Objective Magneto- optical crystals are pivotal components that determine the performance of magneto- optical devices. Through the hybridization between the excited Bi3+ 6p orbital and Fe3+ 3d orbital, the modification of superexchange induces a strong mixing between crystal field states of varying energies, greatly enhancing the magneto-optic effect in ferrite. Doping Bi3+ ions emerges as a key approach to enhancing the magneto-optical properties of commercial Y3Fe5O12 (YIG) crystals. However, despite being the only known room temperature single-phase multiferroic material, there is scarce literature reporting on the magneto- optical properties and its applications in magneto-optical devices of the perovskite BiFeO3 with a high concentration of Bi3+. This can be attributed to its unique spiral G-type antiferromagnetic structure, which exhibits weak macroscopic magnetism. Additionally, due to its trigonal crystal system and birefringence effect, BiFeO3 demonstrates a considerably feeble magneto-optical effects. In the present study, stable pure phase cubic BiFeO3 single crystals are grown by doping Sr2+ and Ti4+ ions. This eliminates the birefringence effect of the trigonal BiFeO3 and induces strong magnetic and magneto-optical effects, providing a useful reference for exploring high-quality, large-size new magneto-optical crystals suitable for high-performance magneto-optical devices. Methods Bi2O3 is chosen as the self-flux solvent, and a series of crystals including Bi1-xSrxFeO3 and Bi1-xSrxFe1-xTixO3 (x= 0. 0.5) are grown by using the molten salt method. The crystal structure and lattice parameters of Sr:BiFeO3 and Sr/ Ti:BiFeO3 are determined by XRD spectra analysis and Rietveld refinement. The structure and morphology changes of BiFeO3 crystals are observed by scanning electron microscopy (SEM). Elemental valence states in the crystals are analyzed using X-ray photoelectron spectroscopy (XPS), while magnetic properties and magneto-optical performance are characterized by a vibrating sample magnetometer and magneto- circular dichroism spectroscopy respectively. Results and Discussions The Rietveld refinement results show that Bi0.7Sr0.3FeO3 and Bi0.7Sr0.3Fe0.7Ti0.3O3 crystals belong to the Pm3m space group of the cubic crystal system. The cell parameters of Bi0.7Sr0.3FeO3 and Bi0.7Sr0.3Fe0.7Ti0.3O3 crystals are 3.9517 A and 3.9447 A, respectively. The SEM images also prove that BiFeO3 changes from a triangular columnar crystal to regular cubic crystals. When the cooling rate of crystal growth is controlled within the range of 1. 10 degrees C/h, the size of cubic crystal grains are 20. 50 mu m. Sub-millimeter size crystal grains are obtained when the cooling rate of crystal growth is 0.5 degrees C/h. However, as the grain size increases, the distance between the stress field of the dislocation packing group and the dislocation source in the crystal also increases, resulting in a stronger stress field and subsequent grain deformation. XPS spectra show that doping of heterovalent elements leads to the production of Fe2+ and high-valence iron. The saturated hysteresis loop and MCD spectra indicate that the magnetic and magneto-optical properties of BiFeO3 crystal can be significantly enhanced by doping of Sr2+ and Ti4+ ions, but the coercivity has not significantly changed. The saturation magnetization of Bi0.7Sr0.3Fe0.7Ti0.3O3 is observed to be 0.31 (A center dot m(2))/ kg, which is approximately four times that of BiFeO3, while exhibiting a significant MCD ellipticity value (phi F) of 179 (degrees)/ cm, in contrast to the negligible MCD signal produced by BiFeO3 (Fig. 7 and Fig. 8). This can be attributed to the introduction of Sr2+ and Ti4+ ions, leading to the elimination of the birefringence effect, as well as the suppression of the periodic spiral spin magnetic structure and providing additional electronic transition pathways. Consequently, this enhances both the magnetic and magneto-optical properties of BiFeO3 crystals. Conclusions A series of non-birefringent cubic Bi1-xSrxFeO3 ( x= 0.3, 0.4, 0.5) and Bi1-xSrxFe1-xTixO3 ( x= 0.2, 0.3, 0.4, 0.5) crystals are grown by using the molten salt method. The introduction of Sr2+ and Ti4+ ions causes lattice distortion of BiFeO3 and inhibits its periodic helical spin magnetic structure. Especially, when Ti4+ ions are introduced to replace part of Fe3+, the helical G-type antiferromagnetic structure of BiFeO3 will be broken, thereby releasing part of the spin magnetic moment of Fe ions. This results in the magnetism and magneto-optical effects of Sr: BiFeO3 and Sr/ Ti:BiFeO3 are significantly stronger than that of BiFeO3. The saturation magnetization of Bi0.7Sr0.3Fe0.7Ti0.3O3 is approximately 4 times that of BiFeO3. Its MCD. F value is observed to be 179 (degrees)/ cm, which is about 4.5 times that of YIG, a popular commercial magneto- optical material tested under the same conditions. With high saturation magnetization, low coercivity and strong magneto-optical effect, Sr/ Ti:BiFeO3 crystals are expected to be used as core magneto- optical materials in magneto-optical modulation, magneto-optical sensing, magneto- optical imaging and other devices, and are hopefully applied in optical communication, laser display, biomedicine, etc.
