Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Stack Sampling”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Controlling Oxidation of Nb in Oxygen Abundant Environments.

Superconducting Radio Frequency (SRF) cavities, the core of modern particle accelerators, rely on high-purity niobium (Nb) to achieve high quality factors and accelerating gradients. However, native oxidation in ambient conditions introduces a complex oxide layer with increased surface defects and contamination, particularly from carbon. This research explores a controlled oxidation processes in an oxygen-abundant environment to try to control this complex oxide stack, with the goal of improving surface purity and uniformity. Using standard surface preparation techniques—including electropolishing, vacuum baking, and sonication—followed by a custom oxidation protocol, we compared structural and chemical changes through Confocal Microscopy, Scanning Electron Microscopy (SEM), and X-Ray Photoelectron Spectroscopy (XPS). Results show that iterative oxidation cycles can significantly reduce carbon-related defects, even without baking. Aditionally, baked samples with a specific oxidation technique demonstrated a measurable 25\% decrease in carbide formation.

Romero, Juan [Fermilab]↗
DOE OSTI2025

Assessment of thermally driven local structural phase changes in 1⁢𝑇′−MoTe 2

The role of layer disorder is important in establishing the topological phases of MoTe 2 . A rich tapestry of atomic ordering influences the structural phase transitions (SPTs), but there is little understanding of the mechanistic details of the phase transition. An atomistic level study was conducted to investigate the local structure of the 1⁢𝑇′ and 𝑇 𝑑 phases of MoTe 2 by using the pair distribution function (PDF) technique. While the average structure exhibits an SPT and coexistence of phases as a function of temperature, the local structure showed the suppression of SPT. The sample retained its monoclinic structure at all temperatures in short-range order. A sharp PDF peak observed at short distances indicated a strong atom-atom correlation between the Mo and Te atoms within the Mo octahedra. In addition, a large-box modeling of the PDF data indicated a preferential motion of Te atoms towards 𝑐 axis at all temperatures. Structural defects, such as stacking faults, likely result in the coexistence of phases in the average structure and suppress the local SPT of MoTe 2 . These results are stepping stones to understand the long-debated origins of structural, vibrational, and electronic properties of MoTe 2 and similar transition metal dichalcogenides.

2-dimensional systems↗
DOE OSTI2011

Andreev spectroscopy of FeSe: Evidence for two-gap superconductivity

Current-voltage characteristics (CVCs) of Andreev superconductor-constriction-superconductor (ScS) contacts in polycrystalline samples of FeSe with the critical temperature T{sub C} = (12 {+-} 1) K have been measured using the break-junction technique. In Sharvin-type nanocontacts, two sets of subharmonic gap structures were detected due to multiple Andreev reflections, indicating the existence of two nodeless superconducting gaps {Delta}{sub L} = (2.75 {+-} 0.3) meV and {Delta}{sub S} = (0.8 {+-} 0.2) meV. Well-shaped CVCs for stacks of Andreev contacts with up to five contacts were observed due to the layered structure of FeSe (the intrinsic multiple Andreev reflections effect). An additional fine structure in the CVCs of Andreev ScS nanocontacts is attributed to the existence of a Leggett mode. A linear relation between the superconducting gap {Delta}{sub L} and the magnetic resonance energy E{sub magres} Almost-Equal-To 2{Delta}{sub L} is found to be valid for layered iron pnictides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2013

Preliminary Geological Map of the Peace Vallis Fan Integrated with In Situ Mosaics From the Curiosity Rover, Gale Crater, Mars

A geomorphically defined alluvial fan extends from Peace Vallis on the NW wall of Gale Crater, Mars into the Mars Science Laboratory (MSL) Curiosity rover landing ellipse. Prior to landing, the MSL team mapped the ellipse and surrounding areas, including the Peace Vallis fan. Map relationships suggest that bedded rocks east of the landing site are likely associated with the fan, which led to the decision to send Curiosity east. Curiosity's mast camera (Mastcam) color images are being used to refine local map relationships. Results from regional mapping and the first 100 sols of the mission demonstrate that the area has a rich geological history. Understanding this history will be critical for assessing ancient habitability and potential organic matter preservation at Gale Crater.

