Correlation between structure, oxygen content and the multiferroic properties of Sr doped BiFeO3

In this study a series of Bi1−xSrxFeO3−δ (0 ⩽ x ⩽ 0.45) multiferroic samples have been prepared in order to study the effect of divalent Sr content on the crystal structure, dielectric and magnetic properties. Rietveld refinement of X-ray diffraction data revealed a structural phase transition from rhombohedral to pseudo cubic phase at x = 0.25. Mössbauer measurements showed that Fe ions retained their trivalent state while the Sr2+ ion substitution at Bi3+ site results in oxygen deficiency and in tetrahedral coordination for some of the Fe-O ions. For x ⩾ 0.25 all the compositions are ferromagnetic with a strong magnetization enhancement between x = 0.15 and x = 0.25. The ferromagnetic onset is consistent with the structural transformation at x = 0.25 that is understood to destroy the parent antiferromagnetic cycloidal spin structure. Oxygen annealing was observed to result in a decrease of the magnetic moment suggesting that oxygen vacancies also contribute to the observed magnetization. The observed decrease in the values of magnetic coercivity at low temperatures is suggested to be indicative of magnetoelectric coupling in these multiferroic samples. The dielectric response depends in general on the crystal structure and oxygen vacancies while the x = 0.45 composition shows a marked anomaly in ε(T). This anomaly is explained in terms of relaxation effects originating presumably in nanoscale polar inhomogeneities.