16 research outputs found
COMBINATORIAL INVESTIGATION OF RARE-EARTH FREE PERMANENT MAGNETS
The combinatorial high throughput method allows one to rapidly study a large
number of samples with systematically changing parameters. We apply this method to
study Fe-Co-V alloys as alternatives to rare-earth permanent magnets. Rare-earth
permanent magnets derive their unmatched magnetic properties from the
hybridization of Fe and Co with the f-orbitals of rare-earth elements, which have
strong spin-orbit coupling. It is predicted that Fe and Co may also have strong
hybridization with 4d and 5d refractory transition metals with strong spin-orbit
coupling. Refractory transition metals like V also have the desirable property of high
temperature stability, which is important for permanent magnet applications in
traction motors.
In this work, we focus on the role of crystal structure, composition, and
secondary phases in the origin of competitive permanent magnetic properties of a
particular Fe-Co-V alloy. Fe38Co52V10, compositions are known as Vicalloys. Fe-CoV
composition spreads were sputtered onto three-inch silicon wafers and patterned
into discrete sample pads forming a combinatorial library. We employed highthroughput
screening methods using synchrotron X-rays, wavelength dispersive
spectroscopy, and magneto-optical Kerr effect (MOKE) to rapidly screen crystal
structure, composition, and magnetic properties, respectively. We found that in-plane
magnetic coercive fields of our Vicalloy thin films agree with known bulk values
(300 G), but found a remarkable eight times increase of the out-of-plane coercive
fields (~2,500 G). To explain this, we measured the switching fields between in-plane
and out-of-plane thin film directions which revealed that the Kondorsky model of
180° domain wall reversal was responsible for Vicalloy’s enhanced out-of-plane
coercive field and possibly its permanent magnetic properties. The Kondorsky model
suggests that domain-wall pinning is the origin of Vicalloy’s permanent magnetic
properties, in contrast to strain, shape, or crystalline anisotropy mechanisms
suggested in the literature. We also studied the thickness dependence of an Fe70Co30-
V thin film library to consider the unique effects of our thin film libraries which are
not found in bulk samples. We present results of data mining of synchrotron X-ray
diffraction data using non-negative matrix factorization (NMF). NMF can
automatically identify pure crystal phases that make up an unknown phase mixture.
We found a strong correlation between magnetic properties and crystal phase quantity
using this valuable visualization.
In addition to the combinatorial study, this dissertation includes a study of
strain controlled properties of magnetic thin films for future applications in random
access memories. We investigated the local coupling between dense magnetic stripe
domains in transcritical Permalloy (tPy) thin films and ferroelectric domains of
BaTiO3 single crystals in a tPy/BaTiO3 heterostructure. Two distinct changes in the
magnetic stripe domains of tPy were observed from the magnetic force microscopy
images after cooling the heterostructure from above the ferroelectric Curie
temperature of BaTiO3 (120°C) to room temperature. First, an abrupt break in the
magnetic stripe domain direction was found at the ferroelectric a-c-domain
boundaries due to an induced change in in-plane magnetic anisotropy. Second, the
magnetic stripe domain period increased when coupled to a ferroelectric a-domain
due to a change in out-of-plane magnetic anisotropy. Micromagnetic simulations
reveal that local magnetic anisotropy energy from inverse magnetostriction is
conserved between in-plane and out-of-plane components
Combinatorial search of superconductivity in Fe-B composition spreads
We have fabricated Fe-B thin film composition spreads in search of possible
superconducting phases following a theoretical prediction by Kolmogorov et
al.^1 Co-sputtering was used to deposit spreads covering a large compositional
region of the Fe-B binary phase diagram. A trace of superconducting phase was
found in the nanocrystalline part of the spread, where the film undergoes a
metal to insulator transition as a function of composition in a region with the
average composition of FeB_2. The resistance drop occurs at 4K, and a
diamagnetic signal has also been detected at the same temperature. The
superconductivity is suppressible in the magnetic field up to 2 Tesla.Comment: 11 pages, 4 figure
Observation of the superconducting proximity effect in the surface state of SmB6 thin films
The proximity effect at the interface between a topological insulator (TI)
and a superconductor is predicted to give rise to chiral topological
superconductivity and Majorana fermion excitations. In most TIs studied to
date, however, the conducting bulk states have overwhelmed the transport
properties and precluded the investigation of the interplay of the topological
surface state and Cooper pairs. Here, we demonstrate the superconducting
proximity effect in the surface state of SmB6 thin films which display bulk
insulation at low temperatures. The Fermi velocity in the surface state deduced
from the proximity effect is found to be as large as 10^5 m/s, in good
agreement with the value obtained from a separate transport measurement. We
show that high transparency between the TI and a superconductor is crucial for
the proximity effect. The finding here opens the door to investigation of
exotic quantum phenomena using all-thin-film multilayers with high-transparency
interfaces
Combinatorial Discovery of Lanthanum-Tantalum Oxynitride Solar Light Absorbers with Dilute Nitrogen for Solar Fuels Applications
Oxynitrides with the photoelectrochemical stability of oxides and desirable band energetics of nitrides comprise a promising class of materials for solar photochemistry. Challenges in synthesizing a wide variety of oxynitride materials has limited exploration of this class of functional materials, which we address using a reactive cosputtering combined with rapid thermal processing method to synthesize multi-cation–multi-anion libraries. We demonstrate the synthesis of a La_xTa_(1–x)O_yN_z thin film composition spread library and its characterization by both traditional thin film materials characterization and custom combinatorial optical spectroscopy and X-ray absorption near edge spectroscopy (XANES) techniques, ultimately establishing structure-chemistry-property relationships. We observe that over a substantial La–Ta composition range the thin films crystallize in the same perovskite LaTaON2 structure with significant variation of anion chemistry. The relative invariance in optical band gap demonstrates a remarkable decoupling of composition and band energetics so that the composition can be optimized while retaining the desirable 2 eV band gap energy. We also demonstrate the intercalation of diatomic nitrogen into the La_3TaO_7 structure, which gives rise to a direct-allowed optical transition at 2.2 eV, less than half the value of the oxide’s band gap. These findings motivate further exploration of the visible light response of this material that is predicted to be stable over a wide range of electrochemical potential
