Effect of cobalt substitution on structural, impedance, ferroelectric and magnetic properties of multiferroic Bi2Fe4O9 ceramics

Structural, impedance, ferroelectric and magnetic properties were examined in multiferroic Bi₂Fe_4(1-x)Co_4xO₉ (0$\leq$x$\leq$0.02) ceramics synthesized via solid-state reaction method. X-ray diffraction analysis and Rietveld refinement showed secondary phase formation (for x$\geq$0.01) which was subsequently confirmed from room temperature Raman spectroscopy study. The frequency dependence of impedance and electric modulus of the material showed the presence of non-Debye type relaxation in all the samples. The values of the activation energies calculated from imaginary impedance and modulus lie in the range of 0.92-0.99 eV which confirmed that the oxygen vacancies play an important role in the conduction mechanism. Moreover, suitable amount of Co substitution significantly enhanced the remnant polarisation (2P_r) from 0.1193 $μ$C/cm² (x=0) to 0.2776 $μ$C/cm² (x=0.02). Besides, room temperature M-H measurement showed improved ferromagnetic hysteresis loop for all the modified samples. The remnant magnetization (M_r) and coercive field (H_c) increased from 0.0007 emu/gm and 42 Oe for x=0 to 0.1401 emu/gm and 296 Oe for x=0.02. The improved ferroelectricity was due to Co 3d-O 2p hybridization and enhanced magnetization originated from the partial substitution of Co³⁺ ions leading to breakdown of balance between the anti-parallel sub lattice magnetization of Fe³⁺ ions.