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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

Defect-mediated ferromagnetism in correlated two-dimensional transition metal phosphorus trisulfides

Controlling the magnetic spin states of two-dimensional (2D) van der Waals (vdW) materials with strong electronic or magnetic correlation is important for spintronic applications but challenging. Crystal defects that are often present in 2D materials such as transition metal phosphorus trisulfides (MPS 3 ) could influence their physical properties. Here, we report the effect of sulfur vacancies on the magnetic exchange interactions and spin ordering of few-layered vdW magnetic Ni 1-x Co x PS 3 nanosheets. Magnetic and structural characterization in corroboration with theoretical calculations reveal that sulfur vacancies effectively suppress the strong intralayer antiferromagnetic correlation, giving rise to a weak ferromagnetic ground state in Ni 1-x Co x PS 3 nanosheets. Notably, the magnetic field required to tune this ferromagnetic state (<300 Oe) is much lower than the value needed to tune a typical vdW antiferromagnet (> several thousand oersted). These findings provide a previously unexplored route for controlling competing correlated states and magnetic ordering by defect engineering in vdW materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of Controlled Artificial Disorder on the Magnetic Properties of EuFe 2 (As 1– x P x ) 2 Ferromagnetic Superconductor

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe 2 ( As 1 – x P x ) 2 ( x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, T c ( H ) , shows an extraordinarily large decrease. It starts at T c ( H = 0 ) ≈ 24 K in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only T c moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in T c to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in EuFe 2 ( As 1 – x P x ) 2 superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered Eu 2 + lattice act as efficient pairbreakers leading to a significant T c reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of T m (less than 1 K) after the irradiation drives T c to below T m . Our results suggest that FM and SC phases coexist microscopically in the same volume.

36 MATERIALS SCIENCE↗
NASA NTRS1973

Theory of disordered Heisenberg ferromagnets

A Green's function technique is used to calculate the magnetic properties of Heisenberg ferromagnets in which the exchange interactions deviate randomly in strength from the mean interaction. Systems of sc, bcc, and fcc topologies and of general spin values are treated. Disorder produces marked effects in the density of spin wave states, in the form of enhancement of the low-energy density and extension of the energy band to higher values. The spontaneous magnetization and the Curie temperature decrease with increasing disorder. The effects of disorder are shown to be more pronounced in the ferromagnetic than in the paramagnetic phase.

Stubbs, R. M.↗
NASA NTRS1975

Microwave resonance thermomagnetic analysis - A new method for characterizing fine-grained ferromagnetic constituents in lunar materials

Microwave resonance thermomagnetic analysis (MRTA) is the name given to a newly evolved technique for inferring the natures of fine-grained ferromagnetic constituents in lunar materials. Based on standard ferromagnetic resonance (FMR) procedures, the method makes use of the microwave skin effect for diagnosing the presence of metallic iron. Modelling experiments carried out on well-characterized iron and magnetitelike precipitates produced independently in simulated lunar glasses, coupled with published data for magnetite, provide a potential basis for detecting and discriminating between iron metal and ferric iron spinel, even when both are present in an unknown sample. Application of the technique to the lunar samples indicates the possible existence of magnetitelike phases in amounts up to about 0.3 wt% in soils from seven samples regions of the moon. These findings do not require any special geologic processes for their explanation, although some evidence supports the suggestion that fumarolic activity may have occurred in the lunar highlands.

Griscom, D. L.↗
NASA NTRS1974

Ferromagnetic resonance studies of thermal effects on lunar metallic Fe phases

Ferromagnetic resonance results of annealing experiments are discussed which illustrate the thermal effects on lunar metallic iron phases already present in a lunar fines sample. Spectral features of ferromagnetic resonance produced in the sample by heat treatments at temperatures between 600 and 1025 C are described which resemble those detected in lunar breccias and crystalline rocks. A correlation is shown to exist between these features and the degree of thermal metamorphism. It is noted that this correlation can be used as a built-in geothermometer or probe to investigate the thermal history and degree of metamorphism of a lunar sample containing metallic iron phases. The thermal history of a metaclastic rock is analyzed in this way, and it is shown that thermal metamorphism is an effective process for increasing relaxation times or the stability of the natural remanent magnetization carried by single-domain metallic iron particles.

