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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 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Electronic, structural and magnetic properties of Mn (1+x) Pt (1-x) Sb

Electronic and magnetic properties of half-metallic Heusler alloys can be modified by tuning their chemical compositions. We have carried out a combined theoretical and experimental investigation of Mn (1+x) Pt (1-x) Sb (0 ≤ x ≤ 0.5) alloys. Our first-principles calculations indicate that the stoichiometric MnPtSb exhibits nearly half-metallic band structure, but a robust half-metallicity can be achieved in Mn-rich compositions Mn (1+x) Pt (1-x) Sb (0 ≤ x ≤ 0.5) with x = 0.25 and higher in their cubic structures. In addition, while MnPtSb exhibits ferromagnetic alignment, Mn (1+x) Pt (1-x) Sb are ferrimagnetic for all non-zero values of x. We have also synthesized cubic MnPtSb and Mn 1.25 Pt 0.75 Sb alloys using arc melting and annealing. The magnetic properties of these alloys are consistent with our theoretical predictions. Furthermore, these results indicate that the Mn-rich Mn (1+x) Pt (1-x) Sb alloys have potential for spin-transport-based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Self-induced spin-orbit torques in metallic ferromagnets

Herein we present a phenomenological theory of spin-orbit torques in a metallic ferromagnet with spin-relaxing boundaries. The model is rooted in the coupled diffusion of charge and spin in the bulk of the ferromagnet, where we account for the anomalous Hall effects as well as the anisotropic magnetoresistance in the corresponding constitutive relations for both charge and spin sectors. The diffusion equations are supplemented with suitable boundary conditions reflecting the spin-sink capacity of the environment. In inversion-asymmetric heterostructures, the uncompensated spin accumulation exerts a dissipative torque on the order parameter, giving rise to a current-dependent linewidth in the ferromagnetic resonance with a characteristic angular dependence. We compare our model to recent spin-torque ferromagnetic resonance measurements, illustrating how rich self-induced spin-torque phenomenology can arise even in simple magnetic structures.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Enhanced harmonic generation accompanying Ferromagnetic resonance in thin permalloy elliptical disks

Throughout this work, large amplitude magnetization dynamics in elliptical permalloy nano-disks are studied via simulation of the Landau-Lifshitz equation. When the external magnetic field is applied in the plane of the disk two low-lying resonant modes exist. The higher frequency mode corresponds to the Kittel-like uniform ferromagnetic resonance mode and is consistent with previous experimental observations. The second mode with the lower frequency, termed the "cap mode", is localized near the two ends of the ellipse. At certain fields the ratio of the frequencies of these modes is equal to two or three and here one can efficiently generate the Kittel mode as the 2nd or 3rd harmonic of the "cap mode" when the magnitude of the input microwave field is of order 0.1 Oe, which can easily be achieved in practice. The nonlinear responses depend strongly on the equilibrium, field-dep-endent, behavior of the magnetization which is non-uniform and displays features not captured by the Stoner-Wohlfarth model for an ellipsoid.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Formation of Mn-rich interfacial phases in Co 2 Fe x Mn 1-x Si thin films

Here, we report the formation of Mn-rich regions at the interface of Co 2 Fe x Mn 1-x Si thin films grown on GaAs substrates by molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) with electron energy loss (EEL) spectrum imaging reveals that each interfacial region: (1) is 1–2 nm wide, (2) occurs irrespective of the Fe/Mn composition ratio and in both Co-rich and Co-poor films, and (3) displaces both Co and Fe indiscriminately. We also observe a Mn-depleted region in each film directly above each Mn-rich interfacial layer, roughly 3 nm in width in the x = 0 and x = 0.3 films, and 1 nm in the x = 0.7 (less Mn) film. We posit that growth energetics favor Mn diffusion to the interface even when there is no significant Ga interdiffusion into the epitaxial film. Element-specific X-ray magnetic circular dichroism (XMCD) measurements show larger Co, Fe, and Mn orbital to spin magnetic moment ratios compared to bulk values across the Co 2 Fe x Mn 1-x Si compositional range. The values lie between reported values for pure bulk and nanostructured Co, Fe, and Mn materials, corroborating the non-uniform, layered nature of the material on the nanoscale. Finally, SQUID magnetometry demonstrates that the films deviate from the Slater-Pauling rule for uniform films of both the expected and the measured composition. The results inform a need for care and increased scrutiny when forming Mn-based magnetic thin films on III-V semiconductors like GaAs, particularly when films are on the order of 5 nm or when interface composition is critical to spin transport or other device applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoelectronics with proximitized materials

