Ho and Ti Co-Substitution Tailored Structural Phase Transition and Enhanced Magnetic Properties of BiFeO₃ Thin Films

The polycrystalline thin films of BiFeO₃ (BFO) and Bi_0.90Ho_0.10Fe_1-xTi_xO (x = 0, 0.025, 0.05, 0.10, 0.15, and 0.20) were successfully synthesized by the simple sol-gel method. X-ray diffraction and Raman spectra revealed the substitution of Bi and Fe by Ho and Ti, respectively, and correspondingly a structural phase transition from the rhombohedral phase to orthorhombic phase. The field-emission scanning electron microscopy and transmission electron microscopy images indicated that the average size of the particles was decreased and the surface homogeneous agglomeration was enhanced with the increased concentration of Ti to x = 0.05. The X-ray photoelectron spectroscopy measurements illustrated that Fe³⁺ and O^2- ions tended to increase with the Ti concentration increase, which accounted for the enhanced super-exchange interaction between Fe³⁺ and O^2-. Because of the reduced concentration of oxygen vacancies, Ho and Ti ions with a smaller ionic radius and denser surface structure, the Ho and Ti co-substituted films with an appropriate concentration of Ti (x = 0.05) showed an optimal saturation magnetization (M _s) of 44.23 emu/cm³ and remanent magnetization (M _r) of 4.62 emu/cm³, which were approximately 1.8 times and 1.9 times than that of the pure BFO, respectively. This work opened up an effective way to modulate the structure and properties of BFO-based materials.