## 3 Thin Films  
The high coercive field can make difficult the needed poling for the Aurivillius structures, which can be a drawback for the potential applications for these materials. In this sense, the use of thin films can overcome this issue, besides expanding the applicability for this family of compounds. By reducing the material thickness, lower voltages (tens of V) are needed to apply a high electric field. This can help to facilitate the films integration in microelectronic devices. The ferroelectric properties in Aurivillius thin films depend on several parameters as composition, microstructure or crystallographic orientation, which in turn is markedly dependent on the substrate. Different processing routes have been used during years to fabricate the thin films. Among them, sol-gel and spin-coating, Metal Organic Decomposition, Chemical Solution Deposition, Pulsed Laser Deposition or Magnetron Sputtering can be highlighted, at temperatures ranging from 600 to 800 \({}^{\circ}\)C. The peculiar dependence of the ferroelectric polarization with the crystalline structure and the anisotropic growth habit mark the dependence of the properties with the film texture. Thus, films with grains growing with the c-axis in normal to the substrates will have a very low spontaneous polarization in the normal plane [64], which will make the application more difficult. On the contrary, grains align with the a (or b) axis in the normal direction will maximize the ferroelectric response. This is the case of the work by [PERSON] et al., which showed a remnant polarization of 32 \(\upmu\)C/cm\({}^{2}\) in Bi\({}_{3.25}\)La\({}_{0.75}\)Ti\({}_{3}\)O\({}_{12}\). The substrate determines the preferential growth by the adequate lattice matching with the film. Thus, a (100)-oriented growth of BLT was achieved using a YSZ(100) single-crystal substrates covered by a thin strained (110)-oriented SrRuO\({}_{3}\) layer. The strain increases SrRuO\({}_{3}\) lattice parameter, reducing the BLT-SrRuO\({}_{3}\) lattice misfit along the BLT[001]\(\backslash\)SrRuO\({}_{3}\)[001] direction. It favors a better nucleation conditions for the growth of (100)-oriented BLT grains [65]. At least, a mixed orientation is needed to have a ferroelectric signal [64]. High values of remnant polarization were achieved in other doped Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\)-based materials, i.e., 44 \(\upmu\)C/cm\({}^{2}\) by Zr doping [66] or 19 \(\upmu\)C/cm\({}^{2}\) in Bi\({}_{3.15}\)Nd\({}_{0.85}\)Ti\({}_{3}\)O\({}_{12}\) thin films [67]. Other Aurivillius compositions have also been prepared and  
Figure 8: Thermal evolution of the real part of the piezoelectric coefficient d\({}_{31}\) for recrystallized and sintered ceramics (taken from [63]). characterized. Thus, [PERSON] et al. processed thin films with SrBi\({}_{2}\)Nb\({}_{2}\)O\({}_{9}\), SrBi\({}_{2}\)Ta\({}_{2}\)O\({}_{9}\), SrBi\({}_{4}\)Ti\({}_{4}\)O\({}_{15}\) and different solid solutions among them. Maxima values of 19 and 25 \(\upmu\)C/cm\({}^{2}\) were achieved in (SrBi\({}_{2}\)Nb\({}_{2}\)O\({}_{9}\))\({}_{0.80}\)-(SrBi\({}_{2}\)Ta\({}_{2}\)O\({}_{9}\))\({}_{0.20}\) and (SrBi\({}_{2}\)Ta\({}_{2}\)O\({}_{9}\))\({}_{0.80}\)-(SrBi\({}_{4}\)Ti\({}_{4}\)O\({}_{15}\))\({}_{0.20}\), respectively [68]. [PERSON] et al. reported values of \(P_{r}\) = 15.7 \(\upmu\)C/cm\({}^{2}\) in non c-oriented SrBi\({}_{2}\)Nb\({}_{2}\)O\({}_{9}\) films [69] (Figure 9). The interest in Aurivillius thin films is also related to the fatigue-free characteristics for this family of compounds. Fatigue is the ability of the material to cyclically switch its ferroelectric polarization without reducing the amount of switched charge. This property makes this material a potential candidate for nonvolatile ferroelectric random access memory FeRAM applications. The positive and negative polarization can be taken as \"1\" and \"0\" states. There are several advantages with respect to traditional memories, as high speeds or low voltages needed for the data writing.