Ferroelectric Bloch-line switching: A paradigm for memory devices?

Vortices inside polar domain walls in ferroelastic materials can form ordered arrays resembling Bloch-lines in magnets. The Bloch lines are energetically degenerate with dipoles oriented perpendicular to the wall. By symmetry, these dipoles are oriented at +90° or −90° relative to the wall dipoles. These two states have the same energy and can be inverted by modest applied electric fields. As the majority of wall dipoles are oriented inside the wall, perpendicular to the Bloch line vortex, weak depolarization fields exist for the wall dipoles but not for Bloch lines. The Bloch line density depends on the density of the twin walls and the elastic anisotropy of the crystal structure. We estimate that distances between twin boundaries are as small as 50 nm and Bloch lines can form with some densities of 100 Bloch lines in an area of 100 × 100 nm2. The local dipole moment in the Bloch line is equivalent to the displacement of Ti in BaTiO3. Switchable Bloch lines can be detected by their macroscopic dipole moment and can constitute the functional part of a memory device