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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 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Robust ferromagnetism in Mn-doped MoS{sub 2} nanostructures

Layered transition metal dichalcogenides (TMDs) have attracted extensive attention due to their interesting properties originating from an effective honeycomb lattice and strong spin-orbit coupling, and have potential applications in catalysis, lithium batteries, photonic, electronic, and valleytronic devices. Introducing magnetism in the TMDs can lead to the interesting functionalities such as magnetic order and carrier spin polarization with potential applications in spintronics. Here, we demonstrate an effective approach to induce robust ferromagnetism in MoS{sub 2} nanostructures by transition metal doping. After doping with a few percent Mn{sup 2+}, the magnetism of MoS{sub 2} nanostructures is enhanced dramatically. Moreover, the magnetic properties are strongly temperature dependent, which is clearly different from the behavior of defect-induced magnetism. Our approach opens up the possibility for tuning the spin and magnetic properties in two-dimensional nanostructures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Magnetic properties of low-moment ferrimagnetic Heusler Cr 2 CoGa thin films grown by molecular beam epitaxy

Recently, theorists have predicted many materials with a low magnetic moment and large spin-polarization for spintronic applications. These compounds are predicted to form in the inverse Heusler structure; however, many of these compounds have been found to phase segregate. In this study, ordered Cr 2 CoGa thin films were synthesized without phase segregation using molecular beam epitaxy. The present as-grown films exhibit a low magnetic moment from antiferromagnetically coupled Cr and Co atoms as measured with superconducting quantum interface device magnetometry and soft X-ray magnetic circular dichroism. Electrical measurements demonstrated a thermally-activated semiconductor-like resistivity component with an activation energy of 87 meV. Finally, these results confirm spin gapless semiconducting behavior, which makes these thin films well positioned for future devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Perspective: Interface generation of spin-orbit torques

We present that most of the modern spintronics developments rely on the manipulation of magnetization states via electric currents, which started with the discovery of spin transfer torque effects 20 years ago. By now, it has been realized that spin-orbit coupling provides a particularly efficient pathway for generating spin torques from charge currents. At the same time, spin-orbit effects can be enhanced at interfaces, which opens up novel device concepts. Here, we discuss two examples of such interfacial spin-orbit torques, namely, systems with inherently two-dimensional materials and metallic bilayers with strong Rashba spin-orbit coupling at their interfaces. We show how ferromagnetic resonance excited by spin-orbit torques can provide information about the underlying mechanisms. In addition, this article provides a brief overview of recent developments with respect to interfacial spin-orbit torques and an outlook of still open questions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Interface magnetization transition via minority spin injection

The interface magnetization of n-type BaTiO 3 /La 0.7 Sr 0.3 MnO 3 heterojunction is selectively probed by magnetic second-harmonic generation at 80 K. Additionally, the injection of minority spins at the interface causes a sudden, reversible transition of the spin alignment of interfacial Mn ions from ferromagnetic to antiferromagnetic exchange coupled, while the bulk magnetization remains unchanged. We attribute the emergent interfacial antiferromagnetic interactions to weakening of the double-exchange mechanism caused by the strong Hund’s rule coupling between injected minority spins and local magnetic moments. Lastly, the effect is robust and may serve as a viable route for electronic and spintronic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Spin wave and spin flip in hexagonal LuMnO 3 single crystal

Manipulation and control of spin wave and spin flip are crucial for future developments of magnonic and spintronic devices. We present that the spin wave in hexagonal LuMnO 3 single crystal can be selectively excited with laser polarization perpendicular to the c -axis of hexagonal LuMnO 3 and photon energy ~1.8 eV. The selective excitation of spin wave also suggests that the spin flip can be selectively controlled in hexagonal manganites. Here, a microscopic model of the spin wave generation correlated with the four-spin-flip in hexagonal manganites is suggested to account for the line-shape of the observed spin wave.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Variable-temperature inelastic light scattering spectroscopy of nickel oxide: Disentangling phonons and magnons

