Parameters controlling magnetic transitions in manganites

Using description of structural and physical properties of perovskites in terms of the tolerance factor t(x,T,d), which is dependent on composition, temperature, and oxygen-content, we have established synthetic methods and studied composition-structure-properties for new manganites La$_{1-x-y}$Sr$_{x}$Ba$_{y}$MnO$_{3-d}$ far beyond solubility limits normally achieved during the synthesis in air. Parameters controlling magnetic transitions have been identified through examples of single-valent compounds of RMnO$_{3}$ (the Mn-O-Mn bond angles that can be equivalently described in terms of the tolerance factor) and Sr$_{1-x}$Ca$_{x}$MnO$_{3}$ (the tolerance factor and the variance of A-site ion sizes), and the mixed-valent La$_{0.5}$Sr$_{0.5-y}$Ba$_{y}$MnO$_{3}$ (the tolerance factor, variance of sizes, and the local strains described in terms of the elongated Mn-O bonds). By using an example of kinetically stable, atomically-ordered layered-perovskites RBaMn$_{2}$O$_{6}$ we show that the increase of Curie temperature T$_{c}$ and enhancement of colossal magneto-resistive effect at room temperature, can be achieved through reduction of variance of sizes and local strains. Work at NIU was supported by the NSF-DMR-0302617 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357.