Coexisting ferroelectric and magnetic morphotropic phase boundaries in Dy-modified BiFeO₃-PbTiO₃ multiferroics

Morphotropic phase boundary (MPB) in ferroelectric solid solutions can significantly enhance the dielectric and piezoelectric performances of the materials. Similarly, magnetic MPB has been found to exist in a few ferromagnets and is proved to be greatly beneficial to the magnetostrictive response. In this letter, we report the finding of a magnetic MPB in the multiferroic 0.66Bi_1-xDy_xFeO₃-0.34PbTiO₃ system, which overlaps the structural MPB of ferroelectric phases. A (weak) ferromagnetic state is induced at room temperature by Dy for the rhombohedral compositions with x ≥ 0.10. Upon cooling down from room temperature, the tetragonal compositions (x ≤ 0.05) show only one magnetic phase transition from the paramagnetic to antiferromagnetic phase, while the rhombohedral compositions exhibit a series of magnetic phase transitions from the (weak) ferromagnetic state to an antiferromagnetic order then to another weak ferromagnetic phase. A magneto-structural phase diagram has been established that reveals the presence of a magnetic MPB and a ferroelectric MPB. More significantly, the ferroelectric and magnetic MPB regions are found to overlap with each other, pointing to interesting kinds of multiferroic couplings.