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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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Critical-Element-Free Permanent-Magnet Materials Based on Ce 2 Fe 14 B

Developing a critical-element-free low-cost permanent magnet is an urgent necessity in view of rapidly developing technologies and the associated huge market demand for Nd 2 Fe 14 B-based magnets. Here, inspired by the abundant and low-cost nature of Ce and these high-performance Nd 2 Fe 14 B permanent magnets, we explore whether it is in fact possible to attain a useful performance in alloys based on the sister material Ce 2 Fe 14 B, employing both experimental and theoretical efforts. Experimentally, we study Ce 2 Fe 14 B with Co, La, and Zr substitutions. The Zr substitution is explored in view of Zr’s frequent role in enhancing magnetic anisotropy in permanent magnets, while the Co and La substitutions serve to remedy the too-low Curie point of 433 K in the base alloy. While we find no Zr-related anisotropy enhancement either experimentally or theoretically, the cosubstitution of La and Co indeed improves the Curie temperature as well as the magnetization, M s , with a potential energy product as high as 38 MG Oe. These properties together suggest optimization of the alloy LaCeFe 12.7 Co 1.3 B (with only 7 wt % cobalt) as a critical-element-free permanent magnet. While the substituted elements do not enhance magnetic anisotropy, from theory, we find a substantial increase, to a first anisotropy constant, K1, as high as 4.24 MJ/m 3 , associated with Bi substitution for Ce. In this paper, our experimental and theoretical results demonstrate the great potential of La, Co, and Bi substitutions in developing low-cost and critical-element-free Ce 2 Fe 14 B-based permanent magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spontaneous Polarization in an Ultrathin Improper-Ferroelectric/Dielectric Bilayer in a Capacitor Structure at Cryogenic Temperatures

To determine the effect of depolarization and the critical thickness in improper-ferroelectric hexagonal-ferrite thin films, we investigate the polarization switching of a ferroelectric/dielectric bilayer in capacitor structures at 20 K. Experimentally, we show that the spontaneous polarization persists throughout the studied thickness range (3 to 80 unit cell), even with a thick (10-nm) dielectric layer, suggesting no practical thickness limit for applications. By fitting the effect of depolarization using the phenomenological theory, we show that the spontaneous polarization remains finite when the thickness of the ferroelectric layer approaches zero, providing a hint for the absence of critical thickness. We also find that the interfacial effects limit the multidomain formation and govern the polarization switching mechanisms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Robust block magnetism in the spin ladder compound BaFe 2 Se 3 under hydrostatic pressure

The majority of the iron-based superconductors (FeSCs) exhibit a two-dimensional square lattice structure. Recent reports of pressure-induced superconductivity in the spin-ladder system, BaFe 2 X 3 (X=S, Se), introduce a quasi-one-dimensional prototype and an insulating parent compound to the FeSCs. In this work, we report x-ray, neutron diffraction, and muon spin relaxation experiments on BaFe 2 Se 3 under hydrostatic pressure to investigate its magnetic and structural properties across the pressure-temperature phase diagram. A structural phase transition was found at a pressure of 3.7(3) GPa. Neutron diffraction measurements at 6.8(3) GPa and 120 K show that the block magnetism persists even at these high pressures. A steady increase and then fast drop of the magnetic transition temperature T N and greatly reduced moment above the pressure P s indicate potentially rich and competing phases close to the superconducting phase in this ladder system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Field-free platform for Majorana-like zero mode in superconductors with a topological surface state

Superconducting materials exhibiting topological properties are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. In this letter, we explore the possibility of a field-free platform for generating Majorana zero energy excitations by depositing magnetic Fe impurities on the surface of candidate topological superconductors, LiFeAs and PbTaSe 2 . We use scanning tunneling microscopy to probe localized states induced at the Fe adatoms on the atomic scale and at sub-Kelvin temperatures. We find that each Fe adatom generates a striking zero-energy bound state inside the superconducting gap, which do not split in magnetic fields up to 8 T, underlining a nontrivial topological origin. Finally, our findings point to magnetic Fe adatoms evaporated on bulk superconductors with topological surface states for exploring Majorana zero modes and quantum information science under field-free conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Magnetocaloric effect in the vicinity of the magnetic phase transition in NdCo 2–x Fe x compounds