We report the results of an investigation of the temperature dependence of the magnon and phonon frequencies in NiO. A combination of Brillouin-Mandelstam and Raman spectroscopies allowed us to elucidate the evolution of the phonon and magnon spectral signatures from the Brillouin zone center (GHz range) to the second-order peaks from the zone boundary (THz range). The temperature-dependent behavior of the magnon and phonon bands in the NiO spectrum indicates the presence of antiferromagnetic (AF) order fluctuation or a persistent AF state at temperatures substantially above the Néel temperature (TN=523 K). Tuning the intensity of the excitation laser provides a method for disentangling the features of magnons from acoustic phonons in AF materials without the application of a magnetic field. Our results are useful for the interpretation of the inelastic-light scattering spectrum of NiO and add to the knowledge of its magnon properties important for THz spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Magnetic domain configuration of (111)-oriented LaFeO 3 epitaxial thin films

In antiferromagnetic spintronics control of the domains and corresponding spin axis orientation is crucial for devices. Here we investigate the antiferromagnetic axis in (111)-oriented LaFeO 3 SrTiO 3 , which is coupled to structural twin domains. The structural domains have either the orthorhombic a- or b-axis along the in-plane <1$\bar{1}$0> cubic directions of the substrate, and the corresponding magnetic domains have the antiferromagnetic axis in the sample plane. Six degenerate antiferromagnetic axes are found corresponding to the <1$\bar{1}$0> and <11$\bar{2}$> in-plane directions. This is in contrast to the biaxial anisotropy in (001)-oriented films and reflects how crystal orientation can be used to control magnetic anisotropy in antiferromagnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Synthesis and superconductivity of In-doped SnTe nanostructures

In x Sn 1-x Te is a time-reversal invariant candidate 3D topological superconductor derived from doping the topological crystalline insulator SnTe with indium. The ability to synthesize low-dimensional nanostructures of indium-doped SnTe is key for realizing the promise they hold in future spintronic and quantum information processing applications. But hitherto only bulk synthesized crystals and nanoplates have been used to study the superconducting properties. Here for the first time we synthesize In x Sn 1-x Te nanostructures including nanowires and nanoribbons, which show superconducting transitions. In some of the lower dimensional morphologies, we observe signs of more than one superconducting transition and the absence of complete superconductivity. We propose that material inhomogeneity, such as indium inhomogeneity and possible impurities from the metal catalyst, is amplified in the transport characteristics of the smaller nanostructures and is responsible for this mixed behavior. Our work represents the first demonstration of In x Sn 1-x Te nanowires with the onset of superconductivity, and points to the need for improving the material quality for future applications

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Topological spin dynamics in cubic FeGe near room temperature

Understanding spin-wave dynamics in chiral magnets is a key step for the development of high-speed, spin-wave based spintronic devices that take advantage of chiral and topological spin textures for their operation. Here, we present an experimental and theoretical study of spin-wave dynamics in a cubic B20 FeGe single crystal. Using the combination of waveguide microwave absorption spectroscopy (MAS), micromagnetic simulations, and analytical theory, we identify the resonance dynamics in all magnetic phases (field polarized, conical, helical, and skyrmion phases). Finally, because the resonance frequencies of specific chiral spin textures are unique, a quantitative agreement between our theoretical predictions and experimental findings for all resonance frequencies and spin wave modes enables us to unambiguously identify chiral magnetic phases and to demonstrate that MAS is a powerful tool to efficiently extract a magnetic phase diagram.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Moving towards the magnetoelectric graphene transistor

Here, the interfacial charge transfer between mechanically exfoliated few-layer graphene and Cr 2 O 3 (0001) surfaces has been investigated. Electrostatic force microscopy and Kelvin probe force microscopy studies point to hole doping of few-layer graphene, with up to a 150 meV shift in the Fermi level, an aspect that is confirmed by Raman spectroscopy. Density functional theory calculations furthermore confirm the p-type nature of the graphene/chromia interface and suggest that the chromia is able to induce a significant carrier spin polarization in the graphene layer. A large magnetoelectrically controlled magneto-resistance can therefore be anticipated in transistor structures based on this system, a finding important for developing graphene-based spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tailoring Magnetic Skyrmions by Geometric Confinement of Magnetic Structures

