Effect of annealing atmosphere on magnetism for Fe-doped BaTiO3 ceramic

Ba(Ti0.3Fe0.7)O3 ceramic was prepared by solid-state reaction and post-annealed in vacuum and oxygen, respectively. The as-prepared and annealed samples are all single-phase, crystallizing in a 6H–BaTiO3-type hexagonal perovskite structure. Room-temperature ferromagnetism is exhibited in all ceramics. For the as-prepared sample, the super-exchange interactions of Fe3+ in different occupational sites (pentahedral and octahedral sites) are expected to produce the ferromagnetism observed. After annealing in vacuum, the magnetization is reduced while the exchange mechanism remains unchanged. On the contrary, O2 annealing can effectively enhance the magnetization due to the presence of Fe4+, an unusual valence for iron. The simultaneous presence of Fe3+ and Fe4+ allows new exchange mechanism responsible for the ferromagnetic interaction. The exchange couplings of Fe ions with mixed valences (Fe3+ and Fe4+) determine the magnetic behavior.