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.

Bi₂O₃ is chosen as the self-flux solvent, and a series of crystals including Bi_1-xSr_xFeO₃ and Bi_1-xSr_xFe_1-xTi_xO₃ (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.

The Rietveld refinement results show that Bi_0.7Sr_0.3FeO₃ and Bi_0.7Sr_0.3Fe_0.7Ti_0.3O₃ crystals belong to the Pm3¯m space group of the cubic crystal system. The cell parameters of Bi_0.7Sr_0.3FeO₃ and Bi_0.7Sr_0.3Fe_0.7Ti_0.3O₃ crystals are 3.9517 Å and 3.9447 Å, 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 ℃/h, the size of cubic crystal grains are 20-50 μm. Sub-millimeter size crystal grains are obtained when the cooling rate of crystal growth is 0.5 ℃/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 Bi_0.7Sr_0.3Fe_0.7Ti_0.3O₃ is observed to be 0.31 (A·m²)/kg, which is approximately four times that of BiFeO₃, while exhibiting a significant MCD ellipticity value (ψ_F) of 179 (°)/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.

A series of non-birefringent cubic Bi_1-xSr_xFeO₃ (x=0.3, 0.4, 0.5) and Bi_1-xSr_xFe_1-xTi_xO₃ (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 Bi_0.7Sr_0.3Fe_0.7Ti_0.3O₃ is approximately 4 times that of BiFeO₃. Its MCD ψ_F value is observed to be 179 (°)/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.