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 θ and the interatomic distances through the band-width relation W ~ cosθ/ n (n ~ 3.5). It was observed previously that TN ~ W2 ~ for RMO3 with M = Cr3+ (t3), Fe3+ (t3e2), and Ni3+ (t6e1) when are constant. We have observed similar dependence for Mn3+ (t3e1). When the 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 TN for Sr1−xCaxMn4+O3 because of a local variance of bond angles Δθ. By removing the effects of stretched bonds and size disorder we have increased Curie temperatures of the layer ordered manganites R1−yBayMnO3. Frequently interesting compounds with θ ~ 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 θ 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.