Gale Crater↗
DOE OSTI2016

Ge{sub 0.83}Sn{sub 0.17} p-channel metal-oxide-semiconductor field-effect transistors: Impact of sulfur passivation on gate stack quality

The effect of room temperature sulfur passivation of the surface of Ge{sub 0.83}Sn{sub 0.17} prior to high-k dielectric (HfO{sub 2}) deposition is investigated. X-ray photoelectron spectroscopy (XPS) was used to examine the chemical bonding at the interface of HfO{sub 2} and Ge{sub 0.83}Sn{sub 0.17}. Sulfur passivation is found to be effective in suppressing the formation of both Ge oxides and Sn oxides. A comparison of XPS results for sulfur-passivated and non-passivated Ge{sub 0.83}Sn{sub 0.17} samples shows that sulfur passivation of the GeSn surface could also suppress the surface segregation of Sn atoms. In addition, sulfur passivation reduces the interface trap density D{sub it} at the high-k dielectric/Ge{sub 0.83}Sn{sub 0.17} interface from the valence band edge to the midgap of Ge{sub 0.83}Sn{sub 0.17}, as compared with a non-passivated control. The impact of the improved D{sub it} is demonstrated in Ge{sub 0.83}Sn{sub 0.17} p-channel metal-oxide-semiconductor field-effect transistors (p-MOSFETs). Ge{sub 0.83}Sn{sub 0.17} p-MOSFETs with sulfur passivation show improved subthreshold swing S, intrinsic transconductance G{sub m,int}, and effective hole mobility μ{sub eff} as compared with the non-passivated control. At a high inversion carrier density N{sub inv} of 1 × 10{sup 13 }cm{sup −2}, sulfur passivation increases μ{sub eff} by 25% in Ge{sub 0.83}Sn{sub 0.17} p-MOSFETs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The impact of aqueous medium on zeolite framework integrity

Understanding the zeolite framework stability in aqueous phase is crucial to develop stable catalysts. Al K–edge, extended X–ray absorption fine structure and 27Al MAS NMR spectroscopies in combination with DFT calculations have been used to monitor both qualitative and quantitative structural changes of two well–characterized samples with BEA structure. The effects of various properties on stability were explored, including Al concentration, Al distribution, particle size and structural defects. As the samples were degraded by treatment in hot liquid water, the local structure about the Al T–site remained mostly intact, including the Al–O–Si angles and bond distances, while the nano–scale crystalline structure as measured by XRD and TEM was disrupted. The combined data suggest a three–step mechanism in which, initially, the HBEA framework crystallinity decreases via hydrolysis of T–O bonds along polymorph stacking faults and inter–grain boundaries in a mode similar to crack propagation in glass. With prolonged exposure, amorphization occurs via hydrolysis of surface Si–OH groups propagating inward through the zeolite lattice. In parallel, cracks propagate within the crystalline micro–domains along paths through specific T–O–T groups. Authors thank B. W. Arey (PNNL) for HIM measurements, T. Huthwelker for support during Al XAFS measurements at the Swiss Light Source (PSI, Switzerland) and M. Y. Hu (PNNL) for support during NMR experiments. This work was supported by the U. S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. NMR experiments were performed at the Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research, and Physical Science Laboratory both located at Pacific Northwest National Laboratory (PNNL). PNNL is a multi–program national laboratory operated for DOE by Battelle under Contract No. DE–AC05–76RL01830.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2026

Enhancing dimensionality prediction in hybrid metal halides via feature engineering and class-imbalance mitigation

