G.H. Jonker

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 with perovskite structure IV Conducting and non-conducting compounds

Synopsis A further experimental contribution to the knowledge of the magnetic interactions in compounds with perovskite structure is given. LaMnO3 is an antiferromagnetic compound, but it is shown that it would be ferromagnetic if the cubic perovskite structure, present at high temperatures, could be preserved at low temperature. Besides the positive magnetic interaction between Mn3+- and Mn4+ -ions an interaction of the same sign is found between Mn3+- and Cr3+ -ions. A survey is given of the results of this and former investigations.

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.