Tsay, F.-D.↗
NASA NTRS1982

Ferromagnetic resonance intensity - A rapid method for determining lunar glass bead origin

Ferromagnetic resonance intensity, I(s), relies on the absence or presence of single-domain Fe formed during impact melting by autoreduction of Fe(2+) in the melt to distinguish volcanic from impact glass spherules in the lunar soil. SEM inspection of individual glass bead surfaces gave reliable evidence of the mode of glass genesis. Ferromagnetic resonance intensity was tested against the Apollo 15 and 17 volcanics. I(s) values of less than one were found in 94% of the volcanic glasses, implying the absence of single-domain Fe, while 75% of the impact glasses had elevated I(s) values. These samples lack single-domain Fe, and may represent impact melts of bedrock with little or no regolith contribution. Both the primary discrimination of volcanic and impact glasses, and the secondary one of bedrock-derived impact melts from impact melts partially or entirely derived from regolith, have been demonstrated for the I(s) technique.

Stone, C. D.↗
NASA NTRS1992

Formation of a metastable ferromagnetic tau phase during containerless melt processing and rapid quenching in Mn-Al-C alloys

Solidification of selected Mn-Al-C alloys during containerless levitation and rapid quenching has yielded the first report for a ferromagnetic metastable tau phase formed directly from the melt. Complete solidification to tau phase was interrupted by the competitive evolution of an equilibrium epsilon phase during recalescence. The amount of undercooling required to produce the metastable ferromagnetic tau phase in a Mn(0.55)Al(0.433)C(0.017) alloy during solidification was estimated as approximately 470 K based on differential thermal analysis results. When the alloy carbon content was increased to 3.4 at. pct, transition in structure development occurred so that the samples exhibited gamma 2 phase formation as well as tau and epsilon phases.

Kim, Y. J.↗
NASA NTRS1993

Formation of a metastable ferromagnetic tau phase during containerless melt processing in Mn-Al-C alloys

This paper reports the production of a metastable ferromagnetic tau phase directly from the melt in Mn(0.55)Al(0.433)C(0.017), using a containerless processing method involving levitation melting followed by quenching. Using the results from differential thermal analysis measurements and an analysis of the phase equilibria, it was found that the minimum amount of undercooling level required for ferromagnetic metastable tau phase formation in this alloy was Delta T = 87 K. The attainment of this undercooling may be facilitated by the application of containerless melt processing.

Kim, Y. J.↗
NASA NTRS2000

High frequency, small signal MH loops of ferromagnetic thin films

A method is presented for transforming the high frequency bias susceptibility measurements of ferromagnetic thin films into the form of a MH loop with, depending upon the measurement geometry, the y-axis zero crossing giving a measure of the coercive force or anisotropy field. The loops provide a measure of the quantitative and qualitative high frequency switching properties of ferromagnetic thin films. c2000 American Institute of Physics.

NASA Discipline Life Sciences Technologies↗
NASA NTRS2018

I-Xe Dating of Aqueous Alteration in the CI Chondrite Orgueil: I. Magnetite and Ferromagnetic Separates

The I-Xe system was studied in a ferromagnetic sample separated from the Orgueil CI carbonaceous chondrite with a hand-held magnet and in two magnetite samples, one chemically separated before and the other one after neutron irradiation. This work was done in order to investigate the effects of chemical separation by LiCl and NaOH on the I-Xe system in magnetite. Our test demonstrated that the chemical separation of magnetite before irradiation using either LiCl or NaOH, or both, does not contaminate the sample with iodine and thus cannot lead to erroneous I-Xe ages due to introduction of uncor-related128*Xe. The I-Xe ages of two Orgueil magnetite samples are mutually consistent within experimental uncertainties and, when normalized to an absolute time scale with the reevaluated Shallowater aubrite standard, place the onset of aqueous alteration on the CI parent body at 4564.3 ± 0.3 Ma, 2.9 ± 0.3 Ma after formation of the CV Ca-AI-rich inclusions (CAIs). The I-Xe age ofthe ferromagnetic Orgueil separate is 3.4 Ma younger, corresponding to a closure of the I-Xe system at 4560.9 ± 0.2 Ma. These and previously published I-Xe data for Orgueil (Hohenberg et al., 2000) indicate that aqueous alteration on the CI parent body lasted for at least 5 Ma. Although the two magnetite samples gave indistinguishable I-Xe ages, their temperature release profiles differed. One of the two Orgueil magnetites released less radiogenic Xe than the other, 80% of it corresponding to the low-temperature peak of the release profile, compared to only 6% in case of the second Orgueil magnetite sample. This could be due to the difference in iodine trapping efficiencies for magnetite grains of different morphologies. Alternatively, the magnetite grains with the lower radiogenic Xe concentrations may have formed at a later stage of alteration when iodine in an aqueous solution was depleted.

