Nonlinear dielectric properties and polarization in thin ferroelectric P(VDF-TrFE) copolymer films

Thin VDF-TrFE copolymer films of molar composition 70/30 and thickness from 25 nm to 175 nm have been prepared by spin coating on a glass substrate covered with an aluminum electrode. The linear, second and third order permittivities ε₁, ε₂ and ε₃ of poled and unpoled samples have been studied versus temperature in heating and cooling cycles up to 120°C, i.e., above the Curie temperature. The polarization and its temperature dependence are derived from ε₁ and ε₂ . This allows a non-destructive readout of the polarization state. It is found that above the Curie temperature and after cooling to room temperature a non-switchable polarization remains. This nonswitchable polarization points from the bottom to the top electrode and is not influenced by the initial polarization direction.