Ferroelectric domain breakdown.

We observe a stringlike domain penetration from a ferroelectric surface deep into the crystal bulk induced by a high voltage atomic force microscope tip. The domains, which resemble channels of an electrical breakdown, nucleate under an electric field of around 10(7) V/cm at the ferroelectric surface, and grow throughout the crystal bulk where the external electric field is practically zero. A theory explaining the shape of the formed domains is presented.