Crystallographic and magnetic identification of secondary phase in orientated Bi₅Fe_0.5Co_0.5Ti₃O ₁₅ ceramics

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO₃)nBi₄Ti ₃O₁₂, can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi₅Fe_0.5Co_0.5Ti₃O ₁₅, where n = 1 with half the Fe³⁺ sites substituted by Co³⁺ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe₂O₄. It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was M_r ≈ 76 memu/g which dropped down to almost zero (M _r ≈ 0.8 memu/g) at 460°C, far lower than the anticipated for CoFe₂O₄.