Fe₂O₃@BaTiO₃Core−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, Fe 2 O 3 @BaTiO 3 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 (Fe 2 O 3 /Fe 3 O 4, BaFe 12 O 19 /Ba 12 Fe 28 Ti 15 O 84 ). The formation of new magnetic phases occurs during sintering or post-annealing via reaction between BaTiO 3 and Fe 2 O 3 . 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 Fe 2 O 3 (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 % Fe 2 O 3 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%.