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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Relationship between A-site cation and magnetic structure in 3d–5d–4f double perovskite iridates Ln 2 NiIrO 6 ( Ln = La, Pr, Nd)

We report a comprehensive investigation of Ln 2 NiIrO 6 (Ln=La, Pr, Nd) using thermodynamic and transport properties, neutron powder diffraction, resonant inelastic x-ray scattering, and density-functional theory (DFT) calculations to investigate the role of A-site cations on the magnetic interactions in this family of hybrid 3d–5d–4f compositions. Magnetic structure determination using neutron diffraction reveals antiferromagnetism for La 2 NiIrO 6 , a collinear ferrimagnetic Ni and Ir state that is driven to long-range antiferromagnetism upon the onset of Nd ordering in Nd 2 NiIrO 6 , and a noncollinear ferrimagnetic Ni and Ir sublattice interpenetrated by a ferromagnetic Pr lattice for Pr 2 NiIrO 6 . For Pr 2 NiIrO 6 , heat-capacity results reveal the presence of two independent magnetic sublattices, and transport resistivity indicates insulating behavior and a conduction pathway that is thermally mediated. Here, a first principles DFT calculation elucidates the existence of the two independent magnetic sublattices within Pr 2 NiIrO 6 and offers insight into the behavior in La 2 NiIrO 6 and Nd 2 NiIrO 6 . Resonant inelastic x-ray scattering is consistent with spin-orbit coupling splitting the t 2 g manifold of octahedral Ir 4+ into a J eff = 1/2 and J eff = 3/2 state for all members of the series considered.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Quantum Phase Engineering of Two-Dimensional Post-Transition Metals by Substrates: Toward a Room-Temperature Quantum Anomalous Hall Insulator

In this work, we propose a new strategy to engineer topological and magnetic properties of two-dimensional (2D) hexagonal lattices consisting of post-transition metals. Our first-principles calculations demonstrate that substrates serve as templates to form 2D lattices with high thermodynamic stability, where their topological properties as well as magnetic properties sensitively change as a function of lattice constants, i.e., the system undergoes a first-order phase transition from nonmagnetic to ferromagnetic state above a critical lattice constant. Consequently, substrates can be used to explore versatile magnetic, electronic, and quantum topological properties. We establish phase diagrams of versatile quantum phases in terms of exchange coupling and spin–orbit coupling effectively tuned by the lattice constants. We further reveal the first room-temperature quantum anomalous Hall (QAH) effect, i.e., Sn on 2√3 × 2√3 graphane is a QAH insulator with a large spin–orbit coupling gap of ~0.2 eV and a Curie temperature of ~380 K by using the 2D anisotropic Heisenberg model.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Ferroelectric incommensurate spin crystals

Ferroics, especially ferromagnets, can form complextopological spin structures such as vortices and skyrmions when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric-insulator superlattices such as PbTiO 3 /SrTiO 3 , which was later shown to be a model system owing to its high depolarizing field. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii-Moriya interaction (DMi) has not been experimentally observed. Here we examine a domain structure in a single PbTiO 3 epitaxial layer sandwiched between SrRuO 3 electrodes. In this work, we observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials. These findings further blur the border between emergent ferromagnetic and ferroelectrictopologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Electronic and Magnetic Properties of Oligomers and Chains of Poly(benzodifurandione) (PBDF), A Highly Conducting n-Type Polymer

The seminal development of highly electrically conducting polyacetylene via oxidative or reductive treatment (“doping”) has continuously inspired the search for other conducting π-conjugated polymers. Recently, poly(benzodifurandione), PBDF, was reported to have unexpected solubility given the absence of side chains and to exhibit an unprecedented, high electrical conductivity upon reduction (“n-doping”), with protons acting as counterions. Here, we theoretically investigate the electronic and magnetic properties of PBDF by taking long oligomers and one-dimensional (1D) periodic chains as model systems. With the oligomer models, we characterize the formation of polarons and bipolarons in n-doped PBDF. Our results indicate that singlet bipolarons tend to be the energetically most favorable species when protons bind to two closely located carbonyl groups in nearest-neighbor benzodifurandione moieties. The calculations on the 1D periodic chain models show that the positions of the protonated carbonyl groups determine the metallic, semiconducting, or insulating nature of a PBDF chain. When the protonated carbonyl groups are all situated on the same side of a PBDF chain, a stable helical chain configuration is found that exhibits ferromagnetic behavior. Our findings elucidate the mechanism of polaron and bipolaron formation in long oligomers of n-doped PBDF and highlight the fascinating electronic and magnetic properties of periodic 1D chains. Furthermore, these studies also provide a stepping stone for investigations of PBDF thin films, for which two- and three-dimensional structures must be considered.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Electrical Interrogation of Thickness‐Dependent Multiferroic Phase Transitions in the 2D Antiferromagnetic Semiconductor NiI 2

