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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Quantum anomalous Hall insulator phase in asymmetrically functionalized germanene

Here, using first-principles computations, we discuss topological properties of germanene in buckled as well as planar honeycombs with asymmetric passivation via hydrogen and nitrogen (GeHN) atoms. GeHN in the planar structure is found to harbor a quantum anomalous Hall (QAH) insulator phase. Our analysis indicates that the buckled GeHN also possesses a QAH phase under tensile strain. We computed the associated Chern numbers and edge states to confirm the presence of the QAH state. In particular, chiral edge bands connecting conduction and valence bands were found at the edges of a planar zigzag GeHN nanoribbon. By considering a range of buckling distances, we demonstrate how the system undergoes the transition from the trivial to the QAH phase between the buckled and planar structures. Finally, we show CdTe(111) to be a suitable substrate for supporting buckled germanene in the QAH phase. Our results suggest that functionalized germanene could provide a robust QAH-based platform for spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Dirac-electron-mediated magnetic proximity effect in topological insulator/magnetic insulator heterostructures

The possible realization of dissipationless chiral edge current in a topological insulator/magnetic insulator heterostructure is based on the condition that the magnetic proximity exchange coupling at the interface is dominated by the Dirac surface states of the topological insulator. We report a polarized neutron reflectometry observation of Dirac-electron-mediated magnetic proximity effect in a bulk-insulating topological insulator (Bi 0.2 Sb 0.8 ) 2 Te 3 /magnetic insulator EuS heterostructure. We are able to maximize the proximity-induced magnetism by applying an electrical back gate to tune the Fermi level of topological insulator to be close to the Dirac point. A phenomenological model based on diamagnetic screening is developed to explain the suppressed proximity-induced magnetism at high carrier density. Our work paves the way to utilize the magnetic proximity effect at the topological insulator/magnetic insulator heterointerface for low-power spintronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Coexistence of type-II Dirac point and weak topological phase in Pt 3 Sn

Intriguing topological phases may appear in both insulating and semimetallic states. Topological insulators exhibit topologically nontrivial band inversion, while topological Dirac/Weyl semimetals show “relativistic” linear band crossings. Here, we report an unusual topological state of Pt 3 Sn, where the two topological features appear simultaneously. Based on first-principles calculations, we show that Pt 3 Sn is a three-dimensional weak topological semimetal with topologically nontrivial band inversion between the valence and conduction bands, where the band structure also possesses type-II Dirac points at the boundary of two electron pockets. The formation of the Dirac points can be understood in terms of the representations of relevant symmetry groups and the compatibility relations. The topological surface states appear in accordance with the nontrivial bulk band topology. As a result, the unique coexistence of the two distinct topological features in Pt 3 Sn enlarges the material scope in topological physics, and is potentially useful for spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Magnetic order induces symmetry breaking in the single-crystalline orthorhombic CuMnAs semimetal

Recently, orthorhombic CuMnAs has been proposed to be a magnetic material where topological fermions exist around the Fermi level. Here we report the magnetic structure of the orthorhombic Cu 0.95 MnAs and Cu 0.98 Mn 0.96 As single crystals. While Cu 0.95 MnAs is a commensurate antiferromagnet below 360 K with a propagation vector of k = 0,Cu 0.98 Mn 0.96 As undergoes a second-order paramagnetic to incommensurate antiferromagnetic phase transition at 320 K with k = (0.1,0,0), followed by a second-order incommensurate to commensurate antiferromagnetic phase transition at 230 K. In the commensurate antiferromagnetic state, the Mn spins order parallel to the crystallographic b axis but antiparallel to their nearest neighbors, with the spin orientation along the b axis. This magnetic order breaks S 2z , the two-fold rotational symmetry around the c axis, resulting in finite band gaps at the crossing point and the disappearance of the massless topological fermions. Furthermore, our first-principles calculations suggest that orthorhombic CuMnAs can still host spin-polarized surface states and signature induced by nontrivial topology, which makes it a promising candidate for antiferromagnetic spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anisotropic magnetocrystalline coupling of the skyrmion lattice in MnSi

In this paper, we investigate the anisotropic nature of magnetocrystalline coupling between the crystallographic and skyrmion crystal (SKX) lattices in the chiral magnet MnSi by magnetic field-angle resolved resonant ultrasound spectroscopy. Abrupt changes are observed in the elastic moduli and attenuation when the magnetic field is parallel to the [011] crystallographic direction. These observations are interpreted in a phenomenological Ginzburg-Landau theory that identifies switching of the SKX orientation to be the result of an anisotropic magnetocrystalline coupling potential. Finally, our paper sheds new light on the nature of magnetocrystalline coupling potential relevant to future spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Stabilization and control of Majorana bound states with elongated skyrmions

