130 research outputs found
High-Symmetry Polarization Domains in Low-Symmetry Ferroelectrics
We present experimental evidence for hexagonal domain faceting in the
ferroelectric polymer PVDF-TrFE films having the lower orthorhombic
crystallographic symmetry. This effect can arise from purely electrostatic
depolarizing forces. We show that in contrast to magnetic bubble shape domains
where such type of deformation instability has a predominantly elliptical
character, the emergence of more symmetrical circular harmonics is favored in
ferroelectrics with high dielectric constant
Piezoresponse force microscopy for polarity imaging of GaN
The polarity distribution of GaN based lateral polarity heterostructures is investigated by piezoresponse force microscopy (PFM). Simultaneous imaging of surface morphology, as well as the phase and magnitude of the piezoelectric response, is performed by PFM on a GaN film with patterned polarities on a c-Al2O3 substrate. We demonstrate that the polarity distribution of GaN based lateral polarity heterostructures can be deduced from the phase image of the piezoresponse with nanometer scale spatial resolution
Domain wall saddle point morphology in ferroelectric triglycine sulfate
Ferroelectric domain walls, across which there is a divergence in polarization, usually have enhanced electrical conductivity relative to bulk. However, in lead germanate, head-to-head and tail-to-tail walls are electrically insulating. Recent studies have shown that this is because, when oppositely oriented domains meet, polar divergence is obviated by a combination of domain bifurcation and suspected local dipolar rotation. To explore the uniqueness, or otherwise, of this microstructure, we have used tomographic piezoresponse force microscopy to map three-dimensional domain morphologies in another uniaxial ferroelectric system: triglycine sulfate. This mapping reveals an abundance of domain wall saddle points, which are characteristic of interlocking bifurcated domains. Conducting atomic force microscopy, performed close to the saddle points, showed no evidence for highly localized conducting domain wall sections, across which a divergence in polarization might be implied; this supports the notion that localized dipolar rotation occurs to minimize any potential polar discontinuity. Overall, our study, therefore, confirms that mutual domain bifurcation and suspected local dipolar rotation are not unique to lead germanate and instead may be widely present in other uniaxial ferroelectrics
Imaging Mechanism of Piezoresponse Force Microscopy in Capacitor Structures
The image formation mechanism in Piezoresponse Force Microscopy (PFM) of
capacitor structures is analyzed. We demonstrate that the spatial resolution is
a bilinear function of film and top electrode thicknesses, and derive the
corresponding analytical expressions. For many perovskites, the opposite
contributions of d31 and d33 components can result in anomalous domain wall
profiles. This analysis establishes the applicability limits of PFM for
polarization dynamics studies in capacitors, and applies to other structural
probes, including focused X-ray studies of capacitor structures.Comment: 20 pages, 3 figures, 2 tables, 1 Aappendix, to be submitted to Appl.
Phys. Let
Epitaxial Bi2FeCrO6 Multiferroic Thin Films
We present here experimental results obtained on Bi2FeCrO6 (BFCO) epitaxial
films deposited by laser ablation directly on SrTiO3 substrates. It has been
theoretically predicted, by Baettig and Spaldin, using first-principles density
functional theory that BFCO is ferrimagnetic (with a magnetic moment of 2 Bohr
magneton per formula unit) and ferroelectric (with a polarization of ~80
microC/cm2 at 0K). The crystal structure has been investigated using X-ray
diffraction which shows that the films are epitaxial with a high crystallinity
and have a degree of orientation depending of the deposition conditions and
that is determined by the substrate crystal structure. Chemical analysis
carried out by X-ray Microanalysis and X-ray Photoelectron Spectroscopy (XPS)
indicates the correct cationic stoichiometry in the BFCO layer, namely
(Bi:Fe:Cr = 2:1:1). XPS depth profiling revealed that the oxidation state of Fe
and Cr ions in the film remains 3+ throughout the film thickness and that both
Fe and Cr ions are homogeneously distributed throughout the depth.
Cross-section high-resolution transmission electron microscopy images together
with selected area electron diffraction confirm the crystalline quality of the
epitaxial BFCO films with no identifiable foreign phase or inclusion. The
multiferroic character of BFCO is proven by ferroelectric and magnetic
measurements showing that the films exhibit ferroelectric and magnetic
hysteresis at room temperature. In addition, local piezoelectric measurements
carried out using piezoresponse force microscopy (PFM) show the presence of
ferroelectric domains and their switching at the sub-micron scale.Comment: Accepted for publication in Philosophical Magazine Letter
Enhanced electric conductivity at ferroelectric vortex cores in BiFeO3
In many large ensembles, the property of the system as a whole cannot be understood from studying the individual entities alone ¿ these ensembles can be made up by neurons in the brain, transport users in traffic networks or data packages in the Internet. The past decade has seen important progress in our fundamental understanding of what such seemingly disparate 'complex systems' have in common; some of these advances are surveyed here
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