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

The chiral sillenite Bi 25 FeO 40 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 Bi 26‐ x ‐ y Co x Fe y O 40 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 Bi 25 FeO 40 crystal. The magnetization of Bi 19.0 Co 3.5 Fe 3.5 O 40 increases by 27 times and the magneto‐optical effect of Bi 19.8 Co 3.0 Fe 3.2 O 40 at 720, 777, 1288, 1388, and 1790 nm are greater than Bi:YIG and undoped Bi 25 FeO 40 . Bi 26‐ x ‐ y Co x Fe y O 40 crystals with enhanced magneto‐optical properties by Co‐Fe codoping are promising for novel multifunctional magneto‐optical devices.