While materials design for many device applications usually relies on adding impurities, recent advances in scaling-down heterostructures with improved interfacial properties offer a different way to transform a large class of materials. A given material can be drastically changed by inheriting properties leaking from its neighboring regions, such as magnetism, superconductivity, or spin-orbit coupling. While these proximity effects often have a short range and are considered negligible, the situation is qualitatively different in atomically thin and two-dimensional materials where the extent of proximity effects can exceed their thickness. Consequently, proximitized materials have a potential to display novel properties and device opportunities, absent in any of the constituent region of the considered heterostructures. Such proximitized materials could provide platforms for a wide range of emerging applications: from seamless integration of memory and logic, to fault-tolerant topologically protected quantum computing.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Spin-Superfluidity and Spin-Current Mediated Non-Local Transport

Some strategies for reducing energy consumption in information processing devices involve the use of spin rather than charge to carry information. This idea is especially attractive when the spin current is a collective one carried by the condensate of a magnetically ordered state, rather than a quasiparticle current carried by electrons or magnons. In this Chapter we explain how easy-plane magnets can be viewed as Bose-Einstein condensates of magnons, defined in terms of quanta of the spin-component perpendicular to the easy plane, and how they can carry dissipationless spin-currents that induce non-local interactions between electrically isolated conducting channels. We comment specifically on important differences between superconductivity in normal/superconducting/normal circuits and spin-superfluidity in normal/magnetic/normal circuits.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Synthesis, Stability, and Magnetic Properties of Antiperovskite Co 3 PdN

Experimental synthesis and characterization of theoretically predicted compounds are important steps in the materials discovery pipeline. Here, we report on the synthesis of Co 3 PdN, which was recently predicted to be a stable magnetic antiperovskite. The Co 3 PdN thin films were grown by reactive sputtering and were confirmed to form in an antiperovskite crystal structure. The thermal stability of the compound is demonstrated up to 600 K by in situ X-ray diffraction, though the phase persists at slightly higher temperatures (700 K) in an air-free magnetometer. Both ab initio calculations and magnetization measurements find Co 3 PdN to be ferromagnetic with an experimentally determined Curie temperature of T C = 560 ± 5 K. The saturation magnetization of 1.2 μ B /Co found in the experiment is slightly lower than the 1.7 μ B /Co value expected by theory. A narrow magnetic hysteresis loop with a coercive field of 100 Oe at low temperature suggests that Co 3 PdN might be useful in electronic applications requiring fast switching of the magnetization vector. While prior prediction of Co 3 PdN showed a gapped electronic band structure for each spin channel, we show that this was due to incomplete sampling of Brillouin zone paths and that band crossings exist along R-X|M and X|M-R paths. The metallic nature of Co 3 PdN is further confirmed by temperature-dependent transport measurements, which also show a considerable anomalous Hall effect. Altogether, this work represents an appreciable step toward understanding the synthesis, structure, stability, and properties of a new magnetic material.

14 SOLAR ENERGY↗
DOE OSTI2022

Strategies for Computer-Aided Discovery of Novel Open-Shell Polymers

Organic π-conjugated polymers with a triplet ground state have been the focus of recent research for their interesting and unique electronic properties, arising from the presence of the two unpaired electrons. These compounds are usually built from alternating electron-donating and electron-accepting monomer pairs which lower the HOMO–LUMO gap and yield a triplet state instead of the typical singlet ground state. In this paper, we use density functional theory calculations to explore the design rules that govern the creation of a ground-state triplet conjugated polymer and find that a small HOMO–LUMO gap in the singlet state is the best predictor for the existence of a triplet ground state, compared to previous use of a pro-quinoidal bonding character. Furthermore, this work can accelerate the discovery of new stable triplet materials by reducing the computational resources needed for electronic-state calculations and the number of potential candidates for synthesis.

