Magnetic and Mössbauer Studies on Oxygen Deficient Perovskite, La₀.₆Sr₀.₄FeO_3-δ

Samples of La _0.6 Sr _0.4 Fe _3-δ with varying oxygen vacancy contents were prepared by heating them in different gas flows. Magnetization measurement showed that samples with low oxygen vacancies have a magnetic ordering temperature in the range of 300-325 K while those with 9%-12% oxygen vacancies have a magnetic ordering temperature of 800 K and higher. Mössbauer spectra at 300 K exhibit paramagnetic or weak magnetic characteristics for the N ₂ , O ₂ , and air-quenched samples, whereas an average hyperfine field of 52 T is found for the CO/CO ₂ reduced samples. The heat treatment in the reducing atmosphere creates oxygen vacancies and increases unit cell volume. However, the Fe-O bond length remains nearly constant, resulting in distortion/rotation of the oxygen octahedra which increases the Fe-O-Fe bond angle as much as 12 degrees. This dramatically affects the Fe-O-Fe superexchange coupling and plays a key role in the increase of the Nèel temperatures.