J. H. van Santen

Ferromagnetic compounds of manganese with perovskite structure

Abstract 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.

Magnetic compounds wtth perovskite structure III. ferromagnetic compounds of cobalt

Synopsis Polycrystalline mixed crystals (La, Sr) CoO 3 have been prepared. Perovskite structure is found for all compositions. Ferromagnetism is observed for intermediate Sr concentrations. Curves are given for the saturation magnetizations, the paramagnetic Curie temperatures, and the effective paramagnetic moments as a function of composition. It is suggested that the ferromagnetism observed is caused essentially by a positive Co 3+ — Co 4+ interaction. The sign of the exchange interaction is discussed in connection with the theories of Anderson and Polder, and of Zener.

On a generalization of the Lorentz-Lorenz formula

Abstract The well-known Lorentz-Lorenz formula holds only for a crystal in which all atoms have environments with cubic symmetry. In this paper a derivation is given of an analogous formula, valid for the more general case that the environments of atoms are not necessarily cubic, the macroscopic symmetry of the crystal being still cubic. The formula is applied to the perovskite lattice.