An interpretation of the magnetic properties of the perovskite-type mixed crystals La1−xSrxCoO3−λ

Abstract It is pointed out that trivalent cobalt with an octahedral co-ordination of oxygen ions has a low-spin and a high-spin state of comparable energy. The anomalous magnetic properties of the perovskite-type mixed crystals La 1− x Sr x CoO 3− λ are interpreted in the light of this fact. It is assumed that the tetravalent cobalt ion is always in the low-spin state in this system. It is shown how the geometry of the perovskite-type lattice lends itself to the formation of two sublattices, each consisting of a set of (111) planes, in one of which the cobalt ions are predominantly in a high-spin state and in the other in a low-spin state. Experiments indicate that the magnetic interaction between trivalent cobalt ions in the high-spin state is antiferromagnetic, whereas the interaction between the low-spin tetravalent cobalt ions and the high-spin trivalent cobalt ions is ferromagnetic. These interactions are considered in relation to modified superexchange and the semicovalent-exchange models. The ferromagnetic Co 3+ −Co IV interaction permits a sharp distinction to be made between the two, although there is always some ambiguity in qualitative models. Further, the ferromagnetic interaction can be understood without recourse to Z ener 's double-exchange mechanism.