113 research outputs found
Localized Control of Curie Temperature in Perovskite Oxide Film by Capping-layer- induced Octahedral Distortion
With reduced dimensionality, it is often easier to modify the properties of
ultra-thin films than their bulk counterparts. Strain engineering, usually
achieved by choosing appropriate substrates, has been proven effective in
controlling the properties of perovskite oxide films. An emerging alternative
route for developing new multifunctional perovskite is by modification of the
oxygen octahedral structure. Here we report the control of structural oxygen
octahedral rotation in ultra-thin perovskite SrRuO3 films by the deposition of
a SrTiO3 capping layer, which can be lithographically patterned to achieve
local control. Using a scanning Sagnac magnetic microscope, we show increase in
the Curie temperature of SrRuO3 due to the suppression octahedral rotations
revealed by the synchrotron x-ray diffraction. This capping-layer-based
technique may open new possibilities for developing functional oxide materials.Comment: Main-text 5 pages, SI 6 pages. To appear in Physical Review Letter
Dependence of electronic structure of SrRuO3 and the degree of correlation on cation off-stoichiometry
We have grown and studied high quality SrRuO3 films grown by MBE as well as
PLD. By changing the oxygen activity during deposition we were able to make
SrRuO3 samples that were stoichiometric (low oxygen activity) or with ruthenium
vacancies (high oxygen activity). Samples with strontium vacancies were found
impossible to produce since the ruthenium would precipitate out as RuO2. The
volume of the unit cell of SrRuO3 becomes larger as more ruthenium vacancies
are introduced. The residual resistivity ratio (RRR) and room temperature
resistivity were found to systematically depend on the volume of the unit cell
and therefore on the amount of ruthenium vacancies. The RRR varied from ~30 for
stoichiometric samples to less than two for samples that were very ruthenium
poor. The room temperature resistivity varied from 190 microOhm cm for
stoichoimetric samples to over 300 microOhm cm for very ruthenium poor samples.
UPS spectra show a shift of weight from the coherent peak to the incoherent
peak around the Fermi level when samples have more ruthenium vacancies. Core
level XPS spectra of the ruthenium 3d lines show a strong screened part in the
case of stoichiometric samples. This screened part disappears when ruthenium
vacancies are introduced. Both the UPS and the XPS results are consistent with
the view that correlation increases as the amount of ruthenium vacancies
increase.Comment: 21 pages, 5 figures, submitted to Physical Review
Misfit Strain Accommodation in Epitaxial ABO3 Perovskites: Lattice Rotations and Lattice Modulations
We present a study of the lattice response to the compressive and tensile
biaxial stress in La0.67Sr0.33MnO3 (LSMO) and SrRuO3 (SRO) thin films grown on
a variety of single crystal substrates: SrTiO3, DyScO3, NdGaO3 and
(La,Sr)(Al,Ta)O3. The results show, that in thin films under misfit strain,
both SRO and LSMO lattices, which in bulk form have orthorhombic (SRO) and
rhombohedral (LSMO) structures, assume unit cells that are monoclinic under
compressive stress and tetragonal under tensile stress. The applied stress
effectively modifies the BO6 octahedra rotations, which degree and direction
can be controlled by magnitude and sign of the misfit strain. Such lattice
distortions change the B-O-B bond angles and therefore are expected to affect
magnetic and electronic properties of the ABO3 perovskites.Comment: Submitted to Phys. Rev. B 13 pages, 9 figure
Optimized fabrication of high quality La0.67Sr0.33MnO3 thin films considering all essential characteristics
In this article, an overview of the fabrication and properties of high
quality La0.67Sr0.33MnO3 (LSMO) thin films is given. A high quality LSMO film
combines a smooth surface morphology with a large magnetization and a small
residual resistivity, while avoiding precipitates and surface segregation. In
literature, typically only a few of these issues are adressed. We therefore
present a thorough characterization of our films, which were grown by pulsed
laser deposition. The films were characterized with reflection high energy
electron diffraction, atomic force microscopy, x-ray diffraction, magnetization
and transport measurements, x-ray photoelectron spectroscopy and scanning
transmission electron microscopy. The films have a saturation magnetization of
4.0 {\mu}B/Mn, a Curie temperature of 350 K and a residual resistivity of 60
{\mu}{\Omega}cm. These results indicate that high quality films, combining both
large magnetization and small residual resistivity, were realized. A comparison
between different samples presented in literature shows that focussing on a
single property is insufficient for the optimization of the deposition process.
