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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Fabrication and Investigation of Indium Nitride Possessing Ferromagnetic Properties

An overview of the scientific literature since 2000 on InN doping with impurities giving it ferromagnetic properties and on the magnetic properties of InN is presented. According to theoretical and experimental studies, InN doped with transition metals and rare earth elements possesses ferromagnetic properties at temperatures above room temperature and is a material promising for spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Field-Dependent Magnetic Domain Behavior in van der Waals Fe 3 GeTe 2

Two-dimensional magnetic van der Waals (vdW) materials can show a variety of topological nontrivial spin textures, such as Bloch- or Néel-type stripe, skyrmion, or bubble domains under certain external stimuli. It is critical to understand the magnetic domain behavior in vdW materials in order to control their size and density in response to external stimuli, such as electric and magnetic fields. In this study, we examine the magnetic field dependence of topologically non-trivial magnetization spin textures in vdW Fe 3 GeTe 2 . Néel-type stripe domains and skyrmions are formed depending on the magnetic field-cooling protocol used during in situ Lorentz transmission electron microscopy (LTEM) experiments. Use of quantitative reconstruction of magnetic induction maps and micromagnetic simulations allow for the understanding of the LTEM results of Néel-type stripe domains as well as skyrmions. In addition, the deformation of skyrmion contrast is observed as a result of the introduction of an in-plane magnetic field. We demonstrate the stability of the stripe domains and skyrmions in response to an externally applied magnetic field due to an energy barrier for domain wall annihilation. Our results establish an understanding of the energy landscape that governs the behavior of the topologically non-trivial spin textures in vdW materials which can be harnessed for spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Recovery of edge states of graphene nanoislands on an iridium substrate by silicon intercalation

Finite-sized graphene sheets, such as graphene nanoislands (GNIs), are promising candidates for practical applications in graphene-based nanoelectronics. GNIs with well-defined zigzag edges are predicted to have spin-polarized edge-states similar to those of zigzag-edged graphene nanoribbons, which can achieve graphene spintronics. However, it has been reported that GNIs on metal substrates have no edge states because of interactions with the substrate. In this work, we used a combination of scanning tunneling microscopy, spectroscopy, and density functional theory calculations to demonstrate that the edge states of GNIs on an Ir substrate can be recovered by intercalating a layer of Si atoms between the GNIs and the substrate. We also found that the edge states gradually shift to the Fermi level with increasing island size. This work provides a method to investigate spin-polarized edge states in high-quality graphene nanostructures on a metal substrate.

density functional theory↗
DOE OSTI2021

Hidden symmetries, spin and charge of artificial magnetic monopoles

Here, we discuss the non-Abelian artificial magnetic monopoles associated with n-level energy crossing in quantum systems. We found that hidden symmetries reveal themselves as observables such as spin, hypercharge, and other physical degrees of freedom. We illustrated our results on concrete examples of two and three energy-level crossings. Our findings can be helpful for modeling various phenomena in physical systems such as SU(N) topological insulators and spintronic devices based on topological insulators.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Blowing polar skyrmion bubbles in oxide superlattices

Particle-like topological structures such as skyrmions and vortices have garnered ever-increasing interests due to their rich physical insights and potential broad applications in spintronics. Here, in this work, we discover the reversible switching between polar skyrmion bubbles and ordered vortex arrays in ferroelectric superlattices under an electric field, reminiscent of the Plateau-Raleigh instability in fluid mechanics. An electric field phase diagram is constructed, showing a wide stability window for the observed polar skyrmions. A “volcano”-like pontryagin density distribution is formed, indicating the formation of a smooth circular skyrmion. The topological charge Q at different applied field is calculated, verifying the field-driven topological transition between Q = 0 and Q = ±1 states. This study is a demonstration for the computational design of field-induced topological phase transitions, giving promise for the design of next-generation nanoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Differences in Sb 2 Te 3 growth by pulsed laser and sputter deposition

