Multiferroic properties of Bi5.75R0.25Fe1.4Ni0.6Ti3O18 (R = Eu,Sm, Nd,Bi and La) ceramics

An improved technique was used to prepare the ceramic samples of Bi_5.75R_0.25Fe_1.4Ni_0.6Ti₃O₁₈ (BRFNT, R = Eu, Sm, Nd, Bi, and La). The five-layer Aurivillius phase of the samples was confirmed by X-ray diffraction (XRD) without detectable impurities. BRFNT samples exhibit the pseudo-tetragonal except the orthorhombic BSmFNT samples. The characteristic plate-like morphology was revealed by field emission transmission electron microscopy (FETEM) images. At ambient temperature, all the samples present both ferroelectric and magnetic properties and BEuFNT shows the best ferroelectric behavior with its remanent polarization as high as 14.9 μC/cm² under 155 kV/cm, which is three times higher than that of Bi₆Fe_1.4Ni_0.6Ti₃O₁₈. Moreover, the remanent magnetization of BEuFNT is increased up to 1.20 emu/g compared to that of Bi₆Fe_1.4Ni_0.6Ti₃O₁₈ which is only 0.53 emu/g. With increasing radius of the introduced A-site ions, the ferroelectric phase transition temperature (T_CE) is decreased while the magnetic phase transition temperature (T_CM) fluctuates. The decrease in both T_CE and T_CM corresponds to the upward shift of the related Raman modes and vice versa, indicating that the T_CE and the T_CM depend not only on the t factor, but also on the strength of the covalent bonds.