Anomalous temperature dependence of magnetic coercivity and structure property correlations in Bi0.75A0.25FeO3 (A = Sr, Pb, and Ba) system

We have systematically investigated the effects of divalent dopants (Sr2+, Pb2+, Ba2+) of varying ionic sizes on the structural and electronic properties of BiFeO3. These include multiferroic, magnetic, Mossbauer and optical measurements, and, in particular, the temperature dependence of the magnetic coercivity. Mossbauer results showed that Fe ions retained their trivalent state, and divalent ion substitution at the trivalent site results in generation of oxygen vacancies. Optical measurements showed that the d-d transition energies develop a small blue shift (90-100 meV), consistent with decreased crystal field splitting while the direct band gap also showed blue shift as a consequence of decreased chemical pressure. The saturation magnetization of the system increases with increasing size of the dopant. This enhancement is ascribed to the local lattice distortions induced by the size difference between the host and the dopant, generation of oxygen vacancies and the suppression/destruction of the antiferro-magnetic spiral spin structure. We found an anomalous but systematic decrease in the magnetic coercivity (H-c) at low temperatures, which is explained in terms of an effective magnetic anisotropy that includes the effects of magnetoelectric coupling. The almost linear decrease in Hc with decreasing temperature is qualitatively explained within the effective anisotropy model and the Landau mean field formulation. Our findings give further insight into the coercivity mechanism of Bi(0.75)A(0.25)FeO(3) (A = Sr-2, Ba2+, and Pb2+) bulk ferromagnets and suggest that the coercivity of such multiferroics can be tailored by the combination of magnetic anisotropy and magnetoelectric interaction.