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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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Thermal transport in yttrium iron garnet at very high magnetic fields

The ferrimagnetic insulator yttrium iron garnet (YIG) is one of the most important materials in the active fields of insulator-based spintronics and spin caloritronics. Nevertheless, and despite the fact that this material has been studied for over six decades, the thermal properties of magnons in YIG have not been sufficiently characterized, mainly because at not very low temperatures they are overwhelmed by the contribution of phonons. Here, we report measurements of the thermal conductivity in YIG under magnetic fields up to 31.4 T to increase the magnon energy gap, to suppress the magnon contribution, and to isolate that of the phonons relative to theirbehavior at zero field. We observe that at a temperature of 20 K, even with a field as large as 31.4 T, the magnon contribution is not completely suppressed. In conclusion, the magnon thermal conductivity, measured by subtracting the value of the total thermal conductivity at 31.4 T from the value at zero field, has a peak at 16 K, with an amplitude thatis over five times larger than the one obtained by measuring under a field of only 7 T, as previously reported.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Selective-area growth and magnetic characterization of MnAs/AlGaAs nanoclusters on insulating Al{sub 2}O{sub 3} layers crystallized on Si(111) substrates

We report on selective-area metal-organic vapor phase epitaxy and magnetic characterization of coupled MnAs/AlGaAs nanoclusters formed on thin Al{sub 2}O{sub 3} insulating layers crystallized on Si(111) substrates. Cross-sectional transmission electron microscopy reveals that poly-crystalline γ-Al{sub 2}O{sub 3} grains are formed after an annealing treatment of the amorphous Al{sub 2}O{sub 3} layers deposited by atomic layer deposition on Si(111) substrates. The 〈111〉 direction of the γ-Al{sub 2}O{sub 3} grains tends to be oriented approximately parallel to the 〈111〉 direction of the Si substrate. We observe that hexagonal MnAs nanoclusters on AlGaAs buffer layers grown by selective-area metal-organic vapor phase epitaxy on partially SiO{sub 2}-masked Al{sub 2}O{sub 3} insulator crystallized on Si(111) substrates are oriented with the c-axis along the 〈111〉 direction of the substrates, but exhibit a random in-plane orientation. A likely reason is the random orientation of the poly-crystalline γ-Al{sub 2}O{sub 3} grains in the Al{sub 2}O{sub 3} layer plane. Magnetic force microscopy studies at room temperature reveal that arrangements of coupled MnAs nanoclusters exhibit a complex magnetic domain structure. Such arrangements of coupled MnAs nanoclusters may also show magnetic random telegraph noise, i.e., jumps between two discrete resistance levels, in a certain temperature range, which can be explained by thermally activated changes of the complex magnetic structure of the nanocluster arrangements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Ultrafast dynamics of localized magnetic moments in the unconventional Mott insulator Sr 2 IrO 4

Here, we report a time-resolved study of the ultrafast dynamics of the magnetic moments formed by the ${{J}_{\text{eff}}}=1/2$ states in Sr 2 IrO 4 by directly probing the localized iridium 5d magnetic state through resonant x-ray diffraction. Using optical pump–hard x-ray probe measurements, two relaxation time scales were determined: a fast fluence-independent relaxation is found to take place on a time scale of 1.5 ps, followed by a slower relaxation on a time scale of 500 ps–1.5 ns.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Detection of uncompensated magnetization at the interface of an epitaxial antiferromagnetic insulator

We have probed directly the temperature and magnetic field dependence of pinned uncompensated magnetization at the interface of antiferromagnetic FeF 2 with Cu, using FeF 2 -Cu-Co spin valves. Electrons polarized by the Co layer are scattered by the pinned uncompensated moments at the FeF 2 -Cu interface giving rise to giant magnetoresistance. We determined the direction and magnitude of the pinned uncompensated magnetization at different magnetic fields and temperatures using the angular dependencies of resistance. The strong FeF 2 anisotropy pins the uncompensated magnetization along the easy axis independent of the cooling field orientation. Most interestingly, magnetic fields as high as 90 kOe cannot break the pinning at the FeF 2 -Cu interface. This proves that the pinned interfacial magnetization is strongly coupled to the antiferromagnetic order inside the bulk FeF 2 layer. Studies as a function of FeF2 crystalline orientation show that uncompensated spins are only detected in a spin valve with (110) crystal orientation, but not in valves containing FeF 2 (100) and FeF 2 (001). This observation is in agreement with symmetry-related considerations which predict the equilibrium boundary magnetization for the FeF 2 (110) layer.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Vanadium oxide coatings to self-regulate current sharing in high-temperature superconducting cables and magnets

