Local conduction at domains and domains walls is investigated in BiFeO3 thin films containing mostly 71o domain walls. Measurements at room temperature reveal conduction through 71o domain walls. Conduction through domains could also be observed at h
igh enough temperatures. It is found that, despite the lower conductivity of the domains, both are governed by the same mechanisms: in the low voltage regime electrons trapped at defect states are temperature-activated but the current is limited by the ferroelectric surface charges; in the large voltage regime, Schottky emission takes place and the role of oxygen vacancies is that of selectively increasing the Fermi energy at the walls and locally reducing the Schottky barrier. This understanding provides the key to engineering conduction paths in oxides.
Recent works have shown that the domain walls of room-temperature multiferroic BiFeO3 (BFO) thin films can display distinct and promising functionalities. It is thus important to understand the mechanisms underlying domain formation in these films. H
igh-resolution x-ray diffraction and piezo-force microscopy, combined with first-principles simulations, have allowed us to characterize both the atomic and domain structure of BFO films grown under compressive strain on (001)-SrTiO3, as a function of thickness. We derive a twining model that describes the experimental observations and explains why the 71o domain walls are the ones commonly observed in these films. This understanding provides us with a new degree of freedom to control the structure and, thus, the properties of BiFeO3 thin films.
Thin films of TbMnO3 have been grown on SrTiO3 substrates. The films grow under compressive strain and are only partially clamped to the substrate. This produces remarkable changes in the magnetic properties and, unlike the bulk material, the films d
isplay ferromagnetic interactions below the ordering temperature of ~40K. X-ray photoemission measurements in the films show that the Mn-3s splitting is 0.3eV larger than that of the bulk. Ab initio embedded cluster calculations yield Mn-3s splittings that are in agreement with the experiment and reveal that the larger observed values are due to a larger ionicity of the films.
Metastable manganite perovskites displaying the antiferromagnetic so-called E-phase are predicted to be multiferroic. Due to the need of high-pressures for the synthesis of this phase, this prediction has only been confirmed in bulk HoMnO3. Here we r
eport on the growth and characterization of YbMnO3 perovskite thin films grown under epitaxial strain. Highly-oriented thin films, with thickness down to ~30nm, can be obtained showing magneto-dielectric coupling and magnetic responses as those expected for the E-phase. We observe that the magnetic properties depart from the bulk behavior only in the case of ultrathin films (d< 30nm), which display a glassy magnetic behavior. We show that strain effects alone cannot account for this difference and that the film morphology plays, instead, a crucial role.
Ferroelectrics display spontaneous and switchable electrical polarization. Until recently, ferroelectricity was believed to disappear at the nanoscale; now, nano-ferroelectrics are being considered in numerous applications. This renewed interest was
partly fuelled by the observation of ferroelectric domains in films of a few unit cells thickness, promising further size reduction of ferroelectric devices. It turns out that at reduced scales and dimensionalities the materials properties depend crucially on the intricacies of domain formation, that is, the way the crystal splits into regions with polarization oriented along the different energetically equivalent directions, typically at 180o and 90o from each other. Here we present a step forward in the manipulation and control of ferroelectric domains by the growth of thin films with regular self-patterned arrays of 90o domains only 7 nm wide. This is the narrowest width for 90o domains in epitaxial ferroelectrics that preserves the film lateral coherence, independently of the substrate.