For high quality films, all properties have to be adressed. For LSMO devices,
the thin film quality is crucial for the device performance. Therefore, this
research is important for the application of LSMO in devices.Comment: Accepted for publication in Journal of Physics D - Applied Physic
Enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2
Superconducting quantum interference device (SQUID) microscopy shows stripes
of increased diamagnetic susceptibility in underdoped, but not overdoped,
single crystals of Ba(Fe1-xCox)2As2. These stripes of increased diamagnetic
susceptibility are consistent with enhanced superfluid density on twin
boundaries. Individual vortices avoid pinning on or crossing the stripes, and
prefer to travel parallel to them. These results indicate a relationship
between superfluid density, local strain, and frustrated magnetism, and
demonstrate two mechanisms for enhancing critical currents.Comment: 16 pages, 4 figure
Prepyramid-to-pyramid transition of SiGe islands on Si(001)
The morphology of the first three-dimensional islands appearing during
strained growth of SiGe alloys on Si(001) was investigated by scanning
tunneling microscopy. High resolution images of individual islands and a
statistical analysis of island shapes were used to reconstruct the evolution of
the island shape as a function of size. As they grow, islands undergo a
transition from completely unfacetted rough mounds (prepyramids) to partially
{105} facetted islands and then they gradually evolve to {105} facetted
pyramids. The results are in good agreement with the predictions of a recently
proposed theoretical model
Variation in superconducting transition temperature due to tetragonal domains in two-dimensionally doped SrTiO<sub>3</sub>
Strontium titanate is a low-temperature, non-Bardeen-Cooper-Schrieffer
superconductor that superconducts to carrier concentrations lower than in any
other system and exhibits avoided ferroelectricity at low temperatures. Neither
the mechanism of superconductivity in strontium titanate nor the importance of
the structure and dielectric properties for the superconductivity are well
understood. We studied the effects of twin structure on superconductivity in a
5.5-nm-thick layer of niobium-doped SrTiO embedded in undoped
SrTiO. We used a scanning superconducting quantum interference device
susceptometer to image the local diamagnetic response of the sample as a
function of temperature. We observed regions that exhibited a superconducting
transition temperature 10% higher than the temperature at
which the sample was fully superconducting. The pattern of these regions varied
spatially in a manner characteristic of structural twin domains. Our results
emphasize that the anisotropic dielectric properties of SrTiO are
important for its superconductivity, and need to be considered in any theory of
the mechanism of the superconductivity.Comment: 14 pages, 11 figures, Supplemental Information available at
http://stanford.edu/group/moler/papers/Noad_STOsuperconductivity_SI.pd
Optimized fabrication of high-quality La0.67Sr0.33MnO3 thin films considering all essential characteristics
In this paper, an overview of the fabrication and properties of high-quality La0.67Sr0.33MnO3 (LSMO) thin films is given. A high-quality LSMO film combines a smooth surface morphology with a large magnetization and a small residual resistivity, while avoiding precipitates and surface segregation. In the literature, typically only a few of these issues are adressed. We therefore present a thorough characterization of our films, which were grown by pulsed laser deposition. The films were characterized with reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, magnetization and transport measurements, x-ray photoelectron spectroscopy and scanning transmission electron microscopy. The films have a saturation magnetization of 4.0 µB/Mn, a Curie temperature of 350 K and a residual resistivity of 60 µΩ cm. These results indicate that high-quality films, combining both large magnetization and small residual resistivity, were realized. A comparison between different samples presented in the literature shows that focussing on a single property is insufficient for the optimization of the deposition process. For high-quality films, all properties have to be adressed. For LSMO devices, the thin-film quality is crucial for the device performance. Therefore, this research is important for the application of LSMO in devices
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