High-temperature superconductors such as REBa 2 Cu 3 O 7-δ (REBCO, RE = rare earth) enable high-current cables and high-field magnets. By removing the turn-to-turn insulation in a magnet application, recent experiments demonstrated that REBCO magnets can self-protect against catastrophic damage during a superconducting-to-normal transition (quench), i.e., when the stored magnetic energy rapidly converts to heat. The current can bypass the hot spot during a quench, thereby reducing the localized heat dissipation. The removal of the insulation between turns, however, leads to excessive eddy currents during current ramping, thereby forcing a much-prolonged magnet charging time. To address this issue, we investigate vanadium oxide (VO x ) coatings as a temperature-dependent self-switching medium that automatically manages current sharing. VO x coatings (with 1.70 ≤ × ≤ to 2.07) were deposited by reactive cathodic arc deposition, initially on insulating glass to determine the electrical properties, and later on commercial REBCO tapes. The coatings are x-ray amorphous but with a short-range crystalline ordering according to Raman spectrometry. The resistivity of VO x decreased by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process is compatible with commercial REBCO tapes as evidenced by the negligible change in the critical current caused by the coating process. Here, the results from current sharing experiments and circuit analysis suggest that the VO x coating can effectively self-regulate current sharing in REBCO magnets, suppress excessive eddy currents, and enable self-protection during quenches.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1999

Non-Equilibrium Superconductivity and Magnetic Pair Breaking in Perovskite Half-Metallic Ferromagnet-Insulator-Superconductor (F-I-S) Heterostructures

The effect of spin-polarized quasiparticle currents on the critical current density (J-c) of cuprate superconductors is studied in perovskite F-I-S heterostructures as a function of insulator thickness and of underlying magnetic materials. A pulsed current technique is employed to minimize extraneous Joule heating on the superconductor. At temperatures near T-c, F-I-S samples with insulator thicknesses\1e2nm show precipitous decrease in J_c as current injection (I_m) is increased. In contrast, J_c in a controlled sample with a substituted non-magnetic material (N-I-S) exhibit no dependence on I_m. Similarly, a F-I-S sample with a 10 mn insulating barrier also show little J_c effect versus I_m. At low temperatures with I_m = 0, significant suppression of J-c is observed only in the thin barrier F-I-S samples, although T_c and the normal-state resistivity of all samples are comparable. These phenomena can be attributed to the Cooper pair breaking induced by externally-injected and internally-reflected spin-polarized quasiparticle currents. We estimate an order of magnitude range for the spin diffusion length of 100 nm to 100\ mum.

Fu, C.-C.↗
DOE OSTI2019

Atomically Thin CrCl 3 : An In-Plane Layered Antiferromagnetic Insulator

The recent discovery of magnetism in atomically thin layers of van der Waals (vdW) crystals has created new opportunities for exploring magnetic phenomena in the two-dimensional (2D) limit. In most 2D magnets studied to date, the c-axis is an easy axis, so that at zero applied field the polarization of each layer is perpendicular to the plane. In this work, we demonstrate that atomically thin CrCl 3 is a layered antiferromagnetic insulator with an easy-plane normal to the c-axis, that is, the polarization is in the plane of each layer and has no preferred direction within it. Ligand-field photoluminescence at 870 nm is observed down to the monolayer limit, demonstrating its insulating properties. We investigate the in-plane magnetic order using tunneling magnetoresistance in graphene/CrCl 3 /graphene tunnel junctions, establishing that the interlayer coupling is antiferromagnetic down to the bilayer. From the temperature dependence of the magnetoresistance, we obtain an effective magnetic phase diagram for the bilayer. Our findings suggest that CrCl 3 should be useful for studying the physics of 2D phase transitions and for making new kinds of vdW spintronic devices.

2D magnetic insulator↗
DOE OSTI2024

Hidden magnetism and split off flat bands in the insulator metal transition in VO 2

Transition metal d -electron oxides with an odd number of electrons per unit cell are expected to form metals with partially occupied energy bands, but exhibit in fact a range of behaviors, being either insulators, or metals, or having insulator-metal transitions. Traditional explanations involved predominantly electron-electron interactions in fixed structural symmetry. The present work focuses instead on the role of symmetry breaking local structural motifs. Viewing the previously observed V-V dimerization in VO 2 as a continuous knob, reveals in density functional calculations the splitting of an isolated flat band from the broad conduction band. This leads past a critical percent dimerization to the formation of the insulating phase while lowering the total energy. In VO 2 this transition is found to have a rather low energy barrier approaching the thermal energy at room temperature, suggesting energy-efficient switching in neuromorphic computing. Interestingly, sufficient V-V dimerization suppresses magnetism, leading to the nonmagnetic insulating state, whereas magnetism appears when dimerization is reduced, forming a metallic state. This study opens the way to design novel functional quantum materials with symmetry breaking-induced flat bands.

