Layer-structured bismuth complex oxides Bi₇Fe_3−xNi_xTi₃O₂₁ (0 ≤ x ≤ 2) (BFNT) were synthesized using a low-temperature combustion synthesis method. X-ray diffraction patterns and high-resolution transmission electron microscopy analysis indicated that the samples presented a six-layer Aurivillius structure. Substituting Fe sites by Ni ions inside the lattice was found to be effective in enhancing the multiferroic properties at or above the room-temperature. The sample with a composition of x = 1 exhibited a large remnant magnetization (2M_r = 1.32 emu g⁻¹) that is about five hundred times higher than that in un-substituted Bi₇Fe₃Ti₃O₂₁ ceramics. The work is an important step in the effort to find a single phase and a fully functioning multiferroic material.