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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Building blocks for correlated superconductors and magnets

Recent efforts at Los Alamos to discover strongly correlated superconductors and hard ferromagnets are reviewed. While serendipity remains a principal engine of materials discovery, design principles and structural building blocks are beginning to emerge that hold potential for predictive discovery. Successes over the last decade with the so-called “115” strongly correlated superconductors are summarized, and more recent efforts to translate these insights and principles to novel hard magnets are discussed. While true “materials by design” remains a distant aspiration, progress is being made in coupling empirical design principles to electronic structure simulation to accelerate and guide materials design and synthesis.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Realization of an intrinsic ferromagnetic topological state in MnBi 8 Te 13

Novel magnetic topological materials pave the way for studying the interplay between band topology and magnetism. However, an intrinsically ferromagnetic topological material with only topological bands at the charge neutrality energy has so far remained elusive. Using rational design, we synthesized MnBi 8 Te 13 , a natural heterostructure with [MnBi 2 Te 4 ] and [Bi 2 Te 3 ] layers. Thermodynamic, transport, and neutron diffraction measurements show that despite the adjacent [MnBi 2 Te 4 ] being 44.1 Å apart, MnBi 8 Te 13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. First-principles calculations and angle-resolved photoemission spectroscopy measurements reveal it is an axion insulator with sizable surface hybridization gaps. Our calculations further demonstrate the hybridization gap persists in the two-dimensional limit with a nontrivial Chern number. Therefore, as an intrinsic ferromagnetic axion insulator with clean low-energy band structures, MnBi 8 Te 13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized magnetoelectric effect.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ferromagnetic moment in the magnetoelectric antiferromagnet Co 4 ⁢Ta 2 ⁢O 9 : Evidence of the 𝑃⁢1 magnetic space group

Exploring ferromagnetic moments in magnetoelectric materials is of both fundamental and technological importance. It enables the control of the correlated state using an uncompensated moment. However, its material realization is challenging owing to its exclusive microscopic mechanisms. Here, we report a nearly isotropic ferromagnetic moment in the magnetoelectric antiferromagnet Co 4 ⁢Ta 2 ⁢O 9 , by performing dc and ac magnetic susceptibility, magnetization, and neutron diffraction measurements on identical single crystals. Combined with the group theory analysis, we demonstrate a 𝑃⁢1 magnetic space group. This implies that physical activities can occur along any crystallographic direction, which explains the unusual magnetoelectric property of Co 4⁢ Ta 2⁢ O 9 . The ground state is classified as the M-type altermagnet based on the magnetic point group. Our study provides essential information to resolve the debate on the magnetic ground state and understand the magnetoelectric properties of the related compound with the prospect of observing and examining the 𝑃⁢1 magnetic space group or ferromagnet in ferroelectrics.

Antiferromagnets↗
DOE OSTI2014

Exchange bias and antiferromagnetic interfacial exchange coupling in the mixed-spin oxide Li{sub 2/7}Ni{sub 5/7}O

We present a detailed study on magnetic properties of mixed spin oxide Li{sub 2/7}Ni{sub 5/7}O, where small ferromagnetic (FM) clusters (with averaged diameter of ∼18 Å) immersed with an antiferromagnetic (AFM) host. The exchange bias (EB) with shifts of the hysteresis loop along both the field and magnetization axes is found, which is due to the interfacial interaction between the FM clusters and the AFM host. An AFM interfacial exchange coupling is deduced from the exchange interactions between Ni ions. The type and strength of this interfacial exchange interaction are discussed in terms of the EB at low temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Modulation of interlayer exchange coupling strength in magnetic tunnel junctions via strain effect

Interlayer exchange coupling of two ferromagnetic electrodes separated by a thin MgO tunnel barrier is investigated using magneto-optical Kerr effect. We find that the coupling field can be reduced by more than 40% as the thickness of a top Ta capping layer increases from 0.5 to 1.2 nm. In contrast, a similar film stack with an additional 3 nm Ru capping layer displays no such dependence on Ta thickness. Transmission electron microscopy study shows that the oxidation of the exposed Ta capping layer induces changes in the crystalline structures of the underlying films, giving rise to the observed reduction of the interlayer coupling field.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Spin-driven ordering of Cr in the equiatomic high entropy alloy NiFeCrCo

