Substitution effects on orbital ordering and multiferroicity in some perovskite-like candidates for advanced functional materials

Substitution is a feasible means in designing novel functional materials with perovskite-like structure. Both A- and B- sites of the perovskite structure could be affected. We show that the magnetic interactions and other electronic properties of important materials such as colossal magnetoresistive (CMR) hexagonal double perovskites and manganates, or multiferroic RMn₂O₅ (R=rare earth metal) could be influenced but to understand why the compound does not display the expected properties calls for detailed information on microscopic level. Indeed, unlike ferromagnetic and half metallic Sr₂FeMoO₆, which is a paradigmatic CMR compound, lack of long-range magnetic ordering and spin glass behavior was established in Ba₂MSbO₆ (M=Fe, Co) due to a significant antisite disorder. In the parent charge ordered Bi_0.5A_0.5Fe_xMn_1-xO₃ (A=Ca, Sr) substitution with Fe³⁺ for Mn³⁺ destroyed the charge order known to hamper CMR behavior without to induce magnetoresistive effect. Similarly, although partial substitution of Mn seems to be the way to increase the crosslink between magnetization and electric polarization in multiferroic YbMn₂O₅ we found that it introduces a low level of disorder between the two transition metal positions in the YbFeMnO₅ structure. The lack of evidence for a crystallographic phase transition to a polar space group rules out expectations of a spontaneous electric polarization. In addition, the observed collinear magnetic structure with k = 0 does not permit a spin polarization and, therefore, YbMnFeO₅ is not expected to be a multiferroic compound.