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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Crystal Growth and Elemental Homogeneity of the Multicomponent Rare-Earth Garnet (Lu 1/6 Y 1/6 Ho 1/6 Dy 1/6 Tb 1/6 Gd 1/6 ) 3 Al 5 O 12

We report high-entropy aluminum garnets were grown as bulk single crystals using the micro-pulling-down method, taking the synthesis of complex ceramics a step further from the conventional preparation of polycrystalline materials. We studied the effects of growth parameters on the elemental distribution in high optical quality crystals of (Lu 1/6 Y 1/6 Ho 1/6 Dy 1/6 Tb 1/6 Gd 1/6 ) 3 Al 5 O 12 containing six cations (yttrium and rare-earths) taken in equimolar amounts. A single garnet structure was confirmed by powder X-ray diffraction. Electron microprobe measurements were obtained to correlate the radial distribution of rare-earth elements with pulling rates and molten zone height. The nature of the elemental distribution in the radial direction was associated with ionic radius: smaller rare-earths concentrated in the center of the crystal, while larger rare-earths segregated toward the outer edge of the cylindrical crystal. Faster pulling rates led to a flattening of the concentration profiles toward the nominal concentration, promoting a more homogeneous radial elemental distribution, while varying the molten zone height did not have a significant effect. The demonstrated success with crystal growth enables the practical availability of single crystals of multicomponent aluminum garnets for further discovery of new phenomena and applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tunable Magnetic Exchange between Rare-Earth Metal 5d and Iron 3d States: A Case Study of the Multiple Magnetic Transitions in Gd 6 FeBi 2 and the Solid Solutions Dy 6– x Gd x FeBi 2 (1 ≤ x ≤ 5) with Curie Temperatures in the Range 120–350 K

In this paper, the room-temperature magnet Gd 6 FeBi 2 was comprehensively characterized by means of temperature-dependent single-crystal X-ray diffraction, magnetization measurements, and experimental electron density and electronic structure computations. This work explores the electron-spin effects of Fe on this structure and suggests that Gd6FeBi2 shows structural features and exchange interactions, which are clearly distinguishable from other analogues, including the solid solutions Dy 6–x Gd x FeBi 2 (0 ≤ x ≤ 5). The unique traits of Gd 6 FeBi 2 and its derivatives encompass abnormal variations of lattice parameters with the temperature, as well as the presence of multiple magnetic transitions of complex nature. Based on the comprehensive analyses, it can be suggested that the unique magnetic response in this material is the result from tunable Fe spin states, which are coupled with strong Gd 5d–Fe 3d interactions, which differ from other members of this large, structural family. In the ground state, according to density functional theory calculations, the Fe atom carries two net electrons, whose spins are coupled in an antiparallel fashion to the spins of the Gd electrons. Near the Curie temperature, the Fe net moment is reduced intermittently, coupled with a multiple of magnetic transitions. The magnetic correlations are manifested in unexpected variations of the lattice parameters as a function of temperature, suggesting spin–lattice interactions. Furthermore, this study emphasizes the important role of Gd 5d electrons in tuning Fe 3d-based magnetic contribution, which enables better understanding on the spin coupling in other related compounds and sheds light on the development of new magnetic and spintronic compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Anomalous magnetic noise in an imperfectly flat landscape in the topological magnet Dy 2 Ti 2 O 7

