Sr2/3Y1/3CoO8/3+δ: Transition from insulating antiferromagnet to metallic ferromagnet by control of the oxygen content

Magnetic and electronic properties of the oxygen deficient ordered perovskite, Sr2/3Y1/3CoO8/3+δ, have been studied for two different oxygen contents corresponding to δ=0.00 and 0.04 in the chemical formula. For the former, at low temperature, the background state is antiferromagnetic insulating (TN=290K,ρ10K=4×105Ωcm) as expected from the presence of trivalent cobalt in the high spin-state. Remarkably, the more oxidized compound with a cobalt oxidation state of ≈3.08 is a ferromagnetic half-metal with TC=225Kandρ10K=2×10-3Ωcm. Consistently, upon application of an external magnetic field, the spin-scattering reduction in the TC vicinity is responsible for a weak negative magnetoresistance. These dramatic changes of the physical properties for such a slight increase of the cobalt oxidation state are interpreted as a result of the structural disordering created by the extra oxygens. The thermoelectric power measurements, showing a sign change of the Seebeck coefficient as the oxygen content increases, indicate that electrons moving in a metallic eg band dominate the transport properties of the ferromagnetic and metallic compound. This suggests the existence of an orbital ordering in the pristine compound, related to an ordered array of CoO4 tetrahedra, which can be collapsed by the presence of these extra oxygen anions.