Domain wall kinetics of lithium niobate single crystals near the hexagonal corner

A mesospheric approach based on a simple microscopic 2D Ising model in a hexagonal lattice plane is proposed to explain macroscopic "asymmetric in-out domain wall motion" observation in the (0001) plane of MgO-doped stoichiometric lithium niobate. Under application of an electric field that was higher than the conventional coercive field (E-c) to the ferroelectric crystal, a natural hexagonal domain was obtained with walls that were parallel to the Y-axis of the crystal. When a fraction of the coercive field of around 0.1E(c) is applied in the reverse direction, this hexagonal domain is shrunk (moved inward) from the corner site into a shape with a corner angle of around 150 degrees and 15 degrees wall slopes to the Y-axis. A flipped electric field of 0.15E(c) is then applied to recover the natural hexagonal shape, and the 150 degrees corner shape changes into a flat wall with 30 degrees slope (moved outward). The differences in corner domain shapes between inward and outward domain motion were analyzed theoretically in terms of corner and wall site energies, which are described using the domain corner angle and wall slope with respect to the crystal Y-axis, respectively. In the inward domain wall motion case, the energy levels of the evolving 150 degrees domain corner and 15 degrees slope walls are most competitive, and could co-exist. In the outward case, the energy levels of corners with angles >180 degrees are highly stable when compared with the possible domain walls; only a flat wall with 30 degrees slope to the Y-axis is possible during outward motion. (C) 2015 AIP Publishing LLC.