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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Direct Observation of Magnetic Bubble Lattices and Magnetoelastic Effects in van der Waals Cr 2 Ge 2 Te 6

Abstract Ferromagnetic van der Waals (vdW) materials are of large current interest for the fundamental study of low‐dimensional magnetism and for potential applications in multilayer heterostructures. Cr 2 Ge 2 Te 6 (CGT) is particularly exciting because it is a ferromagnetic semiconductor with tunable electronic and magnetic properties. Controlling the magnetic domain structure of CGT is a requirement for understanding its novel interface physics and for tuning behavior for potential devices. Herein, cryo‐Lorentz transmission electron microscopy is performed in the temperature range of 12–50K to directly image the magnetic domain structures in CGT. A rich phase diagram of domain structures including stripe domains, magnetic bubble lattices of mixed‐chirality, and topologically‐protected lattices of homochiral magnetic bubbles is observed. The types and chiralities of the bubbles can be controlled by topographical changes in the CGT flakes. Additionally, it is observed that in‐plane strain and magnetoelastic coupling can align and organize both bubble lattices and stripe domains. This study provides insights into creating and controlling complex magnetic domain structures for integration into multilayer heterostructures and for future studies of 2D magnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2019

( LaCo O 3 ) n / ( SrCo O 2.5 ) n superlattices: Tunable ferromagnetic insulator

Ferromagnetic insulators have great potential for spintronic applications. For such applications, it is essential to find materials with a robust and controllable ferromagnetic insulating phase. Yet, because ferromagnetism in functional transition-metal oxides is usually coupled to metallicity, ferromagnetic insulators are very rare and independent control of their magnetic and electrical properties is difficult. In this study, the electrical, magnetic, and optical properties of (LaCoO 3 ) n /(SrCoO 2.5 ) n superlattice films are investigated for the manipulation of the ferromagnetic insulating phase. While the superlattices remain insulating irrespective of the periodicity n, the electronic structure and magnetic state vary drastically. Superlattices with large periodicities n of 10 and 20 show a ferromagnetic transition at a critical temperature T C of ~80K. With decreasing periodicity and increasing interface density of the superlattices, system with n =4 becomes almost nonmagnetic, while in systems with n =2 and 1, a reentrant ferromagnetic phase is observed at T C of ~180 and ~225K, respectively. Optical spectroscopy reveals that the fine control of the magnetic ground state is achieved by the modified electronic structure associated with the spin-state transition. Our findings indicate an important design principle to create and manipulate the ferromagnetic insulating properties of Co-based oxide thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Effect of Sb incorporation on structure and magnetic properties of quaternary ferromagnetic semiconductor (Ga, Mn)(As, Sb) thin films

GaAs-based quaternary ferromagnetic semiconductor (Ga, Mn)(As, Sb) has been successfully prepared by molecular-beam epitaxy. High-resolution x-ray diffraction measurements indicate that the lattice constant has a notable alteration with changing Sb content. Magnetic measurements demonstrate the same evolution of the Curie temperature T{sub C} and the effective Mn content x{sub eff} with increasing Sb content. The incorporation of low Sb content is of benefit to increasing x{sub eff}, thus, increasing T{sub C}. However, higher Sb content degrades the crystal quality, resulting in a decrease of x{sub eff}. Experimental results show that T{sub C} is proportional to the product of x{sub eff} and p{sup 1/3}, which is consistent with the Zener Model. The exchange energy N{sub 0}β is calculated to be −1.09 eV, which is similar to that of (Ga, Mn)As.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Domain engineering of the metastable domains in the 4f-uniaxial-ferromagnet CeRu 2 Ga 2 B

In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu 2 Ga 2 B. We demonstrate a feasible control both globally and locally of domain shapes and sizes by the external field as well as a smooth transition from single stripe to bubble domains, which opens the door to future applications based on magnetic domain tailoring.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Fast chirality reversal of the magnetic vortex by electric current

The possibility of high-density information encoding in magnetic materials by topologically stable inhomogeneous magnetization configurations such as domain walls, skyrmions, and vortices has motivated intense research into mechanisms enabling their control and detection. While the uniform magnetization states can be efficiently controlled by electric current using magnetic multilayer structures, this approach has proven much more difficult to implement for inhomogeneous states. Here, we report direct observation of fast reversal of magnetic vortex by electric current in a simple planar structure based on a bilayer of spin Hall material Pt with a single microscopic ferromagnetic disk contacted by asymmetric electrodes. The reversal is enabled by a combination of the chiral Oersted field and spin current generated by the nonuniform current distribution in Pt. Our results provide a route for the efficient control of inhomogeneous magnetization configurations by electric current.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Spin Texture Control and Magnetic Gap Engineering in a Ferromagnetic Insulator–Topological Insulator Sandwiched Heterostructure

