Magnetoelectric coupling in multiferroic BiFeO ₃ by co-doping with strontium and nickel

We report the effects of the Sr ²⁺ and Ni ²⁺ co-doping of BiFeO ₃ on the crystal structure and multiferroic properties of Bi _1−x Sr _x Fe _1-y Ni _y O ₃ (x = 0.05, 0.0 ≤ y ≤ 0.10, and Δy = 0.05) that is synthesized using assisted high-energy ball milling. The mixtures of Bi ₂ O ₃ , Fe ₂ O ₃ , SrO and NiO were milled for 5 h, pressed at 900 MPa, and sintered at 800 °C in order to obtain cylindrical test pieces. X-ray diffraction and Rietveld refinement elucidated the effects of Sr ²⁺ and Ni ²⁺ on the crystal structure. Co-doping with Sr[sbnd]Ni in suitable proportions stabilizes rhombohedral BiFeO ₃ . High contents of Ni ²⁺ promote the precipitation of secondary phases in the forms of NiFe ₂ O ₄ and Bi ₂₅ FeO ₄₀ . The magnetic behavior was examined by means of vibrating sample magnetometry. The results showed a change in the magnetic order from antiferromagnetic for the undoped sample to the ferromagnetic order for the co-doped samples. This change is attributed to the modulations in the magnetic moment due to crystal structure distortions. All samples show high relative permittivity values, which were enhanced by doping with Sr ²⁺ . Ni ²⁺ cations increase the dielectric dissipation factor; this enhancement is related to their interactions with cations of a different oxidation state, such as Fe ³⁺ , Fe ²⁺ , Ni ²⁺ , Bi ³⁺ and Sr ²⁺ in the crystal structure of BiFeO ₃ . The magnetoelectric coupling that was evaluated using magnetodielectric measurements was above 4% at 1 kHz for the higher applied magnetic field of 18 kOe.