Nanoscale magnetic skyrmions have interesting static and transport properties that make them candidates for future spintronic devices. Control and manipulation of the size and behavior of skyrmions is thus of crucial importance. Here, using a Ginzburg-Landau approach, we show theoretically that skyrmions and skyrmion lattices can be stabilized by a spatial modulation of the uniaxial magnetic anisotropy in a thin film of a centro-symmetric ferromagnet. Remarkably, the skyrmion size is determined by the ratio of the exchange length and the period of the spatial modulation of the anisotropy, at variance with conventional skyrmions stabilized by dipolar and Dzyaloshinskii–Moriya interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Spatial dependence of the super-exchange interactions for transition-metal trimers in graphene

This study examines the magnetic interactions between spatially variable manganese and chromium trimers substituted into a graphene superlattice. Using density functional theory, we calculate the electronic band structure and magnetic populations for the determination of the electronic and magnetic properties of the system. To explore the super-exchange coupling between the transition-metal atoms, we establish the magnetic ground states through a comparison of multiple magnetic and spatial configurations. Through an analysis of the electronic and magnetic properties, we conclude that the presence of transition-metal atoms can induce a distinct magnetic moment in the surrounding carbon atoms as well as produce a Ruderman-Kittel-Kasuya-Yosida-like super-exchange coupling. It is hoped that these simulations can lead to the realization of spintronic applications in graphene through electronic control of the magnetic clusters.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evolution of ferromagnetism in two-dimensional electron gas of LaTiO 3 /SrTiO 3

Understanding, creating, and manipulating spin polarization of two-dimensional electron gases at complex oxide interfaces present an experimental challenge. For example, despite almost a decade long research effort, the microscopic origin of ferromagnetism in LaAlO 3 /SrTiO 3 heterojunctions is still an open question. Here, by using a prototypical two-dimensional electron gas (2DEG) which emerges at the interface between band insulator SrTiO 3 and antiferromagnetic Mott insulator LaTiO 3 , the experiment reveals the evidence for magnetic phase separation in a hole-doped $\textbf{Ti}$ $d^1$ $\textbf{t}_{2_g}$ system, resulting in spin-polarized 2DEG. The details of electronic and magnetic properties of the 2DEG were investigated by temperature-dependent d.c. transport, angle-dependent X-ray photoemission spectroscopy, and temperature-dependent magnetoresistance. The observation of clear hysteresis in magnetotransport at low magnetic fields implies spin-polarization from magnetic islands in the hole rich LaTiO 3 near the interface. We present these findings to emphasize the role of magnetic instabilities in doped Mott insulators, thus providing another path for designing all-oxide structures relevant to spintronic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

High spin polarization in epitaxial Fe 4 N thin films using Cr and Ag as buffer layers

Fe 4 N thin films with (001) texture were prepared by reactive sputtering on MgO substrates, utilizing either a Cr or Ag buffer layer to facilitate the epitaxial growth. X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry measurements show that the Fe 4 N thin film grown on the Ag buffer layer is superior to that grown on the Cr buffer layer. The point contact Andreev reflection measurement was then conducted, and the spin polarizations were determined to be 61.1% and 81.3% for Fe 4 N thin films with Cr and Ag buffer layers, respectively. The 81.3% spin polarization is significantly higher than the ratio reported previously for Fe 4 N and is comparable with that of state-of-the-art Heusler alloys. This result is in agreement with the theoretical prediction on the discrepancy between the two differently defined spin polarizations for Fe 4 N. Moreover, our study indicates that an optimized growth process for Fe 4 N thin films is crucial for achieving a high spin polarization and that true half-metallicity could potentially be realized with Fe 4 N. Furthermore the high spin polarization of Fe 4 N combined with its low fabrication temperature and simple composition makes Fe 4 N a competitive candidate to be a half-metallic ferromagnet in spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Low magnetic damping and large negative anisotropic magnetoresistance in half-metallic Co 2-x Mn 1+x Si Heusler alloy films grown by molecular beam epitaxy

