin-orbit coupling has been observed by utilizing the spin pumping method. In the ferromagnetic resonance condition, the spin pumping driven by the dynamical exchange interaction of a permalloy film injects a pure spin current into the adjacent ZnO layer. This spin current gives rise to a DC voltage through the ISHE in the ZnO layer, and the DC voltage is proportional to the microwave excitation power. The effect is sizeable even when the spin backflow is considered.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Giant volume magnetostriction in the Y{sub 2}Fe{sub 17} single crystal at room temperature

An investigation of the Y{sub 2}Fe{sub 17} compound belonging to the class of intermetallic alloys of rare-earth and 3d-transition metals is presented. The magnetization, magnetostriction, and thermal expansion of the Y{sub 2}Fe{sub 17} single crystal were studied. The forced magnetostriction and magnetostriction constants were investigated in the temperature range of the magnetic ordering close to the room temperature. The giant field induced volume magnetostriction was discovered in the room temperature region in the magnetic field up to 1.2 T. The contributions of both anisotropic single-ion and isotropic pair exchange interactions to the volume magnetostriction and magnetostriction constants were determined. The experimental results were interpreted within the framework of the Standard Theory of Magnetostriction and the Landau thermodynamic theory. It was found out that the giant values of the volume magnetostriction were caused by the strong dependence of the 3d-electron Coulomb charge repulsion on the deformations and width of the 3d-electron energy band.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interplay between out-of-plane anisotropic L1{sub 1}-type CoPt and in-plane anisotropic NiFe layers in CoPt/NiFe exchange springs

Films of L1{sub 1}-type CoPt/NiFe exchange springs were grown with different NiFe (Permalloy) layer thickness (t{sub NiFe} = 0–10 nm). X-ray diffraction analysis reveals that the characteristic peak position of NiFe(111) is not affected by the CoPt-layer—confirming the absence of any inter-diffusion between the CoPt and NiFe layers. Magnetic studies indicate that the magnetization orientation of NiFe layer can be tuned through varying t{sub NiFe} and the perpendicular magnetic anisotropy of L1{sub 1}-type CoPt/NiFe films cannot sustain for t{sub NiFe} larger than 3.0 nm due to the existence of exchange interaction at the interface of L1{sub 1}-CoPt and NiFe layers. Magnetic force microscopy analysis on the as-grown samples shows the changes in morphology from maze-like domains with good contrast to hazy domains when t{sub NiFe} ≥ 3.0 nm. The three-dimensional micro-magnetic simulation results demonstrate that the magnetization orientation in NiFe layer is not uniform, which continuously increases from the interface to the top of NiFe layer. Furthermore, the tilt angle of the topmost NiFe layers can be changed over a very wide range from a small number to about 75° by varying t{sub NiFe} from 1 to 10 nm. It is worth noting that there is an abrupt change in the magnetization direction at the interface, for all the t{sub NiFe} investigated. The results of present study demonstrate that the tunable tilted exchange springs can be realized with L1{sub 1}-type CoPt/NiFe bilayers for future applications in three-axis magnetic sensors or advanced spintronic devices demanding inclined magnetic anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ab initio calculations on magnetism induced by composite defects in magnesium oxide

The local magnetic state induced by the composite defects, composed of an oxygen vacancy and a nitrogen substituting oxygen, in magnesium oxide has been studied by using ab initio calculation based on density functional theory. The calculated results show that local magnetic moment can be induced by the composite defects around the oxygen vacancy, when the exchange split of the oxygen vacancy is enhanced either by the hybridization between the N-p and nearest neighbor O-p orbitals or by applying on-site Coulomb repulsion (U) and exchange interaction (J). We show that the magnetic state induced by the composite defect is energetically more stable than the non-magnetic state. In addition, we show that the U and J applied on the p-orbitals of N and O atoms may significantly impact the calculated magnetic state of the composite defect, resulting in magnetic state for a configuration that is non-magnetic by generalized gradient approximation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Perpendicular magnetic tunnel junction with thin CoFeB/Ta/Co/Pd/Co reference layer