Combinatorial Discovery of Lanthanum-Tantalum Oxynitride Solar Light Absorbers with Dilute Nitrogen for Solar Fuels Applications
Oxynitrides with the photoelectrochemical stability of oxides and desirable band energetics of nitrides comprise a promising class of materials for solar photochemistry. Challenges in synthesizing a wide variety of oxynitride materials has limited exploration of this class of functional materials, which we address using a reactive cosputtering combined with rapid thermal processing method to synthesize multi-cation–multi-anion libraries. We demonstrate the synthesis of a La_xTa_(1–x)O_yN_z thin film composition spread library and its characterization by both traditional thin film materials characterization and custom combinatorial optical spectroscopy and X-ray absorption near edge spectroscopy (XANES) techniques, ultimately establishing structure-chemistry-property relationships. We observe that over a substantial La–Ta composition range the thin films crystallize in the same perovskite LaTaON2 structure with significant variation of anion chemistry. The relative invariance in optical band gap demonstrates a remarkable decoupling of composition and band energetics so that the composition can be optimized while retaining the desirable 2 eV band gap energy. We also demonstrate the intercalation of diatomic nitrogen into the La_3TaO_7 structure, which gives rise to a direct-allowed optical transition at 2.2 eV, less than half the value of the oxide’s band gap. These findings motivate further exploration of the visible light response of this material that is predicted to be stable over a wide range of electrochemical potential
HIV-1 Infection of DC: Evidence for the Acquisition of Virus Particles from Infected T Cells by Antigen Uptake Mechanism
Dendritic cells (DC) play a pivotal role in transmission and dissemination of HIV-1. Earlier studies reported that DC present at the site of infection trap virus particles via DC-SIGN and transfer the virus to the interacting naïve T cells. This prompted us to ask the question whether DC could acquire virus from infected T cells during DC-T cell interaction. To address this, we investigated the likely transfer of virus from HIV-1 infected T cells to DC and the underlying mechanisms involved. Results indicate that DC acquire virus from infected T cells via antigen uptake mechanism and this results in infection of DC with expression of proteins directed by viral DNA. Further studies with HIV-1 lacking the Env protein also resulted in infection of DC. The use of antibodies against DC-SIGN and DC-SIGN-R ruled out a role for receptor in the infection of DC. Additional data show that DC infection is directly correlated with the ability of DC to take up antigen from infected T cells. Overall, these studies provide evidence to suggest that HIV-1, besides infecting immune cells, also utilizes immunological mechanism(s) to acquire and disseminate virus
Understanding reasons clinicians obtained endotracheal aspirate cultures and impact on patient management to inform diagnostic stewardship initiatives
Simultaneous imaging of the ferromagnetic and ferroelectric structure in multiferroic heterostructures
By measuring the spin polarization of secondary electrons and the intensity of backscattered electrons generated in a scanning electron microscope, we are able to simultaneously image the ferromagnetic domain structure of a ferromagnetic thin film and the ferroelectric domain structure of the underlying ferroelectric substrate upon which it is grown. Simultaneous imaging allows straightforward, quantitative measurements of the correlations in these complex multiferroic systems. We have successfully imaged domains in CoFe/BFO and Fe/BTO, two systems with very different ferromagnet/ferroelectric coupling mechanisms, demonstrating how this technique provides a new local probe of magneto electric/strictive effects in multiferroic heterostructures.ISSN:2166-532
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Combinatorial Discovery of Lanthanum–Tantalum Oxynitride Solar Light Absorbers with Dilute Nitrogen for Solar Fuel Applications
Oxynitrides with
the photoelectrochemical stability of oxides and
desirable band energetics of nitrides comprise a promising class of
materials for solar photochemistry. Challenges in synthesizing a wide
variety of oxynitride materials has limited exploration of this class
of functional materials, which we address using a reactive cosputtering
combined with rapid thermal processing method to synthesize multi-cation–multi-anion
libraries. We demonstrate the synthesis of a La<sub><i>x</i></sub>Ta<sub>1–<i>x</i></sub>O<sub><i>y</i></sub>N<sub><i>z</i></sub> thin film composition spread
library and its characterization by both traditional thin film materials
characterization and custom combinatorial optical spectroscopy and
X-ray absorption near edge spectroscopy (XANES) techniques, ultimately
establishing structure-chemistry-property relationships. We observe
that over a substantial La–Ta composition range the thin films
crystallize in the same perovskite LaTaON<sub>2</sub> structure with
significant variation of anion chemistry. The relative invariance
in optical band gap demonstrates a remarkable decoupling of composition
and band energetics so that the composition can be optimized while
retaining the desirable 2 eV band gap energy. We also demonstrate
the intercalation of diatomic nitrogen into the La<sub>3</sub>TaO<sub>7</sub> structure, which gives rise to a direct-allowed optical transition
at 2.2 eV, less than half the value of the oxide’s band gap.
These findings motivate further exploration of the visible light response
of this material that is predicted to be stable over a wide range
of electrochemical potential