Greatly Enhanced Magnetic and Magneto‐Optical Properties of Bismuth Iron Oxide Sillenite Crystal with Cobalt–Iron Codoping

The chiral sillenite Bi25FeO40 exhibits versatile properties including electro‐optics, magneto‐optics, optical rotation, and photorefraction, showing great potential in multi‐functional magneto‐optical devices. However, the weak magnetism of this crystal leads to a poor magneto‐optical property, which is extremely hindering its practical applications. Combining with the sillenite structure, the formation energy and the magnetic‐optical effect are calculated based on density functional theory. The codoping of strongly magnetic Co and Fe ions is predicted to enable high doping concentrations. The hybridization between Bi 6p orbitals and Fe/Co 3d orbitals significantly enhances the optical transition and thus greatly improves the magneto‐optical properties. The millimeter‐sized Bi26‐x‐yCoxFeyO40 crystals are successfully prepared by hydrothermal methods. The characterizations on structure, TG‐DSC, magnetism, and magneto‐optical effect confirm the theoretical predictions that the Co‐Fe codoped crystals maintain the chiral bismuth ferrite structure with multifunctional properties and high thermal stability. The Co‐Fe codoping significantly enhances the magnetism and magneto‐optical properties of Bi25FeO40 crystal. The magnetization of Bi19.0Co3.5Fe3.5O40 increases by 27 times and the magneto‐optical effect of Bi19.8Co3.0Fe3.2O40 at 720, 777, 1288, 1388, and 1790 nm are greater than Bi:YIG and undoped Bi25FeO40. Bi26‐x‐yCoxFeyO40 crystals with enhanced magneto‐optical properties by Co‐Fe codoping are promising for novel multifunctional magneto‐optical devices. DFT calculation is a feasible way for predicting the magneto‐optical Faraday rotation angle of Bi25FeO40 sillenite crystal. A series of cubic Bi26‐x‐yCoxFeyO40 crystals are prepared by hydrothermal method. Bi19.8Co3.0Fe3.2O40 crystal has a stronger magneto‐optical effect than the commercial Bi:YIG crystal in 1500–1800 nm. It opens up new possibility for the creation of novel multiple functions magneto‐optical devices.