We present a machine learning (ML) framework for predicting the structural dimensionality of hybrid metal halides (HMHs), including organic-inorganic perovskites, using a combination of chemically-informed feature engineering and advanced class-imbalance handling techniques. This study is motivated by the small and highly imbalanced nature of experimentally available HMH datasets, which limits the applicability and reliability of conventional ML approaches. The dataset, consisting of 494 HMH structures, is highly imbalanced across dimensionality classes (0D, 1D, 2D, 3D), posing significant challenges to predictive modeling. To mitigate this limitation, the dataset was augmented to 1336 samples using the synthetic minority oversampling technique, enabling improved learning of underrepresented dimensionality classes while preserving chemically meaningful feature relationships. We developed interaction-based descriptors designed to capture coupled steric and polarity effects relevant to dimensionality prediction, which are not readily captured by standard single-parameter or composition-only descriptors. These descriptors are integrated into a multi-stage workflow combining feature selection, ensemble stacking, and performance optimization. Our approach significantly improves F1-scores for underrepresented classes, achieving robust cross-validation performance across all dimensionalities. This work demonstrates a generalizable strategy for extracting reliable and interpretable structure–dimensionality relationships from limited experimental data, enabling pre-synthesis screening of organic cations and providing a practical blueprint for small-data ML in hybrid materials systems.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Radioactive Liquid Waste Treatment Facility Discharges in 2011

This report documents radioactive discharges from the TA50 Radioactive Liquid Waste Treatment Facilities (RLWTF) during calendar 2011. During 2011, three pathways were available for the discharge of treated water to the environment: discharge as water through NPDES Outfall 051 into Mortandad Canyon, evaporation via the TA50 cooling towers, and evaporation using the newly-installed natural-gas effluent evaporator at TA50. Only one of these pathways was used; all treated water (3,352,890 liters) was fed to the effluent evaporator. The quality of treated water was established by collecting a weekly grab sample of water being fed to the effluent evaporator. Forty weekly samples were collected; each was analyzed for gross alpha, gross beta, and tritium. Weekly samples were also composited at the end of each month. These flow-weighted composite samples were then analyzed for 37 radioisotopes: nine alpha-emitting isotopes, 27 beta emitters, and tritium. These monthly analyses were used to estimate the radioactive content of treated water fed to the effluent evaporator. Table 1 summarizes this information. The concentrations and quantities of radioactivity in Table 1 are for treated water fed to the evaporator. Amounts of radioactivity discharged to the environment through the evaporator stack were likely smaller since only entrained materials would exit via the evaporator stack.

03 NATURAL GAS↗
DOE OSTI2014

Star formation and black hole growth at z ≅ 4.8

We report Herschel/SPIRE, Spitzer and Wide-field Infrared Survey Explorer observations of 44 z ≅ 4.8 optically selected active galactic nuclei (AGNs). This flux-limited sample contains the highest mass black holes (BHs) at this redshift. Ten of the objects were detected by Herschel and five show emission that is not clearly associated with the AGNs. The star formation (SF) luminosity (L{sub SF}) obtained by fitting the spectral energy distribution (SED) with standard SF templates, taking into account AGN contribution, is in the range 10{sup 46.62}-10{sup 47.21} erg s{sup –1} corresponding to SF rates of 1090-4240 M{sub ☉} yr{sup –1}. Fitting with very luminous submillimeter galaxy SEDs gives SF rates that are smaller by 0.05 dex when using all bands and 0.1 dex when ignoring the 250 μm band. A 40 K graybody fits to only the 500 μm fluxes reduce L{sub SF} by about a factor of two. A stacking analysis of 29 undetected sources gives significant signals in all three bands. A SF template fit indicates L{sub SF} = 10{sup 46.19-46.23} erg s{sup –1} depending on the assumed AGN contribution. A 40 K fit to the stacked 500 μm flux gives L{sub SF} = 10{sup 45.95} erg s{sup –1}. The mean BH mass (M{sub BH}) and AGN luminosity (L{sub AGN}) of the detected sources are significantly higher than those of the undetected ones. The spectral differences are seen all the way from UV to far infrared wavelengths. The mean optical-UV spectra are similar to those predicted for thin accretion disks around BHs with similar masses and accretion rates. We suggest two alternative explanations to the correlation of L{sub SF}, L{sub AGN} and M{sub BH}, one involving no AGN feedback and the second involving moderate feedback that affects, but does not totally quench, SF in three-quarters of the sources. We compare our L{sub SF} and L{sub AGN} to lower redshift samples and show a new correlation between L{sub SF} and M{sub BH}. We also examine several rather speculative ideas about the host galaxy properties including the possibility that the detected sources are above the SF mass sequence (MS) at z ≅ 4.8, perhaps in mergers, and most of the undetected sources are on the MS.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Aggregation and Solubility of a Model Conjugated Donor–Acceptor Polymer