36 MATERIALS SCIENCE↗
DOE OSTI2016

New Class of 3D Topological Insulator in Double Perovskite

In this work, we predict a new class of 3D topological insulators (TIs) in which the spin–orbit coupling (SOC) can more effectively generate band gap. Band gap of conventional TI is mainly limited by two factors, the strength of SOC and, from electronic structure perspective, the band gap when SOC is absent. While the former is an atomic property, the latter can be minimized in a generic rock-salt lattice model in which a stable crossing of bands at the Fermi level along with band character inversion occurs in the absence of SOC. Thus large-gap TIs or TIs composed of lighter elements can be expected. In fact, we find by performing first-principles calculations that the model applies to a class of double perovskites A 2 BiXO 6 (A = Ca, Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, surface Dirac cones are robust against the presence of dangling bond at boundary.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Origin of Chirality Induced Spin Selectivity in Photoinduced Electron Transfer

Here we propose a mechanism by which spin-polarization can be generated dynamically in chiral molecular systems undergoing photoinduced electron transfer. The proposed mechanism explains how spin-polarization emerges in systems where charge transport is dominated by incoherent hopping, mediated by spin-orbit and electronic exchange couplings through an intermediate charge transfer state. We derive a simple expression for the spin-polarization that predicts a nonmonotonic temperature dependence, consistent with recent experiments, and a maximum spin-polarization that is independent of the magnitude of the spin-orbit coupling. We validate this theory using approximate quantum master equations and the numerically exact hierarchical equations of motion. The proposed mechanism of chirality induced spin selectivity should apply to many chiral systems, and the ideas presented here have implications for the study of spin transport at temperatures relevant to biology and provide simple principles for the molecular control of spins in fluctuating environments.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Ultrasensitive Sub-monolayer Palladium Induced Chirality Switching and Topological Evolution of Skyrmions

Chiral spin textures are fundamentally interesting, with promise for device applications. Stabilizing chirality is conventionally achieved by introducing Dzyaloshinskii-Moriya interaction (DMI) in asymmetric multilayers, where the thickness of each layer is at least a few monolayers. Here we report an ultrasensitive chirality switching in (Ni/Co) n multilayer induced by capping with only 0.22 monolayer of Pd. Using spin-polarized low-energy electron microscopy, we monitor the gradual evolution of domain walls from left-handed to right-handed Néel walls and quantify the DMI induced by the Pd capping layer. We also observe the chiral evolution of a skyrmion during the DMI switching, where no significant topological protection is found as the skyrmion winding number varies. This corresponds to a minimum energy cost of <1 attojoule during the skyrmion chirality switching. Here, our results demonstrate the detailed chirality evolution within skyrmions during the DMI sign switching, which is relevant to practical applications of skyrmionic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Low-Field Regime of Magnon Transport in PLD-Grown YIG Films

The diffusive propagation of magnons in the archetypal magnetic insulator yttrium iron garnet (YIG) is being actively explored for low-power and low-loss data communication. However, operation under external magnetic fields reduces the magnon diffusion length and attenuates the voltage amplitude at measurement terminals of magnonic devices. Here, we explore the low-field and field-free regime of diffusive magnon transport in YIG films, demonstrating that the field-induced suppression of magnon diffusion length can be fully inhibited only at the zero-field limit. Even a modest field of 10 mT attenuates the nonlocal spin voltage by ~20% in an ~1 μm long transport channel. We further identify the often overlooked in-plane uniaxial magnetic anisotropy as the dominant factor governing magnon transport in the low-field regime. Using Stoner-Wohlfarth macrospin simulations, we quantify the anisotropy parameters and reveal a 10-fold enhancement at low temperatures, a key finding for field-free operation of magnonic devices under cryogenic conditions.

36 MATERIALS SCIENCE↗