Spin-driven ordering of Cr in an equiatomic fcc NiFeCrCo high entropy alloy (HEA) was predicted by first-principles calculations. Ordering of Cr is driven by the reduction in energy realized by surrounding anti-ferromagnetic Cr with ferromagnetic Ni, Fe, and Co in an alloyed L1{sub 2} structure. The fully Cr-ordered alloyed L1{sub 2} phase was predicted to have a magnetic moment that is 36% of that for the magnetically frustrated random solid solution. Three samples were synthesized by milling or casting/annealing. The cast/annealed sample was found to have a low temperature magnetic moment that is 44% of the moment in the milled sample, which is consistent with theoretical predictions for ordering. Scanning transmission electron microscopy measurements were performed and the presence of ordered nano-domains in cast/annealed samples throughout the equiatomic NiFeCrCo HEA was identified.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetism in Na-filled Fe-based skutterudites

The interplay of superconductivity and magnetism is a subject of ongoing interest, stimulated most recently by the discovery of Fe-based superconductivity and the recognition that spin-fluctuations near a magnetic quantum critical point may provide an explanation for the superconductivity and the order parameter. We investigate magnetism in the Na filled Fe-based skutterudites using first principles calculations. NaFe 4 Sb 12 is a known ferromagnet near a quantum critical point. We find a ferromagnetic metallic state for this compound driven by a Stoner type instability, consistent with prior work. In accord with prior work, the magnetization is overestimated, as expected for a material near an itinerant ferromagnetic quantum critical point. NaFe 4 P 12 also shows a ferromagnetic instability at the density functional level, but this instability is much weaker than that of NaFe 4 Sb 12 , possibly placing it on the paramagnetic side of the quantum critical point. NaFe 4 As 12 shows intermediate behavior. We also present results for skutterudite FeSb 3 , which is a metastable phase that has been reported in thin film form.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electric-field control of electromagnon propagation and spin-wave injection in a spiral multiferroic/ferromagnet composite

We consider theoretically a composite chain consisting of a multiferroic helimagnet coupled to a conventional ferromagnet and inspect the conversion of electromagnon excitation into spin waves and vice versa. We demonstrate an electric-field control of spin-wave injection realized by electrically exciting an electromagnon that propagates with an intrinsic frequency larger than the gap of the spin wave in the ferromagnet. The efficiency of the conversion of the electromagnon into spin waves depends strongly on the strength of the magnetoelectric coupling at the interface and the intrinsic frequency of the multiferroic helimagnets. The phenomena predicted here suggest that a multiferroic/ferromagnet composite offers new opportunities for spin-wave injection, conversion, and control using electric field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Computational search for itinerant n -type and p -type magnetic semiconductors: Arsenopyrites as bipolar magnetic semiconductors

We report a computational search for three-dimensional bulk ferromagnetic semiconductors based on itinerant Stoner physics. Stoner-based ferromagnetic semiconductors require an unusual combination of high band edge density of states and moderate transport effective mass. Nonetheless, several potential materials are found. Here, we identify arsenopyrites, exemplified by FeAsS, as materials that, with suitable doping, exhibit ferromagnetic semiconducting behavior for both n- and p-type dopants. The existence of a material that becomes ferromagnetic for both p and n types may enable device concepts, for example, ferromagnetic p–n junction devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Half metallic ferromagnetism in tri-layered perovskites Sr{sub 4}T{sub 3}O{sub 10}(T = Co, Rh)

First-principles density functional theory (DFT) is used to investigate the electronic and magnetic properties of Sr{sub 4}Rh{sub 3}O{sub 10}, a member of the Ruddlesden-Popper series. Based on the DFT calculations taking into account the co-operative effect of Coulomb interaction (U) and spin-orbit couplings (SOC), Sr{sub 4}Rh{sub 3}O{sub 10} is found to be a half metallic ferromagnet (HMF) with total magnetic moment μ{sub tot} = 12 μ{sub B} per unit cell. The material has almost 100% spin-polarization at the Fermi level despite of sizable SOC. Replacement of Rh atom by the isovalent Co atom is considered. Upon full-replacement of Co, a low-spin to intermediate spin transition happens resulting in a HMF state with the total magnetic moment three-time larger (i.e., μ{sub tot} = 36 μ{sub B} per unit cell), compared to Sr{sub 4}Rh{sub 3}O{sub 10}. We propose Sr{sub 4}Rh{sub 3}O{sub 10} and Sr{sub 4}Co{sub 3}O{sub 10} as candidates of half metals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Itinerant ferromagnetism in transition metal dichalcogenide moiré superlattices