In conjugated polymers, solution-phase structure and aggregation exert a strong influence on device morphology and performance, making understanding solubility crucial for rational design. Using atomistic molecular dynamics (MD) and free-energy sampling algorithms, we examine the aggregation and solubility of the polymer PTB7, studying how side-chain structure can be modified to control aggregation. We demonstrate that free-energy sampling can be used to effectively screen polymer solubility in a variety of solvents but that solubility parameters derived from MD are not predictive. We then study the aggregation of variants of PTB7 including those with linear (octyl), branched (2-ethylhexyl), and cleaved (methyl) side chains, in a selection of explicit solvents and additives. As a result, energetic analysis demonstrates that while side chains do disrupt polymer backbone stacking, solvent exclusion is a critical factor controlling polymer solubility.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Direct fabrication and characterization of vertically stacked Graphene/h-BN/Graphene tunnel junctions

We have used a lithography free technique for the direct fabrication of vertically stacked two-dimensional (2D) material-based tunnel junctions and characterized by Raman, AFM, XPS. We fabricated Graphene/h-BN/Graphene devices by direct deposition of graphene (bottom layer), h-BN (insulating barrier) and graphene (top layer) sequentially using a plasma enhanced chemical vapor deposition on Si/SiO 2 substrates. The thickness of the h-BN insulating layer was varied by tuning the plasma power and the deposition time. Samples were characterized by Raman, AFM, and XPS. The I-V data follows the barrier thickness dependent quantum tunneling behavior for equally doped graphene layers. The resonant tunneling behavior was observed at room temperature for oppositely doped graphene layers where hydrazine and ammonia were used for n-doping of one of the graphene layers. The resonance with negative differential conductance occurs when the band structures of the two electrodes are aligned. The doping effect of the resonant peak is observed for varying doping levels. The results are explained according to the Bardeen tunneling model.

2D-materials↗
DOE OSTI2019

Phase changes in fluoropolymer binders

Motivation: Understanding fluoropolymer binder structure and aging response continues to be an important goal for the prediction and assessment of polymer bonded explosive compositions within the stockpile. As part of ongoing efforts within NNM, we are developing new methods to quantify crystallization and chemical degradation within aged Fluoropolymer materials. Significance: Time, temperature and radiation are known to alter the mechanical properties of fluoropolymers as crystalline domains nucleate and grow within the material. However, current methods do not accurately quantify and assess the degree or rapidity of these physical aging processes. For this reason, we are developing methods that directly measure crystalline domain, composition, size and shape and cross-correlating them with traditional techniques. Progress: Most recently, we thermally aged fluoropolymer binder, FK800, for various durations up to 23 days at several temperatures including: 25C (red), 35C (green), and 60C (blue). We used wide angle X-ray scattering (WAXS) to observe when crystalline peaks emerged, signaling a phase transition from amorphous to partially crystalline. Some of these data are summarized in Figure 1. As expected, samples held at 25C (red), which is below the glass transition temperature at 28C, remain amorphous – the WAXS data (Figure 1b) lacks sharp peaks indicative of crystallinity and the ultra-small angle X-ray scattering (USAXS) also has a smooth background without peaks indicative of structure (Figure 1a). By contrast, samples held at 35C and 60C have new peaks in both the USAXS and WAXS data. The structure is clearer after subtracting off the amorphous background from the WAXS data (Figure 1c). These data show that samples aged at 60C are more crystalline as determined by the more defined peaks and also have a larger lattice constant as seen by the shift in peak position as compared to the 35C samples. The corresponding USAXS data (Figure 1a) indicate that the size of the crystals was larger at 60C than at 35C as seen by the shift in the knee position to lower q-values. Interestingly, the shape of the scatter curve also changed, which indicates a change in morphology. Atomic force microscopy (AFM) was used to image the fluoropolymer (Figure 1c-e). At 35C, ~50nm long stacked crystal plates are observed. High resolution imaging shows that the stacks consist of 2-6 lamellae that are spaced approximately 3nm apart. At 60C, the individual lamella appear both thicker and longer and are intertwined to fill the surface. Morphological data such as these images help to interpret & model volume averaging techniques like X-ray scattering and NMR. We also expect the shift from isolated islands to intertwined clusters to impact mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2017