Chemistry↗
DOE OSTI2019

Magnetic field-tuned Fermi liquid in a Kondo insulator

Abstract Kondo insulators are expected to transform into metals under a sufficiently strong magnetic field. The closure of the insulating gap stems from the coupling of a magnetic field to the electron spin, yet the required strength of the magnetic field–typically of order 100 T–means that very little is known about this insulator-metal transition. Here we show that Ce $${}_{3}$$ 3 Bi $${}_{4}$$ 4 Pd $${}_{3}$$ 3 , owing to its fortuitously small gap, provides an ideal Kondo insulator for this investigation. A metallic Fermi liquid state is established above a critical magnetic field of only $${B}_{{\rm{c}}}\approx$$ B c ≈ 11 T. A peak in the strength of electronic correlations near $${B}_{{\rm{c}}}$$ B c , which is evident in transport and susceptibility measurements, suggests that Ce $${}_{3}$$ 3 Bi $${}_{4}$$ 4 Pd $${}_{3}$$ 3 may exhibit quantum criticality analogous to that reported in Kondo insulators under pressure. Metamagnetism and the breakdown of the Kondo coupling are also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Magnetic Properties of Iron-Doped Bi{sub 2}Se{sub 3}, a Topological Insulator

We study the dc magnetization of a Bi{sub 1.94}Fe{sub 0.06}Se{sub 3} single crystal in the temperature range of 1.9 to 400 K in applied magnetic fields up to 70 kOe for two sample orientations in the applied field. We register three magnetic phase transitions and observe considerable anisotropy of the magnetic properties. The coercive force varies with temperature in a nonmonotonic manner. For the sample in a paramagnetic state, we estimate its magnetic moment per iron atom; its Curie–Weiss temperature is found to be negative. Arrot–Belov plots are constructed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Insulating ground state and 2−𝑘 magnetic structure of candidate Weyl hydrogen-atom K2⁢Mn3⁢(AsO4)3

The ideal Weyl "hydrogen-atom" semimetal exhibits only a single pair of Weyl nodes and no other trivial states at the Fermi energy. Such a material would be a panacea in the study of Weyl quasi-particles, allowing direct unambiguous observation of their topological properties. The alluaudite-like K2⁢Mn3⁢(AsO4)3 compound was recently proposed as such a material. Here, we use comprehensive experimental work and first-principle calculations to assess this prediction. We find K2⁢Mn3⁢(AsO4)3 crystallizes in the 𝐶⁢2/𝑐 symmetry with a quasi-one-dimensional Mn sublattice, growing as small needle-like crystals. Bulk property measurements reveal magnetic transitions at ≈8 and ≈4 K, which neutron scattering experiments show correspond to two distinct magnetic orders, first a partially ordered ferrimagnetic 𝐤𝟏=(0,0,0) structure at 8 K and a second transition of 𝐤𝟐=(1,0,0) at 4 K to a fully ordered state. Below the second transition, both ordering vectors are necessary to describe the complex magnetic structure with modulated spin magnitudes. Both of the best-fit magnetic structures in this work are found to break the symmetry necessary for the generation of Weyl nodes, though one of the magnetic structures allowed by 𝐤𝟏 does preserve this symmetry. However, the crystals are optically transparent and ellipsometry measurements reveal a large band gap, undermining expectations of semimetallic behavior. Density functional theory calculations predict an insulating antiferromagnetic ground state, in contrast to previous reports, and suggest potential frustration on the magnetic sublattice. Given the wide tunability of the alluaudite structure, we consider ways to push the system closer to a semimetallic state.

