56 research outputs found
Mapping Structural Heterogeneity at the Nanoscale with Scanning Nano-structure Electron Microscopy (SNEM)
Here we explore the use of scanning electron diffraction coupled with
electron atomic pair distribution function analysis (ePDF) to understand the
local order as a function of position in a complex multicomponent system, a hot
rolled, Ni-encapsulated, ZrCuNiAl bulk metallic
glass (BMG), with a spatial resolution of 3 nm. We show that it is possible to
gain insight into the chemistry and chemical clustering/ordering tendency in
different regions of the sample, including in the vicinity of nano-scale
crystallites that are identified from virtual dark field images and in heavily
deformed regions at the edge of the BMG. In addition to simpler analysis,
unsupervised machine learning was used to extract partial PDFs from the
material, modeled as a quasi-binary alloy, and map them in space. These maps
allowed key insights not only into the local average composition, as validated
by EELS, but also a unique insight into chemical short-range ordering
tendencies in different regions of the sample during formation. The experiments
are straightforward and rapid and, unlike spectroscopic measurements, don't
require energy filters on the instrument. We spatially map different quantities
of interest (QoI's), defined as scalars that can be computed directly from
positions and widths of ePDF peaks or parameters refined from fits to the
patterns. We developed a flexible and rapid data reduction and analysis
software framework that allows experimenters to rapidly explore images of the
sample on the basis of different QoI's. The power and flexibility of this
approach are explored and described in detail. Because of the fact that we are
getting spatially resolved images of the nanoscale structure obtained from
ePDFs we call this approach scanning nano-structure electron microscopy (SNEM),
and we believe that it will be powerful and useful extension of current 4D-STEM
methods
Electron-beam-induced ferroelectric domain behavior in the transmission electron microscope: Toward deterministic domain patterning
Magnetic properties of Co2C and Co3C nanoparticles and their assemblies
Nano-composite material consisting of Co2C and Co3C nanoparticles has recently been shown to exhibit unusually large coercivities and energy products. Experimental studies that can delineate the properties of individual phases have been undertaken and provide information on how the coercivities and the energy product change with the size and composition of the nanoparticles. The studies indicate that while both phases are magnetic, the Co3C has higher magnetization and coercivity compared to Co2C. Through first principles electronic structure studies using a GGA+U functional, we provide insight on the role of C intercalation on enhancing the magnetic anisotropy of the individual phases
Quantum paramagnetism in a non-Kramers rare-earth oxide: Monoclinic
Little is so far known about the magnetism of the monoclinic
layered perovskites that replace the spin-ice supporting pyrochlore structure
for . We show that high quality monoclinic PrTiO
single crystals with a three-dimensional network of non-Kramers Pr ions
that interact through edge-sharing super-exchange interactions, form a singlet
ground state quantum paramagnet that does not undergo any magnetic phase
transitions down to at least 1.8 K. The chemical phase stability, structure,
and magnetic properties of the layered perovskite PrTiO were
investigated using x-ray diffraction, transmission electron microscopy, and
magnetization measurements. Synthesis of polycrystalline samples with the
nominal compositions of PrTiO () showed
that deviations from the PrTiO stoichiometry lead to secondary
phases of related, structures including the perovskite phase PrTiO
and the orthorhombic phases PrTiO and PrTiO. No
indications of site disordering (stuffing and anti-stuffing) or vacancy defects
were observed in the PrTiO majority phase. A procedure for growth
of high-structural-quality, stoichiometric single crystals of PrTiO
by the traveling solvent floating zone (TSFZ) method is reported.
Thermo-magnetic measurements of single-crystalline PrTiO reveal an
isolated singlet ground state that we associate with the low symmetry crystal
electric field environments that split the -fold degenerate
spin-orbital multiplets of the four differently coordinated Pr ions into
36 isolated singlets resulting in an anisotropic temperature independent
van-Vleck susceptibility at low . A small isotropic Curie term is associated
with 0.96(2)\% non-interacting Pr impurities.Comment: 18 pages, 7 figures, 6 table
High Throughput Discovery of Lightweight Corrosion-Resistant Compositionally Complex Alloys
Compositionally complex alloys hold the promise of simultaneously attaining
superior combinations of properties such as corrosion resistance,
light-weighting, and strength. Achieving this goal is a challenge due in part
to a large number of possible compositions and structures in the vast alloy
design space. High throughput methods offer a path forward, but a strong
connection between the synthesis of a given composition and structure with its
properties has not been fully realized to date. Here we present the rapid
identification of light weight highly corrosion-resistant alloys based on
combinations of Al and Cr in a Cantor-like base alloy (Al-Co-Cr-Fe-Ni).
Previously unstudied alloy stoichiometries were identified using a combination
of high throughput experimental screening coupled with key metallurgical and
electrochemical corrosion tests, identifying alloys with excellent passivation
behavior. Importantly, the electrochemical impedance modulus of the
exposure-modified, air-formed film at the corrosion potential was found as an
accurate non-destructive predictor of corrosion and passivation
characteristics. Multi-element EXAFS analyses connected more ordered type
chemical short range order in the Ni-Al 1st nn shell to poorer corrosion. This
report underscores the utility of high throughput exploration of
compositionally complex alloys for the identification and rapid screening of
vast stoichiometric space
Evidence of a magnetic transition in atomically thin Cr2TiC2Tx MXene
Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr2TiC2Tx MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr2TiC2Tx at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr2TiAlC2 MAX phase). This work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials
Mineralogy and petrology of comet 81P/wild 2 nucleus samples
The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk
Electron beam induced domain motion in ferroelectric RKTP observed by transmission electron microscopy
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