On O¹⁶–O¹⁸ Isotope Effects in Manganese Perovskites

Abstract

Large isotope effects which have been found by others [1,2] in some colossal magneto resistance manganite perovskites, AMnO₃, are evidence of unusually strong interactions between the lattice and magnetism. We offer a model which is based upon the approximate degeneracy of two Mn⁺³ states: one with high spin S = 2, and the other with low spin S = 1. These states have different radii and different electron form factors. They thus have different force constants governing the interaction with neighboring oxygen ions which provide the sought for link between magnetism and oxygen mass. The experiments can be understood with the reasonable assumption that the LS has greater force constants than HS. The dependence of changes in T_c with isotopic substitution as a function of the A-ion radius, the metal insulator transitions and the Mossbauer effect changes, are discussed in terms of this model.