Structure, Morphology, and Magnetic Properties of CoFe <sub>2</sub>O <sub>4</sub>-BaTiO <sub>3</sub> and CoFe <sub>2</sub>O <sub>4</sub>-Bi <sub>4</sub>Ti <sub>3</sub>O <sub>12</sub> Core-Shell Nanoparticles

Finding suitable ferromagnetic materials is quite challenging from the different application points of view; however, the performance of heterostructures of metal oxides is mainly associated with the interfacial coupling of the two-component structures. Magnetic core-shells of CoFe2O4-BaTiO3 and CoF2O4-Bi4Ti3O12 have been synthesized at different ratios by two-step wet chemical procedures (co-precipitation and sol-gel methods). The nature of the crystalline samples was analyzed by XRD, SEM, EDS, VSM, and TEM, including particle size, morphological change, and interphase effects. The alteration of core-shell nature of CoFe2O4-BaTiO3 and CoFe2O4-Bi4Ti3O12 was seen if the concentration of BaTiO3 or Bi4Ti3O12 was varied, and the two nanocomposites CoFe2O4-BaTiO3and CoFe2O4-Bi4Ti3O12 presented different types of morphologies. Regarding crystal structure characterization, a tetragonal unit cell for BaTiO3, orthorhombic for Bi4Ti3O12, and cubic for the CoFe2O4 were preserved as majority phases in the composites. The role of substrate morphology, interface stoichiometry, and cation intermixing were also analyzed as the random orientation of BaTiO3 or Bi4Ti3O12 over the CoFe2O4 surface depends on the ferrite phase percentage. The results show that the 50:50 ratio for CoFe2O4-BaTiO3 and CoFe2O4-Bi4Ti3O12 is suitable for the core-shell structures, observing a relatively homogeneous coating with an average size of around 20 nm. As a result, a superparamagnetic behavior was detected, although both systems have exhibited a complex magnetic behavior (negative magnetization).