Tuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltites

We report on the magnetic, resistive, and structural studies of perovskite La1∕3Sr2∕3CoO_3-δ. By using the relation of synthesis temperature and oxygen partial pressure to oxygen stoichiometry obtained from thermogravimetric analysis, we have synthesized a series of samples with precisely controlled δ=0.00--0.49. These samples show three structural phases at δ=0.00--0.15, ≈0.25, ≈0.5, and two-phase behavior for other oxygen contents. The stoichiometric material with δ=0.00 is a cubic ferromagnetic metal with the Curie temperature TC=2740.3em0exK. The increase of δ to 0.15 is followed by a linear decrease of TC to ≈1600.3em0exK and a metal-insulator transition near the boundary of the cubic structure range. A further increase of δ results in the formation of a tetragonal 2aₚ×2aₚ×4aₚ phase for δ≈0.25 and a brownmillerite phase for δ≈0.5. At low temperatures, these are weak ferromagnetic insulators (canted antiferromagnets) with magnetic transitions at Tₘ≈230 and 1200.3em0exK, respectively. At higher temperatures, the 2aₚ×2aₚ×4aₚ phase is G-type antiferromagnetic between 2300.3em0exK and ≈3600.3em0exK. Low temperature magnetic properties of this system for δ<1∕3 can be described in terms of a mixture of Co³⁺ ions in the low-spin state and Co⁴⁺ ions in the intermediate-spin state and a possible spin transition of Co³⁺ to the intermediate-spin state above TC. For δ>1∕3, there appears to be a combination of Co²⁺ and Co³⁺ ions, both in the high-spin state with dominating antiferromagnetic interactions.