Quantum materials that combine magnetism with topological order are emerging as key platforms for next-generation spintronics and low-energy electronics. They enable the realization of emergent quantum phenomena, such as the quantum anomalous Hall effect and axion insulator states. The ferromagnetic insulator (FMI)/topological insulator (TI)/FMI sandwich structure of a single-septuple layer (1SL) MnBi 2 Te 4 /four-quintuple layer (4QL) Bi 2 Te 3 /1SL MnBi 2 Te 4 holds great potential to achieve such desirable quantum phenomena at an elevated temperature, owing to its large Dirac point band gap and high Curie temperature. Here, for this work, spin- and angle-resolved photoemission spectroscopy (spin-ARPES) is employed to directly verify that the band gap arises from broken time-reversal symmetry via proximity-driven magnetization. This study demonstrates direct control of the spin state via external magnetic fields and unambiguously confirms the exchange interaction as the gap-opening mechanism. The robust magnetic gap and controllable spin texture make this heterostructure a suitable candidate for spintronic applications and magnetic topological quantum phases.

exchange gap↗
DOE OSTI2015

Ferromagnetic domain behavior and phase transition in bilayer manganites investigated at the nanoscale

Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here in this article, we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La 2-2x Sr 1+2x Mn 2 O 7 with x = 0:38, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curie temperature within the a - b plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. Lastly, the ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Charge transport and antiferromagnetic ordering in nitroxide radical crystals

Nonconjugated radical polymers and small molecules are employed as functional materials in organic electronic devices. Furthermore, the unpaired electrons on these materials have permanent magnetic moments, making these materials promising candidates for organic magnets. Through molecular design, strong antiferromagnetic and ferromagnetic ordering have been achieved in conjugated materials. However, the magnetic properties of nonconjugated radical polymers have only shown weak magnetic interactions among the open-shell sites due to the large mean separation between radicals in typical materials. Here, we have designed, synthesized, and crystalized two open-shell molecules that used molecular engineering to control the assembly of the open-shell sites into a strong antiferromagnetically ordered network. The strong antiferromagnetic interaction is evidenced by a high paramagnetic-to-antiferromagnetic transition temperature of ~40 K. This high transition temperature was a result of a high spin exchange coupling constant J of about –20 cm –1 , which was suggested by both experimental and computed coupling parameters given by the energy difference between high-spin and low-spin broken-symmetry structures. In addition, a single-crystal electrical conductivity of ~10 –3 S m –1 was achieved, which indicated the potential of this material in electronic applications. As a result, this work provides an insight into a design strategy for radical-based electronic and magnetic materials through proper molecular structure modifications.

36 MATERIALS SCIENCE↗
NASA NTRS1999

A magnetostatic-coupling based remote query sensor for environmental monitoring

A new type of in situ, remotely monitored magnetism-based sensor is presented that is comprised of an array of magnetically soft, magnetostatically-coupled ferromagnetic thin-film elements or particles combined with a chemically responsive material that swells or shrinks in response to the analyte of interest. As the chemically responsive material changes size the distance between the ferromagnetic elements changes, altering the inter-element magnetostatic coupling. This in turn changes the coercive force of the sensor, the amplitude of the voltage spikes detected in nearby pick-up coils upon magnetization reversal and the number of higher-order harmonics generated by the flux reversal. Since the sensor is monitored through changes in magnetic flux, no physical connections such as wires or cables are needed to obtain sensor information, nor is line of sight alignment required as with laser telemetry; the sensors can be detected from within sealed, opaque or thin metallic enclosures.