Abstract 2D magnetic materials hold promise for quantum and spintronic applications. 2D antiferromagnetic materials are of particular interest due to their relative insensitivity to external magnetic fields and higher switching speeds compared to 2D ferromagnets. However, their lack of macroscopic magnetization impedes detection and control of antiferromagnetic order, thus motivating magneto‐electrical measurements for these purposes. Additionally, many 2D magnetic materials are ambient‐reactive and electrically insulating or highly resistive below their magnetic ordering temperatures, which imposes severe constraints on electronic device fabrication and characterization. Herein, these issues are overcome via a fabrication protocol that achieves electrically conductive devices from the ambient‐reactive 2D antiferromagnetic semiconductor NiI 2 . The resulting gate‐tunable transistors show band‐like electronic transport below the antiferromagnetic and multiferroic transition temperatures of NiI 2 , revealing a Hall mobility of 15 cm 2 V −1 s −1 at 1.7 K. These devices also allow direct electrical probing of the thickness‐dependent multiferroic phase transition temperature of NiI 2 from 59 K (bulk) to 28 K (monolayer).

2D magnets↗
DOE OSTI2023

Axion optical induction of antiferromagnetic order

Using circularly polarized light to control quantum matter is a highly intriguing topic in physics, chemistry and biology. Previous studies have demonstrated helicity-dependent optical control of chirality and magnetization, with important implications in asymmetric synthesis in chemistry; homochirality in biomolecules; and ferromagnetic spintronics. We report the surprising observation of helicity-dependent optical control of fully compensated antiferromagnetic order in two-dimensional even-layered MnBi 2 Te 4 , a topological axion insulator with neither chirality nor magnetization. Furthermore, to understand this control, we study an antiferromagnetic circular dichroism, which appears only in reflection but is absent in transmission. We show that the optical control and circular dichroism both arise from the optical axion electrodynamics. Our axion induction provides the possibility to optically control a family of PT-symmetric antiferromagnets (P, inversion; T, time-reversal) such as Cr 2 O 3 , even-layered CrI3 and possibly the pseudo-gap state in cuprates. In MnBi 2 Te 4 , this further opens the door for optical writing of a dissipationless circuit formed by topological edge states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Time-Resolved Spectroscopy of Insulator-Metal Transitions: Exploring Low-Energy Dynamics in Strongly Correlated Systems. Final Report

