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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 1,675 records · Page 93
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Spin-orbit coupling controlled ground states in the double perovskite iridates $\textit{A}_2\textit{B}$IrO$_6$ ($\textit{A}$ = Ba, Sr; $\textit{B}$ = Lu, Sc)

Iridates with the 5$d^4$ electronic configuration have attracted recent interest due to reports of magnetically ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $\textit{J}$ = 0 electronic ground state for each Ir 5+ ion. The major focus of prior research has been on the double perovskite iridates Ba 2 YIrO 6 and Sr 2 YIrO 6 , where the nature of the ground states (i.e., ordered vs nonmagnetic) is still controversial. Here, in this work, we present neutron powder diffraction, high-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates Ba 2 LuIrO 6 , Sr 2 LuIrO 6 , Ba 2 ScIrO 6 , and Sr 2 ScIrO 6 that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $\textit{J}$ = 0 electronic ground states for the Ir 5+ ions in all cases, with similar values for Hund's coupling $J_H$ and the spin-orbit coupling constant $λ_{\text{SOC}}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do exhibit weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large $λ_{\text{SOC}}$ is the key driving force behind the electronic and magnetic ground states realized in the $5d^4$ double perovskite iridates, which agrees well with conventional wisdom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Magnetism in EuAlSi and the Eu 1−𝑥 ⁢Sr 𝑥 ⁢ AlSi solid solution solid solution

The magnetic properties of EuAlSi, a compound comprising a honeycomb lattice of Al/Si atoms and a triangular lattice of Eu atoms, are presented. By means of single-crystal x-ray diffraction, we find that EuAlSi crystallizes in an AlB 2 -type structure with space group 𝑃⁢6/mmm and unit cell parameters 𝑎 = 4.2229⁢ (10) ⁢Å and 𝑐 = 4.5268 ⁢(12)⁢ Å. Our magnetic measurements indicate that EuAlSi is a soft ferromagnetic material with 𝑇 Curie = 25.8 K. The susceptibility follows the Curie-Weiss law at high temperatures, which allowed us to determine the paramagnetic Curie temperature 𝜃 𝑃 = 36.2 ⁢(1) ⁢K and an effective magnetic moment 𝜇 eff = 8.07 ⁢(1)⁢ µ 𝐵 /Eu. This value is in agreement with the theoretical value of 7.9 µ 𝐵 for Eu 2+ free ion. Moreover, we have prepared the Eu 1−𝑥 ⁢Sr 𝑥 ⁢ AlSi solid solution, where the atoms in the triangular lattice were systematically exchanged, in order to study the evolution of the collective quantum properties from the ferromagnetic EuAlSi toward the superconducting SrAlSi. Across the Eu 1−𝑥 ⁢Sr 𝑥 AlSi solid solution, the unit cell parameters change linearly, following Vegard’s law, and making the system reliable for studying composition dependence of the interplay between the crystal structure and physical properties. As the Sr content increases, i.e., 𝑥 increases, we note a consistent reduction of 𝜇 eff and 𝑇 Curie . Long-range magnetic order in Eu 1−𝑥 ⁢Sr 𝑥 ⁢ AlSi persists up to 𝑥 = 0.95, whereas superconductivity is only observed for samples with 𝑥 > 0.97.

Walicka, Dorota I. [University of Geneva (Switzerl↗
DOE OSTI2018

Structural, electronic, magnetic, and magnetocaloric properties in metallic antiperovskite compound Mn3GaC

Highlights: • We have studied the magnetocaloric effect of the metallic antiperovskite compound Mn{sub 3}GaC. • We used the ab-initio calculations, the Monte Carlo simulations and mean field theory. • A second-order ferromagnetic-paramagnetic phase transition about T{sub C} ∼ 249 K. • The magnetic moment and the exchange coupling interactions are calculated. - Abstract: The structural, electronic, magnetic, and magnetocaloric properties of the metallic antiperovskite compound Mn{sub 3}GaC were investigated using several theoretical methods such as: First principle calculations, Monte Carlo simulations and mean field theory. The metallic antiperovskite compound Mn{sub 3}GaC exhibits a second-order ferromagnetic-paramagnetic phase transition around T{sub C} = 249 K. Using the first principle calculations, the magnetic moment and the exchange coupling interactions values are 1.37 μ{sub B} and J{sub 1} = 35.78meV, J{sub 2} = 40.16meV, respectively. The total magnetization, the susceptibility and the specific heat of this compound are calculated. The critical temperature obtained is in good agreement with the experimental results. Obviously, the large MCE with no hysteresis loss is obtained around T{sub C}. The maximum values of the magnetic entropy change (ΔS{sub mag}), adiabatic temperature change (ΔT{sub ad}) and the relative cooling power (RCP) are 13.41 J/kg.K, 15.96 K, 748 J/kg respectively, under applied an external magnetic field of h = 5.0 T.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Magnetoelectric properties at the Co/AlN(0001) interface