Noise generated by motion of charge and spin provides a unique window into materials at the atomic scale. From temperature of resistors to electrons breaking into fractional quasiparticles, “listening” to the noise spectrum is a powerful way to decode underlying dynamics. Here, we use ultrasensitive superconducting quantum interference device (SQUIDs) to probe the puzzling noise in a frustrated magnet, the spin-ice compound Dy 2 Ti 2 O 7 (DTO), revealing cooperative and memory effects. DTO is a topological magnet in three dimensions—characterized by emergent magnetostatics and telltale fractionalized magnetic monopole quasiparticles—whose real-time dynamical properties have been an enigma from the very beginning. We show that DTO exhibits highly anomalous noise spectra, differing significantly from the expected Brownian noise of monopole random walks, in three qualitatively different regimes: equilibrium spin ice, a “frozen” regime extending to ultralow temperatures, and a high-temperature “anomalous” paramagnet. We present several distinct mechanisms that give rise to varied colored noise spectra. In addition, we identify the structure of the local spin-flip dynamics as a crucial ingredient for any modeling. Thus, the dynamics of spin ice reflects the interplay of local dynamics with emergent topological degrees of freedom and a frustration-generated imperfectly flat energy landscape, and as such, it points to intriguing cooperative and memory effects for a broad class of magnetic materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Low-temperature high-frequency dynamic magnetic susceptibility of classical spin-ice Dy 2 Ti 2 O 7

In this study, radio-frequency (14.6 MHz) AC magnetic susceptibility, , of Dy 2 Ti 2 O 7 was measured using self-oscillating tunnel-diode resonator. Measurements were made with the excitation AC field parallel to the superimposed DC magnetic field up to 5 T in a wide temperature range from 50 mK to 100 K. At 14.6 MHz, a known broad peak of from kHz—range audio-frequency measurements around 15 K for both [111] and [110] directions shifts to 45 K, continuing the Arrhenius activated behavior with the same activation energy barrier of E a ≈ 230 K. Magnetic field dependence of along [111] reproduces previously reported low-temperature two-in-two-out to three-in-one-out spin configuration transition at about 1 T, and an intermediate phase between 1 and 1.5 T. The boundaries of the intermediate phase show reasonable overlap with the literature data and connect at a critical endpoint of the first order transition line, suggesting that these features are frequency independent. An unusual upturn of the magnetic susceptibility at T → 0 was observed in magnetic fields between 1.5 T and 2 T for both magnetic field directions, before fully polarized configuration sets in above 2 T.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Robust Kagome Electronic Structure in Topological Quantum Magnets XMn 6 Sn 6 (X = Dy, Tb, Gd, Y)

Crystal geometry can greatly influence the emergent properties of quantum materials. As an example, the kagome lattice is an ideal platform to study the rich interplay between topology, magnetism, and electronic correlation. In this work, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of XMn 6 Sn 6 (X=Dy,Tb,Gd,Y) family compounds. Here, we observe the Dirac fermion and the flat band arising from the magnetic kagome lattice of Mn atoms. Interestingly, the flat band locates in the same energy region in all compounds studied, regardless of their different magnetic ground states and 4f electronic configurations. These observations suggest a robust Mn magnetic kagome lattice across the XMn 6 Sn 6 family, thus providing an ideal platform for the search for, and investigation of, new emergent phenomena in magnetic topological materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Δ$\textit{K}=0 \textit{M}1$ Excitation Strength of the Well-Deformed Nucleus 164 Dy from $\textit{K}$ Mixing

We report the size of a Δ$\textit{K}=0 \textit{M}1$ excitation strength has been determined for the first time in a predominantly axially deformed even-even nucleus. It has been obtained from the observation of a rare $\textit{K}$-mixing situation between two close-lying $J^π = 1^+$ states of the nucleus 164 Dy with components characterized by intrinsic projection quantum numbers $\textit{K}$ = 0 and $\textit{K}$ = 1. Nuclear resonance fluorescence induced by quasimonochromatic linearly polarized γ-ray beams provided evidence for $\textit{K}$ mixing of the 1 + states at 3159.1(3) and 3173.6(3) keV in excitation energy from their γ-decay branching ratios into the ground-state band. The Δ$\textit{K}$ = 0 transition strength of $B(M1; 0^+_1 → 1^+_{K=0}) = 0.008(1)μ^2_N$ was inferred from a mixing analysis of their $\textit{M}$1 transition rates into the ground-state band. It is in agreement with predictions from the quasiparticle phonon nuclear model. This determination represents first experimental information on the $\textit{M}$1 excitation strength of a nuclear quantum state with a negative $\mathcal{R}$-symmetry quantum number.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Structure of the (110) Ln ScO 3 ( Ln =Gd,Tb,Dy) surfaces

