Ferroelectric properties and the origin of the magnetism in the Bi₆Fe₂Ti₃O₁₈ thin films

The Fe-containing Aurivillius phase Bi₆Fe₂Ti₃O₁₈ (BFFT) thin films were successfully prepared using the chemical solution deposition technique. The microstructure, ferroelectricity, and magnetic properties were investigated. A defined hysteresis loop was obtained with the remnant polarization (2P _r) 22.6 μC/cm² at 2 kHz under the electric field ~600 kV/cm. The frequency dependence of hysteresis loops was also studied to exclude contribution of the leakage current to polarization in BFFT thin film. Furthermore, the electromechanical responses of the films were investigated using piezoresponse force microscopy. These results indicate the intrinsic ferroelectricity in the present BFFT films. It was found that the BFFT film shows room-temperature weak ferromagnetism with the saturated magnetization (M _s) ~2.9 emu/cm³. To explore the origin of the observed weak ferromagnetism, the magnetism of the same stoichiometric ceramics and the films was compared. The BFFT ceramics show antiferromagnetic interaction background, while the magnetism of the BFFT film may originate from the spin canting of the coupling of the Fe³⁺-based and Fe²⁺-based sublattices. The existence of Fe²⁺ ions in the BFFT films was also proved by the XPS spectrum.