AlN nitride is a large-band-gap polar material that has gained interest due to its ability to become ferroelectric and its compatibility with Si- and GaN-based technologies. This compound could, therefore, be a good candidate to design new artificial multiferroics if combined with a ferromagnetic electrode. For this work, we performed first-principles calculations to investigate the set up of the magnetoelectric coupling at the Co/AlN(0001) interface. Our results describe a complex interfacial atomic structure with variable local magnetic properties as a function of the atom alignment. We predict an average variation of interface spin magnetization, when the polarization is reversed, of 3.83 𝜇B nm −2 , close to the values already reported in the literature at metal/oxide interfaces. This confirms the potential of this AlN-based ferroelectric compound to be used in future voltage-controlled spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Transition-Metal-Related Quantum Emitters in Wurtzite AlN and GaN

Transition-metal centers exhibit a paramagnetic ground state in wide-bandgap semiconductors and are promising for nanophotonics and quantum information processing. Specifically, there is a growing interest in discovering prominent paramagnetic spin defects that can be manipulated using optical methods. Here, we investigate the electronic structure and magneto-optical properties of Cr and Mn substitutional centers in wurtzite AlN and GaN. We use state-of-the-art hybrid density functional theory calculations to determine level structure, stability, optical signatures, and magnetic properties of these centers. The excitation energies are calculated using the constrained occupation approach and rigorously verified with the complete active space configuration interaction approach. Our simulations of the photoluminescence spectra indicate that $Cr$$^{1+}_{Al}$ in AlN and $Cr$$^{1+}_{Ga}$ in GaN are responsible for the observed narrow quantum emission near 1.2 eV. We compute the zero-field splitting (ZFS) parameters and outline an optical spin polarization protocol for $Cr$$^{1+}_{Al}$ and $Cr$$^{1+}_{Ga}$. Our results demonstrate that these centers are promising candidates for spin qubits.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Interaction modifiers in artificial spin ices

The modification of geometry and interactions in two-dimensional magnetic nanosystems has enabled a range of studies addressing the magnetic order, collective low-energy dynamics and emergent magnetic properties in, for example, artificial spin-ice structures. The common denominator of all these investigations is the use of Ising-like mesospins as building blocks, in the form of elongated magnetic islands. Here, we introduce a new approach: single interaction modifiers, using slave mesospins in the form of discs, within which the mesospin is free to rotate in the disc plane1. We show that by placing these on the vertices of square artificial spin-ice arrays and varying their diameter, it is possible to tailor the strength and the ratio of the interaction energies. We demonstrate the existence of degenerate ice-rule-obeying states in square artificial spin-ice structures, enabling the exploration of thermal dynamics in a spin-liquid manifold. Furthermore, we even observe the emergence of flux lattices on larger length scales, when the energy landscape of the vertices is reversed. In conclusion, the work highlights the potential of a design strategy for two-dimensional magnetic nano-architectures, through which mixed dimensionality of mesospins can be used to promote thermally emergent mesoscale magnetic states.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Strain-tunable magnetism at oxide domain walls