Moiré materials are artificial crystals formed at van der Waals heterojunctions that have emerged as a highly tunable platform that is able to realize much of the rich quantum physics of electrons in atomic scale solids, and in several cases even new quantum phases of matter. Here we use finite-size exact diagonalization methods to explore the physics of single-band itinerant electron ferromagnetism in semiconductor moiré materials. As a result, we predict where ferromagnetism is likely to occur in triangular-lattice moiré systems, and where it is likely to yield the highest Curie temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Scaling behavior of spin gap of the bond alternating anisotropic spin-1/2 Heisenberg chain

Scaling behavior of spin gap of a bond alternating spin-1/2 anisotropic Heisenberg chain has been studied both in ferromagnetic (FM) and antiferromagnetic (AFM) cases. Spin gap has been estimated by using exact diagonalization technique. All those quantities have been obtained for a region of anisotropic parameter Δ defined by 0≤Δ≤1. Spin gap is found to develop as soon as the non-uniformity in the alternating bond strength is introduced in the AFM regime which furthermore sustains in the FM regime as well. Scaling behavior of the spin gap has been studied by introducing scaling exponent. The variation of scaling exponents with Δ is fitted with a regular function.

36 MATERIALS SCIENCE↗
DOE OSTI2014

New opportunities in microwave electronics with ferromagnetic nanowires

A series of monolithic tunable microwave band-stop filters and phase shifters have been designed, fabricated, and characterized on arrays of ferromagnetic nanowires (FMNWs) constituted of Ni, Co, CoFeB, and NiFe alloy embedded in anodic alumina templates (AAO). The aim of the present investigation is to significantly boost the operating frequency of devices in very low applied magnetic field. The stop-band response can cover from 8 to 40 GHz with an applied magnetic field up to 5 kOe using these materials. In addition, we observed a substantial increase in frequency tunability (>110%) for Ni and NiFe alloy based filter. Frequency bandwidth/linewidth can be tuned with the proper choice of magnetic material, broad band for Co nanowires to narrow band for CoFeB nanowires. The amorphous nature of CoFeB reduces linewidth considerably and hence increases resonance absorption. It is also observed that the frequency linewidth (Δf) decreases with increasing applied magnetic field. The same filter can be used as a tunable phase shifter. For Ni nanowire based phase shifter, a maximum shift of 75°/cm at 4 kOe was observed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interfacial Dzyaloshinskii-Moriya interaction of antiferromagnetic materials

The interface between a ferromagnet (FM) or antiferromagnet (AFM) and a heavy metal (HM) results in an antisymmetric exchange interaction known as the interfacial Dzyaloshinskii-Moriya interaction (iDMI) which favors non-collinear spin configurations. The iDMI is responsible for stabilizing noncollinear spin textures such as skyrmions in materials with bulk inversion symmetry. Interfacial DMI values have been previously determined theoretically and experimentally for FM/HM interfaces, and, in this work, they are calculated for the metallic AFM MnPt and the insulating AFM NiO. The heavy metals considered are W, Re, and Au. Values for the iDMI, exchange, and anisotropy constants are determined for different AFM and HM thicknesses. The iDMI values of the MnPt heterolayers are comparable to those of the common FM materials, and those of NiO are lower. In few-layer films of (001) MnPt, the high spin orbit coupling of the Pt layers can give rise to a small DMI in the absence of a HM layer.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Correlation between charge transfer and exchange coupling in carbon-based magnetic materials

