Crystal structure, magnetic properties, and mossbauer studies of La_0.6Sr_0.4FeO_3-δprepared by quenching in different atmospheres

Abstract

Samples of La_0.6Sr_0.4FeO_3-δ compounds prepared by quenching in different gaseous environments were studied by x-ray diffraction, neutron diffraction, magnetization measurements, and Mossbauer spectroscopy (MS). All materials are single phase and crystallize in the rhombohedral perovskite structure. Samples prepared in flowing air, N₂, and O₂ yielded oxygen vacancies ranging from 0% to 1%. The oxygen vacancy concentration increases from 6.8% to 9.6% as the ratio of CO/CO₂ changes from 10:90 to 90:10. The air-, N₂-, and O₂-quenched samples have a magnetic ordering temperature in the range of 300-325K. The magnetic ordering temperature increases for all the samples subjected to the reducing CO/CO₂ atmosphere. The neutron data refinements and magnetization data indicate that the Fe sublattice of La_0.6Sr_0.4FeO_3-δ has an antiferromagnetic structure below the magnetic ordering temperature. The Fe atoms possess a magnetic moment of 3.8μ_B and a hyperfine field of 53 T in the CO/CO₂-quenched samples. It is found that the heat treatment in the CO/CO₂ atmosphere creates more oxygen vacancies, changes the Fe valence states, and increases the unit cell volume. In the meantime, the Fe-O-Fe bond angle increases. These dramatically affect the Fe-O-Fe superexchange coupling. The change of the Fe-O-Fe bond angle and the change of the Fe valence states in the CO/CO₂ heat treatment play a key role in the increase of the magnetic ordering temperatures and the magnetic moment. Therefore by creating oxygen vacancies or having excess oxygen, the exchange interaction of Fe-O and the valence state of Fe ions are affected, and lead to large changes in the magnetic properties, such as the magnetic ordering temperature, the magnetic moments, and the hyperfine interactions in the pervoskite structure.