Research On The Control Of Domain Structure And Polarization State In Ferroelectric Thin Films

Ferroelectric materials are one kind of materials that have spontaneous polarization,dielectric properties of piezoelectric and pyroelectric property in a certain temperature range,which can be used in sensor switch drive and memory,and other fields.Ferroelectric domain structure has a great influence on the physical properties of ferroelectric thin films,in particular,due to the substrate surface strain effect and the influence of size effect,more complex domain structure and physical properties can be produced with rich physical phenomena.In this paper,ferroelectric phase field model is established with the consideration of electrical boundary conditions and substrate strains.The domain structure and polarization fields of PbTiO₃thin film and PMN-0.36PT thin film were further investigated under various condition.Based on the time evolution of the Ginzburg-Landau theory,combined with the phase field method,we build total free energy to study the domain structure evolution and polarization field.Finite difference and finite element methods are used to solve the total free energy equation including the landau free energy,gradient of depolarization field,elastic energy,and the external forces.By solving time-dependent Ginzburg-landau equation,electrostatic potential energy,phase,polarization and domain structures can be studied.Results show that,electrode distribution and substrate strain boundary conditions have vital effects on the domain structures and corresponding physical properties.Main content is listed as following:(1)Establish the time-dependent Ginzburg-Landau free energy,and numerical solution based on finite difference and finite element methods.Phase field model of PbTiO₃and domains in the thin film structure can be changed over time.We also discussed the formation and development of domain structure and the steady state,and to explore the influence of boundary conditions on the domain structure.(2)Domain structure of PbTiO₃ film and PMN-0.36PT film were studied under short-circuit and open-circuit electrical boundary conditions.Existence of low symmetry phase were explored in the frame work of Ginzburg-Landau theory.(3)The domain structure and its stability of PbTiO₃ thin film under stress constraint were studied.Possible domain structure and its causes were discussed,and the strain conditions and controllability of domain structure mutation were analyzed in detail.(4)Domain structures in PMN-PT thin film and its stability under stress constraints were investigated and compared with lead titanate.The effect of substrate strain on the domain of PMN-PT thin film was calculated for further control of the domain structures through micro clamping strain and polarization status inside the domain structure.Results will further impact the design of functional materials with high quality and properties.