Structural and Magnetic studies of Orthoferrite BaFeO3−δ

Abstract

The orthoferrites BaFeO3−δ can adopt several competing structural phases depending on the oxygen content (3 −δ) of the formula unit which can be varied continuously between δ = 0 to δ = ½. The end members at δ = 0 and δ = ½ correspond, respectively, to struc-tures with cubic and monoclinic symmetries. The δ = ½ member having the monoclinic symmetry has the well-known Brownmillerite type structure. For intermediate values of δ, structures with hexagonal, tetragonal, rhombohedral and cubic symmetries have been controversially reported by several authors. In this thesis, we investigated the conditions required for the formation of these various phases and those for growing single-crystal of the Brownmillerite phase Ba2 Fe2O5 (δ = ½). Effect of La doping on the structural and physical properties of BaFeO3−δ were also studied. The polycrystalline samples of BaFeO3−δ and their La-doped variants were obtained by using the conventional solid-state reaction route under varying heat-treatment conditions. The single-crystal of the Brownmillerite phase is grown using the floating-zone method associated with a four-mirror image furnace. We find that BaFeO3−δ synthesized under air in the temperature range 1183-1373 K and with a cooling rate of 450 K/h crystallizes with a hexagonal struc-ture. The same samples when annealed at 1373 K under flowing oxygen tended to be impure containing small quantities of unidentifiable impurities. Attempts to synthesize the tetragonal phase under different sintering conditions always resulted in a multiphase sample consisting of a mixtures of hexagonal and monoclinic phases. The Brownmillerite phase, on the other hand, could only be synthesized at temperatures above 1473 K in air with subsequent quenching. Using this as the primary information, attempts were made to grow single crystals of the Brownmillerite phase using a rapid growth technique in an image furnace. Preliminary work suggests that the floating-zone can be stabilized under synthetic air with growth speeds ranging from 5 to 10 mm/h. La-doping in the hexagonal BaFeO3−δ tends to stabilize the tetragonal phase. While 1 % La-doped sample retains the hexagonal structure, the sample with higher La-doping contained increasing amounts of the tetragonal phase. The sample with 20 % La-doping is found to be purely tetrago-nal. Magnetization measurements were performed on pure BaFeO3−δ (hexagonal) and 20 % La-doped BaFeO3−δ (tetragonal) samples. Both samples show weak ferromagnetism at low temperatures as inferred from hysteresis loops in their isothermal magnetization measured at T = 2 K. The temperature variation of magnetization of the undoped sample exhibits magnetic anomalies near T = 250 K, 170 K and 110 K. In the La doped sample corresponding anomalies were present, however, they appear at lower temperatures of T = 225 K, 70 K and 40 K, respectively. The possible origins of this magnetic anomalies is discussed. The resistivity measurements show an increase of the electrical conductivity due to the electron-doping in the La-doped samples.