Switching polarisation (1)
In an electric field, E, a polarised material lowers its (volume-normalized) free energy by –P.E, (where P is the polarisation). Any dipole moments which lie parallel to the electric field are lowered in free energy, while moments that lie perpendicular to the field are higher in free energy and moments that lie anti-parallel are even higher in free energy, (+P.E).
This introduces a driving force to minimise the free energy, such that all dipole moments align with the electric field.
Let us start by considering how dipole moments may align in zero applied field:
These two moments are stable, because they sit in potential energy wells. The potential barrier between them can be represented on a free energy diagram:
This material is considered to be homogenous. If the polarisation points left then we have:
The electric field alters the free energy profile, resulting in a ‘tilting’ of the potential well:
An increase in the electric field will result in a greater tilt, and lead to the dipole moments switching, leading to:
Next we must look at the more realistic scenario in which domains form.