Switching polarisation (2)
Consider a material which is fully polarised, so that all of the dipole moments are aligned in the same direction. Then apply a reversed electric field over it. New domains with a reversed polarisation nucleate at the electrodes. They then grow towards the other electrode, forming needle domains. When they reach the other electrode, they then grow laterally until the polarization of the entire sample is reversed.
This shows the origin of the hysteresis loop. The removal of the field will leave some polarisation behind, and only when the field is reversed does the polarisation start to lessen as new, oppositely poled domains form. They grow quickly however, giving a large change of polarisation for very little electric field. But to form an entirely reversed material, a large switching field is required. This is because of both defects in the crystal structure, in a manner similar to zener drag, and also to do with stray field energy. The polarisation of the material goes from a coupled pattern, with 180° boundaries, to a state in which many heads and tails are separated. This leads to the increase in stray field energy. Therefore, to attain this state, lots of energy has to be put in by a larger field.
Here we show a how a minor hysteresis loop fits into the major loop above.
The part of curve shown fits into the major hysteresis curve.
There are three sections to this curve.
1) Reversible domain wall motion.
2) Linear growth of new domains.
3) New domains reaching the limit of their growth.