Integration of high density spin transfer torque magnetoresistance random access memory requires a thin stack (less than 15 nm) of perpendicular magnetic tunnel junction (p-MTJ). We propose an innovative approach to solve this challenging problem by reducing the thickness and/or moment of the reference layer. A thin reference layer structure of CoFeB/Ta/Co/Pd/Co has 60% magnetic moment of the conventional thick structure including [Co/Pd] multilayers. We demonstrate that the perpendicular magnetization of the CoFeB/Ta/Co/Pd/Co structure can be realized by anti-ferromagnetically coupling to a pinned layer with strong perpendicular anisotropy via Ruderman-Kittel-Kasuya-Yosida exchange interaction. The pMTJ with thin CoFeB/Ta/Co/Pd/Co reference layer has a comparable TMR ratio (near 80%) as that with thick reference layer after annealing at 280 °C. The pMTJ with thin reference layer has a total thickness less than 15 nm, thereby significantly increasing the etching margin required for integration of high density pMTJ array on wafers with form factor of 300 mm and beyond.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effect of NaOH Concentration and Adding Sequences on Structural and Magnetic Properties of Ni{sub 0.4}Co{sub 0.6}Fe{sub 2}O{sub 4} Nanopowders Prepared Via Co-precipitation Route

Ni{sub 0.4}Co{sub 0.6}Fe{sub 2}O{sub 4} nanopowders were prepared via the co-precipitation route followed by annealing treatment. The structural and magnetic properties of the as-synthesized samples were determined by XRD, FT-IR, TG-DSC, and PPMS measurements, respectively. The XRD patterns indicated a single-phase cubic spinel structure for all the Ni-Co ferrite samples, regardless of adding sequences of the reactants or NaOH concentration. The analysis of the XRD patterns revealed that the enhancement in lattice constant with increasing NaOH concentration is related to the prevention of oxidization of Co{sup 2+} ions in the Ni-Co ferrite lattice. The FT-IR spectra indicated that samples prepared in the B process have fewer impurities than those prepared in the A process. The enhancement in saturation magnetization with the increase in sodium hydroxide concentration could be attributed to the strengthening of super-exchange interaction between A and B sublattices, due to replacements of Co{sup 3+} ions (magnetic moment of 0 μ{sub B}) by Co{sup 2+} ions (magnetic moment of 3 μ{sub B}) at B sublattices. The obvious increase in the coercivity field with the increase in concentration of NaOH solutions can be interpreted in terms of enhancement of magneto-crystalline anisotropy that originated from gradual substitutions of Co{sup 3+} ions with Co{sup 2+} ions at the octahedral sites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Control of ferromagnetism in (In{sub 0.9}Fe{sub 0.1}){sub 2}O{sub 3} via F doping of electron carriers

Highlights: • F doping was achieved by a process of low temperature reaction with PVDF. • RTFM was obtained in the F-doped (In{sub 0.9}Fe{sub 0.1}){sub 2}O{sub 3.} • Magnetism and electric resistivity can be controlled by the content of doped F. • The FM can be ascribed to a long range exchange interaction induced by carriers. - Abstract: Ferromagnetism in (In{sub 0.9}Fe{sub 0.1}){sub 2}O{sub 3} was obtained by fluorine (F) doping. The ferromagnetism can be controlled by changing the electron carrier concentration via F doping. With increasing the F concentration, the electron carrier concentration increases, and samples undergo a paramagnetic insulator to ferromagnetic metal transition. For the ferromagnetic samples, the anomalous Hall effect (AHE) was observed. These results indicate that electron carriers play an important role in inducing the ferromagnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evidence of improved ferroelectric phase stabilization in Nd and Sc co-substituted BiFeO{sub 3}

