5.3.9 Ferroelectric Switching and Domains

Application of an electric field to a ferroelectric ceramic leads to the alignment of antiparallel or off-aligned dipoles to reorient themselves along the field.

At sufficiently large fields, all the dipoles will be aligned along the field and the direction reverses by reversing the direction of the field. This phenomenon of polarization reversal takes place by way of nucleation and growth of favourably oriented domains into the unfavourably oriented domains and associated domain wall motion.

If we assume that our hypothetical crystal has an equal number of positive and negative domains in the virgin states, then the net polarization of the crystal will be zero. Now what happens when field E  is applied? The plot that we get is something like shown above where P is polarization in μC/cm²and E is electric field across the sample in V/cm . The process is something like this:

Initial polarization P  increases linearly with the increasing electric field and the crystal behaves like a dielectric because the applied field is not large enough to switch any of the domains oriented opposite to its direction. This linear region is shown as AB .

Further increase in the field strength forces nucleation and growth of favourably oriented domains at the expense of oppositely oriented domains and polarization starts increasing rapidly (BC) until all the domains are aligned in the direction of the electric field i.e. reach a single domain state (CD) when polarization saturates to a value called saturation polarization (P_S). The domains reversal actually takes place by formation of new favourably oriented domains at the expense of unfavourable domains. Now, when the field is decreased, the polarization generally does not return to zero but follows path DE and at zero field some of the domains still remain aligned in the positive direction and the crystal exhibits a remanent polarization (P_R). To bring the crystal back to zero polarization state, a negative electric field is required (along the path EF) which is also called the coercive field (E_C).

Further increase of electric field in the opposite direction will cause complete reversal of orientation of all domains in the direction of field (path FG) and the loop can be completed by following the path GHD.
This relation between P and E is called a ferroelectric hysteresis loop which is an important characteristic of a ferroelectric crystal. The principle feature of a ferroelectric crystal is not only the presence of spontaneous polarization but also the fact that this polarization can be reversed by application of an electric field.

Domain switching in a previously polarized ferroelectric sample can also be viewed in the following animation.