  We have studied the polarization fatigue of La and Mg co-substituted BiFeO3
thin film, where a polarization peak is observed during the fatigue process.
The origin of such anomalous behavior is analyzed on the basis of the defect
evolution using temperature-dependent impedance spectroscopy. It shows that the
motion of oxygen vacancies (VO..) is associated with a lower energy barrier,
accompanied by the injection of electrons into the film during the fatigue
process. A qualitative model is proposed to explain the fatigue behavior, which
involves the modification of the Schottky barrier upon the accumulation of VO..
at the metal-dielectric interface.
