  Over the years attempts have been made to compensate for the inherent
weaknesses in the bulk state of the multiferroic BiFeO3, such as high leakage
current and the absence of ferromagnetic correlation, and exploit its
magnetoelectric potential by forming solid solutions with other perovskites.
Studies in the recent few years have shown that alloying of BiFeO3 with BaTiO3,
both with and without additives, can induce both ferroelectric and
ferromagnetic switching. While the coexistence of both the ferroic orders is
encouraging from the view point of technological applications, the origin of
ferromagnetism in this system remains elusive. Here, we synthesized such
compositions and carried out a detailed structural analysis employing magnetic
separation of the powder particles. We found that the origin of ferromagnetism
lies in the spontaneous precipitation of a minor ( ~ 1 wt %) barium hexaferrite
(BaFe9O19) phase, often undetected in routine x-ray diffraction studies of
powders sampled from the entire specimen. We also demonstrate that inspite of
the insignificant fraction the ferrimagnetic phase, this self-grown composite
exhibit noticeably enhanced ferroelectric switching in the presence of external
magnetic field. We obtained a dc magnetoelectric coupling of ~ 9 x 10-8 s/m, a
value which is comparable to what has been reported for layered
ferroelectric/ferromagnetic laminates and bilayer thin film
ferroelectric-ferromagnetic hetrostructures. Our study suggests that reasonably
large magnetoelectric coupling is realizable in simple 0-3
ferroelectric-ferromagnetic bulk composites provided synthesis strategies are
developed which induces spontaneous precipitation of the ferromagnetic phase in
small volume fraction to ensure good insulating behaviour of the composite thus
developed.