High quality van der Waals chalcogenides are important for phase change data storage, thermoelectrics, and spintronics. Using a combination of statistical design of experiments and density functional theory, we clarify how out-of-equilibrium van der Waals epitaxial deposition methods can improve the crystal quality of Sb 2 Te 3 films. We compare films grown by radio frequency sputtering and pulsed laser deposition (PLD). The growth factors that influence the crystal quality for each method are different. For PLD grown films a thin amorphous Sb 2 Te 3 seed layer most significantly influences the crystal quality. In contrast, the crystalline quality of films grown by sputtering is rather sensitive to the deposition temperature and less affected by the presence of a seed layer. This difference is somewhat surprising as both methods are out-of-thermal-equilibrium plasma-based methods. Non-adiabatic quantum molecular dynamics simulations show that this difference originates from the density of excited atoms in the plasma. The PLD plasma is more intense and with higher energy than that used in sputtering, and this increases the electronic temperature of the deposited atoms, which concomitantly increases the adatom diffusion lengths in PLD. In contrast, the adatom diffusivity is dominated by the thermal temperature for sputter grown films. These results explain the wide range of Sb 2 Te 3 and superlattice crystal qualities observed in the literature. Finally, these results indicate that, contrary to popular belief, plasma-based deposition methods are suitable for growing high quality crystalline chalcogenides.

36 MATERIALS SCIENCE↗
DOE OSTI2016

General solution of the Dirac equation for quasi-two-dimensional electrons

The general solution of the Dirac equation for quasi-two-dimensional electrons confined in an asymmetric quantum well, is found. The energy spectrum of such a system is exactly calculated using special unitary operator and is shown to depend on the electron spin polarization. This solution contains free parameters, whose variation continuously transforms one known particular solution into another. As an example, two different cases are considered in detail: electron in a deep and in a strongly asymmetric shallow quantum well. The effective mass renormalized by relativistic corrections and Bychkov–Rashba coefficients are analytically obtained for both cases. It is demonstrated that the general solution transforms to the particular solutions, found previously (Eremko et al., 2015) with the use of spin invariants. The general solution allows to establish conditions at which a specific (accompanied or non-accompanied by Rashba splitting) spin state can be realized. These results can prompt the ways to control the spin degree of freedom via the synthesis of spintronic heterostructures with the required properties.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Strain induced novel quantum magnetotransport properties of topological insulators

Recent theoretical and experimental researches have revealed that the strained bulk HgTe can be regarded as a three-dimensional topological insulator (TI). Motivated by this, we explore the strain effects on the transport properties of the HgTe surface states, which are modulated by a weak 1D in-plane electrostatic periodic potential in the presence of a perpendicular magnetic field. We analytically derive the zero frequency (dc) diffusion conductivity for the case of quasielastic scattering in the Kubo formalism, and find that, in strong magnetic field regime, the Shubnikov–de Haas oscillations are superimposed on top of the Weiss oscillations due to the electric modulation for null and finite strain. Furthermore, the strain is shown to remove the degeneracy in inversion symmetric Dirac cones on the top and bottom surfaces. This accordingly gives rise to the splitting and mixture of Landau levels, and the asymmetric spectrum of the dc conductivity. These phenomena, not known in a conventional 2D electron gas and even in a strainless TI and graphene, are a consequence of the anomalous spectrum of surface states in a fully stained TI. These results should be valuable for electronic and spintronic applications of TIs, and thus we fully expect to see them in the further experiment. - Highlights: • The strain removes the degeneracy in inversion symmetric Dirac cones. • The strain gives rise to the splitting and mixture of the Landau levels. • The strain leads to the asymmetric spectrum of the dc conductivity. • Shubnikov de Haas oscillations are shown to be superimposed on Weiss oscillations. • Interplay between strain and electric field causes different occupancy of TI states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Highly insulating ferromagnetic cobaltite heterostructures

Ferromagnetic insulators are rather rare but possess great technological potential in, for example, spintronics. Individual control of ferromagnetic properties and electronic transport provides a useful design concept of multifunctional oxide heterostructures. We studied the close correlation among the magnetism, atomic structure, and electronic structure of oxide heterostructures composed of the ferromagnetic perovskite LaCoO 3 and the antiferromagnetic brownmillerite SrCoO 2.5 epitaxial thin film layers. By reversing the stacking sequence of the two layers, we could individually modify the electric resistance and saturation magnetic moment. Lastly, the ferromagnetic insulating behavior in the heterostructures was understood in terms of the electronic reconstruction at the oxide surface/interfaces and crystalline quality of the constituent layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Dy uniform film morphologies on graphene studied with SPA-LEED and STM

