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

Abstract

We report on the magnetic, resistive, and structural studies of perovskite La${}_{1/3}$Sr${}_{2/3}$CoO${}_{3-\hat{I}´}$. 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 $\hat{I}´=0.00-0.49$. These samples show three structural phases at $\hat{I}´=0.00-0.15$, $\approx 0.25$, $\approx 0.5$, and two-phase behavior for other oxygen contents. The stoichiometric material with $\hat{I}´=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_{\mathrm{C}}=274$ K. The increase of $\hat{I}´$ to 0.15 is followed by a linear decrease of $T_{\mathrm{C}}$ to $\approx$ 160 K and a metal-insulator transition near the boundary of the cubic structure range. Further increase of $\hat{I}´$ results in formation of a tetragonal $2a_p\times 2a_p\times 4a_p$ phase for $\hat{I}´\approx 0.25$ and a brownmillerite phase for $\hat{I}´\approx 0.5$. At low temperatures, these are weak ferromagnetic insulators (canted antiferromagnets) with magnetic transitions at $T_{\mathrm{m}}\approx 230$ and 120 K, respectively. At higher temperatures, the $2a_p\times 2a_p\times 4a_p$ phase is $G$-type antiferromagnetic between 230 K and $\approx$360 K. Low temperature magnetic properties of this system for $\hat{I}´<1/3$ 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 $T_{\mathrm{C}}$. For $\hat{I}´>1/3$, there appears to be a combination of Co${}^{2+}$ and Co${}^{3+}$ ions, both in the high-spin state with dominating antiferromagnetic interactions.

RevTeX, 9 pages, 7 figures, to be published in Physical Review B