Lead-Free BNT-BT_0.08/CoFe₂O₄ Core-Shell Nanostructures with Potential Multifunctional Applications

Herein we report on novel multiferroic core-shell nanostructures of cobalt ferrite (CoFe₂O₄)-bismuth, sodium titanate doped with barium titanate (BNT-BT_0.08), prepared by a two-step wet chemical procedure, using the sol-gel technique. The fraction of CoFe₂O₄ was varied from 1:0.5 to 1:1.5 = BNT-BT_0.08/CoFe₂O₄ (molar ratio). X-ray diffraction confirmed the presence of both the spinel CoFe₂O₄ and the perovskite Bi_0.5Na_0.5TiO₃ phases. Scanning electron microscopy analysis indicated that the diameter of the core-shell nanoparticles was between 15 and 40 nm. Transmission electron microscopy data showed two-phase composite nanostructures consisting of a BNT-BT_0.08 core surrounded by a CoFe₂O₄ shell with an average thickness of 4-7 nm. Cole-Cole plots reveal the presence of grains and grain boundary effects in the BNT-BT_0.08/CoFe₂O₄ composite. Moreover, the values of the dc conductivity were found to increase with the amount of CoFe₂O₄ semiconductive phase. Both X-ray photoelectron spectroscopy (XPS) and Mössbauer measurements have shown no change in the valence of the Fe³⁺, Co²⁺, Bi³⁺ and Ti⁴⁺ cations. This study provides a detailed insight into the magnetoelectric coupling of the multiferroic BNT-BT_0.08/CoFe₂O₄ core-shell composite potentially suitable for magnetoelectric applications.