Magnetic properties of cobalt in oxide lattices

Abstract

The abnormally low value of the magnetic susceptibility of Co 3 O 4 and ZnCo 2 O 4 and the uncertainty about the valence of the cobalt ions in these spinels has led to the investigation of mixed crystals of CoO and ZnO. The magnetic susceptibility of the ( Co, Zn) O mixed crystals, measured up to 1200°K, could be represented by ζ = α + C T − θ with a fairly large temperature-independent paramagnetism (α = 0.5 × 10 −3 e.m.u.) and a magnetic moment for the Co 2 + ions in tetrahedral interstices of 4.05 μ β. The susceptibility measurements on several samples of ZnCo 2 O 4 indicated that Co 3 + ions are present in the octahedral holes. They showed only a small temperature-independent paramagnetism of 0.1 × 10 −3 e.m.u. It appears that in this spinel tervalent cobalt behaves in a similar way as in the numerous six-coordinated complex compounds. The temperature dependence of the molar susceptibility of Co 3O 4 : ζ m = α + C T − θ with α = 0.7 × 10 −3 e.m.u. and μ = 4.14μ β is now easily explained on the assumption that this compound is a normal 2–3 spinel. The tervalent cobalt ions in the octahedral interstices do not exhibit a permanent moment, which explains the low susceptibility values. The Co 3 + ions in the octahedral of the perowskite LaCoO 3 seem to be normally paramagnetic. This difference in behaviour of tervalent cobalt in the octahedra of various oxygen lattices and the behaviour of divalent cobalt in tetrahedra can be qualitatively understood with the aid of the ⪡crystal field⪢ theory.