140 research outputs found
Structural evolution of epitaxial SrCoOx films near topotactic phase transition
Control of oxygen stoichiometry in complex oxides via topotactic phase
transition is an interesting avenue to not only modifying the physical
properties, but utilizing in many energy technologies, such as energy storage
and catalysts. However, detailed structural evolution in the close proximity of
the topotactic phase transition in multivalent oxides has not been much
studied. In this work, we used strontium cobaltites (SrCoOx) epitaxially grown
by pulsed laser epitaxy (PLE) as a model system to study the oxidation-driven
evolution of the structure, electronic, and magnetic properties. We grew
coherently strained SrCoO2.5 thin films and performed post-annealing at various
temperatures for topotactic conversion into the perovskite phase
(SrCoO3-{\delta}). We clearly observed significant changes in electronic
transport, magnetism, and microstructure near the critical temperature for the
topotactic transformation from the brownmillerite to the perovskite phase.
Nevertheless, the overall crystallinity was well maintained without much
structural degradation, indicating that topotactic phase control can be a
useful tool to control the physical properties repeatedly via redox reactions.Comment: 16 pages, 4 figure
Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films
Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960
nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3
bottom electrodes. X-ray characterization shows that the structure evolves from
angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted
rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness.
Despite this significant structural evolution, the ferroelectric polarization
along the body diagonal of the distorted pseudo-cubic unit cells, as calculated
from measurements along the normal direction, barely changes.Comment: Legend in Fig.3 corrected and et
Strain-coupled ferroelectric polarization in BaTiO3-CaTiO3 superlattices
We report on growth and ferroelectric (FE) properties of superlattices (SLs)
composed of the FE BaTiO3 and the paraelectric (PE) CaTiO3. Previous theories
have predicted that the polarization in (BaTiO3)n/(CaTiO3)n SLs increases as
the sublayer thickness (n) increases when the same strain state is maintained.
However, our BaTiO3/CaTiO3 SLs show a varying lattice-strain state and
systematic reduction in polarization with increasing n while
coherently-strained SLs with n=1, 2 show a FE polarization of ca. 8.5 uC/cm^2.
We suggest that the strain coupling plays more important role in FE properties
than the electrostatic interlayer coupling based on constant dielectric
permittivities.Comment: 7 pages, 4 figure
Growth control of oxygen stoichiometry in homoepitaxial SrTiO3 films by pulsed laser epitaxy in high vacuum
In many transition metal oxides (TMOs), oxygen stoichiometry is one of the
most critical parameters that plays a key role in determining the structural,
physical, optical, and electrochemical properties of the material. However,
controlling the growth to obtain high quality single crystal films having the
right oxygen stoichiometry, especially in a high vacuum environment, has been
viewed as a challenge. In this work, we show that through proper control of the
plume kinetic energy, stoichiometric crystalline films can be synthesized
without generating oxygen defects, even in high vacuum. We use a model
homoepitaxial system of SrTiO3 (STO) thin films on single crystal STO
substrates. Physical property measurements indicate that oxygen vacancy
generation in high vacuum is strongly influenced by the energetics of the laser
plume, and it can be controlled by proper laser beam delivery. Therefore, our
finding not only provides essential insight into oxygen stoichiometry control
in high vacuum for understanding the fundamental properties of STO-based thin
films and heterostructures, but expands the utility of pulsed laser epitaxy of
other materials as well
Strongly coupled phase transition in ferroelectric/correlated electron oxide heterostructures
We fabricated ultrathin ferroelectric/correlated electron oxide
heterostructures composed of the ferroelectric Pb(Zr0.2Ti0.8)O3 and the
correlated electron oxide (CEO) La0.8Sr0.2MnO3 on SrTiO3 substrates by pulsed
laser epitaxy. The hole accumulation in the ultrathin CEO layer was
substantially modified by heterostructuring with the ferroelectric layer,
resulting in an insulator-metal transition. In particular, our thickness
dependent study showed that drastic changes in transport and magnetic
properties were strongly coupled to the modulation of charge carriers by
ferroelectric field effect, which was confined to the vicinity of the
interface. Thus, our results provide crucial evidence that strong ferroelectric
field effect control can be achieved in ultrathin (10 nm) heterostructures,
yielding at least a 100,000-fold change in resistivity
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