Ferromagnetic properties of barium titanate ceramics doped with cobalt, iron, and nickel

The influence of annealing in strongly reducing atmosphere on the magnetic properties of hexagonal BaTiO₃ + 0.04 BaO + x/2 Co₂O₃ (0.0025 ≤ x ≤ 0.10) ceramics was investigated. The samples air-sintered at 1673 K were subsequently tempered at 1473 K in H₂/Ar stream. While the as-sintered samples exclusively exhibit paramagnetic behavior, the annealed samples show distinct saturation in the field dependence of the magnetization at 300 K measured in the range between −90 and 90 kOe. Besides, the field dependence of the magnetization is hysteretic with coercive fields in the order of 100 Oe. Both properties point to ferromagnetic regions which were identified as precipitations of metallic cobalt by TEM and EDX. The cobalt precipitations were exclusively found in tetragonal grains which were completely Co-free outside the precipitations. Obviously, these tetragonal grains were formed during the annealing process when the Co content of the formerly hexagonal grains was concentrated into metallic Co particles by diffusion processes. Hence, the Co-free matrix of the grains transformed into the cubic phase which is the equilibrium phase of undoped BaTiO₃ at the annealing temperature 1473 K and during cooling to room temperature into tetragonal phase. The size of the metallic precipitations ranges from about 20 to 100 nm. A reduction both of the annealing temperature to 1373 K and of the annealing time from 120 to 30 min did not change the minimum particle size, but now, the very rare precipitations only occurred at triple points or grain boundaries. EPR measurements confirmed the occurrence of the ferromagnetic precipitations. While at 300 K, the as-sintered samples did not show any Co EPR signal, the annealing in strongly reducing atmosphere caused a strong and broad line which is attributed to the ferromagnetic resonance signal of the Co precipitations. The investigations were extended for the dopants, iron or nickel, with a nominal sample composition of BaTiO₃ + 0.04 BaO + 0.01 Fe₂O₃ or 0.02 NiO, respectively. The field dependence of the magnetization as well as the EPR spectra showed similar results compared to the case of Co-doped samples. Hence, also in Fe- or Ni-doped BaTiO₃ ceramics, ferromagnetic properties are caused by annealing in strongly reducing atmosphere.