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Multiferroic materials have drawn increasing interest due to the coexistence of ferromagnetism (FM) and ferroelectricity (FE), which provides significant potentials for applications in spintronics, information storage, and sensors, etc. In this paper, the multiferroic Bi6Fe2-xCoxTi3O18 (BFCT-x,x=0-2.0) ceramics are prepared by the solid-state reaction. The BFCT-x samples belong to Aurivillius structure containing five perovskite layers clapped between two Bi-O layers. The lattice constants a, b, and c of BFCT-x samples increase simultaneously with increasing cobalt content up to 0.6 and then decrease with further addition of cobalt. The magnetic and ferroelectric properties, and their corresponding Curie temperatures are measured. At room temperature (RT), the magnetism of the BFCT-0, BFCT-1.8 and BFCT-2.0 samples can be understood by the presence of the antiferromagnetic (AFM) interaction with the dominant paramagnetism (PM) state, which is consistent with the linear behavior of the M-H plot. The Fe3+-O-Fe3+ and Co3+-O-Co3+ interactions present in the BFCT-x samples lead to AFM. The BFCT-0.21.0 samples show saturated magnetic loops, while the BFCT-1.2 sample is far from saturation even under an applied magnetic field of 10 kOe. The M-H curve of BFCT-1.6 sample shows a weak ferromagnetism. The Co content (x=0.2-1.6) dependences of 2Ms and 2Mr have been recorded. Both the 2Ms and 2Mr experience first-increase-then-decrease variation tendency with their maximal values of ～ 4.49 emu/g and ～ 0.89 emu/g located at x =0.6 and x =1.0, respectively. As the cobalt content varies from x=0.2 to x=1.2, the paramagnetic-ferromagnetic phase transition temperature (TMC) decreases from 752 to 372 K. At RT, the BFCT-x samples are ferroelectric, and the maximum and minimum values of remnant polarization (2Pr) are about 8.0 up C/cm2 (x=0.6) and 1.1 up C/cm2(x=1.2), respectively. The 2Pr of the BFCT-0.6 is about three times larger than that of Bi5Fe2Ti3O18 (x=0) sample. Furthermore, the dependence of 2Pr on Co content first increases with Co doping when x qslant 0.6, and decreases from x=0.8 to x=1.2, and then increases again. The ferroelectric Curie temperature Tc of the BFCT-x samples increases with increasing x up to 0.8 and then decreases with further increasing cobalt content. It is noteworthy that the Tc of BFCT-1.0 is 2 K lower than that of BFCT-0.6, while the 2Pr decreases by 63%. It is seen that the 2Pr and 2Mr increase simultaneously with increasing Co content (below 0.6). When 0.8 x qslant 1.0, the 2Mr increases while 2Pr decreases with increasing Co content. After x1.2, the 2Mr decreases while 2Pr increases with increasing Co content. The repelling between the FE and FM as discussed above may result from the magnetic-crystalline and ferroelectric-crystalline anisotropy. The mechanism of this phenomenon is not quite clear and needs further investigation.

Bi5Fe1−xCoxTi3O15 (x = 0.0, 0.2, 0.4, 0.5, 0.6, and 0.8) multiferroic ceramics are synthesized in two steps using the solid state reaction technique. X-ray diffraction patterns show that the samples have four-layer Aurivillius phases. At room temperature (RT), the samples each present a remarkable coexistence of ferromagnetism (FM) and ferroelectricity (FE). The remnant polarization (2Pr) reaches its greatest value of 14 μC/cm2 at x = 0.6. Remnant magnetization (2Mr) first increases and then decreases, and the greatest 2Mr is 7.8 menu/g when x = 0.5. The magnetic properties for x = 0.4 are similar to those for x = 0.6, indicating that the magnetic properties originate mainly from the coupling between Fe3+ and Co3+ ions, rather than from their own magnetic moments.

Multiferroic ceramics of Bi 5 Fe 1- x Co x Ti 3 O 15 ( x =0.0—0.6) are synthesized by the conventional solid state reaction, and their microstructure,magnetic and ferroelectric properties are investigated. X-ray diffraction patterns show that a four-layer Aurivillius phase is formed in each sample with a suggested structural transformation at heavy doping of Co ions. The Raman results demonstrate that Co ions enter into the lattice,occupy the B -sites in the BFCT- x and have an effect on the order occupancy of cations at B sites. It is found that the Co ion modification induces remarkable ferromagnetism (FM) at room temperature with a greatest remanent magnetization (2 M r) of 2.3 memu/g at x =0.5,which is three orders of magnitude larger than that of Bi 5 Ti 3 FeO 15 (BFTO). All samples with Co doping have ferroelectricity (EM). The remanent polarization (2 P r) reaches a value of 11 μC/cm 2 at x =0.1,which is about 38.2% higher than that of BFTO,and then decreases when 0.2≤ x ≤0.4 and increases again at x =0.5 and 0.6. Of all materials,the sample of x =0.5 is best in both good FE and FM.

