Effects of Nd-doping on multiferroic properties of Bi<sub>6−x</sub>Nd<sub>x</sub>Fe<sub>1.4</sub>Ni<sub>0.6</sub>Ti<sub>3</sub>O<sub>18</sub> polycrystalline

Abstract

Single phase polycrystalline Nd-modified BNFNT-<i>x</i> series samples are obtained from the precursors of the same chemical formula, and prepared by using the citric acid-nitrate method. The X-ray photoelectron spectroscopy measurement indicates that a slight Nd modification does not exert significant influence on the stability of the octahedral FeO<sub>6</sub>, nor NiO<sub>6</sub> nor TiO<sub>6</sub>. When the molar concentration of Nd exceeds 0.25, the stability of BiO layer is cemented and conducive to the insulating role of BiO layer. It is seen that a small quantity of Nd substitution for bismuth can improve the ferroelectric polarization (2<i>P</i><sub>r</sub>) of ～ 19.7 <inline-formula><tex-math id="Z-20190127014418-3">\begin{document}$ \mu {\rm C/cm }^2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20181287-e-lijj-revised_Z-20190127014418-3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20181287-e-lijj-revised_Z-20190127014418-3.png"/></alternatives></inline-formula>. The room-temperature magnetization (2<i>M</i><sub>s</sub>) can reach a maximal value of ～ 4.132 emu/g (1 emu/g = 10<sup>−3</sup> A·m<sup>2</sup>/g)in the BNFNT-0.20 sample. Two anomalies are observed in the temperature-dependent dielectric loss spectrum: one is situated in the temperature range from 200 K to 400 K and the other is located in the vicinity of 900 K. It is considered that the loss anomaly found near 900 K might be associated with the viscous motion of ferroelectric domain walls. In addition, the loss peak shown in a temperature range from 200 K to 400 K shifts toward the higher temperature with measuring frequency increasing, indicating the characteristics of dielectric relaxor behavior. The activation energy is evaluated to be 0.287−0.366 eV, which suggests that the relaxor is associated with the electrons transfer and hop between Fe<sup>3+</sup> and Fe<sup>2+</sup>. The room-temperature magnetization (2<i>M</i><sub>s</sub>) has reached a maximal value of ～ 4.132 emu/g in the BNFNT-0.20 sample. The lattice distortion due to the introduction of Nd changes the angle of such antiferromagnetic coupling bonds as Fe<sup>3+</sup>—O—Fe<sup>3+</sup>, Fe<sup>3+</sup>—O—Ni<sup>3+</sup> and Ni<sup>3+</sup>—O—Ni<sup>3+</sup>, which leads the AFM spin states to break, and thus increases the magnetic properties. While with further modification of Nd, the drastic lattice distortion reduces the occupation of the B-sites of the magnetic ions, which might be responsible for further deteriorating the magnetic properties.