Dynamic Scaling Behavior Of Ferroelectric Domain Walls Studied By Scanning Probe Microscopy

Ferroelectric materials are divided into domains inner their body,which contains spontaneous polarization with different directions.Two neighbouring domains are separated by domain walls,a thin area of polarization transition region.Due to the thin thickness of domain walls,detailed research is restricted by characterization methods,and until now there has not been an effective method to study microscopic dynamics of domain walls.In this thesis,we are trying to adopt the dynamic scaling method from surface growth studies to the study of ferroelectric domain walls.We want to macroscopically calculate the boundary evolution during a domain's growth,and to determine the universality class of this system,and then bring the characteristics of this class into ferroelectricity system to help us better understand the microscopic process such as nucleation and wall expansion.In this thesis,we studied the domain evolution process of two typical ferroelectric materials,lithium niobate and bismuth ferrite.The contents are divided into the following two parts:Firstly,a method based on scanning probe microscopy to control and record domain growth was devised.We used this method to observe the evolution of one single domain,and used the scaling concept to study the roughness process.Important scaling exponents were determined,including roughness exponent ?=1.4,growth exponent ?=0.37 and dynamic exponent z=4,and they indicated that the growth can be described by one dimensional molecular beam epitaxy model,which stresses on the influence of local shape on nucleation rate.Secondly,imaging quality of low temperature piezoresponse microscopy was optimised,and the dependence of domain wall velocity on temperature was studied by this facility.Our result indicated that the domain wall motion under tip field was belongs to disordered creep motion,and the activation energy of this motion under different temperature was determined to be 2.30±0.05 MV/cm,2.80±0.05 MV/cm and 4.70±0.10 MV/cm under 290 K,40 K and 10 K,respectively.Our result conforms to the theoretically strong dependence of creep motion on temperature,revealing the important role that thermal fluctuation plays during ferroelectric polarization switch.