Crystallized Domain Walls in HfO₂ Enable Chessboard-Type Ultra-High-Dense Ferroelectric Memory

Ferroelectric domain walls (DWs) are interfacial buffering regions between oppositely polarized domains, whose stability and density critically influence the endurance and scalability of ferroelectric memories. In conventional ferroelectrics, accommodating oppositely polarized regions requires DWs to spread over several unit cells through a high-symmetry transition region, incurring substantial energetic costs and limiting device scalability. Here, the reported DWs in HfO₂ are sharply localized within nearly a single-unit-cell width as low-symmetry structures, resulting in remarkably low DW energies along arbitrary crystallographic directions. Strikingly, these DWs exhibit distinct low-energy crystalline configurations, including the experimentally observed Pbca DW structures and previously unreported phases in HfO₂. The sharply localized DWs ultimately stabilize ultrasmall square sub-nm² ferroelectric domains separated by sizable energy barriers, thereby enabling chessboard-type ultra-high-density domains reaching ∼400 Tbit/cm². Inspired by experimentally observed anomalous behaviors such as sluggish DW dynamics and imprint effects, our discovery establishes that DWs formed during polarization switching can themselves constitute distinct and stable bulk-like structural phases, expanding the conventional paradigm of DWs into a new structural building block for emergent phases. Consequently, this discovery provides a streamlined route toward enhanced ferroelectric functionality and improved CMOS-compatible device performance, including lower operating voltages and ultra-high-density memories.