Spin State of Cobalt Ions and the Magnetic Properties of Heavily Ba‐Substituted Cobaltites with Perovskite Structure

Abstract

The evolution of the magnetic and magnetotransport properties and of the crystal structure in the cobaltites La0.45Ba0.55CoO3‐δ and La0.4Ba0.6CoO3‐δ depending on the oxygen content is investigated by various methods. The oxygen content decrease leads to a stabilization of the oxidation state 3+ of the cobalt ions and to a transition from ferromagnetic conducting state into an insulating antiferromagnetic long‐range one. Application of a magnetic field gradually transforms the antiferromagnetic state into a ferromagnetic one. This transition is accompanied by a large magnetoresistance effect. There is no obvious temperature dependence of the average cubic unit cell parameter, however, temperature evolution of diffraction peak profiles suggests a phase separation scenario apparently due to a spin crossover. It is assumed that the ferromagnetic component corresponds to the collective intermediate spin state of cobalt ions while antiferromagnetic phase corresponds to a mixed high/low spin state.