Order/Disorder Effects in Perovskite Manganites

Indirect magnetic interactions between transition metals in perovskites ABO$_{3}$ are commonly described in terms of superexchange and double-exchange interactions that depend on the bond angle B-O-B. By useing the tolerance factor parameterization procedure we have designed and obtained new substituted manganites with randomly-mixed and layer-ordered structures having identical chemical compositions. The order/disorder effects on magnetic properties have been investigated as a function of the bond angle disorder for both the A- and Mn- sites. By comparing ordered (containing planes of the RO$_{2}$ and BaO$_{2})$ and disordered ferromagnetic compounds La$_{1-x}$Ba$_{1+x}$Mn$_{2}$O$_{6}$, we have demonstrated that by decreasing local structural and charge disorder, an increase of T$_{c}$ of over 100 K can be achieved. Similar effects have been demonstrated for Sr$_{2}$MnGaO$_{6-d}$ where transitions from spin-glass seen for the disordered compound to antiferromagnetic behavior seen for the ordered material (containing planes of the MnO$_{2}$ and GaO$_{2-d})$, have been observed. Supported by NSF (DMR-0302617) and U.S. Department of Energy, BES -- Materials Sciences (W-31-109-ENG-38)