Narrow Band Gap and Room-temperature Ferromagnetism in KNb_1−x Fe_xO_31−δ

We have investigated the structure, optical and magnetic properties of ferroelectric KNb_1−xFe_xO_3−δ (x=0, 0.01, 0.03, 0.05, 0.10, 0.15, 0.20, 0.25) synthesized by a traditional solid-state reaction method. According to the X-ray diffraction and the results of Rietveld refinement, all the samples maintain orthorhombic distorted perovskite structures with Amm2 space group without any secondary phase, suggesting the well incorporation of Fe ions into the KNbO₃ matrix. With the increase of Fe concentration, the band gap of each sample is decreased gradually, which is much smaller than the 3.18 eV band gap of pure KNbO₃. Through X-ray photoelectron spectrum analysis, the increased density of oxygen vacancy and Fe ions may be responsible for the observed decrease in band gap. Compared with the pure KNbO₃, Fe doped samples exhibit room-temperature weak ferromagnetism. The ferromagnetism in KNb_1−xFe_xO_3−δ with low-concentration dopants (x=0.01–0.10) can be attributed to the bound magnetic polaron mediated exchange. The enhancement of magnetism for the high-concentration (x=0.10–0.20) doped samples may arise from the further increase of magnetic Fe ions.