C iii] Emission in Star-forming Galaxies at z ∼ 1

The C iii]λλ1907, 1909 rest-frame UV emission doublet has recently been detected in galaxies during the epoch of reionization (z > 6), with a high equivalent width (EW; 10 Å, rest frame). Currently, it is possible to obtain much more detailed information for star-forming galaxies at significantly lower redshift. Accordingly, studies of their far-UV spectra are useful for understanding the factors modulating the strength of C iii] emission. We present the first statistical sample of C iii] emission measurements in star-forming galaxies at z ∼ 1. Our sample is drawn from the DEEP2 survey and spans the redshifts 0.64 ⩽ z ⩽ 1.35 (〈z〉=1.08). We find that the median EW of individual C iii] detections in our sample (1.30 Å) is much smaller than the typical value observed thus far at z > 6. Furthermore, out of 184 galaxies with coverage of C iii], only 40 have significant detections. Galaxies with individual C iii] detections have bluer colors and lower luminosities on average than those without, implying that strong C iii] emitters are in general young and low-mass galaxies without significant dust extinction. Using stacked spectra, we further investigate how C iii] strength correlates with multiple galaxy properties (M {sub B}, U − B, M {sub *}, star formation rate, specific star formation rate) and rest-frame near-UV (Fe ii* and Mg ii) and optical ([O iii] and Hβ) emission line strengths. These results provide a detailed picture of the physical environment in star-forming galaxies at z ∼ 1, and motivate future observations of strong C iii] emitters at similar redshifts.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2010

Miniature Piezoelectric Shaker for Distribution of Unconsolidated Samples to Instrument Cells

The planned Mars Science Laboratory mission requires inlet funnels for channeling unconsolidated powdered samples from the sampling and sieving mechanisms into instrument test cells, which are required to reduce cross-contamination of the samples and to minimize residue left in the funnels after each sample transport. To these ends, a solid-state shaking mechanism has been created that requires low power and is lightweight, but is sturdy enough to survive launch vibration. The funnel mechanism is driven by asymmetrically mounted, piezoelectric flexure actuators that are out of the load path so that they do not support the funnel mass. Each actuator is a titanium, flextensional piezoelectric device driven by a piezoelectric stack. The stack has Invar endcaps with a half-spherical recess. The Invar is used to counteract the change in stress as the actuators are cooled to Mars ambient temperatures. A ball screw is threaded through the actuator frame into the recess to apply pre-stress, and to trap the piezoelectric stack and endcaps in flexure. During the vibration cycle of the flextensional actuator frame, the compression in the piezoelectric stack may decrease to the point that it is unstressed; however, because the ball joint cannot pull, tension in the piezoelectric stack cannot be produced. The actuators are offset at 120 . In this flight design, redundancy is required, so three actuators are used though only one is needed to assist in the movement. The funnel is supported at three contact points offset to the hexapod support contacts. The actuator surface that does not contact the ring is free to expand. Two other configurations can be used to mechanically tune the vibration. The free end can be designed to drive a fixed mass, or can be used to drive a free mass to excite impacts (see figure). Tests on this funnel mechanism show a high density of resonance modes between 1 and 20 kHz. A subset of these between 9 and 12 kHz was used to drive the CheMin actuators at 7 V peak to peak. These actuators could be driven by a single resonance, or swept through a frequency range to decrease the possibility that a portion of the funnel surface was not coincident with a nodal line (line of no displacement). The frequency of actuation can be electrically controlled and monitored and can also be mechanically tuned by the addition of tuning mass on the free end of the actuator. The devices are solid-state and can be designed with no macroscopically moving parts. This design has been tested in a vacuum at both Mars and Earth ambient temperatures ranging from 30 to 25 C

Sherrit, Stewart↗
DOE OSTI2019

Effect of Strain Rate on the Tensile Behavior of CoCrFeNi and CoCrFeMnNi High-Entropy Alloys