The surface reconstruction on the (110) surface of LnScO 3 (Ln=Gd,Tb,Dy) has been investigated using x-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron diffraction coupled with higher-level density-functional theory methods. The experimental techniques generate constraints on the surface chemistry and structure that are used to inform the theoretical comparison of a number of potential surface structures. The resulting structure is then compared to experimental aberration-corrected transmission electron microscopy results using multislice simulation. The surfaces of both single-crystalline substrates and hydrosauna synthesized nanoparticles exhibit a Sc-rich double layer with a Sc 3 O 4 termination.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Homogeneous superconducting gap in Dy Ba 2 Cu 3 O 7 - δ synthesized by oxide molecular beam epitaxy

Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate Dy Ba 2 Cu 3 O 7 - δ DBCO). We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film—it is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CU O 2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one (at ~ 18 meV ) corresponding to the superconductivity and the other one (at ~ 90 meV ) to the pseudogap. Finally, while the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Crystal structures and comparisons of huntite aluminum borates RE Al 3 (BO 3 ) 4 ( RE = Tb, Dy and Ho)

Three huntite-type aluminoborates of stoichiometry RE Al 3 (BO 3 ) 4 ( RE = Tb, Dy and Ho), namely, terbium/dysprosium/holmium trialuminium tetrakis(borate), were synthesized by slow cooling within a K 2 Mo 3 O 10 flux with spontaneous crystallization. The crystal structures were determined using single-crystal X-ray diffraction (SC-XRD) data. The synthesized borates are isostructural to the huntite [CaMg 3 (CO 3 ) 4 ] structure and crystallized within the trigonal R 32 space group. The structural parameters were compared to literature data of other huntite RE Al 3 (BO 3 ) 4 crystals within the R 32 space group. All three borates fit well into the trends calculated from the literature data. The unit-cell parameters and volumes increase linearly with larger RE cations whereas the densities decrease. All of the crystals studied were refined as inversion twins.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Crystal structures and comparisons of potassium rare-earth molybdates KRE(MoO 4 ) 2 (RE = Tb, Dy, Ho, Er, Yb, and Lu)

Six potassium rare-earth molybdates KRE(MoO 4 ) 2 (RE = Tb, Dy, Ho, Er, Yb, and Lu) were synthesized by flux-assisted growth in K 2 Mo 3 O 10 . The crystal structures were determined using single-crystal X-ray diffraction data. The synthesized molybdates crystallize with the orthorhombic Pbcn space group (No. 60). Trendlines for unit-cell parameters were calculated using data from the current study. The unit-cell parameters a and c increase linearly whereas b decreases with larger RE cations, based on crystal radii. The unit-cell volumes increase linearly and the densities decrease linearly with larger RE cations. The average distances between the RE cations and the nearest O atoms increase with larger cations whereas the average distances of Mo—O and K—O do not show specific trends.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Development of Dy-free Nd-Fe-B-based sintered magnet through grain boundary engineering using Pr-Cu alloys

For this study, Dy-free sintered magnets were fabricated by mixing Nd-Fe-B powder with various amounts of Pr-Cu powder followed by the common procedures for making Nd-Fe-B based sintered magnets. With the Pr-Cu addition increasing from 0 to 10 wt.%, the obtained magnets’ H cj increases from the original 14.5 kOe to 18.6 kOe. The highest (BH) max achieved was 35.0 MGOe with 7.5 wt.% PrCu. The distribution of the Pr and Cu elements was primarily at grain boundary and triple junctions, leading to a reduced coupling among grains, thus an enhanced H cj .