The use of graphene for microelectronics and spintronic applications requires strategies for metals to wet graphene and to grow layer-by-layer. This is especially important when metals will be used as electrical contacts or as spin filters. Extensive work in the literature so far has shown that this is very challenging, since practically all metals grow 3D, with multi-height islands forming easily. Reasons for the 3D morphology are the much weaker metal carbon bond when compared to the metal cohesive energy and the role of Coulomb repulsion of the poorly screened charges at the metal graphene interface. We employed the complementary techniques of SPA-LEED and STM to study the growth of Dy on graphene. It was found that under kinetic limitations it is possible to fully cover graphene with a bilayer Dy film, by growing well below room temperature in stepwise deposition experiments. Lastly, the Dy film, however, is amorphous but ways to crystallize it within the 2D morphology are possible, since long range order improves at higher growth temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The structural modification and magnetism of many-layer epitaxial graphene implanted with low-energy light ions

Modifying the properties of graphene has gained wide interest for a plethora of potential applications, including spintronics. One approach has demonstrated that proton irradiation can induce ferromagnetism in graphene as well as in graphite. However, little is known about how the protons interact with graphene, the mechanism that creates the ferromagnetism, or whether the protons remain in the graphene. Here we report an investigation, broadly relevant to graphitic carbon, using low-energy (360–2000 eV) ions of hydrogen, deuterium, and helium implanted into multilayer epitaxial graphene. Complementary x-ray and neutron reflectivity demonstrate that essentially all of the implanted hydrogen remains chemisorbed in graphene. In situ x-ray diffraction reveals significantly different rates of interlayer expansion of the multilayer graphene. Analysis of these data demonstrates that the interlayer expansion arises entirely from the interstitials created by the ions and not from hydrogen that remains in the graphene. The results also establish a quantitative measure of the layer expansion due to carbon interstitials. Magnetometry and x-ray diffraction studies show that the magnetic moment relates to the amount of interstitial carbon rather than the amount of hydrogen, demonstrating that the induced room-temperature ferromagnetism arises directly from the disrupted bonding of the carbon lattice.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Thermodynamics and kinetics of Pb intercalation under graphene on SiC(0001)

SiC-supported graphene intercalated by a two-dimensional Pb monolayer can provide an appealing platform for spintronic applications. Such a monolayer structure is thermodynamically ultrastable, as observed in recent experiments. However, important fundamentals such as the structure of intercalated phases, locations of intercalated atoms, thermodynamic preference for intercalation, and intercalation pathways for this system have not yet been understood conclusively. Here, in this work, extensive density functional theory calculations are performed to assess Pb intercalation thermodynamics and kinetics under graphene on SiC(0001). We find that intercalation of isolated Pb atoms is strongly disfavored over adsorption on top of graphene. However, intercalation of complete Pb layers in the gallery between SiC and graphene buffer layer is strongly favored over supported Pb monolayers and moderately favored over formation of supported large three-dimensional Pb islands. We also find that initiation of intercalation either by individual Pb atoms directly penetrating graphene or by hopping under a static graphene step edge is energetically prohibitive at experimental temperatures. Consequently, more complex intercalation pathways are operative and further analyzed. We demonstrated that once an intercalated Pb monolayer forms around a graphene step edge, a facile Pb mass transport by Pb vacancy-mediated diffusion can be triggered for continued growth of the intercalated monolayer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Quantum anomalous Hall insulator phases in Fe-doped GaBi honeycomb

We discuss electronic and magnetic properties of the Fe-doped GaBi honeycomb using first principles calculations. Our analysis shows that the pristine GaBi honeycomb transitions from being a two-dimensional quantum spin Hall (QSH) insulator to a quantum anomalous Hall (QAH) insulator when it is doped with one Fe atom in a 4 × 4 GaBi honeycomb. Additionally, the QAH phase in Fe-doped GaBi is found to be robust in that it maintains its Chern number (C = 1) under fairly large strains (~ 4%) and supports a gap as large as 112 meV at 2.21% strain. The QAH phase is also retained when the Fe-doped GaBi is placed on a CdTe substrate, suggesting that Fe-doped GaBi films could be useful for spintronics applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Strain induced electronic structure, and magnetic and structural properties in quaternary Heusler alloys ZrRhTiZ (Z = Al, In)