I-Xe systematics↗
DOE OSTI2020

Ferromagnetic resonance in single vertices and 2D lattices macro-dipoles of elongated nanoelements: measurements and simulations

We report broadband ferromagnetic resonance measurements of the in-plane magnetic field response of three- and four-fold symmetric vertices formed by non-contacting permalloy nano-ellipses together with extended lattices constructed from them. Complementing the experimental data with simulations, we are able to show that, as far as the most intense FMR responses are concerned, the spectra of vertices and lattices can largely be interpreted in terms of a superposition of the underlying hysteretic responses of the individual ellipses, as elemental building blocks of the system. Here, this property suggest that it is possible to understand the orientation of the individual magnetic dipole moments in a dipole network in terms of dynamic measurements alone, thereby offering a powerful tool to analyze the alignment statistics in frustrated systems that are exposed to various magnetic histories.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Direct Imaging of Antiferromagnet-Ferromagnet Phase Transition in van der Waals Antiferromagnet CrSBr

The advent of van der Waals (vdW) ferromagnetic (FM) and antiferromagnetic (AFM) materials offers unprecedented opportunities for spintronics and magneto-optic devices. Combining magnetic Kerr microscopy and density functional theory calculations, the AFM-FM transition is investigated and a surprising abnormal magneto-optic anisotropy in vdW CrSBr associated with different magnetic phases (FM, AFM, or paramagnetic state) is discovered. This unique magneto-optic property leads to different anisotropic optical reflectivity from different magnetic states, permitting direct imaging of the AFM Néel vector orientation and the dynamic process of the AFM-FM transition within a magnetic field. Using Kerr microscopy, not only the domain nucleation and propagation process is imaged but also the intermediate spin-flop state in the AFM-FM transition is identified. In conclusion, the unique magneto-optic property and clear identification of the dynamics process of the AFM-FM phase transition in CrSBr demonstrate the promise of vdW magnetic materials for future spintronic technology.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Universal Superconductivity in FeTe and All‐Iron‐Based Ferromagnetic Superconductor Heterostructures

Abstract Ferromagnetism (FM) and superconductivity (SC) are two of the most famous macroscopic quantum phenomena. However, nature normally does not allow SC and FM to coexist without significant degradation. Here, the first fully iron‐based SC/FM heterostructures, composed of Fe(Te,Se) and Fe 3 GeTe 2 , are introduced, and it is shown that this system exhibits both strong FM and high‐temperature SC with an atomically sharp interface. From this study, it is also discovered that minute level of various cationic dopants can drive otherwise non‐superconducting FeTe films into a SC state. This suggests that the ground state of FeTe is so close to the SC state that it can be driven in and out of the SC state with various other perturbations. Altogether, this shows that Fe‐Te‐based heterostructures provide a unique opportunity to manipulate magnetism, superconductivity, and topological physics, paving the way toward new superconducting technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Topological Spin Textures in an Insulating van der Waals Ferromagnet

Generation and control of topological spin textures constitutes one of the most exciting challenges of modern spintronics given their potential applications in information storage technologies. Of particular interest are magnetic insulators, which due to low damping, absence of Joule heating and reduced dissipation can provide energy-efficient spin-textures platform. Here, it is demonstrated that the interplay between sample thickness, external magnetic fields, and optical excitations can generate a prolific paramount of spin textures, and their coexistence in insulating CrBr 3 van der Waals (vdW) ferromagnets. Using high-resolution magnetic force microscopy and large-scale micromagnetic simulation methods, the existence of a large region in T-B phase diagram is demonstrated where different stripe domains, skyrmion crystals, and magnetic domains exist and can be intrinsically selected or transformed to each-other via a phase-switch mechanism. Lorentz transmission electron microscopy unveils the mixed chirality of the magnetic textures that are of Bloch-type at given conditions but can be further manipulated into Néel-type or hybrid-type via thickness-engineering. The topological phase transformation between the different magnetic objects can be further inspected by standard photoluminescence optical probes resolved by circular polarization indicative of an existence of exciton-skyrmion coupling mechanism. The findings identify vdW magnetic insulators as a promising framework of materials for the manipulation and generation of highly ordered skyrmion lattices relevant for device integration at the atomic level.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Direct Observation of Room–Temperature Magnetic Skyrmion Motion Driven by Ultra–Low Current Density in Van Der Waals Ferromagnets

