| Bi Fe O3(BFO) is one of the few multiferroic materials, which the multiferroic material that exhibit the coexistence of magnetic and ferroelectric orderings at room temperature. The ferroelectric Curie and anti-ferromagnetic Néel temperatures of the BFO is TC=1103 K and TN=647 K, respectively. In recent years, Bi Fe O3 has attracted a lot of researching interests due to the potential applications in nonvolatile resistancerandom access memory(RRAM) device, sensors and magneto-electric devices. But the pure Bi Fe O3 thin films show weak ferromagnetism owing to its naturalproperty and the high depolarization field due to the large leakage current, which hinders its actual application.In this work, Bi Fe1-xMnxO3(BFMnxO), Bi Fe0.96Mn0.04CrxO3(BFMCxO), Bi1-xSmxFe0.96Mn0.04Cr0.02O3(BSxFMCO), Bi0.89-xSrxSm0.11Fe0.96Mn0.04Cr0.02O3(BSrxSFMC) and Bi1-xRExFe1-yTMyO3/Ni Fe2O4(RE=Sm, Sr; TM=Cr, Mn) thin films were successfully prepared on FTO/glass(Sn O2: F) substrates via a sol–gel spin-coating method. The structures, leakage current mechanism, dielectric, ferroelectric and magnetic properties were also investigated systematically. The main conclusions are as follows:(1) For Mn2+-doped Bi Fe1-xMnxO3 thin films, Mn2+-doping can induce a structure transition from a single trigonal for the pure BFO thin film to co-existence of trigonal and tetragonal phase for the BFMx=0.04 O thin film. Moreover, the leakage current densities of the BFMxO thin films are 10-5~10-6 A/cm2, which is about two 2~3 of magnitude lower than that of the BFO thin film(10-3 A/cm2). The results indicate that Mn2+-doping can reduce the leakage current densities of the BFMxO thin films. Mn ions are in the mixed-valence state of Mn4+ and Mn2+ in the BFMx=0.04 O thin film, and reduce the concentration of Fe2+ of the BFMx=0.04 O thin film. The higer ferroelectric remnant polarization value of Pr=76.20 μC/cm2 and saturated magnetization value of Ms=4.45 emu/cm3 is obtained in the BFMx=0.04 O thin film.(2) Cr-doping can further induce lattice distortion of the BFMCxO thin films, and hinder a trend of grain growth. Cr-doping can further improve the ferroelectric property of BFMCxO thin film. At an applied electric of 727 k V/cm, the highest ferroelectric remnant polarization value of Pr=102.65 μC/cm2 is obtained in the BFMx=0.04 O thin film. Compared with the BFMO thin film, the BFMCx=0.02 O thin film shows a lower saturated magnetization value of 1.05 emu/cm3, which can be attribute to the clusters structure of Mn are destroyed by Cr-doping. Meanwhile, Cr-doping can hinder the interaction of Mn and strong localization of electrons and holes. Moreover, Mn and Cr ions are in the mixed-valence state of Mn4+/2+ and Cr3+/6+ respectively for the BFMCx=0.02 O thin film, and further reduce the concentration of Fe2+.(3) Compared with the BFMCO thin film, Sm-doping can further decrease the leakage current density of the BSxFMCO thin film, and also improved the breakdown field. The better ferroelectric remnant polarization value of Pr=89.42 μC/cm2 and saturated magnetization value of 1.22 emu/cm3 is obtained in the BSx=0.11 FMCO thin film. Moreover, Mn and Cr ions are in the mixed-valence state of Mn4+/2+ and Cr3+/6+ respectively for the BSx=0.11 FMCO thin film. BSx=0.11 FMCO thin film show the low concentration of Fe2+ than the pure BFO. At the high applied electric field, the leakage current mechanism of the BSx=0.11 FMCO thin film blongs to the F–N transmission mechanism.(4) For the BSrxSFMC thin films, Sr2+-doping can induced a structure transition and in favour of the grain growth. Meanwhile, Sr2+-doping can also decrease and increase the leakage current densities and breakdown field of the BSrxSFMC thin films, respectively. The leakage current densities and the breakdown fields are 10-6 A/cm and 630 k V/cm, respectively. The best dielectric, ferroelectric and magnetic properties are obtained in the x=0.04 sample. The dielectric constent is εr=290 at the 1 k Hz. At an applied electric field of 727 k V/cm and the mesauring frequency of 1 k Hz, the remnant polarization value 2Pr=173.43 μC/cm2 and saturated magnetization value of Ms=2.3 emu/cm3 are obtained in the x=0.04 sample.(5) Compared with the one ion doping, multi-ion co-doping can easier induced a structure transition for Bi Fe O3 thin films. The tetragonal phase is increased with the increase of the ion species in the thin films. Moreover, all the thin films show a high degree of(110) preferred orientation. Ions doping at the Fe-site with Mn or(Mn, Cr) ion exhibited a bigger crystal grain, which indicates the Mn or(Mn, Cr) ions doping at Fe-site is in favour of the grain growth for the BMFO and BFMCO thin films. But the Sm or(Sm, Sr) ions doping at Bi-site can induce grain refinement in the BSFMCO and BSr SFMCO thin films. The best ferroelectric and magnetic properties are obtained in the BSrx=0.04 SFMC thin film.(6) The Bi1-xRExFe1-yTMyO3/Ni Fe2O4 composite thin films of the multilayered structure by combining multi-ion co-doped BFO with spinel ferromagnetic materials of Ni Fe2O4 have improved the ferroelectric and magnetic properties of BFO thin films simultaneously. The best ferroelectric remnant polarization value of Pr=88.82 μC/cm2 and magnetic saturated magnetization value of Ms=35.18 emu/cm3 is obtained in the BSr SFMCO/NFO thin film. |
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