Enhancing polarization retention in BiFeO3 domain wall nanodevices through controlled space charge redistribution

Abstract

Ferroelectric thin films with binary switchable polarization offer promising applications in domain wall (DW) memory. However, achieving reliable polarization retention in size-reduced devices remains challenging. In this study, we present a planar ferroelectric BiFeO3 DW nanodevice demonstrating stable polarization retention through electrical cycling. By precisely controlling the device size, including the width and gap of planar Pt electrodes, we observed a back-switching process in switched polarization within 20 min, as characterized by ex-situ piezoresponse force microscopy (PFM). To overcome this issue, long-time alternating voltage pulse cycling effectively suppressed the back-switching behavior, leading to symmetrized polarization coercive voltages of the device. PFM characterization further confirmed the persistence of the artificially created DW between the electrode pair. This improved polarization retention was attributed to the migration of mobile charged defects, such as oxygen vacancies, at the domain boundary. The presence of these defects generated a local built-in field, which screened the net polarization induced by the charged DW. Upon erasing the DW, the local built-in field gradually dissipated during opposite voltage poling, resulting in restored poor polarization retention and asymmetric coercive voltage in the nanodevices. This work presents a promising approach to utilize defect redistribution for stabilizing the switched polarization in the development of high-density ferroelectric DW memory.