${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x\mathrm{Fe}{\mathrm{O}}_3$ ceramics with $x=0$ , 0.1, 0.2, and 0.3 have been synthesized by solid state reaction, starting from metal oxides. A series of structure transformations is found to depend upon the doping level. Below 10% La doping, ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x\mathrm{Fe}{\mathrm{O}}_3$ maintains the rhombohedral structure of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ . However, for ${\mathrm{Bi}}_{0.8}{\mathrm{La}}_{0.2}\mathrm{Fe}{\mathrm{O}}_3$ and ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}\mathrm{Fe}{\mathrm{O}}_3$ , the structures change to the orthorhombic and tetragonal, respectively. La doping significantly reduces electric leakage and leads to successful observation of electrical polarization hysteresis loops. Doping with La also enhances the ferromagnetic moment, due to the broken cycloid spin structure caused by the changes in the crystalline structure.

We report the significant improvement of the ferroelectric properties of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin film through control of electrical leakage by Nb doping. A very large remanent electrical polarization value of $80\mu \mathrm{C}∕{\mathrm{cm}}^2$ was observed in ${\mathrm{Bi}}_{0.8}{\mathrm{La}}_{0.2}{\mathrm{Nb}}_{0.01}{\mathrm{Fe}}_{0.99}{\mathrm{O}}_3$ thin film on $\mathrm{Pt}∕\mathrm{Ti}∕\mathrm{Si}{\mathrm{O}}_2∕\mathrm{Si}$ substrate. The doping effect of Nb in reducing the movable charge density due to oxygen vacancies in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ was confirmed by the dielectric measurements. A very small loss was observed in the Nb and La codoped $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin film. As well as the improvement in the ferroelectric properties, the magnetic moment was also enhanced due to the doping of La.

Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. However, single-phase materials with such cross-coupling properties at room temperature exist rarely in nature; new design of nano-engineered thin films with a strong magneto-electric coupling is a fundamental challenge. Here we demonstrate a robust room-temperature magneto-electric coupling in a bismuth-layer-structured ferroelectric Bi₅Ti₃FeO₁₅ with high ferroelectric Curie temperature of ~1000 K. Bi₅Ti₃FeO₁₅ thin films grown by pulsed laser deposition are single-phase layered perovskit with nearly (00l)-orientation. Room-temperature multiferroic behavior is demonstrated by a large modulation in magneto-polarization and magneto-dielectric responses. Local structural characterizations by transmission electron microscopy and Mössbauer spectroscopy reveal the existence of Fe-rich nanodomains, which cause a short-range magnetic ordering at ~620 K. In Bi₅Ti₃FeO₁₅ with a stable ferroelectric order, the spin canting of magnetic-ion-based nanodomains via the Dzyaloshinskii-Moriya interaction might yield a robust magneto-electric coupling of ~400 mV/Oe·cm even at room temperature.