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 La1/3Sr2/3CoO3−�. 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 � = 0.00 0.49. These samples show three structural phases at � = 0.00 0.15, � 0.25, � 0.5, and two-phase behavior for other oxygen contents. The stoichiometric material with � = 0.00 is a cubic ferromagnetic metal with the Curie temperature TC = 274 K. The increase ofto 0.15 is followed by a linear decrease of TC to � 160 K and a metal-insulator transition near the boundary of the cubic structure range. Further increase ofresults in formation of a tetragonal 2ap ×2ap ×4ap phase for � � 0.25 and a brownmillerite phase for � � 0.5. At low temperatures, these are weak ferromagnetic insulators (canted antiferromagnets) with magnetic transitions at Tm � 230 and 120 K, respectively. At higher temperatures, the 2ap × 2ap × 4ap phase is G-type antiferromagnetic between 230 K and �360 K. Low temperature magnetic properties of this system for � 1/3, there appears to be a combination of Co 2+ and Co 3+ ions, both in the high-spin state with dominating antiferromagnetic interactions.