Polycrystalline BiFeO{sub 3} and Bi{sub 0.9}Nd{sub 0.1}Fe{sub 1−x}Sc{sub x}O{sub 3} (x = 0, 0.05, and 0.10) multiferroic compounds were prepared using conventional solid-state route. X-ray diffraction studies and Raman measurements indicated that the compounds were crystallized in rhombohedral structure with R3c space group. Weak ferromagnetism was induced due to the suppression of canted spin structure in the substituted compounds. Both remanent magnetization (M{sub r}) and coercive field (H{sub c}) were enhanced in Nd and Sc substituted compounds. Further, Néel temperature T{sub N} was decreased from 644 K for BiFeO{sub 3} to 550 K for Bi{sub 0.9}Nd{sub 0.1}Fe{sub 0.9}Sc{sub 0.1}O{sub 3} compound due to weakening of magnetic exchange interactions between B-site cations in the substituted compounds. Enhanced and nearly well saturated electrical polarizations were observed in Sc substituted compounds, which are attributed to the strengthening of covalent hybridization between Bi and O ions and reduction in oxygen vacancies. The remnant polarization was enhanced to 12.5 μC/cm{sup 2} in Bi{sub 0.9}Nd{sub 0.1}Fe{sub 0.9}Sc{sub 0.1}O{sub 3} compound. Impedance studies revealed that insulating character of substituted compounds is enhanced and electrical relaxations are of non-Debye type.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Two-tone nonlinear electrostatic waves in the quantum electron–hole plasma of semiconductors

Longitudinal electrostatic waves in the quantum electron–hole plasma of semiconductors are considered taking into account the degeneracy of electrons and holes and the exchange interaction. It is found in the framework of linear theory that the dispersion curve of longitudinal waves has two branches: plasmon and acoustic. An expression for the critical cutoff frequency for plasma oscillations and an expression for the speed of sound for acoustic vibrations are derived. It is shown that the plasma wave always exists in the form of a superposition of two components, characterized by different periods and wavelengths. Two nonlinear solutions are obtained within nonlinear theory: one in the form of a simple superposition of two tones and the other in the form of beats.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Non-Heisenberg magnetism in a quaternary spin-gapless semiconductor

Understanding of spin-gapless semiconductors with fully spin-polarized charge carriers is critically important because of their promise for spintronic applications. Here, we report non-collinear spin structures, magnetic ground state, and effective exchange interactions of the spin-gapless semiconductor CoFeCrAl investigated with noncollinear density functional calculations. The ground state of CoFeCrAl is ferrimagnetic and has a spin configuration with ↓ Fe, ↑ Co and ↑ Cr spins. In our constrained calculations, the magnetizations of the Fe, Co, and Cr sublattices are rotated by various angles θ, which give rise to three sets of noncollinear spin structures. For all three elements, the magnetic energy increases with the angle, which reconfirms the ferrimagnetic spin structure. During rotation, the magnitudes of the Co and Cr spins remain almost unchanged, whereas that of Fe strongly decreases as a function of the angle θ. This indicates that the finite-temperature behavior of CoFeCrAl is characterized by a pronounced non-Heisenberg behavior of the ↓ Fe moments, whereas the ↑ Co and ↑ Cr moments are Heisenberg-like. Here, we discuss how this feature affects the finite-temperature behavior of the alloy beyond the commonly considered intersublattice Heisenberg exchange.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Mn concentration dependent structural and optical properties of a-plane Zn{sub 0.99-x}Mn{sub x}Na{sub 0.01}O

Non-polar a-plane (112{sup ¯}0) Zn{sub 0.99−x}Mn{sub x}Na{sub 0.01}O (x = 0, 0.02, 0.03, 0.04, and 0.05) thin films have been prepared on r-plane (112{sup ¯}0) sapphire substrates by pulsed laser deposition (PLD). The influences of Mn content on the structural and optical properties of the films have been studied. It is indicated that moderate Mn doping facilitates the non-polar growth of ZnO, and all the doped films are single phase with a hexagonal wurtzite structure. The transmittance spectra suggested that all films are transparent in the visible region exhibiting a transmittance above 80%. Mn{sup 2+} doped Zn{sub 0.99}Na{sub 0.01}O shows an initial decrease of optical band gap (OBG) for small concentration of Mn, followed by a monotonic increase. The anomalous decrease in OBG for low concentrations of Mn is attributed to the strong exchange interaction present between the sp electrons of the host matrix and d electrons of Mn. Emission studies were also performed showing suppressed blue-shifted ultraviolet band and dominant violet-blue bands, which might originate from the zinc vacancy (V{sub Zn}) and zinc interstitial (Zn{sub i}) defects. The intensity of defect-related emission peaks is Mn doping-level-dependent as well and the results coincide well with that from the structural analyses.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Electronic Structure of Above-Room-Temperature van der Waals Ferromagnet Fe 3 GaTe 2

