Coercive fields in ferroelectrics: A case study in lithium niobate and lithium tantalate

The experimentally measured coercive electric fields for domain reversal in ferroelectrics are typically many orders of magnitude lower than the estimates from phenomenological free-energy theory. This letter specifically investigates the influence of polarization gradients at pre-existing 180° domain walls in ferroelectrics on coercive fields for domain wall motion. It is shown that the ratio of theoretical coercive field without and with a preexisting domain wall is directly proportional to the ratio x_o/a, where a is the lattice parameter and 2x_o is the polarization wall width. This factor is 7.5-45 for a 20-120 nm wall width, the latter width determined here as the experimental upper limit for polarization wall width in lithium tantalate.