NASA Discipline Life Sciences Technologies↗
DOE OSTI2014

Electronic structures and ferromagnetism of SnO{sub 2} (rutile) doped with double-impurities: First-principles calculations

The electronic and magnetic properties of double-impurities-doped SnO{sub 2} (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed for double impurities (M1 and M2) from M1 = Cr, and M2 = Mn, and Re. The origins of ferromagnetism are shown to be different in the two cases. For Sn{sub 1-2x}Cr{sub x}Mn{sub x}O2, the hybridization between Cr-3d and O-2p results in Cr becoming ferromagnetic with a magnetic moment of about 5.0 μ{sub B} per supercell. The Cr-and Mn-doped SnO{sub 2} system exhibits half-metallic ferromagnetism. The strong ferromagnetic couplings between local magnetic moments can be attributed to p-d hybridization. In contrast, in (Cr, Re) codoped TiO{sub 2}, the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Re, which would lead to the ferromagnetic through the double-exchange mechanism in transition metal oxides. Since there are two possible couplings between the impurities, we studied both configurations (ferromagnetic and antiferromagnetic (AF)) for double-impurities-doped SnO{sub 2}. Our calculations show that a ferromagnetic alignment of the spins is energetically always more stable than simple AF arrangements, which makes these materials possible candidates for spin injection in spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

YbV 3 ⁢Sb 4 and EuV 3 ⁢Sb 4 vanadium-based kagome metals with Yb 2+ and Eu 2+ zigzag chains

Here, in this work, we present Yb⁢V 3 ⁢Sb 4 and Eu⁢V 3 ⁢Sb 4 , two compounds exhibiting slightly distorted vanadium-based kagome nets interleaved with zigzag chains of divalent Yb 2+ and Eu 2+ ions. Single crystal growth methods are reported alongside magnetic, electronic, and heat capacity measurements. Yb⁢V 3⁢ Sb 4 is a nonmagnetic metal with no collective phase transitions observed between 60 mK and 300 K. Conversely, Eu⁢V 3 ⁢Sb 4 is a magnetic kagome metal exhibiting easy-plane ferromagneticlike order below T C =32 K with hints of modulated spin texture under low field. With Yb⁢V 3 ⁢Sb 4 and Eu⁢V 3 ⁢Sb 4 we demonstrate another direction for the discovery and development of vanadium-based kagome metals while incorporating the chemical and magnetic degrees of freedom offered by a rare-earth sublattice.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Dynamic generation of spin-wave currents in hybrid structures

Spin transport through the interface in a semiconductor/ferromagnetic insulator hybrid structure is studied by the nonequilibrium statistical operator method under conditions of the spin Seebeck effect. The effective parameter approach in which each examined subsystem (conduction electrons, magnons, phonons) is characterized by its specific effective temperature is considered. The effect of the resonant (electric dipole) excitation of the spin electronic subsystem of conduction electrons on spin-wave current excitation in a ferromagnetic insulator is considered. The macroscopic equations describing the spin-wave current caused by both resonant excitation of the spin system of conduction electrons and the presence of a nonuniform temperature field in the ferromagnetic insulator are derived taking into account both the resonance-diffusion propagation of magnons and their relaxation processes. It is shown that spin-wave current excitation is also of resonant nature under the given conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Microwires fabricated by glass-coated melt spinning

The glass-coated melt spinning method offers a route for the manufacture of metal filaments with a few micrometers in diameter in a single operation directly from the melt. Cobalt-based amorphous wires, Cu-15.0 atomic percent (at. %) Sn shape-memory wires, and Ni{sub 2}MnGa (atomic percent) ferromagnetic wires were successfully produced by this method. The cobalt-based amorphous wire is flexible, and Cu-15.0 at. % Sn shape-memory wires have the tensile elongation of 14%. However, because of chemical reaction with glass and oxidation, it is hard to make Cu–Al–Ni shape-memory wires and Ni–Nb–Sn amorphous wires. Conditions for preparing these materials were summarized, and the differences of the solidification processes among glass-coated amorphous cobalt-based wires, Cu-15.0 at. % Sn shape-memory wires, and Ni{sub 2}MnGa wires were analyzed and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Effects of Ferromagnetic Disks on AC Losses in HTS Pancake Coils with Nonmagnetic and Magnetic Substrates

Performance improvement of high-temperature superconducting (HTS) pancake coil is of paramount importance for its application in electrical facilities. In this paper, the utilization of ferromagnetic disks including weak magnetic material and strong magnetic material is investigated through numerical simulations for the aim of reducing the AC loss. Both the nonmagnetic and magnetic substrates are considered for the rare-earth-barium-copper-oxide coated conductor tapes wounded into the pancake coil. In order to analyze the influences of ferromagnetic disks on the maximum allowable currents and AC losses of nonmagnetic substrate-based coil and magnetic substrate-based coil, a self-consistent model based on the A-formulation is adopted to calculate the critical current and a 2D axisymmetric model built on the H-formulations is established to calculate the AC loss. According to the simulation results, it is found that ferromagnetic disks especially the strong magnetic disk are slightly detrimental to the critical currents of the pancake coils. Nevertheless, the presence of large background field weakens the effects of ferromagnetic disks. The simulation results also indicate that strong magnetic disks achieve about 50% reduction of the AC loss in the nonmagnetic substrate-based coil, and the AC loss of the magnetic substrate-based coil can be increased or reduced with strong magnetic disks depending on the applied current amplitude. The utilization of strong magnetic disks leads to the enhancement of the AC loss in the magnetic substrate-based coil at lower current and to the reduction of it at larger current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Nanoscale Skyrmions in a Nonchiral Metallic Multiferroic: Ni 2 MnGa

