D. P. Song

We investigate the effect of high magnetic-field annealing on the microstructural, ferroelectric, and magnetic properties of Bi₆Fe_1.4Co_0.6Ti₃O₁₈ thin films. The magnetic field can lower the energy barrier for nucleation and improve the grain connectivity. The application of magnetic field of 6T parallel to the film plane can substantially enhance the remnant polarization P_r from 18.1 to 29 μC/cm² as a result of the variation in grain size and growth orientation caused by magnetic field annealing. Moreover, the remnant magnetization M_r is substantially improved from 2.48 to 4.56 emu/cm³ arising from the enhanced exchange coupling due to the better grain connectivity. These results demonstrate that high magnetic-field annealing is an effective way to optimize multiferroic properties of the Aurivillius compounds.

We prepared V-doped Bi₆Fe₂Ti₃O₁₈ thin films on Pt/Ti/SiO₂/Si (100) substrates by using a chemical solution deposition route and investigated the doping effect on the microstructure, dielectric, leakage, and ferroelectric properties of Bi₆Fe₂Ti₃O₁₈ thin films. The Bi_5.97Fe₂Ti_2.91V_0.09O₁₈ thin film exhibits improved dielectric properties, leakage current, and ferroelectric properties. The incorporation of vanadium resulted in a substantially enhanced remnant polarization (2P_r) over 30 μC/cm² in Bi_5.97Fe₂Ti_2.91V_0.09O₁₈ thin film compared with 10 μC/cm² in Bi₆Fe₂Ti₃O₁₈ thin film. It is demonstrated that the improved properties may stem from the improvement of crystallinity of the films with the contribution of suppressed oxygen vacancies and decreased mobility of oxygen vacancies caused by the V-doping. The results will provide a guidance to optimize the ferroelectric properties in Bi₆Fe₂Ti₃O₁₈ thin films by chemical solution deposition, which is important to further explore single-phase multiferroics in the n = 5 Aurivillius thin films.

Single-phase polycrystalline SrBi₅Ti₄CrO₁₈ and SrBi₅Ti₄Cr_0.5Co_0.5O₁₈ were synthesized by a modified Pechini method. Both samples have an orthorhombic structure with the space group B2cb. The valence state of Cr is suggested to be +3 and the Co ions exist in the form of Co²⁺ and Co³⁺ based on the results of x-ray photoelectron spectroscopy. The sample SrBi₅Ti₄CrO₁₈ exhibits the paramagnetic state, whereas SrBi₅Ti₄Cr_0.5Co_0.5O₁₈ undergoes a ferrimagnetic transition at 89 K originating from the antiferromagnetic coupling of Cr-based and Co-based sublattices. In addition, SrBi₅Ti₄Cr_0.5Co_0.5O₁₈ shows a typical spin-glass behavior below 89 K with zν = 6.02 and τ₀ = (1.75 ± 0.33) × 10⁻¹⁴ s as evidenced by the results of the frequency dependence of ac susceptibility and magnetic relaxation measurements. In particular, both the dielectric constant and dielectric loss of SrBi₅Ti₄Cr_0.5Co_0.5O₁₈ exhibit the characteristics of dielectric relaxation around 89 K with the activation energy of (0.14 ± 0.02) eV, which can be ascribed to the electron hopping of Co²⁺-V_O-Co³⁺ through the bridging oxygen vacancies.

We have successfully synthesized the Aurivillius compounds SrBi₅Ti₄MnO₁₈ and SrBi₅Ti₄Mn_0.5Co_0.5O₁₈ using a modified Pechini method. Both samples have an orthorhombic structure with the space group B2cb. The valence state of Mn is suggested to be +3 and the doped Co ions exist in the form of Co²⁺ and Co³⁺ based on the results of x-ray photoelectron spectroscopy. The sample SrBi₅Ti₄MnO₁₈ exhibits a dominant paramagnetic state with the existence of superparamagnetic state as evidenced by the electron paramagnetic resonance results, whereas SrBi₅Ti₄Mn_0.5Co_0.5O₁₈ undergoes a ferrimagnetic transition at 161 K originating from the antiferromagnetic coupling of Co-based and Mn-based sublattices, and a ferromagnetic transition at 45 K arising from the Mn³⁺-O-Co³⁺ (low spin) interaction. The sample SrBi₅Ti₄Mn_0.5Co_0.5O₁₈ exhibits two dielectric anomalies. One corresponds to a relaxor-like dielectric relaxation which follows the Vogel-Fulcher function and the other dielectric relaxation obeys the Arrhenius law arising from the collective motion of oxygen vacancies. In addition, the sample SrBi₅Ti₄Mn_0.5Co_0.5O₁₈ exhibits a magnetodielectric effect caused by the Maxwell-Wagner effect because of the conductivity of the sample. This is demonstrated by the fact that the activation energy in dielectric loss process is close to that for dc conductivity and the magnetodielectric effect is sensitive to the measured frequency.