High-entropy alloys (HEAs), a novel class of metal alloys, have been receiving increasing attention from the scientific community. HEAs have the potential to be used in critical load-bearing applications in replacement of conventional alloys such as stainless steel and nickel-base superalloys. Tensile experiments at quasi-static to dynamic strain rates (10{sup −4}-10{sup 3} s{sup −1}) were performed on two single-phase face-centered cubic HEAs, CoCrFeNi and CoCrFeMnNi. Electron backscatter diffraction was used to study the microstructure of the samples before the experiments, and transmission electron microscopy was performed postmortem. The dominant deformation mechanisms were dislocation slip at quasi-static strain rates with the addition of deformation nano-twins at dynamic strain rates. Ultimate dynamic tensile strength and ductility improved with the increase in strain rate, which can be attributed to the activation of deformation nano-twins in HEAs. CoCrFeNi and CoCrFeMnNi both have low stacking fault energies, which could promote twinning at high strain rates to accommodate plastic deformation. The strain rate sensitivity of the flow stress increased with increasing strain rate, beginning with negligible strain rate sensitivity in the quasi-static range to high strain rate sensitivity in the dynamic range. CoCrFeMnNi showed greater strain rate sensitivity of flow stress. CoCrFeNi, with less configurational entropy, had higher mechanical properties and strain-hardening rates compared to CoCrFeMnNi, which was attributed to the weakening effect of the addition of Mn on the solid solution hardening.

36 MATERIALS SCIENCE↗
DOE OSTI2018

3D electron tomographic and biochemical analysis of ER, Golgi and trans Golgi network membrane systems in stimulated Venus flytrap (Dionaea muscipula) glandular cells

The insect-trapping leaves of Dionaea muscipula provide a model for studying the secretory pathway of an inducible plant secretory system. The leaf glands were induced with bovine serum albumin to secrete proteases that were characterized via zymogram activity gels over a 6-day period. The accompanying morphological changes of the endoplasmic reticulum (ER) and Golgi were analyzed using 3D electron tomography of glands preserved by high-pressure freezing/freeze substitution methods. Secretion of multiple cysteine and aspartic proteases occurred biphasically. The majority of the Golgi was organized in clusters consisting of 3–6 stacks surrounded by a cage-like system of ER cisternae. In these clusters, all Golgi stacks were oriented with their cis-most C1 cisterna facing an ER export site. The C1 Golgi cisternae varied in size and shape consistent with the hypothesis that they form de novo. Following induction, the number of ER-bound polysomes doubled, but no increase in COPII vesicles was observed. Golgi changes included a reduction in the number of cisternae per stack and a doubling of cisternal volume without increased surface area. Polysaccharide molecules that form the sticky slime cause swelling of the trans and trans Golgi network (TGN) cisternae. Peeling of the trans-most cisternae gives rise to free TGN cisternae. One day after gland stimulation, the free TGNs were frequently associated with loose groups of oriented actin-like filaments which were not seen in any other samples. These findings suggest that the secretory apparatus of resting gland cells is “overbuilt” to enable the cells to rapidly up-regulate lytic enzyme production and secretion in response to prey trapping.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2011

Exchange anisotropy, engineered coercivity and spintronics in atomically engineered L1 0 heterostructures