Several forms of carbon-based magnetic materials, i.e. single radicals, radical dimers, and alternating stacks of radicals and diamagnetic molecules, have been investigated using density-functional theory with dispersion correction and full geometry optimization. Our calculated results demonstrate that the C{sub 31}H{sub 15} (R{sub 4}) radical has a spin of ½. However, in its [R{sub 4}]{sub 2} dimer structure, the net spin becomes zero due to antiferromagnetic spin-exchange between radicals. To avoid antiferromagnetic spin-exchange of identical face-to-face radicals, eight alternating stacks, R{sub 4}/D{sub 2m}/R{sub 4} (with m = 3-10), were designed. Our calculated results show that charge transfer (Δn) between R{sub 4} radicals and the diamagnetic molecule D{sub 2m} occurs with a mechanism of spin exchange (J) in stacks. The more electrons that transfer from R{sub 4} to D{sub 2m}, the stronger the ferromagnetic spin-exchange in stacks. In addition, our calculated results show that Δn can be tailored by adjusting the electron affinity (E{sub a}) of D{sub 2m}. The correlation between Δn, E{sub a}, m, and J is discussed. These results give some hints for the design of new ferromagnetic carbon-based materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Phase separated behavior in yttrium doped CaMnO{sub 3}

Highlights: • Electron doped manganite. • Magnetic phase separation observed. • Structural transition coupled with magnetic transition. - Abstract: The effect of electron doping on the structural, transport, and magnetic properties of Mn (IV)—rich Ca{sub 1−x}Y{sub x}MnO{sub 3} (x ≤ 0.2) samples have been investigated using neutron diffraction, neutron depolarization, magnetization and resistivity techniques. The temperature dependence of resistivity follows the small polaron model and the activation energy exhibits a minimum for x = 0.1 sample. A phase separated magnetic ground state consisting of ferromagnetic domains (∼7 μm) embedded in G-type antiferromagnetic matrix is observed in the sample, x = 0.1. The transition to the long-range magnetically ordered state in this sample is preceded by a Griffith’s phase. On lowering temperature below 300 K a structural transition from orthorhombic structure (Pnma) to a monoclinic structure (P2{sub 1}/m) is observed in the case of x = 0.2 sample. The ferromagnetic behavior in this case is suppressed and the antiferromagnetic ordering is described by coexisting C-type and G-type magnetic structures corresponding to the monoclinic and orthorhombic phases, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Voltage-controlled ferromagnetism and magnetoresistance in LaCoO{sub 3}/SrTiO{sub 3} heterostructures

A LaCoO{sub 3}/SrTiO{sub 3} heterostructure grown on Si (001) is shown to provide electrically switchable ferromagnetism, a large, electrically tunable magnetoresistance, and a vehicle for achieving and probing electrical control over ferromagnetic behavior at submicron dimensions. Fabrication of devices in a field-effect transistor geometry enables application of a gate bias voltage that modulates strain in the heterostructure via the converse piezoelectric effect in SrTiO{sub 3}, leading to an artificial inverse magnetoelectric effect arising from the dependence of ferromagnetism in the LaCoO{sub 3} layer on strain. Below the Curie temperature of the LaCoO{sub 3} layer, this effect leads to modulation of resistance in LaCoO{sub 3} as large as 100%, and magnetoresistance as high as 80%, both of which arise from carrier scattering at ferromagnetic-nonmagnetic interfaces in LaCoO{sub 3}. Finite-element numerical modeling of electric field distributions is used to explain the dependence of carrier transport behavior on gate contact geometry, and a Valet-Fert transport model enables determination of spin polarization in the LaCoO{sub 3} layer. Piezoresponse force microscopy is used to confirm the existence of piezoelectric response in SrTiO{sub 3} grown on Si (001). It is also shown that this structure offers the possibility of achieving exclusive-NOR logic functionality within a single device.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electric‐Field‐Driven Reversal of Ferromagnetism in (110)‐Oriented, Single Phase, Multiferroic Co‐Substituted BiFeO 3 Thin Films

Abstract While multiferroic materials are attractive systems for the promise of ultra‐low‐power‐consumption computational technologies, electric‐field‐induced magnetization reversal is a key challenge for realizing devices at scale. Though significant research efforts have been working toward the realization of a material which couples ferroelectricity and ferromagnetism, there are few, even composite, systems which are practical for device scale applications at room temperature. Co‐substituted multiferroic BiFe 0.9 Co 0.1 O 3 is a promising candidate system, due to coupled ferroelectricity and weak ferromagnetism at room temperature. Here, it is theoretically indicated that the ferroic orders in this material are statically coupled, where an in‐plane 109° ferroelectric switching event can result in the reversal of this out‐of‐plane component of magnetization, and the electric field‐induced magnetization reversal is experimentally observed. Such an in‐plane poling configuration is particularly desirable for device applications.

Chemistry↗