Microstructure and properties of Co-, Ni-, Zn-, Nb- and W-modified multiferroic BiFeO 3 ceramics

Abstract

BiFeO 3 polycrystalline ceramics were prepared by the mixed oxide route and a chemical route, using additions of Co, ZnO, NiO, Nb 2O 5 and WO 3. The powders were calcined at 700 °C and then pressed and sintered at 800–880 °C for 4 h. High density products up to 96% theoretical were obtained by the use of CoO, ZnO or NiO additions. X-ray diffraction, SEM and TEM confirmed the formation of the primary BiFeO 3 and a spinel secondary phase (CoFe 2O 4, ZnFe 2O 4 or NiFe 2O 4 depending on additive). Minor parasitic phases Bi 2Fe 4O 9 and Bi 25FeO 39 reduced in the presence of CoO, ZnO or NiO. Additions of Nb 2O 5 and WO 3 did not give rise to any grain boundary phases but dissolved in BiFeO 3 lattice. HRTEM revealed the presence of domain structures with stripe configurations having widths of typically 200 nm. In samples prepared with additives the activation energy for conduction was in the range 0.78–0.95 eV compared to 0.72 eV in the undoped specimens. In co-doped specimens (Co/Nb or Co/W) the room temperature relative permittivity was ∼110 and the high frequency dielectric loss peaks were suppressed. Undoped ceramics were antiferromagnetic but samples prepared with Co or Ni additions were ferromagnetic; for 1% CoO addition the remanent magnetization ( M R) values were 1.08 and 0.35 emu/g at temperatures of 5 and 300 K, respectively.