Transition metal oxides (TMOs) have attracted much attention in recent years. The strong coupling between charge, spin, and orbital degrees of freedom creates novel functional properties. The research at the interplay between ferromagnetism, ferroelectricity and conductivity provides a promising way to the development of future spintronic devices. The interface characterization of multifunctional oxide heterostructures is crucial to understand deeply the magnetic response, spin injection and spin transport. In year one, we characterized optically the magnetic property with the method of magnetization-induced second-harmonic generation (MSHG), which is a sensitive technique to selectively probe the interface where the spatial inversion symmetry is broken. The interface of n-type BaTiO 3 (BTO)/La 0.67 Sr 0.33 MnO 3 (LSMO) forms a Schottky barrier and displays a good diode effect. By applying an external electric field across the sample, we observe the magnetoelectric (ME) coupling effect to modulate the interface magnetization. The voltage dependent magnetic contrast sharply vanishes at positive voltage (applied to LSMO contact) and reveals a ferromagnetic (FM)-to- antiferromagnetic (AFM) transition occurring at the interface, while the magneto-optic Kerr effect (MOKE) indicates that the bulk property is not affected. The strain mediated mechanism and ferroelectric (FE) polarization induced charge medicated mechanism are both discussed, but they do not play an important role in the observed magnetization transition. A new mechanism is proposed - the injected minority spins through strong Hund's interaction with the local magnetic moments at positive voltage weaken the double-exchange interaction of nearby Mn e g electrons, thereby reducing the ferromagnetic (FM) ordering. The dominant superexchange interaction of localized t 2g electrons finally leads to the AFM configuration. By reducing the electron carrier concentration of BTO, a positive voltage shift of the transition is observed. The oxygen richer sample with lower carrier concentration requires a higher voltage to increase the hole doping level of LSMO to induce the interfacial AFM phase. By inserting a thin (1nm) La 0.5 Ca 0.5 MnO 3 layer between BTO and LSMO, the interface magnetic response from LSMO is enhanced. In year two, a non-multiferroic heterostructure SrTiO 3 (STO)/ La 0.5 Ca 0.5 MnO 3 (LCMO)/La 0.67 Sr 0.33 MnO 3 (LSMO) was studied. In this system, the interface FM-AFM transition is observed at negative voltage. The STO layer may act as a hole-donating layer, which induces the hole transfer to LSMO making it overdoped, while the LCMO interlayer at the 0.5 hole doping level displays complicated CE type AFM phase. Thus the AFM phase is dominant at the interface and electron accumulation is required to induce the FM phase by applying a negative voltage. For oxygen rich sample, the magnetic MSHG loop reappears at high positive voltage (around +2.5 V). At high doping level (x > 0.5), LSMO exhibits A-type AFM order, consisting of AFM superexchange coupling between adjacent layers and intralayer FM double-exchange coupling. The less tensile strain of STO and LSMO due to a smaller lattice mismatch as compared to BTO would result in a weaker interlayer coupling . Thus, interfacial A-type AFM order can occur in STO/LCMO/LSMO heterostructure at high hole doping concentration. The study of ME coupling of multifunctional oxide heterostructures is definitely important to the development and performance of future spintronic devices. The results presented here regarding the interfacial magnetic transition controlled by an external electric field offer some new perspective to study the spin transport of magnetic tunnel junctions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures

Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in the heavy metal/ferromagnet bilayer systems. We observe the USMR in Pt/α-Fe 2 ⁢O 3 bilayers where the α-Fe 2 ⁢O 3 is an antiferromagnetic (AFM) insulator. Systematic field and temperature dependent measurements confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven by the imbalance of creation and annihilation of AFM magnons by spin orbit torque due to the thermal random field. However, unlike its ferromagnetic counterpart, theoretical modeling reveals that the USMR in Pt/α-Fe 2 ⁢O 3 is determined by the antiferromagtic magnon number with a non-monotonic field dependence. In conclusion, our findings extend the generality of the USMR which pave the ways for the highly sensitive detection of AFM spin state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Spectral weight reduction of two-dimensional electron gases at oxide surfaces across the ferroelectric transition

The discovery of a two-dimensional electron gas (2DEG) at the LaAlO 3 /SrTiO 3 interface has set a new platform for all-oxide electronics which could potentially exhibit the interplay among charge, spin, orbital, superconductivity, ferromagnetism and ferroelectricity. In this work, by using angle-resolved photoemission spectroscopy and conductivity measurement, we found the reduction of 2DEGs and the changes of the conductivity nature of some ferroelectric oxides including insulating Nb-lightly-substituted KTaO 3 , BaTiO 3 (BTO) and (Ca,Zr)-doped BTO across paraelectric-ferroelectric transition. We propose that these behaviours could be due to the increase of space-charge screening potential at the 2DEG/ferroelectric regions which is a result of the realignment of ferroelectric polarisation upon light irradiation. This finding suggests an opportunity for controlling the 2DEG at a bare oxide surface (instead of interfacial system) by using both light and ferroelectricity.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Giant spin Seebeck effect through an interface organic semiconductor

Interfacing an organic semiconductor C 60 with a non-magnetic metallic thin film (Cu or Pt) has created a novel heterostructure that is ferromagnetic at ambient temperature, while its interface with a magnetic metal (Fe or Co) can tune the anisotropic magnetic surface property of the material. Here, we demonstrate that sandwiching C60 in between a magnetic insulator (Y 3 Fe 5 O 12 :YIG) and a non-magnetic, strong spin–orbit metal (Pt) promotes highly efficient spin current transport via the thermally driven spin Seebeck effect (SSE). Experiments and first principles calculations consistently show that the presence of C 60 reduces significantly the conductivity mismatch between YIG and Pt and the surface perpendicular magnetic anisotropy of YIG, giving rise to enhanced spin mixing conductance across YIG/C 60 /Pt interfaces. As a result, a 600% increase in the SSE voltage (V LSSE ) has been realized in YIG/C 60 /Pt relative to YIG/Pt. Temperature-dependent SSE voltage measurements on YIG/C 60 /Pt with varying C 60 layer thicknesses also show an exponential increase in V LSSE at low temperatures below 200 K, resembling the temperature evolution of spin diffusion length of C 60 . Our report emphasizes the important roles of the magnetic anisotropy and the spin diffusion length of the intermediate layer in the SSE in YIG/C 60 /Pt structures, providing a new pathway for developing novel spin-caloric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Block orbital-selective Mott insulators: A spin excitation analysis