36 MATERIALS SCIENCE↗
DOE OSTI2021

Role of Pr-Vacancies and O-Interstitials on the Activity and Stability of (Pr 1−x Ln x ) 2 NiO 4 (Ln = La, Nd, Pm, Sm, Gd, Tb, Dy, and Ho) towards Oxygen Reduction Reactions: A DFT Study

Praseodymium nickelate, Pr 2 NiO 4 (PNO), is a promising electrode to promote oxygen reduction reaction (ORR) in a solid oxide fuel cell, but it exhibits phase transformation during electrochemical operation. The origin of the simultaneous phase transformation and high electrochemical performance remains obscure. We carried out a systematic density functional theory study to elucidate the mechanism for this conjugated phenomenon. Charge, electronic structure, and normal-mode analysis suggest the presence of peroxide. Our study shows that the formation of peroxide (O 2 2– ) is attributed to both oxygen interstitials and Pr vacancies. The peroxide species limits the oxygen ion migration due to the additional energy required to break its O–O bond, which leads to a decrease in ORR activity. Subsequently, we investigate the diffusion paths of Pr-ions while comparing them with those of other Ln 3+ ions (La, Nd, Pm, Sm, Gd, Tb, Dy, and Ho) in PNO. The formation energies for various Ln 3+ cation occupancies are calculated, as well as segregation energies in CeO 2 (111) surfaces. Lastly, criteria for effective Ln 3+ dopants are developed. La, Nd, and Pm are proposed as potential substituents in PNO to obtain a stable structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1990

Millimeter wave surface resistance of RBa2Cu3O(7-delta) (R=Y,Eu,Dy,Sm,Er) superconductors

The measurements are reported of the millimeter wave surface resistance R(sub s) at 58.6 GHz of bulk samples of RBa2Cu3O(7-delta) (R = Y,Eu,Dy,Sm,Er) and of YBa2Cu3O(7-delta) superconducting films, in the temperature range from 20 to 300 K. The bulk samples were prepared by cold pressing the powders of RBa2Cu3O(7-delta) into one in. disks. The powders were prepared by several sinterings in one atmosphere of oxygen at 925 C, with grindings between sinterings, to obtain the superconducting phase. The thin films were deposited on SrTiO3 and LaGaO3 substrates by pulsed laser ablation. Each sample was measured by replacing the end wall of a gold-plated Te sub 013 circular mode copper cavity with the sample and determining the cavity quality factor . From the difference in the Q-factor of the cavity, with and without the sample, the R(sub s) of the sample was determined.

Miranda, F. A.↗
NASA NTRS1994

Preparation and Dielectric Measurements of the Rare Earth Green Phases R2BaCuO(5-x) (R = Y, Sm, Gd, Dy, Ho, Er, Yb)

It has been demonstrated that R2BaCuO(5-x) (R = Y, Sm, Gd, Dy, Ho, Er, Yb) does not undergo significant densification unless the sintering temperatures are near the incongruent melting point or the sintering times are long. Good quality powders of Y2BaCuO(5-x) have been synthesized by using oxide raw materials or precursors such as acetates and nitrates. The acetates- and the nitrates-derived yttrium green phase resulted in finer particle sizes, acceptable dielectric properties and lower melting temperatures than those processed via oxide raw materials. The hot pressing technique has been employed to produce a dense R2BaCuO(5-x) (R=Y,Gd) substrate with satisfactory dielectric properties. Reactivity to reducing conditions, i.e. graphite die, limited the optimization of the properties. A high sensitivity to the annealing atmosphere has been demonstrated in Y2BaCuO,.,,. Oxygen treatment at 950 OC has been shown to improve the dielectric properties while treatment in nitrogen, at the same temperature, degraded desirable properties. A high sensitivity to the annealing atmosphere has been demonstrated in Y2BaCuO(5-x). Oxygen treatment at 950 C has been shown to improve the dielectric properties while treatment in nitrogen, at the same temperature, degraded desirable properties. The dielectric constants of the rare earth green phases R2BaCuO(5-x) were found to be low. Relaxation peaks were detected at low temperatures (T less than 150 K) and at high temperatures (150 less than T greater than 420 K). The dielectric losses and conductivities at 77 K were measured to be in the range of 10(exp -4) and 10(exp -12) (Omega-cm)(exp -1), respectively. Many parameters were found to exhibit dependencies on the rare earth cation sizes.

Gonzalez-Titman, Carlos↗
DOE OSTI2020

Materials Data on Dy(FeO2)2 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