In this work, electronic structure, and magnetic and structural properties of quaternary Heusler alloys ZrRhTiZ (Z = Al, In) have been analyzed from first principles calculations. The ferromagnetic ground state and thermodynamical stability of these compounds are supported by relative total energies, and by derived formation and cohesive energies, respectively. The investigated Curie temperatures of the compounds exceed room temperature indicating that these compounds are promising candidates for beyond room temperature spintronics and magneto-electronics applications. Both compounds follow the Slater-Pauling 18 electron rule and exhibit half-metallic ferromagnetic behavior with magnetic moments of 2 μB with 100% spin polarization in one spin channel at the Fermi-level. The effect of uniform strain on half-metallicity of these compounds inverts the band gap from one spin channel to another spin channel with a mixed spin (magnetic moment) regime that can be mapped to an exchange bias like effect observed in some of the magnetic Heusler compounds. The half metallic ferromagnetic behavior is retained within –2 to 2% strain contrasting with the anomalous spin flip in –1% and 1% strain in ZrRhTiZ (Z = Al, In).

36 MATERIALS SCIENCE↗
DOE OSTI2021

Unraveling peculiar magnetism and band topology in Mn 3 Sb

Magnetic, pseudogap, topological, magnetostructural, and elastic behaviors of Mn 3 Sb have been unraveled. The ferrimagnetism (FIM) is described by localized and delocalized electron magnetism resulting in different magnetic moments on Mn atoms, confirming the neutron diffraction data. The identified magnetostructural properties are due to the non-equivalent Mn atoms in its lowest symmetry structure. The electronic structure is also unique due to variable valance states of Mn atoms. The magnetic moment (4.10 μB) of non-equivalent Mn1 atom is antiparallely aligned with the magnetic moments (2.34 μB) of Mn 2 and Mn 3 atoms. The estimated Curie temperature, TC, is higher than the room temperature, which may have above the room temperature applications in spintronic devices. The band structure and density of states (DOS) show the characteristics of band topology (opening of a gap in Dirac-like band features) and pseudo-gap, respectively, around the Fermi level. While expanding the unit cell, the tetragonal FIM ground state transforms to the cubic primitive FIM phase, however, the contraction transforms it to the L1 2 ferromagnetic (FM) phase. The estimated elastic constants, bulk to shear modulus ratio, and elastic anisotropy factor indicate that Mn3Sb exhibits mechanically stable, ductile, and anisotropic behaviors, respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Interplay of pressure and temperature in modulating the magnetic properties of Cr 2 O 3

The spin-lattice interaction is pivotal in tailoring materials properties and is crucial for developing novel spintronic devices. Here, in the current work, we aim to explore the effects of pressure and temperature on the magnetic properties of Cr 2 O 3 to gain insights into its spin-lattice interaction. Through high-pressure neutron diffraction experiments, we observed an enhancement of the magnetic Bragg intensity with increasing pressure and it becomes more pronounced as the temperature increases. Results from first-principles calculations reveal a strengthening of the easy-axis magnetic anisotropy, doubling from 0 to 20 GPa. The exchange parameters, calculated based on this spin orientation, show an enhancement of the dominant magnetic interactions and an increase of magnetic ordering temperature when pressure increases. These findings suggest that the contributions from these two mechanisms are responsible for the observed increase in magnetic Bragg intensity.

Cr2O3↗
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 OSTI2020

Extend NdJ relationship with the size, multiple exchanges and Dzyaloshinskii-Moriya interaction for Néel skyrmions in hexagonal magnetic interfaces

For the integration with other parts of spintronic devices, it is desired to minimize the size of skyrmions to a nanometer scale. To provide a theoretical guidance for this endeavour, we established an extended NdJ relationship between the skyrmion size and fundamental magnetic properties of two-dimensional hexagonal ferromagnetic lattices, including multiple exchanges between different neighbors (J 1 -J 4 ), Dzyaloshinskii-Moriya interactions (d 1 and d 2 ) and on-site magnetic anisotropy energy. We demonstrated the applicability of this approach through density functional theory and spin spiral calculations for several interface, e.g., Co/Gra, Rh/Gra, Co/Pt 3 and MgO/Co/Pt 3 , and showed that the size of Néel skyrmions can be effectively reduced by either enhancing d or more conveniently reducing J through cancelation of J i s.

36 MATERIALS SCIENCE↗