Phase stability, oxygen nonstoichiometry and magnetic properties of BiFeO3−δ

The phase stability of polycrystalline $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ (BFO) was measured using coulometric titration in the technologically important temperature range for growth of epitaxial thin films, $680°\mathrm{C}–800°\mathrm{C}$ . BFO exhibits only a very narrow oxygen nonstoichiometry, i.e., $\delta <0.01$ . For the most nonstoichiometric sample obtainable at $800°\mathrm{C}$ (containing a maximum ${\mathrm{Fe}}^{2+}$ fraction of $2\mathrm{mol}\%$ ) the saturation magnetization was $3.1\times {10}^{-4}{\mu }_B∕\mathrm{Fe}$ , a factor of around 2 higher than a nominally fully oxygenated sample. Hence, the contribution of ${\mathrm{Fe}}^{2+}$ to the ferromagnetism is small. The decomposition products of BFO below the oxygen phase stability line are ${\mathrm{Bi}}_{25}\mathrm{Fe}{\mathrm{O}}_{40-x}$ and $\gamma \text{-}{\mathrm{Fe}}_2{\mathrm{O}}_3$ .