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

摘要

We report on the magnetic, resistive, and structural studies of perovskite La_(1/3)Sr_(2/3)CoO_(3-δ). By using the relation of synthesis temperature and oxygen partial pressure to oxygen stoichiometry obtained from thermo-gravimetric 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 T_C=274 K. The increase of δ to 0.15 is followed by a linear decrease of T_C to ≈ 160 K 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 2a_p X 2a_p X 4a_p 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 T_m ≈ 230 and 120 K, respectively. At higher temperatures, the 2a_p x 2a_p X 4a_p phase is G-type antiferromagnetic between 230 K and ≈360 K. Low temperature magnetic properties of this system for δ < 1/3 can be described in terms of a mixture of Co~(3+) ions in the low-spin state and Co~(4+) ions in the intermediate-spin state and a possible spin transition of Co~(3+) to the intermediate-spin state above T_C. For δ > 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.