We identified and investigated some of the scientific and technically most challenging issues in thin film magnetism focusing on epitaxially grown layers of specific L1 0 ordered, intermetallic, heterostructures with well-controlled crystallography and interface structures. Specifically, we addressed antiferromagnetic/ferromagnetic heterostructures, exhibiting exchange bias (EB) in both in-plane (MnPd/Fe) and perpendicular (IrMn/(Co/Pt) n ) geometries, and ferromagnetic/ferrimagnetic (Co/Y 3 Fe 5 O 12 ) bilayers with strong interlayer exchange coupling and exhibiting spin reorientation transitions. In the former case, the work included experimental and theoretical studies to gain more fundamental insight into the origin and magnitude of EB, as well as to address important aspects of EB such as the asymmetry in the magnetic reversal mechanism, the role of interfacial structure, including compensated or uncompensated spins, AF domains, competing anisotropies and the angular dependence of the magnetization reversal process. Exchange bias is central to many magnetic technologies and driven by the fast development of nanotechnology, there is much interest in understanding the phenomenon of exchange bias on the nanoscale. By patterning, as the FM domain size reaches a lateral scale comparable to the AF domain size (~100nm), each nano-element can be treated as a separate and isolated exchange bias system that behaves independently. Therefore, the non-averaged, intrinsic, exchange bias, in all its complexity, can be studied. Such size and dimensionality effects, particularly in structures with lateral dimension of the order of their domain sizes, were studied by developing and implementing a novel Nano-imprint as well as convention optical lithography/patterning. However, one limitation of the NIL method is that after imprinting the material-deposition or -evaporation has to be done at around room temperature in order to keep the resists structure undisturbed. As a result, the method is unsuitable for epitaxial growth, since the latter often involves growth at elevated temperatures higher than the glass transition temperature of the resist. Therefore, a mask transfer NIL process was developed to grow epitaxial nanostructure arrays at elevated temperatures where organic resists are rendered unstable. In the case of the metal/oxide heterostructures, the domain structure of the metal is carefully modulated by that of the underlying oxide, opening the possibility of carrying out novel experiments to study spin-dependent domain-wall scattering and quantify domain wall resistance in mesoscopic geometries. Utilizing state-of-the-art characterization methods, using synchrotron radiation and electron holography, we addressed the critical role of all aspects of the microstructure, at relevant length scales, in determining these specific magnetic properties. Two significant highlights of this project were the use of photoemission electron microscopy (PEEM) work to elucidate their asymmetric magnetization reversal mechanism and the use of element-specific X-ray magnetic reflectivity and x-ray resonant scattering to probe buried interfaces, both of importance in understanding the fundamental physics of exchange bias. In the latter case, a complex magnetic interfacial configuration in Fe/MnPd, consisting of a 2-monolayer-thick induced ferromagnetic region, and pinned uncompensated Mn moments that reach far deeper (~13 Å), both in the antiferromagnet, were found. Such epitaxial EB samples also show in-plane reorientation transitions, determined by the competition between the interface exchange coupling and the intrinsic uniaxial energies, and is driven by the temperature, as well as the thickness of MnPd and Fe layers. Complementing these results, work on multilayers show that perpendicular EB arise from a complex interplay between unidirectional anisotropy at the terminating FM/AFM interface, the perpendicular anisotropy of the FM/nonmagnet(NM) multilayer stack and the overall magnetostatic energy of the structure. Collaborative work with Prof. R. Stamps (UWA) in modeling and analysis of slow-dynamics, using an inductive ferromagnetic resonance technique, were also carried out. Further details of our research is presented below broadly in five thematic areas. Overall, this research allowed us to obtain a deeper understanding of the range of related magnetic phenomena and establish pathways for potential technological applications of these thin film and patterned heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Combined microscopy studies of complex electronic materials. Final report

The aim of this project was to study the electronic properties of exotic electronic materials which are only available as micron-scale samples or have microscopic inhomogeneity, using a special combination of multiple submicron scanning microscopy techniques allied to control of temperature, magnetic and electric fields and mechanical strain. In nanobeams of VO 2 , with its still mysterious metal-insulator phase transition, we determined the phase diagram and demonstrated that hydrogen diffusion is strongly channelled along one crystal axis. Using graphene, we developed and applied the scanning photo-Nernst effect, in which a current is generated when a laser is focused on the edge a sample in a magnetic field. We demonstrated that CrI 3 , a two-dimensional layered magnet, retains true Ising ferromagnetization down the monolayer level due to its out-of-plane anisotropy. In exfoliated WTe 2 , a layered topological semimetal, we discovered that monolayers are the first monolayer topological insulator and we imaged the helical edge states with scanning microwave microscopy. We also discovered that the same material when stacked in a few layers is a ferroelectric metal, and that the monolayer can be converted to a superconducting state by electron doping with a gate a temperatures below 1 K. Additionally, we developed the capability to perform momentum-resolved electronic spectroscopy on operating nanoscale devices by applying micron-scale angle resolved photoemission spectroscopy to two-dimensional heterostructure devices incorporating graphene and transition metal dichalcogenide semiconductors. Using it, we measured the band gaps of a series of monolayer semiconductors and observed band renormalization as a function of electron doping in monolayer WSe 2 .

36 MATERIALS SCIENCE↗