Applying stress to a ferroelastic material results in a nonlinear strain response as domains of different orientations mechanically switch. The ability to write, erase and move domain walls between such ferroelastic domains suggests a method for making nanoelectronics where the domain wall is the device. However, little is known about the magnetic properties of such domain walls. A fascinating model system is SrTiO 3 , where the ferroelastic domain walls display strain-tunable polarity and enhanced conductivity. Here, we reveal a long-range magnetic order with modulations along the ferroelastic domain walls in SrTiO 3 and SrTiO 3 -based heterointerfaces, which manifests itself as a striped pattern in scanning superconducting quantum interference device maps of the magnetic landscape. In conducting interfaces, the magnetism is coupled to itinerant electrons with clear signatures in magnetotransport measurements. The magnetic state is also coupled dynamically to the lattice and can be reversibly tuned by applying local external forces. As a result, this study raises the possibility of designing nanoscale devices based on domain walls where strain-tunable ferroelectric, ferroelastic and ferromagnetic orders may coexist.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Magnetic molecules lose identity when connected to different combinations of magnetic metal electrodes in MTJ-based molecular spintronics devices (MTJMSD)

Abstract Understanding the magnetic molecules’ interaction with different combinations of metal electrodes is vital to advancing the molecular spintronics field. This paper describes experimental and theoretical understanding showing how paramagnetic single-molecule magnet (SMM) catalyzes long-range effects on metal electrodes and, in that process, loses its basic magnetic properties. For the first time, our Monte Carlo simulations, verified for consistency with regards to experimental studies, discuss the properties of the whole device and a generic paramagnetic molecule analog (GPMA) connected to the combinations of ferromagnet-ferromagnet, ferromagnet-paramagnet, and ferromagnet-antiferromagnet metal electrodes. We studied the magnetic moment vs. magnetic field of GPMA exchange coupled between two metal electrodes along the exposed side edge of cross junction-shaped magnetic tunnel junction (MTJ). We also studied GPMA-metal electrode interfaces’ magnetic moment vs. magnetic field response. We have also found that the MTJ dimension impacted the molecule response. This study suggests that SMM spin at the MTJ exposed sides offers a unique and high-yield method of connecting molecules to virtually endless magnetic and nonmagnetic electrodes and observing unprecedented phenomena in the molecular spintronics field.

42 ENGINEERING↗
DOE OSTI2025

The electronic structure, crystal fields, and magnetic anisotropy in RECo 5 magnets

The current progress in describing rare-earth-based magnets' electronic structure and magnetic properties is discussed. We use several currently popular electronic structure methods to show the typical values of critical parameters that define the physics of RECo 5 (RE = rare earth atom) materials. The magnetic moments and magnetic anisotropy of 4f atoms are obtained using several approaches, including anisotropic 4f-charge density-constrained DFT and DFT+HI methods. We also suggest the introduction of "penalty" functional for obtaining correct variational total energy in the traditional Hund's rule-constrained DFT-based techniques. The applicability and future extensions are discussed. The proposed combination of methods is potentially suitable for high-throughput computational searches of new rare-earth-containing magnetic materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Magnetic structure and magnetoelectric effect in the centrosymmetric antiferromagnet Cu 2 ( MoO 4 ) ( SeO 3 )

Magnetic properties of Cu 2 ⁢(MoO 4 )⁢(SeO 3 ), an S = $\frac{1}{2}$ centrosymmetric antiferromagnet (AFM), were investigated using superconducting quantum interference device magnetometry, neutron diffraction, and magnetoelectric (ME) measurements. Here, the magnetic susceptibility measurements indicate a broad peak at ~50 K, followed by a phase transition into AFM order at T N = 23.6⁢(1) K. Above T N , a fit to the Curie-Weiss law gives a Curie-Weiss temperature Θ CW = -68⁢(1) K, suggesting the dominant AFM coupling. Neutron powder diffraction reveals that the C⁢u 2+ spins are aligned AFM along the c axis with weak noncollinearity under the magnetic space group of P2$^{'}_{1}$/c. The ME response indicates that a nondiagonal component of a ME tensor is active, supporting the simultaneous spatial and time reversal symmetry breaking under P2$^{'}_{1}$/c.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

A versatile X-ray phase retarder for lock-in XMCD measurements

X-ray magnetic circular dichroism (XMCD) is a technique commonly used to probe magnetic properties of materials with element and orbital selectivity, which requires the use of circularly polarized (CP) X-rays. It is possible to accomplish XMCD experiments with fixed CP and alternating the magnetic field orientation, but most reliable data are obtained when alternating the magnetization orientation and the polarization between right and left helicities. A versatile strategy has been developed to perform XMCD experiments using a hard X-ray quarter-wave plate, at both polychromatic dispersive and conventional monochromatic optics, in combination with synchronous data acquisition. The switching frequency waveform is fed into a lock-in amplifier to detect and amplify the XMCD signal. The results on a reference sample demonstrate an improvement in data quality and acquisition time. The instrumentation successfully generated 98% of CP X-rays switching the beam helicity at 13 Hz, with the possibility of faster helicity switching once it is installed at the new Brazilian fourth-generation source, SIRIUS.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS2010