In the present paper, the magnetocaloric effect (MCE) of NdCo 2-x Fe x (x = 0, 0.2, 0.4, 0.6) compounds was investigated by magnetization measurements. Here, the temperature-dependent high-resolution synchrotron x-ray diffraction study shows a magnetostructural transition from the paramagnetic cubic phase to the ferromagnetic tetragonal phase below their Curie temperatures. Differential scanning calorimetry analysis shows the absence of thermal hysteresis, indicating the second-order nature of the magnetostructural phase transition in these compounds. The maximum values of magnetic entropy change (ΔS M ) and wide operating temperature (OT) are obtained under a field change of 5 T, which are 7.33 Jkg -1 K -1 , 6.45 Jkg -1 K -1 , 5.71 Jkg -1 K -1 , 4.70 Jkg -1 K -1 and 78 K, 82 K, 85 K, 92 K for x = 0, 0.2, 0.4, and 0.6, respectively. The corresponding values of relative cooling power (RCP) are 529.96 Jkg -1 , 497.25 Jkg -1 , 470.55 Jkg -1 , and 428.31 Jkg -1 . The observed wide OT range and large RCP values are comparable with Gd, Gd 5 Ge 2 Si 2 , and some rare-earth-based giant magnetocaloric materials, making this series of compounds suitable for magnetic refrigeration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Structural, magnetic, and electronic evolution of the spin-ladder system BaFe 2 S 3-x Se x with isoelectronic substitution

We reWe report experimental studies of a series of BaF e 2 S 3 - x Se x ( 0 ≤ $x$ ≤ 3 ) single crystals and powder specimens using x-ray diffraction, neutron-diffraction, muon-spin-relaxation, and electrical transport measurements. A structural transformation from $C$ $mcm$ ( BaFe 2 Se 3 ) to $P$ $nma$ ( BaFe 2 Se 3 ) was identified around $x$ = 0.7 - 1 . Neutron-diffraction measurements on the samples with $x$ = 0.2, 0.4, and 0.7 reveal that the Néel temperature of the stripe antiferromagnetic order is gradually suppressed from ~120 to 85 K, while the magnitude of the ordered Fe 2+ moments shows very little variation. Similarly, the block antiferromagnetic order in BaFe 2 Se 3 remains robust for 1.5 ≤ $x$ ≤ 3 with negligible variation in the ordered moment and a slight decrease of the Néel temperature from 250 K ( $x$ = 3 ) to 225 K ( $x$ = 1.5 ). The sample with $x$ = 1 near the $C$ $mcm$ and $P$ $nma$ border shows coexisting, two-dimensional, short-range stripe- and block-type antiferromagnetic correlations. The system remains insulating for all $x$ , but the thermal activation gap shows an abrupt increase when traversing the boundary from the $C$ $mcm$ stripe phase to the $P$ $nma$ block phase. The results demonstrate that the crystal structure, magnetic order, and electronic properties are strongly coupled in the BaFe 2 S 3-x Se x system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Orbital selective spin waves in detwinned NaFeAs

