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Register a new userFerroelectric nanodomains in epitaxial PbTiO3 films grown on SmScO3 and TbScO3 substrates
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Domain structures of 320 nm thin epitaxial films of ferroelectric PbTiO3 grown by MOCVD technique in identical conditions on SmScO3 and TbScO3 perovskite sub- strates have been investigated by Raman spectroscopy and piezoresponse force microscopy techniques. Phonon frequency shifts and typical domain structure motifs are discussed. The results reveal strikingly different domain structure architecture: domain structures of the PbTiO3 film grown on SmScO3 have dominantly a-domain orientation while strongly preferential c-domain orientation was found in the PbTiO3 film grown on the TbScO3 substrate. Differences between the two cases are traced back to the film-substrate lattice mismatch at the deposition temperature.
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The mean-field Landau-type theory is used to analyze the polarization properties of epitaxial ferroelectric thin films grown on dissimilar cubic substrates, which induce biaxial compressive stress in the film plane. The intrinsic effect of the film surfaces on the spontaneous polarization is taken into account via the concept of the extrapolation length. The theory simultaneously allows for the influence of the misfit strain imposed on the film lattice by a thick substrate. Numerical calculations are performed for PbTiO3 and BaTiO3 films under an assumption of the polarization reduction in surface layers. The film mean polarization is calculated as a function of film thickness, temperature, and misfit strain. It is shown that the negative intrinsic size effect is reduced in epitaxial films due to the in-plane compression of the film lattice. At room temperature, strong reduction of the mean polarization may take place only in ultrathin films (thickness ~ 1 nm). Theoretical predictions are compared with the available experimental data on polarization properties of BaTiO3 films grown on SrRuO3 coated SrTiO3.
The discovery of stable room temperature ferroelectricity in Croconic Acid, an organic ferroelectric material, with polarization values on par with those found in inorganic ferroelectric materials and highest among organic ferroelectric materials, has opened up possibilities to realize myriads of nano-electronic and spintronic devices based on organic ferroelectrics. Such possibilities require an adequate understanding of the ferroelectric properties of Croconic Acid grown on surfaces that are commonly employed in device fabrication. While several macroscopic studies on relatively larger crystals of Croconic Acid have been performed, studies on thin films are only in their early stages. We have grown thin films of Croconic Acid on gold and cobalt surfaces, which are commonly used in spintronic devices as metallic electrodes, and studied the ferroelectric response of the films using ex-situ Piezoresponse Force Microscopy at room temperature. We show that the polarization reversal in Croconic Acid domains is sensitive to the substrate surface. Using the same experimental protocol, we observe the robust polarization reversal of a single, mostly in-plane electrical domain for a cobalt substrate, whereas polarization reversal is hardly observed for a gold substrate. We attribute this difference to the influence of substrates on the Croconic Acid molecular networks. Our study suggests that to realize devices one has to take care about the substrate on which Croconic Acid will be deposited. The fact that polarization switching is robust on cobalt surface can be used to fabricate multifunctional devices that utilize the cobalt-Croconic Acid interface.
There is growing evidence that domain walls in ferroics can possess emergent properties that are absent in bulk materials. For example, 180 domain walls in the ferroelectric-antiferromagnetic BiFeO3 are particularly interesting because they have been predicted to possess a range of intriguing behaviors; including electronic conduction and enhanced magnetization. To date, however, ordered arrays of such domain structures have not been reported. Here, we report the observation of 180 stripe nanodomains in (110)-oriented BiFeO3 thin films grown on orthorhombic GdScO3 (010)O substrates, and their impact on exchange coupling to metallic ferromagnets. Nanoscale ferroelectric 180 stripe domains with {112 } domain walls were observed in films < 32 nm thick to compensate for large depolarization fields. With increasing film thickness, we observe a domain structure crossover from the depolarization field-driven 180 stripe nanodomains to 71 domains determined by the elastic energy. Interestingly, these 180 domain walls (which are typically cylindrical or meandering in nature due to a lack of strong anisotropy associated with the energy of such walls) are found to be highly-ordered. Additional studies of Co0.9Fe0.1/BiFeO3 heterostructures reveal exchange bias and exchange enhancement in heterostructures based-on BiFeO3 with 180 domain walls and an absence of exchange bias in heterostructures based-on BiFeO3 with 71 domain walls; suggesting that the 180 domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. This is further confirmed by X-ray circular magnetic dichroism studies, which demonstrate that films with predominantly 180 domain walls have larger magnetization than those with primarily 71 domain walls. Our results could be useful to extract the structure of domain walls and to explore domain wall functionalities in BiFeO3.
The influence of the deposition pressure PO2 and substrate temperature TS during the growth of Bi2FeCrO6 thin films grown by pulsed laser deposition has been investigated. It is found that the high volatility of Bi makes the deposition very difficult and that the growth of pure Bi2FeCrO6 thin films on SrTiO3 substrates is possible only in a narrow deposition parameter window. We find that the pure Bi2FeCrO6 phase is formed within a narrow window around an oxygen pressure PO2 =1.210-2 mbar and around a substrate temperature TS=680 degC. At lower temperature or higher pressure, Bi7.38Cr0.62O12+x_also called (b*Bi2O3)and Bi2Fe4O9 /Bi2(Fe,Cr)4O9+x phases are detected, while at lower pressure or higher temperature a (Fe,Cr)3O4 phase forms. Some of these secondary phases are not well known and have not been previously studied. We previously reported Fe/Cr cation ordering as the probable origin of the tenfold improvement in magnetization at saturation of our Bi2FeCrO6 film, compared to BiFeO3. Here, we address the effect of the degree of cationic ordering on the magnetic properties of the Bi2FeCrO6 single phase. Polarization measurements at room temperature reveal that our Bi2FeCrO6 films have excellent ferroelectric properties with ferroelectric hysteresis loops exhibiting a remanent polarization as high as 55-60 miroC/cm2 along the pseudocubic (001) direction.
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.
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