Co 2-x Mn 1+x Si films with various composition x were epitaxially grown using molecular beam epitaxy (MBE). High crystallinity and atomic ordering in the prepared Co 2-x Mn 1+x Si films were observed, and their magnetic damping and anisotropic magnetoresistance (AMR) effect were systematically investigated. An ultra-low magnetic damping constant of 0.0007 was obtained in the Co 2-x Mn 1+x Si film with a valence electron number (NV) of about 29.0. Additionally, a relatively large negative AMR effect was observed in the Co 2-x Mn 1+x Si films that had a N V of about 29.0. This low damping and the large negative AMR effect indicate that epitaxial Co 2-x Mn 1+x Si films with high atomic ordering grown by MBE possess a high-spin polarization. Half-metals have potential for enhancing the performance of various kinds of spintronic devices, such as magnetic sensors and magnetic random access memory (MRAM) devices, because the conduction electrons are completely spin-polarized due to the Fermi level (EF) being in the energy gap of one of the spin channels. Among the half-metals, Co-based Heusler alloys are expected to show half-metallic properties at room temperature due to their high Curie temperature. In the past decade, high tunnel magnetoresistance (TMR) and current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) have been experimentally demonstrated using Co-based Heusler alloy electrodes. Furthermore, further improvement of their half-metallic properties is required to obtain improved performance in magnetic tunnel junctions (MTJs) and CPP-GMR devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Temperature dependence of the Dzyaloshinskii-Moriya interaction in Pt/Co/Cu thin film heterostructures

Magnetic materials that exhibit chiral domain walls are of great interest for spintronic devices. In this work, we examine the temperature-dependent behavior of the Dzyaloshinskii-Moriya interaction (DMI) in Pt/Co/Cu thin film heterostructures. We extract the DMI strength, D , from static domain spacing analysis between 300 K and 500 K and compare its temperature dependence to that of the magnetic anisotropy, K u , and saturation magnetization, M s . Consistent with expected scaling in thin films, M s exhibits Bloch-law temperature scaling and K u scales as M s 2.1±0.1 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thickness and angular dependent ferromagnetic resonance of ultra-low damping Co 25 Fe 75 epitaxial films

Metallic ferromagnets with ultra-low damping are highly desirable for charge-based spintronic applications. In this work, we systematically investigate the magnetic dynamics of Co 25 Fe 75 epitaxial films with a Gilbert damping constant as low as 7.1×10 -4 . The in-plane angular dependence of ferromagnetic resonance (FMR) was measured on various thicknesses Co 25 Fe 75 films grown on MgO and MgAl 2 O 4 , from which the mechanisms for FMR linewidth broadening can be distinguished and quantified. The thickness dependencies of the magnetic anisotropy and inhomogeneous broadening of the linewidth are good indicators of crystal quality and magnet uniformity. Additionally, it is shown that anisotropic two-magnon scattering is induced by defects at the surfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

THz-frequency magnon-phonon-polaritons in the collective strong-coupling regime

Strong coupling between light and matter occurs when the two interact such that new hybrid modes, the so-called polaritons, are formed. Here, we report on the strong coupling of both the electric and the magnetic degrees of freedom to an ultrafast terahertz (THz) frequency electromagnetic wave. In our system, optical phonons in a slab of ferroelectric lithium niobate are strongly coupled to a THz electric field to form phonon-polaritons, which are simultaneously strongly coupled to magnons in an adjacent slab of canted antiferromagnetic erbium orthoferrite via the magnetic-field component of the same THz pulse. We juxtapose experimental results of bare slabs consisting of the two materials with a photonic crystal cavity, consisting of a two-dimensional array of air holes cut into the hybrid slab. In both cases, the strong coupling leads to the formation of new magnon-phonon-polariton modes, which we experimentally observe in the time domain as a normal-mode beating and which corresponds in the frequency domain to an avoided crossing. Finally, our simple yet versatile waveguide platform provides a promising avenue through which to explore ultrafast THz spintronics, quantum electrodynamics, sensing, and spectroscopic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