Exploring the extraordinary properties around morphotropic phase boundary is important for the applications of functional oxide materials. A better understanding on the domain transition kinetics around morphotropic phase boundary under external fields will provide valuable information for the novel device designs. Based on my thermodynamic study of PMN-PT, the domain switching kinetics by electrical and mechanical stimulation in relaxor-based ferroelectric PMN-PT is studied by combined approach of phase-field modeling and in-situ TEM (by University of Sydney). Utilizing the significantly small but asymmetric energy barriers among different domains, we realize recoverable ferroelastic domains and ferroelectric switching via ferroelastic nanoscale hierarchical domain structures. We hope these discoveries can promote new ideas in the application fields of non-volatile memory and controllable ferroelastic transitions.
Superlattices, composed of two or more periodically repeated atomic/nanoscale oxide layers, offer additional degrees of freedom in addition to composition and strain, to engineer the domain structures and thus their responses under an external field. The main objective of this work is to determine whether enhanced piezoelectric responses, which are closely tied to dielectric permittivity, can be achieved through engineering the monoclinic phase by taking advantage of the electrostatic coupling and elastic forces among different layers in the ferroelectric superlattices. Thus, we select traditional ferroelectric PbTiO3 and BaTiO3 as the two individual but coupled layers. First of all, there is a large difference between the pseudocubic lattice parameters of the two individual layers, thus there are many available substrates which can simultaneously exert tensile and compressive strains onto PbTiO3 and BaTiO3 layers, respectively. Secondly, the strong electrostatic coupling among the two ferroelectric layers can induce polarization rotation from in-plane to out-of-plane, resulting large volume of monoclinic phases, which are important for piezoelectric enhancements. We also find a minimum tetragonality exists at room temperature when the PbTiO3 volume fraction is around 90%, resulted from its different sensitivity to epitaxial strains and the electrostatic coupling of the two polarized layers in this PbTiO3/BaTiO3 superlattice. More details can be found in our papers published in JAP and PRB.
For many applications, the functionality of oxide materials and devices is ultimately determined by the concentration and diffusion dynamics of oxygen vacancies. The role of oxygen vacancies in phenomena such as fatigue and degradation in ferroelectrics is very important. The objective of this study is to reveal the dynamics of oxygen vacancies in the ferroelectric capacitors. The ferroelectric material is considered as n-type semiconductor where oxygen vacancies acted as single donors. The results showed that two Schottky barriers near the electrode/ferroelectric interfaces existed in both the 90° and 180° domain structures. It has also been observed that while an intrinsic electrostatic potential drop across the 90° domain wall created the electric fields which drove the electrons and oxygen vacancies aggregate on the different sides of the domain wall, the 180° domain wall had insignificant interaction with the potential, and no electron or vacancy accumulation in 180° domain structure was observed. Thus, polarization fatigue and leakage of ferroelectric capacitors are possibly originating from 90° domain walls. Polarization charge density is believed to be the origin of this difference. This understanding on the intrinsic interaction between the charged defects and domain walls provides the experimental researchers important suggestions to improve the performances of ferroelectric devices through designing ferroelectric domain structures. Details can be found in our PRB paper.
More works on structural materials:
- Effects of film thickness and mismatch strains on magnetoelectric coupling in vertical heteroepitaxial nanocomposite thin films.
- Interface and surface effects on 180 stripe domain structure in ferroelectric thin films.
- Vortex domain structure in free standing nanoparticles (JAP and JPD) and epitaxial nanodots.
- Domain structure on electrocaloric effects in ferroelectric thin films.
Upcoming works:
- Strain-controlled displaced morphotropic phase boundary in epitaxial relaxor Pb(Mg1/3Nb2/3)O3-PbTiO3 film.
Liang Hong 