We have been systematically developing strategies for making new perovskite manganites with novel magnetic and electronic properties. This effort requires two-fold approach: development of understanding the dependence of properties on chemical and structural factors and development of ability for synthesis of desired compounds. We show that similar to other single valent 3d systems the magnetic superexchange interactions in RMnO3 manganites (R = Rare or Alkaline Earth's) are dependent on the Mn-O-Mn bond angle that is a function of interatomic distances R-O and Mn-O. The local structural disorder on the R-site suppresses magnetic interactions. Recently, by comparing disordered (randomly mixed R/Ba) and ordered (forming Ba/R/Ba layers along the c-axes) perovskites we have demonstrated much more conspicuous effects of structural and charge disorder for mixed-valent manganites. We show that by stabilizing ordered structure; i.e., by suppressing local structural and charge disorder, for R = La and Pr a substantial increase of Tc (from 340 and 200 to 365 and 335 K, respectively) can be achieved.
To achieve these compounds, we have been systematically developing special synthesis techniques for extending chemical composition ranges far beyond those previously achieved, developing rules for predicting what compositions it should be possible to make by these special techniques, and establishing methods for selectively ordering or disordering mixtures of metal atoms on the R or B sites. Our design rules for the synthesis of metal-oxide compounds enable the chemical compositions and crystal structures of these materials to be controlled in order to achieve the desired magnetic and electronic properties for applications.
Work at NIU was supported by the NSF-DMR-0105398 and by the State of Illinois under HECA. At ANL work was supported by the U.S. Department of Energy, Division of Basic Energy Science ? Materials Sciences, under contract No. W-31-109-ENG-38.