Multiferroic properties of sol–gel derived Bi₅Fe_1-xCo_xTi₃O₁₅ thin films

Co-doped Bi_5FeTi_3O_15 thin films (BFCT-x, Bi_5Fe_1-xCo_xTi_3O_15) were prepared using a sol--gel technique. XRD patterns confirm their single phase Aurivillius structure, and the corresponding powder Rietveld analysis indicates the change of space group around x=0.12. The magnetic hysteresis loops are obtained and ferromagnetism is therefore confirmed in BFCT-x thin films. The remanent magnetization (M_\rm r) first increases and reaches the maximum value of 0.42 emu/cm^3 at x=0.12 due to the possible Fe^3+--O--Co^3+ ferromagnetic coupling. When x = 0.25, the M_\rm r increases again because of the dominant Fe^3+--O--Co^3+ ferromagnetic coupling. The remanent polarization (2P_\rm r) of BFCT-0.25 was measured to be as high as 62 \mu C/cm^2, a 75% increase when compared with the non-doped BFCT-0 films. The 2P_\rm r remains almost unchanged after being subjected to 5.2\times 10^9 read/write cycles. Greatly enhanced ferroelectric properties are considered to be associated with decreased leakage current density.