Spin-Crossover Phenomena in Perovskite Cobaltites : Their History and Current Status(Interdisciplinary)

Abstract

The perovskite cobaltites have been arousing widespread interest of researchers owing to the various spin states of Co^<3+>(3d)^6 ions and crossover among them. It has been known since 1966 that LaCoO_3 shows a spin crossover, or a spin-state transition, around 100K from the low spin (LS) nonmagnetic ground state with S=0 to a magnetic state. The relevant magnetic spin state of Co^<3+> still remains controversial. Although there have been many experimental results supporting the Jahn-Teller active intermediate spin (IS) state with S=1 as the magnetic state, there is instead a spectroscopic evidence for the high spin (HS) state with S=2. We argue a possibility that IS and HS states both excited in the 100K transition coexist above the transition temperature. The inhomogeneous spin phase may be caused by strong interactions among Co^<3+> ions in the magnetic spin states. We also review the magnetic and electronic anomaly around 500K which is commonly observed in RECoO_3 (RE = rare earth elements).