Taddei, Keith M [Argonne National Laboratory]↗
DOE OSTI2018

Crossover from itinerant to localized magnetic excitations through the metal-insulator transition in NaOsO 3

NaOsO 3 undergoes a metal-insulator transition (MIT) at 410 K, concomitant with the onset of antiferromagnetic order. The excitation spectra have been investigated through the MIT by resonant inelastic x-ray scattering (RIXS) at the Os L 3 edge. Low resolution (ΔE ~ 300 meV) measurements over a wide range of energies reveal that local electronic excitations do not change appreciably through the MIT. This is consistent with a picture in which structural distortions do not drive the MIT. In contrast, high resolution (ΔE ~ 56 meV) measurements show that the well-defined, low-energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in NaOsO 3 . In particular, the presence of weak correlations in the paramagnetic phase implies a degree of departure from the ideal Slater limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Block Magnetic Excitations in the Orbitally Selective Mott Insulator BaFe 2 Se 3

Iron pnictides and selenides display a variety of unusual magnetic phases originating from the interplay between electronic, orbital, and lattice degrees of freedom. Using powder inelastic neutron scattering on the two-leg ladder $BaFe_2Se_3$, we fully characterize the static and dynamic spin correlations associated with the Fe 4 block state, an exotic magnetic ground state observed in this low-dimensional magnet and in Rb 0.89 Fe 1.58 Se 2 . All the magnetic excitations of the Fe 4 block state predicted by an effective Heisenberg model with localized spins are observed below 300 meV and quantitatively reproduced. However, the data only account for 16(3)$μ^2_B$ per Fe 2+ , approximatively 2/3 of the total spectral weight expected for localized S=2 moments. Our results highlight how orbital degrees of freedom in iron-based magnets can conspire to stabilize an exotic magnetic state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Characterization of Candidate Insulation Resins for Training Reduction in High Energy Physics Magnets

Superconducting magnets are critical components in particle accelerators and are used to generate and sustain the large magnetic fields needed for High Energy Physics programs. One significant issue with current epoxy insulated Nb$_{3}$Sn magnets is the long training process required before stable magnet performance can be realized. It is believed that training can be significantly reduced by addressing magnet quenching through improvements in the epoxy electrical insulation. In this work, two approaches for insulation modification have been undertaken: (1) addition of thermally conductive fillers to help with quench management and (2) development of insulation resins with high strain capability at cryogenic temperatures. This paper will discuss the characterization of these insulation systems to verify their performance prior to evaluation in subscale Nb$_{3}$Sn canted cosine theta accelerator dipole magnets.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Magnetic and Electronic Properties of Gd-Doped Topological Insulator Bi{sub 1.09}Gd{sub 0.06}Sb{sub 0.85}Te{sub 3}

The recent realization of quantum anomalous Hall effect and Majorana fermions observation enhance interest in magnetism investigation in topological insulators. In this work, the electronic and magnetic structure of the Gd-doped topological insulator Bi{sub 1.09}Gd{sub 0.06}Sb{sub 0.85}Te{sub 3} were systematically studied by means of angle-resolved photoemission spectroscopy, resonance photoemission spectroscopy (ResPES) and SQUID magnetometry. Resonant features related to the Gd density of states near the Fermi level are experimentally observed. Study of magnetic structure showed antiferromagnetic ordered bulk at low temperatures as well as presence of hysteresis loop at elevated temperatures. Finally, possible mechanism of magnetism and its relation to observed electronic features are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Detection of the metal–insulator transition in disordered systems of magnetic nanoislands

We have studied the conductivity and permittivity of a series of nanoisland-type FeNi films with an effective thickness of up to 3.2 nm on different substrates. It has been observed that the quantity Re ε changes its sign at effective thickness d{sup *} ≈ 1.5–1.8 nm, because of the metal–insulator transition. Analysis of the temperature dependences of the conductivity has confirmed the existence of the metal–insulator transition at the same thickness d{sup *}. It has been concluded that the introduced effective permittivity can serve as a characteristic of island metal systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Magnetic Imaging of Domain Walls in the Antiferromagnetic Topological Insulator MnBi 2 Te 4

The control of domain walls or spin textures is crucial for spintronic applications of antiferromagnets. Despite many efforts, it has been challenging to directly visualize antiferromagnetic domains or domain walls with nanoscale resolution, especially in magnetic field. Here, we report magnetic imaging of domain walls in several uniaxial antiferromagnets, the topological insulator MnBi 2 Te 4 family, using cryogenic magnetic force microscopy (MFM). Our MFM results reveal higher magnetic susceptibility inside the domain walls than in domains. Domain walls in these antiferromagnets form randomly with strong thermal and magnetic field dependence. Here, the direct visualization of these domain walls and domain structures in the magnetic field will not only facilitate the exploration of intrinsic topological phenomena in antiferromagnetic topological insulators but will also open a new path toward control and manipulation of domain walls or spin textures in functional antiferromagnets.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