Electrical Switching of Perovskite Thin-Film Resistors

Electronic devices that exploit electrical switching of physical properties of thin films of perovskite materials (especially colossal magnetoresistive materials) have been invented. Unlike some related prior devices, these devices function at room temperature and do not depend on externally applied magnetic fields. Devices of this type can be designed to function as sensors (exhibiting varying electrical resistance in response to varying temperature, magnetic field, electric field, and/or mechanical pressure) and as elements of electronic memories. The underlying principle is that the application of one or more short electrical pulse(s) can induce a reversible, irreversible, or partly reversible change in the electrical, thermal, mechanical, and magnetic properties of a thin perovskite film. The energy in the pulse must be large enough to induce the desired change but not so large as to destroy the film. Depending on the requirements of a specific application, the pulse(s) can have any of a large variety of waveforms (e.g., square, triangular, or sine) and be of positive, negative, or alternating polarity. In some applications, it could be necessary to use multiple pulses to induce successive incremental physical changes. In one class of applications, electrical pulses of suitable shapes, sizes, and polarities are applied to vary the detection sensitivities of sensors. Another class of applications arises in electronic circuits in which certain resistance values are required to be variable: Incorporating the affected resistors into devices of the present type makes it possible to control their resistances electrically over wide ranges, and the lifetimes of electrically variable resistors exceed those of conventional mechanically variable resistors. Another and potentially the most important class of applications is that of resistance-based nonvolatile-memory devices, such as a resistance random access memory (RRAM) described in the immediately following article, Electrically Variable Resistive Memory Devices (MFS-32511-1).

Liu, Shangqing↗
DOE OSTI2018

Mn-induced Ferromagnetic Semiconducting Behavior with Linear Negative Magnetoresistance in Sr 4 (Ru 1-x Mn x ) 3 O 10 Single Crystals

Triple-layered Sr 4 Ru 3 O 10 is a unique ferromagnet with the central RuO 6 layer behaving differently from two outer layers both crystallographically and magnetically. Here, we report that the partial substitution of Ru by smaller Mn gives rise to modification in crystal structure, electronic and magnetic properties of Sr 4 (Ru 1-x Mn x ) 3 O 10 . Through the single crystal X-ray diffraction refinement, we find that (Ru/Mn)O 6 octahedral rotation is no longer detectable at x ≥ 0.23, leading to the tetragonal structure. The magnetization measurements indicate the ferromagnetic transition temperature T C decreases from 105 K for x = 0 to 30 K for x = 0.41, with the reduced magnetic moment as well. Remarkably, Mn doping results in the change of magnetic anisotropy from the easy c axis in x = 0 to the easy ab plane seen in x = 0.34 and 0.41. Such change also removes the ab -plane metamagnetic transition observed in x = 0. Furthermore, the electrical resistivity increases with increasing x showing semiconducting behavior with Δ ~ 10 meV for x = 0.34 and 30 meV for x = 0.41. Under applied magnetic field, the magnetoresistance exhibits negative and linear field dependence in all current and field configurations. These results clearly indicate Sr 4 (Ru 1-x Mn x ) 3 O 10 is a novel ferromagnetic semiconductor with exotic magnetotransport properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Pressure effects on spin-lattice coupling of CdCr 2 S 4

In this work, results of DC magnetization and x-ray diffraction measurements under hydrostatic pressure are reported for the CdCr 2 S 4 spinel compound. Magnetization measurements were performed in a polycrystalline sample up to 14 kbar in the 20–220 K temperature range. At low applied magnetic fields an anomalous susceptibility behavior, associated to magnetic clusters, persists up to 14 kbar. X-ray synchrotron diffraction spectra acquired under pressure show a strong linear correlation between the variation of lattice parameters under pressure and the variation of the Cr–Cr distance. In contrast, the S-Cr-S angles remain constant. Clearly, the CdCr 2 S 4 magnetic properties are strongly sensitive to pressure variations and the Cr local distortions observed in this compound promote a competition between direct antiferromagnetic and super-exchange ferromagnetic interactions within such short-range magnetic clusters.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tunable dimensional crossover and magnetocrystalline anisotropy in Fe 2 P -based alloys

