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تسجيل مستخدم جديدTuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltites
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We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-delta}$. 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 $delta=0.00-0.49$. These samples show three structural phases at $delta=0.00-0.15$, $approx0.25$, $approx0.5$, and two-phase behavior for other oxygen contents. The stoichiometric material with $delta=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_{rm C}=274$ K. The increase of $delta$ to 0.15 is followed by a linear decrease of $T_{rm C}$ to $approx$ 160 K and a metal-insulator transition near the boundary of the cubic structure range. Further increase of $delta$ results in formation of a tetragonal $2a_ptimes 2a_p times 4a_p$ phase for $deltaapprox 0.25$ and a brownmillerite phase for $deltaapprox0.5$. At low temperatures, these are weak ferromagnetic insulators (canted antiferromagnets) with magnetic transitions at $T_{rm m}approx230$ and 120 K, respectively. At higher temperatures, the $2a_ptimes 2a_p times 4a_p$ phase is $G$-type antiferromagnetic between 230 K and $approx$360 K. Low temperature magnetic properties of this system for $delta<1/3$ 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 $T_{rm C}$. For $delta>1/3$, there appears to be a combination of Co$^{2+}$ and Co$^{3+}$ ions, both in the high-spin state with dominating antiferromagnetic interactions.
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