Bandgap narrowing and ferroic modulation in KNbO₃ ceramics composited by spinel AFe₂O₄ (A = Mg, Zn, Ni, Co)

Lead-free multiferroic composite ceramics, KNbO₃-AFe₂O₄ (KNO-AFO, A = Mg, Zn, Ni, Co), are synthesized. The microstructure and property characterizations reveal that the introduced AFO can intrinsically modulate the optical/ferroelectric/ferromagnetic properties due to the independent existence of the two phases in the composite ceramics, including the significantly narrowing of the optical bandgap from the intrinsic ~3.2 eV of KNO down to ~1.2 eV, of which the modulation behavior is in line with the prediction of the first-principles calculations, and more importantly, the maintaining of the excellent ferromagneticity (with a remnant magnetization value of 0.8 – 36.5 emu/g) and ferroelectricity (~1 – 4 µC/cm²) but for slightly deteriorating of the leakage as compared to the anti-magnetic KNO. In addition, different spinel ferrites composited to KNO present different functions for the ferroelectric/ferromagnetic properties, i.e., the ortho-spinel phase mainly retaining the ferroelectricity of the composite ceramics while the anti-spinel phase responsible for the ferromagnetism, especially in the KNO-CFO ceramics that not only have the strongest narrowing effect of the optical bandgap but also demonstrate the most excellent magnetic properties. This makes the composite ceramics promising for a wide range of applications in ferric-based storage and photovoltaics, and should offer more options for the research of multifunctional materials and devices.