We present a comprehensive study of the spin excitations—as measured by the dynamical spin structure factor S(q,ω)—of the so-called block-magnetic state of low-dimensional orbital-selective Mott insulators. We realize this state via both a multi-orbital Hubbard model and a generalized Kondo-Heisenberg Hamiltonian. Due to various competing energy scales present in the models, the system develops periodic ferromagnetic islands of various shapes and sizes, which are antiferromagnetically coupled. The 2×2 particular case was already found experimentally in the ladder material BaFe2Se3 that becomes superconducting under pressure. Here we discuss the electronic density as well as Hubbard and Hund coupling dependence of S(q,ω) using density matrix renormalization group method. Several interesting features were identified: (1) An acoustic (dispersive spin-wave) mode develops. (2) The spin-wave bandwidth establishes a new energy scale that is strongly dependent on the size of the magnetic island and becomes abnormally small for large clusters. (3) Optical (dispersionless spin excitation) modes are present for all block states studied here. In addition, a variety of phenomenological spin Hamiltonians have been investigated but none matches entirely our results that were obtained primarily at intermediate Hubbard U strengths. Our comprehensive analysis provides theoretical guidance and motivation to crystal growers to search for appropriate candidate materials to realize the block states, and to neutron scattering experimentalists to confirm the exotic dynamical magnetic properties unveiled here, with a rich mixture of acoustic and optical features.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

First- and second-order magnetic anisotropy and damping of europium iron garnet under high strain

Understanding and tailoring static and dynamic properties of magnetic insulator thin films is important for spintronic device applications. Here, we grow atomically flat epitaxial europium iron garnet (EuIG) thin films by pulsed laser deposition on (111)-oriented garnet substrates with a range of lattice parameters. By controlling the lattice mismatch between EuIG and the substrates, we tune the strain in EuIG films from the compressive to the tensile regime, which is characterized by x-ray diffraction. Using ferromagnetic resonance (FMR), we find that, in addition to the first-order perpendicular magnetic anisotropy which depends linearly on the strain, there is a significant second-order one that has a quadratic strain dependence. Inhomogeneous linewidth of the FMR increases notably with increasing strain, while the Gilbert damping parameter remains nearly constant (≈ 2 × 10 –2 ). Finally, these results provide valuable insight into the spin dynamics in ferrimagnetic insulators and useful guidance for material synthesis and engineering of next-generation spintronics applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spin transport modified by magnetic order

Spintronics has developed over the years from initially generating spin-polarized charge currents from exchange interaction in ferromagnets, into a more general concept, where electronic spin current can be generated and manipulated by spin–orbit interactions and in addition magnonic spin currents can occur in magnetically ordered materials even if they are insulating. Thus the interplay between spin transport and magnetic order has been a very active research field, which has developed many new ideas, such as thermal spin transport and electronic spin currents with unusual symmetries generated by magnetically ordered materials. This article discusses a few selected examples in order to illustrate some of the complexities that can be encountered in these systems. A particular focus is on the contributions from Chia-Ling Chien and his research group, who often explored many of the topics at the forefront of spintronics research, while critically examining unproven assumptions made by other researchers in the field.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Triplet Resonating Valence Bond State and Superconductivity in Hund’s Metals

A central idea in strongly correlated systems is that doping a Mott insulator leads to a superconductor by transforming the resonating valence bonds (RVBs) into spin-singlet Cooper pairs. Here, we argue that a spin-triplet RVB (tRVB) state, driven by spatially, or orbitally anisotropic ferromagnetic interactions can provide the parent state for triplet superconductivity. We apply this idea to the iron-based superconductors, arguing that strong on site Hund’s interactions develop intra-atomic tRVBs between the $t_{2_g}$ orbitals. On doping, the presence of two iron atoms per unit cell allows these interorbital triplets to coherently delocalize onto the Fermi surface, forming a fully gapped triplet superconductor. In this work, this mechanism gives rise to a unique staggered structure of on site pair correlations, detectable as an alternating π phase shift in a scanning Josephson tunneling microscope.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Magnetic states of graphene proximitized Kitaev materials

