Tunable polarization and magnetization at room-temperature in narrow bandgap Aurivillius Bi₆Fe_2−xCo_x/2Ni_x/2Ti₃O₁₈

Aurivillius compound Bi₆Fe_2-xCo_x/2Ni_x/2Ti₃O₁₈ (xBFCNT, 0≤x≤1) ceramics synthesized by a conventional solid state method, can exhibit simultaneously visible-light response, ferroelectric and ferromagnetic orders at room-temperature. The effects of structural phases and lattice distortions on electron transitions, polarizations and orbital orderings have been systematically investigated. Narrow band gaps of xBFCNT were confirmed and modified from 2.12 eV to 1.28 eV with increasing x by ultraviolet-visible-near infrared spectrophotometer. Co and Ni ions co-doping is found to induce ferromagnetic behavior but affects adversely the ferroelectric characteristics. In particular, the x=0.4 composition show obvious ferromagnetism with maximum remnant magnetization M_r (0.5 emu/g) and saturation magnetization M_s (2.4872 emu/g), due to spin canting of Fe/Ni/Co-based sub-lattices. These results reveal rich physical phenomena and open an avenue to design promising solar-energy conversion devices and multiferroic applications.