Fe2O3@BaTiO3 Core-Shell Particles as Reactive Precursors for the Preparation of Multifunctional Composites Containing Different Magnetic Phases

Well-designed reactive precursors and templates allow for careful control of solid-state reactions at the nanoscale level, thus enabling the fabrication of materials with specific microstructures and properties. In this study, Fe2O3@BaTiO3 core-shell particles have been used as precursors for the in situ fabrication of multifunctional composites containing a dielectric/ferroelectric phase and two magnetic phases with contrasting coercivities (Fe2O3/Fe3O4, BaFe12O19/Ba12Fe28Ti15O84). The formation of new magnetic phases occurs during sintering or post-annealing via reaction between BaTiO3 and Fe2O3. The starting powders have been prepared using a multistep process that combines colloidal chemistry methods and a solid-state reaction. The nature and the amount of the magnetic phases and, consequently, the final magnetic properties of the composite can be controlled by varying the relative amount of Fe2O3 (30 or 50 vol %), the densification method (conventional or spark plasma sintering), and the processing temperature. The composites show constricted magnetic hysteresis loops with a coercivity of 0.1-2.5 kOe and a saturation magnetization of 5-16 emu/g. Composites obtained from powders containing 30 vol%Fe2O3 show, at temperatures of 20-80 °C and frequencies between 10 kHz and 1 MHz, a relative dielectric constant of 50 and dielectric losses of <10%.