We show that elongated magnetic skyrmions can host Majorana bound states in a proximity-coupled two-dimensional electron gas sandwiched between a chiral magnet and an s-wave superconductor. Our proposal requires stable skyrmions with unit topological charge, which can be realized in a wide range of multilayer magnets, and it allows quantum information transfer by using standard methods in spintronics via skyrmion motion. Finally, we also show how braiding operations can be realized in our proposal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Topological phase transitions and quantum Hall effect in the graphene family

Monolayer staggered materials of the graphene family present intrinsic spin-orbit coupling and can be driven through several topological phase transitions using external circularly polarized lasers and static electric or magnetic fields. We show how topological features arising from photoinduced phase transitions and the magnetic-field-induced quantum Hall effect coexist in these materials and simultaneously impact their Hall conductivity through their corresponding charge Chern numbers. We also show that the spectral response of the longitudinal conductivity contains signatures of the various phase-transition boundaries, that the transverse conductivity encodes information about the topology of the band structure, and that both present resonant peaks which can be unequivocally associated with one of the four inequivalent Dirac cones present in these materials. As a result, this complex optoelectronic response can be probed with straightforward Faraday rotation experiments, allowing the study of the crossroads between quantum Hall physics, spintronics, and valleytronics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Chiral surface and edge plasmons in ferromagnetic conductors

The recently introduced concept of "surface Berry plasmons" is studied in the concrete instance of a ferromagnetic conductor in which the Berry curvature, generated by spin-orbit (SO) interaction, has opposite signs for carrier with spins parallel or antiparallel to the magnetization. By using collisionless hydrodynamic equations with appropriate boundary conditions, we study both the surface plasmons of a three-dimensional ferromagnetic conductor and the edge plasmons of a two-dimensional one. The anomalous velocity and the broken inversion symmetry at the surface or the edge of the conductor create a "handedness" whereby the plasmon frequency depends not only on the angle between the wave vector and the magnetization, but also on the direction of propagation along a given line. In particular, we find that the frequency of the edge plasmon depends on the direction of propagation along the edge. These Berry curvature effects are compared and contrasted with similar effects on plasmon dispersions induced by an external magnetic field in the absence of Berry curvature. Here, we argue that Berry curvature effects may be used to control the direction of propagation of the surface plasmons via coupling with the magnetization of ferromagnetic conductors, and thus create a link between plasmonics and spintronics.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Induced versus intrinsic magnetic moments in ultrafast magnetization dynamics

Ferromagnetic metal alloys are today commonly used in spintronic and magnetic data storage devices. These multicompound structures consist of several magnetic sublattices exhibiting both intrinsic and induced magnetic moments. Here, we study the response of the element-specific magnetization dynamics for thin film systems based on purely intrinsic (CoFeB) and partially induced (FePt) magnetic moments using extreme ultraviolet pulses from high-harmonic generation (HHG) as an element-sensitive probe. In FePt, on the one hand, we observe an identical normalized transient magnetization for Fe and Pt throughout both the ultrafast demagnetization and the subsequent remagnetization. On the other hand, Co and Fe show a clear difference in the asymptotic limit of the remagnetization process in CoFeB, which is supported by calculations for the temperature-dependent behavior of the equilibrium magnetization using a dynamic spin model. Furthermore, in this work, we provide a vital step toward a comprehensive understanding of ultrafast light-induced magnetization dynamics in ferromagnetic alloys with sublattices of intrinsic and induced magnetic moments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Emergent c -axis magnetic helix in manganite-nickelate superlattices

Here, the nature of the magnetic order in (La 2/3 Sr 1/3 MnO 3 ) 9 /(LaNiO 3 ) 3 superlattices is investigated using x-ray resonant magnetic reflectometry. We observe a new -axis magnetic helix state in the (LaNiO 3 ) 3 layers that had never been measured in nickelates, and which mediates the ~130° magnetic coupling between the ferromagnetic (La 2/3 Sr 1/3 MnO 3 ) 9 layers, illustrating the power of x-rays for discovering the magnetic state of complex oxide interfaces. Resonant inelastic x-ray scattering and x-ray absorption spectroscopy show that Ni-O ligand hole states from bulk LaNiO 3 are mostly filled due to interfacial electron transfer from Mn, driving the Ni orbitals closer to an atomic-like 3d 8 configuration. We discuss the constraints imposed by this electronic configuration to the microscopic origin of the observed magnetic structure. The presence of a magnetic helix in (La 2/3 Sr 1/3 MnO 3 ) 9 /(LaNiO 3 ) 3 is crucial for modeling the potential spintronic functionality of this system and may be important for designing emergent magnetism in novel devices in general.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Time-resolved study of nonlinear three-magnon processes in yttrium iron garnet films

