Molecular Dynamics Simulation Of Vortex Domain Structures Formation And Evoultion In Defective Ferroelectrics

Ferroelectric materials are attracting increasing attention due to a wide spectrum of excellent functional properties.It is promising to take advantage of these properties in designing and manufacturing advanced functional electronic,optoelectronic,and mechatronic devices.Low dimensional ferroelectrics have novel properties in comparison with bulk counterparts,such as formation and evolution patterns of special polarization domain configurations at nanoscale,and this can expand many potential engineering applications.However,it is inevitable to introduce and generate defects during the preparation and service of ferroelectric devices,and the polarization properties of ferroelectric materials will be strongly affected by these defects.Therefore,it is of great scientific and engineering significance to study the formation and evolution mechanism of polarization domains in nano-ferroelectric thin films with defects.In this dissertation,a defective crystal cell model of perovskite ferroelectric is established,a charge compensation method for defective crystal cells is also proposed,and the calculation formulas of characteristic physical quantity is derived.The configuration features and evolution behavior of nano-scale polarization domains in ferroelectric thin films with different defects under mechanical loading are investigated using the molecular dynamics method based on the shell model.The main creative research achievements are introduced as follows.(1)The shapes,sizes and position relationships of polarization vortexes in superlattices,as well as their motion and annihilation processes are investigated in three dimensions.It is observed for the first time that long-range ordered array of clockwise-counterclockwise vortexes occur simultaneously in different material layers.The vortexes continually change their locations,shapes,and sizes and possibly walk across the bimaterial interface with increasing compressive strain.The vortexes near the material interface or boundaries are inclined to annihilate first by vanishing at the interface or boundary,while the vortexes inside the material annihilate by moving towards and colliding with the antivortexes.(2)The formation and evolution mechanisms of polarization domains in barium titanate thin films containing oxygen vacancies are investigated.The unique domain structure of a clockwise closure domain surrounding a head-to-head domain caused by oxygen vacancies is observed for the first time.The effect of oxygen vacancy lattice volume fraction on the polarization domains is revealed.As the oxygen vacancies increase,around the vacancy lattice,the closure domain gradually switches from clockwise to counterclockwise,while inside the vacancy lattice,the polarization domains evolves from a random configuration to a clockwise vortex,and finally to a segmented stripe-shaped multi-domain structure.For a single oxygen vacancy,its location region where various closure domains can occur is obtained.(3)The whole process from initiation to annihilation of a polarization vortex induced by a void under compressive loading is investigated,and it is found that a void can enhance the stability of polarization vortexes.It is shown that the increase of the void volume fraction will cause the clockwise vortex to gradually turn to counterclockwise and completely reverse at a certain proportion.The effects of the size,shape,orientation and position of the void on the domain switching of polarization vortexes are revealed,and the conditions for the nucleation of clockwise and counterclockwise vortex domains around a void are given.This dissertation demonstrates the feasibility of controllling the polarization vortex configurations in ferroelectric nanofilms by controlling the size and position of oxygen vacancy lattice and void,and can provide a theoretical basis for the design and development of logic storage devices based on ferroelectric polarization vortexes.