The recent discovery of room-temperature ferromagnetism in 2D van der Waals (vdW) materials, such as Fe 3 GaTe 2 (FGaT), has garnered significant interest in offering a robust platform for 2D spintronic applications. Various fundamental operations essential for the realization of 2D spintronics devices are experimentally confirmed using these materials at room temperature, such as current-induced magnetization switching or tunneling magnetoresistance. Nevertheless, the potential applications of magnetic skyrmions in FGaT systems at room temperature remain unexplored. In this work, the current-induced generation of magnetic skyrmions in FGaT flakes employing high-resolution magnetic transmission soft X-ray microscopy is introduced, supported by a feasible mechanism based on thermal effects. Furthermore, direct observation of the current-induced magnetic skyrmion motion at room temperature in FGaT flakes is presented with ultra-low threshold current density. This work highlights the potential of FGaT as a foundation for room-temperature-operating 2D skyrmion device applications.

2D ferromagnets↗
DOE OSTI2021

Epitaxial Stabilization of Metastable 3C BaRuO 3 Thin Film with Ferromagnetic Non-Fermi Liquid Phase

Thin films of perovskite ruthenates of the general formula ARuO 3 (A = Ca and Sr) are versatile electrical conductors for viable oxide electronics. They are also scientifically intriguing, as they exhibit nontrivial electromagnetic ground states depending on the A-site element. Among them, realization of the cubic perovskite (3C) BaRuO 3 in thin film form has been a challenge so far, because the 3C phase is metastable with the largest formation energy among the various polymorph phases of BaRuO 3 . In this study, 3C BaRuO 3 thin films are successfully prepared employing epitaxial stabilization. Here the 3C BaRuO 3 thin films show itinerant ferromagnetism with a transition temperature of ≈48 K and a non-Fermi liquid phase. The epitaxial stabilization of the 3C BaRuO 3 further enables to make a standard comparison of perovskite ruthenates, thereby establishing the importance of the Ru-O orbital hybridization in understanding the itinerant magnetic system.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Separation of Artifacts from Spin–Torque Ferromagnetic Resonance Measurements of Spin–Orbit Torque for the Low–Symmetry Van der Waals Semi–Metal ZrTe 3

Spin-orbit torques generated by exfoliated layers of the low-symmetry semi-metal ZrTe 3 are measured using the spin-torque ferromagnetic resonance (ST-FMR) technique. When the ZrTe 3 has a thickness greater than about 10 nm, artifacts due to spin pumping and/or resonant heating can cause the standard ST-FMR analysis to overestimate the true magnitude of the torque efficiency by as much as a factor of 30, and to indicate incorrectly that the spin-orbit torque depends strongly on the ZrTe 3 layer thickness. Artifact-free measurements can still be achieved over a substantial thickness range by the method developed recently to detect ST-FMR signals in the Hall geometry as well as the longitudinal geometry. ZrTe 3 /Permalloy samples generate a conventional in-plane anti-damping spin torque efficiency ξ$^{FL}_{∥}$ = 0.014 ± 0.004, and an unconventional in-plane field-like torque efficiency |ξ$^{FL}_{∥}$| = 0.003 ± 0.001. As a result, the out-of-plane anti-damping torque is negligible. It is suggested that artifacts similarly interfere with the standard ST-FMR analysis for other van der Waals samples thicker than about 10 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Synchrotron X-ray spectroscopic study of the antiferromagnetic-ferromagnetic transition in Ni-doped FeRh epitaxial thin films

Here we investigate the change of magnetism and electronic structure across the antiferromagnetic (AFM)-ferromagnetic (FM) transition of Ni-doped FeRh epitaxial thin films by x-ray magnetic circular dichroism (XMCD) and hard x-ray photoemission spectroscopy (HAXPES). The Rh L edge XMCD results indicate that the remnant FM phase at low temperature possesses smaller Rh moment than the normal FM phase which can undergo AFM-FM transition. The HAXPES results confirm an increase of Rh 4d density of state at the Fermi level and a possible "well-screened" state at the Ni 2p photoemission in the FM phase. FM fluctuation of the inter-site Fe-Fe exchange coupling within the AFM phase can be interpreted from the temperature dependence of the Fe 2p HAXPES results.

36 MATERIALS SCIENCE↗