| Materials properties are frequently predicted to change drastically at the nanoscale. Here, new phenomena can emerge that are only observable with an extraordinary control at the microscopic level. An example of this arose from the recent studies of interfaces between two Mott insulators. The interface turned out to be metallic and therefore appears as a separate object with distinct properties. Our knowledge of the materials nanoscale has improve thanks to novel experimental techniques such as scanning probe microscopy (SPM), refined thin film deposition methods or novel x-ray scattering and spectroscopic techniques. As these advanced tools become more common, the nanoobjects will also become more easily available in labs worldwide, providing new access routes to the nanoworld. Apart from the above-mentioned artificial interfaces between atomically-flat layers, in common functional materials a variety of other atomically-confined objects and interfaces exist, which are predicted to display novel phenomena. These can be domain walls, twin planes, the areas around dislocations, etc., in which the long-range periodicity of the crystal structure, and therefore the magnetic or the ferroelectric ordering, breaks down. Such features can be considered as individual entities having their own electronic structure, chemistry and symmetry. These nano-objects are present in many functional materials, whose macroscopic properties are well-known to us, but they are inaccessible using the standard macroscopic characterization techniques. We propose to investigate domain walls and other sources of nanoscale symmetry breaking, such as the boundaries between two phases in nanocomposites and nanostructures. With the recent progress in theoretical modeling and thin film growth techniques, the density and configuration of nanowalls can be tuned to a great extent. Standard diffraction and characterization techniques (SQUID or standard ferro-piezoelectric measurements) are not longer adequate to characterize these materials. Scanning probe techniques are essential in a modern physical chemistry lab and the proposed project would allow further development of our expertise in this direction, ultimately aimed at the design of novel nanomaterials that exploit these or similar mechanisms. We will focus on oxides, mainly perovskites, other ABO3 and spinel compounds, because of the simplicity of their structures and the variety of functional properties that they display, of interest in science and technology: Ferromagnetism, ferroelectricity, piezoelectricity, ferroelasticity, etc. Many of these materials present two or more of these properties. We are particularly interested in exploiting this multi-functionality, that is the coupling between the different degrees of freedom, especially between the electrical and magnetic ones (the so-called multiferroicity). Moreover, we have ample experience working with magnetic, ferroelectric and multiferroic oxides and this knowledge of the macroscopic scale is the starting point of the proposed research. We will synthesize complex oxides in thin film form with well-defined nanoscales. This is possible by depositing the materials epitaxially into adequate single-crystal substrates, using Pulsed Laser Deposition (already available in our lab). Several materials will be studied: In insulating ferromagnets, such as BiMnO3, domain walls will be characterized in other to investigate the local multiferroic properties (appearance of electrical polarization at the wall) that should arise at the walls between some ferromagnetic domains. This has been recently suggested by theorists as a novel mechanism for multiferroicity . Ferromagnetic ferroelectric BiMnO3 and antiferromagnetic ferroelectric BiFeO3 are among the most promising single-phase multiferroics for room temperature applications. The use of local probes will help understand the magneto-electric coupling by observing direcly (anti-) ferromagnetic and ferroelectric domain walls and their interaction. In mismatched nanostructures and nanocomposites of magnetic materials, the boundaries between two phases are sources of strain gradients. There inversion symmetry breaks down and local piezoelectric response is expected at the nanometer scale even in non-piezoelectric (centrosymmetric) materials , leading to local multiferroicity. Besides coupling magnetism and ferroelectricity, there are various other coupling mechanisms to exploit symmetry breaking in nanowalls. Theory suggests that in selected metallic ferromagnetic oxides such as Fe3O4, anti-phase domain walls can couple antiferromagnetically. This would result in a spin-valve behavior. Switching the magnetization switches the resistivity potentially by many orders of magnitude. In manganite perovskites, such as (La,Ca)MnO3, particular twin planes are expected to exhibit Charge Ordering. This type of nanowall would result in spin-polarized tunneling. While such behavior affects the bulk properties, the true nature can only be assessed by investigating these properties on a microscopic scale. The general idea of this proposal is to learn from these model systems how nature solves symmetry breaking, elastic mismatching and chemistry unbalances at the molecular level, giving place to combined functionalities that are not common in the macroscopic world. The ultimate goal would be to make use of this by artificially scaling these systems to workable sizes with efficient designs. |
