Ferroelectric Self-Poling, Switching, and Monoclinic Domain Configuration in BiFeO 3 Thin Films
Self-poling of ferroelectric films, i.e., a preferred, uniform direction of the ferroelectric polarization in as-grown samples is often observed yet poorly understood despite its importance for device applications. The multiferroic perovskite BiFeO 3 , which crystallizes in two distinct structural polymorphs depending on applied epitaxial strain, is well known to exhibit self-poling. This study investigates the effect of self-poling on the monoclinic domain configuration and the switching properties of the two polymorphs of BiFeO 3 (R' and T') in thin films grown on LaAlO 3 substrates with slightly different La 0.3 Sr 0.7 MnO 3 buffer layers. Our study shows that the polarization state formed during the growth acts as “imprint” on the polarization and that switching the polarization away from this self-poled direction can only be done at the expense of the sample's monoclinic domain configuration. We observed reduction of the monoclinic domain size and found that it was largely reversible; hence, the domain size is restored when the polarization is switched back to its original orientation. This is a direct consequence of the growth taking place in the polar phase (below T c ). Finally, switching the polarization away from the preferred configuration, in which defects and domain patterns synergistically minimize the system's energy, leads to a domain state with smaller (and more highly strained and distorted) monoclinic domains.