Single-layer α-ruthenium trichloride (α–RuCl 3 ) has been proposed as a potential quantum spin liquid. Graphene/RuCl 3 heterobilayers have been extensively studied with a focus on the large interlayer electron transfer that dopes both materials. Here we examine the interplay between the competing magnetic state of RuCl 3 layer and graphene electronic properties. We perform self-consistent Hartree-Fock calculations on a Hubbard-Kanamori model of the 4d 5 t 2g electrons of α–RuCl 3 and confirm that out-of-plane ferromagnetic and zigzag antiferromagnetic states are energetically competitive. We show that the influence of hybridization between graphene and RuCl 3 bands is strongly sensitive to the magnetic configuration of RuCl 3 and the relative orientations of the two layers. We argue that strong hybridization leads to graphene magnetoresistance and that it may tilt the balance between closely competing magnetic states. Furthermore, Our analysis can be applied to any van der Waals heterobilayer system with weak interlayer hybridization and allows for arbitrary lattice constant mismatch and relative orientation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Nematic order driven by superconducting correlations

The interplay of nematicity and superconductivity has been observed in a wide variety of quantum materials. To explore this interplay, we consider a two-dimensional (2D) array of nematogens, local droplets with Z 3 nematicity, coupled to a network of Josephson junction wires. Using finite temperature classical Monte Carlo simulations, we elucidate the phase diagram of this model and show that the development of superconducting correlations and the directional delocalization of Cooper pairs can promote nematogen ordering, resulting in long-range nematic order. We obtain the transport properties of our model within an effective resistor network picture. We discuss these ideas in the context of the 2D electron gas at the (111) KTaO 3 interface and the doped topological insulators Nb x Bi 2 Se 3 and Cu x Bi 2 Se 3 . Overall, our work makes contact with Phil Anderson’s numerous contributions to broken symmetries driven by the saving of kinetic energy, including double exchange ferromagnetism and the interlayer tunneling theory of high Tc csuperconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Probing polarization-tunable sub-terahertz spin pumping in bimodal α-Fe 2 O 3 /Pt

Hematite (α-Fe 2 O 3 ) is an insulating antiferromagnet possessing a room temperature canted phase induced by the super-exchange Dzyaloshinskii-Moriya Interaction (DMI), resulting in enhanced spin pumping and spin transport over uniaxial antiferromagnets. In this work we resonantly excite the quasi-ferromagnetic (acoustic) and purely antiferromagnetic (optical) spin-dynamical modes in bulk and thin-film heterostructures of α-Fe 2 O 3 /Pt at room temperature, inducing inverse-spin-Hall-voltages up to 255 GHz. Following recent measurements of optical-mode spin-pumping in α-Fe 2 O 3 , we observe a novel and exotic microwave polarization mapping along with amplified spin-charge conversion in the optical branch and discuss the implications of layer thickness on acoustic and optical spin-modes. The results provide a deeper understanding of spin-wave selection rules for canted antiferromagnets and their potential use in nanoscale terahertz spintronics.

interfaces↗
DOE OSTI2021

Defect-engineered room-temperature ferromagnetism in quasi-two-dimensional nitrided CoTa 2 O 6

Thermal ammonolysis of quasi-two-dimensional (quasi-2D) CoTa 2 O 6 yields the O 2– /N 3– and anionic vacancy-ordered Co 2+ $Ta^{5+}_{2}$O 6–x N 2x/3 $\square$ x/3 (x $\leqslant$ 0.15), which exhibits a transition from antiferromagnetism to defect-engineered above-room-temperature ferromagnetism. First-principles calculations reveal that the origin of ferromagnetism is a particular CoO 5 N configuration with N located at Wyckoff position 8j, which breaks mirror symmetry about the ab plane. A pressure-induced electronic phase transition is also predicted at ~24.5 GPa, accompanied by insulator-to-metal transition and magnetic moment vanishing.

36 MATERIALS SCIENCE↗