The effect of Sr concentration on the crystal and magnetic structures of La$_{1-x}$Sr$_x$FeO$_{3-\gamma}$ orthoferrite

The structural and magnetic features of substituted La$_{1-x}$Sr$_x$FeO$_{3-\gamma}$ have been studied, in both synthesized and vacuum-annealed samples, depending on the Sr content (for $x$ = 0.33 and 0.50), using X-ray diffraction, Mössbauer spectroscopy and magnetic measurements. It has been shown that in the as-prepared samples, the volume of the pseudocubic perovskite cell decreased and the number of vacancies and Fe$^{4+}$ ions increased with an increase in the Sr concentration. The Néel temperatures were determined for two compositions of the compound: $T_\mathrm{N}\approx$ 233 K for the La$_{0.5}$Sr$_{0.5}$FeO$_{3-\gamma}$ sample and $T_\mathrm{N}\approx$ 385 K for the La$_{0.67}$Sr$_{0.33}$FeO$_{3-\gamma}$ sample. The contributions of Fe$^{3+}$ ions with different local environments were redistributed under vacuum annealing. That led to changes in the $J_\mathrm{F}/J_\mathrm{AF}$ ratio, a shift in the balance towards increasing antiferromagnetism, a noticeable increase in the Néel temperature $T_\mathrm{N}$, and a decrease in the width of the magnetic hysteresis loops and the magnetization value.