Electronic structure calculations are used to examine the magnetic properties of Fe 2 P-based alloys and the mechanisms through which the Curie temperature and magnetocrystalline anisotropy can be optimized for specific applications. It is found that at elevated temperatures the magnetic interaction in pure Fe 2 P develops a pronounced two-dimensional character due to the suppression of the magnetization in one of the sublattices, but the interlayer coupling is very sensitive to band filling and structural distortions. This feature suggests a natural explanation of the observed sharp enhancement of the Curie temperature by alloying with multiple elements, such as Co, Ni, Si, and B. The magnetocrystalline anisotropy is also tunable by electron doping, reaching a maximum near the electron count of pure Fe 2 P. These findings enable the optimization of the alloy content, suggesting co-alloying of Fe 2 P with Co (or Ni) and Si as a strategy for maximizing the magnetocrystalline anisotropy at and above room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Thermodynamics and Magnetism of SmFe12 Compound Doped with Co and Ni: An Ab Initio Study

Ni-doped Sm(Fe1−xCox)12 alloys are investigated for their magnetic properties. The Sm(Fe,Co)11M1 compound (M acts as a stabilizer) with the smallest (7.7 at.%) rare-earth-metal content has been recognized as a possible contender for highly efficient permanent magnets thanks to its significant anisotropy field and Curie temperature. The early transition metals (Ti-Mn) as well as Al, Si, and Ga stabilize the SmFe12 compound but significantly decrease its saturation magnetization. To keep the saturation magnetization in the range of 1.4–1.6 T, we suggest replacing a certain amount of Fe and Co in the Sm(Fe1−xCox)12 alloys with Ni. Ni plays the role of a thermodynamic stabilizer, and contrary to the above-listed elements, has the spin moment aligned parallel to the spin moment of the SmFe12 compound, thereby boosting its saturation magnetization without affecting the anisotropy field or Curie temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Nanocrystalline Nickel-Substituted Lanthanum Cobaltites Synthesized by Urea Combustion Method: Magnetic and Transport Properties

We have systematically investigated the effect of Ni substitution on the structural, magnetic and transport properties of LaCoO{sub 3}. Single-phase rhombohedral structure with space group R-3c is confirmed by the Rietveld refinement of X-ray diffraction data in all samples. The unit cell parameters were found to increase with Ni doping. The FM and AFM were observed to coexist as confirmed by M–H hysteresis. Positive values of Θ for nickel-doped samples are consistent with the existence of FM interactions. The effective magnetic moment decreases with Ni substitution. All the samples exhibit temperaturedependent semiconducting behaviour. The electrical resistivity decreases considerably with increasing Ni content and with increasing temperature. The transport mechanism has been investigated by fitting a nearest-neighbour small polaron hopping (SPH) model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetocaloric effect observations near room temperature in few-layered chromium telluride (Cr 2 Te 3 )

Transition metal telluride compositions are explored extensively for their unique magnetic behavior. Few-layered chromium telluride (Cr 2 T e3 ) exhibits a near-room-temperature phase transition, where the material can be effectively used in applications such as magnetic refrigeration. Compared to existing magnetocaloric materials, Heusler alloys, and rare-earth-based alloys, the large-scale synthesis of mechanically exfoliated Cr 2 Te 3 involves less complexity, resulting in a stable composition. Compared to existing tellurides, Cr 2 Te 3 exhibited a large change in magnetic entropy (|ΔS M |) of 1.88 J k g−1 K −1 at a magnetic field of 4 T. A refrigeration capacity (RC) of ∼82 J kg −1 was determined from the change in magnetic entropy versus temperature curve. The results were comparable with those for existing Cr-based compounds. First-principles density functional theory (DFT) confirmed the magnetic properties of Cr 2 Te 3 , including a near-room-temperature Curie temperature, T C , consistent with experimental results. Here, structural transition was also observed using first-principles DFT, which is responsible for the magnetic behavior.

36 MATERIALS SCIENCE↗