The existence of orbital-dependent electronic correlations has been recognized as an essential ingredient to describe the physics of iron-based superconductors. NaFeAs, a parent compound of iron-based superconductors, exhibits a tetragonal-to-orthorhombic lattice distortion below T s ≈ 60 K, forming an electronic nematic phase with two 90° rotated (twinned) domains, and orders antiferromagnetically below T N ≈ 42 K. We use inelastic neutron scattering to study spin waves in uniaxial pressure-detwinned NaFeAs. By comparing the data with combined density functional theory and dynamical mean-field theory calculations, we conclude that spin waves up to an energy scale of E crossover ≈ 100 meV are dominated by d yz -d yz intraorbital scattering processes, which have the twofold (C 2 ) rotational symmetry of the underlying lattice. On the other hand, the spin wave excitations above E crossover , which have approximately fourfold (C 4 ) rotational symmetry, arise from the d xy -d xy intraorbital scattering that controls the overall magnetic bandwidth in this material. In addition, we find that the low-energy (E ≈ 6 meV) spin excitations change from approximate C 4 to C 2 rotational symmetry below a temperature T* (>T s ), while spin excitations at energies above E crossover have approximate C 4 rotational symmetry and are weakly temperature dependent. Here, these results are consistent with angle-resolved photoemission spectroscopy measurements, where the presence of a uniaxial strain necessary to detwin NaFeAs also raises the onset temperature T* of observable orbital-dependent band splitting to above T s , thus supporting the notion of orbital selective spin waves in the nematic phase of iron-based superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Orbital selectivity of layer-resolved tunneling in the iron-based superconductor Ba 0.6 K 0.4 Fe 2 As 2

Here, we use scanning tunneling microscopy/spectroscopy to elucidate the Cooper pairing of the iron pnictide superconductor Ba 0.6 K 0.4 Fe 2 As 2 . By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer identity. By cross referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct Fe-3$\textit{d}$ orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a general concept that, for complex multiorbital material, tunneling on different terminating layers can feature orbital selectivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Three-dimensional Ising ferrimagnetism of Cr-Fe-Cr trimers in FeCr2Te4

We carried out a comprehensive study of magnetic critical behavior in single crystals of ternary chalcogenide Fe Cr 2 Te 4 that undergoes a ferrimagnetic transition below T c ~ 123 K. Detailed critical behavior analysis and scaled magnetic entropy change indicate a second-order ferrimagnetic transition. Critical exponents β = 0.30 ( 1 ) with T c = 122.4 ( 5 ) K , γ = 1.22 ( 1 ) with T c = 122.8 ( 1 ) K , and δ = 4.24 ( 2 ) at T c ~ 123 K suggest that the spins approach the three-dimensional Ising ( β = 0.325 , γ = 1.24 , and δ = 4.82 ) model coupled with attractive long-range interactions between spins that decay as J ( r ) ≈ r - 4.88 . Our results suggest that the ferrimagnetism in Fe Cr 2 Te 4 is due to itinerant ferromagnetism among the antiferromagnetically coupled Cr-Fe-Cr trimers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb 0.25 Bi 2 Se 3

Recently, the niobium (Nb) doped topological insulator Bi 2 Se 3 , in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi 2 Se 3 layers, has been shown to exhibit both superconductivity with $T_c$ ≃ 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth $λ_{\text{eff}}(T)$ in the layered topological superconductor candidate Nb 0.25 Bi 2 Se 3 . In this work the exponential temperature dependence of $λ^{–2}_{\text{eff}}(T)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature $T_c$ = 2.9 K and the gap to $T_c$ ratio 2Δ /$k_BT_c$ = 3.95(19). We also reveal that the ratio Tc/λ–2eff is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field μSR experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of Nb 0.25 Bi 2 Se 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Prediction of superconductivity and topological aspects in single-layer β -Bi 2 Pd

Topological superconductors, characterized by topologically nontrivial states residing in a superconducting gap, are a recently discovered class of materials having Majorana fermions. The interplay of superconductivity and topological states gives rise to opportunities for achieving such topological superconductors in condensed matter systems. Up to now, several single-material topological superconductors in this form have been theoretically predicted and experimentally confirmed. Here, using the first-principles calculations, we study the superconducting single-layer β-Bi 2 Pd. In this work, the electronic density of states near Fermi level of this monolayer are dominated by the Bi-p and Pd-d orbitals, forming a two-band Fermi surface with multiclass sheets. The presence of soft phonon bands, in cooperation with the electron susceptibility, accounts for electron-phonon superconductivity of single-layer β-Bi 2 Pd. With the centrosymmetric structure, single-layer β-Bi 2 Pd possesses a continuous gap over the whole Brillouin zone and topological Dirac-like states at its one-dimensional boundary. The present findings would lead to the expectation of one-dimensional topological superconductivity and Majorana bound states in a monolayer candidate of β-Bi 2 Pd with intrinsic full-gap superconductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Spinodal electronic phase separation during insulator-metal transitions