Magnetic skyrmions belong to a set of topologically nontrivial spin textures at the nanoscale that have received increased attention due to their emergent behavior and novel potential spintronic applications. Discovering materials systems that can host skyrmions at room temperature in the absence of external magnetic field is of crucial importance not only from a fundamental aspect, but also from a technological point of view. So far, the observations of skyrmions in bulk metallic ferromagnets have been limited to low temperatures and to materials that exhibit strong chiral interactions. In this paper, we show the formation of nanoscale skyrmions in a nonchiral multiferroic material, which is ferromagnetic and ferroelastic, Ni 2 MnGa at room temperature without the presence of external magnetic fields. By using Lorentz transmission electron microscopy in combination with micromagnetic simulations, we elucidate their formation, behavior, and stability under applied magnetic fields at room temperature. Finally, the formation of skyrmions in a multiferroic material with no broken inversion symmetry presents new exciting opportunities for the exploration of the fundamental physics of topologically nontrivial spin textures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetic, Optoelectronic, and Thermodynamic Properties of Sr{sub 2}CrXO{sub 6} (X = La and Y): Half-Metallic and Ferromagnetic Behavior

The effects of spin polarization on the structure, magnetic, and optoelectronic properties of Cr-based series of double perovskites Sr{sub 2}CrXO{sub 6} (X = La and Y) have been studied by using the full-potential linearized augmented plane-wave method (FP-LAPW), based on the density functional theory (DFT) as implemented in the Wien2k code, within the generalized gradient approximation (GGA), GGA + U, and GGA plus Trans-Blaha-modified Becke–Johnson (TB-mBJ) as the exchange correlation. Our results show a similar half-metallic ferromagnetic ground state for both materials. From the electronic properties, it is found that Sr{sub 2}CrYO{sub 6} has a direct band gap at (Γ-Γ) direction and Sr{sub 2}CrLaO{sub 6} has an indirect band gap at (Γ-W) direction. Furthermore, we have computed the optic and thermodynamic properties which are investigated for the first time. Consequently, the magnetic, optoelectronic, and thermodynamic properties show these compounds are promising for high technological applications, namely spintronic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnons in one-dimensional k-component Fibonacci structures

We have studied the magnon transmission through of one-dimensional magnonic k-component Fibonacci structures, where k different materials are arranged in accordance with the following substitution rule: S{sub n}{sup (k)}=S{sub n−1}{sup (k)}S{sub n−k}{sup (k)} (n≥k=0,1,2,…), where S{sub n}{sup (k)} is the nth stage of the sequence. The calculations were carried out in exchange dominated regime within the framework of the Heisenberg model and taking into account the RPA approximation. We have considered multilayers composed of simple cubic spin-S Heisenberg ferromagnets, and, by using the powerful transfer-matrix method, the spin wave transmission is obtained. It is demonstrated that the transmission coefficient has a rich and interesting magnonic pass- and stop-bands structures, which depends on the frequency of magnons and the k values.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Transverse barrier formation by electrical triggering of a metal-to-insulator transition

Application of an electric stimulus to a material with a metal-insulator transition can trigger a large resistance change. Resistive switching from an insulating into a metallic phase, which typically occurs by the formation of a conducting filament parallel to the current flow, is a highly active research topic. Using the magneto-optical Kerr imaging, we found that the opposite type of resistive switching, from a metal into an insulator, occurs in a reciprocal characteristic spatial pattern: the formation of an insulating barrier perpendicular to the driving current. This barrier formation leads to an unusual N-type negative differential resistance in the current-voltage characteristics. We further demonstrate that electrically inducing a transverse barrier enables a unique approach to voltage-controlled magnetism. By triggering the metal-to-insulator resistive switching in a magnetic material, local on/off control of ferromagnetism is achieved using a global voltage bias applied to the whole device.

36 MATERIALS SCIENCE↗