In this work, the temporal aspects of two nonlinear spin-wave processes, i.e., three-magnon splitting and confluence, were investigated in a yttrium iron garnet film. Three-magnon splitting involves the conversion of a magnon at the microwave pumping frequency f p into two magnons, each with a frequency near f p /2, while confluence refers to the subsequent combination of two of the split magnons into one with frequency f c < f p . Time-resolved Brillouin scattering measurements confirm that these processes occur close to the driving antenna (<1.7mm away), which is expected. Here, the data indicate, however, that magnons with larger group velocities are more likely to undergo confluence, which is not predicted by existing theory. Understanding the details of the splitting and confluence processes may have important implications for the use of short-wavelength spin waves for spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effects of lattice geometry on the dynamic properties of dipolar-coupled magnetic nanodisk arrays

Here, we have studied the impact of lattice geometry on the dynamic properties of close-spaced arrays of circular nanomagnets, also known as magnonic crystals. To this end, we prepared 2D nanomagnet arrays with both square and hexagonal lattice symmetries (300-nm disk diameter, 400-nm center-to-center distance) and performed broadband ferromagnetic resonance (FMR) measurements. Micromagnetic simulations were used to interpret distinct features of the measured resonance spectra. The FMR bias field was applied along two distinct principal directions for each lattice, and a sample with well-separated, decoupled disks was measured for reference. We found that the interdisk dipolar coupling has a strong impact on the FMR for these 2D magnonic crystals. Distinctly different oscillation modes were found for the individual nanomagnets, dependent on lattice symmetry and direction of the bias field. Moreover, we find that spectral peak splitting from excitation of edge and center modes, as well as the damping, depends on the lattice symmetry and the orientation of the bias field. These findings demonstrate that lattice geometry has a strong influence on the excited spin-wave spectrum and is a relevant design parameter for spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Long-range magnetic order stabilized by acceptors

Tuning magnetic order in magnetic semiconductors is a long-sought goal. Here, we propose that a proper mix of acceptors with magnetic dopants may quench local magnetic order in favor of the long-range one. Using Mn and acceptor codoped LiZnAs as an example, we show, by first-principles calculations, the emergence of a long-range magnetic order. Furthermore, this intriguing observation may be understood based on a crossover between an acceptor-free magnetism and a band-coupling magnetism. Our findings pave the way for a precise control of magnetic order in future spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electrical and optical properties of iron in GaN, AlN, and InN

Iron is a common trace impurity in the group-III nitrides. Iron is also intentionally introduced in III-nitride electronic devices to create semi-insulating substrates and in the context of spintronics and quantum information applications. Despite the wide-ranging consequences of iron's presence in III-nitrides, the properties of iron impurities in the nitrides are not fully established. Here, we investigate the impact of iron impurities on the electrical and optical properties of GaN, AlN, and InN using first-principles calculations based on a hybrid functional. We report formation energies of substitutional and interstitial iron impurities as a function of the Fermi-level position. We also investigate complexes of Fe with substitutional oxygen on the nitrogen site, with nitrogen vacancies and with hydrogen interstitials. In GaN and AlN, iron on the cation site is amphoteric. We discuss the role of the Fe-induced acceptor level and its impact on nonradiative recombination in the context of loss mechanisms in light emitters, and current collapse in high-electron-mobility transistors. In InN, we find the iron interstitial to be the most favorable configuration, where it acts as a shallow double donor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields

Two-dimensional materials provide a unique platform to explore the full potential of magnetic proximity-driven phenomena, which can be further used for applications in next-generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene–ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. Additionally, we discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the nonlocal spin signals in graphene, which is due to spin relaxation by thermally induced transverse fluctuations of the FMI magnetization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Topological Phase Transitions in the Photonic Spin Hall Effect

The recent synthesis of two-dimensional staggered materials opens up burgeoning opportunities to study optical spin-orbit interactions in semiconducting Dirac-like systems. In this work, we unveil topological phase transitions in the photonic spin Hall effect in the graphene family materials. It is shown that an external static electric field and a high frequency circularly polarized laser allow for active on-demand manipulation of electromagnetic beam shifts. The spin Hall effect of light presents a rich dependence with radiation degrees of freedom, and material properties, and features nontrivial topological properties. Finally, we discover that photonic Hall shifts are sensitive to spin and valley properties of the charge carriers, providing an unprecedented pathway to investigate spintronics and valleytronics in staggered 2D semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Electron Accumulation and Emergent Magnetism in LaMnO 3 /SrTiO 3 Heterostructures

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here in this paper we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO$_{3}$/SrTiO$_{3}$ (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO$_{3}$. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the LaMnO$_{3}$, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO$_{3}$ films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