Fe 3 GaTe 2 , a recently discovered van der Waals ferromagnet, demonstrates intrinsic ferromagnetism above room temperature, necessitating a comprehensive investigation of the microscopic origins of its high Curie temperature ($T$ C ). In this study, we reveal the electronic structure of Fe 3 GaTe 2 in its ferromagnetic ground state using angle-resolved photoemission spectroscopy and density functional theory calculations. Our results establish a consistent correspondence between the measured band structure and theoretical calculations, underscoring the significant contributions of the Heisenberg exchange interaction ($J$ ex ) and magnetic anisotropy energy to the development of the high-$T$ C ferromagnetic ordering in Fe 3 GaTe 2 . Intriguingly, we observe substantial modifications to these crucial driving factors through doping, which we attribute to alterations in multiple spin-splitting bands near the Fermi level. In conclusion, these findings provide valuable insights into the underlying electronic structure and its correlation with the emergence of high-$T$ C ferromagnetic ordering in Fe 3 GaTe 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Enhancement of Curie temperature in Mn{sub 2}RuSn by Co substitution

The Co-substituted Mn{sub 2}RuSn nanomaterials, namely, Mn{sub 2}Ru{sub 0.5}Co{sub 0.5}Sn and Mn{sub 2}Ru{sub 0.35}Co{sub 0.65}Sn have been synthesized and investigated. The presence of Co in the Mn{sub 2}RuSn (a = 6.21 Å) decreased the lattice parameter, where a = 6.14 Å and 6.12 Å for the as prepared Mn{sub 2}Ru{sub 0.5}Co{sub 0.5}Sn and Mn{sub 2}Ru{sub 0.35}Co{sub 0.65}Sn, respectively. The samples show a ferrimagnetic spin order with relatively small coercivities, similar to those of soft magnetic materials. There is a substantial increase in the Curie temperature (T{sub c} = 448 K for Mn{sub 2}Ru{sub 0.5}Co{sub 0.5}Sn and 506 K for Mn{sub 2}Ru{sub 0.35}Co{sub 0.65}Sn) of Mn{sub 2}RuSn (T{sub c} = 272.1 K) due to Co substitution, which is a result of strengthening of the positive exchange interaction in this material. These materials are highly stable against heat treatment of up to 450 °C. The first-principles calculations are consistent with our experimentally observed structural and magnetic properties. They also provide insight on how the magnetic and electronic structures change when Ru is replaced with Co in Mn{sub 2}RuSn.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Comparative X-Ray Absorption Analysis of the Spectrum of Vacant Electronic States in Cobalt and Nickel Tetraphenylporphyrin Complexes

The energy distributions and the properties of the lower vacant electronic states in cobalt and nickel tetraphenylporphyrin complexes CoTPP and NiTPP are studied by X-ray absorption spectroscopy. Quasimolecular analysis of the experimental absorption spectra measured in the region of the 2p and 1s ionization thresholds of complexing metal atoms, as well as the 1s thresholds of ligand atoms (nitrogen and carbon), is based on the comparison of the corresponding spectra with each other and with the spectra of the simplest nickel porphyrin NiP. It has been established that, despite a general similarity of the spectra of nitrogen and carbon in CoTPP and NiTPP, the fine structure of the 2p and 1s absorption spectra of cobalt and nickel atoms are radically different. The observed differences in the spectra of cobalt and nickel are associated with the features of the energy distribution of vacant 3d electron states. The presence in CoTPP of the partially filled valence 3db{sub 2g} molecular orbital (MO) results in the appearance in the cobalt spectra of a low-energy band, which is absent in the spectrum of nickel in NiTPP and leads to a doublet structure of transitions to b{sub 1g} and e{sub g} MOs due to the exchange interaction between 3d electrons in partially filled 3db{sub 2g} and 3db{sub 1g} or 3de{sub g} MOs. The spectrum of vacant states in CoTPP differs from that in NiTPP also due to the smaller energy distance between 3db{sub 1g} and e{sub g} MOs and the different positions of nonbonding MOs with the C2p character of the porphine ligand.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