Modified magnetization and electron transition behavior in Bi₂Fe₄O₉, Bi₂Fe₄O₉-CoFe₂O₄ and Bi₂Fe₄O₉-NiFe₂O₄

Mullite Bi₂Fe₄O₉ (BFO) and its composites with spinel CoFe₂O₄ (CFO) and NiFe₂O₄ (NFO) have been synthesized by solid-state reaction, exhibiting visible-light response and enhanced ferromagnetic order simultaneously at room-temperature. Independent phases have been detected in composite samples by XRD, Raman spectroscopy and further confirmed by SEM images. Calculated through XRD patterns, it can be found that not only the lattice parameters and crystallite size, but also the growth preference orientation of BFO could be effected by spinel magnetic content. Optical spectroscopy shows three absorption edges existed which indicates multiple electron transitions for BFO and its composites. Interband electron transition occurs at 1.56, 0.92 and 0.47 eV for BFO, BFO-NFO and BFO-CFO respectively. Furthermore, M-H hysteresis loops suggest enhanced ferromagnetism in composite samples due to magnetic spinel CFO and NFO phase. Different curve shape and remnant magnetization imply the different intrinsic magnetic nature of CFO and NFO.