立方相BiFeO_(3)磁光晶体的熔盐法制备与性能分析

Preparation by Molten Salt Method and Properties Analysis of Cubic Phase BiFeO_(3) Magneto-Optical Crystals

采用熔盐法,以Bi_(2)O_(3)为自助溶剂,成功制备了Sr/Ti∶BiFeO_(3)系列晶体。X射线衍射谱和Rietveld精修结果显示,Sr^(2+)、Ti^(4+)离子掺杂使BiFeO_(3)的结构从三方相转变为无双折射的立方相。这一结构转变也通过扫描电子显微镜的观察得到了证实。Sr/Ti∶BiFeO_(3)的X射线光电子能谱表明,随着Sr^(2+)、Ti^(4+)的引入,BiFeO_(3)的Fe元素价态发生改变。高价Fe离子的生成使BiFeO_(3)产生晶格畸变,并对其周期性螺旋自旋磁结构产生抑制作用,从而使BiFeO_(3)的宏观磁性得到提高,但矫顽力没有发生明显变化。晶体磁滞回线和磁圆二色效应光谱显示,Bi_(0.7)Sr_(0.3)Fe_(0.7)Ti_(0.3)O_(3)的饱和磁化强度较BiFeO_(3)提高了3倍,其磁光效应达到商用Y_(3)Fe_(5)O_(12)的4.5倍。

Objective Magneto-optical crystals are pivotal components that determine the performance of magneto-optical devices.Through the hybridization between the excited Bi^(3+) 6p orbital and Fe^(3+) 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 Bi^(3+) ions emerges as a key approach to enhancing the magneto-optical properties of commercial Y_(3)Fe_(5)O_(12)(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 BiFeO_(3) with a high concentration of Bi^(3+). 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, BiFeO_(3) demonstrates a considerably feeble magneto-optical effects. In the present study, stable pure phase cubic BiFeO_(3) single crystals are grown by doping Sr^(2+) and Ti^(4+) ions. This eliminates the birefringence effect of the trigonal BiFeO_(3) 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 Bi_(2)O_(3) is chosen as the self-flux solvent, and a series of crystals including Bi1-xSrxFeO_(3) and Bi_(1-x)Sr_(x)Fe_(1-x)Ti_(x)O_(3)(x=0-0.5) are grown by using the molten salt method. The crystal structure and lattice parameters of Sr∶BiFeO_(3) and Sr/Ti∶BiFeO_(3) are determined by XRD spectra analysis and Rietveld refinement. The structure and morphology changes of BiFeO_(3) 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 Bi_(0.7)Sr_(0.3)FeO_(3) and Bi_(0.7)Sr_(0.3)Fe_(0.7)Ti_(0.3)O_(3) crystals belong to the Pm3m space group of the cubic crystal system. The cell parameters of Bi_(0.7)Sr_(0.3)FeO_(3) and Bi_(0.7)Sr_(0.3)Fe_(0.7)Ti_(0.3)O_3 crystals are 3.9517 A and 3.9447 A, respectively. The SEM images also prove that BiFeO_(3) 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 Fe^(2+) and high-valence iron.The saturated hysteresis loop and MCD spectra indicate that the magnetic and magneto-optical properties of BiFeO_(3) crystal can be significantly enhanced by doping of Sr^(2+) and Ti^(4+) ions, but the coercivity has not significantly changed. The saturation magnetization of Bi_(0.7)Sr_(0.3)Fe_(0.7)Ti_(0.3)O_3 is observed to be 0.31(A·m^(2))/kg, which is approximately four times that of BiFeO_3, while exhibiting a significant MCD ellipticity value(ψ_(F)) of 179(°)/cm, in contrast to the negligible MCD signal produced by BiFeO_(3)(Fig. 7 and Fig. 8). This can be attributed to the introduction of Sr~(2+) and Ti^(4+) 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 BiFeO_(3) crystals.Conclusions A series of non-birefringent cubic Bi_(1-x)Sr_xFeO_3(x=0.3, 0.4, 0.5) and Bi_(1-x)Sr_(x)Fe_(1-x)Ti_xO_(3)(x=0.2, 0.3,0.4, 0.5) crystals are grown by using the molten salt method. The introduction of Sr^(2+) and Ti~(4+) ions causes lattice distortion of BiFeO_(3) and inhibits its periodic helical spin magnetic structure. Especially, when Ti^(4+) ions are introduced to replace part of Fe^(3+), the helical G-type antiferromagnetic structure of BiFeO_(3) 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∶BiFeO_(3) and Sr/Ti∶BiFeO_(3) are significantly stronger than that of BiFeO_(3). The saturation magnetization of Bi_(0.7)Sr_(0.3)Fe_(0.7)Ti_(0.3)O_(3) is approximately 4 times that of BiFeO_(3). 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∶BiFeO_(3) 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.