Fatigue failure in ferroelectrics has been intensively investigated in thepast few decades. Most of the mechanisms discussed for ferroelectric fatiguehave been built on the "hypothesis of variation in charged defects", whichhowever are rarely evidenced by experimental observation. Here, using acombination of complex impedance spectra techniques, piezoresponse forcemicroscopy and first-principles theory, we examine the microscopic evolutionand redistribution of charged defects during the electrical cycling in BiFeO3thin films. The dynamic formation and melting behaviors of oxygen vacancy (VO)order are identified during the fatigue process. It reveals that the isolatedVO tend to self-order along grain boundaries to form a planar-alignedstructure, which blocks the domain reversals. Upon further electrical cycling,migration of VO within vacancy clusters is accommodated with a lower energybarrier (~0.2 eV) and facilitates the formation of nearby-electrode layerincorporated with highly concentrated VO. The interplay between the macroscopicfatigue and microscopic evolution of charged defects clearly demonstrates therole of ordered VO cluster in the fatigue failure of BiFeO3 thin films.
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