A dielectric loss peak with relaxation-type characteristic is observed in Bi5TiNbWO15 over 200–400°C. The modified Cole–Cole relation by introducing relaxation strength as another important fitting parameter is used to describe this temperature-dependent behaviour of dielectric relaxation process. This peak is considered to be associated with the oxygen vacancies inside the grains and with its activation energy by relaxation determined to be 0.76 eV. The obtained broadening factor α is around 0.4, which indicates a strong correlation between those relaxation units. It is confirmed that the behaviour of this peak is due to the combined effects of the dielectric relaxation and electrical conduction by the thermal motion of oxygen vacancies. These results are further confirmed in Bi5TiNbWO15 samples through oxidization atmosphere treatment and Nd modification respectively.

The ferroelectric ceramics samples of SrBi4-x/3Ti4-xMoxO15(x=0.00, 0.003, 0.012, 0.03, 0.06, 0.09) have been prepared by the conventional solid-state reaction method. X-ray diffraction analysis shows that the crystal structure of SrBi4Ti4O15 (SBTi) is not affected by Mo-doping. The scanning electronics microscope (SEM) images indicate that all the samples consist of sheet grains and the grain size is reduced by Mo-doping while the ferroelectric properties are improved significantly. The remanent polarization (2Pr) increases at first，then decreases with the increasing Mo content. At x=0.06, the 2Pr maximizes at a value of 26.5 μC/cm2, which about 117% higher in comparison with that of SBTi. And the coercive field of SBTi is increased only about 20%. The Curie temperature of the samples varied slightly after doping, which implies that the good thermal stability of SBTi is not sacrificed.

High quality Sr2Bi4Ti5O18 (SBTi) ferroelectric thin films on Pt/Ti/SiO2/Si(100) substrates were successfully prepared using the sol-gel method.The microstructure, ferroelectric properties and fatigue characteristics of SBTi thin films were investigated.The results show that the films with smooth surface are of single phase of SBTi and randomly oriented. The well-saturated ferroelectric hysteresis loops and the fatigue properties were obtained by hysteresis measurements using a metal-ferroelectric-metal structure. The remnant polarization (2Pr) and coercive field (2Ec) reached to 24.0μC/cm2 and 137.8kV/cm, respectcively in the electric field of 275kV/cm. No fatigue was observed up to 4.4×1010 switching cycles.

Ferroelectric Sr 2 Bi 4-x Dy x Ti 5 O 18 (SBDT-x, x= 0—0.20) ceramic samples were prepared using the conventional solid-state reaction method. x-ray diffraction patterns (XRD) of SBDT-x ceramics show that doping with a small amount of Dy does not change the crystal structur e of Sr 2 Bi 4 Ti 5 O 18 (SBTi). Their Curi e temperature (T c ) and dielectric loss (tanδ) decrease with Dy dopin g amount. The remnant polarization (2P r ) of SBDT-x increases at first , then decreases with increasing of Dy content. When Dy content is 0.01, the 2P r reaches a maximum value of 20.1 μC·cm -2 . The variation of 2P r for SBDT-x relates to space charge density, internal strain an d structure distortion. (Bi 2 O 2 ) 2+ layer acting as an insulating layer and space charge storage plays an important role in their ferroelectric properties in BLSFs. The incorporation of doping ions into (Bi 2 O 2 ) 2+ layer may destroy its original function and deteriorate the ferroelectric property.

Lanthanum doped bismuth layer-structured ferroelectrics and its intergrowth type, SrBi 4-x La x Ti 4 O 15 , Sr 2 La x Bi 4-x Ti 5 O 18 , and (Bi,La) 4 Ti 3 O 12 -Sr(Bi,La) 4 Ti 4 O 15 , have been prepared by solid-state reaction method. The dielectric properties of those materials are measured. The results show that SrBi 4-x La x Ti 4 O 15 (x=1.0), Sr 2 La x Bi 4-x Ti 5 O 18 (x≥0.5)， (Bi,La) 4 Ti 3 O 12 -Sr(Bi,La) 4 Ti 4 O 15 (x=1.50) exhibit typical relaxational characteristics. The degree of diffuseness and relaxation increased with increasing La 3+ content. The origin of relaxation is the microdomain induced by La 3+ doing. The micromechanism corresponding to the transition between normal ferroelectrics and relaxors is explained as the transition from the macrodomain of normal ferroelectrics to the microdomain of relaxors.