Electronic phase transitions such as insulator-metal transitions are common in strongly correlated systems. Here, using a combination of thermodynamic linear-stability analysis and phase-field simulations and employing VO 2 as a prototypical example, we predict that an insulator-metal transition driven by photoexcitation may involve an intermediate, modulated charge density state with a temperature-dependent characteristic wavelength. It is shown that such an intermediate two-phase electronic state is formed through a spinodal mechanism and that its formation can be generic for insulator-metal transitions driven by fast stimuli. This transient electronic phase separation is expected to stimulate future experimental and computational efforts.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetic order and fluctuations in the quasi-two-dimensional planar magnet Sr( Co 1-x Ni x ) 2 As 2

We use neutron scattering to investigate spin excitations in Sr ( Co 1 - x Ni x ) 2 As 2 , which has a c -axis incommensurate helical structure of the two-dimensional (2D) in-plane ferromagnetic (FM) ordered layers for 0.013 ≤ x ≤ 0.25 . By comparing the wave vector and energy dependent spin excitations in helical ordered Sr ( Co 0.9 Ni 0.1 ) 2 As 2 and paramagnetic SrCo 2 As 2 , we find that Ni doping, while increasing lattice disorder in Sr ( Co 1 - x Ni x ) 2 As 2 , enhances quasi-2D FM spin fluctuations. However, our band structure calculations within the combined density functional theory and dynamic mean field theory ( DFT + DMFT ) failed to generate a correct incommensurate wave vector for the observed helical order from nested Fermi surfaces. Furthermore, since transport measurements reveal increased in-plane and c -axis electrical resistivity with increasing Ni doping and associated lattice disorder, we conclude that the helical magnetic order in Sr ( Co 1 - x Ni x ) 2 As 2 may arise from a quantum order-by-disorder mechanism through the itinerant electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Nonmonotonic crossover in electronic phase separated manganite superlattices driven by the superlattice period

Studying manganite superlattices [(LCMO) 2n / (PCMO) n ] t made of La 0.625 Ca 0.375 MnO 3 (LCMO) and Pr 0.625 Ca 0.375 MnO 3 (PCMO), we found an unexpected behavior varying the period n. At small n, the ensemble is a three-dimensional ferromagnetic metal due to interfacial charge transfer. At large n , the LCMO layers dominate transport. However, rather than a smooth interpolation between these limits a sharp transport and magnetic anomaly is found at an intermediate critical PCMO thickness n*. Magnetic force microscopy reveals that the phase-separation length scale also maximizes at n* where, unexpectedly, it becomes comparable to that of the (La 1-y Pr y ) 0.625 Ca 0.375 MnO 3 (LPCMO) alloy. Finally, we conjecture the phenomenon originates in a disorder-related length scale: Large charge-ordered clusters as in LPCMO can only nucleate when Pr-rich regions reach a critical size related to n*.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Field-induced topological Hall effect and double-fan spin structure with a $c$-axis component in the metallic kagome antiferromagnetic compound YMn 6 Sn 6

Geometric frustration in the kagome lattice makes it a great host for the flat electronic band, nontrivial topological properties, and novel magnetism. In this work, we use magnetotransport measurements to map out the field-temperature phase diagram of the centrosymmetric YMn 6 Sn 6 with a Mn kagome lattice and show that the system exhibits the topological Hall effect (THE) with an in-plane applied magnetic field around 240 K. In addition, our neutron diffraction results demonstrate that the observed THE cannot arise from a magnetic skyrmion lattice, but instead from an in-plane field-induced double-fan spin structure with c-axis components. This paper provides a platform to understand the influence of a field-induced novel magnetic structure on magnetoelectric response in topological kagome metals.

36 MATERIALS SCIENCE↗