Effect of oxygen content variations on structural and magnetic features in La$_{0.5}$Sr$_{0.5}$FeO$_{3-\delta}$

The dependences of the magnetic moment on the temperature $M(T)$ and the magnetic field strength $M(H)$ of polycrystalline samples La$_{0.5}$Sr$_{0.5}$FeO$_{3-\delta}$ before and after vacuum annealing are studied in detail. Four critical temperatures are visible on the curves $M(T)$ of the sample before annealing. The temperature $T_1=T_{\mathrm{N}}\approx$ 230 K corresponds to the Néel temperature of the compound La$_{0.5}$Sr$_{0.5}$FeO$_{3-\delta}$, at which a weak “parasitic” ferromagnetism is established in the sample. The temperature $T_2=T_{\mathrm{V}}\approx$ 195 K corresponds to the Vervey temperature, below which the localization of charge carriers occurs in the sample. The temperature range from $T_3\approx$ 170 K to $T_4\approx$ 80 K apparently corresponds to the formation of a frustrated spin glass state in the sample. Vacuum annealing leads to a noticeable increase in the Néel temperature $T_{\mathrm{N}}$. The magnetometry data are also confirmed by the data of Mössbauer and Raman spectroscopy.