The Dielectric Response Of Ferroelectric Thin Films With Surface Transition Layers

Ferroelectric thin films have been one of the most active study areas from both fundamental researches and technologies point of view. Ferroelectric thin films are widely applied to microsystems, memories and high frequency electrical components due to their unique properties such as dielectric, ferroelectric and piezoelectric properties. With the increase of science and technology, the prepared techniques of ferroelectric thin films tend to perfect and the miniaturization for the ferroelectric thin films based devices is possible. However, the size effects result in the physical properties of ferroelectric thin films seriously degrade compared to those of the corresponding bulk materials. So far, the size effect remains a hotspot topic. At the same time, the artificial synthetical ferroelectric bilayer films, multilayers, superlattice structures and graded films have been aroused great interest for physical reseach scholars. It is surprising to find that different compositions of ferroelectric thin film layers with suitable combination can improve the devices performance or create new functional properties. For example, we can obtain the antiferroelectric behaviors by ferroelectric multilayers or superlattices. In these structures, the couplings and interactions between layers can intensively affect the growth and physical properties of films. They create new candidate materials and privide a new opportunity for the device applications of ferroelectric thin films. The abnormal phenomena mainly come from the characters of interactions between layers, but originate from some unavoidable factors synchronously, such as impurities, defects, surface and interface stress, and interfacial effects between different components and so on, and these factors are native. They may bring about local differences in thin film structures, thus, it will produce structure transition zones near the surface or interface of thin films. Therefore, on basic investigation of the effects of the surface or interface transition layers, not only has the significance of theoretical research but also has very practical research value. Under such background, the dielectric response of ferroelectric thin films with surface or interface transition layers is investigated, based on the mean-field approximate soft mode theory, Landau phenomenological theory and mean-field approximate Ising mode theory.Within the framework of mean-field approximate soft mode theory, the dynamic properties of ferroelectric thin films with surface transition layers are first studied. The results show that the surface transition layer is the essential reason for the differences of the dynamic characters between the thin films and bulk materials. At arbitrary temperture, the existence of surface transition layers makes the average damping factor augment and frequency of soft mode fall. Moreover, the peaks of the curves of frequency or temperature dependence of the real and imaginary parts of the average dielectric constant widen, the peak values reduce, and the peak positions shift to the low frequency or low temperature.Using the GLD phenomena theory, we study the polarization and dielectric susceptibility of ferroelectric bilayer films with the surface transition layer. The results show that the surface transition layer is disadvantaged to the system at ferroelectric phase, but ferroelectric interfacial coupling (the interaction between two thin films in the ferroelectric bilayer film) favors the system at ferroelectric phase. Due to the competition between the surface transition layer and ferroelectric interfacial coupling, the system should be a transition point of dielectric property, if the actions of the ferroelectric interfacial coupling are less than those of surface transition layer, the size-driven phase transition in the system may occur and the dielectric susceptibility will augment; on the contrary, the size-driven phase transition in the system cannot take place, and the dielectric susceptibility will reduce. The anomalous behaviors of ferroelectric bilayer films are attributed to the competition of the surface transition layer and ferroelectric interfacial coupling. We can obtain different cases of physical properties of ferroelectric bilayer films by controlling the parameters of surface transition layer and ferroelectric interfacial coupling. We explain the seemingly contradictory phenomena observed in experiments by introducing the surface transition layer in the ferroelectric bilayer film.Within the framework of mean-field approximate Ising mode theory, we study the hysteresis loop and dielectric susceptibility of ferroelectric bilayer films with antiferroelectric interfacial coupling by introducing the surface transition layer. We choose two different ferroelectric layers A and B, the bulky pseudo spin interactions are J Aand J B, the transverse fields are ? aand ? b, respectively. The results show that the actions of the surface transition layer are disadvantaged to the system at antiferroelectric character, but the antiferroelectric interfacial coupling favors the system at antiferroelectric character. Moreover, we predict that for a given surface transition layer thickness, there are two critical pseudo-spins interactions of surface transition layer J b1 and J b2, and two critical tunnel frequencys of surface transition layer ? a1and ? a2. When J b1 < Jb