Ferromagnetic compounds of manganese with perovskite structure

Various manganites of the general formula La ³⁺Mn ³⁺O ₃^2--Me ²⁺Mn ⁴⁺O ₃^2- have been prepared in the form of polycrystalline products. Perovskite structures were found, i.a. for all mixed crystals LaMnO ₃CaMnO ₃, for LaMnO ₃SrMnO ₃ containing up to 70% SrMnO ₃, and for LaMnO ₃BaMnO ₃ containing less than 50% BaMnO ₃. The mixed crystals with perovskite structure are ferromagnetic. Curves for the Curie temperature versus composition and saturation versus composition are given for LaMnO ₃CaMnO ₃, LaMnO ₃SrMnO ₃, and LaMnO ₃BaMnO ₃. Both types of curves show maxima between 25 and 40% Me ²⁺Mn ⁴⁺O ₃^2-; here all 3 d electrons available contribute with their spins to the saturation magnetization. The ferromagnetic properties can be understood as the result of a strong positive Mn ³⁺Mn ⁴⁺ exchange interaction combined with a weak Mn ³⁺Mn ³⁺ interaction and a negative Mn ⁴⁺Mn ⁴⁺ interaction. The Mn ³⁺Mn ⁴⁺ interaction, presumably of the indirect exchange type, is thought to be the first clear example of positive exchange interaction in oxidic substances.