Antiferromagnet–ferromagnet transition in the cobaltites

Abstract

The three series oxygen-deficient cobaltites La₀.₅Ba₀.₅CoO₃₋δ, LnBaCo₂O₅.₅ and Sr₂YCo₄O₁₀.₅ have been studied. It has been shown that La₀.₅Ba₀.₅CoO₃ is an insulating ferromagnet whereas La₀.₅Ba₀.₅CoO₂.₇₅ is a pure antiferromagnet in which the oxygen vacancies are disordered. The oxygen-vacancies ordering leads to appearance of the ferromagnetic component apparently due to a formation of the noncollinear magnetic structure. The antiferromagnet–“ferromagnet” transition is accompanied by a giant magnetoresistance. It is suggested that in the ferromagnetic oxidized compounds Co³⁺ and Co⁴⁺ ions adopt intermediate spin state whereas for antiferromagnetic (Co⁴⁺-free) compositions Co³⁺ ions have high-spin state (pyramids CoO₅) and dominant low-spin state (octahedra CoO₆). In both ferromagnetic and antiferromagnetic compounds the superexchange via oxygen plays an essential role in a formation of the magnetic properties.