Parameters controlling magnetic interactions in perovskite manganites

Magnetic superexchange interactions in single-valent perovskites (R,A)MO3 (R,A = Rare and Alkaline Earth's, M = transition metal) are known to dependent on the M-O-M bond angle theta and the interatomic distances < M-O > through the band-width relation W similar to cos theta/< M-O >(n) (n similar to 3.5). It was observed previously that T-N similar to W-2 similar to < cos(2)theta > for RMO3 with M = Cr3+ (t(3)), Fe3+ (t(3)e(2)), and Ni3+ (t(6)e(1)) when < M-O > are constant. We have observed similar dependence for Mn3+ (t(3)e(1)). When the < M-O > bonds were stretched beyond their equilibrium values in mixed-valent La0.5Sr0.5-xBaxMn3.5+O3, a strong suppression of magnetic interactions was observed. In addition, size disorder on the A-site suppressed T-N for Sr1-xCaxMn4+O3 because of a local variance of bond angles Delta theta. By removing the effects of stretched < M-O > bonds and size disorder we have increased Curie temperatures of the layer ordered manganites R1-yBayMnO3. Frequently interesting compounds with theta similar to 180 deg are not possible to obtain due to the limited range of sizes and charges of the A and R ions. Recently, in an attempt to increase the bond angle theta in compounds, which are not possible to obtain through simple A-site substitutions, we have studied the double substituted R- and Mn-site perovskites. Properties of these compounds depend mostly on dilution of the magnetic Mn-O lattice in addition to the previously discussed parameters.