Design Rules for Manganites with Novel Magnetic and Electronic Properties

We have been systematically developing strategies for making new perovskite manganites with novel magnetic and electronic properties. This effort requires a two-fold approach: understanding the dependence of properties on chemical and structural factors and development of the ability to synthesize desired compounds. We show that, similar to other single valent 3d systems, the magnetic superexchange interactions in AMnO$\text{}_{3}$ manganites (A = rare or alkaline earth's) are dependent on the Mn-O-Mn bond angle which is a function of interatomic distances A-O and Mn-O. The local structural disorder on the A-site suppresses magnetic interactions. Recently, by comparing disordered (randomly mixed La/Ba) and ordered (forming Ba/La/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 the ordered structure; i.e., by suppressing local structural and charge disorder on the A-site, a substantial increase in T$\text{}_{C}$ from 340 to 365~K can be achieved. A similar control of order on the Mn-site was achieved for SrMn$\text{}_{1-y}$Ga$\text{}_{y}$ O$\text{}_{3-\text{}_{y}\text{}_{/}\text{}_{2}$ compounds near y=0.5, while SrMn$\text{}_{1-y}$Fe$\text{}_{y}$O$\text{}_{3}$ compounds are always randomly mixed. 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 which compositions should be possible to produce by these special techniques, and establishing methods for selectively ordering or disordering mixtures of metal atoms on the A- or Mn-sites.