MAterials Design : Blocks and Layers Assembly for Spin-Tronic – MAD-BLAST

Our project finds its basics in the MAD-BLOCK project previously submitted as an "ANR blanche" proposal in 2008. Here, the main comments concerning the strong fundamental versus applied weight of the proposed sciences have been re-balanced by introducing an additional panel dedicated to spintronic/spin valves systems. Moreover, the choice of the target-materials has been refined in good connection with the feasibility and cohesion of the project, and the background and excellence of each of the new associated laboratories. The project can be divided in two interacting parts : i) Design of new sandwiched materials : The goal is to create new materials usable for thin film sandwiches and multilayers in which magnetic thin films are "naturally" separated by non-magnetic spacers in a similar manner as in the case of spin-valve or artificially antiferromagnetically coupled systems. The original approach is that both magnetic and non-magnetic layers are part of the same material, which, in our project, belongs to the class of so-called 2D cobaltites. These materials are very versatile since their design deals with the development of the well-known concept (in supra-molecular chemistry) of 'self assembly building units' here applied in the field of metal-oxides and derivatives (halogeno-oxides). The present project is based on recent results about new or well-known cobaltites. The crystal structure of these emerging cobaltites such as Ca3Co4O9 or NaxCoO2 reflects the stacking of several building sub-units with different magnetic and transport specificities. We plan to take advantage of these features in order to modify/create new functionalized materials. For instance, recent investigation of the Ba-Co-O/X (X= O, F, Cl, Br) systems has led to a number of new mixed-valent CoIII/CoIV materials that turned out to display complex magnetic properties. From the structural point of view, the concerned compounds and their dimensionality can be deduced from each other by the reorganization of structural blocks isolated by anionic layers. We have investigated the particular dependence of the magnetic orderings on the connectivity of the concerned blocks and empirical rules have been found. So far, our results are in good connection with the conservation of the intra-block properties and the role of the inter-block connectivity on the local Co moments and on the sign and strength of the magnetic exchanges. Here we propose to develop this concept to predict and design new layered cobaltites using several available blocks ([CoO2], [Co8O8] ') and/or local modification (chemical substitution, valence changes '). This approach will be completed by ab-initio calculations to validate the expected stability of materials and their magnetic ordering. The compounds will be structurally (X-Ray diffraction (XRD), neutron diffraction (ND) and TEM techniques such as electron diffraction (ED) and/or high resolution imaging (HREM)) and physically characterized with a special focus on the magnetic and transport properties. A particular attention will be given to the modification of these properties when the material is prepared in thin film form. ii) From the bulk to spin-tronic systems : The design of different cobaltite systems in which the thickness of the magnetic and non-magnetic layers are modulated by the number and type of building blocks used for each layer, could be very interesting for the field of spintronic applications. In a similar manner as the Fe/Cr multilayers, the cobaltites are constituted of natural stacks of magnetic and non-magnetic layers. Although the magnetic ordering temperature in such systems are rather modest and far below room temperature, from the fundamental point of view, cobaltites can constitute a model multilayered system in which the magnetic / non-magnetic interfaces can be easily controlled since no interdiffusion is allowed. The interactions between the magnetic layers could therefore be modulated by the thickness of the non-magnetic spacer. By reducing the total thickness of the cobaltite down to few nanometers (ideally one or two periods of cobaltite), we may expect obtaining a spin-valve type structure when introducing this layer between two magnetic electrodes. Magnetic and transport measurements will be further performed and correlated to band structure calculations.