Phase separation in inhomogeneous mixed-valence systems: Application to manganites

Abstract

Manganites are inhomogeneous mixed-valence systems in which the Mn ions coexist in two magnetic configurations, Mn4+ and Mn3+. We consider a simple alloy-analogy model with a charge interaction between nearest neighbor Mn ions. In addition there is an antiferromagnetic (AF) (superexchange) interaction between nearest neighbor ions of equal valence, while ions of different valence are coupled ferromagnetically (FM) (double exchange). Assuming classical spins, we show that the system is unstable to phase separation, i.e., to the coexistence of AF and charge ordered FM domains. In a magnetic field the FM domains increase at the expense of the AF ones.