Weak ferromagnetic state in the polar phase of Bi1−xCaxFe1−x/2Nbx/2O3 multiferroics

In this paper, we report on the crystal structure, magnetic and local ferroelectric properties of the Bi1−xCaxFe1−x/2Nbx/2O3 (x ≤ 0.2) compounds prepared by a solid state reaction method. The chemical substitution has been found to reduce the polar ionic displacements in the acentric R3c structure and give rise to the formation of the non-polar Pnma phase at x = 0.2. The substitution-driven decrease of spontaneous polarization is accompanied by the suppression of the cycloidal antiferromagnetic order specific to the parent BiFeO3. As a result, a weak ferromagnetic and ferroelectric state is formed at x ≈ 0.18. Lattice defects contribute to the instability of the cycloidal structure, thus providing the antiferromagnetic + weak ferromagnetic phase coexistence observed over a wide range of Ca/Nb concentrations.